Given the important cost reduction of the membrane bioreactor technology in the last years, this advanced treatment process has now become cost-competitive with other conventional technologies. A cost estimation analysis undertaken with few remaining unsewered and remote areas of Berlin showed that the implementation of semi-central sanitation scheme with a local membrane bioreactor plant would lead to similar costs than the connection to the central sewer, but with a superior effluent quality. For such small systems, some design issues have to be considered in order to optimise the costs and the operation regime, such as plant capacity increase, buffer capacity, process configuration and membrane flux.