Activated Sludge Process Fundamentals

Learn activated-sludge process fundamentals, including aeration basins, microorganisms, mixed liquor, return and waste activated sludge, secondary clarification, organic loading, solids control, and process stability.

The activated-sludge process uses microorganisms to remove biodegradable material from wastewater. Operators maintain a controlled population of microorganisms in an aeration basin, provide oxygen and mixing, separate the biological solids in a secondary clarifier, return part of those solids to the process, and waste excess solids to control the biomass inventory.

Successful activated-sludge operation depends on maintaining the right balance among wastewater loading, microorganisms, oxygen, mixing, solids inventory, return sludge, waste sludge, and clarification. When one part of the process changes, several other operating conditions can change with it.

What Is Activated Sludge?

Activated sludge is a biological wastewater-treatment process in which microorganisms grow as suspended biological solids and consume biodegradable pollutants.

The process typically includes:

an aeration basin;
a secondary clarifier;
return activated sludge;
waste activated sludge.

Basic Process Flow

A simplified activated-sludge flow path is:

primary-treated or raw wastewater enters the aeration basin;
wastewater mixes with biological solids;
air or oxygen supports biological treatment;
mixed liquor flows to the secondary clarifier;
biological solids settle;
clarified effluent leaves the clarifier;
part of the settled sludge returns to the aeration basin;
excess sludge is wasted from the system.

The Role of Microorganisms

Activated sludge depends on microorganisms that use wastewater pollutants as food.

Important microbial groups can include:

bacteria;
protozoa;
other microorganisms associated with the biological floc.

Organic Matter as Food

Biodegradable organic material provides food for the microorganisms.

As microorganisms consume organic matter, they:

produce new cells;
release metabolic end products;
reduce wastewater BOD.

Oxygen Is Required for Aerobic Treatment

Most conventional activated-sludge systems rely on aerobic biological activity.

Oxygen is supplied so microorganisms can oxidize organic material.

Aeration Basin

The aeration basin is where wastewater and biological solids are mixed under controlled aerobic conditions.

The basin must provide:

adequate oxygen;
adequate mixing;
sufficient contact between wastewater and microorganisms.

Mixing

Mixing helps keep biological solids suspended and distributes:

oxygen;
food;
microorganisms.

Poor mixing can create areas with:

solids settling;
low dissolved oxygen;
uneven treatment.

Mixed Liquor

Mixed liquor is the mixture of wastewater and suspended biological solids in the aeration basin.

Mixed Liquor Suspended Solids

MLSS means mixed liquor suspended solids.

MLSS represents the concentration of suspended solids in the aeration basin.

These solids include:

active microorganisms;
inactive biological material;
inert suspended solids.

Mixed Liquor Volatile Suspended Solids

MLVSS means mixed liquor volatile suspended solids.

MLVSS is often used as an estimate of the organic or biological portion of the mixed liquor solids.

MLSS and MLVSS Are Not Identical

MLSS includes all suspended solids in the mixed liquor.

MLVSS represents the portion lost during the volatile-solids test and is commonly used as an approximation of biological solids.

Biological Floc

Activated-sludge microorganisms form aggregates called floc.

Good floc should:

remain suspended in aeration;
settle effectively in the secondary clarifier;
produce clear supernatant.

Secondary Clarification

The secondary clarifier separates biological solids from treated wastewater.

Its main functions are to:

settle activated sludge;
produce clarified effluent;
concentrate sludge for return or wasting.

Activated Sludge Must Settle

Biological treatment alone is not enough.

If the biomass does not settle well, suspended solids can leave the clarifier and increase:

effluent TSS;
effluent BOD;
downstream solids loading.

Return Activated Sludge

Return activated sludge, or RAS, is settled biological sludge pumped from the secondary clarifier back to the aeration basin.

Why RAS Is Returned

RAS helps maintain enough biological solids in the aeration basin.

Without adequate return, microorganisms would leave the process with the clarifier underflow or effluent and the aeration-basin solids concentration would decline.

RAS Rate

The RAS flow influences:

clarifier sludge blanket;
solids return;
underflow concentration;
aeration-basin solids inventory.

Too Little RAS

An insufficient return rate can contribute to:

rising clarifier sludge blanket;
solids accumulation in the clarifier;
reduced solids return.

Too Much RAS

An excessive RAS rate can:

increase hydraulic flow through the clarifier;
reduce sludge concentration in the return stream;
create unnecessary pumping.

Waste Activated Sludge

Waste activated sludge, or WAS, is sludge intentionally removed from the activated-sludge process.

Why Sludge Must Be Wasted

Microorganisms reproduce as they consume wastewater pollutants.

If excess solids are never removed:

solids inventory increases;
MLSS rises;
oxygen demand can increase;
clarifier loading can increase;
sludge age changes.

Wasting Controls Biomass Inventory

Sludge wasting is one of the most important activated-sludge control actions.

Changing WAS affects the process more slowly than changing an air valve or RAS pump because biomass inventory changes over time.

Increasing Wasting

Increasing WAS generally tends to reduce:

system solids inventory;
MLSS over time;
sludge age.

Decreasing Wasting

Decreasing WAS generally tends to increase:

system solids inventory;
MLSS over time;
sludge age.

Solids Retention Time

Solids retention time, or SRT, describes how long biological solids remain in the treatment process on average.

It is also commonly called:

sludge age;
mean cell residence time.

Why SRT Matters

SRT influences:

microbial population;
solids production;
nitrification capability;
settling characteristics;
oxygen demand.

Young Sludge

A relatively low SRT can produce younger sludge.

Possible characteristics include:

rapid biological growth;
higher sludge production;
less complete nitrification where nitrification is required.

Older Sludge

A higher SRT produces older biomass.

Possible characteristics can include:

lower net biomass production;
greater endogenous respiration;
greater opportunity for nitrification.

Food-to-Microorganism Ratio

The food-to-microorganism ratio, or F/M ratio, compares the organic food entering the process with the amount of microorganisms available to treat it.

The concept helps operators understand whether the biomass has:

relatively high food loading;
relatively low food loading.

High F/M Conditions

High F/M means relatively more food is available for the biological solids present.

This can be associated with:

rapid organism growth;
high oxygen demand;
younger sludge conditions.

Low F/M Conditions

Low F/M means relatively less food is available for the amount of biomass present.

This can be associated with:

older sludge;
more endogenous respiration;
lower net biological growth.

Organic Loading

The process should be evaluated using mass loading, not concentration alone.

A common relationship is:

Organic Load, lb/day = Flow, MGD × Concentration, mg/L × 8.34

Organic-Load Example

Flow to the aeration basin is 3 MGD and influent BOD is 180 mg/L.

Load = 3 × 180 × 8.34

Load = 4,504 lb/day

The process receives approximately 4,500 lb/day of BOD.

Flow and Concentration Must Be Evaluated Together

If influent BOD concentration remains constant but flow increases, total organic load increases.

Wet-Weather Loading

Wet weather can create unusual activated-sludge conditions.

Possible effects include:

higher hydraulic flow;
lower influent concentration because of dilution;
shorter hydraulic detention time;
higher clarifier hydraulic loading.

Hydraulic Loading and Organic Loading Are Different

Hydraulic loading describes water flow.

Organic loading describes the mass of biodegradable material entering the process.

A plant can experience high hydraulic loading even when influent BOD concentration is relatively low.

Dissolved Oxygen

Dissolved oxygen, or DO, indicates the concentration of oxygen dissolved in the mixed liquor.

DO is affected by:

aeration rate;
organic loading;
ammonia loading;
temperature;
biomass concentration.

Low DO

Insufficient DO can contribute to:

poor organic removal;
reduced nitrification;
undesirable biological conditions;
odor problems.

Excessive DO

More air is not always better.

Excessive aeration can:

waste energy;
create unnecessary turbulence;
provide little additional treatment benefit once oxygen requirements are satisfied.

Oxygen Demand Changes with Load

When organic or ammonia loading increases, oxygen demand can increase.

A fixed air setting may therefore produce different DO under changing influent conditions.

Temperature

Temperature affects:

biological reaction rates;
oxygen solubility;
nitrification;
settling behavior.

Cold Weather

Lower temperature generally slows biological reactions.

Nitrifying organisms can be especially sensitive to cold conditions.

Warm Weather

Higher temperature can increase biological activity while reducing the amount of oxygen water can hold.

pH

Activated-sludge microorganisms operate best within a suitable pH range.

Rapid pH changes can indicate:

industrial discharge;
chemical upset;
loss of alkalinity;
sampling or instrument error.

Alkalinity

Alkalinity helps buffer pH.

It is especially important in systems performing nitrification because nitrification consumes alkalinity.

Nitrification

Nitrification is the biological oxidation of ammonia to more oxidized forms of nitrogen.

Successful nitrification generally requires:

adequate SRT;
adequate DO;
suitable temperature;
sufficient alkalinity;
absence of significant toxicity.

Secondary Clarifier Sludge Blanket

The sludge blanket is the settled sludge layer in the secondary clarifier.

Operators monitor blanket depth because excessive accumulation can increase the risk of solids carryover.

Rising Sludge Blanket

Possible causes include:

insufficient RAS;
poor settling;
high hydraulic loading;
excessive solids inventory.

Settling Tests

Operators commonly observe mixed-liquor settling using a settleometer or similar test.

Useful observations include:

settling rate;
final settled volume;
clarity of supernatant;
floc appearance.

Thirty-Minute Settled Sludge Volume

A common observation is the volume occupied by settled sludge after 30 minutes.

This result is often used with MLSS to evaluate sludge settleability.

Sludge Volume Index

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

SVI is useful for comparing settling behavior over time.

Use Trends Rather Than One SVI Result

SVI should be interpreted with:

settleometer appearance;
clarifier performance;
MLSS;
microscopic observations where available;
process history.

Good Settling

Good activated sludge generally forms floc that:

settles reasonably quickly;
compacts adequately;
leaves clear supernatant.

Poor Settling

Poor settling can lead to:

high effluent TSS;
high effluent BOD;
loss of biomass;
unstable process control.

Bulking

Bulking sludge settles poorly and occupies excessive volume.

Bulking can be associated with excessive growth of filamentous organisms or other process imbalances.

Filamentous Organisms

Filamentous organisms can interfere with settling when they become excessive.

Possible contributing conditions include:

low DO;
nutrient imbalance;
septic influent;
certain loading conditions.

Pin Floc

Pin floc refers to small biological particles that do not settle or capture efficiently.

It can contribute to:

cloudy effluent;
higher effluent suspended solids.

Rising Sludge

Settled sludge can rise in a secondary clarifier when gas becomes trapped in the sludge blanket.

One possible cause is denitrification occurring in the clarifier.

Clarifier Solids Carryover

Solids can leave the clarifier because of:

poor settleability;
hydraulic overload;
high sludge blanket;
mechanical problems;
rising sludge.

Effluent TSS and BOD

Clarifier solids carryover can increase both:

effluent TSS;
effluent BOD.

This is why high effluent BOD does not always mean biological oxidation has failed.

Biological Treatment Versus Solids Separation

When effluent quality deteriorates, operators should determine whether the primary problem is:

biological treatment;
clarifier solids separation;
both.

Process Control Data

Important activated-sludge data can include:

influent flow;
influent BOD or COD;
MLSS;
MLVSS;
DO;
RAS flow;
WAS flow;
sludge blanket;
settling test results;
effluent TSS;
effluent BOD;
ammonia where relevant.

Trend Data Together

One value rarely explains the whole process.

For example, rising MLSS should be interpreted with:

wasting rate;
influent loading;
settleability;
clarifier blanket;
effluent TSS.

Process Changes Have Different Response Times

Changing airflow can affect DO quickly.

Changing RAS can affect clarifier solids movement relatively quickly.

Changing WAS can affect system biomass and sludge age over a much longer period.

Do Not Expect Immediate Response to Wasting Changes

Large repeated WAS adjustments made before the process has time to respond can create unstable solids control.

Example: DO Falls Suddenly

Review:

blower operation;
airflow;
influent organic load;
ammonia load;
MLSS;
DO instrument accuracy.

Example: MLSS Is Rising

Review:

WAS rate;
influent loading;
RAS concentration;
clarifier inventory;
solids production.

Example: MLSS Is Falling

Possible causes include:

excessive wasting;
solids washout;
low influent loading;
clarifier solids loss.

Example: Sludge Blanket Is Rising

Review:

RAS rate;
settling characteristics;
clarifier flow;
MLSS;
mechanical sludge removal.

Example: Effluent TSS Rises but Ammonia Remains Good

This pattern can suggest that biological treatment remains effective while solids separation is deteriorating.

Example: Effluent Ammonia Rises

Review:

SRT;
DO;
temperature;
alkalinity;
ammonia loading;
toxicity.

Example: Effluent BOD and TSS Rise Together

Investigate secondary clarification and solids carryover before assuming the aeration basin alone is responsible.

Example: Settling Test Worsens Gradually

Review long-term trends in:

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

Example: Settling Changes Suddenly

A sudden change can indicate:

toxic discharge;
major loading change;
pH upset;
sampling error.

Influent Toxicity

Toxic substances can inhibit biological activity.

Possible symptoms include:

loss of oxygen uptake;
rising effluent BOD;
loss of nitrification;
poor floc;
unusual odors.

Septic Influent

Septic wastewater can create:

odor;
low dissolved oxygen;
reduced sulfur compounds;
different biological conditions.

Hydraulic Overload

High flow can cause problems even when biological loading is moderate.

Possible effects include:

shorter aeration detention;
higher clarifier overflow rate;
solids washout.

Organic Overload

High organic loading can increase:

oxygen demand;
biological growth;
sludge production.

Operator Observation Matters

Useful observations include:

foam;
odor;
floc appearance;
clarifier surface condition;
sludge blanket behavior.

Foam

Foam can have several causes and should not be diagnosed by color alone.

Evaluate it with:

sludge age;
loading;
microscopic observations;
process history.

Process Stability

A stable activated-sludge process usually shows consistent relationships among:

loading;
MLSS;
DO;
wasting;
settling;
effluent quality.

Avoid Overcorrecting

Large changes to several controls at once can make it difficult to determine what actually improved or harmed the process.

Controlled Process Changes

When practical:

identify the most likely cause;
change the appropriate control;
document the change;
allow enough response time;
evaluate the result.

Common Activated-Sludge Fundamentals Mistakes

Assuming MLSS represents only living microorganisms.
Confusing RAS with WAS.
Changing wasting repeatedly before the biomass has time to respond.
Using DO alone to judge the entire process.
Ignoring clarifier solids inventory.
Ignoring influent mass loading.
Looking only at concentration and not flow.
Assuming high effluent BOD always means poor biological oxidation.
Ignoring solids carryover when effluent TSS rises.
Making several major process changes simultaneously.

A Practical Daily Activated-Sludge Review

Review influent flow and loading.
Review aeration-basin DO.
Review MLSS and MLVSS trends.
Review RAS flow.
Review WAS operation.
Review settling-test results.
Review clarifier sludge blankets.
Review effluent TSS and BOD.
Review ammonia where nitrification is required.
Document abnormal conditions and process changes.

A Practical Rising-MLSS Review

Verify the MLSS result.
Review WAS rate and operating time.
Review influent organic loading.
Review clarifier solids inventory.
Review settleability.
Review effluent solids loss.
Adjust wasting according to process goals and facility procedures.
Trend the response over an appropriate period.

A Practical Poor-Settling Review

Verify the settling test.
Review MLSS and SVI trends.
Review DO.
Review influent loading.
Review RAS and WAS operation.
Review sludge blanket depth.
Review microscopic observations where available.
Identify whether the condition is biological, hydraulic, or mechanical.

A Practical Effluent-Deterioration Review

Verify the effluent laboratory result.
Determine whether BOD, TSS, ammonia, or several parameters changed.
Review influent flow and loading.
Review aeration DO.
Review MLSS and solids age.
Review settling and clarifier performance.
Review RAS and WAS operation.
Identify whether the primary problem is biological treatment, solids separation, or both.

What to Remember for the Exam

Activated sludge uses suspended microorganisms to remove biodegradable wastewater pollutants.
The basic process includes aeration, secondary clarification, RAS, and WAS.
Mixed liquor is the mixture of wastewater and biological solids in the aeration basin.
MLSS measures total suspended solids in mixed liquor, while MLVSS estimates the volatile or organic portion.
Biological floc must both treat wastewater and settle effectively.
RAS returns settled biological solids from the secondary clarifier to the aeration basin.
WAS removes excess biological solids and is a major control of biomass inventory and sludge age.
Increasing wasting generally reduces solids inventory and SRT over time.
Decreasing wasting generally increases solids inventory and SRT over time.
SRT describes how long biological solids remain in the process.
F/M compares organic food loading with the amount of microorganisms available.
Organic loading in lb/day can be calculated as MGD × mg/L × 8.34.
Hydraulic loading and organic loading are different.
DO is affected by aeration, organic loading, ammonia loading, temperature, and biomass.
Excessive aeration can waste energy without improving treatment.
Secondary clarifiers separate biological solids from treated wastewater.
A rising sludge blanket can indicate inadequate RAS, poor settling, high hydraulic loading, or excessive solids inventory.
Clarifier solids carryover can increase both effluent TSS and BOD.
Activated-sludge troubleshooting should combine loading, DO, MLSS, RAS, WAS, settling, clarifier, and effluent data.
Good process control depends on making deliberate changes, allowing adequate response time, and evaluating trends rather than isolated numbers.