Heterogeneous Nitrification in a Full-Scale Rapid Sand Filter Treating Groundwater

Experiments were conducted to determine ammonium removal kinetics in an operating biologically active sand filter at a waterworks treating anaerobic groundwater. The ammonium load varied between 0.7-3 g N/h/m(2) (concentration ranged from 0.23-0.78 mg N/L), and the inlet water flux varied between 1.7-5.1 m/h. Ammonium profiles and salt breakthrough curves were obtained in four different locations of the filter six times during two filter runs. The experiments show that the nitrification in the rapid sand filter was heterogeneous. The ammonium profiles exhibited variation in time and in space, vertically and laterally within the filter. The nitrification rate constants were calculated by using the ammonium profiles and the local pore velocity data obtained from the breakthrough curves. The local pore velocities were estimated to vary from 0.02-20.5 m/h. The nitrification rate constants varied randomly in time, and it was not possible to determine a clear order of the nitrification reaction. The average zero-order nitrification rate constant was 7 mg N/L/h, and the average first-order nitrification rate constant was 28 h(-1) with an average 1.5 g/h/m(2) ammonium load. The first-order nitrification rate constant was closely related to the water pore velocity, which implies that the rate is strongly determined by the resistance to mass transport in the diffusion boundary layer around the sand grains. DOI: 10.1061/(ASCE)EE.1943-7870.0000653. (C) 2013 American Society of Civil Engineers.