What is a Septic System?

A septic system is an onsite wastewater treatment system. Septic systems are used when a property creating wastewater is not connected to sewer mains. A septic tank is the vessel into which wastewater (from toilets, washing machines, showers, sinks etc.) is directed via an inlet pipe. The tank separates the solids and liquids. The solids material separates into two layers with the heavier material settling to the bottom of the tank forming a ‘sludge’ layer and the lighter solids material floating on the surface forming a ‘solids raft’ which floats on the clarified liquids, known as ‘sullage’ or ‘greywater’, in the middle. The solids material is retained and digested by naturally occurring anaerobic bacteria in the tank. The resulting sullage/greywater flows out of the tank via the discharge pipe to an absorption trench (also known as a drain or leach field) or to another tank for further treatment and return to the groundwater system. 

The tank may be constructed of concrete, plastic or fibreglass and can vary in size and holding capacity. Older domestic septic tanks are usually 1850L or 2400L in capacity; most modern installations use 2850L or 3300L tanks. Commercial premises may use a domestic size tank or lager capacity tanks, dependant on the numbers of staff or public using the facilities.

A septic tank may also be referred to as a ‘solids tank’ or the ‘primary tank’ in a septic system involving more than one tank. The ‘secondary tank/s’ may also be referred to as a ‘liquids tank/s’ or ‘secondary tank/s’.

*Information on multi-tank systems coming soon.


Why pump out a Septic System?

As the diagram pictured illustrates, after a period of use (usually 3-4 years) the sludge layer increases in volume in the bottom of the tank. The solids raft on the surface also thickens. As more waste enters the tank there is nowhere for it to rise or sink to settle and be broken down by the residing anaerobic bacteria. When the septic tank reaches this stage it is no longer operating correctly. This results in the overflowing of raw solids material via the sullage/greywater flowing from the outlet pipe of the tank. These solids will be carried with the sullage/greywater and contaminate the absorption trench, or flow into the secondary tank (liquids tank) of a multi-tank septic system. As these solids build up in the absorption trench or secondary tank they can cause a foul odour, be very damaging, and prevent these parts of the septic system from operating efficiently an effectively.  


What is an Absorption Trench?

An absorption trench, also known as a drain field or leach field, further treats the sullage/greywater that flows from a septic tank before returning the treated water to the natural groundwater system. An absorption trench is generally a series of perforated plastic distribution pipes laid in gravel trenches over a layer of soil. As sullage/greywater flows from the septic tank via the outlet pipe into the trench, the water level rises and passes through pores in the plastic trench liner into the rubble outside. The water is drawn up through the rubble by capillary action towards the ground surface. When passing through the rubble, followed by sand and soil layers, the nutrients or contaminants are treated by naturally occurring bacteria, further purifying the water.

Finally, as the water nears the ground surface it is drawn up via evaporation or transpiration from the grass covering by the sun. The UV rays of the sun further act as a purifying action on the wastewater. If a septic tank is not pumped out when required, solids matter can be carried via sullage/greywater into the trench, forming a layer on the sides of the trench liner that blocks the pores in the liner preventing the passage of the water through to the rubble layer. If let go too long, the solids may even be forced through the pores into the rubble, filling it with a thick sludge that prevents the trench from operating properly and drying out by the usual process. This results in a contaminated boggy area in the region of the absorption trench. The efficiency of an absorption trench can also be affected by heavy or prolonged periods of rain.  


Locating an Absorption Trench

You should consult your local council to ensure all requirements are met when installing your septic system and absorption trench. Generally, an absorption trench should be positioned in a region that receives good sunlight, has permeable soil, runs perpendicular to a sloping gradient, and is an appropriate distance from dwellings/buildings. 


PLEASE NOTE: The information provided is a GUIDE ONLY and not intended in any way to substitute advice regarding your specific requirements from a qualified installer and/or your local council.