Breakdown of organic wastes entering a wastewater treatment plant is accomplished by using a biomass or blend of beneficial microscopic organisms, bacteria, and solids. This converts the non-settleable solids (dissolved and colloidal matter) into settleable solids, carbon dioxide, water, and energy. The biomass must be kept alive by maintaining the proper concentration of dissolved oxygen throughout the treatment process.
When not enough dissolved oxygen is present, at least some of the biomass will die, and the organic wastes will not be properly treated. If this happens, the plant may have to replace the beneficial "bugs" which, in most cases, proves to be very expensive and time consuming. Conversely, when there is too much dissolved oxygen, usually from over aerating, the treatment plant is needlessly wasting costly energy.
The "starred" areas in Figure 1 below show where typical applications for measuring and controlling dissolved oxygen are found in various stages of a wastewater treatment plant.