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 25 degrees C was investigated. The combined system was fed with high strength synthetic sewage having chemical oxygen demand (COD) of 2500 mg L-1. The organic loading rate (OLR) of the UASB reactor was increased gradually from 1.1 to 3.8 gCOD Lr -1 d-1. At steady state condition, the UASB reactor achieved removal efficiency up to 83.5% of total COD (CODtot), 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 CODtot, VFA and protein to 91.0%, 99.9% and 98.2%, respectively. When the OLR of the UASB increased to 4.4 g COD Lr -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 >2.0 mg L-1, 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 <2.0 mg L-1 (period C). The maximum accumulated nitrite concentration in periods A, B and C were 90.0, 0.9 and 75.8 mg NO- 2 -N L-1, 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.