The design and performance of an anaerobic reactor strongly depends on the solids retention time, operational temperature, and the biodegradability and concentration of the entrapped solids, which are interrelated parameters. Domestic sewage is a complex type of wastewater, characterised by a high fraction suspended solids and mostly of relatively low temperatures. The Hydrolysis of retained particles is in general considered as the rate-limiting step of the overall digestion process and is highly influenced by process temperature and solids retention time. When using UASB reactors, the SRT should be long enough to provide methanogenic conditions as illustrated in Table 6. The reduction in operational temperature does not only retards the Hydrolysis step but also leads to a significant decrease in the maximum growth and substrate utilisation rates.