A cell-based biosensor system was designed for monitoring an anaerobic process for treatment of high biochemical oxygen demand (BOD) levels in wastewater samples from a factory processing concentrated rubber latex. The BOD biosensor used immobilized mixed culture of microorganisms as the biological sensing element and an oxygen electrode as the transducer. The assay principle is based on the determination of the oxygen consumption rate caused by microbial respiration. Synthetic wastewater according to the OECD specifications was used as standard solution for calibration of the BOD biosensor. Response time of the sensor was 10-15 min. The BOD of the influent and the effluent from an anaerobic reactor was measured using both the cell-based biosensor system and a standard method (BOD5). Good agreement was achieved between the results from the two assay methods with a percentage difference of less than 10%. However, when exposing the mixed culture to wastewaters from other industrial plants the agreement between the results of the two assays was poor. The anaerobic treatment of the wastewater from the concentrated latex process resulted in a COD removal efficiency of 97% at a hydraulic retention time (HRT) of 50 days. The BOD biosensor was successfully applied to off-line and on-line monitoring of the anaerobic reactor treatment process. (c) 2008 Elsevier Inc. All rights reserved.