New Dosing upgrades enhance process performance at Sewage Treatment Works

Three separate upgrades to the chemical dosing plant at Thames Water’s Hogsmill Sewage Treatment Works in Surbiton near Kingston-upon-Thames in England, have improved efficiency and performance at the site and are also designed to meet the latest phosphate discharge consents into the River Thames. Working with partners, the upgrades were designed, built and installed by chemical dosing specialist, Gee & Company.

The first of these was a custom-built traditional phosphate removal system, dosing poly aluminium chloride to achieve discharge consents. Initially, the system was to have dosed ferric sulphate, but a change to poly aluminium chloride was made for the process reason of its faster flocculation.

The second of the upgrades at Hogsmill relates to the tertiary treatment that is required in order to achieve the solids consent - given the high discharge volumes generated at the plant. The brief called for the eradication of historic ‘gassing’ problems that had been experienced in dosing 14% commercial sodium hypochlorite into the rapid gravity sand filters (RGFs), to inhibit algae growth.

The third and final upgrade at Hogsmill relates to the not uncommon difficulty of filamentous bulking within the biological activated sludge process. In such instances, the bacteria within the biomass become so prevalent that that their filament formation causes them to clump together. This impacts on the efficiency of the BOD reduction process and settlement properties of the activated sludge, leading to carry over in the settlement tanks.