The invention relates to a microbial fuel cell arrangement comprising a cell reactor. The cell reactor comprises a membrane, which has an active surface and a support surface, as well as a pore size of ≤10 nm and/or a divalent ion rejection of ≥50%; an anode and a cathode, which are connected with each other through an external electrical circuit; an influent inlet for liquid medium arranged at the active surface side of the membrane and at least one permeate outlet arranged at the support surface side of the membrane; an influent line connected to the influent inlet; a concentrate outlet, arranged at the active surface side of the membrane and connected to a concentrate line; and pressurisation means for creating pressure difference between the active surface side and support surface side of the membrane. The invention relates also to a method for operating a microbial fuel cell.