What’s Your Activated Sludge Telling You?A Practical Guide to MLSS, MLVSS, and Activated Sludge Troubleshooting
The aeration basin is talking. Are you listening?
Before your final clarifier starts sending solids over the weir, your activated sludge may already be giving you clues that something isn't right.
Maybe the sludge isn't settling like it used to. Maybe the blanket is getting deeper. Maybe your MLSS keeps climbing even though the plant doesn't seem to be getting any better.
That's where MLSS and MLVSS can help.
These two laboratory tests are among the most useful tools an activated sludge operator has for figuring out what's happening inside the aeration basin.
Mixed Liquor Suspended Solids (MLSS) is the total concentration of suspended solids in your aeration basin.
It includes your microorganisms—but also dead cells, organic material, grit, and other suspended solids.
In simple terms:
MLSS tells you how much solids you have.
Too little biomass and you may not have enough microorganisms to handle the incoming load.
Too much, and you can create problems with sludge age, oxygen demand, settling, and clarifier capacity.
But don't get hung up on one "perfect" MLSS number. The trend is often more important than the number itself.
MLVSS: How Much of That Sludge Is Biological?
Mixed Liquor Volatile Suspended Solids (MLVSS) measures the portion of the MLSS that is volatile.
It's commonly used as an indicator of the organic and biological portion of your solids inventory.
For example:
MLSS = 3,000 mg/L
MLVSS = 2,400 mg/L
That's an MLVSS/MLSS ratio of 80%.
Now imagine your MLSS climbs to 3,500 mg/L—but your MLVSS barely changes.
You may not have more healthy biomass.
You may simply have more stuff in the tank.
That's a good reason to investigate.
What Does Healthy Activated Sludge Look Like?
Healthy sludge generally produces:
Well-formed floc
Good settling
Good compaction
Relatively clear supernatant
A manageable clarifier blanket
Stable MLSS and MLVSS
Good effluent quality
Look at a settleometer and you should see the sludge form a reasonably compact blanket rather than remaining suspended in the water column.
Put a sample under the microscope and you should see a biological community that makes sense for your process conditions.
But when things start going wrong, the sludge usually gives you clues.
When Your Sludge Starts Acting Strange
"My sludge won't settle."
You may be dealing with bulking, filamentous growth, poor floc formation, or another process problem.
Check:
SVI
DO
MLSS/MLVSS
Sludge age
F/M
Microscopic characteristics
Possible outcome: Adjust the process to improve floc formation and settling.
"My sludge is settling really fast—but something doesn't look right."
Very rapid settling isn't automatically good.
It can be associated with older sludge and other changes in the biological population.
Check your:
MLSS
MLVSS
Sludge age/MCRT
WAS rate
Clarifier blanket
Microscopic examination
Possible outcome: Adjust wasting to bring the biological solids inventory back toward the desired operating range.
"My MLSS keeps going up."
Don't automatically assume that more MLSS means more biological treatment.
Compare your MLSS and MLVSS.
If MLSS is increasing while MLVSS isn't keeping pace, investigate whether inorganic solids are accumulating.
Possible outcome: Identify and address the source of fixed solids rather than simply continuing to adjust wasting.
"My MLSS keeps dropping."
You may be wasting too much sludge, losing solids through the clarifier, or simply not producing enough biomass for the incoming load.
Look at:
WAS rate
RAS rate
MLSS/MLVSS
Effluent TSS
SVI
Influent loading
Possible outcome: Restore an adequate biological solids inventory.
The Clarifier Doesn't Lie
Your aeration basin and secondary clarifier are part of the same system.
If the mixed liquor isn't settling properly, the clarifier is eventually going to let you know.
Watch for:
Rising sludge blankets
Poor compaction
Floating sludge
Pin floc
Cloudy effluent
Increasing effluent TSS
When you see these conditions, don't just adjust the RAS pump and hope for the best.
Find out what the sludge is doing first.
What Should Operators Test?
A good activated sludge process-control program can include:
MLSS
How much suspended solids are in the aeration basin?
MLVSS
How much of those solids are volatile/organic?
Settleability & SVI
How well is the sludge settling and compacting?
Dissolved Oxygen
Are the microorganisms getting the oxygen they need?
RAS & WAS Solids
How much solids are being returned and wasted?
Sludge Blanket
Is the clarifier keeping up?
pH & Alkalinity
Are conditions suitable for biological treatment and nitrification?
Ammonia, Nitrate & Nitrite
Is nitrogen removal working as expected?
BOD/COD
How much "food" is entering the biological process?
Microscopic Examination
What are the microorganisms telling you?
Sometimes one laboratory test answers the question.
More often, the answer comes from looking at several results together.
Don't Chase Numbers. Watch the Story.
Suppose your plant has:
MLSS | SVI | |
Monday | 2,800 mg/L | 95 |
Wednesday | 2,950 mg/L | 115 |
Friday | 3,100 mg/L | 145 |
None of those MLSS numbers may look particularly alarming by themselves.
But the trend is telling you something.
Your solids are increasing—and your settling is getting worse.
That's the kind of warning you want to catch before the final clarifier starts sending sludge over the weir.
When in Doubt, Test the Sludge
Activated sludge problems can be frustrating because there are often several possible causes.
Should you:
Waste more?
Waste less?
Change RAS?
Increase aeration?
Look for filamentous growth?
Check alkalinity?
Guessing can make the problem worse.
Laboratory testing gives you something much better:
Data.
Let Environmental Labworks Help You Read the Sludge
When your activated sludge isn't behaving the way it normally does, bring us a sample.
Environmental Labworks can provide process-control testing for:
MLSS
MLVSS
Settleability / SVI
TSS
BOD
COD
Ammonia
Nitrate/nitrite
pH
Alkalinity
Microscopic examination
We can test mixed liquor, RAS, WAS, influent, and effluent samples to help you get a clearer picture of what's happening in your treatment process.
Your sludge is talking.
Let the laboratory help you understand what it's saying.
Environmental LabworksLaboratory data for better wastewater treatment.
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