In the UK, approximately 12 million tonnes of wastewater is processed each day = 140 tonnes per second (on average).
Further to this a considerable amount of rainfall enters the wastewater network.
Around 53 million tonnes per annum of untreated sludge is output from 8,500 wastewater treatment works servicing small villages to large cities.
The untreated sludge is transported (from the smaller wastewater treatment works) and processed at around 200 Sludge Treatment Centres (usually located at the larger wastewater treatment works).
Sewage Sludge Treatment
The main objective of sludge treatment is to create a product that is safe and acceptable to recycle to agriculture. This includes reducing or eliminating potentially harmful micro-organisms e.g. E. Coli and Salmonella spp. in biosolids and reducing the fermentability of the final product to acceptable levels. Various treatment technologies are employed to produce biosolids; about 73% by anaerobic digestion and 22% by lime stabilisation.
Biosolids can also be treated by thermal drying and composting sometimes with green waste. Process control points and monitoring are used to ensure that continuous treatment processes operate within defined limits and therefore a consistent quality of biosolids output can be expected.