Abstract: Total ammonia nitrogen in a volume of sludge in an aeration tank is measured and compared to a target ammonia nitrogen setpoint to calculate an airflow target for adjusting the measured total ammonia nitrogen toward the target ammonia nitrogen setpoint. The airflow target may be an airflow rate or, for an SBR, an airflow duration, and airflow into the aeration tank is adjusted according to the airflow target. A target solids retention time setpoint is also calculated from information relating to the airflow target and the solids retention time is adjusted toward the target solids retention time setpoint. The waste activated sludge flow rate for the aeration tank may be adjusted to adjust the solids retention time for the aeration tank toward the target solids retention time setpoint.