Fort Resolution Wastewater Management Study

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FORT RESOLUTION WASTEWATER MANAGEMENT STUDY Tricia Hamilton, AECOM Ken Johnson, AECOM ABSTRACT In response to concerns expressed by community members, community stakeholders, and regulatory authorities, the GNWT retained AECOM, to complete a wastewater management planning study in Fort Resolution. The concerns with the wastewater management were the capacity and environmental impact of the existing system, as well as the requirements and cost for improving and replacing the existing system.

The existing sewage lagoon system is a facultative/infiltration process consisting of six cells linked by channels. Wastewater ultimately percolates through the sandy soil in a northerly direction to the wetlands approximately 400 metres from site. Detention in the lagoon, and the infiltration process through the sandy soil provides treatment of the wastewater.

The sewage lagoon is approaching the end of its service life with its current configuration, as indicated by decreasing available freeboard. To meet the 20-year wastewater generation demand, a new lagoon should be constructed using the same facultative/infiltration process as the current lagoon. The volume of the new lagoon should be approximately 18,000 m³, with a depth of 3 metres. The cost of the new facultative/infiltration lagoon is estimated to be approximately $900,000.

COMMUNITY INFORMATION The community of Fort Resolution is geographically situated on Resolution Bay and immediately south of the Slave River delta. It is located at 61° 10' 16" N latitude and 113° 40' 20" W longitude, approximately 145 km southeast of Yellowknife. Fort Resolution is accessible by road, and is approximately 160 km by road from Hay River.

The community is located approximately 160 metres above mean sea level and 4 metres above the Great Slave Lake. The land to the east and south of Fort Resolution has a gentle slope towards Great Slave Lake.

The surficial soils are mainly deltaic, however, to the north of the community this pattern is interrupted by several massive outcrops of limestone bedrock. Fort Resolution is within the southern margin of the discontinuous permafrost zone, and as such, shallow permafrost is expected in undeveloped, forest-shaded areas. The climate in Fort

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Figure 1. Fort Resolution Area Resolution may be characterized by long cold winters and short cool summers. The daily average temperature is -2.9 C. The July mean high is 21.1°C and the mean low is 10.6 C. The January mean high is -18.4 C and mean low is -27.6 C. Fort Resolution currently uses a waste management site (sewage and solid waste) located approximately 1.5 km north of the community centre. This site is located approximately 0.9 km east of the Fort Resolution airport, and this present-day site was first put in use in approximately 1979.

Raw water is pumped from Great Slave Lake through a submerged intake line into a wet well beneath the truck fill station; the truck fill station is located on the northwest edge of the community. The raw water is treated with a package water treatment plant before discharge into the water trucks which delivers approximately 80 m³/day of potable water to the community.

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Surface water runoff south of the airport ridge will likely discharge into Resolution Bay, and north of the airport ridge will discharge into Nagle Bay. Both bays ultimately discharge into Great Slave Lake.

A subsurface hydrogeological drilling investigation was conducted in the vicinity of the current waste site in 1992. Piezometers were installed to monitor and sample groundwater, and the water depth was found to be 1 to 2 metres below the ground surface. The soil characteristics were reported to be fine-grained sand for the first 3 metres below grade, sand and with silt layers to from 3 to 6 metres, and clay with sand and silt below 6 metres. The results of this hydrogeological investigation concluded that the groundwater flow gradient is toward the wetland located some 500 metres north of the waste site. The report also concluded that effluent flow from the lagoon into the groundwater system should take approximately 12 years to reach the wetland north of the waste site.

SEWAGE INFRASTRUCTURE The sewage collection in Fort Resolution is contracted out with an annual contract value of approximately $150,000. Sewage is collected from 225 buildings around Fort Resolution and trucked 1.5 kilometres to the current lagoon site. The sewage is then emptied into the laoon at the truck dump on the north side of the lagoon. Approximately 12 to 15 trucks with 9,100 litres of sewage are collected in the community each day, five days a week.

Fort Resolution's sewage lagoon operates as a facultative and infiltration (soil absorption) lagoon. In the warmer seasons, while retained in the facultative lagoon, sewage will undergo biodegradation by bacteria, algae and plants. Sun and wind-mixed oxygen near the surface of the lagoon permit photosynthesis and aerobic (oxygen-consuming) reactions. Anaerobic (oxygen-deficient) degradation can also occur in deeper areas of the lagoon.

Due to the porous, sandy soils around Fort Resolution's lagoon, sewage flows down into the soil matrix and may percolate through the unsaturated soil layer into the saturated soil (groundwater). The wastewater will then enter the groundwater flow. As sewage infiltrates through the soil, treatment occurs.

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Figure 2: Infiltration Lagoon The unsaturated zone is the layer of soil between the ground surface and the water table.

This zone has efficient treatment capabilities through filtration, biodegradation, absorption and adsorption; these processes will decrease coliform bacteria, biodegradable material, nitrogen and phosphorous. Studies suggest that the ideal unsaturated zone is between 0.9 to 1.2 metres of soil. However, a significant reduction of coliform bacteria has been measured in approximately 30 centimetres of unsaturated soil.

The saturated zone is the wet soil below the groundwater level. The saturated zone also facilitates treatment through filtration, biodegradation, absorption and adsorption, although not as efficiently as the unsaturated zone. In addition, denitrification may also occur, where organic carbon is available.

The groundwater table at the Fort Resolution site is very high; therefore the available unsaturated soil depth is limited. However, the saturated soil zone is extensive, both below the lagoon site, and to the north of the lagoon site toward the wetland area.

Ultimately, the saturated flow discharges to a wetland system approximately 400 metres north of the sewage lagoon over a time period of approximately twelve years. This wetland system ultimately discharges into Nagle Bay, and from there into Great Slave Lake.

Fort Resolution's sewage lagoon consists of a series of six cells. The first cell (the most northerly) was excavated in approximately 1979, and additional cells were sequentially excavated to the south during the history of the site usage (1979 – 1981, 1985 – present), whenever the lagoon appeared to be reaching capacity or when fill material was needed.

The cells were constructed using an excavator; after a cell was constructed, a channel was excavated to connect the cell to the rest of the lagoon system.

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Figure 3. Fort Resolution Sewage Lagoon Configuration Based upon the population projection outlined, the generation of sewage waste is estimated currently to be 77 m³ per day and in 20 years the generation rate is estimate to be 84 m³ per day.

Wastewater generated in Fort Resolution is primarily domestic in source and characteristics. The wastewater quality from the community may be considered to be a "high strength" waste because of the use of a trucked sewage and water system. The "high strength" condition is typical for trucked sewage and water systems due to the low water usage, which results in a low dilution of the raw sewage.

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Table 1: Wastewater Characteristics of the Sewage Lagoon Fort Resolution Sewage Lagoon Parameter 5-Day Biochemical Oxygen Demand (BOD5) Total Suspended Solids E. Coli Total Coliforms Ammonia-N pH

Unit

Truck Dump (2008)

Cell 6 (2008)

mg/L

421

151

mg/L MPN/100 mL MPN/100 mL mg/L pH Units

224 1,600,000 >1,600,000 66 7.2

190 110,000 1,600,000 23.8 7.4

From the truck dump in Cell 1 to Cell 6, the sample analysis demonstrates a 93% reduction of E Coli; a 64% reduction in BOD5; a 64% reduction in ammonia; and a 15% reduction in suspended solids. The relatively low reduction in suspended solids is a result of the algae growth in the lagoon, which is responsible for the BOD5 and ammonia reductions.

These reductions in BOD5, suspended solids and coliforms demonstrate that the retention in the facultative lagoon contributes significantly to the overall wastewater treatment process.

A water sample was collected from standing water in the area north of the lagoon, prior to the wetland system, which may be representative of groundwater characteristics as the sewage lagoon effluent ultimately discharges into the wetlands. The laboratory analysis for this sample is presented in as "Surface Water Near Wetland".

The BOD5 and nitrogen (in the form of ammonia) concentrations for the truck dump and for Cell 6 are included in Figure 4 for comparison purposes, and are representative of the wastewater quality before the effluent infiltrates into the soil. The BOD5 and ammonianitrogen values may be indicative of the potential impact of the sewage lagoon on the groundwater system as the groundwater moves toward the wetland.

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450

421

Concentration (mg/L)

400 350 300 BOD5

250

Ammonia Nitrogen

200 151 150 100

66

50

24

36

19

4

0

0 Truck Dump (2008)

Lagoon (Cell 6) (2008)

Groundwater Average (1992)

Surface Water Near Wetland (2008)

Sample Location

Figure 4: BOD5 and Nitrogen in Ammonia Concentrations These results indicate a significant overall nitrogen removal from the lagoon system through the groundwater system, as well as significant removal from the groundwater average around the lagoon to the surface discharge near the wetland. The results also indicate a significant reduction in the BOD5 value from the groundwater average concentration to the surface water concentration, demonstrating that significant degradation occurs as wastewater travels down gradient with the groundwater.

It should be noted that these observations are based upon a very limited number of samples, two samples taken from the lagoon in 2008, an average of groundwater samples in 1992, and one sample of standing water (representative of groundwater conditions discharging into the wetlands) in 2008.

The existing sewage lagoon configuration is nearing the end of its operating capacity.

From site observations of freeboard (distance from the water level to the top of the lagoon pond) at the truck fill station on June, 2008, there was approximately 0.55 metres of freeboard remaining in the lagoon.

LAGOON DEVELOPMENT Fort Resolution’s current sewage lagoon is nearing the end of its capacity, and Fort Resolution requires a new sewage lagoon. While desludging the current lagoon would remove some biosolids, it would not effectively remove the interface of sludge

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preventing sewage from infiltrating into the ground. Excavating a new lagoon cell in the existing system will extend the life span of the current lagoon, but after that time infiltration rates will once again slow and the community will once again have to address this problem.

Fort Resolution’s sewage lagoon was excavated with limited engineering consideration.

Cells have been excavated when the lagoon nears capacity or when fill material is required. This is a problematic method of operating a sewage lagoon facility.

Facultative / Infiltration Lagoon The efficiency of an infiltration sewage lagoon decreases over time due to sludge buildup. Solids settle in the bottom of the lagoon, forming a low permeability barrier that reduces the available surface area for wastewater to infiltrate into the surrounding soil.

The sludge should be periodically removed from the lagoon ('desludged') to allow better infiltration of the lagoon.

Figure 5: Two-Celled Infiltration Lagoon With Cobble Berm One method of reducing the impact of sludge build-up in a lagoon is to have a 'settling cell'. This is a lagoon system consisting of two engineered lagoon cells, where the smaller, first cell provides retention time for solids in the sewage to settle before flowing to the second cell, where infiltration occurs.

Due to the cold northern climate, having the cells be connected with a pipe or channel is problematic. A cobble berm separating two cell will reduce the risk of freezing blockage over winter. The top of the cobble berm will be below the outside perimeter, to allow sewage to travel over the cobble berm into the second cell should the permeability of the cobbles be reduced. This design will lessen the impact of sludge build-up on infiltration rates, and desludging may only be required in the first cell.

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A geotechnical and engineering investigation is required to determine the depth to groundwater and verify hydraulic loading rates. To allow for wastewater to percolate through the unsaturated soil matrix, the lagoon should not intersect the groundwater table.

The sewage lagoon should be at least 30 centimetres above the groundwater table. Thus, the lagoon may have to be extended above ground through use of berms.

Lagoon Sizing As infiltration rates are expected to significantly decrease during colder months, the lagoon must be sized to be able to retain the community’s wastewater production for seven months from October until April. Wastewater infiltration is expected to primarily occur in the warmer months from May until September, the projected 2027 sewage production of 30,702 m³ (2027 annual generation rate) must have the capacity to infiltrate into the soil over five months.

Thus, the lagoon must have a capacity of at least 17,910 m³ to be sized for 20 years (2027). For aerobic treatment of sewage while retained in the lagoon, the lagoon should be no more than 3 metres deep. As such, to provide a volume of 17,910 m³, an area of at least 5,572 m² is required.

The hydraulic loading rates of the soil at the Fort Resolution waste site was determined to be in the range from 1.7x10-5 cm/s (0.0147 m³/m²day) to 4.6x10-3 cm/s (3.97 m³/m²day). The average loading rate of 6.7 x 10 4 cm/s (0.58 m³/m²day) is used for this calculation.

Due to higher solids content, wastewater loading rates are estimated to be between 10%15% of hydraulic loading rates. An average of 12.5% of the hydraulic loading rate was assumed for the wastewater loading rate for Fort Resolution. Therefore, the wastewater loading rate is estimated to be 8.38 x 10-5 cm/s (0.0725 m³/m²day).

The annual loading rate is calculated by multiplying the wastewater loading rates by the number of days infiltration is anticipated to occur (May until September). Thus the annual loading rate is predicted to be 11.03 m³/m² year. Given that the annual sewage production in 2027 is predicted to be 30,702 m³, to provide sufficient infiltration capacity the lagoon must be at least 2,783 m².

A 3 metre deep infiltration lagoon with a volume of 17,910 m³ and an area of 5,970 m² should provide sufficient volume and area for cold weather retention and warm weather infiltration of Fort Resolution's wastewater.

If the settling cell is 10% the volume of the infiltration cell, the settling cell should have a volume of 1,791 m³ and an area of 597 m².

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Thus, the size recommended is a 3 metre deep lagoon with a volume of 17,910 m³ and an area of 5,970 m². The actual length and width of the lagoon can be selected to best account for site conditions and tree clearing requirements.

CONCLUSIONS AND RECOMMENDATIONS Fort Resolution's sewage lagoon, composed of six cells excavated as required, is nearing capacity with very little freeboard. The current site has successfully operated for approximately 25 years and there is plenty of available land space in the area. A redevelopment of the current site and improved operation and maintenance practices may address community concerns regarding the site location.

Water samples were obtained during the course of this study, and it was observed that while retained in the lagoon there was a significant reduction of E Coli, BOD5 and ammonia concentrations. Groundwater and surface water samples suggest a significant reduction in nitrogen and BOD5 concentrations attributed to the infiltration process.

Wastewater from the lagoon experiences aerobic treatment while retained in the lagoon, infiltrates the sandy soil, where treatment occurs in the soil matrix, and enters the groundwater flow to the wetlands approximately 400 metres north. During retention time in the wetlands, further treatment occurs due to natural biological activity and settling.

As the soil in Fort Resolution is porous sand historically capable of handling the loading of the community's sewage which discharges with the groundwater into a wetland, a facultative/infiltration lagoon is recommended. A facultative/infiltration lagoon is also less expensive ($0.90 M) than a retention lagoon ($1.7 M).