A microbial membrane reactor for use in flow systems comprises a first element to receive the microorganisms and a second element to receive the flow channels. The two elements have planar interior surfaces which are pressed together separated by a membrane impervious to the microorganisms. The flow channels arranged in or on the surface of the second element serve to transport the liquid along the membrane so that the liquid can interact with the microorganisms arranged on the other side of the membrane. The geometry of the flow pathway and thus the geometry of the flow channels can be easily altered, in respect of the length, width and height of the channel, and adapted to practical requirements.