Spontaneous formation of the active layer in biological filters for removal of iron, manganese and ammonium from ground water usually takes a few months. Inoculation of the new filters with well-established filter material is a common start-up procedure. An operational approach for inoculation of new filters using backwash sludge, instead of filter material, is elaborated here. The sludge is obtained from an already well-established biofilter used for removal of iron, manganese and ammonium. This procedure reduces the start-up time of the filters to 2 weeks only. The successful application of the described start-up procedure was demonstrated in two new plants treating ground water of different qualities. In one of these plants, the ground water contained increased concentrations of iron, manganese and ammonium, and in the other, only manganese concentration was increased.

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