Nitrification in porous media during rapid, unsaturated water flow

A substantial nitrification in rapid infiltration (RI) systems for wastewater treatment is a prerequisite for obtaining good N removal by denitrification. The purpose of this study is to investigate nitrification in porous media at conditions corresponding to RI treatment systems. Nitrification in six 50-cm porous media columns (98% weathered granite or sand and 2% field soil) during unsaturated leaching at constant flow rates of synthetic wastewater was investigated. Concentrations of NH4-N between 20 and 60 mg l−1 were applied and vertical concentration profiles of NO3-N, NO2-N and NH4-N were measured for 54 d at 30°C (three columns) and for 140 d at 10°C (three columns). A time lag in nitrification of 20 d was found at 10°C. Complete nitrification was obtained after 3–5 at 30°C and after approximately 50 d at 10°C. Assuming first-order nitrification at steady-state, the corresponding first order reaction rate coefficients (k1) for NO3 production in the columns were estimated to be between 0.4 and 1 h−1 at 10°C and between 6 and 9 h−1 at 30°C. Steady-state NO3 profiles were obtained between 1.5 and up to 9 weeks after the experiments were started. At the actual soil-air contents (0.10 cm3 air phase cm−3 soil), oxygen limitations were not observed during the experiments. Nitrogen loadings (water flow times N concentration) above 100 mg N 1−1 cm h−1 (1 g N m−2 h−1) caused NH4 accumulation in the columns at 10°C and should probably be avoided during operation of RI system.