[1] To meet growing demand a large self-contained double filtration plant was built at the Walton works in 1925.
Raw water from the Knight and Bessborough reservoirs was passed rapidly through the primary filter beds containing coarse sand to remove suspended solids.
The water percolates through the sand at a rate of 10 cm an hour, gradually leaving a fine film of sediment.
To clean the main filter beds all the water was drained out, and the layer of sediment, with some 2 cm of sand under it, was scraped off manually.
[2] Following successful operation at Walton, similar two stage filtration plants were built at Kempton Park and Stoke Newington.
[3][4] A riverside wharf was located at Walton water works for unloading coal and sand from barges.
When the Queen Elizabeth II reservoir was commissioned in 1962 it also supplied raw water to the Walton treatment works.
[6] Seasonal algal growth in the reservoirs led to problems with the primary filters at the water treatment works.
The success led to the construction of a larger scale plant at Kempton Park water treatment works.
After ferric sulphate dosing the water entered a baffled hydraulic flocculator with 7 – 17 passes and a contact time of about 15 minutes.
After treatment in the CoCoDAFF water passes through post-ozone and granular activated carbon (GAC) contactors then through slow sand filters before disinfection where sodium hypochlorite is introduced to contact tanks.
[10] When the feed water quality is good the flocculators are bypassed and the CoCoDAFF units are operated in contact filtration mode.