Septic Tank

A house not connected to a public sewer has to treat its own wastewater, and a septic system is the usual way of doing it. Everything the drains carry away, toilet and washing water together, flows into a buried tank and settles into three layers: heavy solids on the bottom as sludge, grease and light material floating on top as scum, and a clearer liquid between them. Only that middle layer leaves, going out to the drainfield, perforated pipes laid in gravel trenches where the soil does the actual cleaning.

The tank can be opened and pumped out; the drainfield is a living soil process that mostly cannot be repaired. For the soil to filter, the ground beneath the trenches must be unsaturated, its pores holding air rather than water, so the system is sited by what the earth will accept and by required distances from wells and streams. Percolation testing, permitting and installation are licensed work almost everywhere, and tank size follows the building rather than the people in it: about 3800 litres (1000 gallons) for a three-bedroom, two-bathroom house.

The usual failure is hydraulic overload, water arriving faster than solids can settle and faster than the soil can absorb; using less water indoors is the strongest protection there is. Roots wrap around and puncture the buried pipes, so nothing woody is planted over the field, and driving or paving over it compacts the soil and shuts out the air the treatment needs. A saturated field stops accepting water and backs sewage up indoors, and the tank must not be pumped while the ground is flooded because an empty one can float out of it. Slow drains, odour, soggy ground and unusually green grass over the lines are the early warnings. Prevention is pumping every three to seven years, or sooner once sludge and scum fill about a third of the tank, and keeping oils, wipes, chemicals and coffee grounds out of the drains.