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Assessment of the Takla Landing Contour Trench
Wastewater Disposal System

Technical Series 90-100

Key Messages

  • Takla Landing, a community of about 350 people in Northern B.C., installed an innovative contour trench wastewater disposal system.
  • Wastewater is discharged into 2, 260 m trenches several times a day - filling the trenches completely. In this way the effluent moves horizontally away from the trench side wall rather than downward towards the groundwater.
  • Compared to conventional tile bed systems the contour trench:
    • uses less space;
    • costs less; and
    • allows a greater degree of wastewater renovation through the soil.
  • Based on the Takla results, the system does not contaminate surrounding groundwater, even when placed in moderately permeable sands and gravels. However, there were serious concerns about the use of a geotextile for this application.

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 System

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.

Cross-section of the Trench
FIGURE 1. Cross-section of the Trench Enlarged Image

Advantages

In general, the contour trench system offers the following advantages over conventional ground disposal.

Space Requirements

The system requires little space and can be installed in existing communities using sites that are unfavourable for most other applications.

Reduced Contamination

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

Cost data developed for the system indicates that it is less expensive than an equivalent conventional tile field system.

Potential Application

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.

Uniqueness of the Takla Landing System

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.

Objectives of the Study

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.

Conclusions

The following conclusions were drawn from the study at Takla Landing:

  1. While the initial indications of the study were that the geotextile successfully promoted the growth of a biomat in a ground disposal system, there are serious doubts regarding the long-term use of the material in this application. After approximately one year of use, the geotextile biomat had very few wastewater degradation organisms present and appeared to have clogged up with organic material to the point that its hydraulic conductivity was in doubt.

  2. There were no statistically significant improvements in the degree of wastewater renovation achieved in the contour trench ground disposal system through the use of the geotextile as a trench bottom infiltration media. The coliform, nutrient and carbon removal characteristics of the geotextile and insitu sand sections of the trench were not significantly different.

  3. Serious localized ponding and uneven flow distribution in the trench made it impossible to compare the wastewater renovation characteristics of the specified sand with the insitu sand and geotextile sections of the trench.

  4. Based on the results of this study, the contour trench system did not contaminate the surrounding groundwater when placed in moderately permeable sands and gravels; however, a monitoring program is required to determine the long-term effects of the system on the local groundwater.

  5. Successful operation of the contour trench system depends upon an even flow distribution along the length of the trench. In this study, however, there was uneven flow distribution which may have been attributable to one or more of the following:
    • post-construction settlement of the trench;
    • post-construction settlement of the distribution pipes; and
    • malfunctioning of the dosing chamber.

  6. The depths of the perforated disposal pipe and the trench bottom (0.65 m and 1.2 m respectively) were sufficient to prevent severe freezing under prolonged extreme cold temperature conditions.

  7. Specific benefits derived from this study for the supporting parties include:
    • sampling ports and permanent wells to monitor the operation of the system and to gauge the potential of the system for contaminating the groundwater and lake;
    • identification of probable differential settlement along the length of the system;
    • identification of problems with the system's dosing syphon;
    • data of operation of a contour trench system in a remote northern community;
    • field testing data regarding the use of geotextile as a surface filtration medium in ground disposal systems;
    • field testing data on the use of contour trench technology in moderately permeable granular soils;
    • data regarding the importance of operational components of the system; and
    • field testing data on the use of contour trench technology in a remote native community.

Recommendations

  1. Geotextile should not be used as a surface filtration medium in ground disposal systems until the issue of long-term clogging with organic material is fully addressed. This will be best done at the bench or pilot scale under carefully controlled conditions.

  2. The dosing chamber design should be modified to ensure that syphon breaking occurs every time the chamber is emptied and under all flow regimes.

  3. A thorough investigation of the existing system should be conducted to review problems of uneven flow distribution, differential settlement and malfunctioning of the dosing chamber.

  4. In areas subject to prolonged cold weather (i.e. -20 C for two to three weeks), the disposal pipes should be installed at depths greater than 0.5 m in coarse granular soils.

  5. A thorough groundwater monitoring program should be continued to determine the long term effect of the contour trench disposal system on local groundwater quality.

Resources

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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