Advances in Water Treatment and Enviromental Management

DEVELOPMENTS IN ION EXCHANGE DENITRIFICATION 193preset fraction of the flow “by-passes” each successive plate has improved its accuracy. Any number ofsuccessive cycles can be run without interruption, with termination of the production run controlled eitherby nitrate breakthrough, or to a fixed volume of water treated.The model is far quicker than direct experimentation. Its speed gives us the freedom to explore even unlikelyalternatives, with occasional column tests to check the accuracy of prediction. With conventional resins themodel’s predictions appear to be quite reliable, and we have had occasions when the computer showed uperrors in the experimental work.Results for the selective resins are not quite so accurate, due probably to their non-ideal behaviour. Thepredictions nevertheless give a useful qualitative guide, even if they need to be checked rather more thoroughly.Work continues to improve the model’s performance in this respect.The simulation’s printout shows the condition of the resin in each plate within the column at all points ofthe cycle, which has proved particularly useful, as it gives an insight into the workings of all stages of theprocess.Now that the facility to simulate the process quickly and cheaply has been more widely appreciated, otherinvestigators in Britain have taken up the technique.5. THE ‘WASHOUT’ PROCESSIn any real applications, only a part—usually 30 to 50%—of an ion exchanger’s total capacity is utilised:this is called its “working capacity”. Increasing levels of brine regeneration increase the resins’ workingcapacity, but with diminishing returns at higher regeneration levels.Examination of the movement of ions within the column, provided by the computer simulation, showedus early in our investigations that in the regeneration step sulphate comes off the column very readily,while a zone of high nitrate merely tends to move backwards within the column without much of itactually being removed. Most of the working capacity was provided by capacity which has been occupiedby sulphate.Ion exchange resins’ affinity for polyvalent ions is well known to fall sharply with rising concentration, butthe degree to which this affects the ions’ relative regeneration efficiency was unexpected.We set about using the model to explore means of putting this phenomenon to practical use, which eventuallyled us to the “Washout” process for denitrifying high-sulphate waters, whose cycle runs as follows:a) The production run continues normally until nitrate breakthrough, or until a predetermined volumeof water has been treated.b) In the “washout” step a sulphate-rich solution displaces nitrate and other ions on the resin to waste,and converts the resin largely to the sulphate form.c) In the regeneration which follows, the sulphate-laden resin is regenerated so easily that most or allthe resulting eluate consists of sodium sulphate, which is saved.d) This eluate is diluted with raw water to provide the high-sulphate washout solution (see b) above) inthe next cycle.The ease with which a sulphate-laden resin can be regenerated results in a better regeneration, which inpractice means a higher working capacity, or lower regenerant usage, or both. Resin is delivered by themanufacturers in the all-chloride form. Over the first few cycles in use the process progressively accumulatesa recycled stock of sulphate. With low-sulphate waters this stock remains low and the improvement is notlarge. The greatest improvement in performance which is obtained from the use of the “Washout” process(which has been patented) on moderately high-sulphate waters, using conventional rather than selectiveresin. For very high-sulphate waters the selective resins would still be more economical.Figure 1 summarises a number of field trials of conventional denitrification of a raw water whose compositionis shown in Table 1. Two different nitrate-selective ion exchange resins, and one conventional Type II gelanion resin were regenerated with NaCl.