Advances in Water Treatment and Enviromental Management

Chapter 19DEVELOPMENTS IN IONEXCHANGE DENITRIFICATION: THE‘WASH-OUT’ PROCESSG S Solt (Cranfield Institute of Technology, UK), I J Fletcher(Hydrotechnica Ltd, Shewsbury, UK)SUMMARYIon exchange is at present the only denitrification process being installed in Britain. There is much room forfurther improvement of the process, which suffers from several disadvantages. As development of ion exchangeby direct experimentation is extremely tedious, a computer simulation has been developed specifically forthis problem. It has led the way to an improved process for denitrifying high-sulphate waters using conventionalType II anion resin, which appears to compete favourably with the use of nitrate-selective resin. The processdepends on the rapid change in the resin’s affinity for polyvalent ions with changing concentration in theliquid. Further development continues.1. HISTORICAL BACKGROUNDIn recent decades nitrate levels in groundwater sources have been rising steadily in many areas, largely asthe result of modern agricultural methods, though there has been fierce dispute just how this comes about.In Britain these disputes have for long obscured the essential issue alternative methods of lowering nitratelevels in order to meet the EEC Directive on potable water having been exhausted, many groundwatersources now require denitrification. Whatever changes are made in agriculture, the number of such sourcesis likely to go on increasing over the next few years at least.In Britain, all denitrification plants currently being built or designed are based on anion exchange. Ionexchange has the advantage of being a well-tried industrial process raising no novelty in engineering orcontrol. Biological or membrane processes may be used in future, but at present fall short on combinedconsiderations of cost, treated water quality, reliability and ease of operation.In the ion exchange process water passes through a column of anion exchange resin in the chloride (orchloride/bicarbonate) form. The resin has a higher affinity for nitrate ions and takes them up in exchangefor chloride (and bicarbonate). When its capacity is exhausted, the unit is taken out of service and regeneratedwith brine (or brine and bicarbonate).2. PROBLEMS OF ANION EXCHANGE DENITRIFICATIONWhile its engineering is conventional, the process presents some chemical problems when put to this purpose:a) Conventional anion exchange resin has an affinity for sulphate which is even higher than for nitrate,so that sulphate is also removed, wasting resin capacity and regenerating chemicals.b) Replacement by chloride, of sulphate, nitrate and some of the bicarbonate in the raw water increasesthe chloride: bicarbonate ratio in the product, and in many cases results in a corrosive water.© 1991 Elsevier Science Publishers Ltd, EnglandWater Treatment—Proceedings of the 1st International Conference, pp. 191–198191