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Abstract:In this study, optimal working conditions for the hydrolytic-acidogenic step in two stageAnaerobic Sequencing Batch Reactor (ASBR) system during anaerobic co digestion of tannery wastewater (TWW) and dairy wastewater (DWW) were investigated. The study was carried out at laboratory scale digesters with 0.4L working volume in semi continuous mode operation under mesophilic temperature (38±2°C). Four mixing ratios of substrates (100:0, 75:25, 50:50, 25:75) at hydraulic retention time (HRT) of 5, 3 and 1days and equivalent organic loading rate (OLR) were examined to assess the effect of each operational parameter on process performance of acidogenic step in the two stageASBR. Based on the result obtained, 50:50 substrate (TWW and DWW) mixing ratio(in terms of COD), 3days HRT and OLR of 3.22gCOD/l/daywere selected as optimum operational conditions for enhanced performance of hydrolytic-acidogenic stage during two stage anaerobic co digestion of the two agro industrial wastes. In addition to this, the degree of acidification was mainly affected by higher OLR and the best result (55.5%) was achieved in the reactor 50:50 operated at 3days HRT and 2.63 g COD/l/day OLR. In general co digestion of tannery wastewater with dairy wastewater is more important than using tannery wastewater alone for its enhanced performance. This is because of the positive synergistic effects of the co-substrates (dairy wastewater) in terms of wellbalanced nutrients, an appropriate C:N ratio and a stable buffering capacity.