Potential of a Combination of UASB and DHS Reactor as a Novel Sewage Treatment System for Developing Countries: Long-Term Evaluation

A novel municipal wastewater treatment system, consisting of a combination of an upflow anaerobic sludge blanket (UASB) and down-flow hanging sponge (DHS) posttreatment unit, was continuously evaluated for more than three years with raw sewage as an influent. The system was installed at a sewage treatment site and operated at 25±3°C. This paper reports on the results of a long term monitoring of the system. The whole experimental period was divided into three distinct phases with different operating conditions. Organic pollutants were only partially removed in anaerobic UASB pretreatment unit. The remaining organics as well as nitrogenous compounds were almost completely removed by the DHS posttreatment unit. In all phases the system demonstrated removal efficiency consistently over 95% for unfiltered biochemical oxygen demand (BOD), 80% for unfiltered-chemical oxygen demand and 70% for suspended solids. The system produced an excellent effluent quality with only 4–9 mg/L of residual unfiltered BOD. Dissolved oxygen in the final effluent was 5–7 mg/L although no aeration was provided to DHS system. Moreover, excess sludge production from DHS was negligible thus eliminating secondary sludge that is troublesome to dispose off. The system also exhibited substantial stability against twofold hydraulic shock load and fourfold organic shock load. The results suggested that the proposed system may be a competitive solution for municipal sewage treatment under variable conditions.