Municipal wastewater treatment by upflow anaerobic sludge blanket (UASB) reactors

The objective of the present study was to examine the feasibility of treating municipal wastewater by a UASB system under low-temperature conditions. Two UASB reactors were started-up at 20°C. Reactors were operated at temperatures of 20, 32, 15, 11 and 6°C applying several hydraulic retention times (HRTs) ranging from 48 to 3 h during an operational period of approximately 900 days.; It was found that UASB reactors could be successfully started-up at 20°C. Start-up lasted for about 60 days with a COD removal efficiency ranging from 80 to 85% at the end of this period. The performance of reactors was found to be optimum at an HRT of 6 h at all operating temperatures. It was found that the performance of UASB reactors was good and stable up to a low-temperature of 11°C. The performance of UASB reactor was not very stable at 6°C, even though 30 to 50% of COD removal could be achieved. Gas production increased with a decrease in HRT but decreased with a decrease in temperature. Sulfate reduction played an important role in COD reduction by sulfate reducing bacteria (SRB). COD mass balance calculation demonstrated a satisfactory level of reactor stability up to a low temperature of 11°C. The settling characteristics of sludge particle improved (SVI decreased from 60 +/- 1 to 18 +/- 2 mL/g) over the operational period because of the aggregation of biomass. Fecal coliform reduction efficiency decreased with a decrease in temperature. It was found that UASB reactor at 15T recovered faster than at 20°C after shutdown period of 50 days. Digital image analysis and scanning electron microscopic observations of sludge samples demonstrated that biomass aggregation (granulation) progressed over time during 20°C operation of the UASB reactors. Fractal dimension analysis of sludge aggregates suggested that aggregated sludge particles were between the category of granular and flocculent type of sludge aggregates known as pellets. Rod type, segmented filamentous-type, and cocci-type anaerobic bacterial cells were observed. The effect of operating temperatures on biokinetic parameters was investigated using the Arrhenius model and it was found that the change in temperature affected the kinetic constants. The solid distribution equation for optimization of the sludge blanket height was modified by incorporating gas production rate as a function of temperature. Residence time distribution curves showed that the flow regime during 32 and 20°C treatment fell between the flow regimes of an ideal complete mixed and an ideal plug type of reactors.; This study demonstrated that the UASB system could be applied successfully for the treatment of municipal wastewater under low temperature conditions. (Abstract shortened by UMI.)...