Abstract: A method and apparatus to automatically control the concentration of microorganisms in an activated sludge waste treatment system, consisting of establishing certain factors by laboratory measurements and field observations to determine the quantity of organisms or "sludge" to be removed during a predetermined period of time in order to maintain the proper microbial balance for most efficient operation of the system by the provision of automatic control means to remove a predetermined amount of mixed liquor directly from the aeration tank during such predetermined period of time, allowing the sludge to settle in a settling tank to thicken same, returning the excess water back to the aeration tank, and removing the predetermined amount of concentrated sludge from the settling tank during such predetermined period.