Dosing upgrades enhance process performance at Hogsmill Sewage Treatment Works

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Three separate upgrades to the chemical dosing plant at Thames Water’s Hogsmill Sewage Treatment Works in Surbiton near Kingston-upon-Thames 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 treatment system is fully contained within a large concrete bund built by the civil engineering contractor, Laing O’ Rourke. It contains four Gee custom-built BS 4994 Category 1 chemical tanks, constructed from Celmar/GRP, rated to an SG of 1.8. The entire system was built and completed very quickly, within a start-to-finish programme of four months.

Each of the tanks has a storage capacity of 72m³. For ease of access and chemical delivery, the tank configuration provides two separate fill points, with two fill tanks and two balanced satellite tanks.

Dosing of the chemical is carried out from three polypropylene dosing skids within a GRP kiosk housing Signal series 300 process pumps, working at a capacity of 1200 litres/hour. The pumps are hydraulically operated double diaphragms, a design considered necessary to combat frictional losses and back-pressures in the 200 metre long dosing lines that serve the point of injection into the process.


Supporting the system from an ICA and process control perspective is a Form 4 style panel, a design concept that utilises separate panels bolted together to simulate a full Form 4 panel, a particularly cost-effective solution for the site.

Hypo ‘gassing’ problems solved
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.

Working in conjunction with Engenica/Thames Water, Gee’s approach was to eradicate the existing long dosing lines, where the gas locking was occurring, which also greatly reduced the active chemical content of hypochlorite. The novel solution was to deploy a Michael Smith Systems KTB container (a Gee patented design) which was a central chemical storage and distribution unit, used in combination with an entirely new and custom-built ring main arrangement, which would feed chemical to each of the six RGFs

The central chemical storage system feeds the ring main, which consists of a 1½ inch diameter reinforced PVC flexible hose housed within a 125mm diameter fusion-welded polythene pipe. This outer pipe is designed to act as a means of safe containment, should any leak occur within the inner hose.

Chemical catch pots are also fitted at periodic low-level points within the ring main, into which any excess chemical would run. As an additional safeguard, each catch pot also features a switch, linked with both an alarm and an automatic system shut-down. This solution eliminates any possibility of chemical leaks contaminating the surrounding area.


Where the new ring main passes each of the six RGFs at Hogsmill, it feeds sodium hypochlorite to a local dosing kiosk. Each of these pump kiosks is of Gee design and manufacture, and houses a Signal S400 dosing pump as well as monitors and de-gassing sensor devices – in addition to the usual dosing accessories of multifunction valves and calibration vessels.

Central storage of the chemical is carried out within the KTB unit, which is a packaged tank and bund arrangement, complete with controls. The tank is of fabricated PVC, reinforced with steel and GRP, with a capacity of 6,000 litres. The customary chemical dosing pumps – normally associated with his type of plant – are replaced with chemically-resistant, centrifugal recirculation pumps.

The innovative Gee-designed ring main dosing system is reportedly a resounding success and has overcome all the traditional gas-locking problems on the site.

Return sludge chlorination
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.

The process solution selected was to introduce an oxidising biocide for return sludge chlorination within the activated sludge plant, utilising sodium hypochlorite. By carefully controlling its use, it would eliminate the problem of bulking, by killing only the excess filaments.

For this application and for reasons of cost-effectiveness, Gee elected to use a factory-built, packaged system. Pre-tested before it even leaves the factory, it could be brought to site and placed on a pre-prepared concrete slab and rapidly commissioned. The package used at Hogsmill contains two balanced PVC/GRP chemical tanks, located within a fabricated and GRP-lined steel bund, together with a walk-in kiosk housing the dosing and other associated equipment.

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