A £2.7m upgrade to Horsham Wastewater Treatment Works is due to be completed this summer.
The Southern Water site, just off the A24, treats wastewater from about 80,000 homes.
The work includes constructing a new biological filter measuring 29 metres in diameter – about the length of two buses.
This will add an extra level of treatment to the wastewater before it is recycled into the River Arun.
Southern Water project manager Julie Anne Stokes said: “This is an important investment which will make big improvements to the plant’s processes.
“When complete, the work will provide a greener solution to the treatment of wastewater in the area.”
Construction, which is being carried out by contractor 4Delivery, is part of a programme which includes:
A £9m upgrade of Goddards Green treatment works (due to complete in 2014), a £1.6m upgrade at Luxford’s Lane treatment works, East Grinstead, and a £3.7m upgrade at Eden Vale treatment works in East Grinstead, both due to be finished this year.