Processing Magazine

What are process controls for dissolved oxygen during biological treatment?

November 5, 2013

Dissolved oxygen (DO) is defined in biological treatment as the relative measure of oxygen dissolved in wastewater available to sustain life, including living bacteria. Biological treatment is defined as an aerobic activated-sludge process in the aeration system for treating sewage and industrial wastewater, using air to supply dissolved oxygen and a biological floc composed of organisms which are living bacteria.

Oxygen is needed by living organisms as they oxidize wastes to obtain energy for growth. Therefore controlling oxygen is required for secondary or biological treatment of wastewater. Dissolved oxygen control processes for activated sludge in the aeration system and also effluent discharge, reclamation & reuse include the following:

Ambient conditions, as well as wastewater conditions, entering activated sludge affect dissolved oxygen control processes in the aeration system. Proper aeration provides a supply of oxygen, a means of dissolving the oxygen into the activated sludge, and a means of keeping microorganisms from settling out of the mixed liquor.

Mixing can be accomplished in many cases with just the action of adding air to the process — if adequately distributed. Note that in high-purity oxygen processes, however, the oxygen flow rate is insufficient for adequate mixing and some type of mechanical mixing is also required. When low Biochemical Oxygen Demand (BOD) wastewater is treated, the minimum air flow rate is often based on mixing rather than dissolved oxygen requirements. This is true for both mechanically aerated and diffused air systems. Typically, oxygen requirements are met when the mixed-liquor dissolved oxygenis at 2 mg/L or more. However, optimal dissolved-oxygenranges can vary significantly from facility to facility. Lowdissolved-oxygenconditions have been observed in high-purity oxygen processes, with dissolved-oxygenlevels as high as 10 to 12 mg/L. It is best to experiment with your system to determine what dissolved-oxygen concentration is adequate.

Substantial presences

Indicators of low dissolved-oxygen conditions include substantial presence of low dissolved-oxygen filamentous bacteria in the activated sludge, turbid effluent, or dark gray or black-colored mixed liquor (often with a putrid odor). The first indicator of low dissolved-oxygen conditions will be the growth of low dissolved-oxygen filamentous microorganisms.

As the dissolved oxygen drops, the quantity of these filamentous microorganisms increases, adversely affecting the settle-ability of the activated sludge. As an operator, it is important to recognize these early warning signs and make corrections to dissolved-oxygen levels before the quality of the effluent deteriorates. If dissolved oxygen continues to drop, even low dissolved-oxygen filamentous microorganisms will not be present in the mixed liquor, and treatment efficiencies will be seriously affected. At this point, effluent turbidity will increase and treatment will deteriorate rapidly.

Under severe conditions, mixed liquor may turn a dark gray or even black color and putrid odors may also be present. Visual observations are good as indicators, but actual measurements of both activated sludge dissolved oxygen and effluent water quality should be taken before a determination of cause is made; for example, the black color may be the result of a dye from an industrial discharger.

The key to avoiding low dissolved-oxygen conditions is to properly monitor your aeration system. A properly monitored aeration system includes a dissolved-oxygen profile of the entire aeration system. A profile merely means measuring the dissolved oxygen in different locations and at different depths throughout the aeration system.

Measuring what’s dissolved

Dissolved oxygen can be measured with stationary or portable devices. Only well-maintained and adequately calibrated dissolved-oxygen meters should be used. Dissolved-oxygen meters often come with electronic zeros for calibration. The accuracy of this zero-calibration feature should be checked with an absolute zero solution, composed of a saturated solution of sodium sulfite (see procedure in the EPA approved manual of Analytical “Standard Methods” for Drinking Water and Wastewater). As with any equipment, the manufacturer's instructions should be followed. Some plants use online dissolved-oxygen analyzers to control air flow. Because these systems only measure dissolved oxygen in one location, additional dissolved-oxygen readings should be taken daily in biological reactors at various locations and depths to ensure that there are no dead spots. Dissolved-oxygen meters must be calibrated every time they are used.

Good airflow not only ensures good dissolved-oxygen concentration, but also good mixing. Reactors should be checked periodically to ensure that all portions of the reactor are being adequately mixed and that there are no dead spots that can produce anaerobic conditions. A procedure to do this is to take mixed-liquor grab samples at different locations and depths in the biological reactors and check for consistent suspended solids levels.

Additional considerations

The dissolved-oxygen level for aeration is dependent on type of treatment and “risk” of low dissolved oxygen impairing performance of aerobic microorganisms. To know that a system is fully mixed and aerobic, dissolved oxygen of 2.0 is a good target — the lower the dissolved oxygen, the more efficient is oxygen transfer. The aerobic organisms — which remove more organic matter — actually are effective as long as they have any oxygen but you risk “dead zones” at the low levels. Most treatment organisms survive but persist in the more inefficient anaerobic mode, i.e., facultative organisms can live in either condition.

Higher dissolved oxygen is often a target but in reality this is for assurance of mixing. If dissolved oxygen is 5.0 or higher there is good comfort that dead zones are minimal since normal currents and mixing will transport the oxygenated mixed liquor throughout the equipment. But if the higher Dissolved Oxygen is by excessive mechanical — or less likely excessive blowers — then there could be a settling problem due to shearing of floc and re-suspension of inert materials.

Dissolved-oxygen effluent at the secondary clarifier is primarily affected by the dissolved-oxygen control processes during the activated-sludge process. Certain general steps for controlling dissolved oxygen have been described above. However, if you have specific dissolved oxygen or other wastewater queries, please submit a question.