Graham Webb

There are a large number of legacy biological filtration plants in the UK, dating back to the 1920s and 1930s. In such plants, the influent wastewater is screened and settled in primary tanks prior to filtration in biological filter beds.  Effluent from these beds passes to humus tanks, where residual solids are allowed to settle out. Final effluent may then be polished through sand filters prior to discharge. Humus tanks are similar in design to other types of primary and secondary settlement tanks. They can be of radial flow, upward flow, or horizontal flow types. The sludge must be removed from the tanks regularly and continuously in a large works.

During periods of heavy rainfall, Eastney Sewage Pumping Station diverts flow to the Fort Cumberland Storm Tank.  During the first flush, the solids content of the flow is higher than normal, which overloads 6mm band screens located upstream of the storm tank. In order to help separate a greater quantity of solids from the flow and enable the screens to work as intended, it was proposed that the storm tank receives the flow and provides a residence time for solids to settle.

Civil engineering structures are used to distribute the flow and solid loads to multiple settling tanks or activated sludge plant (ASP) lanes at a sewage treatment works (STW). Achieving a good solids distribution requires adequate mixing to homogenise the solids, but this can be detrimental to achieving a good flow distribution, which normally requires quiescent conditions.

The Budds Farm Wastewater Treatment Works operated by Southern Water Services Ltd. treats waste water from the Portsmouth and Havant catchments. Prior to 2006, the Wastewater Treatment Works at Budds Farm had eight final clarifiers of 35m internal diameter. In order to meet the requirement of projected AMP4 flows, the existing settlement tanks were modified and two new 35m diameter clarifiers were built to operate alongside the existing eight tanks.