The performance of up-flow anaerobic sludge blanket (UASB) in combination with down-flow hanging sponge (DHS) system for sewage treatment at an average wastewater temperature of 15 °C has been investigated for 6 months. The results showed that a combined system operated at a total HRT of 10.7 h and total SRT of 88 days represents a cost effective sewage treatment process. The average CODtotal and BOD5 total concentrations measured in the final effluent of the total system (UASB + DHS) amounted to 43 and 3.0 mg/l, respectively, corresponding to the overall removal efficiency of 90% for CODtotal and 98% for BOD5 total. The total process provided a final effluent containing a low concentration of 12 mg/l for TSS. Eighty-six percent of ammonia was eliminated at space loading rate of 1.6 kg COD/m3 d and HRT of 2.7 h. The calculated nitrification rate of the DHS system according to the nitrate and nitrite production amounted to 0.18 kg/m3 d. The removal of F. coliform in the UASB reactor only amounted to 0.86 log10. On the other hand, the F. coliform concentration dropped substantially, i.e. by 2.6 log10 in the DHS system resulting only 2.7 × 103/100 ml in the final effluent. The calculated average sludge production for UASB operated at an HRT of 8.0 h amounted to 30 g TSS/d, corresponding to sludge yield coefficient of 0.2 g TSS/g total COD removed, while it was indeed very low only 6.0 g TSS/d corresponding to sludge yield coefficient of 0.09 g TSS/g total COD removed, for DHS system. The DHS profile results revealed that in the first and second segment of DHS system, the CODtotal, BOD5 total and TSS was eliminated, followed by the oxidation of ammonia in the next segments.
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