The Effect of Hydraulic Loading Rates on Nitrogen Removal by Using a Biological Filter Proposed for Ventilated Improved Pit Latrines

Abstract

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.