A full-scale upflow anaerobic sludge blanket (UASB) reactor treating traditional opaque beer brewery wastewater recently installed at an opaque beer factory was studied for 2 years. The total volume of the reactor was 500 m(3) and the hydraulic retention time was approximately 24 h. The aim of the study was to evaluate the performance of the UASB reactor during anaerobic digestion of opaque beer brewery wastewater in terms of treatment efficiency. The untreated opaque beer wastewater has high solids content and high organic matter, which need pretreatment before it is discharged into municipal sewage treatment works. The UASB reactor enables the brewery to meet the requirements of the wastewater discharged into municipal sewerage system of Harare. The average percentage reduction in Chemical Oxygen Demand (COD) was 57%. The total and settleable solids were also reduced by 50 and 90%, respectively. The effluent from the UASB reactor contained higher orthophosphates and nitrogen levels than the influent leading to the accumulation of these nutrients in the system. These results indicated that the UASB plant was effective for treating opaque beer brewery wastewater at ambient temperature to meet the quality of effluent that can be discharged into public water works. (C) 2004 Elsevier Ltd. All rights reserved.