Secondary Clarification & Settleability

Learn secondary clarification and activated-sludge settleability, including clarifier hydraulics, sludge blankets, RAS, SVI, settleometer tests, bulking, rising sludge, solids carryover, and troubleshooting.

Secondary clarification is the final solids-separation step in the activated-sludge process. The clarifier must separate biological floc from treated wastewater, produce a clear effluent, concentrate settled sludge for return to the aeration basin, and prevent excessive solids accumulation.

Good biological treatment can still produce poor effluent if the secondary clarifier does not retain the solids. Operators therefore need to evaluate settleability, sludge blanket depth, return activated sludge, hydraulic loading, solids loading, and clarifier mechanical condition together.

Purpose of the Secondary Clarifier

The secondary clarifier performs several important functions:

settles activated-sludge solids;
produces clarified effluent;
concentrates sludge;
provides sludge for return activated sludge;
provides a location for removal of excess biological solids.

Clarification Is Part of Biological Treatment

Activated sludge must both:

remove wastewater pollutants biologically;
settle effectively afterward.

If biological solids leave with the final effluent, treatment performance can deteriorate even when the microorganisms are removing BOD effectively in the aeration basin.

Mixed Liquor Enters the Clarifier

Mixed liquor flows from the aeration basin into the secondary clarifier.

Inside the clarifier:

flow velocity decreases;
biological floc settles;
clearer water moves toward the effluent weirs;
settled sludge moves toward the sludge-collection system.

Clarifier Zones

A secondary clarifier can be thought of as containing several functional zones:

inlet zone;
settling zone;
sludge zone;
effluent zone.

Inlet Zone

The inlet structure should distribute mixed liquor without creating excessive turbulence or short-circuiting.

Poor inlet conditions can cause:

uneven flow distribution;
solids disturbance;
localized high velocities.

Settling Zone

In the settling zone, biological floc separates from the water by gravity.

Good settleability allows solids to move downward while clarified water moves upward.

Sludge Zone

Settled solids accumulate near the bottom of the clarifier before being removed as RAS or WAS.

Effluent Zone

Clarified water flows over effluent weirs and leaves the secondary treatment process.

Sludge Blanket

The sludge blanket is the layer of settled activated sludge in the lower part of the clarifier.

Blanket depth is an important operator measurement because it shows how much sludge is being stored in the clarifier.

Why Blanket Depth Matters

An excessively high blanket can increase the risk of:

solids carryover;
rising sludge;
septic sludge conditions;
loss of biological solids.

Rising Blanket

A rising sludge blanket can result from:

low RAS withdrawal;
poor sludge settleability;
high hydraulic loading;
high solids loading;
mechanical sludge-removal problems.

Low Blanket

A very low blanket may be normal in some conditions, but operators should also consider:

high RAS rate;
low solids inventory;
poor sludge compaction.

Return Activated Sludge

RAS removes settled solids from the clarifier and returns them to the aeration basin.

RAS rate influences:

blanket depth;
sludge concentration;
clarifier solids inventory;
hydraulic loading.

Increasing RAS

Increasing RAS can help move sludge out of the clarifier more rapidly.

However, excessive RAS can:

increase total clarifier flow;
reduce underflow concentration;
increase pumping energy.

Decreasing RAS

Reducing RAS may increase sludge concentration in the return stream, but if RAS becomes too low, the clarifier blanket can rise excessively.

RAS Does Not Fix Poor Settleability

If sludge itself settles poorly, increasing RAS alone may not correct the underlying biological problem.

WAS and Clarifier Performance

WAS controls long-term biological solids inventory.

Excessive system solids can contribute to:

high clarifier solids loading;
high blankets;
greater oxygen demand;
reduced clarifier capacity.

Settleability

Settleability describes how effectively activated-sludge floc settles and compacts.

Good settleability generally produces:

rapid initial settling;
compact sludge;
clear supernatant.

Settleometer Test

A settleometer test allows operators to observe mixed-liquor settling over time.

Useful observations include:

initial settling rate;
30-minute settled volume;
sludge compaction;
supernatant clarity;
floc appearance.

Thirty-Minute Settled Sludge Volume

A common test measures the volume occupied by settled sludge after 30 minutes.

The result is commonly expressed in mL/L.

Example Settling Observation

If mixed liquor settles to 300 mL/L after 30 minutes, the settled sludge occupies 300 milliliters of each liter of sample.

Sludge Volume Index

Sludge Volume Index, or SVI, relates 30-minute settled sludge volume to MLSS concentration.

A common formula is:

SVI, mL/g = 30-Minute Settled Sludge, mL/L × 1,000 ÷ MLSS, mg/L

SVI Example

Assume:

30-minute settled sludge = 300 mL/L;
MLSS = 3,000 mg/L.

SVI = 300 × 1,000 ÷ 3,000

SVI = 100 mL/g

What SVI Indicates

SVI provides an index of how much volume one gram of sludge occupies after settling.

A rising SVI trend can indicate poorer compaction or bulking conditions.

Do Not Use SVI Alone

SVI should be interpreted with:

settleometer appearance;
clarifier blanket depth;
effluent TSS;
MLSS;
microscopic observations where available.

Settling Curve

Observing sludge volume at several times can show how the sludge settles and compacts.

For example, operators may compare:

5 minutes;
10 minutes;
20 minutes;
30 minutes.

Fast Initial Settling

Rapid initial settling followed by good compaction generally indicates strong settleability.

Slow Settling

Slow settling can indicate:

bulking sludge;
poor floc structure;
unusual biological conditions.

Bulking Sludge

Bulking occurs when sludge settles poorly and occupies excessive volume.

One common cause is excessive filamentous growth.

Filamentous Organisms

Filamentous microorganisms can extend from floc and prevent compact settling when present in excessive amounts.

Conditions that can favor certain filamentous organisms include:

low DO;
septic influent;
nutrient imbalance;
particular F/M conditions;
certain industrial wastes.

Bulking Symptoms

Common symptoms include:

high SVI;
slow settling;
poor compaction;
high sludge blanket;
solids carryover.

Bulking Is a Symptom, Not a Complete Diagnosis

Operators should determine the cause before selecting a corrective action.

Pin Floc

Pin floc consists of very small biological particles that do not settle or become captured effectively.

Pin floc can produce:

cloudy effluent;
elevated final TSS;
small particles in the clarifier overflow.

Young Sludge and Floc Formation

Very young sludge conditions can produce weak or poorly developed biological floc.

Old Sludge and Fine Solids

Excessively old sludge conditions can also contribute to fragile floc and fine suspended solids in some systems.

Rising Sludge

Rising sludge occurs when settled sludge rises toward the clarifier surface after initially settling.

Denitrification in the Clarifier

A common cause of rising sludge is denitrification within the sludge blanket.

Nitrate can be biologically converted to nitrogen gas under low-oxygen conditions.

Gas bubbles can attach to sludge particles and lift them upward.

Conditions Favoring Clarifier Denitrification

Risk can increase when:

nitrate is present;
sludge remains in the clarifier too long;
blanket depth is high;
RAS withdrawal is insufficient.

Rising Sludge Versus Bulking

Bulking sludge settles poorly from the beginning.

Rising sludge may settle normally at first and then float upward later.

Floating Sludge

Floating solids can also result from:

gas formation;
grease;
biological foam;
mechanical disturbance.

Solids Carryover

Solids carryover occurs when activated-sludge solids leave the clarifier in the final effluent.

It can increase:

effluent TSS;
effluent BOD;
downstream solids load.

Causes of Solids Carryover

Possible causes include:

bulking;
rising sludge;
high sludge blanket;
hydraulic overload;
solids overload;
poor inlet distribution;
mechanical failure.

Hydraulic Loading

Secondary clarifiers have limited hydraulic capacity.

As flow increases:

upward water velocity increases;
detention time decreases;
settling becomes more difficult.

Surface Overflow Rate

A basic hydraulic loading calculation is:

Surface Overflow Rate = Clarifier Flow ÷ Surface Area

Surface Overflow Example

A clarifier receives 2.0 MGD and has a surface area of 4,000 ft².

Convert flow:

2,000,000 gal/day ÷ 4,000 ft²

Surface Overflow Rate = 500 gal/day/ft²

Why Overflow Rate Matters

As hydraulic loading increases, the clarifier has less opportunity to separate slower-settling particles.

Wet-Weather Flow

Wet weather can challenge secondary clarification because it may create:

higher hydraulic flow;
shorter detention time;
higher clarifier surface loading;
greater risk of solids washout.

Solids Loading

Clarifier performance depends not only on water flow but also on the amount of suspended solids entering.

A simplified solids-loading relationship is:

Solids Load, lb/day = Flow, MGD × MLSS, mg/L × 8.34

Solids-Load Example

Mixed-liquor flow to the clarifier is 3 MGD and MLSS is 3,500 mg/L.

Solids Load = 3 × 3,500 × 8.34

Solids Load = 87,570 lb/day

High MLSS Increases Clarifier Load

If flow remains the same but MLSS rises, the mass of solids entering the clarifier increases.

Peak Flow and High MLSS Together

The combination of high flow and high MLSS can strongly challenge clarifier performance.

Weir Loading

Effluent weirs distribute clarified water leaving the clarifier.

Uneven or excessive weir loading can contribute to poor effluent distribution.

Level Weirs

Weirs should be level so flow is distributed around the clarifier rather than concentrated at one location.

Dirty Weirs

Algae, solids, or debris on weirs can affect:

flow distribution;
effluent appearance;
maintenance conditions.

Scum Removal

Secondary clarifiers often include equipment for removing floating:

scum;
grease;
foam;
other floating material.

Sludge Collection Equipment

Depending on clarifier design, sludge may be moved using:

scraper mechanisms;
suction systems;
other collection equipment.

Mechanical Problems

Mechanical failures can create symptoms similar to biological problems.

Possible issues include:

broken scraper;
plugged sludge withdrawal line;
failed RAS pump;
blocked suction header.

Clarifier Torque

Some clarifiers monitor drive torque.

Increasing torque can indicate:

excessive sludge accumulation;
mechanical obstruction;
equipment problems.

Clarifier Short-Circuiting

Short-circuiting occurs when wastewater moves through part of the clarifier faster than intended.

Possible causes include:

poor inlet distribution;
uneven weirs;
hydraulic imbalance.

Density Currents

Temperature or solids-density differences can create currents that disturb normal settling patterns.

Parallel Clarifiers

When several clarifiers operate in parallel, flow should be distributed appropriately.

One overloaded clarifier can perform poorly even if total plant flow appears acceptable.

Compare Parallel Clarifiers

Useful comparisons include:

blanket depth;
RAS flow;
effluent clarity;
weir flow;
mechanical condition.

Clarifier Effluent Observation

Visual inspection can reveal:

floating solids;
pin floc;
uneven effluent flow;
scum;
localized solids carryover.

Effluent TSS

Effluent TSS is a key indicator of solids-separation performance.

A rising TSS trend should be compared with:

settleability;
blanket depth;
flow;
MLSS;
RAS operation.

Effluent BOD

Biological solids leaving the clarifier contain organic matter and can increase measured effluent BOD.

Therefore, rising BOD and rising TSS together can point toward solids carryover.

Clear Effluent but High Ammonia

If clarifier effluent is clear but ammonia is high, the primary problem may be biological nitrification rather than solids separation.

High TSS but Good Ammonia Removal

This pattern may indicate that biological treatment is functioning while clarification is failing.

Example: Blanket Rising but Effluent Still Clear

This is an early warning condition.

Review:

RAS rate;
settling;
MLSS;
hydraulic loading;
sludge-removal equipment.

Example: Blanket Stable but Effluent TSS Rising

Possible causes include:

pin floc;
poor floc formation;
localized hydraulic short-circuiting;
weir problems.

Example: Blanket Rises During Wet Weather

Possible causes include:

higher hydraulic loading;
reduced settling time;
solids redistribution.

Example: Sludge Settles Well in Settleometer but Clarifier Performs Poorly

This suggests the problem may be more hydraulic or mechanical than biological.

Review:

flow distribution;
weirs;
RAS equipment;
sludge collection;
hydraulic loading.

Example: Settleometer Shows Poor Settling and Clarifier Blanket Rises

This combination suggests a settleability problem that should be investigated biologically.

Example: Sludge Settles, Then Floats

Investigate:

denitrification;
sludge residence time;
nitrate concentration;
blanket depth;
RAS withdrawal.

Example: SVI Rises Gradually

Review trends in:

DO;
F/M;
SRT;
influent characteristics;
microscopic observations.

Example: SVI Changes Suddenly

Consider:

sampling error;
laboratory error;
toxic discharge;
rapid pH change;
major process upset.

Example: One Clarifier Has High Blanket

If other clarifiers are normal, investigate that clarifier specifically.

Possible causes include:

RAS pump problem;
plugged withdrawal line;
uneven influent flow;
mechanical sludge-removal failure.

Example: All Clarifiers Have High Blankets

Review plantwide conditions such as:

high solids inventory;
poor settleability;
high hydraulic loading;
RAS strategy.

Process Response to Poor Settling

Corrective action should address the actual cause.

Possible control areas include:

DO;
SRT;
F/M;
RAS;
WAS;
influent conditions;
clarifier hydraulics.

Avoid Multiple Uncontrolled Changes

Changing aeration, RAS, WAS, and other controls simultaneously can make it difficult to identify what caused improvement or deterioration.

Trend Before and After Changes

After a process adjustment, monitor:

settleometer results;
SVI;
blanket depth;
effluent TSS;
effluent BOD;
MLSS.

Common Clarification Mistakes

Looking only at effluent appearance.
Ignoring sludge blanket depth.
Assuming every high blanket is caused by low RAS.
Using RAS changes to compensate for biological bulking without investigating the cause.
Ignoring wet-weather hydraulic loading.
Ignoring mechanical sludge-removal equipment.
Using one SVI result without reviewing trends.
Confusing rising sludge with bulking.
Assuming high effluent BOD always means poor aeration-basin treatment.
Ignoring differences among parallel clarifiers.

A Practical Clarifier Review

Review influent flow.
Review MLSS and solids loading.
Measure sludge blanket depth.
Review RAS flow.
Perform or review settling tests.
Calculate and trend SVI where used.
Inspect effluent clarity and weirs.
Inspect sludge-removal equipment.
Review effluent TSS and BOD.
Compare conditions with recent trends.

A Practical Rising-Blanket Review

Verify the blanket measurement.
Review RAS pump operation and actual flow.
Review mixed-liquor settleability.
Review clarifier hydraulic loading.
Review total solids inventory.
Inspect sludge-withdrawal equipment.
Check effluent TSS.
Correct the identified cause.

A Practical Poor-Settleability Review

Verify MLSS and settleometer sampling.
Review 30-minute settled volume and SVI.
Observe sludge compaction and supernatant clarity.
Review DO trends.
Review F/M and SRT.
Review influent pH and industrial loading.
Review microscopic observations where available.
Identify the biological condition before changing process controls.

A Practical Effluent-Solids Review

Verify effluent TSS.
Inspect clarifier effluent visually.
Review sludge blanket depth.
Review settling test results.
Review hydraulic and solids loading.
Review RAS operation.
Inspect mechanical equipment.
Determine whether the cause is biological, hydraulic, mechanical, or a combination.

What to Remember for the Exam

Secondary clarifiers separate activated-sludge solids from treated wastewater.
A clarifier must produce clear effluent while concentrating sludge for return or wasting.
The sludge blanket is the settled solids layer in the lower part of the clarifier.
A rising blanket can result from low RAS, poor settling, high hydraulic loading, high solids loading, or mechanical problems.
RAS moves settled sludge from the clarifier back to the aeration basin.
WAS controls long-term total solids inventory.
Settleability describes how effectively activated-sludge floc settles and compacts.
The settleometer test shows settling rate, settled volume, compaction, and supernatant clarity.
SVI relates 30-minute settled sludge volume to MLSS concentration.
SVI in mL/g can be calculated as settled sludge mL/L × 1,000 ÷ MLSS mg/L.
Bulking sludge settles poorly and commonly produces high SVI and poor compaction.
Excessive filamentous growth is a common cause of bulking.
Pin floc can cause cloudy effluent and elevated TSS even when the sludge blanket is not high.
Rising sludge can result from denitrification and gas formation within settled sludge.
Bulking and rising sludge are different conditions.
High hydraulic flow increases clarifier surface loading and can contribute to solids washout.
Higher MLSS increases the solids mass applied to the clarifier at the same flow.
High effluent TSS can also increase measured effluent BOD.
Clarifier troubleshooting should compare blanket depth, RAS, settleability, MLSS, hydraulic loading, solids loading, equipment condition, and effluent quality.
Good secondary clarification depends on stable biology, proper solids control, adequate hydraulic capacity, effective sludge removal, and careful trend monitoring.