Cross-section of a typical septic tank
Septic tanks are one of the most common onsite systems used to treat wastewater, but they provide only limited (primary) treatment through the settling of solids and the flotation of fats and grease.
Effluent from a septic tank is not suitable for irrigation and must only be applied to land through a covered soil absorption system or a sand mound.
Irrigation with treated effluent is possible when septic tanks are used together with an aerated system that treats wastewater to a higher standard.
The septic tank usually has two chambers, separated by a dividing wall (a baffle) that has an opening halfway between the tank floor and roof to allow only the clarified liquid to move between the chambers. The septic tank usually has a manhole cover with two inspection ports
Wastewater enters the first chamber, where solids settle to the bottom and scum floats to the top. The scum forms an airtight layer that allows anaerobic digestion of the solids (microorganisms breaking down biodegradable material in the absence of oxygen). This reduces the volume of solids.
The clarified liquid between the sludge and scum layers flows through the opening in the dividing baffle into the second chamber, where further settlement occurs. The partially clarified liquid then flows from the outlet into absorption beds or mounds where further nutrients and microorganisms are drawn from the liquid.
Common problems with septic tanks are:
- unanchored tanks rising out of the ground after rainfall
- stormwater accessing the tank because the tank is too low or run-off is not diverted
- high levels of sludge accumulating from lack of maintenance
- tank size too small for size of house (hydraulic load).
Effluent (treated wastewater) from septic tanks may be released onto your property through:
- amended soil mounds
- sand mounds
- absorption trenches and beds, and
- evapotranspiration absorption beds.