Artificial sweetener21 September 2000
ACWa has constructed a unique water treatment process for a British Sugar CHP plant.
Skipton based ACWa Services has fulfilled 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. The resulting process is unique, combining pre-treatment, reverse osmosis and mixed bed demineralisation.
The plant is capable of accepting 15 m3/h of incoming feedwater from British Sugar's existing borehole supplies, with a process design that includes CO2 degassing and duplex mixed bed ion exchange. The main process systems have been supplied as skid mounted units.
Owing to the presence of dissolved iron and manganese within the raw feedwater, it was necessary to include processes within the pre-treatment system that prevented fouling of the membranes in the reverse osmosis plant. The first of these – a splash plate unit – is installed at the inlet to the feed tank to aerate the feedwater and oxidise iron present in the dissolved Fe2+ form to the Fe3+ form. The second is a multi-media filter bed, which removes oxidised iron and manganese.
The filter bed comprises a top layer of mixed sand and manganese dioxide media with progressively coarser layers of sand and gravel beneath it. This mixed medium is automatically backwashed and regenerated by dosing with a sodium hypochlorite solution. The filtered water is then dosed with acid to reduce alkalinity and allow the downstream RO plant to operate at 90 per cent recovery.
Water is then pumped to a single pass RO filtration system, designed to provide permeate of 10–20 mg/l total dissolved solids going to the mixed bed units. The plant uses spirally wound RO elements with polyamide active surfaces and polysulphone backing and support layers. Under pressure the feedwater is forced through the membrane, producing permeate (the product) and leaving behind a concentrate which goes to waste.
This plant consists of five pressure tubes containing a number of 20 bar RO membranes arranged to produce a recovery rate of 90 per cent (permeate to feed). As a consequence, certain salts may reach supersaturation levels, leading to precipitation and fouling of the membranes - a condition minimised by dosing the system with antiscalant.
The RO plant is installed with an inverter driven high pressure pump included to reduce manual intervention. As the membranes become progressively fouled, the pressure required to achieve the feedwater flow rate is automatically adjusted by the distributed control system, eliminating the need for throttling valves to meter the flow.
If the feedwater temperature is low, or membrane fouling is extensive, higher pressure is required. The high pressure pump was selected to cope with the worst case scenario of 20 per cent fouling and 10oC water temperature. Permeate from the RO system then passes to a degassing tower, where CO2 is removed, before it is fed to the mixed bed demineralisation units.
This stage removes the remaining dissolved contamination. Within the mixed bed demineralisation vessels are strong acid cation and strong base anion resins, arranged as multiple cation-anion 'cells' grouped randomly.
During the cation exchange, ionisable salts are converted to their respective acids, whilst negative radicals of the acids are converted to hydroxyl ions during the anion exchange process. The exchanged salt radicals remain in the resin – leaving pure water behind. The strong acid cation resin, similar to that used as a softener, is regenerated with hydrochloric acid, which allows it to remove cations such as calcium, magnesium and sodium. The strong base anion resin is regenerated with sodium hydroxide, enabling it to 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. Fully treated water from the mixed bed demineraliser is devoid of nearly all dissolved solids, resulting in impurity levels of <0.1 µS/cm and less than 0.01 ppm silica.
Mixed bed regeneration
The capacity of the demineralisation beds depends on the total amount of dissolved solids within the RO permeate. As the resin becomes exhausted the quality of the treated permeat falls. Regeneration is automatic, using backwash to separate the different resins, acid and caustic injection and displacement and other processes which remix and rinse the resins and recycle the water until the required quality is achieved, when the unit defaults to standby in readiness for the next cycle. Otherwise, the outlet valve remains closed and a 'regeneration failed' alarm appears.
Whenever the demineralised water tanks are full, the RO plant and degasser shut down automatically. To maintain water quality in the tanks, however, the demineraliser feed pumps continue to operate, using actuated valves to redirect the flow. A unique feature of this particular plant is that the operator is able to select high flow polishing and run both demineralisation tanks in parallel at 30 m3/h - at the touch of a button.