How it works

ON THE face of it septic tank systems are very simple things, a tank and some pipework buried under the ground. Yet they have huge potential to cause pollution and damage to the environment if badly designed or improperly installed.

“They need to be designed by a competent individual; these are not off the shelf items,” said chartered engineer Michael Madden.

They can pollute groundwater supplies and cause infection risks to householders if the system backs up or causes pooling of foul water outside. They are used when access to a sewerage system is not available, providing an independent waste water treatment and disposal service as a replacement, he said. The simplest septic tank systems are little more than a single tank with two chambers that receives waste water from the home, both flushing toilets and “grey” sink water.

The tank in turn is linked to a series of buried outflow pipes where liquid coming from the tank is distributed across an open space or “percolation area”, Mr Madden said. “The septic tank does very very little,” he said. Solid materials sink to the bottom and are partly digested and reduced by naturally occurring bacteria.

The first chamber is designed to trap floating material often discarded in the toilet. Grease and oils are also trapped here, allowing liquid with reduced solids to flow into the second chamber and then on out through the pipe network.

“Soil is the key component for the septic tank system,” he said. It must be able to absorb the liquid flowing into it without pooling or letting it get into the water table. Natural bacteria do the rest, breaking down and neutralising the waste.