(a) Solids Retention Time Control.(1) A wastewater treatment facility design must include equipment for a wastewater treatment facility operator to control the solids retention time (SRT) in the aeration tanks by wasting a measured volume of surplus activated sludge.(2) The engineering report and the operation and maintenance manual must provide the formulas used for determining the SRT.(3) The SRT required for nitrification applies to the aerobic portion of the wastewater treatment facility.(b) Aeration System Control. Aeration system control must regulate the total amount of air supplied and how air is distributed to the aeration tanks.(1) In order to conserve energy, a wastewater treatment facility design may provide the operator with the ability to adjust the airflow in proportion to the oxygen demand of the wastewater.(2) If an adjustable type of airflow control is installed, the aeration equipment must be manually adjustable over the entire range of oxygen demands and must maintain solids in suspension.(c) A wastewater treatment facility with a design flow greater than 0.4 million gallons per day must provide for totalized flow measurement of the return sludge and waste sludge discharges for process control.