Ion exchange is a process in which ions on the surface of a solid are exchanged for ions of a similar charge in a solution with which the solid is in contact (Chapter 9). When all the exchange sites have been replaced, the resin must be regenerated.
Both natural solids, such as the natural clay mineral clinoptilolite, and synthetic ion exchange, can be used in the removal of ammonium ions.
pH control is crucial in the ion exchange process, as the form of the ion exchanger is dependent on the pH, see equations (9.1) to (9.3), unless the ion exchanger is a strong acid or base, and also because the form of the ions to be taken up is dependent on pH. The 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 increases above 8, a shift in the ammonia-ammonium equilibrium toward ammonia begins. Consequently, any operation outside the pH range 4 to 8 results in a decrease in the exchange capacity. Neither ammonia, nitrate or nitrite or organic nitrogen can be bound to clinoptilolite.
Ion Exchange is very effective in removing ammonium from waste water, but is, however, not a very attractive treatment method for removal of high ammoinum concentrations. This is because the regeneration becomes more frequent. The operational costs, therefore, become high due to the elution frequency. Using clinoptilolite clay as matrix in a submerged bio-bed as presented in Chapter 5, diminishes this problem because the micro-organisms (nitrifying bacteria) regenerate the ion exchanger. A combination of ion-exchange and nitrification seems, therefore, to be attractive, as presented in Chapter 5.
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