The paper describes the changes which have been made to the plant since it was built in the 1930s. It was initially designed to treat 50,000 m3/d by a 2-stage activated sludge process with integrated charcoal precipitation and was enlarged 25 years ago to treat 130,000 m3/d dry weather flow. It now treats a population equivalent of 650,000, half of which is industrial effluent mainly from textile and food processing factories.

Sludge treatment has always included digestion and this is to be upgraded to 2-stage thermophilic and mesophilic digestion with submerged combustion sludge heating. The sludge produced has always been disposed of to land, with or without drying, using digester gas, and some has been centrifuged. The gas has also been used for car fuelling but it is now purified and blended into the gas supply of a public utility.

The sewage treatment is also to be upgraded mainly to deal with increased storm water flows.

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