Assessment of the Takla Landing Contour Trench
The community of Takla Landing is located on Takla Lake Band Reserve on the east shore of Takla Lake in northern British Columbia. Figure 1 shows the layout of the community. The population of the community at the time of the study, 1988-1989, was estimated to be approximately 350. Services to the community include a community water system using water from Takla Lake, and an electricity distribution system with power supplied by B.C. Hydro.
The contour trench wastewater disposal system was chosen in 1988 to upgrade the community's wastewater system. It is a relatively new disposal technology for application in soils having moderate to poor permeability characteristics. The Takla Landing project was designed to demonstrate the system in highly permeable coarse grain soils and to test the relative filtering and fecal organism attenuation efficiencies of various trench liner media.
The intent of the system is for the effluent to move horizontally from the trench side wall, as opposed to a conventional system in which the effluent moves downward. (See Figure 2 for a cross-section of the trench.) The contour trench system consists of distribution pipes placed in trenches along a constant elevation (or contour) for distances of 200 to 300 metres perpendicular to slopes of 5 to 15°. The Takla Landing system consists of two parallel trenches, each 260 metres long. A major design objective is to discharge the effluent to the soil along a broad front. The system is fed from a dosing chamber which discharges enough effluent to fill one of the distribution pipes approximately four times a day.
FIGURE 1. Cross-section of the Trench Enlarged Image
In general, the contour trench system offers the following advantages over conventional ground disposal.
The system requires little space and can be installed in existing communities using sites that are unfavourable for most other applications.
Due to the design of the system, there is usually greater separation between it and the local water sources (lakes, rivers, etc.) which allows a greater degree of wastewater renovation through the soil.
Because the effluent discharge is perpendicular to the natural flow of ground water, there is less waste, or effluent discharge, per unit length of soil. This will also result in a greater degree of wastewater renovation through the soil including the removal of nutrients, such as phosphorus.
Cost data developed for the system indicates that it is less expensive than an equivalent conventional tile field system.
The system has the potential for widespread application in remote areas throughout Canada. It is particularly well suited for small communities where the impact of sewage on the water supply is a major concern, or where topographic or other physical constraints make construction of a conventional disposal system impractical. These applications include communities in which housing is constructed along a narrow strip parallel to a lake or river. The system may be successful in communities located in the North with a severe climate. However, the applicability in these communities must first be proven, since the weather is harsh and they are also usually isolated from maintenance sources.
At the time of installation, the concept was still relatively new, and untested in conditions found at Takla Landing. Two of the major differences in conditions are discussed below.
The Takla Landing system was installed in soils with a higher permeability than soils in which the system was used in the past. Instead of moderate to poor permeability characteristics, the soils at Takla Landing are highly permeable sands and gravels. The difference in soils was expected to result in the effluent flowing rapidly from the trench bottom to the ground water table below, instead of flowing primarily downslope, in the lateral direction.
Another difference, as mentioned above, is the harsh northern environment and the lack of access to trained maintenance personnel at the Takla Landing site.
The overall objective of the Takla Landing contour trench assessment project was to provide guidelines for application of contour trench technology in areas with coarse grain soils. In doing so, it would promote the application of the technology nationally for individual households and small community systems in both fine grain and coarse grain soils.
The Takla Landing contour trench assessment project was designed to demonstrate the relative filtering and fecal organism attenuation efficiencies of various trench liner media. The three media used were: a non-woven fabric (a synthetic geotextile); a specified mixture of sand with a specific grain size range; and the coarse local sands and gravels.
In many cases, sand with acceptable characteristics to serve as a biomat formation and filter medium is not readily available for subsurface ground disposal systems. Consequently, the cost of importing this material to the site often becomes exorbitant. The quality control testing also presents added complexities and costs when using a specified sand media on the trench bottom. On the other hand, the geotextile can be economically transported long distances and into remote areas and is easily handled and installed.
The following conclusions were drawn from the study at Takla Landing:
Research Report: Assessment of the Takla Landing Contour Trench Wastewater Disposal System, May 1989, prepared for CMHC, Environment Canada, Health and Welfare Canada, Indian and Northern Affairs Canada, and Takla Lake Band.
Research Consultants: NovaTec Consultants Inc.
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