The experience with the applicability of the UASB reactor for sewage treatment in tropical countries started by the pilot plant constructed in Cali-Columbia during the period 1982–1983. The results obtained from the operation of the Cali plant showed the feasibility of the system under the prevailing environmental and sewage characteristics. After that, hundreds of UASB reactors for treatment of sewage at both full scale and pilot scale (Table 3 & 5) have been operational in several tropical countries like India, Columbia, Brazil and Mexico. The ambient temperature in these countries is rather high throughout the year (20-35 oC) and the wastewater strength is rather low.