Successive systems using yeast and activated sludge (AS) were developed to treat monosodium glutamate manufacturing wastewater (MSGW). The yeast system allowed over 80% removal of chemical oxygen demand (COD) and a rise of pH from 2.5 to 6.5 on treating MSGW directly (COD 25,000 mg/l and NH4+-N 19,000 mg/l). Observation of the microbial community using a scanning electron microscope indicated that the two species of yeast (Candida halophila and Rhodotorula glutinis) were predominant in the biofilm reactor during 2 months of operation. The suspended solids (SS) of effluents from the yeast reactor were mainly composed of yeasts and fermentative bacteria used in glutamate production. This part of SS contained 55.8% protein and 18 amino acids and could be utilized as a source of single cell protein (SCP) as animal food additive. The effluent from the yeast system was fed into the activated sludge system after NH4+-N was reduced to a level of 1000 mg/l through air stripping. The activated sludge system could remove 50-70% of the remaining COD further, and the effluent COD and SS were constantly below 1300 and 70 mg/l, respectively. The combined system biologically removed about 95% COD from MSGW, and the COD could be further reduced to below 360 mg/l with coagulation under an FeCl3 dose of 1400 mg/Fe 1. (c) 2004 Elsevier Ltd. All rights reserved.