On-site greywater treatment and reuse in urban areas bears the potential to reduce huge quantities of wastewater and lower freshwater shortages. Until now dissemination of small, single household applications has been rather limited as simple and high quality water producing, but also cost-effective treatment units have not been developed so far. This paper proposes a new process, based on a concurrently working hollow-fibre membrane as fixed biofilm support and filtration device. Bioreactor characteristics, influence of different aeration rates, membrane flux development, as well as structure and composition of biofilm were monitored to evaluate the performance of the tested pilot unit. The introduced process achieved international water reuse guidelines, worked soundly and could, compared to conventional micro MBR, significantly reduce energy demand (<1.4kWhm(-3)). Fouling control by air scouring and chemical cleaning was not required once flux had stabilized. The biofilm analysis showed a porous, spongy-like structure. Microbiological investigation revealed a community of sheathed bacteria and nematodes that could play an important role in the flux stabilisation effect. In general, the study confirmed the suitability of the presented process for greywater treatment and provides valuable design data for future optimization and systematic analysis. Biotechnol. Bioeng. 2015;112: 484-493. (c) 2014 Wiley Periodicals, Inc.