Managing Dissolved Oxygen in Activated Sludge Plants

Sustaining optimal dissolved oxygen levels in activated sludge plants is necessary for biological treatment of organic material and ammonia. While raw wastewater often contains some amounts of oxygen, aeration systems can increase dissolved oxygen (DO), mixing, and the suspension of microbes through mechanical agitation or diffused aeration. Aerobic microorganisms use this oxygen to breakdown organic waste into inorganic byproducts. The amount of dissolved oxygen consumed by microbes during biological treatment is referred to as biochemical oxygen demand (BOD). According to an article by Triplepoint Water, approximately 1.5 pounds of oxygen is consumed for every pound of BOD oxidized. To oxidize one pound of ammonia, that value increases to 4.57 pounds of oxygen. Most plants aim to maintain around 2 mg/L of DO which allows microbes contained within the center of floc to receive oxygen.

Wastewater operators should regularly monitor oxygen availability in the form of dissolved oxygen. Insufficient oxygen levels will allow aerobic and nitrifying microbes to die and floc to break up. At DO concentrations under 1 mg/L, the potential for filamentous growth increases. On the other end of the spectrum, too much oxygen increases power consumption and, at very high levels, inhibits settling. Research has estimated that aeration can use up to 45 to 75% of a treatment facility’s overall electricity use. With an online DO analyzer equipped to automated controls, the EPA reports that energy costs can be reduced by as much as 50%.

Where and when an operator samples for DO will be determined by the requirements written in the facility’s National Pollutant Discharge Elimination System (NPDES) permit and basic process control. To compare dissolved oxygen levels throughout the day, samples should be collected at the same location. The Ohio EPA’s Activated Sludge Process Control and Troubleshooting Chart Methodology recommends that systems sample within 1-2 feet of the surface water near the discharge of the aeration tank into the clarifier. By collecting multiple samples in the same location throughout the week, operators can reliably determine if DO concentrations are sufficient for treatment while developing a DO profile. In addition, measuring DO at multiple depths and locations in the aeration tank can help find dead spots.  

To supply adequate DO, the Ohio EPA manual includes how to determine blower runtime based on organic loading and system design. We should  still note that temperature, pressure, and salinity can all influence the solubility of oxygen. Additional sampling locations can include the raw wastewater, aerobic/ anaerobic digester, and final effluent. Final effluent with high dissolved oxygen can cause eutrophication in the receiving waters, however low DO can harm aquatic organisms. Some permits set a minimum DO level for effluent to ensure aquatic organisms have the necessary oxygen levels to sustain life.

While every technique and tool has its strengths and weaknesses, operators can measure DO through a Winkler Titration test (see Michigan DEQ Laboratory Training Manual pg.91), electrochemical sensor, or optic sensor. The two sensors mentioned can be purchased as portable handheld meters or stationary devices. For automated blower control and continuous sampling, an online sensor is used. For NPDES compliance monitoring, measurements must be taken through an EPA approved method at the frequency specified in the permit.

When using any DO sensor, the EPA’s Field Measurement of Dissolved Oxygen (SESDPROC-106) procedures require that the equipment be well maintained and operated per manufacturer instructions. Upon initial purchase, probes should be inspected, calibrated, and verified for accuracy. During each additional use the instrument should be calibrated and inspected again. The EPA recommends checking instrument calibration and linearity using at least three dissolved oxygen standards annually. All maintenance and sampling activities should be documented in a logbook per NPDES requirements. Any time a measurement is taken, the temperature of the water and any notable wastewater conditions should also be recorded in the logbook. 

Dissolved oxygen is a frequently monitored parameter in wastewater treatment systems. Operators should have a firm understanding of how dissolved oxygen is involved in wastewater processes and how they can manage DO to achieve compliance. Check out our online document library to find useful resources to learn more.