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Major £31 million upgrade of West Sussex treatment works takes shape

A massive upgrade of Horsham Wastewater Treatment Works in West Sussex is entering its final year, in an effort to improve the site’s processes and capacity.

A massive upgrade of Horsham Wastewater Treatment Works in West Sussex is entering its final year, in an effort to improve the site’s processes and capacity. 

The major scheme, which began in September 2022 and is expected to be completed next spring, will further boost the quality of the final effluent returning to the River Arun – thanks to a range of new hi-tech machinery, control and monitoring systems, as well as the replacement and refurbishment of existing equipment. 

The work by contractor CMDP – a joint venture between Costain and MWH Treatment – is being contained to the site itself to minimise any possible impact on the surrounding areas.   

Meanwhile, landscaping and habitat enhancements on-site will ensure an overall 9% gain in biodiversity within the boundaries of the Works.   

Progress on site includes the installation of two new state of the art inlet screens – measuring 12.8m in length they are amongst the largest installed on a wastewater treatment works in the UK and ensure the vast amount of solid matter is removed from the treatment process.  

The two new Moving Bed Biofilm Reactors (MBBRs) continue to be installed. The purpose of the MBBR within the treatment process is to help remove ammonia from the effluent returning to the river Arun. The MBBRs will be fully operational towards the end of the summer.  


This extensive upgrade and construction project has also had a positive impact on the local community of Horsham, beyond improving wastewater treatment for customers. 

Teaming up with Horsham District Council’s voluntary sector support team, Southern Water colleagues in conjunction with CMDP have regularly taken time away from the scheme to support local organisations in need of volunteers to help worthy causes in and around the town.  

More than 600 hours to date have been spent volunteering to clear overgrown areas at QEII Silver Jubilee School, which caters for children with severe or profound multiple learning difficulties or complex needs, while colleagues have also donated pre-worn baby or children's clothes, or purchased and donated baby products, to the Ten Little Toes Baby Bank charity.  

Meanwhile, machinery and volunteers have been passed on to fellow charity Turning Tides, which has a new centre near the site that provides safe accommodation and support for people who are experiencing homelessness whilst also providing skills, workplace training and wellbeing support.  

CMDP has also contributed £10,000 two years in a row to Southern Water’s Community Centre Energy Grant which is opened annually to support running costs for independently run community centres and spaces where activities such as local foodbanks, support groups and children’s programmes operate. Horsham Matters was one grant recipient.  

Meanwhile, Southern Water and its contractors have also engaged with local schools and colleges to deliver related talks and presentations and attended technology and career evenings. In total, Southern Water has engaged with 52 schools reaching 3,543 students in the Horsham district, while delivery partners have delivered over 50 hours of career talks, curriculum and literacy support and site progress updates to classes since the start of the scheme. 


In an effort to reduce our overall carbon footprint, suppliers have been sought from within a 20-mile radius of the site where possible. For example, this includes the use of Southern Cranes based in Southwater. The success of this approach has resulted in over £2.4m of the £23m spend to date coming from within the local supply chain. 

Jon Quirk, Southern Water Project Manager, said: 

‘We have made some really positive progress across the site – and it is all the more rewarding given how much we have been able to engage with the local community. It is vitally important that we deliver real and tangible positive change for organisations throughout the area in which we work.” 

Further updates about the project will be provided in due course via our website.  

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