Abstract: A method and apparatus for operating an activated sludge plant having a plurality of tandem aeration zones, each receiving mixed liquor from an upstream zone or an upstream source and discharging a mixed liquor to a downstream zone or a downstream process includes a control which determines a parameter at a downstream one of the zones. The parameter is representative of a concentration of ammonia in the mixed liquor in the downstream one of the zones and may be used to control at least one upstream zone. A value of airflow to one of the zones may be determined and used to determine a demand for dissolved oxygen in the mixed liquor in that zone as a function of airflow to that zone. An elevated level of demand may be used to indicate a dump of commercial waste having a high BOD demand. A depressed level of demand may be used to indicate the presence of chemicals that inhibit bacterial respiration.