Looking after domestic sewage treatment systems

Could your septic tank or small sewage package treatment works be contributing to the ‘dirty secret’ hidden in the waters of the River Thame

Poorly preforming septic tanks and small sewage treatment plants can release raw sewage, polluting the water in the ground, in rivers, streams and along the coast. Although one polluting septic tank may only have an effect locally the accumulation of many across the catchment can have an overwhelming impact on water quality and wildlife. Research is showing that sewage is the main source of pollution in the Thame Catchment (Bowes et al, 2014). This will be coming from large treatment works, but a significant proportion will be from small domestic treatment works. Not much likes sitting in raw sewage, so let’s not force our wildlife to.

Owners can ensure that their systems are properly maintained by following best practice guidelines and new General Binding Rules introduced in 2015.

  • Use this homeowner pack to record maintenance and repairs works. Keeping good records will help contractors to fix any problems that arise and will be useful if you want to sell your home.
  • Please use this leaflet for 10 top tips to ensure you don’t pollute your local streams and the River Thame.


What are septic tanks and sewage treatment plants?

Septic tanks

Septic tanks are underground chambers where bacteria break down waste. Solids sink to the bottom forming sludge and the liquid flows into a ‘drainage field’ where more bacteria treat it as it soaks into the ground. These systems must not discharge into a watercourse.

Small sewage package treatment plants

Package treatment plants work in a similar way to septic tanks, but use powered mechanical parts to aerate the bacteria. This makes them more effective at treating waste water. However these treatment plants are not able to remove the nutrient phosphate and when plumbed directly into a watercourse can release high levels of phosphate, polluting watercourses.

The water these systems discharge is often not as good as the manufacturers suggest.


  • M. Bowes, H. Jarvie, P. Naden, G. Old, P. Scarlett,C. Roberts, L. Armstrong, S. Harman, H. Wickham, A. Collins. 2014. ‘Identifying priorities for nutrient mitigation using river
    concentration–flow relationships: The Thames basin, UK’. Journal of Hydrology. 517, pages 1-12.