NITRIFICATION IN POROUS-MEDIA DURING RAPID, UNSATURATED WATER-FLOW

A substantial nitrification in rapid infiltration (RI) systems for was tewater treatment is a prerequisite for obtaining good N removal by de nitrification. The purpose of this study is to investigate nitrificati on in porous media at conditions corresponding to RI treatment systems . Nitrification in six 50-cm porous media columns (98% weathered grani te 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 1(-1) were applied and vertical concentrat ion profiles of NO3-N, NO2-N and NH2-N were measured for 54 d at 30 de grees C (three columns) and for 140 d at 10 degrees C (three columns). A time lag in nitrification of 20 d was found at 10 degrees C. Comple te nitrification was obtained after 3-5 d at 30 degrees C and after ap proximately 50 d at 10 degrees C. Assuming first-order nitrification a t steady-state, the corresponding first order reaction rate coefficien ts (k(1)) for NO3 production in the columns were estimated to be betwe en 0.4 and 1 h(-1) at 10 degrees C and between 6 and 9 h(-1) at 30 deg rees C. Steady-stale NO3 profiles were obtained between 1.5 and up to 9 weeks after the experiments were started. At the actual soil-air con tents (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)) caus ed NH4 accumulation in the columns at 10 degrees C and should probably be avoided during operation of RI system.