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Low Dissolved Oxygen in Aeration Tank: Why MLSS Dropped Despite Running Blowers
Updated: Sep 15
Overview
A running blower does not always mean microorganisms are receiving enough oxygen. In this aeration tank, bubbles were visible, but dissolved oxygen remained low and the active biomass started weakening. Poor oxygen transfer, blocked diffusers or a sudden rise in BOD/COD load can create an aeration tank oxygen deficiency even when aeration equipment is running. This blog explains what causes low dissolved oxygen in aeration tank conditions, how low DO affects MLSS and microbial activity, and why DO should be measured at different tank locations. It also covers the checks operators should make before using supplemental oxygen to support biological treatment.
Table of Contents
The blowers had not stopped. Bubbles were still rising through the aeration tank. Yet the biological treatment was getting weaker.
Our MLSS readings and sludge condition told us something was wrong.
The problem became clearer when we measured dissolved oxygen. Air was entering the tank, but enough oxygen was not reaching the microorganisms.
This is where low dissolved oxygen in aeration tank conditions can be misleading. Seeing bubbles does not tell you how much usable oxygen is actually dissolved in the wastewater.
So why can this happen when the blowers are still running?
Why Is Dissolved Oxygen Low in an Aeration Tank Despite Continuous Aeration?
When asking why dissolved oxygen is low in aeration tank, checking whether the blower is ON is not enough.
The oxygen carried by air bubbles must pass into the wastewater before bacteria can use it.
How well this happens is called oxygen transfer efficiency.
Several problems can reduce it:
diffusers may be dirty or blocked;
air may not be spread evenly across the tank;
wastewater temperature may affect oxygen levels;
organic loading may suddenly rise;
oxygen may be consumed faster than it is supplied.
Any of these can cause aeration tank oxygen deficiency while the blower continues to operate.
Why Does Low DO Affect the Biomass?
Aerobic microorganisms use oxygen while breaking down biodegradable organic matter.
This process is part of microbial respiration wastewater treatment. When there is enough oxygen, the microorganisms can remain active and continue treating the organic load.
When oxygen becomes limited, microbial activity can fall.
Prolonged low DO biological treatment conditions can weaken the active biomass and affect COD and BOD removal.
This was why simply looking at the blower gave us the wrong picture. We needed to look at the oxygen available inside the water.
Can Low DO Cause MLSS Reduction?
A common question is: can low DO cause MLSS reduction?
Long periods of oxygen shortage can contribute to a decline in healthy aerobic biomass. But MLSS needs to be interpreted carefully.
MLSS, or Mixed Liquor Suspended Solids, includes living microorganisms and other suspended biological solids. A lower MLSS reading does not automatically prove that bacteria have died.
Sludge washout, excessive sludge wasting and poor settling can also cause an MLSS decline aeration tank operators notice.
Checking MLVSS along with MLSS can give a better picture of the active organic fraction of the sludge.
What Happens When DO Drops Below 2 ppm?
Operators often want to know what happens when DO drops below 2 ppm.
There is no single DO level that is correct for every biological treatment plant. Around 2 mg/L is often used as an operating reference in activated sludge treatment, but actual requirements vary.
When oxygen becomes too low for the process load, microorganisms may struggle to meet the activated sludge oxygen demand.
Nitrifying bacteria can also become affected when oxygen is limited.
The signs of oxygen deficiency in activated sludge may include poor treatment, unusual odour, changes in sludge colour and settling problems.
Why Increasing Blower Output May Not Be Enough
More air does not always mean better aeration basin performance.
If diffusers are fouled, much of the supplied air may not transfer oxygen well. If a sudden BOD or COD load enters the tank, microorganisms may consume oxygen faster than the aeration system can replace it.
This makes dissolved oxygen control ETP work more than a blower setting.
Operators need to compare oxygen supply with actual oxygen demand.
What Should You Check When DO Keeps Falling?
Before changing treatment, we checked the main conditions affecting oxygen:
Check | What it tells you |
DO at different tank points | Whether low-oxygen zones exist |
Influent BOD/COD | Whether oxygen demand has increased |
Diffuser condition | Whether air transfer is restricted |
Blower output | Whether enough air is being supplied |
MLSS and MLVSS | Biomass quantity and organic fraction |
Sludge settling | Whether biomass is being lost |
Measuring DO at several points is especially useful. One reading may miss a poorly aerated part of the tank.
How to Improve Dissolved Oxygen in Wastewater Treatment
When deciding how to improve dissolved oxygen in wastewater treatment, start with the cause.
Blocked diffusers should be cleaned. Air leaks need repair. Blower output and air distribution should be checked. Sudden organic loading should also be investigated.
When the aeration equipment is working but oxygen demand temporarily exceeds supply, NutriServe Oxy Enhancer can be considered.
It is an oxygen-releasing powder intended to provide supplemental oxygen when DO is low or oxygen demand rises.
When Does Supplemental Oxygen Help?
Supplemental oxygen makes sense when oxygen shortage has been confirmed and the existing aeration system needs temporary support.
NutriServe Oxy Enhancer can be used in aeration tanks such as ASP, MBBR, SBR and MBR systems.
The required amount depends on system volume, oxygen demand and current plant conditions.
It should not be used to hide a faulty blower, blocked diffuser or another mechanical problem.
Conclusion
The biggest mistake in this case was assuming that visible bubbles meant enough oxygen was reaching the biomass.
They did not.
Low dissolved oxygen in aeration tank conditions can develop even while blowers are running because oxygen transfer and oxygen demand matter just as much as air supply.
If an MLSS decline aeration tank problem appears at the same time, check DO, MLVSS, sludge settling, organic loading and aeration equipment before deciding what has happened to the biomass.
Good dissolved oxygen wastewater treatment control starts with measuring what is happening inside the water, rather than judging aeration from the tank surface.
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, stabilise treatment, and keep your plant operating efficiently.
