Water

Designing homes to use water efficiently, reduce costs and cope with more frequent droughts or storms.

Septic tanks

A septic tank is a primary system, that is, treatment is minimal and involves only separation of solids and some preliminary anaerobic (without oxygen) action.

Find out about:

  • how septic tanks work
  • construction and installation.

How septic tanks work

The solids settle to the bottom of the tank and the largely untreated liquid, or effluent, is dispersed through underground drains onto land where it filters through the soil to be treated by bacterial action.

Wastewater flows into the septic tank where solids and liquids separate. Partially decomposed solids settle to form sludge on the tank floor, and lighter materials such as fat and grease form a floating layer of scum. Effluent, which may still contain small particles of solids, flows out of the septic tank to a land-application disposal area.

A single-chamber septic tank 
A single-chamber septic tank

   

The partially decomposed solids that settle on the bottom of the tank must be pumped out approximately every 3–5 years, depending on use.

The septic tank outlet should be below the level of the floating scum layer so the amount of these solids that are dispersed onto the land is limited.

Septic tanks are generally gravity-fed. They must therefore be installed below the level of the house. If this is not possible, waste must be pumped to the tank. Tanks may incorporate tees or baffles at the inlet and outlet pipes to slow incoming wastewater and reduce sludge disturbance. Gas baffles may be incorporated to deflect gas from escaping through the outlet.

Gas baffles to the outlet of a septic tank 
Gas baffles to the outlet of a septic tank

  

Construction and installation

Septic tanks may be factory-built and manufactured from reinforced cement mortar, fibreglass, steel, or plastics such as polyethylene and polypropylene.

Tanks may be installed underground, or above ground.

Below-ground systems are most commonly installed and must:

  • resist loads from the surrounding soil and groundwater
  • resist hydrostatic uplift (tendency to float)
  • prevent surface and groundwater getting in
  • have inspection covers that are not accessible to children
  • be clear of trafficked areas.

An above-ground system may be required if the site is sub-ground rock or has a very high water table. Above ground systems must:

  • be watertight
  • be durable
  • be UV resistant
  • resist earthquake forces
  • have inspection covers that are not accessible to children.