PERFORMANCE AND POTENTIAL IMPROVEMENTS TO ANAEROBIC SEWAGE LAGOON IN FORT MCPHERSON, NT
Ken Johnson, Earth Tech Canada, Edmonton email: ken.johnson@earthtech.ca John Bulmer, Department of Public Works and Services, Government of the Northwest Territories, Inuvik email: John_Bulmer@gov.nt.ca Ron Rusnell, Department of Municipal and Community Affairs, Government of the Northwest Territories, Inuvik email: Ron_Rusnell@gov.nt.ca Michel Lanteigne, Earth Tech Canada, Yellowknife email: michel.lanteigne@earthtech.ca
ABSTRACT The Hamlet of Fort McPherson is a Gwich’in community located at 67o 27’ N and 134o 53’ W in the Northwest Territories. The main residential sanitary sewage system of the community consists of a trucked pickup, and a lagoon treatment system. Effluent from the lagoon is discharged once or twice a year, and enters a wetland, and stream system that ultimately discharges into the Peel River. The area of the lagoon is approximately 1.81 hectares, and the estimated volume is 100,000 cubic metres. The performance of the sewage lagoon displays the characteristics of an anaerobic lagoon. The effluent suspended solids are in the range of 51 to 150 mg/L, and the effluent BOD5 is in the range of 17 to 70 mg/L. The effluent ammonia concentration is in the range of 11 to 34 mg/L. The sewage lagoon has sufficient hydraulic capacity for the next 20 years, and the effluent quality is well below the existing water licence criteria. However, concerns have been raised by regulatory agencies with the toxicity of the ammonia concentrations in the effluent. The “anaerobic” nature of the lagoon may not easily facilitate nitrification without some mechanical process addition. An overview of the downstream wetland areas, and the quality of the discharge into the Peel River suggests that the wetland areas may have capacity for treating the ammonia in the seasonal lagoon discharge. With the support of additional biophysical studies, the sewage treatment “system” for the lagoon discharge could be expanded in the future to include the downstream wetland areas.
KEY WORDS: sewage treatment, cold regions, anaerobic lagoon, wetland discharge
INTRODUCTION Background The Hamlet of Fort McPherson is a Gwich’in community located at 67o 27’ N and 134o 53’ W in the Inuvik Region of the Northwest Territories. The community is located on a high point of land on the east bank of the Peel River about 38 kilometres upstream from its confluence with the Mackenzie River. The level hill where the settlement is located is about 1.9 kilometres long by 0.8 kilometres wide, and is 15 to 21 metres above the summer water level of the Peel Channel. The townsite is 1100 air kilometres northwest of Yellowknife, and the Dempster Highway connects Fort McPherson to Inuvik, 220 kilometres to the northeast, and Whitehorse, Yukon 1200 kilometres to the southwest. Fort McPherson experiences an average of 260 days with frost per year. The mean daily temperature in January is –29 °C, and in July, the warmest month, the mean daily temperature is 15 °C. About 115 mm of rain falls each year, and the the mean annual snowfall is 222 cm (Government of the Northwest Territories, 1982). The sanitary sewage treatment and disposal system for the Hamlet of Fort McPherson is comprised of the trucked sewage system (approximately 85 % of effluent) and a piped sewage system (approximately 15 % of effluent). The trucked sewage system consists of a lagoon constructed in an abandoned shale borrow pit approximately 4 kilometres northeast of the community centre (See Figure 1) (Reid Crowther & Partners Ltd., 1997). The piped sewage system consists of a lake discharge (“Sewage Lake”) immediately to the northwest of the developed community boundary. Effluent from both sewage systems enters a stream system that ultimately discharges into the Peel River, which is 2.5 kilometres downstream of the community centre (Earth Tech Canada, 2002). In 2002 the Hamlet used a total of 39,596 m3 of potable water; the estimated water use is 114 L/c/d, based upon an estimated population of 945 people in 2001 (Government of the Northwest Territories, 2003). The monthly and annual quantities of all wastewater discharged is not metered, but is estimated to equal the quantity of potable water. Approximately 33,300 m3 of wastewater (based upon average operating conditions) was trucked to the sewage lagoon, and the remainder of approximately 6,300 m3 flowed into Sewage Lake (Hamlet of Fort McPherson, 2003). Trucked Lagoon Operation The lagoon is discharged in either the early summer or the fall, depending upon the water level in the lagoon. The discharge timing depends upon when the water level rises within about 1 metres of the top of the lagoon. The lagoon has a limited operating capacity because of a fixed culvert discharge, which limits the operating level variation of the lagoon to about 1.5 metres or 25,000 cubic metres of retention volume.
The lagoon is discharged through the culvert, on the south side of the lagoon, into a 2 kilometre lake. The flow is controlled with a valve which is attached to the end of the culvert on the lagoon side of the culvert (See Figure 2) (Earth Tech Canada, 2003). The flow discharges from the lake approximately to 2 kilometres from the lagoon, and flows into a wetland area before entering a stream that flows another 2 kilometres before discharging into the Peel River (See Figure 1).
System Characteristics and Performance Characteristics of Trucked Lagoon The impoundment receiving the sewage is an abandoned gravel quarry, which is generally a rectangular shaped impoundment with an overall length of 190 metres and an overall width of 90 metres; one small area at the north end of the impoundment is approximately 130 metres wide (See Figure 2) (Earth Tech Canada, 2003). The depth of lagoon various significantly, with depths ranging from 5 to 7 metres below the discharge culvert invert elevation. The area of the lagoon is approximately 1.81 hectares, and the estimated volume of the lagoon is 100,000 m3 (Earth Tech Canada, 2003). The sewage treatment and disposal systems operate under the following water licence parameters: • • • • •
Effluent Faecal Coliforms Effluent BOD5 of seasonal discharge Effluent Suspended Solids of seasonal discharge Effluent pH of seasonal discharge Freeboard minimum in lagoon
106 CFU/100mL 120 mg/L 180 mg/L 6 to 9 1.0 metres
Anearobic Performance of Trucked Lagoon The effluent measurements over the past 7 years (See Figures 3 and 4) (Earth Tech Canada, 2003) demonstrate that the trucked sewage lagoon is well within the water licence parameters (Earth Tech Canada, 2003). The trucked sewage lagoon is a relatively deep (5 to 7 m) manmade impoundment, which operates as an anaerobic lagoon based upon its physical characteristics. The threshold depth for an aerobic pond is less than 1.5 metres, and the threshold depth for an anaerobic pond is greater than 2.5 metres (Metcalf and Eddy Inc., 1979). Very little performance data exists for lagoon systems in the far north. The best comprehensive compilation of performance data has been compiled for lagoons in northern Alberta. The performance characteristics of the Fort McPherson lagoon fall outside the performance characteristics for facultative lagoons in northern Alberta lagoons (less than 2.5 metres deep) (Smith, 1996). The measured range of effluent values for BOD5 (17 to 70 mg/L – 7 year range) in Fort McPherson is above the compiled information for 12 month storage lagoons in northern Alberta (12 to 25 mg/L). The
measured range of effluent values for suspended solids in Fort McPherson (51 to 177 mg/L – 7 year range) is above the compiled information for 12 month storage lagoons in northern Alberta (45 to 80 mg/L). The effluent ammonia concentration in the trucked sewage lagoon remains high, averaging 23 mg/L over 7 years, with a range of 11 to 34 mg/L. These high values suggest that the influent undergoes little or no nitrification. Seasonal Performance of Trucked Lagoon The trucked sewage lagoon is discharged in the early summer and late fall, depending upon the water level in the lagoon. If the lagoon water level rises above the 1 metre freeboard elevation during the winter storage operation, the lagoon is discharged in the spring. Figures 4 and 5 suggest that different effluent characteristics may be expected for spring (before June 30), and fall (after June 30) discharges. A fall discharge may produce higher suspended solids ( 4 year average of 109 mg/L, and a range 46 to 138 mg/l) than a spring discharge (4 year average of 71 mg/L, and a range 42 to 138 mg/L). A fall discharge may produce lower BOD5 (4 year average of 36 mg/L, and a range 22 to 40 mg/L) than spring (4 year average of 51 mg/L, and a range 29 to 88 mg/L). The higher seasonal values for suspended solids in the fall may be attributed to the more quiescent settling conditions in the winter and spring, and the algae growth in the summer months. The lower seasonal BOD5 in the fall may be attributed to the increased biological activity during the summer months. The performance difference for spring and fall discharge has also been documented for the Yellowknife sewage lagoon system. Significant improvements were noted in BOD5 (greater than 85 % removal) and fecal coliforms (less than 1000 CFU/100 mL) in the period following the middle of June where the ambient air temperature reached its warmest for the year (Soniassy, R.N. and Lemon, R., 1986). Characteristics of Peel River Discharge The discharge from both the trucked sewage lagoon (seasonal discharge) enters the continuously flowing small stream that discharges into the Peel River (See Figure 6). The stream near the Peel River has a very small summer discharge (September 2003 observation), but occupies a reasonably large channel with significant stream debris (Earth Tech Canada, 2003). The mean flow in the Peel River itself ranges from 750 to 2000 cubic metres per second during the period between May and September (Environment Canada, 1997). Water samples taken from the stream in early September, 2003 produced the following values (Earth Tech Canada, 2003): • •
Suspended solids average of less than 5 mg/L; BOD5 average of less than 4 mg/L;
• •
Fecal coliforms average of less than 10 CFU/100 mL; Ammonia average of 0.28 mg/L.
There is little visible evidence that this stream is a receiving a sewage discharge based upon the limited inspection completed in September, 2003. Although the stream receives a periodic discharge from the trucked sewage lagoon, it also receives a continuous discharge from the piped sewage lagoon (See Figure 1). A fish was also observed in the stream at the time of the inspection.
Waste Generation and System Capacities The current sewage generation is estimated to be 114 litres per capita per day based upon the most recent water licence reporting (Hamlet of Fort McPherson, 2003). Of this generation, approximately 85 percent (33,000 m3 in 2002) is trucked sewage, and 15 percent (6300 m3 in 2002) is piped sewage. It is anticipated that these proportions may remain reasonably constant because the servicing strategy is expected to maintain the majority of the community on trucked services, and just the community core on piped services. Table 1 presents a preliminary 20 year estimate of sewage generation based upon the GNWT population projections and the 2002 per capita estimate of sewage generation.
Table 1. Future Sewage Generation (Government of the Northwest Territories, 2003) Year
Population
2004 2009 2014 2019 2024 2029 2034 2039 2044
982 1,009 1,030 1,055 1076 1097 1119 1141 1164
Sewage Generation (m3) in Given Year (114 L/c/d or 41.9 m3/c/year) 41,150 42,280 43,157 44,204 45,084 45,964 46,886 47,807 48,771
The estimated volume of the lagoon is 100,000 m3, which provides enough capacity for 12 months of retention beyond the year 2044. It is assumed that 41,000 m3 of trucked sewage will be generated in 2044, which is 85 percent of the total 48,771 m3 estimated in Table 1.
System Improvements and Future Expansion The existing sewage trucked sewage lagoon has sufficient hydraulic capacity for the next 20 years, and beyond, and the effluent quality is well below the existing water licence criteria. The effluent quality could be expected to remain below the existing water licence criteria until the sludge accumulation begins to build up, and ultimately reduces the effluent quality. The addition of primary sewage cells, to reduce the sludge volume in the lagoon, is not a necessary design practice for trucked sewage systems in the far north. Northern retention lagoons are commonly designed with a sludge zone to provide a region for sludge accumulation. Concerns have been raised with the effluent toxicity from the ammonia concentrations in the lagoon influent. The general “anaerobic� nature of the lagoon may not easily facilitate nitrification without the additional of some form of mechanical equipment to the lagoon such as aeration, therefore high ammonia concentrations may be anticipated in the future. The addition of mechanical equipment to the lagoon system is not appropriate technology for the Fort McPherson lagoon because of the northern location of the community, and the distance from the community to the lagoon. The only source of electrical power is in the community itself, which is 4 kilometres away from the lagoon. In anticipation of future demands for effluent quality improvements in toxicity and other parameters, the sewage lagoon system could be expanded to incorporate the downstream water bodies, and wetland areas as part of the overall treatment system (See Figure 6) (Earth Tech Canada, 2003). A preliminary review of these areas (shallow lake area and wetland area), and the discharge quality of the stream into the Peel River suggest that these downstream areas are already providing some degree of treatment to the lagoon discharge. Seasonal wetlands in cold regions have very significant wastewater treatment capabilities. A wetland system in Repulse Bay reduced ammonia concentrations of 50 to 95 mg/L to less that 0.10 mg/L (Johnson, 1994). It would be reasonable to expect that the lake and wetland downstream of the Fort McPherson lagoon could achieve a significant reduction in ammonia concentration; a complete biophysical characterization of the lake and wetland system would be required to estimate the potential reduction. The discharge operation of the trucked sewage lagoon utilizes a culvert with a fixed elevation, which restricts the operating levels in the lagoon to about 1.5 metres (25,000 m3). Future discharge operation of the lagoon should utilize a pumping system in order to draw down the lagoon for a 12 month retention, and a fall discharge (after June 30th).
Conclusions and Recommendations The sewage treatment and disposal facilities for the Hamlet of Fort McPherson have sufficient hydraulic capacity for the next 20 years. In the operation of the lagoon, a fall discharge (after June 30th) may provide an overall higher quality effluent because of the additional retention (12 months in total) over the summer months. To achieve a 12 month retention period, the discharge control culvert may have to be abandoned in favour of a pumping system over the lagoon berm, which would accommodate a greater drawdown in the lagoon water level. The downstream lake and wetland to the lagoon may provide additional treatment of the lagoon effluent with regard to nitrification of the wastewater. Therefore, in responding to existing environmental concerns, and in anticipation of future environmental concerns, and more stringent regulatory guidelines, a biophysical study of the downstream lake and wetland should be undertaken in provide a basis to incorporate the downstream water bodies, and wetland areas as part of the overall treatment system.
References: Earth Tech Canada, Fort McPherson Waste Study - Draft Report. October, 2003. Prepared for the Department of Public Works and Services, Government of the Northwest Territories. October. Environment Canada. 1997. Canadian Hydrological Data, Station: 10MC002 RIVER ABOVE FORT MCPHERSON.
PEEL
Government of the Northwest Territories, Bureau of Statistics. 2003. Government of the Northwest Territories, Bureau of Statistics. December, 1982. Community Water & Sanitation Services. Inuvik Region, Northwest Territories. . Hamlet of Fort McPherson. April, 2003. 2002 Fort McPherson Annual Water Licence Report. Johnson, Kenneth, R. June, 1994. Preliminary Engineering of Sewage Disposal System in the Community of Repulse Bay. Proceedings of the Annual Conference of the Canadian Society for Civil Engineering. Metcalf and Eddy Inc. Wastewater Engineering. 1979. Reid Crowther & Partners Ltd. September, 1997. Operation and Maintenance Manual, Trucked Sewage and Solid Waste Disposal Facilities for Fort McPherson, NWT. Smith, D.W., Technical Editor. 1996. Cold Regions Utilities Monograph, Third Edition, American Society of Civil Engineers.
Soniassy, R.N. and Lemon, R. 1986. Lagoon Treatment of Municipal Sewage Effluent in a Subarctic Region of Canada (Yellowknife, NWT). Water Science Technology, Volume 18, PP 129-139.
Fort McPherson, NWT Sewage Treatment and Disposal Systems Figure 1
Mean Yearly Variation in Effluent Parameters 160 147 140 131 120
120
100 mg / L
95 88
87
80 70 60
57 51
40
20
37
40 35
31
17
0 1997
1998
1999
2000
2001
Year Total Suspended Solids
2002
2003
Figure 3 Biochemical Oxygen Demand
Mean Yearly Variation in Total Suspended Solids for Spring and Fall Discharges 140 131 120
120 106
100 96 87
84 mg / L
80
60 52 40
42
20
0 2000
2001
2002 Year
Fall Discharge
2003
Figure 4 Spring Discharge
Mean Yearly Variation in BOD5 for Spring and Fall Discharges 80
71
70
60 55
mg / L
50
49
40
40 37
30
35 31
29
20
10
0 2000
2001
2002 Year
Spring Discharge
2003
Figure 5 Fall Discharge