The 2006 Annual General Conference of the Canadian Society for Civil Engineering 2006 Congrès général annuel de la Société canadienne de génie civil
Calgary, Alberta, Canada May 23-26, 2006 / 23-26 Mai 2006
Application Of Large Scale At-Grade Sewage Treatment And Disposal In Fort Good Hope, NWT Kenneth R. Johnson1, Amir Agha2, and Mukesh Mathrani1 1 Earth Tech Canada Inc., Edmonton, Alberta, Canada 2 Department of Municipal and Community Affairs, GNWT, Norman Wells, NWT, Canada Abstract: The Charter Community of Fort Good Hope, NWT continues to use an exfiltration trench for disposal and treatment of domestic wastewater. This community of 550 people is located just south of the Arctic Circle, in the continuous permafrost region of the north. Sewage volumes are expected to increase with the community population over the next twenty years, and the existing exfiltration lagoon configuration will ultimately not have the capacity for the increasing volume. A study was completed to evaluate the capacity and efficiency of the existing system, and the opportunity to maintain, or improve this unusual application of at-grade sewage treatment and disposal. Based upon the available site information and the performance of the existing system, it was concluded that the process has the hydraulic and treatment capacity to meet the community’s demand and maintain regulatory compliance.
INTRODUCTION Community Environment
The Charter Community of Fort Good Hope (K'asho Got'ine) is a Dene community in the Sahtu Region of the Northwest Territories, located at 66º 15' N and 128º 3' W. The community lies on a peninsula at the confluence of Jackfish Creek, and the east bank of the Mackenzie River (See Figure 1). The town site is 27 kilometres south of the Arctic Circle, about 805 kilometres northwest by air from Yellowknife, and 145 kilometres northwest of Norman Wells. The terrain surrounding Fort Good Hope generally consists of muskeg, swamp, and areas covered with trees ranging in size from stunted growth up to 12 metres in height. However, several significant glacial and fluvial deposits surround the community, and provide one of the few nearby community deposits of concrete gravel in the NWT. Fort Good Hope is situated within the continuous permafrost zone; the active layer penetrates 0.5 to 1.2 metres below the ground surface in the summer. The community receives a total annual precipitation of 267 millimetres, with an average of 150 millimetres of rain and 132 centimetres of snow each year. Mean high and low temperatures vary between 22.6 and 9.9ºC in July and between -27.5 and -35.0ºC in January. Prevailing winds are from the east and average 9.5 km/h annually. Sewage collection in Fort Good Hope employs sewage pumpout tanks. Pumped out sewage is trucked to an exfiltration trench located at the waste disposal facility about 3.5 kilometres north of the community. The estimated monthly volume of pumpout sewage discharged to the exfiltration trench is 1.6 million litres, which is approximately 140 litres per capita per day (550 estimated population). The exfiltration trench is GC-###--1
approximately 98 metres long, 12 metre wide and up to 3 metres deep; the approximate working volume of the trench is 1500 cubic metres.
Figure 1. Fort Good Hope site plan. 1.2
Existing Sewage Exfiltration Area
The 9.0 hectare waste disposal site (240 metres wide by 375 metres long), and is part of a 90 hectare glacial outwash plain located between the townsite and the Hare Indian River. The average depth of the glacial outwash plain is approximately 10 metres, and the deposit contains approximately 9 million cubic metres of poorly graded gravel. The glacial out wash place is one of a series of granular deposits around Fort Good Hope. The deposit is comprised of medium grained gravel that ranges in soil classification from "poorly graded gravel, gravel-sand mix, little or no fines" to "poorly graded sand, gravelly sand, little or no fines". The gradation of the gravel in the area ranges between 20% and 95% sand, 2% and 7 9% gravel, and 1% to 14% silt-clay. The high permeability of the gravel results in good drainage within the area, and the water table in this area appears to be deeper than 15 metres, while the depth of the sewage trench is up to 3 metres deep. GC-###--2
The exfiltration trench does not have any "controlled" discharge system. Upon discharge into the trench, the sewage flows by gravity to fill the entire trench to a level surface. The sewage then "exfiltrates" from the bottom and sides of the trench into the glacial outwash plain deposit, and then "percolates" downward through the deposit (unsaturated flow) until the groundwater table, and then flows into the groundwater (saturated flow). The exfiltration trench in the glacial outwash plain is situated at an elevation of approximately 230 metres, which is about 225 metres above the Mackenzie River elevation of 75 metres (See Figure 2). The exfiltration trench is about 1200 metres from the Mackenzie River. The exfiltration trench and the outwash plain are both oriented parallel to the Mackenzie River.
SYSTEM OPERATION AND PERFORMANCE
The Water Licence issued to the Charter Community of Fort Good Hope by the Sahtu Land and Water Board refers to an appended "Surveillance Network Program" (SNP) when outlining the effluent quality standards required for compliance of the sewage trench seepage. However, the SNP information does not include any specific discharge criteria; historically, many northern communities have been required to meet the discharge parameters of effluent 120 mg/L for Biological Oxygen Demand (BOD5), and 180 mg/L for effluent Total Suspended Solids (TSS). The exfiltration trench treats the sewage in a much different way than the more conventional sewage retention lagoon. A retention lagoon uses the elements of nature at the earth's surface including heat, sunlight, wind and surface vegetation. An exfiltration trench uses the elements of nature in the available soil "matrix", and the processes of biodegradation, filtration, adsorption and absorption to remove the contaminants in sewage. The trench is currently capable of accommodating the volume of sewage produced by Fort Good Hope based upon the community observations; in fact, the sewage percolates quickly into the gravel. However, the community has observed that the level of liquid in the trench has been steadily increasing with time, which is an anticipated part of the performance of an exfiltration process. Sewage solids accumulate with time over the bottom of the trench, and reduce the permeability of the soil; at the same time, this reduction in permeability also reduces the flow through the soil, and enhances the processes removing the contaminants in the soil. Sampling was performed in the months of June to August 2001 on the seepage that is believed to originate from the sewage exfiltration trench. The samples were taken from a stream of water between the exfiltration trench and the Mackenzie River, with the assumption that the groundwater flows toward the Mackenzie River. The results of the sampling analyses indicated that the BOD5 ranged from less than 2 to 7 mg/L, and that the TSS ranged from less than 3 to 6 mg/L. This limited sample data indicates a very high quality of effluent treatment within the soil matrix beneath the exfiltration trench; this effluent quality may be equated to a tertiary level of treatment. In comparison the commonly used water effluent parameters in northern community water licenses, as discussed previously, the BOD5 measured at Fort Good Hope is well below the target of 120 mg/L and the TSS analyzed was well below 180 mg/L.
Figure 2. Fort Good Hope site plan and profile.
WASTE GENERATION AND SYSTEM CAPACITIES
The production of sewage in Fort Good Hope is expected to increase substantially in the next 20 years. Statistics from the Government of the Northwest Territories projects that aboriginal populations may increase at a rate of 0.5 % annually. Based on the estimated population of Fort Good Hope in the year 2003 of 550, and a water consumption volume of 140 litres (0.140 m3) per person per day (April 2004), an estimated 25,000 m3 of domestic sewage will be produced annually by the year 2015, and 32,000 m3 by the year 2025. The existing exfiltration trench has never overflowed, however, the community is concerned that the trench is filling up higher than it ever has before and could overflow in the near future. The ultimate capacity of the existing system is impossible to calculate given the many factors that influence the hydraulic capacity GC-###--4
of the trench. These factors include the granular material in the base and side of the trench, the solids accumulation in the base and sides of the trench, the influence of seasonal frost in reducing the soil permeability, and the influence of permafrost on the soil permeability. The best indication of system capacity are site observations on the rate of exfiltration from the trench; this activity is essentially a "percolation test" of the soil. From a design perspective, the general operational range or hydraulic loading for a "rapid infiltration" system such as this is 6 to 125 metres per year. The estimated loading rate of the Fort Good Hope exfiltration trench is about 20 metres per year based upon sewage generation of 50 m3 per day, and an estimated infiltration surface of 900 m2.
SOIL EXFILTRATION TREATMENT PROCESSES
Soil exfiltration of wastewater uses the elements of nature in the available soil "matrix", and the processes of biodegradation, filtration, adsorption and absorption to remove the contaminants in sewage. A soil matrix approximately 30 centimetres (12 inches) thick may adequately remove the contaminants in sewage, if it is appropriately engineered and operated. All soils have a natural capability to "filter" contaminants because of the inherent biology, and chemical activities that occur in soil. A soil exfiltration system may produce a tertiary quality effluent if engineered and operated properly. The anticipated effluent characteristics are presented in Table 1. Table 1. Anticipated Effluent Characteristics Effluent Parameter BOD5 TSS Total Nitrogen Total Phosphorous Fecal Coliforms
< 5 mg/L < 2 mg/L < 10 mg/L < 1 mg/L < 10 FC/100 mL
The wastewater contaminants that have been most widely studied for removal by a soil matrix are coliforms, biodegradable material (measured by BOD5), nitrogen and phosphorous. Coliforms and other pathogen organisms are removed by physical straining, and "die off" as a result of the harsh environment of the soil. This harsh environment includes the temperature, the absence of any nutrients for the coliforms, and the natural antibiotics in the soil. Biodegradable material is removed by the bacterial metabolism with the "living filter" of the soil â€“ the natural or introduced bacterial literally consume the biodegradable material as it flows through the soil. Nitrogen compounds, primarily in the form of ammonia, undergo a series of reactions with a soil profile resulting in the transformation, and potentially the complete removal of nitrogen from the soil or the storage of nitrogen in the soil. From a biochemical and chemical perspective, the nitrogen removal occurs as a result of nitrification, denitrification, volatization or chemodenitrification. Within a gravel soil profile the nitrogen transformation may be limited to nitrification and the formation of nitrates, which may occur to an extent of 80% within a 1 metre depth of soil. Phosphorous compounds are "retained" by soil through either a chemical reaction or an adsorption reaction. In the application of these "processes" in the natural environment, there is a recognition that the process occurs at different rates in "unsaturated" and "saturated" zones in the soil (See Figure 3). The saturated and unsaturated zones are defined by the position of the groundwater table â€“ the unsaturated zone is the region above the groundwater table and the saturated zone is in the region below the groundwater table. The efficiency and rate of the various biochemical and chemical processes is substantially higher in the unsaturated zone, and this fact is recognized in most regulations governing the use of soil for effluent disposal, where a minimum unsaturated depth of soil is required for complete treatment to occur. These GC-###--5
depths vary from as little as 30 centimetres for well graded sand to about 1 metre for other soil types. However, the saturated zone still has "treatment" capabilities that are significant. Temperature may have a significant influence on the biochemical and chemical processes within the soil; however, biochemical and chemical process still occur at cold temperatures, but at slower rates. In practical terms, slower rates demand either a lower sewage application on a given soil profile, or an increased soil profile to achieve the same level of treatment.
Figure 3. Soil exfiltration treatment processes. 5.
CONCLUSIONS AND RECOMMENDATIONS
The existing sewage exfiltration trench in Charter Community Fort Good Hope is an appropriate sewage treatment technology for this community based upon the technical process information, and the limited performance data. The process is capable of providing a very high quality sewage effluent before discharge into the receiving environment. A number of improvements may be made to the existing process, both in the construction and the operation and maintenance. The capital improvements include: 1. Constructing an erosion protected discharge into the trench to reduce the accumulation of rocks and sediment in the trench. 2. Constructing an engineered discharge structure beside the trench. GC-###--6
3. Constructing a perimeter fence to isolate the trench. 4. Constructing a water level monitoring post in the trench. The operation and maintenance improvements include: 1. Undertaking regular sampling of the representative discharge from the trench at the base of the granular deposit. 2. Undertaking regular monitoring of the water level in the trench. 3. Undertaking periodic “resting” of the trenches (summer only), where a second trench is needed to meet the treatment capacity for the community. The capacity limitation of the existing trench is difficult to determine, therefore the regular monitoring of the water level will provide the necessary data to determine the timeframe for increasing the capacity of the sewage exfiltration system. When a second trench is required, it should be constructed beyond the existing trench, and not parallel to the existing trench, in order to take advantage of additional treatment capacity in the granular deposit. The design criteria for the trench should include a long narrow excavation with a minimum depth of 3 metres in order to minimize the surface area exposed to the atmosphere, and to maximize the heat retention.
Dillon Consulting Limited. 2003. Fort Good Hope Water Licence Application. Ferguson Simek Clark. 2000. Draft Report: Engineering and Environmental Services Fort Good Hope – Sewage and Solid Waste Assessment. Ferguson Simek Clark. 2000. Sewage and Solid Waste Management Site Operations and Maintenance Manual. Johnson, Kenneth Robert. 1986. Role of Saturated and Unsaturated Zones in Soil Disposal of Septic Tank Effluent. Johnson, Ken and Wilson, Anne. 1999. Sewage Treatment Systems in Communities and Camps of the Northwest Territories and Nunavut Territory. Proceedings of the 1st Cold Regions Specialty Conference of the Canadian Society for Civil Engineering. K’asho Got’ine Chartered Community Council. 2005. Annual Report for Water. Reed, S.C., Crites, R.W., and Middlebrooks, E.J.1995. Natural Systems for Waste Management and Treatment. Terriplan Consultants Ltd. and Ferguson Simek Clark. 2001. Zoning By-Law Background Report.
Fort Good Hope Community Plan and
Thurber Engineering Ltd. 1995. Granular Inventory and Management Plan – Community of Fort Good Hope, NWT.