Two upflow anaerobic sludge blanket (UASB) reactors were operated for approximately 900 days to examine the feasibility of treating municipal wastewater under low temperature conditions. In this paper, a modified solid distribution model has been formulated by incorporating the variation of biogas production rate with a change in temperature. It was found that the model simulated the solid distribution well as confirmed by experimental observations of solid profile along the height of the reactor. Mathematical analysis of tracer curves indicated the presence of a mixed type of flow pattern in the sludge-bed zone of the reactor. It was found that the dead-zone and bypass flow fraction were impacted by the change in operating temperatures. © 2006 ASCE.