Developement and evalution of small scale sequencing batch reactors for treatment of organic-rich abattoir effluent in the Eastern African sub-region
Mutua, David Nzioka
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In the eastern Africa sub-region, facilities for the treatment of domestic and industrial effluents are either inefficient or non-existent thus leading to discharge of high contents of organic matter and nutrients into nearby surface waters. The objective of this study was to develop and evaluate small-scale pilot anaerobic-aerobic/anoxic sequencing batch reactors, operated in series, for treating abattoir effluent. Two reactors (i) Anaerobic and (ii) Aerobic made of plastic, each with a total volume of 250 L and a working liquid volume of 200 L, were setup at Makerere University and operated at ambient temperature. The 24 hour operating cycle consisted of the following periods: (a) filling, 0.25 hrs; (b) reaction, 23 hrs; and (c) decanting, 0.25 hrs for the anaerobic reactor, and (a) filling, 0.25 hrs; (b) reaction, 11 h; (c) settling, 12 hrs and (d) decanting, 0.25 hrs for the aerobic/anoxic reactor. Hydraulic retention time was 2 days for the anaerobic sequencing batch reactor (SBR), and 1 day for the aerobic/anoxic SBR while the organic loading was 12.8 kg COD/m3/day. In the anaerobic SBR, removal efficiencies for total chemical oxygen demand (TCOD), soluble chemical oxygen demand (SCOD), biochemical oxygen demand (BOD5), total solids (TS) and turbidity were 79, 76, 86, 79, and 70% with effluent mean concentrations of 3554±58 mg/l, 762±3 mg/l, 1869±27 mg/l, 2307±21, and 2800±9 FAU, respectively. Electrical conductivity, ammonia-nitrogen, ortho-phosphates and total phosphorus concentrations increased by 38, 80, 81 and 71 %, respectively. The high anaerobic effluent concentrations needed further treatment in aerobic bioreactor. Pollutant removal efficiencies in the aerobic SBR were 75, 90, 86, 52, 63, 94, 51, 82, and 73% for TCOD, SCOD, BOD5, TSS, turbidity, ammonia(NH3), total kjeldahl nitrogen (TKN), orthophosphorus (P043-), and total phosphorus (TP), respectively. The oxidized nitrogen forms were eliminated in a denitrification step that required reducing power in form of organic carbon. To evaluate the potential of using internal carbon source for anoxic denitrification, a 12-h operating cycle consisted of the following periods: (a) filling, 0.25 hrs:: (b) settling, 12 hrs and (d) decanting, 0.25 hrs. When 10% (aerobiC SBR hydraulic volume) of anaerobicallytreated abattoir wastewater was used, TCOD, BOD5, TSS, turbidity, NH4-N, TKN, N02-N, N03-N, P043-, and TP were reduced by 75, 70, 65, 39, 80, 86,100, 91, 59 and 39% respectively. However, whereas pH (7.0) and SCOD (100 mg/l) met national. discharge standards, the other parameters didn't. The SBR treatment performance was enhanced by augmenting the system with efficient microorganisms. To achieve this, a total of 20 microorganisms isolated were evaluated for their pollutant removal potential. Isolates with the highest pollutant removal efficiencies were used to make a microbial consortium which was used to augment bioreactors. The augmented bioreactor exhibited the best overall pollutant removal (over 90% for TCOD, BOD5, TKN, TP, P043- and NH4-N) while the nonaugmented bioreactor had low overall treatment performance (less than 20% for COD, BOD5, TKN, TP, P043- and NH4-N). In conclusion anaerobic-aerobic/anoxic SBR is an efficient system for biological carbon and nutrient removal. Anaerobically treated abattoir wastewater can be used as an internal carbon source for anoxic denitrification. Augmentation of the system with efficient microorganism enhanced its performance. Further processing of the SBR effluent in a constructed wetland would strip the remaining nutrients.