Dissolved oxygen is the parameter that defines whether aerobic biological treatment is working. The microorganisms responsible for breaking down organic matter in activated sludge and MBBR systems need oxygen to function. When dissolved oxygen falls below the level required to sustain aerobic activity, treatment performance deteriorates. When it is maintained well above what is needed, energy is wasted on unnecessary aeration. Dissolved oxygen monitoring makes it possible to manage aeration accurately, keeping biological processes performing effectively while avoiding energy waste.

DO monitoring systems measure the concentration of dissolved oxygen in the mixed liquor of biological treatment tanks using sensors that can be installed permanently in the tank or deployed on retractable holders that allow the sensor to be withdrawn for cleaning and calibration without taking the tank out of service.

Two main sensor technologies are used. Optical DO sensors, also called luminescent or fluorescent sensors, measure dissolved oxygen by exposing a sensor cap containing an oxygen-sensitive fluorophore to the sample and measuring the effect of oxygen concentration on the fluorescence signal. They are stable, low maintenance, and do not consume oxygen during measurement. Electrochemical sensors measure the current generated by the reduction of oxygen at a cathode, and while effective, they require more frequent membrane replacement and calibration than optical sensors.

The output from the DO sensor is used to control aeration blower output or valve positions to maintain dissolved oxygen within the target range for the biological process. In systems with automatic aeration control, the DO signal feeds directly into the control loop, enabling the aeration system to respond continuously to changes in oxygen demand caused by variations in influent load or temperature.

DO monitoring systems are used in sewage treatment plants, effluent treatment plants, and aquaculture systems where managing dissolved oxygen precisely is important to both process performance and energy efficiency.