Do you know your Sludge layer from your small sewage discharge?

Package treatment plant 

Work in a similar way but use mechanical parts to aerate the bacteria, which makes them more effective at treating waste water and means they can discharge treated sewage into a drainage field or directly into flowing water.

Septic tank

Are underground tanks where solids sink to the bottom forming sludge and the liquid flows into a drainage field where bacteria treat it as it soaks into the ground. They’re not allowed to discharge into a watercourse.


Are different because the raw sewage is stored in a sealed tank, rather than being treated and discharged, which means they are not covered by the general binding rules. They must be emptied when full and must not be allowed to overflow or leak.

Small sewage discharges

In rural areas and other parts of the country where there is no public sewer, householders and other property owners usually treat their sewage using septic tanks or small sewage treatment plants. The treated effluent is then discharged either to ground through use of a soakaway, or into a river or stream if using a sewage treatment plant. Discharges are measured by volume. If the discharge is up to 2m3 to ground or 5m3 per day to water, this is known under environmental legislation as a small sewage discharge (SSD). For larger discharges, different rules apply.

General binding rules

General binding rules that apply to all small sewage discharges in England. These rules set the conditions that septic tanks and treatment plants will need to meet in order for them to be used without an environmental permit.

Designated sensitive areas

The designated sensitive areas which will continue to trigger permit requirements for small sewage discharges are:

Discharges to ground:

  • Groundwater Source Protection Zone 1s (SPZ1) for discharges to ground

Discharges to ground or surface water:

  • Special Areas of Conservation (SAC)
  • Special Protection Areas (SPA)
  • Ramsar sites
  • Sites of Special Scientific Interest (SSSIs) designated for biological reasons • Designated Bathing Waters
  • Shellfish Protected Waters
  • Ancient Woodlands
  • Selected protected species and protected habitats
  • Local nature reserves and local wildlife sites which are aquatic in nature


Wastewater that flows out of a septic tank 

Gray Water

Water that does not contain human waste but is formed from bathing, laundry, or other household facilities.

Sludge Layer

The bottom layer of matter in a septic tank that is composed of solids and is heavier than water