The purposes of this study are to clarify the behavior of phosphorus in coagulation/sedimentation process, and to evaluate the effects of phosphorus addition into biological activated carbon (BAC) treatment on the biodegradation of organic substances. Conventional coagulation/sedimentation reduced phosphorus concentration to very low level, that is, 0.002–0.004 mgP.l−1 in water containing less than 0.063 mgP.l−1. In continuous experiment, the biodegradation rate of glucose in the BAC with adsorbed phosphorus before the start of operation was 5 times higher than that in the BAC without adsorbed phosphorus. The rate of increase in bacterial population was higher in the BAC with adsorbed phosphorus compared to the BAC without adsorbed phosphorus. The biodegradation rate of glucose in the BAC without adsorbed phosphorus increased significantly by addition of phosphorus into influent. Therefore, growth and biodegradation activity of attached bacteria on BAC was limited by phosphorus of low concentration in the influent treated by coagulation/sedimentation. Adsorption of phosphorus on activated carbon before the start of operation and/or addition of phosphorus in influent will be effective to improve the biological activity on BAC.