A continuously increase in demand for milk and milk products has led to increase in dairy industries. Subsequently, the wastewater generated also increased which cause environmental problems when discharged to surface water or land. Wastewater generated in dairy contains mostly organic constituents which can be treated with biological treatment methods. This research was carried out to treat dairy wastewater of different strength by using UASB reactor at different HRT. After steady state condition in batch mode, the UASB reactor was run in continuous mode. In continuous mode, the study was divided into three phases, based on increasing wastewater strength and decreasing HRT. The 50% diluted dairy effluent was fed into reactor in phase I, whereas, 100% dairy effluent was fed in phase II and III. The HRT was progressively decreased during study i. e. HRT was 20 hours, 14 hours and 10 hours in phase I, II and III, respectively. The pH of the reactor was in the range of 7.12 to 7.72 throughout the study. The COD reduction efficiency was observed 94.7%, 86.8% and 80.9% in phase I, II and III, respectively. With increase in strength and decrease in HRT, a decrease in COD reduction was observed during the study period. The VFA alkalinity ratio varied from 6.71 to 0.07, 5.68 to 0.07 and 4.17 to 0.15 in phase I, II and III respectively. The optimum pH and VFA alkalinity ratio indicate that the reactor was working properly leading to higher COD reduction rate.
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