Long-term experiments were carried out on a nitrifying biofilter with a zeolite-containing expanded clay aggregate filter media (Filtralite ZL) receiving daily peak loads of ammonia. The filter removed ammonium from domestic wastewater by a combination of nitrification and sorption of ammonium. The filter was biologically regenerated continuously and there was no deterioration in performance after 10 months of continuous experiments. Isotherm experiments confirmed that the Filtralite ZL granules that had been used for more than 8 months had about the same capacity as the unused material. Desorption experiments that were carried out indicated that desorption of ammonium from the filter material was mainly caused by an exchange of cations from the wastewater even at low cation concentrations.

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