DO/Nmaster: machine-learning control of airflow, dissolved oxygen and ammonia
DO/Nmaster is a machine-learning based real-time control system for activated sludge aeration. It replaces the conventional PID control loops with control that learns from the plant's own operating data. For each aeration grid it calculates setpoints for airflow, dissolved oxygen and valve position and sends them to the plant's SCADA system. When ammonia-based aeration control is used, ammonia becomes the master control parameter and the dissolved oxygen setpoints are calculated from it.
Why precise aeration control matters
Aeration is the largest single energy use at most activated sludge plants. Holding dissolved oxygen higher than the process needs wastes energy; holding it too low risks nitrification and sludge settling. Lowering dissolved oxygen safely requires control that holds each setpoint closely, so that the setpoint itself can be set lower without risking ammonia limits.
Measured performance at Pomona (U.S. Department of Energy final report)
| Measure | Result |
|---|---|
| Airflow control error | 0.75% average, 0.25% median (24-hour test, setpoint changed every 3 seconds) |
| Dissolved oxygen control error | 0.06 mg/L average with DO/Nmaster, against 0.20 mg/L with tuned PID control |
| Blower energy | After new blowers had reduced blower energy by 34%, lowering dissolved oxygen with DO/Nmaster and SRTmaster reduced it by a further 30% |
| Nitrogen removal | Total inorganic nitrogen removal about 84%, before and after the change to low dissolved oxygen |
At the City of Chico, a 2018 control project built around DO/Nmaster reduced aeration energy per pound of oxygen demand by 47.6%, with the existing blowers, verified under PG&E's California Wastewater Process Optimization Program.
How it works with the plant's equipment
- DO/Nmaster exchanges data with the existing SCADA system using the standard OPC protocol. The plant's PLCs or DCS continue to operate the valves and blowers.
- It requires no new blowers. Accurate control depends on suitable instruments: fast airflow meters, modulating actuators designed for process control, and dissolved oxygen and ammonia analyzers. These can be bought from any supplier. Ekster & Associates sells none of them, but provides recommended performance specifications.
- DO/Nmaster and SRTmaster share one architecture. SRTmaster holds the sludge age on target, which keeps the biomass stable as dissolved oxygen is lowered.
Operators keep authority
Operators set the setpoints and control limits through the operator interface and can override DO/Nmaster at any time. All incoming data is checked before it is used; faulty readings are detected and not acted on, and operators are alerted. If the control system or its communication link stops, the plant's own control system responds according to the fallback logic programmed for that plant.
Where it is used
- Pomona Water Reclamation Plant, Los Angeles County Sanitation Districts
- City of Chico, California, since 2018
- Metro Water Recovery, Denver: under contract since September 2026