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Understanding your wastewater treatment system

Your system safely treats wastewater and returns it to the ground. Its design adheres to standards set by Alberta and it has been installed by a qualified contractor. After installation and inspection, the safe operation and regular maintenance of your onsite wastewater treatment system (OWTS) are your responsibility.

Your septic system has two primary components:

  1. Initial Treatment: Septic tank/Advanced Treatment Units (ATUs)

  2. Final Treatment: Drainfield/Mound/Other


Initial Treatment

Onsite wastewater treatment systems use a septic tank or treatment plant for initial treatment. The household plumbing collects wastewater and sends it to the septic tank working compartment which acts as a separation chamber. Heavy particles separate from the wastewater and settle to the bottom to form a sludge layer. Lighter particles, mainly soap and grease, separate and float to the top to form a scum layer. Using a baffle device the clearest liquid from the center of the tank flows by gravity to the effluent dosing chamber (see Figure 1).

A pump or siphon in the effluent dosing chamber will deliver the effluent to the final soil treatment component.

All initial treatment devices reduce the amount of organic material, dirt, grease, etc. However, disease-causing organisms (pathogens) are not destroyed by initial treatment and its bacterial action. In initial treatment, methane gas and hydrogen sulphide gas (H2S) are produced in the septic tank. The sludge and scum separated out in the initial treatment stage will need to be pumped out on a regular basis.

Final Treatment Components

The final treatment and recycling of effluent back to groundwater occurs in the soil. Natural processes and soil bacteria will remove or alter the pollutants and pathogens in wastewater. With suitable soil and adequate separation distances from water tables this effluent will safely return to the groundwater. The soil treatment of effluent occurs mostly through the action of aerobic bacteria. Aerobic bacteria require oxygen if they are to do their job. In treating sewage they must have food and water (effluent), air to breathe and a suitable environment in which to live (soil). Thus a sufficient depth of suitable soil is needed to allow the use of treatment field trenches to return effluent to the soil. The Alberta Standard of Practice requires five feet of this suitable soil below the bottom of the treatment field trench for septic effluent and three feet for Class 1 plant or advanced treated effluent. Treatment field trenches are typically two feet wide by two feet deep and will likely total more than 400 lineal feet. In areas not having the required depth of suitable soil, imported suitable fill material will be needed to construct a treatment mound.


A key reason to maintain your wastewater treatment system is to save money! Failing systems are expensive to repair or replace, and poor maintenance is often the culprit. Having your system inspected regularly (once a year for three years and then every three years) is a bargain when you consider the cost of replacing the entire system.

• Have a professional inspect your system (once a year for the first three years and then every

three years) and pump your tank as necessary, generally every 12 to 36 months.

• Use water efficiently

• Don’t dispose of household hazardous wastes in sinks and toilets.

• Plant only grass over and near your septic system. Roots from nearby trees or shrubs might

clog and damage the treatment field.

• Don’t drive or park vehicles on any part of your septic system. Doing so can compact the soil

in your treatment field or damage the pipes, tank, or other septic system.

• Direct water flowing from drains, downspouts, driveways, sump pumps, etc. away from your treatment field as it much remain unsaturated for the bacterial action to take place.

Things to keep in mind.

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