A study was carried out to assess the effect of temperature variations, both decreasing and increasing, on the performance of a mesophilic (35 degreesC) and a thermophilic (55 degreesC) upflow anaerobic filter treating a simulated papermill wastewater. An organic loading rate of 1.95 kg COD per m(3) day was used throughout the study. The temperature of the mesophilic digester (MD) was lowered to room temperature (18-24 degreesC) for 11 days and then returned to 35 degreesC. The temperature of the thermophilic digester (TD) was lowered to 45 degreesC for the I I days followed by a further decrease to 35 degreesC for 6 days. It was then returned to 55 degreesC. To examine the effect of an increase in temperature, the temperature of the MD was raised to 45 degreesC for 6 days, followed by an increase to 55 degreesC for 5 days and that in the TD was raised to 65 degreesC for I I days. The mesophilic digester was affected by the drop in temperature but the TD only showed a marked reaction when the temperature was lowered to 35 degreesC. The temperature decrease caused lower COD removal efficiencies, higher suspended solids (SS) in the effluent, lower biogas production, and accumulation of volatile fatty acids (VFA). However, both reactors were able to recover their efficiencies fully after the temperatures readjusted to their optimum temperature conditions. The temperature increase to 45 degreesC in the MD did not show any detrimental effect, while the temperature increases to 55 and 65 degreesC in MD and TD, respectively, gave immediate drops in the treatment efficiency. However, it appeared that the bacteria in both digesters were able to adapt to the higher temperature after a few days acclimatisation to the temperature shock. After the temperatures readjusted to their optimum temperature conditions, thermophilic microorganisms appeared to be more resilient to the temperature increase than mesophilic microorganisms. (C) 2002 Elsevier Science Ltd. All rights reserved.