There are a large number of advantages in using microbial anaerobic degradation processes in waste water pretreatment. A disadvantage of this method lies in the relatively long doubling times of anaerobic microorganisms which cause long residence times in a continuous stirred tank reactor. Because of the autocatalytic behaviour of the microbial processes and the low carbon incorporation rate which results in low biomass concentrations, decoupling of residence times for substrate and biomass in continuously operated systems is necessary. In this way high space time yields can be achieved under anaerobic conditions as well. From the well known variants for biomass retention, immobilisation by an inert support is thought to be an effective alternative. Experiments on anaerobic treatment of sulfite evaporator condensate by means of immobilized microorganisms were carried out in a 121-fixed bed loop reactor. Porous sinter glass from SCHOTT Mainz with a porosity of about 60 % and a mean pore diameter of 60-100 μm served as carrier. The reactor was operated for several months under steady state conditions at different residence times using an iterative approach. At a residence time of 11 h 84 % of the chemical oxygen demand (COD) was eliminated at loading rates up to 100 kg COD/m3 and day. Online data acquisition and process control are recommended and easily realized by means of a digital computer, in order to increase selection stress on microorganism population and optimize space time yield.