The objective of the study was to investigate the fate and effects of diquat dibromide which is the active ingredient in formulations used to control the growth of roots into sewers when applied as Razorooter and mixed with raw sewage, settled sewage, and activated sludge, and when introduced into activated sludge wastewater treatment systems. Both fully aerobic and biological nutrient removal (BNR) activated sludge systems were used for experimental purpose, and both continuous flow and batch reactors were used. The sorption of diquat by both raw sewage particles and activated sludge suspended solids was determined. Diquat dibromide concentrations ranged from 0.93 to 12.6 mg/L in the influent flow. Both the fully aerobic and two full biological nutrient removal systems were fed municipal sewage spiked with diquat dibromide, and operated at a mixed liquor temperature of 10degreesC and an MCRT of 10 days. One of the BNR systems was a control system. The results showed that only about 20% of the diquat in raw sewage flow was removed by adsorptions to the sewage solids, but 80% or more of the diquat was removed in activated sludge systems. When the influent diquat dibromide concentration was approximately I mg/L, over 99% of the diquat dibromide was removed by the activated sludge process. Some of the removal was believed to be by biodegradation. The diquat dibromide used in this study had no observable detrimental effects on any of the biological processes of the continuous flow fully aerobic and BNR activated systems.