Ammonium removal occurs in both the surface water reservoir and rapid sand filters of Amsterdam Water Supply. Nitrification in the reservoir is best modelled as a first order reaction with a temperature dependent breakdown coefficient. In moderate winters, the increase of ammonium concentration in the filter influent is compensated but stagnation of nitrification in the Winter of '95–'96 led to a two and half month excess of the ammonium maximum admissible concentration. Only after the start of phosphate dosing, full restoration of nitrification was achieved. A model to alert for ammonium breakthrough was developed based on growth characteristics of Nitrosomonas.

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