Nitrification in porous media during rapid, unsaturated water flow

Abstract

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 NH 4-N between 20 and 60 mg l −1 were applied and vertical concentration profiles of NO 3-N, NO 2-N and NH 4-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 ( k 1) for NO 3 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 NO 3 profiles were obtained between 1.5 and up to 9 weeks after the experiments were started. At the actual soil-air contents (0.10 cm 3 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 NH 4 accumulation in the columns at 10°C and should probably be avoided during operation of RI system.