The report consists mainly of a compilation of sixteen research papers and publications of work accomplished during the four-year period. Work on mechanism and kinetics of waste water purification was conducted in batch and in completely mixed continuous flow units. It was also necessary to conduct studies on various aspects of research methodology and technique. Reported in detail are investigations on the use of a colorimetric method for determination of COD, the use of COD values of bacterial sludges in waste water purification research, and a statistical study of the spot plate method for viable cell count. A considerable research effort was also made concerning the kinetics and mechanism of exertion of biochemical oxygen demand in both dilute and high energy systems. The effect of sludge concentration on various aspects of biological activity and the mechanism and kinetics of substrate utilization at high biological solids concentration were investigated.