What MLSS Means
MLSS stands for:
👉 Mixed Liquor Suspended Solids
This tells me how much total solids are in the aeration basin.
That includes:
- microorganisms
- organic solids
- inorganic solids
What MLVSS Means
MLVSS stands for:
👉 Mixed Liquor Volatile Suspended Solids
This represents:
- mostly living biological material
In simple terms:
👉 the active bugs doing treatment
Why Comparing Them Matters
The ratio between MLSS and MLVSS tells me system health.
Typical activated sludge systems often have:
- MLVSS around 70–80% of MLSS
If volatile solids drop too low:
- biology may be weak
- excessive inorganic solids may exist
Example
If:
- MLSS = 3,000 mg/L
- MLVSS = 2,100 mg/L
Then:
That means:
👉 about 70% of solids are active volatile solids.
What I Watch Daily
I use MLSS and MLVSS trends to monitor:
- sludge age
- wasting effectiveness
- biological stability
Trends matter more than single numbers. If you want to connect those trends to loading, read F:M ratio explained for beginners next.
Final Thought
MLSS tells me how many solids I have.
MLVSS tells me how much of that is active biology.
Understanding both helps control the process much better.