The raw chess whey (CW) treatment capacity of a jet loop membrane bioreactor (JLMBR) was evaluated. Raw CW was first characterized for carbonaceous, nitrogenous and phosphorus compounds. The total COD range of the raw CW was between 73 and 86 kg m(-3) and 82% of the total COD was found to be soluble. The JLMBR system, of 321 capacity was operated continuously for 3 months with a sludge age of 1.1-2.8 days and COD loads of 3.5-33.5 kg-COD m(-3) per day. A treatment efficiency of 97% was obtained for 1.6 days of sludge age and COD loads of 22.2 kg-COD m(-3) per day. The sludge flocks observed in the system were highly motile, dispersed and had poor settling properties. The membrane filtration characteristics of this sludge were investigated and increasing MLSS concentration decreased membrane flux values. (C) 2003 Elsevier Ltd. All rights reserved.