the effect of hydraulic loading rates on nitrogen removal by using a biological filter proposed for ventilated improved pit latrines

no on-site sanitation system treats both urine and faecal matter in one process. a laboratory scale biological filter was fed with high concentration of urea (4 g n/l) and 17.1 g cod/l to determine if it will be possible to treat liquid that leach from a ventilated improved pit latrine. the hlr in the proposed biological filter system was calculated to be ca 36 l/m2/d, significantly lower than the rates that are typical applied in standard rate biological filters (in the range of 1000 – 4000 l/m2/d) used to treat domestic wastewater. however, the tkn and cod concentrations in standard rate biological filters are significantly lower, namely ca 60 mg n/l and 500 mg cod/l, compared to the typical nitrogen and cod concentrations of faecal sludge, namely 3 - 5 g n/l and 20 – 50 g cod/l, respectively. the biological filter was operated at 13.0, 23.9, 35.7 and 62.3 l/m2/d, until stable state conditions were obtained. it was possible to remove most of the nitrogen and cod at the applied hydraulic loading rates by a combination of volatilization, nitrification and de-nitrification processes. however, at 62.3 l/m2/d the column efficiency (1.5 m long column) decreased and ammonia concentration in the effluent increased again. the best performance was achieved at a hydraulic loading rate of 35.7 l/m2/d, with an average ammonia concentration of 285.5 (± 9.1) mg n/l.