Process Control
MLSS Control
Mixed liquor suspended solids (MLSS) is a core control parameter of the activated sludge process. Managing it — alongside F/M ratio and SRT — keeps treatment capacity and settling in balance.
What MLSS Is
MLSS is the concentration of suspended solids — mostly active biomass — in the aeration basin, measured in mg/L. Too low and the process lacks treatment capacity; too high and the secondary clarifier can be overloaded and settling suffers. The right value is always bounded by what the clarifier can settle.
How to Increase (or Lower) MLSS
To Raise MLSS
Reduce the wasting rate (less WAS). This lengthens the solids retention time and lets biomass accumulate. It responds gradually over several days — not instantly — so adjust in steps and monitor, always staying within the clarifier's settling capacity to avoid solids carryover.
To Lower MLSS
Increase the wasting rate (more WAS). This shortens SRT and reduces the biomass inventory. Again the change is gradual; over-wasting risks losing nitrification and destabilising the process, so confirm the move against a solids balance.
The Interlinked Parameters
Biomass Inventory
Suspended solids in the basin (mg/L)
Loading Ratio
Food-to-microorganism ratio
Sludge Age
Solids retention time (days)
For a given organic load, raising MLSS lowers the F/M ratio and lengthens SRT — favouring nitrification and stable operation. Lowering MLSS does the opposite. Effective control means managing these three together rather than any one in isolation.
MLSS Troubleshooting Matrix
| Condition | Potential Issue | What to Investigate |
|---|---|---|
| MLSS below target | Insufficient biomass | Reduce wasting; raise MLSS gradually within clarifier limits |
| MLSS above target | Clarifier overload risk | Increase wasting; check sludge blanket and SVI |
| Poor nitrification | SRT too short | Raise MLSS / lengthen SRT via reduced wasting |
| Solids carryover despite normal MLSS | Settleability (bulking) | Check SVI; see sludge bulking diagnosis |
| MLSS swings day to day | Erratic wasting control | Base wasting on a consistent solids balance |
Related Topics
Engineering Note
Frequently Asked Questions
MLSS (mixed liquor suspended solids) is the concentration of suspended solids — mostly active biomass — in the aeration basin, measured in mg/L. It is a core control parameter: too low and the process lacks treatment capacity, too high and the clarifier can be overloaded and settling suffers.
MLSS is raised by reducing the wasting rate (less WAS), which lengthens the solids retention time and lets biomass accumulate. It responds gradually over several days, not instantly. It should be adjusted alongside SRT and F/M targets, and always within the settling capacity of the secondary clarifier so you do not trigger solids carryover.
MLSS, F/M ratio and SRT are linked. For a given organic load, raising MLSS lowers the F/M ratio and lengthens SRT, favouring nitrification and stable operation; lowering MLSS does the opposite. Effective process control means managing these together rather than any one in isolation.
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