How Your Septic Tank Works

Understanding your septic tank and how it works is very important for any homeowner with a septic system.  Often misunderstood and thought to be more of a ‘holding tank’, there is actually a lot of magic, or should I say science, happening within your septic tank.

The Ontario Building Code requires all septic tanks, whether concrete or plastic, be divided into 2 different chambers/compartments.

As the sinks drain and the toilets flush, the wastewater from your home travels to the inlet baffle which directs raw wastewater downward to the mid-level of your septic tank within the first chamber. This first chamber holds wastewater long enough for it to separate into 3 layers:

Layer 1: Scum Layer

The top layer floats on top of the wastewater and looks foamy and bubbly and usually yellowish/brownish in colour and it is basically a layer of fats, oils and greases (FOGs) creating SCUM!

Layer 2: Wastewater

The middle layer is the wastewater which is composed of liquid and is the largest layer where the science really happens. A septic tank has many bacteria living within it which are critical to the operation of a properly working tank. Within this layer, the bacteria in your tank will be working overtime to break down and digest the sewage waste.

Layer 3: Sludge

The bottom layer is the sludge layer which is composed of mostly solids and is the thickest and heaviest layer. It is made up of organic, natural waste that lies undigested. Although the bacteria are still working, it takes a lot more time to work through this layer. Inorganic and foreign materials will end up here too.  This material cannot be consumed by the bacteria allowing this layer to build over time.

Once the separation of these 3 layers has taken place, the wastewater will travel through the baffle wall (which separates the 2 chambers within the tank). The baffle wall keeps most floating scum and sludge in the first chamber and has an opening that allows liquid wastewater to flow to the second chamber of the tank.

From the second chamber, the wastewater will pass from the tank through the effluent filter that is installed at the outlet baffle of the septic tank. The effluent filter traps suspended solids and fine particles that would otherwise clog your septic drain field. The filtered liquids then flow through a pipe that leads to your septic leaching bed where the final treatment takes place.

Diagram of how your septic tank works