Unit Water Treatment Processes
Water softening, which under its earlier name of base exchange softening was the first industrial ion exchange process, is still by far the most widely used: the total production of polystyrene sulphonic resins, used for this purpose, is several times higher than the total output of the corresponding anion exchange materials. For the reasons given in Chapter 3 the process is fundamentally very efficient and the basic operating techniques have changed very little over the last 50 years. The process is usually carried out downflow with a settled resin bed and after a backwash in the upflow direction to regrade the resin bed and remove any loose dirt which may have accumulated in it, regeneration is also carried out down-flow. There is now an increasing general tendency to use counterflow operation, the advantages of which will be discussed later in this chapter. The gain which can be made by counterflow operation in the case of water softening is, however, comparatively marginal and the counterflow process is therefore mainly used in demineralizing plants. One industrial company in the United Kingdom produces a range of packed bed automatic counterflow softening units, but the purpose is to give simplicity in design rather than changes in chemical efficiency. The figures which follow in this section apply equally to co-flow and counterflow regeneration procedures.
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