ObjectivesSingle chamber air cathode microbial fuel cells (MFCs) were investigated with sodium-acetate and peptone as test substrates to assess the potential for application as biosensor to determine the concentration of biodegradable organics in water/wastewater samples.ResultsMFCs provided well-reproducible performance at high (>2000mg COD l(-1)Chemical Oxygen Demand) acetate concentration values. Current in the cells proved to be steady from 25 to 35 degrees C, significant decrease was, however, revealed in the current below 20 degrees C. Direct calculation of non-toxic biodegradable substrate concentration in water/wastewater from the current in MFCs is possible only in the non-saturated substrate concentration range due to the Monod-like dependence of the current. This range was determined by a fitted and verified Monod-based kinetic model. Half saturation constant (K-S) values were calculated at 30 degrees C applying different external resistance values (100, 600 and 1000, respectively). In each case K-S remained below 10mg COD l(-1).ConclusionsBiosensors with this particular MFC design and operation are potentially applicable for detecting as low as 5mg COD l(-1) readily biodegradable substrates, and measuring the concentration of these substances up to 50-70mg COD l(-1).