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In order to assess the long-term treatment of sulfate- and carbon- rich wastewater at low temperatures, three anaerobic biofilm reactors were operated at 20°C, a hydraulic retention time (HRT)of two days and fed a synthetic wastewater containing lactate and sulfate. The reactors were operated for over 900 days. DNA was extracted from the reactors around days 180 and 800. Three clone libraries, methanogenic archaea (MA), sulfate reducing bacteria (SRB), and bacteria, were constructed and quantitative PCR analysis was performed with the DNA. It was found that anaerobic biofilm reactors can be operated at 20°C with an organic load rate (OLR) of 1.3 g-chemical oxygen demand (COD)/L-day or less and an sulfur load rate (SLR) of 0.2 g-S/L-day with no significant deterioration in process performance. With long acclimation periods, OLR as high as 3.4 g COD/L-d and SLR of 0.3 g/L-d can be tolerated, producing effluent volatile-acid COD levels consistently less than 200 mg/L. Effluent dissolved sulfide and hydrogen sulfide levels were around 600 mg S/L and 150 mg S/L, respectively, during this period. In addition to long term operation, the effect of organic shock loading was assessed. The reactors were able to recover from one but not two lactate spikes of approximately 5,000 mg COD/L. It was determined that long-term stability could be achieved in reactors that contained well balanced, stable populations of lactate- and propionate-degrading SRB and aceticlastic methanogens. Significant populations of fermenters present resulted in an imbalance which caused lactate to be routed through an additional pathway where propionate was formed. Greater numbers of MA than bacteria were found in all reactors. This may be attributed to the availability of acetate in the reactors for MA consumption and to using the immobilized fixed bed reactor type. Aceticlastic methanogens were the dominant methanogen, and were observed to remove nearly all acetate produced in all reactors. SRB were observed to remove lactate in microbially balanced reactors, whereas fermenters degraded lactate in reactors with less balanced populations.