Water treatment for British Sugar CHP plant

ACWa Services has received a turnkey contract to design, manufacture, install and commission a water treatment plant to provide ultra pure water for British Sugar's new combined heat and power plant in Bury St Edmunds ­ requiring a combination of pretreatment, reverse osmosis and mixed bed demineralisation.

When completed, the new plant will be capable of accepting 15m3/hr of incoming feed water from British Sugar's existing borehole supplies with a process design which includes water pretreatment, reverse osmosis (RO), carbon dioxide degassing and duplex mixed bed ion exchange. Main process systems are to be supplied as skid mounted units, together with all the necessary ancillary equipment required to operate the plant at optimum capacity on a cost-effective basis.

The filter bed comprises a top layer of mixed sand and manganese dioxide media with progressively coarser layers of sand and gravel beneath it. Any iron and manganese will be filtered out by the mixed media which, to maintain removal efficiency, is automatically backwashed and regenerated by dosing with a sodium hydochlorite solution. The filtered water will then be dosed with acid to reduce alkalinity and allow the downstream RO plant to operate at 90 per cent recovery.

From the pretreatment process, water will be pumped to a single pass RO filtration system, designed to provide permeate of 10-20mg/l TDS for feeding to the mixed bed units. The plant will use spirally wound RO elements with polyamide active surfaces and polysulphone backing/support layers. As pressure is applied to one side of the semi-permeable membrane, the feedwater is separated into two streams ­ product water (permeate) and wastewater (concentrate).

The plant will be installed with five pressure tubes containing a number of 20 bar RO membranes staged in a tapered arrangement to allow a recovery rate of 90 per cent (permeate/ feed). As a consequence of this concentration, certain salts may reach supersaturation levels leading to precipitation and fouling of the membranes ­ a condition which will be minimised by dosing the system with antiscalant.

The RO plant is to be installed with an inverter driven high pressure pump incorporated to reduce manual intervention. As the membranes become progressively fouled, the pressure required to achieve feedwater flowrate will be automatically adjusted by British Sugar¹s distributed control system, completely eliminating the need for throttling valves to meter the flow.

If the feedwater temperature is high, a low pressure is necessary, but if membrane fouling is high, a higher pressure will be required. the high pressure pump was selected to cope with the worst scenarios of 20 per cent fouling and 10oC water temperature. Permeate from the RO system will then pass to a degassing tower which removes carbon dioxide before it is fed to the mixed bed demineralisation units.

The mixed bed demineralisation system will be installed to remove the remaining dissolved contamination and produce a product of extremely high purity. Within the ACWa Services mixed bed demineralisation vessels will be a mixed quantity of strong acid cation and strong base anion resins providing an arrangement for multiple cation-anion Œcells' grouped randomly in series. As permeate passes down-flow through the mixed resin beds the process provides a simultaneous cation and anion exchange to produce high purity water.

During the cation exchange, ionisable salts contained in the influent water are converted to their respective acids, while negative radicals of the acids are converted to hydroxyl during the anion exchange process, resulting in the production of pure water. The strong acid action resin, similar to that used as softener, is regenerated with hydrochloric acid, converting it to the hydrogen form which allows it to remove cations such as calcium, magnesium and sodium from the water. The strong base anion resin is regenerated with caustic soda, enabling it to convert to the hydroxyl form and remove anions such as chloride, sulphate and nitrate together with more weakly ionisable substances such as carbon dioxide and silica.

Purified water from the single cation-anion cell reaction is polished by subsequent cation-anion cells. Final treated water from the mixed bed demineraliser will be devoid of nearly all dissolved solids resulting in a treated quality of <0.1µS/cm and silica levels of less than 0.01ppm. The capacity of the demineralisation beds will depend on the total amount of dissolved solids within the RO permeate. As the resin becomes exhausted there is a reduction in the quality of the treated permeate and the bed has to be generated. Regeneration is automatic, using backwash to separate the different resins, acid and caustic injection and displacement and various processes which remix and rinse the resins and recycle the water until the required quality is achieved.

Whenever the demineralised water tanks are full, the RO plant and degasser shut down automatically. To maintain water quality in the tanks, the demineraliser feed pumps continue to operate, using actuated valves to shut off the suction from the degasser sump while opening a valve to allow suction from the base of the demineraliser storage tanks.

One feature of the plant is that the operator is able to select high flow polishing and run both demineralisation tanks in parallel at 30m3/hr at the touch of a button.


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