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Septic Tanks

Septic tanks are used as basic sewerage treatment plants. Anaerobic bacteria (bacteria which thrive and work best where there is no oxygen) act to break down organic solids.

The tank is working correctly when the water level is up to the outlet pipe leading to the soak away. As a gallon of water enters the tank it should displace a gallon of effluent.

Final treatment of the effluent should be dispersal into the ground by a soak away, you should not connect directly into a nearby watercourse.

Problems in Operation

There are a number of reasons why septic tanks may not work properly:

  • A structural defect to the tank.
  • Inadequate soakaways, sometimes resulting in a direct connection from the tank to an adjacent ditch or watercourse.
  • Insufficient thought being given at an early stage of construction to the depth and length of soakaways.
  • Poorly draining land e.g. clay or heavy subsoil.
  • A high and variable water table.
  • plot size inadequate for septic tank drainage.
  • Increased water usage e.g. from domestic appliances such as washing machines.
  • Overuse of disinfectants, detergents and cleansing materials. Wherever possible biodegradable products should be used. It may also be necessary to add special bacteria to your tank.
  • Inadequate maintenance.
  • Infrequent emptying, it is recommended that tanks are emptied once a year.
  • Soak aways blocked with detergent residue or fat.
  • Inability of the land to absorb any further effluent (sewer sickness)


Where mains drainage is not available the preferable option is to install a package treatment works such as a bio-tech or bio-disc unit. The effluent is treated with aerobic bacteria (bacteria which thrive in the presence of oxygen) so that the liquid that is discharged can be put into the ground or to a watercourse with consent from the Environment Agency.


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