Recent UASB-pilot plant and full scale experiences show that anaerobic pre-treatment is feasible for sewage and domestic wastewaters in tropical regions. This offers developing countries the unique opportunity to become completely self-sufficient in this respect. UASB-systems can be applied at very small and at very big scale, i.e. for off-site and (house and community) on-site treatment. Results obtained in pilot plant and full scale installations demonstrate that these systems are quite effective in removing and stabilizing organic pollutants at liquid detention times as low as 4 hrs, i.e. a BOD reduction up to 75-95 % can be achieved, while an almost complete treatment can be accomplished by combining the method with a small aerobic lagoon. Instead of consuming high grade energy, these systems produce energy. Moreover the excess sludge production is very low, while also the dewatering characteristics of the sludge are excellent.