Assessment of the performance of a down-flow hanging sponge system for treatment of agricultural drainage water

The performance of down-flow hanging sponge (DHS) system for the treatment of agricultural drainage water has been investigated for six months. The reactor was operated at a hydraulic retention time (HRT) of 2.0 h, sludge residence time of 88.5 d, and food to micro-organism ratio of 0.24 kg COD/kg MLVSS/d. The results obtained showed that the average total chemical oxygen demand (CODtot) and biochemical oxygen demand (BOD5) concentrations measured in the final effluent of the system were 62.4 and 17.1 mg/l, respectively, corresponding to the overall removal efficiency of 73.1% for CODtot and 86.7% for BOD5. The reactor provided a final effluent containing a low concentration of 14.7 mg/l for total suspended solids (TSS). Moreover, 84% of ammonia-nitrogen was eliminated at organic loading rate (OLR) of 3.0 kg COD/m(3) d. The calculated nitrification rate of the DHS system according to the nitrate and nitrite production amounted to 0.16 kg NH4-N/m(3) d. Based on these results, the reactor achieved an effluent quality complying for reuse in agricultural purposes. The effect of shock load on the performance of DHS system was investigated. The results revealed that reducing the HRT from 2.0 to 0.5 h and increasing the OLR from 3.80 to 15.33 kg COD/m(3) d would increase the COD and ammonia-nitrogen levels in the effluent from 28 to 81 mg/l and from 1.68 to 3.08 mg/l, respectively. However, the COD and ammonia-nitrogen removal efficiency returned to their original values within 3.0 and 1.0 h, respectively.