Nitrogen removal in a small waste stabilization pond containing rock filters

Abstract

This study analyzes samples taken from 15 separate locations inside a small waste stabilization pond system in New Zealand. The system consisted of six ponds in series with rock filters installed between the final five ponds. The study aimed to determine the relative importance of the ponds and rock filters for nitrogen removal, for a typical effluent quality of 10 g/m3 total nitrogen during the 7 month study period. The results suggest that the most significant nitrogen removal occurred in the ponds rather than the rock filters. Comparison of removal rates to nitrogen accumulation in the sludge and estimates of ammonia volatilization suggest that the most likely mechanism for nitrogen removal is nitrification and denitrification. Possible sites for nitrification are explored in relation to theoretical principles and it is suggested that there may be adequate aerobic surface area on the banks of small-scale ponds to allow significant fixed film nitrogen reduction. A simple model is proposed that predicts nitrogen removal based on a constant removal rate proportional to the aerobic bank surface area. Key words: waste stabilization ponds, nitrogen, rock filters, nitrification, denitrification.