CHEMICAL DOSING
Three steps to success
Designed, built and installed by specialist Gee & Company, three upgrades to the chemical dosing plant at
Thames Water’s Hogsmill Sewage Treatment Works in Kingston-on-Thames have improved efficiency
T
he first of three upgrades at Thames
Water’s Hogsmill Treatment Works 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 civils
contractor Laing O’Rourke. It contains four Gee
custom-built BS 4994 Category One 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
The new phosphate removal system at Thames Water’s Hogsmill STW doses polyaluminium chloride,
72m
3
. For ease of access and chemical stored in these four Celmar/GRP tanks, each of 72 m
3
capacity
delivery the tank configuration provides two
separate fill points, with two fill tanks and two storage and distribution unit, used in replaced with chemically-resistant, centrifugal
balanced satellite tanks. Dosing of the chemical combination with an entirely new and custom- recirculation pumps. The novel Gee-designed
is carried out from three polypropylene dosing built ring main arrangement, which would feed ring main dosing system is reportedly a
skids housing Signal series 300 process chemical to each of the six RGFs resounding success and has overcome all the
pumps, working at a capacity of 1200 The central chemical storage system feeds traditional gas-locking problems on the site.
litres/hour and all contained within a GRP the ring main, which consists of a 1
1
/
2
inch
kiosk. The pumps are hydraulically operated diameter reinforced PVC flexible hose housed Return sludge chlorination
double diaphragms, a design considered within a 125mm diameter fusion-welded The third and final upgrade at Hogsmill relates to
necessary to combat frictional loses and back- polythene pipe. This outer pipe is designed to the not uncommon difficulty of filamentous
pressures in the 200m dosing lines serving the act as a means of safe containment should any bulking within the biological activated sludge
injection point into the process. leak occur within the inner hose. process. In such instances, the bacteria within
Supporting the system from an ICA and Chemical catchpots are also fitted at periodic the biomass become so prevalent that their
process control perspective is a Form four style low-level points within the ring main, into which filament formation causes them to clump
panel, a concept that uses separate panels bolted any excess chemical would run. As an additional together. This impacts on the efficiency of the
together to simulate a full Form form panel, a safeguard, each catchpot also features a switch BOD reduction process and settlement
particularly cost-effective solution for the site. linked with both an alarm and an automatic properties of the activated sludge, leading to
system shut-down. This solution eliminates any carry over in the settlement tanks.
Hypo ‘gassing’ problems solved possibility of chemical leaks contaminating the The process solution proposed by Z-Tech
The second of the Hogsmill upgrades relates to surrounding area. Ltd and Gee was to introduce an oxidising
the tertiary treatment that is required to achieve Where the new ring main passes each of the biocide for return sludge chlorination within
the solids consent, given the high discharge six RGFs at Hogsmill, it feeds sodium the activated sludge plant, using sodium
volumes generated at the plant. The brief called hypochlorite to a local dosing kiosk. Each of hypochlorite. By carefully controlling its use, it
for the eradication of historic ‘gassing’ problems these pump kiosks is of Gee design and would eliminate the problem of bulking by
that had been experienced in dosing 14% manufacture and houses a Signal S400 dosing killing only the excess filaments.
commercial sodium hypochlorite into the rapid pump, monitors and de-gassing sensor devices For this application and for reasons of cost-
gravity sand filters (RGFs) to inhibit algae growth. in addition to the usual dosing accessories of effectiveness, Gee elected to use a factory-built,
Working in conjunction with multifunction valves and calibration vessels. packaged system. Pre-tested before it even left
Engenica/Thames Water, Gee’s approach was to Central storage of the chemical is carried out the factory, it could be brought to site and placed
eradicate the existing long dosing lines where within the KTB unit, which is a packaged tank on a pre-prepared concrete slab and rapidly
the gas locking was occurring, which also greatly and bund arrangement, complete with controls. commissioned. The package used at Hogsmill
reduced the active chemical content of The tank is of fabricated PVC, reinforced with contains two balanced PVC/GRP chemical tanks
hypochlorite. The novel solution was to deploy a steel and GRP, with a capacity of 5,000 litres. located within a fabricated and GRP-lined steel
Michael Smith Systems KTB container (a Gee The customary chemical dosing pumps usually bund, together with a walk-in kiosk housing the
patented design) that was a central chemical associated with his type of plant have been dosing and other associated equipment. ■■■
42 Water & Wastewater Treatment November 2009
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