Secondary Clarifier Carryover: When Floc Structure, Not MLSS, Is the Real Problem

In many municipal wastewater treatment plants, secondary clarifier carryover is still routinely blamed on MLSS being “too high” or “too low.”
In reality, MLSS only tells us how much sludge is present—not whether that sludge can actually settle.

What ultimately determines secondary clarifier performance is floc structure:
its compactness, size distribution, integrity, and resistance to hydraulic and biological stress.

Well-structured flocs can settle efficiently even at elevated MLSS, while poorly formed flocs will escape the clarifier even when MLSS remains within the so-called “normal range.”

Why MLSS Alone Is a Misleading Indicator

MLSS reflects total suspended solids concentration, but it does not describe:

• Floc density
• EPS (extracellular polymeric substances) content
• Microbial cohesion strength
• Resistance to shear and flotation forces

This is why plants often experience:

• Sludge carryover
• “Snow-like” solids in effluent
• Rising SS or turbidity
• Sludge rising or rolling in the clarifier

without any obvious MLSS excursion.

Floc Structure: The True Control Variable

From a physical standpoint, secondary clarification succeeds only when flocs exhibit:

• Sufficient effective density
• Adequate particle size (>100–150 μm)
• Stable EPS matrix binding cells and inorganics
• Low susceptibility to shear, gas attachment, and fragmentation

Once floc architecture collapses, gravity separation fails—regardless of MLSS.

Key Factors That Destroy Floc Structure

1. Prolonged Low Organic Loading (Starvation Conditions)

Under prolonged low F/M conditions, microorganisms shift into endogenous respiration. During this phase, EPS production declines while existing binding materials are gradually degraded. Flocs become hollow, fragile, and prone to breakage.

When F/M drops too low for extended periods (e.g. <0.05 kg BOD/kg MLSS·d):

• Microorganisms enter endogenous respiration
• EPS production declines
• Floc “skeletons” hollow out
• Particles become fragile and easily fragmented

Result: light, fluffy flocs that refuse to settle.

In plants operating under chronic low influent strength, operators sometimes rely on PAC dosing to recover settling performance. While PAC can temporarily re-aggregate fine particles through charge neutralization and adsorption, it cannot replace the biological production of EPS. Without restoring sufficient substrate loading, chemical improvement remains short-lived.

2. Toxic or Inhibitory Influent Shocks

Even low concentrations of industrial toxins such as heavy metals or phenolic compounds can partially suppress microbial activity. The outer layers of flocs are often the first to disintegrate, releasing fine solids into the mixed liquor.

Even low-level exposure to:

• Heavy metals
• Phenols
• Cyanides
• Certain industrial organics

can:

• Inhibit microbial metabolism
• Kill peripheral biomass
• Cause partial floc disintegration

Typical symptoms include:

• Grayish or pale sludge
• Sudden increase in fine particles
• Persistent effluent turbidity

In these situations, operators may observe pale or gray sludge, persistent effluent haze, and poor clarification that does not respond to MLSS adjustments. Coagulants such as ferric salts or PAC can help capture fragmented solids, but recovery is significantly faster when pH and alkalinity are stabilized—often with sodium bicarbonate in systems sensitive to nitrification-induced alkalinity loss.

3. Excessive Aeration and Shear Stress

Over-aeration introduces both mechanical and biochemical stress. Strong shear forces physically tear flocs apart, while elevated dissolved oxygen accelerates oxidation of EPS. The resulting particles are lighter, smaller, and less settleable.

High DO (>4 mg/L) and aggressive mixing introduce a double impact:

• Mechanical shear physically breaks flocs
• Accelerated oxidation degrades EPS stability

The result is filamentous, noodle-like fragments that lose effective settling mass in the clarifier.

In high-shear systems, operators often notice that alum becomes less effective, while high-basicity PAC produces denser, more resilient flocs. In some cases, low-dose cationic PAM is used to strengthen inter-particle bonding, though careful control is required to avoid overdosing and restabilization.

4. Extreme pH Conditions

Floc cohesion is highly sensitive to pH. When pH drifts below 6 or above 9, EPS polymers carry similar charges, increasing electrostatic repulsion and weakening floc integrity. Low-alkalinity systems are particularly vulnerable during nitrification, where rapid pH drops can destabilize flocs within hours.

When influent pH drifts outside the optimal biological range (≈6.5–8.5):

• EPS polymers acquire like charges
• Electrostatic repulsion increases
• Cell walls and binding matrices weaken

Flocs unravel into fine, slow-settling debris that escapes clarification.

Maintaining alkalinity with sodium bicarbonate provides buffering without aggressive pH overshoot, while soda ash is sometimes applied where stronger correction is required. Stable pH conditions allow biological floc formation to recover naturally, reducing reliance on emergency chemical clarification.

5. Excessive Sludge Age (Overextended SRT)

Overextended sludge age leads to biomass aging and EPS hydrolysis. Although aged sludge may initially appear dense, it becomes brittle and fragments easily under clarifier hydraulics. This often results in intermittent carryover that worsens during flow fluctuations.

Overly long sludge age leads to:

• Biomass aging and reduced activity
• EPS hydrolysis
• Increased brittleness

Old sludge may settle quickly at first, but fragments easily under mild hydraulic disturbance—causing downstream carryover.

Short-term improvement can be achieved with coagulant addition, but sustainable recovery depends on adjusting sludge wasting rates to restore an appropriate sludge age. Chemical treatment in this context serves as a stabilizer rather than a primary solution.

6. High Inorganic SS / Low MLVSS Ratio

When influent contains excessive inorganic suspended solids, the active biomass fraction decreases. The biological “glue” holding particles together becomes insufficient, leading to weak, non-cohesive flocs.

When influent contains excessive inert solids:

• Active biomass fraction declines
• Organic binding capacity weakens
• Flocs become mineral-heavy but poorly cohesive

This often produces small, dense but non-cohesive particles that escape with effluent flow.

In such cases, PAC or ferric-based coagulants are particularly effective because they bind inorganic fines and increase overall floc density. Properly selected coagulants significantly reduce effluent SS caused by mineral-dominated carryover.

Typical Symptoms of Floc-Driven Clarifier Failure

How to Diagnose the Real Problem

Instead of reacting only to MLSS, prioritize:

1. SVI and SV30 trends, not single-point values
2. Microscopic floc morphology (compact vs. diffuse)
3. MLVSS/MLSS ratio
4. DO profiles and shear intensity
5. Influent toxicity screening
6. pH stability and alkalinity buffering

Operational Focus: Fix the Structure, Not the Number

Effective corrective actions typically include:

• Restoring appropriate organic loading
• Reducing unnecessary aeration intensity
• Adjusting sludge wasting to avoid over-aging
• Stabilizing influent pH and alkalinity
• Identifying and isolating toxic influent sources
• Short-term use of coagulants (e.g. PAC) as a structural aid, not a substitute for biological health

Final Takeaway

Secondary clarifier carryover is rarely a simple MLSS problem.

It is almost always a floc structure problem disguised as a concentration issue.

Plants that shift their focus from “How much sludge do we have?”
to “What kind of sludge structure are we actually producing?”
will resolve clarifier failures faster—and with fewer reactive chemical or hydraulic fixes.