For the past year a semi-scale pilot plant for the anaerobic pretreatment of potato starch wastewater has been in operation. A reactor of 4 m3 was equiped with a settler and a so called membrane thickener to reduce the amount of sludge which has to pass the membrane. According to other publications about membrane supported anaerobic digesters, organic loadings of more than 6 kg COD/m3d rarely have been achieved by this reactor. A loss of 50% of activity was observed after circulating the sludge 20 times and a 90% loss within 100 cycles. It can be shown that the symbiosis between acedogenic and methanogenic bacteria is the most sensitive part. These observations imply that a ‘gentle’ transport of sludge through the membrane module must be developed, if membrane filtration at anaerobic reactors is to be further utilized. The results for an aerobic batch reactor implied that parallel to the destruction of sludge flocs due to the unsuitable pumps or to high transmembrane pressure, a significant decrease of the specific activity will take place.