Cromhall wetland

About the wetland

The wetland, which is located next to our Cromhall Water Recycling Centre, is a first of its kind in the UK to test the effectiveness of removing phosphorus from sewage effluent before it is released into the nearby Tortworth Brook.

Tested as a ‘tertiary treatment’ – the third stage of the water treatment to remove non-biodegradable pollutants, Environment Agency modelling in 2017 had suggested that a new phosphorus permit of three milligrammes a litre (3mg/l) would be required to enable the Tortworth Brook to meet its Water Framework Directive targets.

Traditionally, we would use ferric dosing to remove phosphorus from the effluent, but this process uses lots of energy and chemicals.

Constructing a wetland is a far more passive, cost-effective and environmentally friendly approach that we are exploring to see if it achieves the same outcome.

What impact has it had?

Over the course of the trial, we have learned a lot about the performance of the wetland and the removal of a range of pollutants beyond just phosphorus.

The one-hectare wetland provided a living lab to three PhD students funded through the NERC FRESH programme – hosted at the universities of Bath and Bristol and supported by Wessex Water – to research cycling and removal mechanisms for nutrients, emerging contaminants, pathogens and microplastics.

The findings from the ongoing PhD research are among those being used to help us improve the performance of the wetland at Cromhall to achieve greater holistic environmental benefits and inform future wetland design projects.

Meanwhile, our own Cromhall Wetland Investigation report, submitted to the Environment Agency in 2022, typically followed more traditional permitting sampling and analysis requirements, using fortnightly or monthly samples with analysis for total phosphorus, to assess whether an annual average permit of 3mg/l is met.

It highlighted good nitrogen and pathogen reduction, improvements for emerging contaminants and a huge increase in aquatic and terrestrial biodiversity.

What’s next?

We’ll be conducting further investigations throughout our next five-year investment period (2025-30), as part of the Water Industry National Environment Programme (WINEP) to assess the benefits of wetlands.

These will assess a greater number of sites and applications throughout our region to understand the environmental benefits and potential drawbacks of wetland solutions compared to more traditional options.

The wetland applications being investigated include tertiary treatment of water recycling centre final effluent, treatment of groundwater-influenced storm overflows, in-river and on-farm wetlands, as well as those upstream of water supply reservoirs, such as our Durleigh wetland.