The ability to remove nitrogen and phosphorus by biological means in the ac
tivated-sludge process represents the most significant refinement of the pr
ocess since its discovery. This paper reviews the development of the activa
ted-sludge process in Johannesburg. Emphasis is placed on the last twenty y
ears' research into biological nutrient removal and the successful incorpor
ation of research findings into the design and operation of full-scale plan
ts of up to 200 Ml/d capacity.