top of page

Revamp your Septic Tank

Updated: May 1

Wiltshire Council is offering fully funded grants for homeowners in certain

areas of the Upper Avon catchment to upgrade their domestic sewage systems(eg septic tanks) and improve water quality.

The River Avon (Hampshire) is an internationally protected river and is suffering from poor water quality. This is caused by agricultural runoff, sewage from people in existing urban areas and new development. Natural England has advised that new housing developments in the Hampshire Avon catchment should not add more 'nutrient pollution' to the river. To help deliver interventions that improve water quality, the council has been awarded up to £9.8m for capital projects via the Local Nutrient Mitigation Fund. This fund will be used to deliver these upgrade projects to improve water quality in the river.

Wiltshire Council is teaming up with Wessex Rivers Trust to improve water quality in the Hampshire Avon catchment (Fig1). Many of the rural parts of the river catchment are not on the mains sewer network so water companies are not treating the sewage. Instead, householders must treat and manage their sewage. We're offering full grants for homeowners to upgrade their septic tanks with more efficient small sewage treatment works. By doing so, this reduces pollution and limits the excess nutrients from human waste entering precious water courses and harming aquatic ecosystems.

The problem

Septic tanks are simply designed to separate out our liquid and solid waste using gravity. The solid matter is partially digested by bacteria, whilst the untreated wastewater soaks into a drainage field in the surrounding soil. These systems can release nutrient-rich wastewater which can eventually reach the rivers, contributing to river pollution.

This results in high nutrient levels within waterways, which increases blooms of algal growth; a process called eutrophication. Algae form green carpets across the water’s surface, blocking light and killing aquatic plants below the surface. This ultimately causes oxygen-starved waters. Of course, with fewer aquatic plants, there are fewer essential microhabitats for small fish and insects and, thus, the whole river and its

ecosystem suffers.

In some of the more rural parts of our catchments, septic tanks are a notable source of nutrient enrichment in our rivers and surrounding landscape. Consequently, a new law requires all old septic tanks that discharge into surface water to be upgraded. This can present an issue when the property is sold.

Part of the solution

Replacing septic tanks with more effective package treatment plants could significantly reduce nutrient enrichment of our precious chalk streams in the Hampshire Avon catchment. In turn, this would restore healthier ecosystems, benefiting plants, insects, and our characteristic species such as salmon, water vole, otter and birds, all of which rely on a clean and healthy river.

The upgraded systems work in a variety of ways, depending on which is recommended for the property, from simple biological treatment, to using bursts of aeration. Not only will they separate solids and liquids, but they also biologically treat the effluent so that it can be discharged safely straight into a water course. Moreover, if space allows, tertiary treatment can be added which remove even more nutrients.

Our unique chalk streams currently need all the help they can get. By ensuring domestic wastewater is treated to the highest of standards, we can maximise the environmental benefits and minimise the pollution potential of the effluent. A campaign called Revamp your tank will actively contribute to improving water quality - supporting the health of the river, wildlife and our communities.

The campaign is targeting our primary key areas in the headwaters:


Upper Wylie Valley

Bourne Valley

The Till

Upper Avon & Woodford Valley

The public can register interest via the Wessex Rivers Trust website:

More information:


bottom of page