How to Control MLSS in an Activated Sludge Plant

Introduction

Mixed liquor suspended solids (MLSS) is one of the most watched operating parameters in an activated sludge plant. It gives a practical indication of how much biomass is present in the aeration system, and it influences oxygen demand, sludge age, settling performance and process stability.

MLSS is not controlled in isolation. In a well-run plant, it is managed together with sludge wasting, sludge age, dissolved oxygen, clarifier performance and the influent load. The aim is not to maximise MLSS, but to keep it in a range that supports treatment without creating avoidable solids handling or settlement problems.

For operators and engineers, the key question is simple: how do you keep MLSS where the process needs it, and how do you respond when it drifts? This article sets out the practical control logic used in activated sludge systems.

What MLSS tells you

MLSS is the concentration of suspended solids in the aeration tank mixed liquor. In operational terms, it is a proxy for biomass inventory, although it also includes inert solids and other suspended matter.

A stable MLSS profile usually supports:

• predictable organic removal
• manageable aeration demand
• stable sludge blanket behaviour in the secondary clarifier
• easier control of sludge age and wasting

However, a higher MLSS concentration is not automatically better. Excessive MLSS can:

• increase oxygen transfer demand
• reduce effective tank volume available for treatment
• raise mixed liquor viscosity and hinder mass transfer
• worsen clarifier loading and sludge settleability
• make washout or bulking events more difficult to recover from

MLSS should therefore be read as part of a wider process picture, not as a standalone performance indicator.

Main levers for controlling MLSS

1) Waste activated sludge rate

The principal control lever for MLSS is the rate at which waste activated sludge is removed from the system. Increasing wasting reduces the biomass inventory over time; reducing wasting allows MLSS to rise.

In practice, wasting should be adjusted gradually and based on trends rather than single readings. Typical operator practice is to review:

• daily or shift MLSS trends
• effluent quality trends
• sludge blanket levels
• clarifier return characteristics
• sludge age or other internal solids inventory indicators

A useful rule is to avoid chasing short-term fluctuations. MLSS responds with a time lag, especially in larger plants. Sudden large changes in wasting can destabilise the process.

2) Return activated sludge balance

Return activated sludge (RAS) does not remove solids from the process, but it strongly affects solids distribution between the clarifier and the aeration tank. If RAS is too low, solids can accumulate in the clarifier, risking blanket rise and solids loss. If it is too high, hydraulic loading and pumping energy may increase without improving process stability.

RAS should be coordinated with MLSS control so that:

• the clarifier remains hydraulically and biologically stable
• solids are returned without undue thickening or compression problems
• wasting can be carried out steadily from a suitable point in the process

RAS adjustments are supportive controls. They do not replace sludge wasting as the main MLSS control mechanism.

3) Influent load and flow variation

MLSS changes naturally as the plant responds to influent load. High organic load periods tend to grow biomass inventory if the process is not wasting enough solids. Low-load periods can allow MLSS to drift down, particularly if wasting is not reduced in time.

Storm inflow and infiltration can also influence apparent MLSS by diluting the mixed liquor and changing clarifier behaviour. In these situations, the operator should distinguish between:

• a genuine change in biomass inventory
• a temporary dilution or hydraulic effect
• a settling or return problem that only appears as an MLSS change

This distinction matters. Treating a hydraulic problem as a solids inventory problem can make control worse.

How to set and adjust MLSS in practice

There is no universal MLSS target that suits every activated sludge plant. The common design value or operating range is site-specific and depends on process configuration, influent strength, temperature, aeration capacity, clarifier loading and sludge settleability.

A practical control approach is to define an operating band rather than a single setpoint. The band should reflect:

• the required treatment performance
• available aeration capacity
• secondary clarifier limits
• sludge handling constraints
• the desired sludge age range

When MLSS is below the desired range, first confirm whether the reading reflects a genuine drop in solids concentration or a sampling issue. If the result is confirmed, the typical response is to reduce wasting and review whether the plant is underloaded or experiencing unusual hydraulic conditions.

When MLSS is above the desired range, increase wasting carefully and confirm that the clarifier can cope with the resulting solids transport. Excessive wasting may reduce MLSS too quickly and can lead to loss of treatment capacity or poor effluent quality.

Good practice is to make one change at a time and allow enough time for the system to respond before making another adjustment.

MLSS, sludge age and process stability

MLSS control is closely linked to sludge age, also called solids retention time. Although the two are not identical, they move together in most activated sludge systems. If wasting is reduced, both MLSS and sludge age tend to rise. If wasting is increased, both tend to fall.

This matters because the biological community in the tank changes with sludge age. Younger sludge may improve responsiveness but can reduce nitrification capacity in some systems. Older sludge may support more complete treatment but can increase oxygen demand and the risk of poor settleability if the inventory becomes excessive.

The operator should therefore think in terms of a balanced solids inventory rather than chasing a single concentration value.

Common signs that MLSS control is drifting

Typical warning signs include:

• rising or falling MLSS trends without an obvious operational reason
• unstable secondary clarifier sludge blanket levels
• poorer final effluent suspended solids
• changes in mixed liquor appearance or floc structure
• increased aeration demand for the same load
• more frequent sludge bulking or pin floc observations

If these signs appear, review the control chain in sequence:

1. Sampling and laboratory reliability
2. Wasting rate and wasting point
3. RAS rate and clarifier solids balance
4. Influent flow, load and storm impacts
5. Aeration and mixing conditions
6. Sludge settleability and filamentous behaviour

This sequence helps avoid unnecessary changes and makes troubleshooting more systematic.

Practical troubleshooting guide

MLSS is rising steadily

Possible causes:

• wasting rate too low
• higher-than-usual organic load
• reduced sludge withdrawal due to pump or valve issues
• improved biomass growth after a load increase

Typical response:

• verify the MLSS trend over several days
• check wasting equipment and sludge draw-off points
• increase wasting incrementally
• confirm that clarifier levels remain stable

MLSS is falling steadily

Possible causes:

• wasting rate too high
• low incoming load
• unplanned solids loss in the final effluent
• poor return sludge capture or clarifier upset

Typical response:

• check for effluent solids carryover
• reduce wasting gradually
• inspect clarifier blanket and return conditions
• confirm whether the plant has become underloaded

MLSS is unstable

Possible causes:

• intermittent wasting control
• variable RAS operation
• stormwater influence
• inconsistent sampling point or method
• process upset affecting settleability

Typical response:

• move to a consistent sampling and measurement routine
• avoid large operational swings
• trend MLSS alongside sludge blanket, effluent solids and aeration demand
• investigate the underlying hydraulic or settleability issue

Measurement and monitoring essentials

MLSS control depends on reliable data. A single measurement has limited value; trends are more useful.

Useful monitoring practices include:

• fixed sampling times and locations
• routine laboratory methods with consistent filtration and drying procedure
• regular checks against mixed liquor settleability and sludge volume observations
• trend charts for MLSS, RAS, wasting rate and final effluent solids

Operators should also check whether what appears to be an MLSS problem is actually a measurement artefact. Mixed liquor can be non-uniform, and poor sampling technique can distort the result.

Where process data are available, MLSS should be reviewed together with dissolved oxygen, clarifier performance and sludge production. That wider view is often more informative than a single concentration reading.

Regulatory context and why it matters

Across the EU, the recast Urban Wastewater Treatment Directive places continued emphasis on biological treatment performance. Secondary treatment is defined as treatment that generally involves biological treatment with a secondary settlement stage, or another process that removes biodegradable organic matter.

For activated sludge plants, this means that stable biomass inventory and reliable solids separation remain fundamental to compliance. The directive entered into force on 1 January 2025, must be transposed into national law by 31 July 2027, and repeals the old directive with effect from 1 August 2027. National implementation details will vary, but the operational principle is the same: the biological stage must be robust and controllable.

For reference, the Annex I secondary-treatment effluent standards are BOD5 25 mg/L or 70-90% reduction, COD 125 mg/L or 75% reduction, and TSS 35 mg/L. Good MLSS control is one of the basic tools that helps the plant stay within those limits.

How this applies to your plant

If your MLSS is drifting, the right response is usually not a single setpoint change. It is a review of wasting, RAS, influent load, clarifier behaviour and sludge settleability, followed by small, measured adjustments.

A good MLSS strategy is site-specific and depends on the plant’s design and operating constraints. If you want a structured way to review your biomass inventory, clarifier performance and control options, Start the Engineering Assessment. For a focused reference on this subject, see the related guide on /mlss-control.

The most effective plants treat MLSS as a managed process variable: monitored routinely, adjusted gradually, and interpreted in the context of the whole activated sludge system.