The results of two pilot studies of an immersed membrane activated sludge process are presented. This process involves coupling a bioreactor with effluent separation by microfiltration hollow fibres immersed directly in the bioreactor. The two pilot studies were conducted at Valley Sanitary District, Indio, California, for 5 months and in Maisons-Laffitte, France, for 1 year. The objectives were to demonstrate the process with high biomass concentration (between 5 and 15 g/l) and sludge ages of 10 and 50 days. The process provided a high degree of treatment in terms of suspended solids (100%) and organic matter (>96% for COD). When operated in nitrification-denitrification mode, 99% ammonia and 80% total nitrogen removal were obtained. Better than 6 log removal of total coliforms and better than approximately 4 log removal of naturally occuring bacteriophages were observed. The effluent was suitably pretreated for reverse osmosis in terms of fouling potential. Sludge production was 0.25 kg dry solids/kg of COD/day, about 50% smaller than a conventional activated sludge process. It was demonstrated that the immersed membrane filtration system was able to operate without chemical cleaning or handling of the membrane modules and had an energy requirement for filtration of only 0.3 kWh/m(3) of wastewater treated. The process provides the benefits of membrane filtration (quality, safety, compactness) without its usual disadvantages (high energy consumption, requirements for frequent cleaning). These features make it a "High Tech Rustic Process".