On-site wastewater treatment systems (OSWwTS) or septic tanks
(EA Septic Tank analysis)

On-site wastewater treatment systems (OSWwTS), a.k.a. septic tanks, are private sewage treatment facilities which typically serve the population not connected to main sewer networks (estimated at 2.1 million people in England and Wales). The number of properties in England and Wales thought likely to be served by septic tank or other small point source is estimated at 300000 to 500000.

This information is important in catchment water quality modelling in many areas in the UK, where the impact of septic tank leakage is a significant source of pollution. The information is incorporated in many modelling approaches, but no standard dataset exists.

In 2009 the Environment Agency produced a spatial analysis of likely septic tank locations in England and Wales which has been used for research purposes. This made use of the National Property Database, sewage treatment network, consents to discharge and other data. The output was of 50m grid cells with the number of properties in that grid cell that are likely to contain a septic tank. Each grid cell contains a likelihood score based on the proximity to urban areas.

This is an internal Environment Agency analysis for risk management purposes.