This article evaluates the performance of 10 large scale upflow anaerobic sludge blanket (UASB) sewage treatment plants (STP), located in Brazil (7), India (2) and the Middle East (1). In general the performance of the UASB plants located in Brazil was excellent with COD removal rates ranging between 50 and 77%. In comparison the performance of the two plants located in India was poor with COD removal rates of 44 and 49 %. Factors such as improper design, poor operating procedures, sufficient maintenance and the presence of high sulphate concentrations were identified as the main reasons for the lower performance. In addition a mathematical model was developed and used to compare the economics of an UASB + activated sludge STP with a primary clarification + activated sludge + digester STP. It was found that the capital cost of the two solutions was similar but the operational cost of the UASB solution was 78 % lower than the primary clarification solution.