The steady-state performance, mainly with respect to substrate conversion, of a bubble column bioreactor with substrate limited microbial growth and cell recycle through a membrane filter is investigated numerically. The dispersion model and the Monod equation are employed for describing flow behaviour and cell growth rate in the reactor, respectively. Moderate longitudinal mixing causes a favourable effect on the substrate conversion. Such an effect is enhanced with increasing recycle ratio and separator concentration efficiency. There exists an optimum Peclet number to maximize the substrate conversion. The relation between the critical recycle ratio at which the optimum Peclet number becomes zero and the corresponding exit substrate concentration is presented.