CSA W203:19 Planning, Design, Operation, and Maintenance of Wastewater Treatment in Northern Communities using Lagoon and Wetland Systems

The North of Canada is unique due to its climate, geography, geology, demography, and social aspects. The North is a region with long periods of extremely low temperatures; a region that is exceptionally large and remote; a region with permafrost and other ground-related engineering challenges; and a region of small, isolated communities with low population density. These unique attributes of northern communities can create challenges associated with climate change and municipal wastewater management.

The treatment of domestic wastewater in Canada’s North has historically been accomplished primarily through long-term detention in wastewater stabilization ponds (WSPs), commonly known as sewage lagoons. Treated effluent from these ponds or lagoons is typically discharged to the receiving environment. Some lagoon systems discharge treated effluent directly to receiving water bodies; however, it is common practice to discharge it to natural land or wetland areas. These lowlands receiving treated effluent from sewage lagoons are referred to in this Standard as wetland treatment areas (WTAs). The function of a WTA is to provide ancillary treatment (polishing) beyond what can typically be accomplished by lagoon systems alone. The treatment performance of lagoon systems is influenced by several factors, including climate (e.g., temperature, solar radiation, windspeed, freeze-thaw cycles, permafrost thaw), organic load rating, depth, and sludge accumulation.

The purpose of the Standard is to address procedures, protocols, and methods for collecting information and evaluating the conditions of wetland treatment area (WTA) siting; life cycle phases; best practices for operations maintenance and monitoring; and closure and remediation considerations.

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Understanding and Assessing Impacts

Planning for a new municipal wastewater treatment system often starts years before the project is executed. The planning and siting phase of constructing a new municipal wastewater treatment system can be organized into five phases:

  • Community consultation engagement;
  • Reviewing background information;
  • Constraint mapping;
  • Assessing the site(s); and
  • Identifying treatment requirements.

There are specific Northern circumstances that could impact the project timeline and should be taken into consideration when planning. For example, wastewater treatment system planning should consider specific regulatory requirements, the logistics of equipment mobilization, and short construction seasons.

Constraint mapping considers areas of local importance through community consultation and engagement as well as consulting traditional knowledge; proximity to community infrastructures such as airports, houses, and roads; aesthetics such as odour and visibility through community consultation and climactic data; distance to surface water bodies; distance from lagoon base to groundwater table; distance from drinking water intake or watershed; and wildlife presence.

After potential location(s) have been chosen based on a constraint mapping exercise, the location(s) shall be assessed for their physical suitability. This includes assessing historical climate data and future climate projections such as air temperature, precipitation types and amounts by season, prevailing wind speed and direction, degree days, solar radiation, as well as assessing expected climate impacts such as melting permafrost, increased winter and summer precipitation, more frequent extreme weather events, flooding, hydrological changes, and more. The Standard recommends using statistically downscaled global climate models and considering multiple climate scenarios. Hydrological data, topographical data, a geotechnical investigation, a granular material assessment, and a hydrogeological investigation should be undertaken at this stage.

Identifying Actions

Key factors to consider in the selection of the lagoon process configuration are:

  • The required effluent quality;
  • Availability of wetland treatment post-lagoon;
  • Storage capacity;
  • Climate; and
  • Level of operational flexibility required.

The Standard outlines two approaches to lagoon systems: single-stage lagoon systems and multi-stage lagoon systems. Lagoon systems consisting of a single treatment stage, or cell, have been commonly used in the North. Physical processes (e.g., sedimentation of solids, coagulation of solids, and other constituents during freeze-thaw processes) reduce concentrations of carbonaceous biochemical oxygen demand, total suspended solids, nutrients, and fecal microorganisms. Depending on the design of the lagoon (depth and organic load rating) and geographic location, additional biological treatment might occur in the lagoon.

Depending on the receiving water environment and effluent quality regulations for a specific treatment site, a single-stage lagoon could be the sole treatment required. If a greater level of treatment is required, the lagoon effluent could be further treated in additional lagoon treatment stages (secondary cells) and/or in a WTA. Further treatment in a WTA may be the preferred alternative due to lower costs and enhanced treatment processes. However, there might be situations where wetland treatment is not feasible due to geographic factors, land use, community preferences, or other factors.

The Standard identifies treatment process design requirements to inform the size and location of the construction of the lagoons and wetlands. This includes wastewater volumes and projections, wastewater quality estimations, non-municipal sources of wastewater, and information on the receiving environment which identifies the aquatic environment, flow and currents, depth, tidal cycle, current use, proximity to community and other operations, and the presence of wildlife.

Additionally, complete life cycle costing shall be developed for new or upgraded lagoon and wetland wastewater treatment systems. A typical life span for a lagoon or wetland system is challenging to identify. If properly operated and maintained, these wastewater treatment systems should remain functional for several decades. However, for the purposes of comparing wastewater treatment alternatives, life cycle costing over a minimum 20-year design horizon should be evaluated.

Implementation

The Standard identifies several aspects of lagoon design to consider: a site assessment, geotechnical design, inlet structures, outlet structures, and site access and security infrastructure. The site assessment should be conducted by a professional with experience working in permafrost environments. Consideration shall be given to soil and rock formations, including rock type and grain size, hydraulic conductivity, moisture content, and bedrock competence. The suitability of on-site materials for berm construction should be assessed, as well as other granular materials available in the area. A thermal analysis is required to assess lagoon stability and shall consider how climate change will affect the ground thermal regime. A snow and wind assessment shall be conducted to assess drifting and impacts of snow accumulation on the truck pad and access roads. Snow piling locations should be considered in the design.

Geotechnical design shall aim to reduce seepage through the lagoon bottom and berms to prevent groundwater and/or surface water contamination. The saturated hydraulic conductivity of compacted native soils and berm materials shall be < 10–9 m/s. Otherwise, an engineered liner is recommended. If a liner is required, the lagoon should be fully lined due to uncertainties associated with permafrost maintenance. Rip-rap, gabions or gabion blankets, or engineered geotextiles can be used as protective materials from water and wind erosion for the inner and outer slopes of the berms that would be subjected to scouring. The Standard also provides guidance on the design of inlet and outlet structures including schematics, location, materials, and slope.

If an additional treatment stage is required, the Standard outlines detailed site assessment requirements for WTAs that include soil type, hydraulic retention time, ideal locations, vegetation assessments, soil hydraulic properties and groundwater flow, evapotranspiration, and the inclusion of climate projections, as well as watershed and watercourse delineation.

Outcomes and Monitoring Progress

In the design phase, an operation and maintenance (O&M) plan shall be developed for the entire wastewater treatment system. Lagoon and wetland operational details should specify the time periods during which the effluent release should occur, as well as the effluent flow rates that should be maintained and applied.

The Standard outlines maintenance and monitoring protocols for both lagoon and wetland systems. Maintenance requirements for lagoon systems include the inspection of berms, dams, and liners for damage and erosion, the inspection of berms for seepage, discharge areas, fences and signage, as well as water levels, the removal of debris, a vegetation assessment, and flow diversion.

The Standard Lagoon water quality monitoring shall analyze CBOD5, TSS, TN, TP, NH3-N, pH, E. coli, and any other parameters specified by regulatory authorities. The samples should be collected from approximately 15 to 30 cm below the water surface at three different locations in the lagoon. This analysis is necessary to ensure that effluent meets the regulatory criteria for discharge to the environment or is within expected concentrations to be loaded to a WTA. Water quality sampling should be accompanied by in-situ measurements of temperature and pH using hand-held water quality meters.

Wetland monitoring shall collect samples during loading at the inlet and outlet and analyze them for CBOD5, TSS, TN, TAN, VSS, TP, NH3-N, pH, E. coli, and other parameters specified by regulatory authorities. Water quality sampling should be accompanied by flow measurements and in-situ measurements of temperature, pH, and DO using hand-held water quality meters.

Resources

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