Low Dissolved Oxygen in Aeration Tank: Why MLSS Dropped Despite Running Blowers

In many wastewater treatment plants, operators feel that if the blowers are functioning, oxygen levels are sufficient. But this isn’t always the case. We recently had a situation where the biomass activity declined rapidly because of the low dissolved oxygen condition of the aeration tank. What was shocking was that the blowers were running all the time.

The outcome was an unanticipated MLSS reduction problem in the aeration tank that compromised treatment efficacy and raised concerns about the biological system’s health. 

Understanding Low Dissolved Oxygen in Aeration Tank

Low dissolved oxygen in aeration tank circumstances occurs when the amount of oxygen delivered to the system is less than the amount of oxygen required by the microorganism. Low oxygen in the aeration tank may be due to inadequate distribution or heavy organic loading even when the blowers are working well.

The microorganisms that break down contaminants rely largely on oxygen. A good activated sludge process requires a continuous oxygen demand by the aerobic bacteria.

When oxygen levels stay low for a long time, this whole biological treatment procedure starts to get into trouble. 

Why Is Dissolved Oxygen Low in Aeration Tank?

Many treatment plant operators pose the question, “Why is dissolved oxygen low in aeration tank?” The answer is usually far more complicated than a bad blower.

Common causes: 

• Increased influent organic load

• Clogged or damaged diffusers

• Poor mixing inside the tank

• High MLSS concentrations

• Inefficient blower performance

• Reduced oxygen transfer efficiency

• Inadequate dissolved oxygen control ETP practices

  
  

Even with blowers operating, these factors can result in low DO conditions in biological treatment processes. 

What Happens When DO Drops Below 2 ppm?

One common question is, “what if DO drops below 2 ppm?”

When dissolved oxygen levels fall below the optimum range, a number of problems start to occur: 

Decreased Microbial Activity

Oxygen is necessary for healthy microbes to survive. The oxygen requirement of aerobic bacteria is usually fulfilled by appropriate aeration.

When there is not enough oxygen, microbial respiration wastewater processes slow down considerably.

Falling Treatment Performance

Dissolved oxygen wastewater treatment efficiency is affected by low oxygen. Lower removal of organic materials, resulting in poor effluent quality.

Reduction of MLSS

We have a lot of operators asking “can low DO cause MLSS reduction?”

The answer is yes. Long-term oxygen deficiencies may deteriorate the conditions of the MLSS aeration tank and decrease the activity of biomass. In extreme circumstances, the microorganisms start dying and so the overall MLSS concentration drops.

Poor Aeration Basin Operation

Reduction of oxygen supply immediately impacts the operation of the aeration basin. Even with continual running of the blower the system fails to reach treatment targets. 

Signs of Oxygen Deficiency in Activated Sludge

If you can identify the indicators of oxygen deficit in the activated sludge early enough, you can avoid serious operational issues.

Check out for the following signs: 

• Sudden MLSS decline aeration tank trends

• Rising COD and BOD levels in treated water

• Foaming or unusual sludge appearance

• Septic odors from the aeration tank

• Reduced settling characteristics

• Increased sludge bulking incidents

• Lower efficiency in dissolved oxygen wastewater treatment

  
  

These indicators of caution are often signs of need for oxygen in aeration tanks which must be addressed immediately. 

The Link Between Activated Sludge Oxygen Demand and DO Levels

Each activated sludge system has its own activated sludge oxygen demand. As the influent loading grows, bacteria use more oxygen.

If the oxygen supply does not rise correspondingly, low dissolved oxygen in aeration tank conditions emerge.

For stable functioning, the oxygen requirement of activated sludge should be balanced with the oxygen availability.

That’s why ETP strategies that are effective in controlling dissolved oxygen are important for treatment plants. 

How to Improve Dissolved Oxygen in Wastewater Treatment

Many operators ask, "how to improve dissolved oxygen in wastewater treatment?"

Several practical steps can help:

Optimize Aeration Equipment

Regular inspection of blowers and diffusers helps maintain better oxygen transfer efficiency.

Monitor DO Continuously

Real-time monitoring supports effective dissolved oxygen control ETP practices and prevents sudden oxygen depletion.

Manage Organic Loading

Excessive loading increases activated sludge oxygen demand, making it difficult to sustain proper DO levels.

Improve Oxygen Availability

Enhancing dissolved oxygen wastewater treatment processes can support healthier biomass and better plant stability.

A Practical Solution for Low DO Biological Treatment Challenges

One of the first responses to chronic low DO biological treatment problems by operators is to increase oxygen availability.

That’s where products like NutriServe Oxy Enhancer may really help you.

NutriServe Oxy Enhancer is an oxygen enhancer that assists biological treatment processes by increasing the availability of oxygen in the system. It helps treatment facilities tackle issues related to low dissolved oxygen levels in aeration tank conditions without large infrastructure upgrades. 

Benefits of NutriServe Oxy Enhancer

• Supports improved oxygen transfer efficiency

• Enhances overall aeration basin performance

• Assists in maintaining healthy microbial populations

• Helps meet aerobic bacteria oxygen requirement

• Supports stable microbial respiration wastewater activity

• Contributes to effective dissolved oxygen wastewater treatment

• Reduces the operational impact of aeration tank oxygen deficiency

  
  

NutriServe Oxy Enhancer can be a useful addition to the treatment program for plants that are subject to recurrent oxygen constraints. 

Conclusion

It can be frustrating to see low dissolved oxygen in the aeration tank settings with blowers working. But the operation of the blower alone does not provide an adequate oxygen supply.

Poor oxygen transfer efficiency, rise in activated sludge oxygen demand and inadequate dissolved oxygen management ETP methods might cause issues in low DO biological treatment.

If you have been asking questions like “can low DO cause MLSS reduction?”, “why is dissolved oxygen low in aeration tank?” or “what happens when DO drops below 2 ppm?”, then the solution frequently rests in knowing the link between oxygen availability and microbial health. 

Understanding the indicators of oxygen deficit in activated sludge, and enhancing dissolved oxygen wastewater treatment techniques, can help treatment plants return to stable operations and prevent additional MLSS decline aeration tank difficulties. Adding supportive solutions such as NutriServe Oxy Enhancer can help.

A key to a healthy biological system is the correct balance of oxygen. The better the treatment, the more oxygen needs are met, the more the microbes flourish, and the more reliable the overall wastewater treatment process becomes.

Improve Oxygen Availability for Healthier Biomass

Running blowers doesn't always mean your aeration tank has enough dissolved oxygen. Poor oxygen transfer can reduce microbial activity, lower MLSS, and impact BOD/COD removal. Improving oxygen availability helps restore biological performance, stabilize treatment, and keep your plant operating efficiently.

Improve Dissolved Oxygen Performance