The pH value is crucial for the ion exchange process, as the form of the ion exchanger is dependent on pH, see equations (9.1) - (9.3), unless the ion exchanger is a strong acid or base, and as the form of the ions to be taken up is dependent on pH.
Optimum ammonium exchange by clinoptilolite occurs within an influent pH range of 4 to 8. If the pH drops below this range, hydrogen ions begin to compete with ammonium for the available ion exchange capacity. As the pH increase above 8, a shift in the ammonia-ammonium equilibrium toward ammonia begins. Consequently, operation outside the pH range 4 to 8 results in a pronounced decrease of exchange capacity.
The rate of exchange increases with decreasing clinoptilolite size. However, the improved rate of exchange is accompanied by disadvantage of higher head loss. A suitable flow rate in practice is 8-10 bed volume / hr, providing that the influent is clarified secondary waste water with less than 30 mg /1 suspended matter. Biological growth which occurs is adequately removed by the regeneration.
The break through is determined by the desired concentration of ammonium in the effluent. A typical break through curve is shown in Fig. 9.4. The corresponding utilization of the ion exchange capacity as a function of the column depth is illustrated in Fig. 9.5. The transition zone will have a certain depth, dependent on the flow rate but independent of the depth of the column; see Fig. 9.6, which is reproduced from Jorgensen et al., (1978).
It implies that a shallow bed utilizes less than a deeper bed, although the flow rate expressed in bed volume per unit of time will mean a smaller actual flow for the shallow bed. A deeper bed, on the other hand, will mean a higher head loss. A compromise between the head loss and the utilization has to be found and a bed depth of 1.5-2 meters is recommended in practice; see Koon and Kaufman (1971) and Suhr and Kepple (1974).
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