Dissolved Oxygen Measurement

Why DO is measured

Aerobic biological treatment needs oxygen in the water for the bacteria that consume organic matter and oxidize ammonia. Too little and treatment suffers; too much and the blowers, which are typically the largest energy consumer at a wastewater plant, run harder than needed. Aeration control holds a dissolved oxygen setpoint, commonly around 2 mg/L in a conventional activated sludge basin and lower in processes that manage nitrogen, by modulating air supply. The DO measurement is the input to that loop and the largest lever on plant energy cost, which is why it deserves better than a sensor that was installed once and forgotten.

Sensor types

For continuous aeration control, optical sensors are now the default. Membrane sensors remain on portable meters and in older installations, and the maintenance they need is the reason many DO control loops were left in manual for years.

Calibration

A DO sensor is calibrated at one point, in water-saturated air, where the oxygen partial pressure is known from the barometric pressure and the temperature. The sensor reads 100 percent saturation at that condition, and the transmitter converts to concentration using the temperature, the pressure, and the salinity of the process water.

1. 1

   Clean the sensor

   Remove the cap guard, rinse, and clean the sensing surface as the manufacturer directs. Calibrating a fouled sensor stores the fouling as the calibration.

2. 2

   Place it in saturated air

   In the calibration cup with a wet sponge, or held in air just above the water surface, out of direct sun, and let it equilibrate to the air temperature for several minutes.

3. 3

   Enter the barometric pressure

   From a local barometer or the transmitter sensor, in the units the transmitter expects. Sea level is 760 mm Hg; a plant at 1,500 m elevation is near 635 mm Hg, and a calibration at the wrong value is wrong by the ratio.

4. 4

   Accept the calibration

   The transmitter sets 100 percent saturation. Check the reported slope or gain against the healthy range in the manual.

5. 5

   Set salinity if needed

   Fresh wastewater is near zero. Brackish or seawater processes need the value, which changes the concentration by several percent.

6. 6

   Verify in process

   Compare with a calibrated portable meter in the same location at the same depth. Agreement within a few tenths of a mg/L is expected.

7. 7

   Record it

   Date, pressure, temperature, slope, comparison, and cap age.

Placement in an aeration basin

An aeration basin is not uniform. DO is highest near the diffusers and at the surface, lowest in dead zones and near the inlet where the oxygen demand is greatest. A plug-flow basin has a DO profile along its length; a complete-mix basin still has gradients. The sensor location decides what the control loop sees, and it is chosen for the control objective.

• For control of a plug-flow basin, one sensor per control zone, at roughly two-thirds of the way through the zone, where the DO represents the zone rather than the inlet demand or the outlet recovery.
• Depth of 1 to 2 m below the surface, away from the surface layer that is aerated by contact with air, and away from a diffuser grid where a bubble plume reads high.
• On a rail or a swing arm that lets the sensor be lifted to the walkway for cleaning without a boat or a crane.
• Away from the inlet mixing zone and the return activated sludge entry, unless the objective is to measure there.
• Two sensors in a zone where the control matters most, so that a fouled sensor is detected by disagreement rather than by a process upset.

Fouling and cleaning

Everything in an aeration basin grows a biofilm, and a coated DO sensor reads low, because the film consumes oxygen at the surface, and slowly. The control loop responds to the low reading by adding air, and the plant over-aerates until someone notices. Cleaning is therefore part of the design, not a maintenance afterthought.

Automatic cleaning

An air blast from the plant air supply, on a timer of every 15 to 60 minutes, or a mechanical wiper on the sensor face. Both extend the manual cleaning interval from days to weeks.

Manual cleaning

On a schedule, typically weekly to monthly depending on the basin, with a soft cloth and water, and a dilute acid for scale where the water is hard. Abrasives damage optical caps.

Cleaning detection

Trend the DO reading and its response to the cleaning cycle. A sensor whose reading jumps up after each cleaning was fouled before it; shorten the interval.

Cap and membrane replacement

Optical caps on the manufacturer interval, usually a year; membranes when the slope falls or the response slows.

Using DO for control

DO control is a slow loop with significant dead time. The sensor itself takes 30 to 90 seconds to respond, the basin takes minutes to change its DO after the air changes, and the blower or valve has its own dynamics. The controller is tuned for that: a slow proportional-integral loop with an execution interval of seconds to a minute, output limits that respect the blower minimum turndown and the diffuser minimum airflow, and a large deadband around setpoint so that the blower does not hunt. Cascade control, in which the DO loop sets an airflow setpoint that an inner flow loop holds, handles the blower and valve nonlinearities better than DO driving the blower directly. The DO signal is validated: a sensor reading that does not respond to an air change for several minutes, or a pair of sensors that disagree beyond a limit, drops the loop to a safe fixed airflow and alarms.