Industrialization has resulted in the formation of huge amount of waste products, which are released into the environment in the form of wastewater leading to environmental pollution and deterioration. Tannery effluent is among one of the dangerous pollutants of the industry. Most of the leather industries in Ethiopia discharge their effluent partially or without any treatment to the nearby water bodies. This creates a serious effect on aquatic biota and surrounding environment due to its high organic loading and chromium content. To minimize the effect of tannery wastewater, it should be treated before the effluent is discharged to the environment. Therefore, the main objective of the study was to use a two-stage laboratory scale Anaerobic Sequence Batch Digester (Reactor) in order to investigate the treatment potential of composite tannery wastewater at mesophilic and thermophilic phases. Two-Stage Anaerobic Sequence Batch Digester was used because it has a conducive environment for micro-organisms at a different temperature. Four sets of conditions were investigated; 1) mesophilic to mesophilic; 2) thermophilic to thermophilic; 3) mesophilic to thermophilic; 4) thermophilic to mesophilic, respectively. The Hydraulic Retention Time (HRT) of the hydrolysis/acidification was between 2 and 3 days and greater than 7 days in acetogenesis /methanogenesis. The Organic Loading Rate (OLR) was wide-ranging between 9.58 to 10.28 kg COD/m3-day throughout the study. The removal efficiency of COD, TN, NO3-N, S-2 and SO4-2 of all digesters were in the range of 57-70, 38-51, 44-61, 90-96 and 57-71%, respectively. While the concentration of NH4+-N showed an increment from the influent by 22-31% in all digester. Generally, treatment of composite tannery wastewater by two-stage ASBR shows significant removal of pollutants at thermophilic - thermophilic phase especially S-2.
Key words: Anaerobic sequential batch reactor, composite tannery wastewater, removal efficiency, organic loading rate.
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