Pilot plant experiments were carried out on a nitrifying biofilter with a zeolite containing expanded clay aggregate filtermedia (Filtralite ZL). The filter removed ammonium from domestic wastewater by a combination of nitrification and ion exchange. An identical filter material, but without sorptive capacity with respect to ammonium, was used as a reference (Filtralite). The experiments demonstrated that Filtralite ZL removed more ammonium at high ammonium loading rates than Filtralite. This was caused by ion exchange of ammonium in addition to nitrification. Under low ammonium loading rates, nitrification of already sorbed ammonium took place. This combined effect of ion exchange and nitrification of ammonium in Filtralite ZL was demonstrated in experiments with constant ammonium loading for periods up to 10 days and as well as in experiments with daily variations in ammonium loading rate. No chemical regeneration was necessary in addition to the biological regeneration during the experimental period of four months.

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