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Anaerobic pre-treatment of domestic sewage using UASB reactor systems offers a number of advantages, e.g., system compactness, negligible or no energy consumption, stabilised excess sludge production, potential for energy recovery, low-cost accessibility of sewage for agricultural reuse purposes, etc. Research on high-rate anaerobic sewage treatment started in the early 1980s with the 64 m 3 UASB pilot plant in Cali, Colombia and various other initiatives in Brazil. Hereafter, the technology was adapted to full-scale conditions and slowly introduced in the market. This chapter describes the anaerobic treatment process for domestic sewage and evaluates the performance of current large-scale reactor systems located in (sub-)tropical areas. Although the perspectives from the early 1980s were confirmed in bench-scale studies, a considerable number of the recently constructed treatment plants deliver disappointing performance results. In many cases, inadequate designs, improper reactor operation and insufficient control are responsible for these poor performances. A brief survey is made on the preconditions that must be met prior to starting and operating sewage treatment plants based on the UASB concept. Adequate maintenance and basic system knowledge at the level of plant manager seem to be indispensable for full-scale success.

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