Integration of membrane technology into a new method for a recirculating aquaculture system (RAS) was investigated. Main objectives were: at first to reduce the water consumption, but not to exceed the critical concentrations for fish toxic water ingredients (especially nitrite and ammonium or ammonia) as well as for suspended particles (especially microbial biomass from the biological treatment); at second to facilitate elimination and pre-dewatering of excess microbial biomass for further utilisation by fermentation. On the basis of respective pre-tests, capillary microfiltration membranes were used for the investigations in pilot scale (in 6 culture tanks, each with a volume of 1 m(3) and a fish stocking density of about 20 to 60 kg/m(3)) at the aquaculture company Fischwirtschaftsbetrieb Andreas von Bresinsky, Oelzschau. In long-term tests particularly two membrane types were tested, i.e. Zenon ZW 500 (PVDF, nominal pore size 0.04 gm, membrane area 10 m(2)) and Eidos HF PP-M6 (PP, nominal pore size 0.1 by 0.7 mu m, membrane area 12.75 m(2)). The results were compared to other separation techniques (sedimentation, flocculation, electro-flocculation, hydrocyclone). Main evaluation aspects were -besides filtration capacity, separation, blocking and cleaning behaviour - the effects of fish mass increase and process stability of the biological water purification itself. In long-term operation (more than one year) it could be proven that the integration of microfiltration is not just technically and economically interesting for the investigated special RAS, but it also results in remarkable reduction in wastewater and residue loads.