A study was carried out to assess the effects of short-term temperature increments on the treatment efficiency and methane production of UASB reactors at a working temperature of 37-39°C. Two different substrates were used to determine the effects on the several bacterial groups involved in the digestion process. One reactor was fed with defined synthetic acidified wastewater the other with unacidified wastewaler from a distillery process. Shocks of 5-24 hrs were applied at temperatures in the range of 45 to 61°C. Up to 45°C no detrimental effects were noticeable. Higher temperatures led to a sharp decrease of the activity of the different microbial populations as a result of elevated decay rates. Propionate oxidation turned out to be the most sensitive for temperature increments, whereas the acidogenic bacteria were least affected. Temperature shocks of 55 and 61°C led to a decrease of 50% of the overall efficiency after 10 and 3 hrs, respectively. By means of batch experiments decay rates of 0.44 and > 10 hr −1 of the methanogenic bacteria were estimated at 55 and 65°C respectively. As temporary inactivation of the mesophilic bacteria during a temperature shock was found to be unlikely, reactor recovery is dependent on the bacterial growth and the biomass retention capacity of the reactor. When unacidified wastewater is treated, a pH decrease has to be considered during a temperature shock.

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