Treatment of high-strength synthetic sewage in a laboratory-scale upflow anaerobic sludge bed (UASB) with aerobic activated sludge (AS) post-treatment

Performance of a combined system up-flow anaerobic sludge blanket (UASB) followed by aerobic treatment activated sludge (AS) for removal of carbonaceous and nitrogenous contaminants at an average temperature of 25C was investigated. The combined system was fed with high strength synthetic sewage having chemical oxygen demand (COD) of 2500mg L¹. The organic loading rate (OLR) of the UASB reactor was increased gradually from 1.1 to 3.8 gCOD L_r ¹ d¹. At steady state condition, the UASB reactor achieved removal efficiency up to 83.5% of total COD (COD_tot), 74.0% of volatile fatty acid (VFA) and 94.0% of protein. The combined system performed an excellent organic removal pushing the overall removal efficiency of COD _tot, VFA and protein to 91.0%, 99.9% and 98.2%, respectively. When the OLR of the UASB increased to 4.4g COD L_r ¹ d ¹, the UASB was overloaded and; thus, its effluent quality deteriorated. In respect to nitrogen removal, both partial nitrification and complete nitrification took place in aerobic post-treatment. When the dissolved oxygen (DO) concentration was >2.0mg L¹, complete nitrification (period B) occurred with an average nitrification efficiency of 96.2%. The partial nitrification occurred due to high OLR to AS during the overloading event (period A) and when DO concentration was <2.0mg L¹ (period C). The maximum accumulated nitrite concentration in periods A, B and C were 90.0, 0.9 and 75.8mg NO₂ -N L¹, respectively. The nitrogen balance results of periods A and C indicated that there was a discrepancy between the amount of ammonium nitrogen removed and the amount of oxidized nitrogen formed. This suggests the occurrence of simultaneous nitrification/denitrification (SND) in aerobic post-treatment.