The degradation of a pollutant in batch and continuous operation is studied. Very often the microbial system used to degrade toxic industrial pollutants is severely substrate inhibited, and it is shown that batch operation is usually ineffective compared to continuous operation in a stirred tank. Recirculation of cells to the continuously stirred tank reactor (CSTR) is particularly advantageous for the carbon-lean waste streams treated here. Not only is the rate of pollutant degradation much higher but the stability of the steady-state operation is greatly enhanced. Finally it is shown that the operation of the CSTR is stabilized by addition of a second microorganism.