In this report an attempt was made to quantify the impact of the use of kitchen food waste disposers to the Dutch sewer system
CONCLUSION In this report an attempt was made to quantify the impact of the use of kitchen food waste disposers to the Dutch sewer systems and waste water treatment plants. Although an overwhelming amount of data in foreign literature is available regarding the impacts of kitchen food waste disposers there are no data based on fieldwork in literature dealing with the Dutch situation. The quantification of the impacts of disposers to the sewer systems and waste water treatment plants in the Netherlands is based both on Dutch statistics and on a part of the mentioned foreign literature and on scientific judgment and on common sense. The impact of the use of a kitchen food waste disposer to the sewer system and the waste water treatment is judged on the basis of the calculated impact at a penetration level of 5 to 10%. As stated before according to the manufacturer a penetration level of 10% should be qualified as the higher end of the possible long-term penetration in view of historical penetration levels in the USA and the United Kingdom. One general conclusion of this study with respect to the impact of kitchen food waste disposers to the Dutch sewer systems and waste water treatment plants is that this impact is minimal and that the adverse effects are negligible. More specific the most important conclusions are: ●
●
●
●
There is no evidence that the use of kitchen food waste disposers leads to clogging of indoor and outdoor sewer pipes. The use of kitchen food waste disposers does not lead to an increase of the hydraulic load to the sewer system. The increase of loads to the biological waste water treatment processes due to the use of kitchen food waste disposers is negligible. There is an increase of 2.5 to 5% in loads to the sludge thickeners and digester. With the existing capacity this increase of loads is negligible.
With respect to the increase of costs for sludge dewatering and sludge incineration due to the use of kitchen food waste disposers the following remarks
In this report an attempt was made to quantify the impact of the use of kitchen food waste disposers to the Dutch sewer system
can be made: â—?
â—?
The costs for dewatering and incineration of the sludge produced by one person who is using a kitchen food waste disposer are approximately f 9.50 per year; With a penetration level of 5% and charging the total Dutch population for the additional costs the waste water treatment costs will increase with approximately f 0.35 per person per year. Compared with the annual waste water treatment costs in 1993 of f 63.00 per person this means an increase of only 0.5%. With 10% penetration the increase in costs is approximately f 0.69 per person per year or 1.1%.
Kitchen Food Waste Disposers Effects On Sewer System and Waste Water Treatment April 1996
dr.ir J. de Koning, prof.ir. J.H.J.M. van der Graaf
TU Delft Faculteit der Civiele Techniek Technische Universiteit Delft Department of Water Management. Environmental & Sanitary Engineering
Waste Management Research Unit
EXECUTIVE SUMMARY There are a number of options available to Local Governments for the collection and disposal of organic wastes – including putrescible wastes (kitchen food scraps) which make a significant contribution to the total organic waste currently going to landfill. All governments in Australia are aiming to reduce the total material going to landfill by 50%. Many of the current options proposed and endorsed by Local Governments for achieving these targets are not substantiated by reasonable scientific evidence. This study was undertaken in an attempt to compare a number of alternatives and try to rank them. The results are summarised below: 1.
Food Waste Disposers (FWD) The principal arguments proposed against the use of these kitchen appliances have to do with the additional loads which they would present to the sewage treatment plants. This study examined the impacts of food waste disposal units (FWD) and compost bins used in the Ashmore suburb of the Gold Coast City in Queensland. The calculations related to FWD units are based on a maximum 100% penetration of the market. This means that all households would have such devices installed, and all kitchen food scraps would be diverted from the normal waste management practice (Wheelie Bin/Landfill) to the sewer and sewage disposal plant. Hydraulic Load. It was shown that the increase in flow would only amount to 0.4% of the existing flow. This must be considered to be trivial. Solids Load. The increase in sludge production as a result of the installation of FWDs is more considerable, and would add 18.1% of the existing production.
Waste Management Research Unit
Organic Carbon (BOD) Load. The increase of BOD was shown to be 16.5% of the existing load. Effect on the Treatment System. Based on the most pessimistic circumstances (Plants presently at full load capacity) the aeration tanks would have to be increased in size by 16.5%. Nutrient Removal (N&P). The incremental nutrient load resulting from 100% use of FWDs would amount to 3.0% total N, and 4.6% total P. Water consumption would increase by approximately 4 litres/household/day and electricity consumption by less than 3 kWh/household/year (costing approximately $0.26/household/year). Summary Hydraulic Load Sludge Activated Sludge aeration tank volume Nutrient Removal 2.
+ 0.4%, negligible + 18% Zero to + 16.5% increase in Very slight increase in load
Compost Bins and Tumblers Studies of different design home composting units included in the “Compostabin” design promoted by the Brisbane City Council, and 3191 “Sunshine tumblers”. These studies were instigated to evaluate the production of methane, carbon dioxide and leachate. These issues have not been addressed adequately before various councils have promoted compost bin use. The “Self Evident” value of these devices has not been subjected to any scientific scrutiny.
Waste Management Research Unit
It is assumed that some of the composters will be properly managed in accordance with the manufacturers’ instructions, but some will not. The study therefore examined the difference in performance of the same devices under “managed” and “unmanaged” circumstances. They were also tested with and without kitchen food scraps (highly putrescible). Volume Reduction “Managed” or “unmanaged” tumblers or bins gave about the same reduction in waste volume. Leachate Production “Managed” tumblers with food scraps added produced considerably more leachate than without food scraps. With food scraps: Total leachate production 2091 ml Leachate production per kg compost 44 ml BOD 101-2434 mg/l Without food scraps: Total leachate production 647 ml Leachate production per kg compost 14 ml BOD 202-1045 mg/l Leachate production ceased after 5 days without food scraps but continued to day 16 with increasing strength right up to day 16 with the addition of food scraps. “Unmanaged” tumblers with food scraps produced more leachate than without such scraps. With food scraps: Total leachate production 10,960 ml
Waste Management Research Unit
Leachate production per kg compost 335 ml BOD 586-4103 mg/l Without food scraps: Total leachate production 4,862 ml Leachate production per kg compost 152 ml BOD 947-2312 mg/l “Managed” bins with food scraps produced more leachate than without scraps. With food scraps: Total leachate production 19,980 ml Leachate production per kg compost 330 ml BOD 74-3188 mg/l Without food scraps: Total leachate production 6,114 ml Leachate production per kg compost 94 ml BOD 49-428 mg/l “Unmanaged” bins with food scraps produced much more leachate than without such scraps. With food scraps: Total leachate production 26,990 ml Leachate production per kg compost 601 ml BOD 374-6956 mg/l Without food scraps: Total leachate production 10,650 ml Leachate production per kg of compost 214 ml BOD 8-301 mg/l In all cases, unmanaged composters produced much greater volumes of leachate than did the corresponding managed units.
Waste Management Research Unit
Similarly, the “strength” of leachate as measured by BOD was higher for unmanaged composters than for the corresponding managed units (except for bins without food scraps). Temperatures Temperatures of all the bins and tumblers, whether managed or unmanaged, with or without food scraps, showed an initial rise followed by a gradual drop in temperature indicating a reduction of biological activity over the period of the test. Gases Carbon dioxide concentrations increased rapidly in both the tumblers, and the bins, either managed or unmanaged and after day 3 the rate of production gradually decreased to the end of the test. The test for the bins was extended for longer than 16 days for the gas production tests, and the carbon dioxide production continued to fall right up to 56 days. This indicates that aerobic conditions were prevalent, and the methane study substantiated this conclusion. Methane concentrations were not readily detectable until the 16th day, and then only in those bins containing food scraps. Thereafter the “unmanaged” bin with food scraps produced higher concentration of methane (up to 70+ml/m3) than the “managed” bin (only 9ml/m3 on day 16). No estimate of the total methane produced can be derived from the experiments as a total gas collection and exchange system would be required to provide essential information on total gas volumes produced. CONCLUSIONS The conclusions which may be drawn from the above data are as follows:
Waste Management Research Unit
FWDs do not present an unmanageable load on the existing sewage treatment facilities. Home Composting devices produce a considerable volume of high strength (measured as BOD) leachate when kitchen food scraps are present in the composters. There is no readily available mechanism for retrieving or managing these leachates. Also, the potential for an environmental impact from compost leachate is greater for unmanaged units than for those well managed. There are two important points to note regarding gas generation. The first is that the amount of carbon dioxide ultimately produced per tonne of organic matter is the same irrespective of the process used. Only the rate of generation is affected by process considerations. The second point relates to the production, under anaerobic conditions, of methane as an intermediary. Methane released to the atmosphere will eventually be converted to carbon dioxide. However while present, methane has a much greater greenhouse effect than the equivalent amount of carbon dioxide. Environmentally therefore, it is desirable to minimise methane release. There is no readily available mechanism for achieving this with household composting. Further, the data indicates that poorly managed compost units will produce methane. In contrast municipal facilities such as landfills and sewage treatment works can be constructed to maximise recovery of methane for use as a fuel prior to conversion to carbon dioxide.
Economic and Environmental Impacts of Disposal of Kitchen Organic Wastes using Traditional Landfill - Food Waste Disposer - Home Composting Waste Management Research Unit - Griffith University A Report Prepared for In-Sink-Erator - August 1994
Waste Management Research Unit
Project Leader:
Professor Philip H. Jones Head of the School of Environmental Engineering Director, Waste Management Research Unit
Project Co-ordinator:
Dr. David Moy Deputy Director, Waste Management Research Unit
Project Team (in alphabetical order): Professor Philip H. Jones Mr. Vincent KampschĂ–er Dr. Jozef Latten Dr. David Moy Dr. Rodger Tomlinson Mr. John Ware Mr. Philip Williams Mr. Trevor Wilson
Waste Management Research Unit School of Environmental Engineering GRIFFITH UNIVERSITY QLD 4111
NYC DEPARTMENT OF ENVIRONMENTAL PROTECTION
The Impact of Food Waste Disposers in Combined Sewer Areas of New York City
EXECUTIVE SUMMARY
Domestic in-sink food waste disposers (FWDs) have been banned in New York City since the 1970s in areas served by combined sewer systems. The intent of the ban was to limit the direct discharge of raw organic wastes into water bodies surrounding the City during wet weather and to prevent possible deterioration of the City’s sewer system. Since that time, a number of cities have allowed the introduction of FWDs and some have mandated their use. There have been no reported significant adverse effects attributed to the use of FWDs and the plumbing industry and others have repeatedly requested that the City discontinue the current ban. In response to the public’s interest in FWDs, the Mayor requested that the City Council reconsider the ban. On September 22, 1995, Mayor Giuliani signed Local Law 74 authorizing the Department of Environmental Protection to conduct a 21- month pilot program to study the potential effects of permitting the use of FWDs in combined sewer areas. The goals of the pilot study, as enumerated in Local Law 74, are to analyze and evaluate: & & & & & & & &
the impact of grease and food solids on the operation of combined sewers; the impact on water consumption; the impact on the nutrient content of raw effluent; the impact of increased pollutant loadings to receiving waters, including increases in Biochemical Oxygen Demand (BOD) and suspended solids; the impact on wastewater treatment processes and sludge management; the impact on the City’s ability to comply with applicable statutes, rules, permits, and orders; the impact on solid waste management; and any other impacts on the environment, public health and safety, and the cost of operating the water and sewer system.1
To accomplish the goals of Local Law 74, DEP, in conjunction with the plumbing industry, representatives of FWD manufacturers and their consultants, and the Department of Sanitation, conducted a comprehensive analysis of the issue categories listed in Local Law 74. The Department has considered the results of the analysis and recommends that the ban on the introduction of FWDs in combined sewer areas of the City be lifted. A discussion of the Department’s recommendedon and a summary of the analyses for each impact area follows: 1
Local Law 74, p. 3.
NEW YORK CITY DEPARTMENT OF ENVIRONMENTAL PROTECTION
ES-2
Recommendation As stated above, the Department has concluded that the prohibition on the introduction of FWDs in combined sewer areas should be lifted. Although impact analysis to the year 2035 may give rise to concern in the out-years, the analysis assumes an extreme worst case scenario that is speculative in nature and not likely to materialize. For example, a maximum FWD penetration rate of 1 percent of households per year usage is predicted by the industry and used in the following analyses. Under this assumption, by 2035, over one-third of the households in the City would have voluntarily chosen to incur the expense of purchasing and installing a unit in their homes. The cost of purchasing and installing these appliances -- $300 to $500 -- argues against a one percent installation rate being sustained for nearly forty years. Furthermore, FWDs have been permitted since 1971 in areas of the City served by storm and sanitary sewers, yet saturation rates appear well below 25 percent, according to reports from industry representatives. However, the penetration rate of 1 percent per year of households is consistent with what industry has used as a maximum. In the absence of published evidence that the maximum would never be reached, the 1 percent per year is used here to project worst-case future impacts. In addition, there are many other uncertainties involved in projecting so long into the future. Only the infrastructure improvement programs currently planned can be factored into the analyses. There may be other changes to the wastewater treatment systems needed in the future, if, for example, standards for water quality become progressively more stringent in coming decades. Plans for such improvements are not likely to be significantly changed by the increases in pollutants noted due to the introduction of FWDs. The results of our analyses raise a cautionary flag at very high penetration rates. We believe it is prudent to monitor the introduction of FWDs to insure that the worst case analyses do not materialize. To that end, DEP will track FWD installation using information provided pursuant to the existing Department of Building permitting requirements for the installation of plumbing appliances, including FWDs. DEP will monitor the number and location of units installed and investigate the affected drainage basins as installation rates indicate a need. In the unlikely event that problems begin to materialize, the Department will immediately inform the City Council and recommend corrective action. That action may take the form of suspending installation approvals of FWDs either in affected areas or city-wide, adjusting water billing structures to insure that users of the FWDs are assessed for the cost of corrective actions, or other mitigative measures. Below is a summary of our pilot study and each impact analysis: Pilot Study Sites Three pilot locations were chosen for the study. Each location included a study group with FWD units installed and a control group without FWDs. The selected study locations were as follows:
FOOD WASTE DISPOSER PILOT STUDY
ES-3
Parkway Village, Queens A garden apartment complex bordered by Union Turnpike, Grand Central Parkway, Main Street and Parsons Boulevard. Pilot study area. Thirteen buildings numbered sixty-three to seventy-five along Grand Central Parkway. Population of 211 people. 34 FWDs were installed within 79 apartments. Control area. Eight buildings numbered one through nine, excluding building eight, along Union Turnpike. Population of 127 people. (One apartment out of 49 had a FWD installed by mistake).
Bay Ridge Towers, Brooklyn Two high-rise towers on 65th Street between 2nd and 4th Avenues. Study area: 350 65th Street. A high rise tower between 3rd and 4th Avenues. 121 FWDs installed within 392 apartments, serving 695 people. Control area: 260 65th Street. A high rise tower between 2nd and 3rd Avenues. 420 apartments with a population of 781 people.
Low-Rise Apartment Buildings along E. 85th Street, Manhattan Four- and five-story, pre-1947 walk-up apartments and one postwar elevator building between 1st and 2nd Avenues on the Upper East Side of Manhattan. A total of 88 FWDs were installed in three buildings. Study area buildings: & 326 E. 85th Street. A walk-up apartment with 11 FWDs installed in 17 apartments; population of 20 people. & 328 E. 85th Street. A walk-up apartment with 13 FWDs installed in 20 apartments; population of 27 people. & 344 E. 85th Street: A building with 64 FWDs installed in 65 apartments; population of 87 people. Control area: 333-339 E. 85th Street: A group of five, four- story walk-up apartment buildings with 66 people.
SAMPLING RESULTS Sampling Parameters The key parameters sampled included TSS (total suspended solids), BOD and BOD(F) (biochemical oxygen demand and its filtrate), COD (chemical oxygen demand), and nutrients including NO2 (nitrite), NO3 (nitrate), NH3 (ammonia), TKN (Total Kjeldahl Nitrogen), PO4 (orthophosphate), TP (Total Phosphorous), and Settleable Solids. The sampling results with and without FWDs are presented in Table ES-1,a-c. To provide a basis for analysis of future impacts, projections of future loadings were made for the
NEW YORK CITY DEPARTMENT OF ENVIRONMENTAL PROTECTION
ES-4
years 2000, 2005, 2010, 2025, 2035 (Table ES-2). For purposes of this study, it was assumed that disposers would be installed at a rate of one percent per year based on the total number of households in the City. DEP considers the near-term analysis years -- 2000 and 2005 -- to be a more reasonable time frame upon which impacts can be measured. Beyond that time horizon, impacts are considered speculative. It should be noted that the data indicate a much greater increase in levels of certain pollutants at the Brooklyn site when compared with the Queens and Manhattan data which are similar. This discrepancy may be due to the presence of a large sinkhole in the street bed of 65th Street. It is possible that soil and sand infiltrated the sewer from the sinkhole and contaminated the data. Therefore, two sampling averages were used in the analyses; one with Brooklyn data and one without. Based on previous measurements of typical New York City sewage, DEP considers the Brooklyn data to lie outside the “normal” range, especially for levels of settleable and total suspended solids. Although these data are included in the report in the interest of completeness, caution should be exercised in interpreting results with the Brooklyn data. The impact conclusions that follow are predicated on DEP’s belief that the Queens and Manhattan data are more representative of what can be expected if FWDs are introduced Citywide.
Table ES-1a. Average Pollutant Concentrations at Control Locations
Parameter
Queens Brooklyn Manhattan (lbs/capita/day) (lbs/capita/day) (lbs/capita/day)
Average Brooklyn, Manhattan, Queens
Without Brooklyn
TSS BOD BOD (F) COD pH NO2 NO3 NH3 TKN PO4 TP Settleable Solids
0.0721 0.0695 0.0369 0.1980
0.0815 0.0700 0.0412 0.2268
0.0587 0.0469 0.0253 0.1363
0.0707 0.0621 0.0345 0.1870
0.0654 0.0582 0.0311 0.1672
0.0000 0.0003 0.0129 0.0190 0.0016 0.0036 0.0010
0.0000 0.0001 0.0108 0.0202 0.0011 0.0023 0.0037
0.0000 0.0002 0.0053 0.0205 0.0014 0.0020 0.0058
0.0000 0.0002 0.0097 0.0199 0.0014 0.0026 0.0035
0.0000 0.0002 0.0091 0.0197 0.0015 0.0028 0.0034
Initial O&G (Grab)
0.0081
0.0218
0.0113
0.0137
0.0097
FOOD WASTE DISPOSER PILOT STUDY Comp O&G Final O&G (Grab) Initial TPH (Grab) Comp TPH Final TPH (Grab)
ES-5
0.0130 0.0078
0.0107 0.0081
0.0102 0.0074
0.0113 0.0078
0.0116 0.0076
0.0009
0.0051
0.0009
0.0023
0.0009
0.0024 0.0012
0.0027 0.0049
0.0013 0.0009
0.0022 0.0023
0.0019 0.0011
Table ES-1b. Average of Study Group Adjusted for 100% Food Waste Disposer Saturation
Parameter
100% FWDs Queens FWD Pop 49.4%
TSS BOD BOD (F) COD pH NO2 NO3 NH3 TKN PO4 TP Settleable Solids Initial O&G (Grab) Comp O&G Final O&G (Grab) Initial TPH (Grab) Comp TPH Final TPH (Grab)
100% FWDs 100% FWDs Brooklyn Manhattan FWD Pop 34.1%
Complete W/O Brooklyn Average Average
0.1197 0.1211 0.0492 0.2807
0.3408 0.2402 0.0963 0.5897
0.1048 0.1397 0.0582 0.2553
0.1884 0.1670 0.0679 0.3752
0.1122 0.1304 0.0537 0.2680
0.0000 0.0002 0.0172 0.0287 0.0018 0.0045 0.0088
0.0000 0.0001 0.0172 0.0390 0.0028 0.0050 0.0300
0.0001 0.0002 0.0088 0.0333 0.0024 0.0032 0.0095
0.0000 0.0002 0.0144 0.0337 0.0024 0.0042 0.0161
0.0000 0.0002 0.0130 0.0310 0.0021 0.0039 0.0092
0.0037
0.0157
0.0035
0.0076
0.0036
0.0178 0.0114
0.0211 0.0454
0.0171 0.0083
0.0187 0.0217
0.0174 0.0098
0.0003
-0.0000
0.0006
0.0003
0.0004
0.0025 0.0007
0.0106 0.0005
0.0013 0.0011
0.0048 0.0008
0.0019 0.0009
NEW YORK CITY DEPARTMENT OF ENVIRONMENTAL PROTECTION
ES-6
Table ES-1c. Differences Between Study and Control Groups Parameter TSS BOD BOD (F) COD pH NO2 NO3 NH3 TKN PO4 TP Settleable Solids Initial O&G (Grab) Comp O&G Final O&G (Grab) Initial TPH (Grab) Comp TPH Final TPH (Grab)
Queens
Brooklyn
Manhattan
Complete Average
W/O Brooklyn Average
0.048 0.052 0.012 0.083
0.2593 0.1703 0.0551 0.3629
0.046 0.093 0.033 0.119
0.1177 0.1049 0.0334 0.1882
0.0468 0.0722 0.0226 0.1008
-0.000 -0.000 0.004 0.010 0.000 0.001 0.008
-0.0000 0.0000 0.0065 0.0188 0.0018 0.0027 0.0263
0.0000 0.0000 0.0035 0.0128 0.0010 0.0012 0.0037
0.0000 -0.0000 0.0047 0.0138 0.0010 0.0016 0.0126
0.0000 -0.0000 0.0039 0.0112 0.0006 0.0011 0.0057
-0.004
-0.0060
-0.0079
-0.0061
-0.0062
0.005 0.004
0.0104 0.0373
0.0069 0.0009
0.0074 0.0139
0.0059 0.0023
-0.001
-0.0051
-0.0003
-0.0020
-0.0005
0.000 -0.001
0.0079 -0.0043
-0.0000 0.0002
0.0026 -0.0016
-0.0000 -0.0002
Table ES-2. City-wide Projections (Influent Pounds Increase Per Day Based on Manhattan and Queens Sampled Data Only) Year 2000 2005 2010 2025 2035
Population To NYC WPCPs 7,454,300 7,498,600 7,610,400 8,018,000 8,087,300
% Saturation Population (1% per year) With FWDs 3 8 13 28 38
223,629 599,888 989,352 2,245,040 3,073,174
TSS
BOD
10,476 28,103 46,347 105,172 143,967
16,137 43,287 71,389 161,997 221,753
BOD Filtrate 5,053 13,555 22,356 50,730 69,443
FOOD WASTE DISPOSER PILOT STUDY Year
COD
2000 2005 2010 2025 2035
22,550 60,489 99,761 226,377 309,882
NH3 (ammonia)
TKN
ES-7
PO4 Total Settleable (ortho-ph) Phosphorous Solids
867 2,514 2,326 6,743 3,836 11,121 8,704 25,237 11,915 34,546
139 373 615 1,394 1,909
237 636 1,049 2,381 3,260
1,284 3,445 5,681 12,891 17,646
Oil & Grease 1,314 3,525 5,813 13,191 18,056
The detailed analyses conducted since Local Law 74 took effect, follows. IMPACT EVALUATION Sewer System The introduction of FWD units may cause increases in suspended solids and oil and grease in the sewer system. According to values in the literature, this increase is about 20 percent per capita for domestic wastewater.2 As a result, there may be an increase in maintenance costs incurred by the City. The following table shows the projected increase in maintenance expense as a result of introducing FWD units at a saturation rate of 1 percent beginning in 1997. This table also shows the estimated dollar cost from the impact of suspended solid deposits and its effect on the sewer cleaning program, sewer back-up (SBU) complaints, and grease removal. To put these figures in perspective, DEP currently spends about $0.5 million for routine contractual cleaning and $6,850,000 responding to SBU complaints. Table ES-3. Maintenance Cost Increases due to Food Waste Disposers, 2000 - 2035. Year
% Sat 1% per Yr.
% Impact Sat x 20%
% $ Increase in Sewer Cleaning Expense *
$ Inc in SBU & Grease Cleaning Expense *
Total $ Expense
2000
3
0.60%
3,000
42,000
45,000
2005
8
1.60
8,000
110,000
118,000
2010
13
2.60
13,000
178,000
191,000
2025
28
5.60
28,000
383,000
411,000
2035
38
7.60
38,000
521,000
559,000
* - Based on 1997 Fixed Dollars
2
Metcalf & Eddy, Inc., Wastewater Engineering: Treatment, Disposal, and Reuse (New York: McGraw-Hill, Inc., Third ed.): 166.
NEW YORK CITY DEPARTMENT OF ENVIRONMENTAL PROTECTION
ES-8
A videotape survey was also conducted as part of the pilot study. Videotaping was conducted before FWDs were installed, during the study and at the study’s completion. No noticeable deposits of suspended material were observed in the videotapes at the end of the relatively brief study period. Based on the analysis, potential future maintenance costs, even if worst case projections prove true, would be considered de minimis, therefore, no potential significant adverse impacts on the City’s sewer system are expected if food waste disposers are permitted in combined sewer areas. Water Consumption The incremental increase in water demand due to the introduction of FWDs for the analysis years 2000, 2005, 2010, 2025, and 2035 was projected. The projections were based on a reasonable estimate of an additional per capita water demand of 1 gallon per capita per day with FWDs. This figure fell roughly between the high and low measurements of water demand obtained for the study. Industry estimates of water demand are somewhat lower. Using the above assumptions, the additional water demand with FWDs would be approximately 3 million gallons per day by 2035, even under worst case assumptions. This represents a minor incremental increase when compared against the system’s 1.3 billion gallon average annual daily water demand. Therefore, no potential significant impacts on the City’s water supply system is expected if food waste disposers are permitted city-wide.
Table ES-4. City-wide Water Demand from Food Waste Disposers
Year
NYC Population Projection
Per cent Saturation (1%/apt/yr)
Population with FWDs
Water demand from FWDs (In million gallons per day)
2000
7,454,300
3%
223,629
0.22
2005
7,498,600
8
599,888
0.60
2010
7,610,400
13
989,352
0.99
2025
8,018,000
28
2,245,040
2.24
2035
8,087,300
38
3,073,174
3.07
Wastewater Treatment and Biosolids Handling The analysis of potential impacts on the City’s ability to treat wastewater and dispose of
FOOD WASTE DISPOSER PILOT STUDY
ES-9
sewage biosolids considered the potential additional capital and operating costs that might be incurred from additional food waste loadings in the waste stream. These costs can be attributed to the need for additional aeration capacity to treat the BOD, additional sludge digesters and dewatering facilities to handle solids, and additional nitrogen control measures. Costs for nitrogen control measures are potentially the most variable because they are dependent on future regulatory control scenarios. Tables ES-5 and ES-6 detail the additional costs DEP forecasts would be incurred to handle additional FWD loadings. The costs presented are cumulative and are in constant 1996 dollars. The results show that in the decade after city-wide introduction of FWDs, increases in costs would be relatively small; approximately $4.1 million in 2005 (based on Queens and Manhattan data) for the most expensive nitrogen control measure. Measured against the estimated 1.525 billion dollar cost of maintaining the City’s water and sewer infrastructure, this represents a de minimis impact.
Table ES-5. Annual Operating and Capital Costs for Wastewater Treatment and Biosolids Handling Using Different Nitrogen Control Technologies (Based on Average Queens and Manhattan Sampling Data) Scenario 1 - Increased Aeration Year
Operating Cost
Capital Cost
2000
$578,600
$700,900
2005
2,400,000
1,800,000
2010
2,500,000
3,100,000
2025
5,700,000
17,400,000
2035
7,800,000
28,800,000
Scenario 2 - Fixed Media Nitrogen Removal Year
Operating Cost
Capital Cost
2000
$578,600
$2,400,000
2005
2,400,000
6,100,000
2010
2,500,000
10,200,000
2025
5,700,000
33,300,000
2035
7,800,000
50,600,000
NEW YORK CITY DEPARTMENT OF ENVIRONMENTAL PROTECTION
ES-10
Scenario 3 - Biofilters Year
Operating Cost
Capital Cost
2000
$1,500,000
$17,700,000
2005
4,800,000
50,140,000
2010
6,300,000
79,400,000
2025
14,600,000
165,700,000
2035
19,800,000
218,800,000
Table ES-6. Annual Operating and Capital Costs for Wastewater Treatment and Biosolids Handling Using Different Nitrogen Control Technologies (Based on Average of Brooklyn, Queens and Manhattan Sampling Data) Scenario 1 - Increased Aeration Year
Operating Cost
Capital Cost
2000
$1,300,000
$2,500,000
2005
3,500,000
6,500,000
2010
5,700,000
12,700,000
2025
13,000,000
54,900,000
2035
17,900,000
83,600,000
Scenario 2 - Fixed Media Nitrogen Removal Year
Operating Cost
Capital Cost
2000
$1,300,000
$4,200,000
2005
3,500,000
11,200,000
2010
5,700,000
20,300,000
2025
13,000,000
72,200,000
2035
17,900,000
107,200,000
Year
Operating Cost
Capital Cost
2000
$2,400,000
$23,223,000
Scenario 3 - Biofilters
FOOD WASTE DISPOSER PILOT STUDY
ES-11
2005
6,300,000
63,400,000
2010
10,400,000
100,703,000
2025
23,600,000
229,500,000
2035
32,200,000
305,900,000
The additional treatment plant costs due to FWDs were based on an assumed 3 gallons per capita per day (gcpd) flow rate. Since the water consumption analysis showed that an average flow per capita would be about 1 gcpd flow, a reconciliation of costs due to the additional flow was performed. Additional costs associated with flow would primarily be due to additional pumping requirements and chlorination. Table 7 shows the projected difference in costs that can be expected. These costs can be subtracted from Tables 5 and 6 for any scenario to obtain the projected costs assuming a 1 gallon per capita water consumption rate.
Table ES-7. Costs Resulting from Assuming Three Gallons per Capita per Day Flow (in dollars) Cost Item/Year
2000
2005
2010
2025
2035
Pumping Cost
$3,947
$10,586
$17,451
$41,785
$57,215
Chlorination Cost
4,982
$13,373
22,127
49,973
68,339
Total
$8,928
$23,959
$39,577
$91,758
$125,554
Impact on Water Rates An estimate of the potential impact of the need for additional sewage treatment capacity on water and sewer rates was also performed. Minor incremental costs due to increased sewer maintenance were also identified, but are too small to affect the water rates. Similarly, increases in revenue from additional water demand generated by FWDs is too small to measure. Increased sewage treatment and biosolids (sludge) handling resulted in near-term increases in the average annual household bill of $3.70 for the average owner-occupied dwelling and $3.15 for the average apartment building unit, if the most stringent nitrogen removal scenario were adopted. These impacts are also considered minor (less than 1 percent over projected water rates) and would not result in any potential displacement of residents. If a lesser amount of nitrogen removal is required, these costs would go down. Projections of water rates beyond 2005 are not presented because they are considered speculative.
NEW YORK CITY DEPARTMENT OF ENVIRONMENTAL PROTECTION
ES-12
Water Quality Open Waters Water quality modeling projected an increase in biochemical oxygen demand (BOD) due to FWDs resulting in a 0.01 milligram per liter decrease in dissolved oxygen (DO) in New York Harbor by 2005 (based on Queens and Manhattan sampling data). This increase is considered de minimis. Although larger decreases would occur using Brooklyn data and an escalation in DO deficits in out years is projected under worst case conditions, DEP considers these impacts to be highly speculative. Tributaries Analysis of tributary waters was conducted by estimating the impacts in a tributary with currently planned improvements, the Flushing Bay drainage basin. Installation of FWDs in this area is predicted to increase BOD and TSS loadings in the total CSO stream by 5.0 percent for BOD and TSS by 2.0 percent over baseline loads, using the Queens and Manhattan data set. Water quality modeling showed greatest effects to be near large CSO outfalls at the mouth of, and in, Flushing Creek with worst case loadings assumed. The percent of time that DO concentrations would be below the “never-less-than” 4.0 mg/L DO standard would increase by approximately 1.5 percent over baseline conditions in and around the immediate proximity of Flushing Creek. For lesser sanitary loads (similar to scenarios that omit Brooklyn loadings) the expected DO decrease would be a fraction of this. In 1995 the NY Harbor Survey recorded average DO (at a single site) in Flushing Bay to be 7.7 mg/L (surface) and 5.3 mg/L (bottom), with a minimum DO of 3.5 mg/L. Summertime percent non-compliance with the NYS DEC never-less-than 4.0 mg/L DO standard was 50 percent. In this context, the above increases are considered de minimis. Effects in the later years would be expected to be more severe, but are considered speculative. Solid Waste The Department of Sanitation (DOS) recognizes the potential for kitchen waste disposals to make a positive impact on New York City residential waste management. Using the DEP projections of Total Suspended Solids, DOS estimated the effect of the diverted waste on its operating costs. The amount of food waste diverted is approximately 3 percent of the DOS total household refuse collection. If it is assumed that 38 percent of the City’s households are equipped with kitchen waste disposals in the year 2035, and that the average equipped household places 50% of the targeted food wastes into disposals (this rate is comparable to the current capture rate for recyclables), the Department would save $4 million in solid waste export costs at current disposal rates.
A BRIEF SUMMARY AND INTERPRETATION OF KEY POINTS, FACTS, AND CONCLUSIONS FOR
University of Wisconsin Study: “LIFE CYCLE COMPARISON OF FIVE ENGINEERED SYSTEMS FOR MANAGING FOOD WASTE” by WILLIAM F. STRUTZ STAFF ENGINEER IN-SINK-ERATOR
APRIL, 1998
In order to develop a factual database relative to the actual merits and concerns for different systems of managing food waste, the National Association of Heating- Plumbing -Cooling Contractors commissioned a University of Wisconsin - Madison, Life Cycle Comparison of five Engineered Systems for Managing Food Waste. The comparison included the required land, total system energy, total system materials, total emissions to the environment and total system costs for each method. Dr. Robert Ham of the Civil Engineering Department and one of the country’s recognized landfill experts was chosen to lead and oversee the study. Carol Diggelman, a graduate student in the Civil and Environmental Engineering Department at UW and a Professor in Environmental Engineering at the Milwaukee School of Engineering in Milwaukee, Wisconsin was chosen to do the research. The results of this four year research project are contained in a 571 page report titled “LIFE-CYCLE COMPARISON OF FIVE ENGINEERED SYSTEMS FOR MANAGING FOOD WASTE” which compares all five systems on the basis of processing 100 kilograms of food waste. The five systems are: 1. Food Waste Disposer plus a Publicly Owned Treatment Works ( FWD / POTW ). 2. Municipal Solid Waste Collection / landfilling ( MSW Collection / Landfilling). 3. Municipal Solid Waste Collection / Composting ( MSW Collection / Composting ). 4. Municipal Solid Waste Collection / Waste To Energy ( MSW Collection / WTE; Incineration ). 5. Food Waste Disposer plus an On-Site (Septic) Sysytem ( FWD / OSS ) The first four systems are based on specific state of the art operational systems. The on-site system design is based on simply increasing the septic tank and drain field size by 25 % to accommodate a food waste disposer. This requirement is based on a typical required increase of 25 % solids loading to the system, based on previous research. Assumptions for the study based on the best available data: 1.The base of 100 kilograms of food waste was chosen as a convenient basis of comparison for the five engineered systems. An average person generates 0.29 pounds of food waste per day. Of this, 75 % or 0.21 pounds per day is processed through a food waste disposer. 100 kilograms of food waste is therefore the amount processed by the “average” U.S. family of 2.63 persons over a period of 382 days, or just slightly over one year. 2.
Typical food waste is 70 % water and 30 % solids.
2
3.
The typical composition of food waste and human waste solids is :
%C Human Waste, Solid Organics
%H
%O %N
%S
59.7 9.5
23.8 7.0
0
50.5 6.72
39.6 2.74
0.44
C10H19O3N Food Waste, Solid Organics C21.53H34.21O12.66N1.00S0.07 4. The final destination of food waste in the U.S.: a. Municipal Solid Waste Collection /Landfill....................................41% b. Food Waste Disposer / Publicly Owned Treatment Works..................................................... 37 %* c. Food Waste Disposer / On-Site (Septic) System......................... 12 %* d. Municipal Solid Waste Collection / Waste To Energy(Incineration)................................................ 10 % e. Municipal Solid Waste Collection / Composting......................................0% *Wastewater food waste includes contributions from dishwashers and kitchen sinks.
CONCLUSION Of the five alternative food waste systems measured, a food waste disposer processing food waste through a publicly owned treatment works has the lowest cost to the municipality; the least air emissions, especially greenhouse gases ! ; converts the food WASTE to a RESOURCE which may be recycled; and as a result overall is the most environmentally friendly and sustainable option for recycling non-edible food RESOURCES. The food waste disposer is also the most convenient method of disposing of food waste and is the most likely to be used as the vehicle for source separation of food waste from the solid waste stream.
LIFE-CYCLE COSTS In terms of life-cycle costs, the systems ranked in this order ( lowest to highest ): 1. 2. 3. 4. 5.
Municipal Solid Waste Collection / Landfilling. Municipal Solid Waste Collection / Composting. Food Waste Disposer / Publicly Owned Treatment Works. Municipal Solid Waste Collection / Waste To Energy ( Incineration ). Food Waste Disposer / On-Site (Septic) System.
3
However, in terms of direct costs to the municipality, the Food Waste Disposer / Publicly Owned Treatment Works combination is by far the lowest cost. The rankings and costs are: 1.Food Waste Disposer / Publicly Owned Treatment Works ............$ 0.49. 2. Municipal Solid Waste Collection / Landfilling ..............................$13.65. 3. Municipal Solid Waste Collection / Composting ............................$16.60. 4. Municipal Solid Waste Collection / Waste To Energy ( Incineration ) ...............$20.30. 5. The Food Waste Disposer / On-Site Septic System is the highest cost at $67.20 but since all costs are borne directly by the homeowner, there is zero cost to the municipality.
Other benefits of the Food Waste Disposer / Publicly Owned Treatment Works are:
ENVIRONMENTAL Environmentally, the disposer is the most convenient and most likely to be used method to achieve source separation of the putrescible waste from the solid waste stream. Typically, 75 % of non-edible food waste may be processed through a food waste disposer. Presently 37 % of U.S. household food waste goes to a POTW. Food waste is typically 70 % water, therefore utilizing a wastewater treatment plant is a more natural method of processing this material than is a method which collects this water and HAULS it to a “solid waste� facility. Removing the putrescible food waste at the kitchen sink and diverting it from the solid waste stream also reduces disease causing vectors such as flies, rodents, roaches, etc. that are attracted to food waste.
RECYCLE FOOD NUTRIENTS Since human wastes influent to wastewater treatment systems is carbon limited ( food carbon is exhaled by humans as carbon dioxide, enriching the sewage in Nitrogen and Phosphorus), the addition of food waste provides additional carbon to enhance the generation of biosolids. The greater the amount of biosolids produced at the POTW, the greater the amount of nutrients, nitrogen and phosphorous, that is assimilated into the biomass, which is removed from the system as sludge and removed from the effluent. When biosolids from the POTW, or septage processed through a POTW is applied to the soil, this is a viable method of recycling. This process is themost beneficial for retaining the food waste nutrients in a form that can be recycled.
4
HELPS LANDFILLS Landfilling is the method of disposing of solid waste that is required for every community in the U.S. Presently 41 % of U.S. food waste goes to landfills. Hauling food waste that is 70 % water to a “solid waste� facility represents over 72 % of the life cycle costs of disposing of a potentially recyclable resource. Adding this water to a well designed landfill also increases the quantity of leachate that is generated. Due to the generally acidic nature of leachate from food waste, more metals are contained in the leachate than if the food waste was not in the landfill. This leachate is then typically hauled to a POTW for treatment to prevent the leachate from contaminating soils and potentially the groundwater (hauling the water not once but twice ). Almost all of the nutrient value of the food is lost in the landfill; the only portion that is potentially recycled is that which is captured in the leachate and processed through the POTW. Eventually almost all of the carbon in the food waste at the landfill is converted to methane. In a well designed landfill about 66% of the methane is recovered and beneficially reused as fuel. However the balance of 34 % of the methane escapes to the atmosphere. The methane gas has up to 25 times the global warming impact of carbon dioxide. Putrescible food waste added to the normal household solid waste also adds to disease causing vector problems such as flies, rodents and roaches while awaiting collection. In cities that have mandated food waste disposers, solid waste collection frequency has been reduced from twice weekly to weekly or even bi-weekly.
BENEFITS OVER COMPOSTING Municipal compost facilities are not as prevalent as landfills. They are considered an additional system and a landfill is still required. Hauling food waste that is 70 % water to a compost facility represents over 59 % of the life cycle costs of the compost operation. Since municipal composting requires more moisture than is available in most materials, the addition of food waste does enhance the composting process. This higher moisture content does require periodic turning of the material to keep the process aerobic. If the process goes anaerobic, then there is the potential for significant odors to be generated and the result is community opposition to composting. A number of facilities in the U.S. have been shut down for odor problems. This typically requires locating the facility away from population centers and hauling the high water content food waste longer distances. Composting also results in the loss of most of the nutrients in the food to the extent that the resulting product is of very low value and typically is not worth the cost of hauling and spreading it onto soil. In some communities the compost is of such low value that it is used as landfill cover. Food waste can be processed through the POTW at a much lower cost to the municipality, retain the nutrients for recycling, and reduce the atmospheric emissions. 5
INDIVIDUAL COMPOST SYSTEMS LIMITATIONS The typical backyard compost system, which is not analyzed in this study is typically not as well maintained as a municipal system. This results in more anaerobic conditions, more odors, more methane generated and released to the atmosphere, more potential for leachate to seep into groundwater and a low nutrient, low quality product. These systems also tend to attract disease causing vectors. Yet, many homeowners perceive composting to be the “ideal” method of recycling food waste. The attached fact sheet based on information from the study contradicts this belief.
WASTE TO ENERGY LIMITATIONS Hauling food waste that is 70 % water to a waste-to-energy facility represents over 48 % of the life cycle costs of the operation. Energy required for evaporation of the water in the food waste results in a very small net energy gain from the incineration of food waste. Instead of the nutrients being captured for recycling, most are given off to the atmosphere as acidic or greenhouse gases. Scrubbers are required in a well designed system to reduce these emissions and are a significant factor in making this the highest cost municipal system. Refer to the attached fact sheet for a comparison of the emissions generated by the various methods.
ON-SITE SYSTEM An 0n-Site (Septic) System ( OSS ), is a requirement for processing wastewater in rural areas which are located beyond the municipal collection systems. As stated earlier, the system for this study was based on a 25 % larger system when a food waste disposer is used. Since this is not a state of the art system, this results in the system with the highest life cycle cost. However, systems with a disposer, an adequate soil type and a “standard” sized system have functioned trouble free for more than ten years in cold climates. State of the art for on-site systems is the use of bio-additives to neutralize any potential additional loading due to food waste. This allows using a “standard” sized system without any additional system cost, significantly reducing the life cycle cost of the FWD / OSS system. A state of the art system such as In-SinkErator’s Septic Disposer using Bio-ChargeTM would reduce the size required for the system and reduce the system cost.
6
LIFE CYCLE ANALYSIS OF FIVE FOOD WASTE MANAGEMENT SYSTEMS FOR 100 KILOGRAMS ( 220.5 POUNDS ) OF FOOD WASTE DISPOSAL OPTION FWD / POTW ( Food Waste Disposer + Publicly Owned Treatment Works )
MSW / LANDFILL ( Municipal Solid Waste collection plus Landfilling )
MSW / COMPOST ( Municipal Solid Waste collection plus Composting )
MSW / WTE FWD / OSS ( Municipal Solid ( Food Waste DisWaste collection poser + On-Site plus Waste To Septic System ) Energy ) ( Incineration )
PARAMETER Land Required ( Square feet ) Rank 1 Energy Required ( Btu ) ( Total - Exportable Food Waste Energy ) Rank 1 Materials Required ( Pounds ) Rank Oxygen Required ( Pounds ) Rank Life Cycle Emissions ( pounds ) Carbon dioxide Rank Methane Rank Total Greenhouse Gases (Carbon dioxide+4*Methane)Rank Acid Gases ( Pounds) Nitrogen & Sulfur Oxides
0.003
0.202 3
45,744
80,112 2
Water Vapor ( Pounds ) Total air emissions
3
Water and waterborne wastes 2800 Rank 3 Solid wastes 4.4 Rank 4 Other ( sludge ) 340 Rank 5 Life Cycle Costs Disposer ( Homeowner cost for separation & convenience ) Low $8.83 High $17.45 Total System Cost Low High
$17.94 Rank 3
Public municipality cost ( external to the home ) Rank 1
$0.49
$16.60
$13.65 2
7
$58.58 5
$20.30 4
$16.60 3
$67.20 5
$20.30 4
310
$8.83 $17.45
$20.30 4
2
480
5 3.3 ( septage ) 4
0
$16.60
$13.65
4800 5
1.3
1 39 ( ash ) 1
3
1
420
0
$13.65 2
3994 2273 5
2 2.7
2 25 (compost ) 3
0
$9.32 Rank 1
75 0
370
6
140 3
2
1
3 ( residues ) 2
343
64 0
370
0 1
5
1
1
200
260
83 0
1.0 4
5
4
2547 2273
2.9
160
110 1
190 5
5
4
120
Rank 2 Total Water Required ( Pounds ) Carrier Water ( Pounds ) Rank 4
140
0.2
24 3
15 5
4
3
24 Rank 3
0.00037
100
<0.05 1
130 4
3
2
0.1 2
140
0.00028
101 3
0 1
5
1
97 1
95
100
5 4
4881 5
5
3
0.00028 1
116.1
67
81 1
925,824 5
2
4
97 2
286,433
89.6
0 1
20.432 5
4
1
25 3
143,299
338.2 4
0.020 2
3
287.4 3
0.814 4
$67.20 5
The brief one page fact sheet titled “ LIFE CYCLE ANALYSIS OF FIVE FOOD WASTE MANAGEMENT SYSTEMS” comparing the five systems for land, energy, materials, emissions and costs has been inserted above. This fact sheet was developed to present the key parameters for each system and also ranks the systems for each parameter. Carol Diggelman’s conclusions, recommendations, and two pages of detailed comparison charts from the original report are copied verbatim and attached for reference. Copies of the 118 page Executive Summary and the 571 page Full Report are available upon request. 1.
2.
3.
4.
5.
As shown on page 11, in general, as total flows to the environment increase, so do total system costs, all per 100 kg food waste. Rank by total system cost is a reasonable predictor of overall rank for the 12 selected parameters-total land, total system materials (minus food waste and carrier water), total system energy (minus food and carrier water energy), water, total system cost, air emissions, acid gases (NOx and SOx), greenhouse gases, wastewater, waterborne wastes, solid waste, and food waste byproducts. Total flows to the environment from wastewater systems are about 10 times those from MSW systems, primarily because of FWD carrier water. The FWD/OSS, the only rural system, ranked either first or second for most parameters. Because a larger fraction of the total FWD/OSS was attributable to the 100 kg of food waste; land, materials, energy and flows to the environment attributable to the 100 kg were higher for the rural system than for the four municipal systems. The FWD/OSS has the highest flows to the environment of the five systems; most is water and waterborne wastes discharged with minimal performance control to the subsurface. About half of the effluent BOD5 is discharged directly to the absorption bed which may contribute to biomass assimilation and clogging in the absorption bed. Although food waste carbon removes some ammonia-nitrogen from wastewater as it is assimilated into biomass, a system stoichiometric excess of ammonia-nitrogren remains to be discharged into the subsurface, potentially bypassing plant root zones to pollute groundwater. The MSW Collection/WTE ranks second highest overall and for total system cost. Burning food waste yields little exportable energy in these systems, so diverting food waste to FWD/POTW systems should be defined as recycling and encouraged, just as diverting other recycables with no heating value, such as metal and glass, is encouraged.
8
6.
7.
8.
9.
10.
11.
12.
The FWD/POTW system ranks in the middle of the five systems overall and for total system materials and total system cost. Most of the cost is for the FWD and is borne by the homeowner; the cost to process food waste through the POTW is less than $0.50 per 100 kg of food waste. The FWD/POTW has the lowest land and total system energy requirements but the highest food waste byproduct, sludge, requiring management. Wastewater collection and treatment systems and MSW collection systems and landfills are required systems for both urban and rural residences for reasons of basic public health and sanitation. When a FWD Is incorporated in a household wastewater collection system, there is redundancy in food waste management and most food waste can be managed through either system. Food waste going into a FWD/POTW system, from which either effluent and/or sludge is/are returned to agricultural soils in compliance with Federal and State regulations and in which methane is collected and combusted to produce electricity, is being effectively recycled. Adding food waste carbon to a carbon limited wastewater system contributes to a net removal of nutrients (nitrogren and phosphorus) from effluent, as nutrients are assimilated with carbon into biomass and removed from the system as sludge. Land requirements for each system give a first approximation of a systemâ&#x20AC;&#x2122;s appropriation of and reduction in net primary productivity (mass of biomass produced per area or per Joule of incident energy). Even though impacts to net primary productivity are beyond the scope of this project, the FWD/POTW system with the lowest land requirements has the lowest impact on net primary productivity from 100 kg of food waste. When coupled with potential increases in net primary productivity from effluent and sludge nutrients, this system is potentially the most sustainable of the five systems. The MW Collection/Compost system ranks lowest overall; it has the lowest total system materials and water requirements and generates the lowest amount of wastewater and waterborne wastes. Food nutrients are returned to soil from compost systems. Composting is an optional food waste management system that increases redundancy in food waste management; however, wastewater collection/treatment and MSW Collection / Landfill systems are still required. The MSW Collection/Landfill system is the default system for food waste management; it ranks next to lowest overall and lowest for cost. It also ranks low for water, wastewater, total air emissions and food waste byproducts.
9
13.
14.
15.
As indicated on page 12, for MSW systems the MSW Collection system contributes from half to 3/4 of the total system cost. Systematic diversion of wet, putrescible food waste from MSW to FWDs has the potential to produce drier, more storable MSW and reduce the need for weekly collection and the cost of MSW collection. The MSW Collection system requires about 17 times the land, about 18% of the total materials, 88% of the total system energy, is about half the high estimate and is about the same as the low estimate of the cost of the FWD; the total flows to the environment for the MSW Collection system are about 18% those of the FWD, because there is no carrier water. If household plumbing were redesigned to use non-potable water for flushing wastes (both human through toilets and food through FWDs), diverting food wastes to municipal wastewater systems becomes a more sustainable choice.
Final Recommendations: 1. Diverting food waste through FWDs to a POTW should be encouraged when solidsâ&#x20AC;&#x2122; handling systems are adequate, methane is combusted to generate energy, and effluent and/or sludge are returned to soil; food waste is effectively being recycled and should be so designated in Federal and State regulations. 2. Benefits to MSW management systems from the systematic use of FWDs should be quantified; because by transferring putrescible FW from solid to wastewater management systems, there a reduction in regulatory requirements for MSW collection systems (weekly collection), landfill systems (daily cover requirement), compost systems (more stringent management requirements) and reduced solidsâ&#x20AC;&#x2122; handling for WTE systems. 3. Separate regulations that give different design requirements for POTWs depending on FWD usage should be challenged, especially if no other household appliance or device is so listed. 4. To make the life-cycle inventory a cost-effective process, there needs to be an accurate, up-to-date data base of unit factors for water and waterborne wastes, air emissions, and solid waste for materials and fuels that is readily available to the public.
10
Comparison of Land, Materials, Energy, and Costs of Five System Used to Manage Food Waste Neg.-negligible; NI-no information NA-Not Applicable Land, ft
2
FWD+ POTW+ MSW Compost+ WTE + Landfill+ FWD OSS OSS POTW FWD Collection Compost Collection W-T-E Collection Landfill Collection Table 4.17 Table 5.19 Table 6.103 Table 7.18 Table 8.17 Table 9.27 Table 7.43 2 2 2 2 2 2 2 2 2 2 2 2 ft /100kg ft /100kg ft /100kg ft /100kg ft /100kg ft /100kg ft /100kg ft /100kg ft /100kg ft /100kg ft /100kg ft /100kg 0.0006 20.43 20.43 0.003 0.003 0.01 0.80 0.81 0.01 0.02 0.19 0.20 lb/100kg lb/100kg lb/100kg lb/100kg lb/100kg lb/100kg lb/100kg lb/100kg lb/100kg lb/100kg lb/100kg lb/100kg
Materials Construction & Landfill Materials 0.1 3143.2 3143.3 7.9 8.0 2.7 5.9 8.6 5.0 7.7 243.7 246.4 Equipment, vehicles 0.1 Neg. 0.1 0.1 0.1 0.2 0.4 0.5 0.1 0.3 0.1 0.3 Electricity 1.4 Neg. 1.4 1.4 2.8 5.4 9.5 14.9 22.1 27.4 0.0 5.4 Natural Gas 0.5 Neg. 0.5 0.0 0.5 NI 0.0 0.0 2.4 2.4 0.6 0.6 Diesel Fuel 0.1 12.9 13 0.1 0.2 1.4 0.2 1.6 1.9 3.3 1.4 2.8 Gasoline 0.7 Neg. 0.7 0.0 0.7 NI 0.0 0.0 0.0 0.0 0.0 0.0 FWD Materials 1.5 0 1.5 0.0 1.5 NA 0.0 0.0 0.0 0.0 0.0 0.0 Water 260.4 3733.4 3993.8 2286.3 2546.7 38.5 25.5 64 36.5 75 44.3 82.8 Food Waste 0.0 220.5 220.5 220.5 220.5 0.0 220.5 220.5 220.5 220.5 220.5 220.5 Total 264.9 7109.9 7374.8 2516.2 2781.1 48.2 261.9 310.1 288.4 336.6 510.5 558.7 Total - FW & CW 264.9 4616.2 4881.1 22.5 287.4 48.2 41.4 89.6 67.9 116.1 290.0 338.2 Energy Btu/100kg Btu/100kg Btu/100kg Btu/100kg Btu/100kg Btu/100kg Btu/100kg Btu/100kg Btu/100kg Btu/100kg Btu/100kg Btu/100kg Embodied Materials 308 526506 526814 5707 6014 18983 13351 32334 2289 21272 6628 25611 Embodied-Process equip./vehicles 1477 Neg. 1477 1021 2498 2027 6975 9002 2068 4095 1635 3662 Electricity 6177 Neg. 6177 6056 12233 23373 41061 64434 99225 122598 NI 23373 Natural Gas 13126 Neg. 13126 416 13542 NI NI NI 61347 61347 15299 15299 Diesel 3717 302149 305866 1659 5376 33856 3549 37405 43108 76963 31877 65733 Gasoline 16780 NI 16780 52 16832 NI NI NI NI NI NI NI FWD Material 47197 0.0 47197 0.0 47197 NA NA NA NA NA NA NA Water 547 7840 8387 4798 5345 81 43 124 77 158 93 174 Total 89329 836495 925824 19708 109037 78320 64979 143299 208113 286433 55531 133851 Total - Exportable FW Energy* 89329 836495 925824 -43585 45744 78320 64979 143299 208113 286433 1792 80112 Costs - $ $17.45 $49.75 $67.20 $0.49 $17.94 $9.90 $6.70 $16.60 $10.39 $20.30 $3.75 $13.65 Exportable Electricity** kWh 0 0 0 19 19 0 0 0 0 0 16 16 * Exportable energy for POTW = 63,293 Btu/100kg FW; for Landfill = 53739 Btu/100kg FW
11
Summary of Life Cycle Emissions from Acquisition, Use and Decommissioning of FWD+ POTW+ MSW FWD OSS OSS POTW FWD Collection Air Emissions lb/100kg lb/100kg lb/100kg lb/100kg lb/100kg lb/100kg Particulates 2.8e-02 2.4e-01 2.7e-01 1.8e-03 3.0e-02 1.6e-02 Nitrogren Oxides 4.5e-02 6.2e-01 6.6e-01 5.1e-02 5.0e-02 6.6e-02 HC (Not Methane) 4.7e-02 2.1e-01 2.5e-01 2.6e-03 4.9e-02 2.7e-02 Sulfur Oxides 6.4e-02 2.9e-01 3.6e-01 5.7e-03 7.0e-02 5.9e-02 Carbon Monoxide 1.3e-01 4.9e-01 6.2e-01 5.8e-03 1.4e-01 5.5e-02 Carbon Dioxide 1.4e+01 1.1e+02 1.3e+02 8.4e+01 9.7e+01 9.6e+00 Aldehydes 1.6e-04 1.1e-02 1.1e-02 5.4e-05 2.1e-03 1.2e-03 Other Organics 2.1e-02 2.1e-01 2.3e-01 1.1e-03 2.2e-02 2.3e-02 Ammonia 4.7e-06 6.9e-05 7.4e-05 3.9e-07 5.1e-06 8.0e-06 Lead 3.5e-06 2.0e-08 3.5e-06 1.1e-08 3.5e-06 2.3e-09 Methane 2.5e-04 1.5e+01 1.5e+01 2.9e-05 2.8e-04 1.5e-04 Kerosene 1.0e-06 2.5e-07 1.3e-06 1.3e-07 1.2e-06 1.1e-06 HCI 1.4e-07 2.1e-05 2.3e-06 1.2e-08 1.5e-07 2.5e-07 Water Vapor - FW 0.0e+00 0.0e+00 0.0e+00 2.4e+01 2.4e+01 0.0e+00 Total Air Emissions 1.4e+01 1.3e+02 1.4e+02 1.1e+02 1.2e+02 9.9e+00 SW / CW 1.6e+00 4.7e+02 4.8e+02 2.8e+00 4.4e+00 9.7e-01 * Other 0.0e+00 3.1e+02 3.1e+02 3.4e+02 3.4e+02 0.0e+00 Water / Waterbone Wastes Water 2.6e+02 3.6e+03 3.8e+03 2.1e+03 2.3e+03 3.9e+01 Acid 1.0e-09 6.6e-02 6.6e-02 6.6e-02 6.6e-02 1.7e-09 Metal Ion 2.1e-05 3.2e-04 3.4e-04 1.8e-06 2.3e-05 3.7e-05 Dissolved Solids 1.3e-02 1.8e-01 1.9e-01 2.5e-+00 2.5e+00 2.1e-02 Suspended Solids 7.1e-03 1.2e-01 1.2e-01 2.0e-01 2.1e-01 1.2e-03 BOD 1.1e-03 3.5e-03 4.6e-03 1.3e-04 1.2e-03 4.5e-03 COD 4.0e-03 8.7e-04 4.8e-03 1.3e-05 4.0e-03 1.3e-04 Phenol 7.0e-08 1.0e-06 1.1e-06 5.8e-09 7.6e-08 1.2e-07 Oil 1.4e-03 2.5e-03 3.9e-03 3.3e-05 1.5e-03 3.1e-04 Sulfuric Acid 2.4e-03 5.5e-04 3.0e-03 3.1e-04 2.7e-03 2.6e-03 Iron 6.1e-04 1.4e-04 7.5e-04 7.6e-05 6.8e-04 6.6e-04 1.7e-06 2.5e-05 2.7e-05 1.4e-07 1.8e-06 2.9e-06 Ammonia + NO3 Chromium 4.1e-09 6.0e-08 6.5e-08 3.4e-10 4.4e-09 7.0e-09 Lead 1.8e-09 2.7e-08 2.8e-08 1.5e-10 2.0e-09 3.1e-09 Zinc 2.7e-08 3.9e-07 4.2e-07 2.2e-09 2.9e-08 4.5e-08 Total Water Wastes 3.0e-02 1.3e+01 1.3e+01 2.8e+00 2.8e+00 3.0e-02 Total 2.8e+02 4.5e+03 4.8e+03 2.5e+03 2.8e+03 4.9e+01
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Five Enhanced Systems for the Management of Food Waste Compost+ WTE + Landfill+ Compost Collection W-T-E Collection Landfill Collection lb/100kg lb/100kg lb/100kg lb/100kg lb/100kg lb/100kg 1.4e-02 3.0e-02 9.4e-03 2.5e-02 -1.2e-02 3.8e-03 3.6e-02 1.0e-01 2.8e+00 2.8e+00 7.8e-03 7.4e-02 1.5e-02 4.3e-02 1.1e-01 1.4e-01 1.8e-01 2.1e-01 6.1e-02 1.2e-01 1.9e-02 7.8e-02 -8.9e-02 -3.0e-02 3.1e-02 8.6e-02 6.5e-02 1.2e-01 3.3e-02 8.8e-02 9.4e+01 1.0e+02 1.3e+02 1.4e+02 7.1e+01 8.1e+01 1.3e-04 1.3e-03 1.5e-03 2.7e-03 1.1e-03 2.3e-03 2.5e-03 2.6e-02 3.0e-02 5.4e-02 2.2e-02 4.5e-02 1.3e-06 9.3e-06 1.0e-05 1.8e-05 6.5e-06 1.5e-05 3.5e-10 2.6e-09 2.9e-09 5.1e-09 1.8e-09 4.1e-09 1.3e-04 2.8e-04 2.2e-04 3.7e-04 5.0e+00 5.0e+00 1.9e-06 3.1e-06 7.4e-08 1.2e-06 -4.1e-06 -2.9e-06 3.8e-08 2.8e-07 3.1e-07 5.6e-07 2.0e-07 4.5e-07 1.6e+02 1.6e+02 2.0e+02 2.0e+02 2.4e+01 2.4e+01 2.6e+02 2.7e+02 3.4e+02 3.5e+02 1.0e+02 1.1e+02 1.7e+00 2.7e+00 3.4e-01 1.3e+00 5.0e+00 6.0e+00 3.9e+01 3.9e+01 3.3e+00 3.3e+00 2.5e+01 2.5e+01
2.1e+01 2.7e-10 5.8e-06 3.4e-03 1.3e-03 3.9e-04 1.1e-04 1.9e-08 1.8e-04 4.6e-03 1.1e-03 4.5e-07 1.1e-09 4.8e-10 7.1e-09 1.1e-02 3.2e+02
5.9e+01 2.0e-09 4.3e-05 2.4e-02 2.4e-03 4.9e-03 2.4e-04 1.4e-07 4.8e-04 7.2e-03 1.8e-03 3.3e-06 8.1e-09 3.6e-09 5.2e-08 4.1e-02 3.7e+02
2.9e+01 2.2e-09 4.7e-05 2.7e-02 3.8e-04 3.2e-05 1.3e-04 1.5e-07 4.6e-04 1.7e-04 4.3e-05 3.7e-06 8.9e-09 3.9e-09 5.7e-08 2.8e-02 3.7e+02
6.7e+01 4.0e-09 8.4e-05 4.8e-02 1.5e-03 4.5e-03 2.6e-04 2.7e-07 7.7e-04 2.8e-03 7.0e-04 6.5e-06 1.6e-08 7.0e-09 1.0e-07 5.9e-02 4.2e+02
1.9e+02 1.4e-09 3.0e-05 1.8e-02 2.5e-02 1.3e-02 2.5e-02 9.7e-08 4.2e-04 -9.6e-03 1.0e-02 2.5e-03 5.7e-09 2.5e-09 3.7e-08 8.5e-02 3.2e+02
2.3e+02 3.2e-09 6.7e-05 3.9e-02 2.7e-02 1.8e-02 2.5e-02 2.2e-07 7.3e-04 -6.9e-03 1.1e-02 2.5e-03 1.3e-08 5.6e-09 8.2e-08 1.2e-01 3.7e+02
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7th June 2007
ENVIRONMENTAL IMPACT STUDY OF FOOD WASTE DISPOSERS FOR
THE COUNTY SURVEYORS’ SOCIETY & HEREFORDSHIRE COUNCIL AND WORCESTERSHIRE COUNTY COUNCIL by Dr Tim Evans BSc MS PhD CChem CEnv FCIWEM MRSC
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Copyright Š TIM EVANS ENVIRONMENT 2007 Stonecroft, Park Lane, Ashtead, KT21 1EU England. tel/fax +44 (0) 1372 272172 email tim@timevansenvironment.com
Page 2 of 53 sustai nable solutions; t reatment , management and rec yc ling; Airbeam Roller Stockpile Covers; bi osoli ds ; composti ng; qualit y ass uranc e; mark eting and s ales; HACCP; s trategic and organisati onal st udi es ; land rest oration; s oil scienc e
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Executive Summary This study examines the financial and environmental impacts of food waste disposers (FWD) and finds that they provide a cost-effective, convenient and hygienic means of separating putrescible domestic kitchen food waste (KFW) at source and diverting it from landfill. The study also finds that this route costs less and has a smaller global warming potential than the routes comprising kerbside collection followed by centralised composting or landfill. Home composting is ideal for garden waste because of both treating and also using the treated material where it is generated (the proximity principle). Bokashi treatment and wormeries have enthusiastic followings but users still need to have somewhere to use the treated material. Some householders are unable (e.g. apartment dwellers) or are not inclined to practise home composting. In terms of Best Value Performance Indicators, FWD reduce BV84 (kilograms of household waste collected per head of population), BV86 (cost of household waste collection per household) and BV87 (cost of waste disposal per tonne municipal waste). The National Audit Office concluded that England will not achieve the Landfill Directive targets without a step change in plans and that emphasising recycling alone is unlikely to be the answer. Part of the problem is lack of infrastructure for treating biodegradable municipal waste and this is linked with the delays consequent on the planning process. H&W (Herefordshire Council and Worcestershire County Council) have been pioneering in promoting installation of FWD. FWD have the benefit of separating at source a difficult fraction of biodegradable MSW (because it is wet and malodorous) and diverting it using existing infrastructure and without entailing any regulatory bureaucracy. The net global warming potential1 (GWP) of separate collection and treatment of KFW by composting is -14 kgCO2e/tKFW allowing for fertiliser offset and carbon sequestration when the compost is used on land. For households with FWD feeding to wastewater treatment works where sludge is treated by anaerobic digestion, the biogas
1
Global Warming Potential is expressed as carbon dioxide equivalent (CO2 e) over 100 years.
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is used as renewable energy and the biosolids are used on land (which is the pathway for Severn Trent Water’s works in H&W and Welsh Water’s works in Herefordshire) the GWP is better than -168 kgCO2e/tKFW2. In contrast, landfill is +743 kgCO2e/tKFW. Assuming that KFW is 17.6% of household waste, the cost of collecting and disposing KFW via the solid waste route in H&W averages £18.63 per household*year and the quantity is 180 kgKFW per household*year (2005/06 actuals). This is the approximate annual saving for each installed FWD. The saving will increase, and the payback period will decrease, as the cost of treating KFW increases with ABPR compliant treatment replacing landfilling. For example, letsrecycle.com estimates the current gate fee for composting KFW at a site that complies with the Animal By-Products Regulations is £42-52 /t. By February 2007, 640 FWD had been installed under the H&W cashback scheme at a total cost of £39,650, i.e. £62 per FWD, which is a payback period [at direct cost current savings] of only 3 years and 4 months. The ground KFW is transferred to the wastewater collection and treatment system and therefore adds somewhat to the costs of the water company. The value to H&W could be even greater when LATS (Landfill Allowance Trading Scheme) is factored into the equation. The LATS penalty is currently £150 per tonne of biodegradable municipal waste landfilled in excess of that permitted by allowances held. There could be additional penalties in the target years 2010, 2013 and 2020. The Local Government Association has warned that prices for allowances could be high from 2008/09 onwards, with a "serious deficit" of allowances potentially arising after 2009/10. Water companies are understandably concerned about changes that might adversely affect demands on water resources or that would increase sewer blockages; field trials in several countries (none has yet been undertaken in the UK) have shown that FWD do not affect water usage or accumulation in sewers significantly. Wastewater treatment works (WwTW) are designed to treat biodegradable material suspended in water, i.e. similar to the output of FWD. Ground KFW has been found actually to improve the composition of wastewater for the advanced nutrient removal processes that are now being demanded of WwTW (this is because it has more carbon
2
This figure is based on direct before and after measurements in a town where 30% of households had FWD installed.
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in relation to nitrogen or phosphorus than normal sewage). The additional cost for water companies depends on the route for treating and using or disposing the sewage sludge; for the route most usual in H&W it would be about ÂŁ0.68 per household*year, this is only 4% of the cost of the MSW-landfill route. However, the cost could be as much as ÂŁ8.38 for a WwTW that incinerates its sludge and does not generate electricity (not the case in the H&W area). Overall, food waste disposers appear to be a very cost effective means of separating putrescible kitchen waste at source and diverting it from landfill. The carbon footprint of FWD feeding to a WwTW with anaerobic digestion (AD) and electricity generation (CHP)3 is competitive with separate collection of KFW delivering to centralised AD with CHP and significantly better than centralised composting. They are convenient and hygienic for householders but do not discourage home composting. They avoid the problems of odour and vermin that can be associated with separate collection via the solid waste route.
3
This is the route in H&W
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CONTENTS 1
Executive Summary .......................................................................................3
2
Brief ...............................................................................................................7
3
Introduction....................................................................................................8
3.1
Waste arisings ........................................................................................8
3.2
Solid waste and landfill........................................................................10
3.3
H&Wâ&#x20AC;&#x2122;s joint municipal waste strategy................................................12
3.4
Food waste disposers ...........................................................................14
3.5
Home composting, Bokashi, wormeries, etc. ......................................17
3.6
Land application of sewage sludge ......................................................18
4
Environmental Impact â&#x20AC;&#x201C; Component Analysis ............................................20
4.1
KFW separation and storage................................................................20
4.2
KFW conveyance.................................................................................22
4.3
KFW treatment.....................................................................................27
4.4
Use or disposal of treated KFW...........................................................35
4.5
Summation of component analysis ......................................................37
5
Cost comparison of FWD and MSW routes ................................................39
6
Conclusions..................................................................................................41
7
Acknowledgements......................................................................................44
8
References....................................................................................................45 Appendix A Acronyms and Abbreviations..................................................47 Appendix B H&W Waste statistics .............................................................48 Appendix C Biogas, electricity and GWP from AD of KFW .....................51 Appendix D Costs and GWP from Surahammar field measurements.........52
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Brief To conduct desktop research into the use of food waste disposers (FWD) in Herefordshire and Worcestershire (H&W) as a means of diverting putrescible domestic kitchen waste from landfill. The study shall: • • • • • • • • •
refer to H&W’s joint municipal waste strategy together with UK and European legislation to evaluate the potential impact of FWD on household waste collection and disposal in the two counties. assess the potential for FWD to impact relevant BVPIs. investigate the potential contribution of FWD towards waste minimisation targets. compare the notional carbon footprint of a typical household with and without FWD. compare the use of FWD to alternative means of disposal of putrescible domestic kitchen waste. prepare a report on the above for free publication. provide ad hoc reports on progress to the CSS Research Fund Board. consult with Worcestershire County Council Waste Management prior to engaging in contact with outside bodies in connection with this research. give prominence to European studies and refer to worldwide studies for subjects considered missing or weak in European studies. Research to refer specifically to wastewater flow and treatment facilities in the Severn Trent Water region and the Welsh Water region and also cover private domestic wastewater treatment facilities.
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Introduction The principles of environmental impact were summarised by Commoner (1971) in his â&#x20AC;&#x2DC;Laws of Ecologyâ&#x20AC;&#x2122;: 1.
Everything is Connected to Everything Else.
2.
Everything Must Go Somewhere.
3.
Nature Knows Best.
4.
There Is No Such Thing as a Free Lunch.
Disposal of kitchen food waste (KFW) is no exception to these laws as will be discussed in this report.
3.1 Waste arisings Parfitt (2002) analysed 70 datasets of domestic waste composition obtained in studies commissioned between 1999 and 2002 across England and Wales. He concluded that kitchen waste comprised 17% of total household waste (Figure 1); it is about 30% of the biodegradable waste. He commented that there is a degree of uncertainty because no two studies employed the same methodology but it indicates the scale of the issue.
Figure 1 Total household waste composition (from Parfitt, 2002) Page 8 of 53
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WRAP (2007) estimated that UK households produce around 6.7 million tonnes of food waste and it warned of the consequences saying: “In the UK, the vast majority of food waste ends up in landfill. As food rots in landfill it can produce methane, one of the most potent greenhouse gases and a significant contributor to climate change. When we throw food away, we also waste all the carbon generated as it was produced, processed, transported and stored. This is particularly important given that the whole food supply chain accounts for around 20% of the UK’s greenhouse gas emissions. We could make carbon savings equivalent to taking an estimated 1 in 5 cars off the road if we avoided throwing away all the food that we could have eaten.”
Hogg et al. (2007) estimated the proportion of food waste in UK household waste (HHW) to be 17.6% (Table 1). It appears that households in Herefordshire and Worcestershire (H&W) are less wasteful than the UK average (Appendix B); the average weight of HHW in H&W in 2005/06 was 1,023 kg/hhd*year, of which food waste would have been 180 kg/hhd*year at 17.6%. Table 1 Estimates of food waste in household waste from Hogg et al. (2007)
Household waste (’000 t) Food waste in HHW ∴ Total food waste (’000 t) Food waste ‘captured’ ∴ Food waste in mixed waste (’000 t) Average food waste per hhd·year
England
Wales
Scotland
N. Ireland
UK
25,688
1,585
2,276
919
30,468
17.5%
18%
18%
19%
17.6%
4,495
285
410
184
5,375
2.00%
2.80%
1.95%
2.17%
2.04%
4,405
277
402
180
5,264 216 kg
Irrespective of the detail of precisely what is included in the statistics, the overwhelming conclusion is that the problem is large and that currently the UK does not have a significant means of capturing and diverting this biodegradable waste from landfill.
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Browne (2005) (former Head of Waste and Passenger Transport Management at WCC) weighed the waste in his own house for 12 months after having had a FWD installed (Figure 2). He also measured the electricity and water use. Browne concluded from measuring his household’s waste for a whole year, following installation of a FWD in September 2004, that 25% by weight of the household’s waste went into the FWD. The cost of electricity to run the FWD for the whole year was less than £1 per person (it used 4.2 kWh). Browne considered that using the FWD did not change water consumption measurably. Even though 25% KFW is at the top end of the range reported by Parfitt, the electricity and water use are comparable with findings in other field studies (see later).
Trial Household
Household Waste Site (Recyclables) 5% Kerbside Residual 11%
Household Waste Site (Residuals) 1%
Waste Disposer 25%
Kerbside Tins/ Plastics 6%
Kerbside Glass 19%
Kerbside Paper 33%
Figure 2 Twelve months' waste analysis (fresh weight) for a Worcestershire household with a FWD (Browne, 2005)
3.2 Solid waste and landfill Member States of the European Union are obliged by the Landfill Directive (CEC, 1999) to reduce the quantity of biodegradable municipal waste going to landfills compared with the quantity produced in the reference year 1995. The directive defines municipal waste as ‘waste from households as well as other waste which, because of its nature or composition, is similar to waste from household’; this definition has been interpreted differently by the different Member States (National Page 10 of 53
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Audit Office, 2006). The European Union chose this strategy in order to reduce the leakage of methane-rich landfill gas rather than the strategy of setting limits on landfill gas leakage and encouraging the operation of landfills as bioreactors. Methane (CH4) is estimated to have a global warming potential (GWP) over 100 years of 23, where carbon dioxide (CO2) is 1 (IPCC, 2001). Reportedly, some Member States have already achieved their targets but others have a long way to go. The UK is amongst the laggards.
The National Audit Office concluded “Without a step
change in existing local authority plans, England will not achieve its share of the reductions in landfill the European Union requires by 2010 and 2013” and “An emphasis on increasing recycling alone is unlikely to enable the … Directive on landfill to be met.” The National Audit Office estimated that if no further action is taken by local authorities beyond that already planned the allowances for sending biodegradable municipal waste to landfill will be exceeded “by approximately 270,000 tonnes in 2010 and by approximately 1.4 million tonnes in 2013. The consequent penalties … could amount to £40 million in 2010 and £205 million in 2013.”
Member States need methods for enabling diversion of biodegradable waste from landfill that are hygienic and convenient for their citizens, have a good environmental footprint and that do not impose excessive cost. The conventional wisdom is that this can be achieved by separation at source, separate collection and centralised composting or anaerobic digestion and/or encouraging home composting and/or mixed waste collection and incineration. However it is questionable whether these necessarily meet the criteria of being considered hygienic and convenient by [some] citizens, having a good environmental footprint and not imposing excessive cost.
When one talks with operators of centralised composting or anaerobic digestion facilities in Denmark, Germany and Norway, which have more than 10 years’ experience of this practise, they complain about the amount of contrary material in the separately collected waste. Kegebein, et al. (2001) reported that in Germany communal biowaste bins generally have high contaminant fractions (plastic, glass,
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metal), which increases the difficulty of treatment and reuse. They also reported that only 22% of the biowaste produced in heavily populated areas is collected through separate collection, and attributed this to a lack of acceptance and high cost (approximately 100 euros/household*year). Evans et al. (2002) reported two longestablished centralised treatment sites in Denmark that had stopped accepting source separated domestic and supermarket waste for composting and for anaerobic digestion because they had been unable to solve the problem of excessive physical contaminants. However, at one of these sites, a post-separation device had been developed that enabled extraction of clean ‘bio-pulp’ from waste with physical contaminants; the bio-pulp digested well and met the Danish quality standards.
In the face of so much negative experience from communities that are thought of in the UK as being disciplined and committed to recycling, it seems bizarre that the mantra of separate [solid] collection being the only answer to recycling of biodegradable waste is still widely preached and accepted in the UK.
Herefordshire Council and Worcestershire County Council (H&W) have been in the vanguard of exploring the potential of FWD as an alternative for people who do not wish to home compost, collect and store kitchen food waste (KFW), etc.
3.3 H&W’s joint municipal waste strategy Herefordshire’s and Worcestershire’s joint municipal waste strategy “Managing waste for a brighter future …” published in November 2004 (H&W, 2004) is thorough and innovative.
The concept of collecting and post-sorting dry recyclables is convenient for householders and effective for recycling/resource-recovery. A key requirement is that householders should not be inclined to ‘hide’ wet waste in the dry recyclable bin because this interferes with the sorting.
If there is inadequately wrapped putrescible waste in residual waste and if it is only collected on alternate weeks (AWC), the residual waste bin is likely to become
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malodorous, especially in hot weather. This is a risk with disposable nappies, incontinence pads, etc. but if there is unwrapped food waste, there is the added risk of rats, flies and maggots. However, Worcester City, Wyre Forest and Bromsgrove report they have not experienced this as an issue with AWC. H&W’s strategy of encouraging exclusion of food waste by incentivising home composting and FWD is forward-thinking. Whilst the use of FWD is convenient and hygienic, it is not really ‘retention’ (as it is described in H&W, 2004) because the waste is transferred to another off-site route; an example of Commoner’s 2nd and 4th laws. Severn Trent Water (who will be the recipients of most of the KFW) appear to have been willing to cooperate as part of sustainable development but when the number of installed FWD becomes significant there will be a material increase in their costs and some equable reimbursement out of the savings from not collecting [wet] KFW might be appropriate.
Experience in many countries has been reported for more than 10 years that kerbside collection of garden waste has the unintended consequence of discouraging home composting and increases the total weight of municipal waste (e.g. BioCycle magazine). Some authorities have adopted kerbside collection of garden waste as a quick win to boost the quantity composted and meet their targets [BV82a and BV82b] but from an environmental perspective it is counter-productive and it is good that H&W has been more imaginative. The innovation (H&W, 2004 section 5.3.8) of providing a greenwaste home shredding service in some areas is excellent; it facilitates and improves home composting, accords with the proximity principle and works towards Best Value Performance Indicator (BVPI) No. 84. BV84a kilograms of household waste collected per head of population. BV84b % change from the previous financial year in kilograms of household waste collected per head of population. Separation of KFW at source and diversion via FWD does not yet count against BV82 (DCLG, 2007) which are defined as: BV82a(ii) Total tonnage of household waste arisings which have been sent by the Authority for recycling.
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BV82b(ii) The tonnage of household waste sent by the Authority for composting or treatment by anaerobic digestion. BV82c(ii) Tonnage of household waste arisings which have been used to recover heat, power and other energy sources. BV82d(ii) The tonnage of household waste arisings which have been landfilled.
FWD divert biodegradable household waste from landfill and since all of the biosolids (sewage sludge) in H&W are recycled to land as biofertiliser, all of the KFW discharged to the wastewater system via FWD would be recycled and most likely would also contribute to biogas production [for renewable fuel use]. Unless a quota allowance is made for each FWD installed the amount that passes via FWDs cannot be quantified. However, the published field trial data are quite consistent and it would therefore be reasonable for Defra to assign an amount of KFW to each installed FWD in the same way that it is considering for home composting in connection with LATS (Landfill Allowance Trading Scheme). Defra (2005) says: â&#x20AC;&#x153;Biodegradable waste composted by householders on their domestic premises benefits WDAs, as it will not be counted in waste arisings figures. However, Defra is considering whether, if the Local Authority is actively promoting home composting, this is enough of a benefit and if there is a way of fully recognising the diversion in the mass balance calculation. WRAP are still in the process of developing such a model that will enable the calculation of the diversion of BMW through home composting.â&#x20AC;? If the case is valid for Local Authorities who promote home composting actively, it should be equally valid for those who promote FWD actively.
3.4 Food waste disposers A FWD is an electro-mechanical device that fits in the drain line from a kitchen sink. The average cost of purchasing and installing a FWD is around ÂŁ150 (In-SinkErator, priv. comm. 2007) and the expected life is around 12 years, thus the cost of ownership of a FWD is less than Bokashi treatment (see 3.5). A FWD is flushed with cold water and spins food waste onto an abrasive ring that reduces the waste to small sized particles (98% of particles are smaller than 2mm diameter). These fine particles
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join the wastewater collection and treatment system. FWD grind rather than smash so glass, stones and metal do not splinter. Thus it can be said that FWD separate kitchen food waste (KFW) at source and divert it from landfills but it does go somewhere and that somewhere is the wastewater system which is designed to convey and treat [biodegradable] material suspended in water. The cold water used for flushing coalesces fat onto the other particles and thus avoids deposition on sewer walls; also, it cools the electric motor.
Around 50% of households in the USA have FWDs; they are used with both mains drainage and septic tanks. The percentage of households with FWDs installed in Europe is much less than in the USA. In the UK, which has the greatest use, only 5% of households have a FWD. However, the situation is very different in commercial kitchens; the inclusion of a FWD is normal when a catering facility is remodelled; 40% of commercial kitchens have FWDs. They should also have, and maintain, grease traps, but sadly this is often not the case and even where there is an obligation to install a grease trap there is often no requirement to maintain them when they have been installed.
Field studies (which will be reviewed in more detail later) showed that 96% of householders trialling FWD continue to use them i.e. that the proportion that give up using them is much smaller than with home composting. The 4% who stopped using them did so because of noise, but since modern FWD are quieter, even this should not be an issue in the future. Field studies have shown that use of FWD has a negligible effect on water consumption, that the ground KFW is conveyed in sewers at normal flow velocities (i.e. well within the design criteria of sewers) and that in practice there is no increase in accumulation in sewers, that only about 3 kWhe/household*year is used by FWD but that the food waste generates at least 33 kWhe/household*year electricity from biogas at
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wastewater treatment works (WwTW) that have anaerobic digestion, which is the most prevalent type of sludge treatment in the UK. Severn Trent Water has almost universal anaerobic digestion at its sludge treatment centres. FWD increase the amount of biosolids produced at a WwTW but the extra cost of wastewater treatment and of treating it by AD with biogas CHP and recycling the biosolids to agriculture (the most prevalent route in the UK) is less than one-tenth of the amount saved by H&W for the solid waste route.
Historically WwTW were required to remove suspended solids, biological oxygen demand (BOD) and ammonia from the water. Suspended solids are collected, together with surplus biomass from removing the BOD as sewage sludge and treated. The ammonia is converted to nitrate. Many WwTWs are now required to remove nitrogen (nitrate as well as ammonia) and phosphorus in addition to solids and BOD. The preferred treatment is â&#x20AC;&#x2DC;biological nutrient removalâ&#x20AC;&#x2122; (BNR) but the wastewater at many WwTW does not have sufficient carbon to sustain the biomass needed for BNR and WwTW have to purchase additional carbon (e.g. methanol) and chemical dosing (commonly iron). FWD assist BNR by adding carbon.
Only 75% of households in the USA are on municipal sewerage; there are many septic tanks; there are also many properties on septic tanks in the H&W area. FWD installation is widespread in the USA because many years ago many municipalities saw the benefits of FWDs and mandated them in all new homes and kitchen refurbishments. Subsequent to that, homebuilders specified FWDs in more than 90% of all new build construction in the USA. Currently around 50% of US households have FWD. In the light of this extensive experience, the USA is therefore probably the best source of advice about the likely effect on septic tank sizing and emptying. The frequencies for septic tank emptying shown in Table 2 were calculated to provide a minimum of 24 hours of wastewater retention assuming 50% digestion of the retained solids and they assume year-round occupancy of the residence.
New York State (2007) describes septic tank emptying as a critical step in septic system care as it extends the life of the infiltration field. It also advises that operating
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a FWD is equivalent to increasing the number of occupants by one, i.e. 4 people living in a house with a 3407 litre septic tank should empty it every 2.3 years, but if they use a FWD it should be emptied every 1.7 years. KFW is more digestible than faecal waste and therefore accumulates more slowly (weight for weight) because faecal waste has already been digested.
Table 2 Septic tank emptying frequency in years (from New York State, 2007)
Septic tank size (litres) 1893 2839 3407 3785 4732 5678 7571 9464
1 5.8 9.1 11.0 12.4 15.6 18.9 25.4 30.9
2 2.6 4.2 5.2 5.9 7.5 9.1 12.4 15.6
Household size - Number of Occupants 3 4 5 6 7 8 1.5 1.0 0.7 0.4 0.3 0.2 2.6 1.8 1.3 1.0 0.7 0.6 3.3 2.3 1.7 1.3 1.0 0.8 3.7 2.6 2.0 1.5 1.2 1.0 4.8 3.4 2.6 2.0 1.7 1.4 5.9 4.2 3.3 2.6 2.1 1.8 8.0 5.9 4.5 3.7 3.1 2.6 10.2 7.5 5.9 4.8 4.0 3.5
9 0.1 0.4 0.7 0.8 1.2 1.5 2.2 3.0
3.5 Home composting, Bokashi, wormeries, etc. Home composting, Bokashi, wormeries, green cone digesters etc. can all treat KFW at source, which is ideal provided there is somewhere to use the treated material. The principles of home composting appear simple. It is only necessary to purchase or construct a bin (or preferably three so that there is a sequence of filling, maturing, mixing and emptying) to chop the material going into the bin, ensure there is an adequate, balanced mix of nitrogenous and carbonaceous materials and that they are mixed periodically and it should work. However, questions about composting are amongst the perennials asked of gardening programmes and periodicals. The Bokashi system uses a pair of proprietary bins (costing £60) in which KFW ferments anaerobically with the aid of bran inoculated with microorganisms; the bran costs about £2.50 per month (i.e. £30 per year). It produces a leachate that can be used as plant food and a digestate that can be added to the compost heap or worked into soil. Wormeries use ‘compost worms’ to convert KFW to vermi-stabilised material that Page 17 of 53
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can be used as a soil improver. â&#x20AC;&#x153;Green coneâ&#x20AC;? is an anaerobic digester that should be sited in a warm sunny location and on soil where the leachate will drain. Dr Julian Parfitt (WRAP, priv. comm. 2006) tried a green cone but abandoned it because of the smell adjacent to the sunny sitting area of his family garden. Whilst the emissions of composting are short-cycle CO2, the anaerobic systems emit CH4 and thus have an adverse carbon footprint.
These treatment-at-source systems have their enthusiastic users, but they are not for everybody. They score well on the proximity principle of treating KFW (and other biodegradable material) at source and of using the treated material at source. However many people, such as those living in apartments or with very small gardens, do not have the opportunity for treatment at source, or do not have the interest or inclination to do treatment at source. Alternatives are needed for these members of society.
3.6 Land application of sewage sludge The use of biosolids as a nutrient-rich soil improver and biofertiliser has been practised for decades. Within the EU it is regulated by national implementations of the sludge directive (CEC, 1986). This was the first soil protection directive; the European Commission says it has been a success because there have been no adverse effects where it has been applied. Compliance with the sludge directive and nitrates directive are cross-compliance requirements of the Single Payment Scheme of the EU Common Agricultural Policy. The scientific literature on the subject is extensive with more than 50,000 publications (Evans, 2004). There is a persistent myth that sewage sludge is heavily contaminated but it is untrue. Control of inputs of pollutants has been a considerable success. Dangerous substances legislation has eliminated some substances, e.g. PCBs. Controls imposed at factories have reduced the concentrations of potentially toxic elements [heavy metals] (Figure 3).
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3000
2500
30
100
Zinc
2000
20
Copper Cadmium Mercury
1500
15
1000
10
500
5
2000
80
Cadmium Zinc
70
1500
60 50
1000
mgCD/kgDS
25
mgZn/kgDS
2500
concentrations of C d and H g m g/kgD S
40 30
500
20 10
1998
0
0 20 00
1993
19 93
1983
19 86
0 1973
19 79
0
19 72
concentrations of C u and Zn m g/kgD S
90
Figure 3 Changes in trace element concentrations in sewage sludge with time (Stockholm, left and West London, right)
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Environmental Impact â&#x20AC;&#x201C; Component Analysis This section will review the information that is available about each step in the process from production of KFW to ultimate use or disposal for the two selected alternatives, i.e. separate collection as solid waste and treatment by composting or anaerobic digestion, compared with source separation by FWD and co-treatment at a WwTW with anaerobic digestion of the sludge. When considering the carbon footprint the direct CO2 evolution from KFW [or compost or digestate] is of no consequence to global warming potential (GWP) because it is short-cycle CO2 but escape of CH4 from whatever source does have GWP as does CO2 from road transport and public-supply electricity generation, etc. (Smith et al., 2001). Landfilling is included in this report as a reference i.e. the current situation.
4.1 KFW separation and storage 4.1.1 Solid waste When KFW is separated at source and separately collected as solid waste, it must be stored on site; almost inevitably, this means a bin in the kitchen and another outside. KFW bins are generally made from petrochemical derived plastic.
KFW is about 75% moisture; in hot weather it becomes smelly quickly and it attracts flies and other vectors. Collection agencies have been advised that separate collection need not cost more than combined collection because the recyclable waste can be collected bi-weekly alternating with non-recyclable waste. This is known as AWC (alternate weekly collection). Understandably, people have objected to AWC of KFW in hot weather because of odour and flies. Some municipalities in southern Europe have found it necessary to collect KFW very frequently (even daily) in the heat of summer to avoid odour. Bags of KFW left out for collection (especially by weekend and other visiting householders) are likely to be opened by foxes, gulls and other scavengers, which creates a mess, odour, etc. Matheson (2005) reported that the main motivation for residents in tower blocks to participate in community composting was their desire to get rid of rats around the communal Paladin food waste collection bins. Page 20 of 53
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The National Pest Technicians Association reported that rat infestations have increased by 39% from 1998-99 to 2004-05 (NPTA, 2007). They attributed this increase to a variety of causes but prominent amongst these was the increased access to food as a result of inappropriate [as NPTA regarded it] recycling of KFW which NPTA considered provided a source of food for rodents and flies. NPTA advised that containers provided to householders should be large enough and properly secure so that the waste is contained safely. NPTA recommended special collection facilities should be made available, particularly in hot summer months, and segregated organic household waste should be stored in such a way as to prevent fly infestation. Provisions should be made to guard against other pest infestations such as rats, mice and urban foxes. NPTA advised alternate weekly collection (AWC) should only be where wheeled bins are provided and cited World Health Organisation advice that AWC is questionable for KFW in hot weather.
Odour development is also an indication of oxygen depletion in the waste and conditions that would favour Clostridium botulinum. Bรถhnel et al. (2002) have reported an increase in botulism in Germany, which they link to separate collection, storage and treatment of biowaste; they report that greenwaste is much less of a risk. They have also found that the conditions favouring botulinum neurotoxin production favour the larvae of flies (Calliphoridae) and postulate they could be vectors.
Wouters et al. (2002) reported that keeping separated food waste in kitchens increases bioaerosols and allergens compared with mixed waste that contains food waste; they concluded this is a respiratory risk to susceptible individuals. It appears that an unintended consequence of obliging people to store food waste might not only be causing them nuisance [odour and vermin] but might additionally be exposing them to health risks.
4.1.2 FWD Using FWD eliminates the need for storing KFW in the home or outside in individual or communal collection bins and would thus satisfy the main concern of
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Matheson’s tower block residents. The KFW is disposed to the FWD as soon as it is produced.
It eliminates the resources and energy embedded in collection bins.
FWDs themselves are constructed of steel and copper [mainly] so their constituent materials are 92.5% recyclable (steel 50%; stainless 9%; iron 20%; copper 8.5%; aluminium 5.0%).
4.2 KFW conveyance 4.2.1 Solid waste Via the solid waste route KFW is transported in refuse lorries with all of the emissions, road wear and accident risk associated with road haulage vehicles. A large proportion of kerbside collected waste is delivered to a Refuse Transfer Station (RTS) from where it is transported to a centralised composting or anaerobic digestion (AD) site by a large refuse transport vehicle (RTV). A smaller proportion will be transported to the composting site by the Refuse Collection Vehicle (RCV). According to Smith et al. (2001), the average emissions of an RCV and a RTV are 0.84 and 0.71 kg CO2 /km and their payloads are 6.67 and 20 t respectively. Neither vehicle runs full all of the time. The RCV travels to its collection round empty, and is not full until the end of the round when it travels to the RTS or composting site, thus its effective load averages approximately 50% of its payload, which is the same as the RTV, which returns from the treatment or disposal site empty. The specific emissions are thus 0.25 kg CO2 /km*t waste and 0.071 kg CO2 /km*t waste respectively. In comparison Smith et al. reported the average emission of a medium sized petrol powered car is 0.21 kg CO2 /km and the payload 0.01 t, which equates to the specific emission for a private car delivering waste to a civic amenity site and returning empty being about 42 kg CO2 /km*t waste. Even if the payload is 100 kg, rather than 10 kg, the specific emission is 4 kg CO2 /km*t waste. It is arguable whether separate collection affects ‘garbage miles’. If the weight of waste on each collection round divides equally between the collections, i.e. if a weekly mixed collection goes to AWC of separated fractions and if each is 50% of the combined weight, the ‘garbage miles’ will be unchanged. However, a KFW collection would be a third collection (dry recyclables, KFW/putrescible and residual)
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and if unacceptable odour is to be avoided it would have to be weekly in hot weather at least. The analyses of Parfitt (2002) and Hogg et al. (2007) agree that KFW is around 17% of the total weight of household waste (HHW). In H&W, 12.6% of HHW is taken to household waste sites [civic amenity sites]. It therefore appears inevitable that separate collection does increase garbage miles and 10 kg CO2e / t KFW has been allowed (Table 3) for separate collection of KFW.
4.2.2 FWD When KFW is eliminated via a FWD it is ground using electricity and then transferred to the sewerage system as a suspension in water. In this section each of these elements will be assessed.
4.2.2.1 Water use Each time they are used, FWD are flushed with cold water, this cools the motor and conveys the food waste out of the grinding chamber. Water resources in south east England, which has the highest population in the UK and has low rainfall, are already under pressure, however the Chartered Institution of Water and Environmental Management has concluded (CIWEM, 2003) â&#x20AC;&#x153;The change in water usage associated with operation of FWD has been measured to be trivial or not significant.â&#x20AC;?
A detailed stratified survey in the USA (Ketzenberger, 1995) reported that FWD were used for about 15 seconds per start irrespective of the number of people in the household; subjectively this seems sensible (because FWD use is linked to food preparation events) and accounts for the range of reported water-use when expressed as litres per capita. A study in Sweden fitted FWDs in a community of 100 apartments (155 adults and 56 children); the duration of use per start was 38 seconds (Nilsson et al. 1990). The per capita water use was 13 L/day less during the 11 months after the FWDs had been installed than the 6 months prior to installation. Another Swedish study (Kalberg et al., 1999) and one from Canada (Jones, 1990) were unable to detect any greater per-capita volume of water used where FWD had been installed. Both Swedish studies found that water use actually decreased during
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the period when FWD were used but they concluded it would not be appropriate to attribute this directly to the fact that FWD had been installed. The Canadian study concluded the influence on water use was not significant within the overall “noise” in measured water use. Whilst this inability to measure an increase in water use when FWDs are installed seems counter intuitive initially, it is perhaps understandable when one thinks about the routine of food preparation, etc. After using the sink it is normal to wash it down to clean it, if there were a FWD this would also flush the FWD.
The studies that have been able to measure water use associated with FWD operation found data ranged from 0.29 L/person*day (large families) to 6.4 L/person*day. The extremes of the range are probably anomalous. There has only been one study of water use in the UK that has included FWD, however the methodology used was fundamentally flawed. Even when the paper was presented, the statistical analysis used was criticised as having been demonstrated to be inappropriate for this type of work (Thackray et al., 1978).
The study by the New York City Department of Environmental Protection (NYDEP, 1999), which was undertaken to inform its decision whether to change the regulations regarding FWD installation, is probably the largest field study ever undertaken. It involved 514 apartments with FWD compared with 535 apartments without FWD; they were divided into 4 localities to reflect some of the city’s diversity. The survey comprised 2014 people in total, i.e. 1.92 people per apartment. The report concluded the average water use attributable to FWD was 3.6 L/person*day. If uses/day averaged 2.2 as in Ketzenberger’s study, this would equate to 3.1 L/use, i.e. the same as Ketzenberger. The overall result of the NYDEP study was that the 18-year restriction on FWD installation in New York City was removed.
4.2.2.2
Electricity
Domestic FWDs typically have a 350 to 500 W motor (0.5 to 0.75 horsepower), if usage averages 2.4 times per day for 16 seconds per use the annual electricity consumption is about 2 to 3 kWh/household*year. Surveys have found that usage
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(starts/day) is largely independent of the number of people in a household because it is determined by food preparation events.
The EU-average electricity generation emission factor (cited by Smith et al., 2001) is 0.45 kg CO2e /kWh (range coal = 0.95 to wind = 0.009 kg CO2e /kWh)4. Thus the annual GWP of the electricity used by a FWD is around 1 kg CO2e /household. If the average KFW per household is 180 kg/year (Appendix B), this equates to approximately 6 kg CO2e /t KFW. 4.2.2.3
Sewers
Sewer systems are designed to remove wastewater to prevent urban flooding and disease; the pipe diameters and gradients are designed such that the flow velocity keeps the typically encountered solids in suspension. During periods when the flow velocity is low, solids might settle but they should be re-suspended when velocities increase. Design standards for â&#x20AC;&#x153;self-cleansing velocityâ&#x20AC;? range from 0.48 m/s to 0.9 m/s (Ashley et al., 2004). An obvious concern is that use of FWD might result in sediment build-up in sewers. The field studies already cited in this paper have checked the effect of FWDs on the conditions in sewers and found no significant accumulations. The times of day when FWDs are used corresponds with times of high flow (Nilsson et al., 1990). In an experimental rig using different types of KFW, sediment-free transport of the output from FWD was observed at 0.1 m/s, i.e. well within the normal design standards (Kegebein et al., 2001). 40-50% of the output was <0.5 mm and 98% was <2 mm by sieve analysis. All of the output passed a 5 mm sieve. The largest particles were fragments of lettuce leaves. Depending on the type of KFW, between 15 and 36% of the output of the FWD was dissolved. The output of the FWD was very finely divided and very biodegradable.
FOG (fat, oil and grease) is a significant problem in sewerage operations, it can reduce the capacity of sewers and even block them; FOG can also accumulate inside the cooling jackets of pumps and cause them to overheat if it is not removed. It appears that FOG undergoes chemical transformations (possibly involving proteins) 4
CO2 e = carbon dioxide equivalent according to the Global Warming Potential (GWP) over 100 years.
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that increase its hardness. Field studies have found that domestic FWDs do not increase FOG; it is supposed that the constituents of FOG coalesce onto food waste particles in the cold water flush and that they are therefore not â&#x20AC;&#x153;freeâ&#x20AC;? to attach/solidify onto sewer surfaces. De Koning (1996) concluded that even in Holland where the gradients of sewers are shallower than elsewhere (and as a consequence sedimentation would be more likely) ground KFW from FWD would not result in sewer obstructions from sedimentation or FOG deposition. WRc in the UK is undertaking (2005-2009) a major collaborative research project into FOG through the sewers and WwTWs (http://www.wrcplc.co.uk/default.aspx?item=316). Most of the UK water companies are subscribing to the project as well as interests in Ireland and possibly the USA. The project includes social science into how people use sewers and how to influence their behaviour. It is important that people do not put FOG down the drain so one objective of the project is to identify how to encourage this good behaviour.
An important question is whether putting more food into the sewers will increase the number of rats. NPTA (2007) is critical of the sewerage operators but as discussed below, the outputs of FWD are not pertinent to the criticism. A spokesperson for the British Pest Control Association [Adrian Meyer, Rodent Control Consultant, priv. comm. 2005] advised that installing FWDs would probably be detrimental to rats and certainly not advantageous because finely ground food waste would be less attractive to sewer rats than un-ground waste. Apparently, nobody really knows how rats find their food in sewers, which are dark, but rats have been seen scooping grains etc. out of the flow. There is invariably identifiable food such as sweet corn grains in the grit and screenings skips at WwTWs; these would have been large enough to be identifiable by rats. However, if they had been through a FWD they would have been liquidised and hence not identifiable by rats; food residues <2mm would be non-identifiable by rats. Alternatively, rats might not feed in sewers at all but merely use them as refuges and feed on the surface from waste bins, etc.
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4.3 KFW treatment Separating KFW makes it easier to sort, recover and recycle other fractions of municipal solid waste (MSW) because KFW is wet and therefore contaminates recyclable materials rendering them more difficult and more costly (or impossible) to sort and/or recycle.
4.3.1 Solid waste The alternative treatments for KFW via the MSW route are landfill, incineration, composting and anaerobic digestion. Landfill will not be discussed because it must be phased out to comply with the Landfill Directive. Autoclave treatment will not be discussed either because it is probably much less suitable for separated KFW than AD because of odour and loss of revenue from biogas; however this should not be taken as questioning the potential for autoclave treatment with residual waste from which dry recyclables and KFW have been removed.
Incineration (Energy from Waste, EfW) is attractive because of its practicability. It is not subject to the problems of physical contaminants that are significant for the other routes. The cities of Aarhus in Denmark and Rotterdam in the Netherlands both decided in 2006 to stop composting of separately collected KFW and supermarket waste because of physical contaminants and to incinerate the wastes instead. Whilst Danish and Dutch citizens accept incineration and appear satisfied that emissions are controlled adequately, this is not the case in the UK where a significant proportion of the public is opposed to incineration. On 9th January 2007 Hull City Council and the East Riding of Yorkshire Council announced that approval had been given for an EfW plant costing ÂŁ30 million to burn 240,000 tonnes of rubbish every year to generate electricity and heat, however this was after a long planning battle and the opposition groups have said they will continue to protest.
Severn Trent Water has two incinerators near Birmingham burning digested sludge, one at Coleshill and the other at Roundhill. The moisture content of KFW is similar to dewatered digested sludge and it might be possible to co-incinerate them if
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Severn Trent Water was interested and if the incinerators had spare capacity, if the EA would grant the necessary variation to the licences and if Ofwat would agree acceptable financial terms. Using an existing incinerator would have the obvious advantage of avoiding some of the planning hurdles but public acceptance would still need to be handled carefully and proactively before malicious misinformation became established. However, it is an expensive option both in terms of transport distances to the incinerators and the cost of operating waste incinerators and their emission controls; the value of electricity and heat from burning KFW are relatively trivial. Smith et al. (2001) found incineration was one of the more expensive options for whole MSW; the putrescible fraction has the lowest net calorific value of any of the combustible fractions5 confirming that income offset would be negligible. Yorkshire Water Services, which operates four sludge incinerators, estimates the cost of sludge incineration at ÂŁ160/tDS (priv. comm. 2006). The incineration option will not be considered further in this report.
The status of KFW in the solid waste route is Animal By-Products Regulations Category III (catering waste) unless it can be proved not to have come in contact with meat. In the solid waste scenario, this would be difficult to assure. Thus, KFW must be treated in an ABPR compliant system licensed by the State Veterinary Service as well as the Environment Agency.
4.3.1.1
Composting
The energy consumption of in-vessel composting (not necessarily ABPR) has been estimated to be 40 kWh electricity per tonne of waste, i.e. 18 kg CO2e/tonne at the EU-average power emission factor. This is the average of the 16 plants surveyed by Wannholt (1998) (cited by Smith et al., 2001). It includes the use of gas cleaning systems to remove odour emissions as well as the electricity used for blowing air to aerate the piles and maintain correct temperature and humidity. The additional requirements of ABPR would probably result in somewhat greater energy use because ABPR defines shredding and two stages of treatment to prevent by-pass. Apparently
5
Net calorific values of plastics, textiles, paper/card and putrescibles, are 31.5, 14.6, 11.5 and 3.98 MJ/t respectively (Smith et al., 2001) for comparison coal that has a CV of 24,000MJ/t Page 28 of 53
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Wannholt reported that the yield of compost was 47% of the weight received and that only 6% of the weight of the waste received was rejected [contaminants] and diverted to landfill or incineration. This is a very low reject rate, Smith et al. proposed 40% yield and 10% reject as a more realistic performance to expect.
There are undoubtedly anaerobic microzones in composting material where the oxygen supply is inadequate to satisfy the oxygen drawdown of the microorganisms. Methane is produced in these microzones but the consensus is that, except in the worst cases, the methane is oxidised to carbon dioxide in the surface layers of the composting material or in the biofilter and that methane emission from composting material can be neglected as not significant for practical purposes.
The question of occupational health issues related to composting has been debated for several years. B端nger et al. (2007) reported significant impairment of lung function etc. of compost workers, compared with office workers; they attributed this to exposure to dust and bioaerosols containing pathogens, glucans and allergens. This reinforces the advice to monitor workers subject to such occupational exposure for the sake of their own health and to protect employers from possible claims for industrial injury.
4.3.1.2
Anaerobic digestion
Anaerobic digestion (AD) has several practical and revenue advantages for separately collected food waste:
a)
whereas composting converts biodegradable carbon to CO2 which does not have GWP because it is short cycle, AD converts it to biogas which is about 65% CH4 and 34% CO2 with traces of other gases; the CH4 is contained and can be used as renewable energy, i.e. it has a negative GWP contribution (because of offsetting fossil fuel) and a significant income generation potential from sales of electricity and Renewables Obligation Certificates (ROCs).
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Figure 4 Co-digestion facility for food, manure and other wastes in Denmark – biogas holder left; two digesters right with the two 70 °C sanitisation towers in their shadow.
b)
Operational experience has shown in Denmark (Evans et al. 2002) and Germany (Hese Umwelt priv. comm. 2006) that it is more practicable to extract the physical contaminants (which have proved inevitable in separately collected food waste) prior to AD than it is with composting. The answer to this issue is a high-pressure screen like that shown in Figure 5.
c)
If regulatory issues (Ofwat and EA) can be overcome, and with Severn Trent’s cooperation it would be possible to use the AD infrastructure that already exists at their larger WwTWs, which would obviate many of the planning issues of developing a treatment site de novo. The factors that might make this interesting to Severn Trent could be financial (Ofwat permitting) and transforming the sludge to “enhanced treated” status plus better dewatering.
Mesophilic anaerobic digestion (MAD) at 33 to 40 °C is a stable and reliable process. The methane-rich biogas can be used as renewable energy. AD and CHP have been used in the UK for sewage sludge for more than 70 years. Performance is described in terms of VS destruction; VS is ‘volatile solids’ actually ‘loss on ignition’, i.e. it is equivalent to organic matter. Typically fully mixed MAD achieves 40% VS destruction, this can be increased to 60% by pretreating the feed using thermal hydrolysis (TH). The yield of biogas depends on the makeup of the material being digested, e.g. fat has a very high gas yield. The yield for sewage sludge is typically about 1.3 m3/kg VS destroyed. Half the additional gas from TH is used in the steam Page 30 of 53
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feed
bar screen
trash biodegradable pulp
Figure 5 DewasterÂŽ for separating bio-pulp from physical contaminants prior to digestion
boiler to drive the process; the other half is available for CHP. TH pressure cooks the feed at 160 °C for 30 minutes, which increases the digestibility of the organic matter, sterilises the feed and reduces its viscosity to such an extent that the solids loading can be trebled and the digesters continue to be fully mixed, i.e. the capacity of existing digesters could be trebled by retrofitting TH (Evans, 2003). TH exceeds the timetemperature requirements for ABPR. The digestate from TH + MAD dewaters much better than from other MAD configurations; e.g. using a conventional belt filter press the cake dry solids increases from about 22%DS to 34%DS. The combined effect of increasing VS destruction and increasing cake %DS is that the mass of cake is halved.
Smith et al. (2001) included AD of separately collected organic fraction of MSW (OFMSW) but they assumed that the digestate has to be composted before it can be used on land. It is unnecessary and counter-productive to compost digestate from an ABPR AD plant because the readily degradable carbon has already been stabilised and there is therefore no necessity to use more energy to create short cycle CO2 when this carbon would be better feeding soil biomass as soil improver. ABPR requires that feed containing Category III material is pre-sanitised (at 70 °C for 1 hour) prior to AD, thus post-composting would have no additional hygienic value. Thirdly post-composting volatilises ammonia, which is a waste of valuable fertiliserreplacement nitrogen.
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A calculation for this study of the biogas yield, electricity generation potential, revenue from electricity inclusive of Renewables Obligation Certificates (ROCs) and the GWP offset is shown in Appendix C for MAD preceded by ABPR-compliant ‘pasteurisation’ or TH. The GWP offset at the EU-average electricity generation emission factor (cited by Smith et al., 2001) which is 0.45 kg CO2e /kWh would be -131.9 and -183.2 kg CO2e /t feed respectively (Appendix C). 4.3.1.3
Landfilling
As a generalisation in this report, the collection of KFW and its delivery to landfill has been assumed to be the same as that for composting or AD. The landfill site has been assumed to be modern and constructed and managed to best practice standards with efficient landfill gas collection and use of that landfill gas for electricity generation. When biodegradable (putrescible) waste is placed in a landfill, the first stage of degradation is aerobic; this releases short cycle CO2 that has no GWP. Degradation switches to anaerobic when the available oxygen has been used; initially the pH decreases because of VFA (volatile fatty acid) production, this mobilises metals, pH later increases as methanogens develop and convert the VFAs to landfill gas. Metals are re-precipitated as the pH increases. Even the best techniques of landfill construction and landfill gas pumping result in some landfill gas leakage, and since this is 40-65% CH4 by volume the GWP is very significant. On the positive side, landfills sequester significant amounts of carbon. Smith et al. (2001) estimated that electricity generation from putrescible waste has a GWP of -32 kgCO2e/t KFW, short-cycle carbon sequestration contributes -272 kgCO2e/t KFW, but fuel use within the landfilling operations is +8 kgCO2e/t KFW and methane from leaking landfill gas contributes +1025 kgCO2e/t KFW resulting in an overall GWP for ‘treatment’ of +729 kgCO2e/t KFW. When 14 kgCO2e/t KFW is added for ‘conveyance’ (i.e. collection in mixed waste through to delivery to the landfill) the total for the route is +743 kgCO2e/t KFW.
4.3.2 FWD KFW is discharged to the sewer even without a FWD in the form of dishwasher output, washing up, sink cleaning after meal preparation, etc. The treatment
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requirements for wastewater and the rules for use of biosolids on land mean that equivalence to ABPR Category III risk management is achieved (Defra priv. comm.).
Kegebein et al. (2001) estimated that where the wastewater treatment works (WwTW) receiving the KFW treated its sludge by AD, the biogas from KFW would amount to approximately 300 MJ/resident*year, which they said corresponds to a heating value of 8 litres of diesel fuel or 183 kWh/household*year (2.2 people per household). At 40% electricity generation efficiency, this is 73 kWhe/household*year electricity generation, which at the EU average for electricity generation is a GWP of -33 kgCO2e/household*year (i.e. a saving). If the average KFW content of household waste is 17.6% (Hogg et al., 2007), the average quantity for H&W is 180 kg KFW/household*year (Appendix B). Thus, the GWP according to the work of Kegebein et al. is -183 kgCO2e/t KFW. This is probably an overestimate because no allowance was made for biodegradation in the sewer and in wastewater treatment but it is a similar order of magnitude to the figure for KFW transported directly to codigestion (Appendix C). More than 50% of UK sewage sludge is treated by AD (Gendebien et al., 1999) and the proportion treated, and the efficiency of biogas production, are both increasing as more water companies seek to gain from the income potential of renewable energy. Most of Severn Trent Waterâ&#x20AC;&#x2122;s sludge treatment centres use AD and so does Hereford WwTW.
As discussed in section 4.2.2 it has proved difficult to measure the impact of FWDs on most of the parameters measurable at a WwTW because of the variations that occur naturally and because there have been few cases where the number of FWD installed has been a sufficiently large proportion of the contributing properties. A notable exception has been the town of Surahammar in Sweden (Kalberg and Norin, 1999). After an initial pilot investigation, Surahammar decided to offer FWD to householders as an alternative to a new refuse collection charge for separate collection. Between May 1997 and October 1998, 1100 of the 3700 households had a FWD installed. No significant difference was found at the WwTW in grit, BOD, COD, N or P or in the quantity of chemical used for P-removal. Kalberg and Norin suggested that changes in these parameters were not visible because of the variation
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that happens because of weather, etc. However, there was a significant change in three parameters. The average weight collected on the 3 mm inlet screens increased from 26 kg/day (average for 1996-97) to 46 kg/day for the period March to December 1998. 3 mm screens are very fine by UK standards; in the UK, 6 mm screens are considered to be the normal fine screens. The amount retained on the 3 mm screens was reduced if the screens were cleaned more frequently (i.e. solids were <3 mm but were retained on other debris). The ratio of BOD7:N increased from approx. 3.7 before May 1997 to 4.5-4.6 mg/L after October 1998, this was greater than the value of 4.2 mg/L that the authors predicted by theory; they speculated the reason for the difference, if it is real, could be the result of denitrification in the sewers. KFW is more carbonaceous than toilet waste. Increasing BOD7:N is desirable for biological nutrient removal (BNR). There was also a significant increase in daily biogas production [averaged over the 4 months September to December] from about 340 m3/d to about 420 m3/d (Figure 6). Biogas production could be considered to be a
value that integrates the impact of FWD inputs over time (see also Appendix D).
average biogas production m 3/day
430 410 390 370 350 330 310 290 270 250 1995
1996
1997
1998
Figure 6 Daily biogas yield averaged for September to December each year (Kalberg and Norin, 1999)
Formerly WwTWs were required to remove suspended solids, BOD and ammonia, now many are required to remove nitrogen and phosphorus as well. The sewage at many WwTWs has insufficient carbon for denitrification and biological phosphorus removal and they therefore have to use supplementary carbon, such as Page 34 of 53
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methanol, to feed their BNR. KFW could be a useful input of much-needed carbon if there were sufficient FWDs.
4.4 Use or disposal of treated KFW Using treated KFW on land as nutrient-rich soil improver completes nutrient cycles and conserves organic matter irrespective of whether it is done via the solid waste route or via FWD and biosolids recycling. The organic matter in treated KFW feeds soils; it increases soil microbial biomass and it improves soil structure. Soils with better soil structure allow more rainwater infiltration, which reduces run-off, they have better reserves of plant-available water in dry periods and they are more resistant to erosion. Furthermore, there is a positive relationship between the amount of soil organic matter and the efficiency of fertiliser use and resilience of plants to soil-borne plant pathogens.
4.4.1 Solid waste 4.4.1.1
Compost
Compost can be used as a soil improver for horticulture, agriculture or land reclamation. There has been considerable interest in using compost as an alternative to peat in growing media; whilst this is technically feasible (Evans and Rainbow, 1998) the pursuit of it has been something of a distraction. Growing media have demanding technical requirements, which are difficult to match with composted greenwaste, let alone KFW, because the pH and nutrient content are high. Peat has very good horticultural properties and its cost as a raw material entering a growing medium factory is only ÂŁ5-8 per m3. Composted KFW has an advantage of proximity to domestic customers but the established growing media producers have the advantages of economy of scale, automation and brand recognition. KFW also comes with the problem of physical contaminants, which are really not tolerated by domestic customers. Using composted KFW as bulk soil improver for â&#x20AC;&#x2DC;professionalâ&#x20AC;&#x2122; [commercial] users is much less difficult.
Smith et al. (2001) estimated that allowing for the decay of compost added to soil over 100 years (which is the conventional time scale for GWP calculation) the use
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of compost on land would sequester the equivalent of 22 kg short-cycle CO2 /t KFW treated by composting. Smith et al. also estimated 36 kg CO2e avoided /t waste for the fertiliser replacement value of the compost; they have somewhat overestimated nitrogen value for H&W conditions because they have used data from pot experiments and southern European field trials. In pot experiments, the density of rooting is much greater than in the open ground, and the temperature in the pot is also greater than open-ground soil temperatures; these factors result in greater extraction of nutrients and greater rates of mineralisation of organic nitrogen than in open soil. The amount of nitrogen available to plants from an organic source depends on microbial mineralisation from organic-N via ammonium-N to nitrate-N. Mineralisation is temperature dependent. Field experiments in Costa Blanca, Spain found 40-60% availability of organic-N where the comparable figure in UK was 20%.
4.4.1.2
Digestate
It is easier to produce digestate that is free of physical contaminants than compost, especially when something like Dewaster速 is used (4.3.1.2). Using digestate on land has the same benefits as using compost and conserves more of the nitrogen fertiliser value. Dewatered digestate is somewhat sticky and therefore not as well suited to manual application as compost, which is friable and easily spread with hand tools. However, there is no difficulty in spreading dewatered digestate on a commercial scale using manure spreader type machines.
The benefits of carbon sequestration and fertiliser replacement are similar to those discussed for compost and within the approximations of this report it is appropriate to use the same 22 kg short-cycle CO2 sequestered /t waste and 36 kg CO2e GWP avoided /t waste for the fertiliser replacement value. The latter is an underestimate because AD conserves nitrogen from the feedstock whereas composting volatilises it as ammonia gas. Thus, digestate contains more nitrogen than compost and the proportion of nitrogen that is plant-available is greater in digestate than it is in compost.
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4.4.2 FWD KFW separated at source and despatched from the premises via a FWD is conveyed by the sewers to a WwTW where the solubilised fraction is treated as wastewater and the settleable solids become part of the sludge. In the case of Severn Trent Water, sludge is anaerobically digested and the digestate is recycled to farmland as with the MSW-AD routes (section 4.4.1.2). The amount of digestate is less than the MSW-AD route because some is biodegraded in the water phase; however, similar assumptions can be made.
4.5 Summation of component analysis The principal components of GWP that have been discussed in this report are summarised in Table 3. The assumptions and approximations have been discussed in the appropriate sections, including the appendices. Some elements have not been quantified because they are too uncertain, such as the GWP of the wheeled bins and disposal of the rejects from the centralised treatment site. Rejects from FWD will go to the residual waste; rejects from MSW composting and AD will also go to residual waste but at a later point of entry to the route. The GWP associated with the additional biogas yield at a WwTW with AD has been derived from two sources; it is encouraging that they are in good agreement. A further apparent omission from Table 3 is the GWP associated with wastewater treatment but this has been shown (Monteith et al. 2005) to be trivial in the context of this study because emissions are mostly short-cycle CO2 in well-managed plants.
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Table 3 Summary of the main GWP contributions (kg CO2e / t KFW) MSW route compost 70째C+AD
TH+AD
FWD route landfill incineration
Kegebein Surahammar
bins, odours, vermin, health
separation and storage
0
0
14.3
14.3
14.3
14.3
14.3
6.2
6.2
RCV separate collection (extra distance)
10
10
10
0
0
0
0
treatment (incl. electricity generated)
18
-132
-183
-24
-2
-183
-119
-22
-22
-22
-272
0
-22
-22
0
0
0
1025
0
0
0
-36
-36
-36
0
0
-36
-36
1.70
3.83
1.84
0
0.30
2.84
2.84
-14
-162
-215
743
13
-232
-168
conveyance (from hhd to treatment)
C-sequestration landfill gas leakage fertiliser offset delivery (from trt 60km round trip in RTV) Total
Table 3 shows that all routes have less GWP than landfill. In terms of the options for source separated KFW, (co)incineration has the worst carbon footprint because of the low net calorific value and the large volume of flue gas associated with KFW. Composting is intermediate but the routes where the KFW is delivered to anaerobic digestion with CHP (via FWD or directly by road) have the best carbon footprint. In the H&W area, sewage sludge is treated at sludge treatment centres and WwTW that have AD. The value would be even greater if all of the hot water [from cooling the engines and recoverable from the hot exhaust gases] could be used. For example, Worcester WwTW is sited next door to a public swimming pool that can use the heat from hot water effectively. In Denmark where district heating infrastructure has been in place for many years, the hot water can be used for heating buildings. Sadly, it is not often the case in the UK at present that the full value of this heat can be used.
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Cost comparison of FWD and MSW routes Waste statistics (quantities and costs) derived from Best Value Performance Indicators are shown in Appendix B. In the context of this study these data have their limitations because they do not categorise the component parts of the waste, but they are the best available. Parfitt (2002) analysed 70 datasets of domestic waste composition obtained in studies commissioned between 1999 and 2002 across England and Wales. He concluded that kitchen waste comprised 17% of total domestic waste (Figure 1). He commented that there is a degree of uncertainty because no two studies employed the same methodology; most were reportedly “dustbin waste”. Hogg et al. (2007) reported a similar percentage of food waste in household waste at 17.6%.
The quantity of kerbside waste collected from households by the local authorities in H&W ranges from 314 to 469 kg/person*year (Appendix B) because, for example, some offer kerbside collection of greenwaste and others do not. The weighted annual averages, from the total BV84a weight collected, total population and total number of households, together with Hogg et al.’s 17.6% for KFW in the domestic waste stream, yield the following: Table 4 Summary of annual cost and quantity household waste savings (see Appendix B)
Description Mass of KFW if it is 17.6% of BV84a
180.1 kg/hhd
Pro rata KFW [BV86] collection cost
£7.72 /hhd
Pro rata KFW [BV87] disposal cost
£10.91 /hhd
Combined pro rata KFW collection and disposal [current] cost
£18.63 /hhd
If KFW were collected separately, treated and recycled in compliance with ABPR the cost would be much more expensive than the average of the household waste costs shown in Table 4. Thus the average combined financial saving for the collection
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agencies and the disposal agency is likely to be in excess of £18.63 /hhd*year for each FWD installed. KFW comprises about 25.9% of the biodegradable waste and, in addition, it is the most difficult fraction because it is so wet. Eliminating KFW at source via FWD immediately contributes to achieving the LFD targets (BV84) and there is a ‘multiplier effect’ in that it also facilitates post-separation and recycling of dry biodegradables. There is an additional multiplier effect if LATS (Landfill Allowance Trading Scheme) is factored into the equation. The LATS penalty is currently £150 per tonne of biodegradable municipal waste landfilled in excess of that permitted by allowances held. There could be additional penalties in the target years 2010, 2013 and 2020. The Local Government Association has warned that current data imply that prices for allowances could be high from 2008/09 onwards, with a "serious deficit" of allowances potentially arising after 2009/10 (letsrecycle.com).
Estimating the cost transfer to the sewerage and wastewater operator is also problematic because of the uncertainties in quantities involved. By definition, KFW is biodegradable and therefore some of it will never reach the WwTW because it will biodegrade in the sewers.
Table 5 Summary of cost transfer to wastewater sector (see Appendix D6)
Description of WwTW and sludge treatment and recycling or disposal
/hhd*y
Anaerobic digestion, CHP, land-application
£0.68
Anaerobic digestion and land-application but no CHP
£3.63
Lime stabilisation and land-application (no AD)
£5.96
AD + CHP + ROC + incineration
£2.18
Incineration (no AD)
£8.38
6
based on the measurements made in the Surahammar field study
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Conclusions This study has examined the environmental, health and financial impacts of using FWD to divert KFW from landfill and concluded that, in agreement with H&Wâ&#x20AC;&#x2122;s joint municipal waste management strategy, FWD can have a very positive role.
Many field studies have shown that FWD have negligible effect on the use of water or energy. If the wastewater treatment works (WwTW) that receives the KFW has anaerobic digestion (AD) and electricity generation the energy balance is very positive (2.5 kWhe /household*year used against at least 33 kWhe /hhd*year generated from the biogas and could be as much as 73 kWhe). The majority of sludge produced by WwTW in Severn Trent Water is treated by AD, as is the sludge at Hereford WwTW. The current trend in the water industry is to increase the efficiency of biogas generation and to exploit its value as renewable energy more effectively.
Laboratory experiments have shown that the output from FWD is finely divided and that the density of particles is such that it is carried easily in the flow velocity used for designing sewers. Field studies have confirmed that FWD do not influence sewer blockage neither are the particles large enough to block the screens at CSOs (combined sewer overflows) â&#x20AC;&#x201C; the screens are 6mm; 98% of the output of FWD was <2 mm and 100% was < 5 mm. When sewage sludge is used on land (which is the route for the majority in the UK), the organic matter in KFW is conserved and the nutrient cycles are completed.
The carbon footprint of FWD use is better than the solid waste route with centralised composting (-168 and -14 kg CO2e GWP /t KFW respectively) and is approximately equivalent to centralised AD. Landfill is +743 kg CO2e GWP /t KFW. At the average rate of KFW production per household in H&W, this is only -30 and -3 kg CO2e GWP / household and +134 kgCO2e GWP / household for landfill. These figures are small by comparison with the annual 10,920 kg CO2e carbon footprint of the average Briton (The Independent, 2006) but look more relevant when compared Page 41 of 53
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with the 100 kg CO2e for lighting. The most significant factor differentiating FWD and centralised composting is whether the readily degradable carbon is stabilised by being converted to carbon dioxide or to methane that is used as renewable fuel. Ultimately, the product of either is short-cycle CO2 but AD produces useful energy (CH4 that burns to CO2) and composting consumes energy. De Koning and van der Graaf (1996) concluded that until the proportion of households with FWD installed exceeds 30% there is unlikely to be any substantive effect on WwTW operating capacity. However, Kalberg and Norin (1999) found that even when 30% of the households connected to a WwTW did have FWD they were unable to measure any change in the power consumption by the air blowers used for secondary treatment of the wastewater (the power consumption is an ‘integrator’ of the load). Even if more than 30% of households installed FWD, it would only be WwTW that are close to the limits of their operating capabilities that would need capital investment in extensions to treatment. For biological nutrient removal (BNR) [of nitrogen and phosphorus] WwTW are often limited because sewage is too ‘weak’; installation of FWDs would be beneficial by adding to the carbonaceous strength of sewage, which would aid BNR.
Sewage pumping is not affected by installation of FWD since it has been found in field studies that FWD do not increase water usage. By transferring KFW from the MSW route to the waterborne route, FWD will add to the cost of wastewater treatment; the amount depends on the routes for sludge treatment and for sludge use or disposal. The most frequent combination in Severn Trent Water is AD with CHP followed by beneficial use of the digested sludge on land, which is the same for Hereford WwTW; the cost increase for this is only about £0.68 per household*year.
The average direct cost saving to the collection and disposal agencies in the Herefordshire and Worcestershire area is more than £18.63 per household*year. The payback on the average cashback payments to date is only 3 years and 4 months. There could be additional financial benefit from LATS trading. The saving will increase, and the payback period will decrease, as the cost of treating KFW increases
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with ABPR compliant treatment replacing landfilling. For example, letsrecycle.com estimates the current gate fee for ABPR compliant composting is ÂŁ42-52 /t.
This study has found that food waste disposers (FWD) provide a convenient and hygienic means for householders to separate kitchen food waste (KFW) at source; they divert it from municipal solid waste landfill. Importantly, FWD do this using existing infrastructure and, by taking wet putrescible matter out of the solid waste stream, they make management of the dry fractions easier and less expensive and avoid odour issues, which have proved so detrimental to public acceptance of AWC. There is no reason that FWD should discourage home composting since FWD are not designed to take garden waste and indeed exclusion of cooked KFW from home composting might encourage home composting.
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Acknowledgements The County Surveyors’ Society contributed funding to this research as Project No. 59 – “Using Food Waste Disposers to Divert Putrescible Kitchen Waste from Landfill”. It was managed by Jeremy Howell-Thomas, Project Development Officer, Waste Management, Worcestershire County Council.
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References Ashley, R.M.; Bertrand-Krajewski, J.-L.; Hvitved-Jacobsen, T. and Verbanck, M (2004) Solids in sewers: characteristics, effects and control of sewer solids and associated pollutants. IWA Publishing, London. BioCycle - Journal of Composting & Organics Recycling. JG Press, Inc., 419 State Avenue, Emmaus PA, 18049 USA Böhnel, H. (2002) Household biowaste containers (bio-bins) – potential incubators for Clostridium botulinum and botulinum neurotoxins. Water, Air and Soil Pollution 140: 335-341 Brighton & Hove (2004) Sustainability Strategy – Waste http://www.brightonhove.gov.uk/downloads/bhcc/sustainability/waste2004-06.pdf Browne, P. (2005) Food Waste Disposers as a means of waste diversion from landfill. County Surveyors Society Waste Committee and unpublished data Bünger, J.; Schappler-Scheele, B.; Hilgers, R. and Hallier, E. (2007) A 5-year follow-up study on respiratory disorders and lung function in workers exposed to organic dust from composting plants. Int. Arch. Occup. Environ. Health 80:306–312 CEC (1986) Council Directive of 12 June 1986 on the protection of the environment, and in particular of the soil, when sewage sludge is used in agriculture (86/278/EEC). Official Journal of the European Communities, No L181/6 12. CEC (1999) Directive on the landfill of waste. (1999/31/EC) Council Directive. Journal of the European Communities 16.7.1999 No L 182/1 CEC (2002) Regulation (EC) No 1774/2002 of the European Parliament and of the Council of 3 October 2002 laying down health rules concerning animal by-products not intended for human consumption. Official Journal of the European Communities L 273/1 10.10.2002 CIWEM (2003) Policy Position Statement (PPS) Food Waste Disposers February 2003 Commoner, Barry (1971) The closing circle; nature, man, and technology. Knopf, New York. DCLG (2007) Best Value Performance Indicators: 2005/06 http://www.communities.gov.uk/pub/119/BestValuePerformanceIndicators200506GuidanceDocum entAmended010405PDF6386Kb_id1136119.pdf (accessed 23 April 2007) Defra (2005) The Landfill Allowance Trading Scheme (LATS): Monitoring the scheme. http://www.defra.gov.uk/environment/waste/localauth/lats/pdf/latsfaq-07.pdf (accessed 10 April 2007) Evans, T.D. (2003) Independent review of retrofitting Cambi to MAD. Water Environment Federation 17th Annual Residuals & Biosolids Conference, 19-22 February 2003, Baltimore Evans, T.D. (2004) Layman’s guide to the use of sludge in agriculture. (unpublished) European Commission, Brussels Evans, T.D., Jepsen, S.-E., Panter, K. P. (2002) A survey of anaerobic digestion in Denmark. 7th CIWEM AquaEnviro European Biosolids & Organic Residuals Conference, 18-20 November 2002 Evans, T. and Rainbow, A. (1998). Wastewater biosolids to garden centre products via composting. Acta Horticulturae no 469, 157-168. Gendebien, A. Carlton-Smith, C. Izzo, M. Hall, J.E. (1999) UK Sewage sludge survey 1996/97 TR P165, WRc, Medmenham, SL7 1FD, England H&W (2004) Managing waste for a brighter future. The Joint Municipal Waste Management Strategy for Herefordshire & Worcestershire 2004-2034. Hogg, D.; Barth, J.; Schleiss, K. and Favoino, E. (2007) Dealing with Food Waste in the UK. WRAP http://www.wrap.org.uk/downloads/Dealing_with_Food_Waste_-_Final__2_March_07.667fd840.pdf (accessed 29 March 2007) IPCC (2001) Intergovernmental Panel on Climate Change - Climate Change 2001: The Scientific Basis. Cambridge University Press, UK. http://www.grida.no/climate/ipcc_tar/wg1/index.htm
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Jones, P. H. (1990) Kitchen garbage grinders (KGGs/food waste disposers) the effect on sewerage systems and refuse handling. Institute for Environmental Studies, University of Toronto. Kalberg, Tina & Norin, Erik, VBB VIAK AB. (1999) Köksavfallskvarnar – effekter på avloppsreningsverk, En studie från Surahammar. VA-FORSK RAPPORT 1999-9. Kegebein, J.; Hoffmann, E. and Hahn, H.H. (2001) Co-Transport and Co-Reuse, An Alternative to Separate Bio-Waste Collection? Wasser. Abwasser 142, 429-434 Ketzenberger, B.A. (1995) Effect of ground food wastes on the rates of scum and sludge accumulation, University of Wisconsin-Madison. Koning, J. de and Graaf, J.H.J.M. van der (1996) Kitchen food waste disposers, effects on sewer system and wastewater treatment. Technical University Delft. letsrecycle.com (2007) Landfill Allowances. http://www.letsrecycle.com/legislation/landfillallowances.jsp (accessed 23 April 2007) Matheson, C. (2005) Case study: Setting up community composting and recycling projects in Hackney. LARAC Conference ‘20:20 Vision – Driving recycling through innovation’. Ministry of Land, Infrastructure and Transport (2005) Summary of Report on Social Experiment of Garbage Grinder Introduction. Compiled by National Institute for Land and Infrastructure Management Ministry of Land, Infrastructure and Transport, Japan. March 2005 Translated by Gitter, M. (2005) Monteith, H.D.; Sahely, H.R.; MacLean, H.L. and Bagley, D.M. (2005) A rational procedure for estimation of greenhouse-gas emissions from municipal wastewater treatment plants. Water Environment Research 77, 390-403 National Audit Office (2006) Reducing the reliance on landfill in England. The Stationery Office, London National Pest Technicians Association (2007) National Rodent Survey Report 2006 http://www.npta.org.uk/ New York City DEP (1999) The impact of food waste disposers in combined sewer areas of New York City. New York City Department of Environmental Protection New York State (2007) Septic system maintenance - septic tank pumping table shows when to clean the septic tank. http://www.inspect-ny.com/septic/tankpump.htm (accessed 06/02/2007) Nilsson, P.; Lilja, G.; Hallin, P.-O.; Petersson, B. A.; Johansson, J.; Pettersson, J.; Karlen, L. (1990) Waste management at the source utilizing food waste disposers in the home; a case study in the town of Staffanstorp. Dept. Environmental Engineering, University of Lund. Parfitt, J. (2002) Analysis of household waste composition and factors driving waste increases. http://www.cabinetoffice.gov.uk/strategy/downloads/su/waste/downloads/composition.pdf (accessed 27 March 2007) Rosenwinkel, K.-H. and Wendler, D. (2001) Influences of food waste disposers on sewerage system, waste water treatment and sludge digestion. Proc. 8th Int’l Waste Management & Landfill Symp. CISA Env. Sanitary Eng. Centre, Sardinia, Italy. Smith, A.; Brown, K.; Ogilvie, S.; Rushton K. and Bates, J. (2001) Waste Management Options and Climate Change: Final Report. Office for Official Publications of the European Communities, Luxembourg Strategy Unit (2002) Waste not, Want not - A strategy for tackling the waste problem in England. http://www.number-10.gov.uk/su/waste/report/00-pdf.html Thackray, J.E.; Cocker, V.; Archbald, G. (1978) The Malvern and Mansfield studies of domestic water usage. Proc. Inst. Civ. Eng. (1978) 37-61 and discussion 483-502 The Independent (2006) Your carbon footprint revealed. reporting research by The Carbon Trust and University of Surrey. 9th December 2006 WRAP (2007) Research Summary: Understanding Food Waste. http://www.wrap.org.uk/document.rm?id=3659 (accessed 10 April 2007) Wouters, I.M., Douwes, J., Doekes, G., Thorne, P.S., Brunekreef, B. and Heederik, D.J. (2000) Increased levels of markers of microbial exposure in homes with indoor storage of organic household waste. Appl. Environ. Microbiol. 66, 627-31
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Appendix A Acronyms and Abbreviations ABPR
Animal By Products Regulations
AD
anaerobic digestion
AWC
alternate weekly collection
BNR
biological nutrient removal
BOD7
biological oxygen demand measured with 7 days incubation
CHP
combined heat and power
CO2e
carbon dioxide equivalent over 100 years
COD
chemical oxygen demand
Defra
Department of Environment Food and Rural Affairs
DS
dry solids (drying at 105 °C)
EA
Environment Agency of England and Wales
EfW
energy from waste
FOG
fat, oil and grease
FWD
food waste disposer
GWP
hhd
global warming potential Herefordshire Council and Worcestershire County Council; also Herefordshire and Worcestershire geographic area household
HHW
household waste
HWS
Household Waste Sites
IPCC
Intergovernmental Panel on Climate Change
KFW
kitchen food waste
kWhe
kilowatt hour of electricity
LFD
landfill directive
MAD
mesophilic anaerobic digestion
MSW
municipal solid waste
NPTA
National Pest Technicians Association
OFMSW
organic fraction of municipal solid waste
Ofwat
Water Services Regulation Authority
RCV
refuse collection vehicle
ROC
Renewables Obligation Certificate
RTS
refuse transfer station
RTV
refuse transfer vehicle
TH
thermal hydrolysis
VFA
volatile fatty acids [fatty acids with a carbon chain of ≤6C atoms]
VS
volatile solids (loss on ignition at 550 C°)
WCA
Waste Collection Authority
WCC
Worcestershire County Council
WwTW
wastewater treatment works
H&W
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Appendix B H&W Waste statistics The data used in Appendix B are from the websites of the individual local authorities, H&W (2004) and 2005/06 BV84 data provided by Worcestershire County Council, Waste Management Services. WCC, WMS was unable to provide data about the average round-trip distances travelled by RCV or RTV and therefore assumptions have been made in Table 7 together with the rationale outlined in section 4.2.1 on conveyance of solid waste. Table 6 Waste and population statistics (2005/06 actual) Bromsgrove Malvern Hills Redditch Worcester City Wychavon Wyre Forest Herefordshire
BV84a 468.8 313.6 414.0 355.8 354.5 365.1 521.7
BV84b -14.15% 0.50% -0.27% -1.76% -7.36% -1.60% 1.42%
BV86 £71.19 £50.52 £50.54 £25.98 £48.96 £41.34 £44.69
Note: Herefs is a unitary authority and its BV84a includes w aste from HWS totalling 24606 tonnes BV84a
kg household w aste collected per head of population
BV84b
annual change in household w aste collected per person
BV86
cost of household w aste collection £/household
BV87
Cost of w aste disposal per tonne municipal w aste
population h'holds total kerbside t Bromsgrove 90,000 42,192 36,859 Malvern Hills 73,800 23,144 31,169 Redditch 79,200 32,789 33,159 Worcester City 93,500 33,267 40,677 Wychavon 115,000 40,768 48,437 Wyre Forest 97,800 35,707 41,758 Herefordshire 177,800 68,152 76,410 totals 727100 276,018 308,469 weighted average kerbside collection cost [from BV84a] BV84a weighted average kerbside collection cost weighted average kerbside collected weight from BV84a BV87 disposal cost per tonne (incl. tax) Worcestershire CC BV87 disposal cost per household Worcs CC total household waste (kerbside+HWS) Herefs total household waste (kerbside+HWS) H&W total household waste (kerbside+HWS) H&W average household waste KFW if 17.6% of total minimum KFW [BV84] kerbside collection cost minimum KFW [BV87] disposal cost combined minimum KFW collection and disposal cost
total £ £2,623,992 £1,574,658 £1,675,856 £1,056,788 £2,371,476 £1,726,276 £2,508,928 £13,537,974 £49.05 /t £43.89 /hhd 894.8 kg/hhd £60.56 /t £61.97 /hhd 291053 t 24606 t 315659 t 1023 kg/hhd 180.1 kg/hhd £7.72 /hhd £10.91 /hhd £18.63 /hhd
averages £62.19 /t £68.04 /t £51.11 /t £31.77 /t £58.17 /t £48.35 /t £36.81 /t
Note 1: the pro rata costs for collection and disposal are derived from combined collection; they would be significantly greater if there was separate collection and treatment
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Note 2: HHW comprises waste collected by the WCA + waste collected by the HWS + all waste collected from ‘Bring’ schemes: it excludes trade waste, fly tipped waste and soil & rubble. MSW comprises all of the components of HHW plus trade waste, fly tipped waste and soil & rubble.
The payload of RTV might have increased since Smith et al. (2001) because maximum permitted gross vehicle weights have increased but since the contribution of RTV is much less than RCV it was not thought worth changing this. The assumption for RCV is that the distance to the start of the collection round and the distance back to the RTS are the same; if collection rounds were approximately radial from the RTS, i.e. the RCV travelled empty a long distance to the start of the round and a short distance full back to the RTS, the CO2 per tonne waste would increase.
Table 7 Estimation of GWP associated with transporting KFW as solid waste (from Smith et al. 2001) payload tonnes
kgCO2/km
kgCO2/ km*t waste
round-trip km
kgCO2/ t waste
RCV
6.67
0.84
0.252
40
10.07
RTV
20
0.71
0.071
60
4.26
total
14.33
Note: vehicles run full 50% of the time
‘Household waste’ means7: − All waste collected by Waste Collection Authorities (WCAs) under Section 45(1) of the Environmental Protection Act 1990, plus − All waste arisings from Civic Amenity (CA) Sites established under Section 51(1)(b) of the Environmental Protection Act 1990, and − Waste collected by third parties for which collection or disposal recycling credits are paid under Section 52 of the Environmental Protection Act 1990.
‘Household waste’ includes waste from the following sources: − Waste collection rounds (including separate rounds for collection of recyclables); − Street cleansing and litter collection; 7
http://www.communities.gov.uk/pub/119/BestValuePerformanceIndicators200506GuidanceDocumentAmended010405PDF6386 Kb_id1136119.pdf
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− Bulky waste collections, where “bulky waste” is defined as o any article of waste which exceeds 25 kilograms in weight o any article of waste which does not fit, or cannot be fitted into: (a)
a receptacle for household waste provided in accordance with section 46 of the Environmental Protection Act 1990; or
(b)
where no such receptacle is provided, a cylindrical container 750 millimetres in diameter and 1 metre in length.
− Hazardous household waste collections; − Garden waste collections; − Drop-off/bring systems; − Park litter (but not grass cuttings, leaves, etc); − House clinical waste collections; − Any other household waste collected by the authority. Household waste does not include: − Incinerator residues; − Beach cleansing wastes (i.e. produced by the specific activity of cleaning up a beach); − Rubble (including soil associated with the rubble) ; − Home composted waste; − Clearance of fly-tipped wastes; − Vehicles (whether abandoned or not); − Re-used waste material; − Grass cuttings, leaves etc in parks
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Appendix C Biogas, electricity and GWP from AD of KFW
The GWP is calculated as the saving from other electricity generated using the EU-average electricity generation emission factor (cited by Smith et al., 2001) which is 0.45 kg CO2e /kWh (range coal = 0.95 to wind = 0.009 kg CO2e /kWh). Two alternative AD processes are considered, one with 70 °C for 1-hour pre-sanitisation and the other with thermal hydrolysis to sterilise and increase the digestibility of the feed.
Table 8 Estimation of GWP associated with AD of separately collected KFW8 Description feed
unit tonne
reject
%
70°C+AD 1
TH+AD 1
10%
0.1
feed dry solids
%DS
30%
0.3
feed volatile solids
%VS
85%
0.85
feed VS
tDS
0.2295
0.2295
feed ash (i.e. non-VS)
tDS
0.0405
0.0405
VS destruction
%
40%
60%
ash in digestate
tDS
0.0405
0.0405
VS in digestate
tDS
0.1377
0.0918
total digestate
tDS
0.1782
0.1323
cake DS
%DS
cake
tonnes
biogas yield /kg VS destroyed
m3
energy value of methane
MJ/Nm3
methane content of biogas
%
energy value of biogas
MJ/Nm
3
conversion MJ to kWh energy value of biogas
kWh/Nm
3
22%
34%
0.810
0.389
1.3
1.3
37.78
37.78
65%
0.65
24.557
24.557
0.2778
0.2778
6.8214
6.8214
biogas yield /t feed
Nm
3
119.34
179.01
methane yield /t feed
Nm3
77.6
116.4
Nm
3
11.934
29.835
net biogas for CHP
Nm
3
107.406
149.175
net energy /t feed
kWh
732.7
1017.6
biogas used for sanitisation or TH
electricity @ generating efficiency = 40%
kWh/t feed
income (incl ROCs) @ 9 p/kWh GWP at EU average 0.45 kg CO2e /kWh
£/t feed kg CO2e/t feed
8
293.1
407.0
£26.38 -131.9
£36.63 -183.2
This estimate is only for the anaerobic digestion step, i.e. it does not include collection and delivery to the AD plant or removal and recycling of the digestate.
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Appendix D Costs and GWP from Surahammar field measurements As discussed in section 4.3.2 the only field study in which there has been a sufficient proportion of the households connected to a single WwTW that have had FWD installed to be able to observe any significant effect at the WwTW was reported by Kalberg and Norin (1999). Base-line observations were made for 2 years before the trial when one-third of the connected properties volunteered to have FWD installed as an alternative to new ‘pay-by-weight’ solid waste charges. Surahammar WwTW has MAD and a significant increase in biogas production was measured (Figure 6). Kalberg and Norin did not attempt to measure the amount of KFW disposed via the FWD but using their data and some reasonable assumptions it is possible to back-calculate the amount of KFW; this is shown in Table 9. The backcalculated value is a similar order of magnitude as the weight of KFW calculated for households in Herefordshire and Worcestershire (Table 6). Furthermore, the estimate of additional biogas derived by Kegebein et al. (2001) is a similar order of magnitude to the field observation of Kalberg and Norin, as do the derived values for GWP.
Table 9 includes estimates of the additional costs that would be incurred by WwTWs that do not have AD and CHP though this does not apply to Severn Trent Water’s WwTWs. The figure of £65 /tDS for the additional cost of wastewater treatment is an assumption based on the ‘Trade Effluent’ charging schemes published by Severn Trent, Yorkshire, Thames and Anglian water companies. Since these charges are audited and approved by Ofwat as fair, it is probably reasonable to use them as a basis for this exercise. Even the most expensive is less than half the saving to the MSW route that would result from KFW diversion and the least expensive is only 4% of the cost of the MSW-landfill route.
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Table 9 Additional cost of wastewater treatment resulting from FWD based on the field measurements of Kalberg and Norin (1999) Number of FWD installed at Surahammar
number of units
extra biogas measured at Surahammar
m3/d 3
1100 70
∴ extra biogas ∴extra biogas
m /y
25550
3
23.23
assumed gas yield from VS destroyed
m3/kgVS destroyed
∴VS destroyed assumed VS destroyed
kg/FWD*y
∴ original VS assumed original VS % of total solids
kg/FWD*y
∴ original TS assume TS of KFW
kgTS KFW /FWD*y
m /FWD*y
% %VS
∴ KFW (fresh weight) per household ∴ non-VS (i.e. ash)
17.87 60% 29.78 80% 37.22
%TS
30%
kg/y
124.1
kg/FWD*y
∴ VS in digestate kg/FWD*y ∴ yield of digestate
1.3
kgVS/FWD*y kgDS/FWD*y
∴ content of VS in digestate assume digestate cake DS
%VS
∴ yield of cake assumed recycling cost
kg cake/FWD*y
∴ digestate recycling cost assume cost of wastewater treatment
£ /FWD*y
∴additional cost for wastewater treatment electricity generated calculated from biogas produced GWP calculated from EU average for electricity generation ∴ GWP calculated to KFW
£/FWD*y
%DS £ /t cake £/tDS received
7.44 11.91 19.36 61.5% 24% 80.65 £15.00 £1.21 £65.00 £2.42
kWh/FWD*y kgCO2e/FWD*y
32.76 -14.74
kgCO2e/t KFW
-118.80
assume electricity value with ROC
£/kWh
£0.09
∴ electricity value with ROC ∴ net additional cost to a WwTW with AD+CHP
£/FWD*y £/FWD*y
£2.95 £0.68
or net additional cost to a WwTW with AD but no CHP
£/FWD*y
£3.63
For a WwTW with lime stabilisation assume lime dose
% on DS
30%
assume cost of lime
£/t
∴ cost of lime stabilising extra sludge ∴ net extra cost for a WwTW using lime stabilisation (no AD)
£/FWD*y
£0.67
£/FWD*y
£5.42
assume cost of incineration (Yorkshire Water)
£/tDS
∴ extra cost of incineration (no AD) ∴ net additional cost Ww treatment + incineration
£/FWD*y £ /FWD*y
£8.38
cost of incineration at a WwTW with AD
£/FWD*y
£3.10
∴ net additional cost Ww treatment + AD + ROC + incineration £/FWD*y
£2.57
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£60
£160.00 £5.96
Environmental Impact Study of Food Waste Disposers for The County Surveyors’ Society, Herefordshire Council and Worcestershire County Council by
Dr Tim Evans BSc MS PhD CChem CEnv FCIWEM MRSC
Synopsis by J Howell-Thomas, Worcestershire County Council 1
Synopsis This study examines the financial and environmental impacts of food waste disposers (FWD) and finds that they provide a cost-effective, convenient and hygienic means of separating kitchen food waste (KFW) at source and diverting it from landfill. The study also finds that this route costs less and has a better carbon footprint than other routes. In terms of Best Value Performance Indicators, FWD reduce BV84 (kilograms of household waste collected per head of population), BV86 (cost of household waste collection per household) and BV87 (cost of waste disposal per tonne municipal waste). Herefordshire Council and Worcestershire County Council have been pioneering in promoting installation of FWD. FWD have the benefit of separating at source a difficult fraction of biodegradable waste and diverting it using existing infrastructure and without entailing any regulatory bureaucracy. KFW is ‘difficult’ because it is wet and increases the difficulty of sorting the ‘dry recyclable’ fraction of waste, also in hot weather it can become smelly. The net global warming potential1 (GWP) of separate collection and treatment of KFW by composting is -14 kgCO2e/tKFW. For households with FWD feeding to wastewater treatment works where sludge is treated by anaerobic digestion, the biogas is used as renewable energy and the biosolids are used on land, the GWP is better than -168 kgCO2e/tKFW2. This is the pathway for Severn Trent Water’s works in Herefordshire and Worcestershire and Welsh Water’s works in Herefordshire. In contrast, landfill is +743 kgCO2e/tKFW. The cost of collecting and disposing KFW via the solid waste route in Herefordshire and Worcestershire averages £18.63 per household*year and the quantity is 180 kgKFW per household*year (2005/06 actuals). This is the approximate annual saving for each installed FWD. By February 2007, 640 FWD had been installed under the Herefordshire and Worcestershire cashback scheme at 1
Global Warming Potential is expressed as carbon dioxide equivalent (CO2 e) over 100 years.
2
This figure is based on direct before and after measurements in a town where 30% of households had FWD installed.
Page 1 of 3
a total cost of ÂŁ39,650, i.e. ÂŁ62 per FWD, which represents a payback period of only 3 years and 4 months. The ground KFW is transferred to the wastewater collection and treatment system and therefore adds to the costs of the water company. Water companies are understandably concerned about changes that might adversely affect demands on water resources or that would increase sewer blockages; field trials in several countries have shown that FWD do not affect water usage or accumulation in sewers significantly. Wastewater treatment works (WwTW) are designed to treat biodegradable material suspended in water, i.e. similar to the output of FWD. Ground KFW has been found actually to improve the composition of wastewater for the advanced nutrient removal processes that are now being demanded of WwTW. The additional cost for water companies depends on the route for treating and using or disposing the sewage sludge; for the route most usual in Herefordshire and Worcestershire it would be about ÂŁ0.68 per household*year, this is only 4% of the cost of the MSW-landfill route. Overall, FWD appear to be a very cost effective means of separating putrescible kitchen waste at source and diverting it from landfill. The carbon footprint of FWD feeding to a WwTW with anaerobic digestion (AD) and electricity generation (CHP)3 is competitive with separate collection of KFW delivering to centralised AD with CHP and significantly better than centralised composting. They are convenient and hygienic for householders but do not discourage home composting. Home composting is ideal for kitchen and garden waste but some householders are unable or are not inclined to practise it. FWD avoid the problems of odour and vermin that can be associated with separate collection via the solid waste route. Herefordshire Council and Worcestershire County Council (H&W) have been in the vanguard of exploring the potential of FWD as an alternative for people who do not wish to home compost, collect and store kitchen food waste (KFW), etc. Field studies have shown that use of FWD has a negligible effect on water consumption, that the ground KFW is conveyed in sewers at normal flow velocities and that in practice there is no increase in accumulation in sewers, that only about 3 kWhe/household*year is used by FWD but that the KFW generates at least 33 kWhe/household*year electricity from biogas at wastewater treatment works (WwTW) that have anaerobic digestion, which is the most prevalent type of sludge treatment in the UK. Field studies have confirmed that FWD do not influence sewer blockage neither are the particles large enough to block the screens at CSOs (combined sewer overflows). When sewage sludge is used on land (which is the route for the majority
3
This is the route in H&W
Page 2 of 3
in the UK), the organic matter in KFW is conserved and the nutrient cycles are completed. FWD increase the amount of biosolids produced at a WwTW but the extra cost of wastewater treatment and of treating it by AD with biogas CHP and recycling the biosolids to agriculture is less than one-tenth of the amount saved by H&W for the solid waste route. Historically WwTW were required to remove suspended solids, biological oxygen demand (BOD) and ammonia from the water. Suspended solids are collected, together with surplus biomass from removing the BOD, as sewage sludge and treated. The ammonia is converted to nitrate. Many WwTWs are now required to remove nitrogen (nitrate as well as ammonia) and phosphorus in addition to solids and BOD. The preferred treatment is ‘biological nutrient removal’ (BNR) but the wastewater at many WwTW does not have sufficient carbon to sustain the biomass needed for BNR and WwTW have to purchase additional carbon (e.g. methanol) and chemical dosing (commonly iron). FWD assist BNR by adding carbon. This study found that FWD provide a convenient and hygienic means for householders to separate KFW at source; they divert it from municipal solid waste landfill. Importantly, FWD do this using existing infrastructure and, by taking wet putrescible matter out of the solid waste stream, they make management of the dry fractions easier and less expensive and avoid odour issues, which have proved so detrimental to public acceptance of alternate weekly waste collections. There is no reason that FWD should discourage home composting since FWD are not designed to take garden waste and indeed exclusion of cooked KFW from home composting might encourage home composting.
2
Publication The full Environmental Impact study is available online free of charge from Worcestershire County Council at www.sinkyourwaste.com.
3
Acknowledgements The County Surveyors’ Society contributed some funding to this work. It was managed by Jeremy Howell-Thomas, Project Development Officer, Waste Management Services, Worcestershire County Council JHowellThomas@worcestershire.gov.uk. . Page 3 of 3