In the Netherlands, biological trickling filters without chemical pre-oxidation are generally applied to treat anaerobic groundwater, containing methane, iron, ammonium and manganese. Previous research showed that all compounds can be removed in one filter step and that not only the ammonia oxidation (by nitrification), but also the iron oxidation is often a biological process, despite oxygen saturated conditions and neutral pH. However, the optimal conditions for each process differs. In this paper, we report the preliminary results of a demonstration plant (40 m3 h–1) with two consecutive trickling filtration steps. The first highly loaded filter removed 1–1.5 ppm of methane and 5–6 ppm of iron with filtration rates up to 30 m h−1. The second filter step removed 5–6 ppm of ammonium and 0.5–0.6 ppm of manganese virtually completely at 2 m h−1. Quantitative (real time) polymerase chain reaction (qPCR) indicated that the growth of methane-oxidizing bacteria was marginal, but biological iron oxidation by Gallionella bacteria accounted for a quarter to over half of the total iron conversion.

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