Ultra violet light is to be used to treat stormwater leaving Southern Water's Chichester Wastewater Treatment Works.
Southern Water is investing in UV disinfection of the stormwater which will significantly improve the quality of the flows released into Chichester Harbour during storms and at times of high ground water levels.
When it rains heavily, stormwater arriving at the treatment works is stored in a tank until the rain subsides, before going through the normal treatment process.
However, if the rain continues once the storage tank is full, to prevent flooding, the stormwater is first passed through 6mm screens and then diverted for immediate release through an outfall into the harbour.
At Chichester this also occurs at times of high groundwater levels when the sewerage system can become inundated with clean groundwater.
In the £2 million scheme, these storm flows will now pass through a UV channel to be disinfected before it is released. The UV plant will be able to treat up to 300 litres of stormwater per second - that's the equivalent of 528 pints every second.
Project Manager Dave Carlisle said: "Ultra violet disinfection has for many years been used at treatment works to help clean wastewater arriving from homes and businesses. However, using it to clean storm flows is a first for Southern Water.
"We recognise that Chichester Harbour is an environmentally sensitive location and we, among others, have a responsibility to help protect it. By providing this additional treatment, we will ensure all flows entering the harbour from our site are of consistent high quality."
Southern Water's Head of Environment & Quality John Spence added: "We have been working in partnership with a range organisations to focus on improving water quality in the harbour including the Environment Agency, Chichester District Council, Chichester Harbour Conservancy, West Sussex County Council and Natural England. This project is part of a number of initiatives that are being taken to ensure the water quality in Chichester Harbour is improved."
The works received a significant upgrade in 2008 when processes were installed to remove nitrogen and UV disinfect the treated wastewater.
The UV scheme is being carried out by contractor 4Delivery, which will also build a pumping station and stand-by generator on the site. Construction is due to be completed in the spring.