Septic tank

The rundown

At a glance

Size & capacity

Sized by the number of people using it. A typical household tank holds roughly 2,700–4,000 litres; busier homes and premises need a bigger tank.

Coping with busy periods

Limited. Settling needs calm, steady conditions — a sudden surge of water pushes solids out of the tank and into the drainage field, where they cause blockages.

Bleach & chemicals

Better than a powered treatment plant, because it relies mostly on settling rather than living bacteria — but bleach, strong cleaners and biocides still harm the process. Use them sparingly.

Wipes, sanitary items & fats

Poor. Wet wipes, sanitary products, nappies, cotton buds, fats and oils do not break down — they fill the tank faster and clog the drainage field. Only the "three Ps" (pee, poo, paper) should go down.

Land footprint

The tank itself is compact, but the drainage field needs a sizeable area of suitable, porous soil, set well away from watercourses, wells and buildings.

Water quality & where it goes

Poor on its own — partly treated only. The drainage field and soil provide the rest of the treatment, so a healthy field is essential.

Care & servicing

Have it emptied (de-sludged) regularly — for most homes about once a year — and never let the sludge build up into the outlet. Protect the drainage field: no driving, building or deep-rooting trees over it.

Signs it is failing

Slow or gurgling drains, smells, sewage backing up indoors, or soggy, unusually lush grass over the drainage field.