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

Pit latrines are the most frequently used sanitation systems in developing countries because of weak infrastructure and poor economic wealth. A modified ventilated improved pit latrine, with a biological filter beneath is proposed to stabilize and to remove the bulk of the nitrogen from the liquid phase. Although the hydraulic loading rate 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; the total Kjeldahl nitrogen and chemical oxygen demand concentrations are significantly higher in faecal sludge, namely 3 5 g /L and 20 – 50 g /L, respectively compared to ca 60 mg/L and 500 mg/L in standard rate biological filters. The biological filter was operated at nitrogen loading rates of 72, 145, 290 and 435 g/m2/d, respectively, until stable state conditions were obtained. The biological filter showed effective nitrogen removal between 72 and 434 g/m2/d and the best total nitrogen removal was obtained at 145 g/m2/d, namely 92 %. These results suggest that it should be possible to remove nitrogen effectively using a biological filter beneath a modified ventilated improved pit latrine.