The feasibility of anaerobic ammonium oxidation (Anammox) in fixed-bed reactors was evaluated on laboratory and pilot scales. Using synthetic wastewater, the specific nitrogen removal rate was increased from 0.05-0.1 kgNm-3reactord-1 to 0.35-0.38 kgNm-3reactord-1 within a year (T = 22-27°C) in all applications. However, the anammox activity was seriously and repeatedly inhibited at prolonged high nitrite concentrations (e.g. six days at 30-50 gNO2-Nm-3) and recovery was always a lengthy process. But even at a moderate nitrite concentration (11 ± 10 gNO2-Nm-3), the observed specific growth rate was only 0.018 d-1 at 26.4 ± 0.8°C, which corresponds to approximately 0.025 d-1 at 30°C (doubling time: 28 days). In a second experimental period for another 250 days, one of the laboratory reactors was fed with partially nitrified sludge liquors from a domestic wastewater treatment plant (WWTP). In this case, the specific elimination rate was as high as 3.5 kgNm-3reactord-1 at 26-27°C. Independently of the feed, the average nitrogen elimination rate lay between 80-85% in all applications. An appropriate hydraulic design is essential to prevent clogging and local nitrite inhibition in fixed-bed reactors.

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