What Size of Septic Tank Does My House Need?

There is a common misconception that the size of your septic tank is correlated to the square footage of your house.  In reality, the size of your septic tank is dependent mainly on the number of bedrooms the house possess.  The number of bedrooms gives a designer and/or installer an idea of the maximum number of occupancy that could potentially live in the house.  According to the 2015 Alberta Private Sewage Systems Standard of Practice, for a single family dwelling and duplex, 2 bedrooms or less; it is assumed that each bedroom would allow for 2 people per bedroom.  However, for a single family dwelling or duplex with 3 or more bedrooms; it is assumed that each bedroom could account for 1.5 people per bedroom.

 The number of maximum potential occupancy would be used to estimate what is known as a peak daily flow rate.  This is accomplished by using empirical data, provided in the 2015 Alberta Private Sewage Systems Standard of Practice, that each potential occupant has then capability of producing 75 imperial gallons of wastewater per day.  This calculation would in turn provide a peak daily flow rate.  For example, a single family dwelling with 3 bedrooms is expected to have a maximum occupancy of 4.5 people.  This would produce a peak flow of approximately 338 imperial gallons per day.

 The working chamber, also known as the primary chamber, will separate the wastewater into three main layers, the scum layer, effluent layer and sludge layer.  The top layer is the scum layer.  Which is made of any material that floats to the surface of the wastewater. The second layer is the effluent layer.  This is the layer that is eventually gets drawn into the dose/pump chamber.  The third layer is the sludge layer.  This is composed of any solids or particulates that have settled to the bottom of the tank.  Calculating the estimated accumulated volume of the scum layer and the sludge layer is the next step in determining an appropriately sized septic tank.  This is done by using the same maximum occupancy and multiplying it by the 88 imperial gallons.  88 Imperial gallons is the expected sludge and scum accumulation per occupant; in accordance with the 2015 Alberta Private Sewage Systems Standard of Practice.  From the previous example the expected sludge and scum accumulation is 396 imperial gallons.

 How does all this information tie into the size of the septic tank?  The combination of the peak daily flow and the sludge and scum accumulation will determine the size of the working compartment the septic tank requires.  For the previous example a septic tank with a minimum working compartment of 734 imperial gallons is required for a 3 bedroom house.  All of these calculations were not dependent on the square footage of the house.  Most importantly they require the number of bedrooms and maximum occupancy of the dwelling.  However, this is not always the case.  Some things to keep in mind is the number of fixtures installed in the house, soaker tubs/Jacuzzi’s, garbage grinders and water softeners.  Also keep in mind any future renovations; like adding an additional bedroom or washroom in the basement.  These can all play a part in the operation of the septic tank.