How to Increase MLSS in Activated Sludge

Introduction

Mixed liquor suspended solids (MLSS) is one of the most watched operating indicators in activated sludge. In practice, operators usually ask how to increase MLSS when they need more biological capacity, better nitrification stability, or more resilience against shock loads.

The first point is that MLSS is not increased by adding a chemical dose or by simply turning up one valve. It rises when the aeration system retains more biomass than it loses through wasting, washout, or poor solids capture. That means the correct response is usually a combination of process control and hydraulic review, not a single action.

This article explains the main levers for increasing MLSS in a controlled way, and when a low MLSS reading is actually a symptom of a broader process issue. For operators looking to review current control practice, see MLSS control guidance.

What MLSS actually tells you

MLSS is the concentration of suspended solids in the aeration basin. It is a practical proxy for the amount of biomass and inert solids held in the biological reactor, but it is not identical to active biomass alone.

A higher MLSS can be useful because it may allow:

greater biological mass in the same tank volume
improved tolerance to organic load variation
better nitrification stability, within the limits of oxygen transfer and sludge age
more flexibility where tank volume is fixed

However, an MLSS increase can also bring drawbacks:

higher oxygen demand and aeration energy requirement
poorer oxygen transfer at higher solids concentration
increased viscosity and reduced mixing efficiency
higher risk of blanket rise or clarifier overload if settlement is weak

For that reason, the target should not be “highest possible MLSS”. It should be a stable concentration that the aeration system, secondary clarifiers and sludge line can support.

The main ways MLSS is increased

1) Reduce unnecessary sludge wasting

In most plants, wasting rate is the most direct control lever. If more sludge is wasted than is needed to maintain the desired sludge age, MLSS will fall. If wasting is reduced, solids remain longer in the system and MLSS rises.

Common operational checks include:

confirm wasting is not higher than intended during low-load periods
check whether wasting pumps are delivering the actual set flow
review whether wasting is taking place from the most appropriate point in the process
verify that automatic wasting control is still aligned with current load and settleability

A cautious reduction in wasting is often the first practical step. But changes should be incremental, with close monitoring of effluent quality, sludge blanket levels and dissolved oxygen.

A useful rule is to change one control variable at a time and observe the system response over several solids retention times, not just a few hours.

2) Improve return activated sludge control

Return activated sludge (RAS) does not create biomass, but it helps keep settled solids circulating in the biological system. If RAS is too low, sludge accumulates in the clarifier and can be lost in the effluent. If it is too high, hydraulic conditions may become unstable without giving any real gain in solids inventory.

To support higher MLSS:

keep RAS consistent with clarifier performance
avoid starving the aeration tank of settled sludge during peak flow periods
watch for erratic RAS pump operation, blocked lines or variable-speed controls that are hunting
confirm the return rate is sufficient to prevent blanket build-up in the clarifier

RAS should be viewed as a solids-transfer tool, not a direct MLSS-setting tool. The plant still needs the right wasting regime and good clarifier capture to hold more solids in the system.

3) Reduce solids loss from the secondary clarifier

If MLSS is low because solids are escaping the plant, increasing biomass production will not solve the problem. The system must first retain the solids already made.

Typical causes of solids loss include:

poor sludge settleability
hydraulic overloading of the clarifier
uneven influent distribution
short-circuiting or scouring in the clarifier
excessive gasification or denitrification in the blanket

Operational responses may include:

improve RAS and wasting balance
reduce peak hydraulic stress where possible
check inlet and outlet conditions in the clarifier
review sludge blanket depth and sludge removal frequency
assess whether filamentous bulking or dispersed growth is reducing settlement

If solids are being washed out, MLSS in the aeration basin may appear stubbornly low even when biological growth is adequate.

4) Optimise aeration and mixing so biomass is retained and active

Higher MLSS means more oxygen demand and more mixing duty. If aeration is weak, biomass can become inactive or settle poorly; if mixing is inadequate, solids can accumulate in dead zones and measurements become unreliable.

When increasing MLSS, review:

blower capacity and air distribution
diffuser fouling or loss of oxygen transfer efficiency
dissolved oxygen control strategy
basin mixing under both normal and peak conditions

Indicative operating practice is to maintain enough dissolved oxygen to avoid prolonged anoxic conditions in the aeration zone, while not over-aerating so much that solids are broken up or energy is wasted. Actual targets are site-specific and depend on process configuration, temperature and nitrogen removal requirements.

A plant that is aeration-limited may not be able to sustain a higher MLSS without first improving oxygen transfer or adding capacity.

5) Check whether the issue is really low biomass concentration

Sometimes the question “how do we increase MLSS?” is asked when the real issue is a measurement or process balance problem.

Before changing operation, verify:

sampling point and sampling method
laboratory test consistency
whether the mixed liquor sample is representative
whether high inorganic content is distorting the result
whether the plant has recently changed industrial loading, storm inflow or wasting pattern

A low MLSS reading can be caused by dilution, erratic sampling, or incomplete mixing in the basin. Likewise, a high MLSS result may reflect carryover of inert solids rather than more active biomass.

6) Review upstream conditions that affect sludge growth and retention

MLSS is influenced by the quality of the feed to the biological stage and the stability of the upstream works. If primary treatment, flow equalisation or inlet screening performance changes, the aeration tank may see a different solids and load profile.

Points to review include:

changes in influent flow profile and wet-weather dilution
primary settling performance, if present
industrial discharges that alter settleability or oxygen uptake
selector performance, where used to manage filamentous growth
shock load frequency and duration

A stable front end helps the biomass inventory remain stable. Frequent hydraulic surges or toxic shocks can force operators to waste cautiously, which may limit the ability to build MLSS.

Practical operating approach

If the goal is to raise MLSS without destabilising the plant, use a staged approach:

Confirm the current MLSS trend using reliable sampling and lab checks.
Review wasting history and reduce unnecessary wasting gradually.
Confirm RAS pumps, clarifier blankets and sludge withdrawal are stable.
Check whether solids are being lost in the secondary clarifier.
Review aeration capacity, oxygen transfer and basin mixing.
Monitor effluent quality, ammonia, sludge volume index and blanket levels after each change.

This is usually a balancing exercise. Raising MLSS too quickly can create more problems than it solves, especially if the clarifier, aeration system or sludge handling line is already near its limit.

Typical indicators to watch during the change

MLSS and mixed liquor volatile suspended solids trend
sludge blanket depth in the final clarifier
effluent suspended solids
settleability and sludge volume index
dissolved oxygen profile across the aeration basin
ammonia or nitrification stability, where relevant
waste sludge solids concentration and actual wasting rate

None of these indicators should be used in isolation. The value of an MLSS increase is only realised if the plant remains stable across the whole solids balance.

When increasing MLSS is not the right answer

There are cases where a higher MLSS target is not the best solution.

Examples include:

clarifiers are already hydraulically overloaded
oxygen transfer is limiting performance
sludge settles poorly and blanket control is marginal
the plant needs a different process configuration rather than more inventory
high MLSS would make dewatering or sludge handling unworkable

In those cases, the better action may be to improve process separation, aeration efficiency, or overall capacity rather than chasing a higher solids concentration.

For plants under capacity pressure, it is often worth carrying out a structured review of the full solids and oxygen balance. If you want to assess whether your current activated sludge system can safely support higher MLSS, Start the Engineering Assessment.

How this applies to your plant

To increase MLSS successfully, treat it as a system-wide solids balance problem. In most cases the practical levers are:

reduce excessive wasting
stabilise RAS and clarifier operation
prevent solids loss
ensure aeration and mixing can support the higher inventory
confirm the measured MLSS is real and representative

If your plant is already close to its clarifier, aeration or sludge-handling limits, the safest outcome may not be a higher MLSS target but a broader process review. The Engineering Assessment helps identify whether your current activated sludge process has room to increase solids inventory, or whether another constraint is governing performance.