The feasibility of the expanded granular sludge bed (EGSB) system for the treatment of malting waste water under psychrophilic conditions was investigated by operating a pilot-scale 225·5 dm3 EGSB-reactor system in the temperature range from 13 to 20°C. The concentration of chemical oxygen demand (COD) in the malting waste water was between 282 and 1436 mg dm−3. The anaerobically biodegradable COD of the waste water was about 73%, as determined in the batch bioassays. During reactor operation at 16°C, the COD removal efficiencies averaged about 56%, at organic loading rates (OLR) ranging between 4·4 and 8·8 kg COD m−3 day−1 and a hydraulic retention time (HRT) of approximately 2·4 h. At 20°C, removal efficiencies were approximately 66% and 72%, respectively, at OLRs of 8·8 and 14·6 kg COD m−3 day−1, corresponding to HRTs of 2·4 and 1·5 h. The specific methanogenic activity with the sludge from the reactor, assessed on acetate and volatile fatty acids mixture as substrates, significantly increased (80%) in time, indicating an enrichment of methanogens and acetogens even at the low temperatures applied. These findings are of considerable practical importance because they indicate that anaerobic treatment of low strength waste waters at low temperature might become a feasible option. © 1997 SCI.