NITROGEN REMOVAL BY DOWN-FLOW HANGING SPONGE REACTOR OPERATED AS A POST TREATMENT UNIT FOR UASB TREATING DOMESTIC SEWAGE

This study investigates the nitrification efficiency of Down-flow Hanging Sponge (DHS) reactor, which had been used as a post treatment unit for Upflow Anaerobic Sludge Blanket (UASB) reactor treating municipal sewage. During the total study period of 300 days, it was observed that NH4+-N removal efficiency of DHS was 62% and removal of total nitrogen was 56% at an HRT of 2.5 h. More importantly, it was also shown that DHS can accommodate a very high hydraulic loading rate of 78 m3/m2.d without any deterioration in its nitrification ability. Contribution of denitrification to nitrogen removal was 40% for which the retained biomass in the reactor was utilized as carbon source adding further merit to the process. The effect of temperature on the nitrification efficiency was also studied. The DHS reactor showed a quick recovery in nitrification as the temperature rose from winter to summer. At warmer temperature (>20oC) the NH4+−N removal efficiency averaged up to 84%. Better nitrification in DHS was possible due to the larger amount of retained biomass amounting up to 26 g-VSS/L of DHS sponge, which had higher rate of substrate utilization compared to other nitrifying sludge.