Septic System Basics


What is a septic tank?

A septic tank is a water-tight container, frequently made of concrete, that’s buried somewhere in your yard and accepts all sewage from your house. “Sewage” includes not only the toilet wastes but shower, sink, kitchen, and washing machine wastewater as well. The tank has no moving parts and is just the first part of your on-site wastewater system. The main function of the septic tank is to remove and store the solids present in the incoming wastewater. In Ontario the septic tank must have at least two chambers or compartments. The first chamber must be larger than the second chamber. The inlet must be fitted with a baffle or tee to prevent scum from interfering with inflow into the tank. The outlet of the septic tank must be equipped with an effluent filter, which prevents solids from leaving the tank. Although the tank is usually buried, the openings on the tank should be equipped with access risers to facilitate maintenance.

What is a conventional septic system?

A conventional septic system consists of a septic tank and a leaching bed consisting of gravel filled trenches constructed in the native soils. This is the most common type of system installed in Ontario.

What is a leaching bed?

A leaching bed consists of a number of individual absorption trenches laid parallel to one another. Each trench consists of a perforated PVC pipe (3 or 4 inch diameter) surrounded by clean gravel. Filter cloth is laid on the top of the gravel to prevent fine particles in the backfill from clogging the spaces between the gravel. Each trench, or lateral, is connected to a header pipe and is sloped away from the header at 0.5%.

Wastewater from the septic tank flows to the header, which evenly distributes the effluent to the laterals. Effluent flows down the perforated pipe and percolates through the gravel into the underlying soil where the bulk of treatment occurs.

Depending on the relative elevations of the septic tank and the leaching bed, a pump may be required to lift effluent from the tank to the header of the leaching bed.

How does a conventional septic system work?

Wastewater generated in the house is transported to the septic tank via the building sewer. Wastewater stays in the septic tank for approximately two days, and during this time the solids settle out and oil and scum float to the top. Only the remaining liquid moves out to the leaching bed. In the leaching bed, the liquid is dispersed into the native soils where it is treated by a variety of chemical, physical and biological means and eventually returned to the groundwater. It is very important that solids are retained in the septic tank and not conveyed to the leaching bed, as they can quickly clog a trench and cause failure of the bed.

What treatment occurs in the septic tank?

The septic tank provides primary treatment of the wastewater from the building. The first compartment of the septic tank is for separating the solid material from the liquid portion of the wastewater. Wastewater flows from the building into the first compartment of the tank, where it is retained for approximately 1.5 days. During that time, solids settle out and sink to the bottom, and oils and greases separate and form a scum layer along the top of the liquid. A relatively clear layer of liquid is created between the scum and solids layers. The remaining clear liquid flows through holes in the middle of the baffle wall into the second chamber of the tank, from where it goes out to the leaching bed. When wastewater flows into the tank, equal volume of effluent is transferred out of the tank and to the leaching bed.

Bacteria present in the wastewater breaks down, or “eats up” solids that have settled to the bottom of the tank. In a septic tank, these bacteria are called anaerobic bacteria because they survive best in low oxygen environments. As long as these bacteria are provided with time and warm temperatures, they can break down some of the solid material in a septic tank. However, the septic tank only accounts for approximately 10 – 20% of the treatment in a conventional system.

What treatment occurs in the leaching bed?

Treatment in the leaching bed occurs through a variety of physical, biological and chemical means. Effluent from the second chamber of the septic tank flows to the leaching bed header and is distributed equally to the laterals. Wastewater then moves from the perforated pipe into the gravel in the absorption trench, and eventually into the underlying soil. As this happens, some solids are filtered out (this occurs as the wastewater percolates all the way down the soil column). This is the physical part of the treatment in the leaching bed.

At the soil/gravel interface, a biomat forms that contains many bacteria. These bacteria are aerobic bacteria, meaning they thrive in the presence of oxygen. Similar to the bacteria in the septic tank, they break down, or eat microscopic solids in the wastewater, but because they are aerobic bacteria, they work much faster. Soil microorganisms also feed and break down wastewater. This is the biological component of the treatment in the leaching bed.

Chemical reactions between certain soil components and nutrients in the wastewater also occur. An example of the is the cation exchange which binds phosphorous to some soil particles.

All together the leaching bed is responsible for 80 – 90% of the treatment in a conventional onsite system. It should be noted that treatment will not occur in saturated soils (full of water, from high or perched water table).

Can I do anything to improve treatment in my system?

In order to fully maximize the treatment of your onsite system, you should only flush biodegradable material down the drains. Caustic or acid fluids, or bleaches, will kill the bacteria in your system, effectively halting treatment. Also avoid flushing things like paint, paint thinners etc. The use of an effluent filter is a relatively inexpensive way to prevent solids from moving from your tank into your leaching bed. Frequently mowing the lawn on your leaching bed will improve evapotranspiration and airflow into the leaching bed, improving treatment in the bed as well.

What is an alternative treatment system?

The connotation generally refers to an on-site system that provides enhanced treatment beyond the level of treatment provided by a conventional septic system. An alternative treatment system usually provides better quality effluent prior to entering the soil absorption system or leaching bed. Generally, the system provides better treatment by adding components to the system beyond the septic tank. Most of the additional components achieve higher levels of treatment by providing an aerobic environment, which will ensure the faster working aerobic bacteria are present to break down wastewater. Alternative treatment systems may consist of multiple components. One component may reduce waste strength or remove pathogens, while another may remove nitrate-nitrogen, others, working in combination, do all three. The type of advanced treatment system that you install will depend upon the water resource you are interested in protecting.

Are there certain areas where Alternative Treatments should be used?
Yes, in sensitive resource areas, like around drinking water supplies, advanced treatment systems are necessary to remove pathogens and nutrients such as nitrate-nitrogen from your household wastewater. Conventional systems do not provide the this level of treatment even when working properly. Also, enhanced treatment systems should be used at existing home sites requiring repairs or replacement where the site cannot accommodate a conventional system.

How do Ontario Authorized Alternative Treatment Systems work? 

Approved alternative treatment systems are listed in the Ontario Building Code, Division C, Supplementary Standard SB-5. These technologies are approved by the Minister of Municipal Affairs & Housing. Alternative treatment systems designed as “Treatment Units” other than septic tanks must meet the requirements of Section of the OBC and must produce either secondary or tertiary quality effluent (also described in the same Section). All of the authorized alternative treatment systems described in the following section of our FAQ’s meet tertiary effluent criteria as specified in the Ontario Building Code.

For more information on understanding your home’s septic system, see “Septic Smart”. In French, see “Les systèmes à fosse septique”

For more information on understanding advanced treatment units, see “Septic Smart: Advanced Treatment Systems”