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Water Quality

Septic Tank and Disposal Field Systems

Description > Risks and Impacts > Best Management Practices


A septic tank is a chamber used for the retention and partial treatment of domestic wastewater. Effluent from the tank is discharged to the land through perforated pipes beneath the ground surface. A septic tank can be used wherever soil characteristics are suitable and where there is sufficient soil depth above the ground water table. The population being served must be sufficiently sparse to allow adequate area for the distribution field required. A septic tank is not normally used to serve a population exceeding 150 people due to the disposal field considerations (MELP, 1978).

Septic tanks themselves do not achieve a high degree of treatment. Their primary purpose is to condition sewage so that it will cause less clogging of the disposal field. Conditioning of the sewage is accomplished by floating or settling out some of the suspended solids which in turn anaerobically decompose in the tank (MELP [now WLAP], 1978). Solids and partially decomposed sludge settle to the bottom of the tank and accumulate. Lightweight materials, such as grease and scum, rise to the top. The partially clarified liquid flows through an outlet structure just below the floating scum layer to a disposal system (e.g., disposal field). Baffles (or tees) at the inlet and outlet ensure proper flow movement. In addition, baffles at the outlet prevent the scum from flowing out of the tank to the disposal field (US EPA, 1980).

drawing of typical sewage system

Typical onsite sewage system (adapted from the Soap and Detergent Association)

A single compartment septic tank will provide the minimum acceptable treatment to household wastewater, whereas multiple compartment tanks or two single tanks in series will perform better. Multiple compartments will improve biochemical oxygen demand (BOD) and total suspended solids (TSS) removal, as additional compartments provide better protection against the carry-over of solids into the discharge pipes (US EPA, 1980).

Septic tanks are sized according to the expected flow from the household with sufficient volume for between 24 and 48 hours retention time. Generally speaking, the longer the retention time or the bigger the tank, the better. Typical recommended septic tank capacities are:

  • 1 or 2 bedrooms — 800 gallons (3.6 m3)
  • 3 bedrooms — 1,000 gallons (4.6 m3)
  • 4 bedrooms — 1,200 gallons (5.5 m3)
  • 5 bedrooms — 1,400 gallons (6.4 m3)
  • 6 bedrooms — 1,600 gallons (7.3 m3)

As the sludge depth in the tank increases, the tank's capacity and detention time decreases, thereby decreasing the efficiency of the treatment. Typically, pumping out of tanks is required approximately every three to five years, however, inspections can determine the rate of sludge and scum accumulation. Septic tanks have no mechanical parts, yet routine inspections will determine if any repairs or maintenance to the system is necessary (US EPA, 1980). The sludge and scum must be handled and disposed of in a manner which will protect public health and the environment. Disposal of these materials should be undertaken by qualified individuals who are familiar with the necessary precautions to prevent ground water contamination, odors and aesthetic and health problems (MELP, 1978).

Further treatment of the septic tank effluent is provided through sub-surface disposal to land. Varying degrees of removal of pathogens, nutrients and solids are achieved as the effluent percolates through the soils of the disposal field. The actual degree of treatment obtained is a function of the climate, the nature of the vegetation and the amount of effluent uptake by the plants, the chemical, physical and biological characteristics of the soil, and the depth of areated soil above a water table (MELP [now WLAP], 1978). Treatment is also influenced by the actual flow rate compared to the design flow rate. The size of the disposal field is designed in proportion to the wastewater flow anticipated and the characteristics of the soil. Trench depth is determined from soil type, the position of the water table and climate. A thorough site evaluation should be conducted before the septic system is installed. Septic system failures are often caused by poorly sited drainfields (NSFC, 1995a).

Septic systems require the use of a distribution system to ensure an evenly distributed flow from the tank to the disposal field. Uneven distribution can overload certain areas of the disposal field, causing it to fail. The most common types of distribution methods include:

  • Distribution Box — a tank-like box with as many outlets as pipes in the disposal field. The effluent flows into the box and through the different outlets to the disposal field. It is important that the distribution box be level because this system relies on gravity to ensure even flow distribution to all outlets. Distribution boxes are easy to inspect and some outlets can be blocked to give certain trenches a rest.

  • Drop Box — a simple tank-like box used for distributing wastewater to disposal fields on sloped sites using only gravity. Inside the drop box, the pipe inlet is higher than the outlets, allowing the wastewater to flow downward to the disposal field trenches. A series of drop boxes can be arranged on a sloped disposal field so that after the highest disposal field is saturated, the flow of wastewater continues to the next drop box and trench below. Drop box outlets can also be capped to control the direction of flow and give the saturated upper trenches a rest.

  • Siphons and Pumps — are used to distribute wastewater in systems where soil and site conditions do not allow distribution by gravity. Siphons are used in systems that require even distribution over a large area (i.e. more than 150 m of pipe). The effluent flows from a dosing tank, to the siphon, then to the disposal field in pressurized doses, making uniform distribution easier to achieve. Siphons work using only air, water pressure and gravity.

Electric pumps also deliver controlled amounts or doses of effluent to the disposal field. Dosing can improve the performance of any disposal field by producing more uniform distribution, but it is especially advantageous for disposal fields with shallow or poor soil conditions. However, electric pumps are more expensive to operate than other distribution systems, require regular maintenance (NSFC, 1995b), and are subject to failure with power outages.

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Risks and Impacts

Poorly maintained septic systems are more likely to fail than systems which are inspected regularly and pumped out as required. Sludge and scum can plug the tile field causing system failure, which typically results in ponding of effluent above the field. Failing septic systems are expensive to repair or replace and poor maintenance is a common cause of early system failures. The cost of maintenance is very little in comparison to repair or replacement. For example, replacing a system can cost as much as $15,000 or more while inspections can cost as little as $75 and pump-outs as low as $200 (NSFC, 1995a).

When septic systems fail, inadequately treated household wastewater is released into the environment. Any contact with untreated human waste can pose significant health risks and untreated wastewater from failing septic systems can contaminate nearby wells, ground water and drinking water sources (NSFC, 1995a).

Chemicals disposed of in a septic system can also pollute local water sources and contribute to system failures. It is important for homeowners to educate themselves on what should and should not be disposed of through a septic system (NSFC, 1995a). Septic tank additives or "starters" should never be used; they are unnecessary, expensive and may cause pollution.

Improperly maintained septic systems can impact the economic health of the community. Failed septic systems can reduce property values and contribute to the pollution of local waterbodies used for commercial and recreational activities (NSFC, 1995a).

The following signs can be an indication of a failing system:

  • slowly draining sinks and toilets;
  • gurgling sounds in the plumbing;
  • plumbing backups;
  • sewage odours in the house or yard;
  • wet or mushy ground in the disposal field;
  • grass growing faster or greener in one particular area of the yard; and
  • tests showing the presence of bacteria in nearby wells.

None of these warning signs can be considered a sure indication that a system has failed, but the appearance of one or more should prompt homeowners to have their system inspected. Septic system failures can also occur without any of these warning signs. For this reason, a yearly inspection of systems is recommended (NSFC, 1995a).

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Best Management Practices

The following BMPs are designed for homeowners and will help ensure that their systems function properly and maximize the lifetime of the system:

  • Make sure that your system meets legal requirements before installing, repairing or upgrading an onsite sewage system. In BC, the Ministry of Health Planning is responsible for septic systems and installation is permitted under the Sewage Disposal Regulation of the Health Act. Contact your local public health authority for permits for repairs, improvements, installations and further information.
  • Sketch a map of your septic system showing the location of all components and keep it with your maintenance and repair records. This will make maintenance easier and be useful to future owners.
  • Keep your septic tank cover accessible for inspections and pumpings. Install risers if necessary.
  • Have your system inspected annually to ensure that it is working properly and to determine when it should be pumped out. By inspecting and pumping your system regularly you can prevent high repair or replacement costs. A professional can do a thorough inspection of the entire system including the disposal field and individual components of the system.
  • Pump out the tank regularly to prevent accumulating solids and clogging the disposal field. The frequency of pump outs will depend on the size of your system, the number of people in the house and the habits of those individuals. A general rule is once every three to five years.
  • Upgrade your system when you upgrade your home (i.e. when you add a bedroom or suite).
  • Practice water conservation. Continual saturation of the disposal field can affect the quality of the soil and its ability to naturally remove contaminants. The following points will help you to use water wisely:
    • repair any leaking faucets or running toilets;
    • use dishwashers and washing machines only when full;
    • avoid letting the water run when washing or brushing your teeth;
    • avoid taking long showers and install water-saving features in faucets and shower heads;
    • install low-flush toilets or put a toilet dam (e.g., brick) in the tank to reduce the capacity of the tank;
    • space out activities requiring heavy water use, like laundry, over several days;
  • Divert roof drains, surface water and sump pumps away from the disposal field. Don't saturate your disposal field with automatic sprinklers.
  • Avoid using garburators — this will reduce the amount of solids and grease you put into the system.
  • Don't use toilets as trash cans — excess solids can clog your drainfield which will cost you money for more frequent pumping. Items that should not be flushed include:

    • coffee grounds
    • disposable diapers
    • sanitary napkins
    • cigarette butts
    • fat, grease or oil
    • dental floss
    • kitty litter
    • tampons
    • condoms
    • paper towels and facial tissue
  • Don't put toxic chemicals (paints, varnishes, thinners, waste oils, photographic solutions, or pesticides) down the drain because they can kill the bacteria at work in your system and can contaminate waterbodies.
  • Use biodegradable household cleaners instead of bleach or other hazardous products (which will kill the good bacteria in your system).
  • Do not use toilet cleaners that are placed in the tank.
  • Avoid flushing pet wastes into the system — bury them away from the drainfield instead.
  • Don't drive, pave or put heavy objects or machinery over the septic system and disposal field. Don't cover the disposal field with a hard surface such as concrete or asphalt since evaporation will be prevented. This area should only have a grass cover which will prevent erosion and help remove excess water.
  • Don't plant trees or shrubs near the drainfield because their roots can damage or plug the pipes. Plant grass instead.
  • Don't use septic tank ‘starters’, additives or similar products. These products usually do not help and can sometimes harm your system. Allow bacteria to act on their own.
  • Use low-phosphate or phosphate free detergents.
  • Don't go down into a septic tank. Toxic gases are produced by the natural treatment processes in septic tanks and can kill in minutes. Extreme care should be taken when inspecting a septic tank, even when just looking in.

For more information, contact the Ministry of Health Planning's Health Protection Branch. General information is available from the National Small Flows Clearinghouse.

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