Monitoring & Troubleshooting

Activated-sludge troubleshooting is the process of identifying why treatment performance has changed and selecting the operating response that addresses the actual cause. The most effective operators do not react to one laboratory result or one visual observation. They compare multiple process indicators, determine when the change began, identify which part of the process changed first, and then make controlled adjustments.

Aeration, solids inventory, secondary clarification, influent loading, temperature, pH, RAS, WAS, and equipment operation are connected. A change in one area can produce symptoms somewhere else. Troubleshooting therefore requires a systemwide view.

Monitor the Process Before It Fails

Routine monitoring helps operators identify developing problems before effluent quality deteriorates.

Useful activated-sludge indicators include:

• influent flow;
• influent BOD or COD;
• influent ammonia;
• pH and alkalinity;
• aeration-basin DO;
• MLSS and MLVSS;
• RAS flow and concentration;
• WAS flow and concentration;
• settling test results;
• SVI;
• secondary clarifier sludge blanket;
• effluent TSS;
• effluent BOD;
• effluent ammonia.

Trends Are More Useful Than Isolated Numbers

A single result shows one condition at one time.

A trend can show:

• direction of change;
• rate of change;
• daily cycles;
• seasonal patterns;
• response to operator adjustments.

Establish Normal Operating Patterns

Troubleshooting is easier when operators know what normal operation looks like.

Useful baseline information includes typical:

• flow;
• DO profile;
• MLSS;
• sludge blanket;
• settling behavior;
• RAS and WAS rates;
• effluent quality.

Ask What Changed First

When several parameters are abnormal, determine which changed first.

For example:

1. influent loading increases;
2. DO decreases;
3. settling later deteriorates;
4. effluent TSS rises.

The first change may provide the best clue to the original cause.

Verify the Data Before Changing the Process

Unexpected data should be verified before major process changes are made.

Check for:

• sampling error;
• laboratory error;
• dirty probes;
• incorrect calibration;
• SCADA signal problems;
• wrong units;
• data-entry errors.

Process Problem or Measurement Problem?

If one measurement changes dramatically while related indicators remain normal, investigate the measurement itself.

For example, a DO probe reading near zero while airflow, basin appearance, and portable DO measurements are normal suggests an instrument problem rather than immediate process failure.

Influent Flow

Flow affects:

• hydraulic detention time;
• clarifier loading;
• mass loading;
• process response time.

High Influent Flow

High flow can contribute to:

• shorter aeration detention time;
• higher secondary clarifier loading;
• solids washout;
• lower influent concentration through dilution during wet weather.

Influent Concentration and Mass Loading

Concentration alone does not describe the total load applied to the plant.

A common relationship is:

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

Loading Example

Influent flow increases from 2 MGD to 3 MGD while BOD remains 200 mg/L.

At 2 MGD:

Load = 2 × 200 × 8.34 = 3,336 lb/day

At 3 MGD:

Load = 3 × 200 × 8.34 = 5,004 lb/day

The concentration did not change, but the biological load increased by about 50 percent.

Dissolved Oxygen

DO provides information about the balance between oxygen supply and biological oxygen demand.

A falling DO can result from:

• higher organic loading;
• higher ammonia loading;
• higher MLSS;
• blower failure;
• reduced airflow;
• diffuser fouling;
• instrument error.

Low DO Troubleshooting

When DO falls, review:

1. probe accuracy;
2. blower operation;
3. airflow;
4. influent loading;
5. ammonia loading;
6. MLSS;
7. diffuser condition.

High DO Troubleshooting

High DO may indicate:

• low biological load;
• excessive airflow;
• low biomass;
• probe error.

If treatment is stable, excessive aeration may waste energy.

MLSS

MLSS is a major indicator of aeration-basin solids concentration.

A rising MLSS trend can result from:

• insufficient wasting;
• increased solids production;
• changing RAS distribution;
• increased influent solids.

Falling MLSS

Possible causes include:

• excessive wasting;
• solids washout;
• low biological growth;
• clarifier solids loss.

Do Not Diagnose MLSS Without Clarifier Data

If MLSS appears stable but clarifier blankets are rising, total process solids inventory may still be increasing.

RAS Monitoring

RAS moves settled biological solids back to aeration.

Operators should monitor:

• RAS flow;
• RAS concentration;
• pump status;
• clarifier blanket response.

RAS Pump Failure

A failed RAS pump can cause:

• rapid blanket rise;
• solids accumulation;
• eventual solids carryover;
• reduced solids return to aeration.

WAS Monitoring

WAS controls long-term solids inventory and SRT.

Do not evaluate wasting only by pump run time.

Actual solids removed depend on:

• waste flow;
• waste concentration;
• operating time.

WAS Equipment Problems

If MLSS rises even though the WAS pump appears to operate normally, verify:

• actual flow;
• valve position;
• line blockage;
• pump performance;
• WAS concentration.

Settleometer Monitoring

A settleometer can provide rapid information about sludge-settling behavior.

Observe:

• initial settling;
• 30-minute settled volume;
• compaction;
• supernatant clarity;
• whether sludge later rises.

SVI Trends

SVI can help track changes in sludge settleability.

A steadily increasing SVI may provide warning before severe clarifier solids loss develops.

Clarifier Sludge Blanket

Blanket measurements help show how much sludge is stored in the secondary clarifier.

A rising blanket can result from:

• insufficient RAS;
• poor settling;
• high hydraulic loading;
• excessive solids inventory;
• sludge-removal equipment problems.

Effluent TSS

Rising effluent TSS commonly indicates deterioration in solids separation.

Possible causes include:

• bulking;
• pin floc;
• rising sludge;
• high clarifier loading;
• high sludge blanket;
• mechanical clarifier problems.

Effluent BOD

High effluent BOD can result from:

• incomplete biological treatment;
• biological solids carryover;
• both conditions together.

Use TSS to Help Interpret BOD

If BOD and TSS rise together, solids carryover should be investigated.

If BOD rises while TSS remains low, biological treatment itself may deserve greater attention.

Effluent Ammonia

Effluent ammonia is a critical indicator where nitrification is required.

Rising ammonia can result from:

• low SRT;
• low DO;
• cold temperature;
• insufficient alkalinity;
• high ammonia load;
• toxic inhibition.

Check Several Nitrification Conditions Together

Do not assume that every ammonia increase is caused by low DO.

Review:

• DO;
• SRT;
• temperature;
• pH;
• alkalinity;
• influent ammonia;
• toxicity potential.

pH and Alkalinity

Rapid pH decline can affect biological activity.

In nitrifying systems, low alkalinity can contribute to falling pH and reduced nitrification.

Temperature

Temperature affects biological reaction rates.

Cold conditions can:

• slow biological reactions;
• reduce nitrifier growth;
• change required SRT.

Industrial Discharges

Industrial or commercial discharges can cause sudden changes in:

• pH;
• organic loading;
• toxicity;
• temperature;
• solids characteristics.

Possible Toxic Shock

Possible signs include:

• sudden loss of nitrification;
• reduced oxygen uptake;
• unusual DO increase despite normal airflow;
• deteriorating floc;
• rising effluent BOD.

High DO Can Sometimes Signal Biological Inhibition

If microorganisms suddenly stop consuming oxygen, DO can rise even though airflow has not increased.

This is why a high DO reading should always be interpreted with loading and biological performance.

Hydraulic Upset

A hydraulic upset is primarily caused by abnormal flow rather than excessive organic loading.

Possible symptoms include:

• short detention time;
• high clarifier loading;
• solids washout;
• diluted influent concentrations.

Organic Overload

An organic overload can cause:

• increased oxygen demand;
• falling DO;
• rapid biomass growth;
• higher sludge production;
• temporary effluent deterioration.

Distinguish Hydraulic and Organic Loading

High flow does not always mean high organic loading.

Wet-weather flow can be hydraulically severe while diluted wastewater produces lower concentrations.

Bulking Sludge

Bulking is characterized by poor sludge settling and compaction.

Possible contributing conditions include:

• low DO;
• certain F/M conditions;
• nutrient imbalance;
• septic influent;
• industrial wastewater characteristics.

Do Not Treat All Bulking the Same Way

The appropriate response depends on the organisms and operating conditions causing the problem.

Rising Sludge

Sludge that settles normally and later rises may indicate gas formation, commonly associated with denitrification in the secondary clarifier.

Review:

• nitrate;
• blanket depth;
• RAS withdrawal;
• sludge residence time in the clarifier.

Pin Floc

Small floc particles can remain suspended and leave with the effluent even when the main sludge blanket is relatively low.

Review:

• sludge age;
• floc development;
• effluent TSS;
• microscopic observations where available.

Foam Troubleshooting

Foam can result from several causes.

Evaluate:

• foam color and persistence;
• SRT;
• F/M;
• influent surfactants;
• microscopic observations;
• process history.

Do Not Diagnose Foam by Color Alone

Visual appearance is useful but should be combined with process data.

Odor Troubleshooting

Strong septic odors can indicate:

• septic influent;
• poor mixing;
• low DO;
• solids deposits;
• anaerobic conditions.

Mechanical Problems Can Look Biological

Before making major process changes, verify equipment such as:

• blowers;
• diffusers;
• RAS pumps;
• WAS pumps;
• clarifier mechanisms;
• flow meters;
• DO probes.

Example: DO Low, Effluent Still Good

This may be an early warning.

Review aeration capacity and loading before effluent quality deteriorates.

Example: DO Low and Ammonia Rising

Possible explanation:

• insufficient oxygen for nitrification.

Also review SRT, temperature, alkalinity, and ammonia load.

Example: DO Normal and Ammonia Rising

Review:

• SRT;
• temperature;
• alkalinity;
• toxicity;
• influent ammonia load.

Example: MLSS Rising, Blanket Rising

This combination can indicate increasing total solids inventory.

Review:

• WAS mass;
• influent loading;
• RAS operation;
• settling.

Example: MLSS Stable, Blanket Rising

Solids may be shifting from the aeration basin into the clarifier.

Review RAS and settling performance.

Example: MLSS Falling, Effluent TSS Rising

This can indicate biomass washout through the secondary clarifier.

Example: Effluent TSS High, Ammonia Low

Biological nitrification may still be functioning while solids separation is failing.

Example: Effluent TSS Low, Ammonia High

Clarification may be functioning normally while nitrification is impaired.

Example: BOD and TSS Rise Together

Investigate solids carryover as well as biological treatment.

Example: DO Suddenly Increases

Possible causes include:

• lower influent loading;
• reduced biomass;
• toxic inhibition;
• excess aeration;
• probe error.

Example: RAS Flow Normal but Blanket Rising

Review:

• actual RAS pump output;
• RAS concentration;
• settleability;
• sludge collection equipment;
• hydraulic loading.

Example: One Clarifier Performs Poorly

If other parallel clarifiers are normal, investigate local conditions such as:

• flow distribution;
• RAS pump;
• sludge scraper;
• weirs;
• withdrawal piping.

Example: All Clarifiers Deteriorate Together

Investigate plantwide conditions such as:

• poor settleability;
• high flow;
• high solids inventory;
• toxic influent;
• process-control changes.

Make One Logical Change at a Time

When possible, change the parameter most directly connected to the identified cause.

Changing several controls simultaneously makes the response difficult to interpret.

Know the Response Time

Different controls affect the process at different speeds.

Examples:

• airflow can change DO within minutes;
• RAS changes can alter blanket behavior relatively quickly;
• WAS changes affect SRT and biomass over days.

Do Not Overcorrect Slow Variables

A common mistake is increasing or decreasing WAS repeatedly before the system has had enough time to respond.

Document Every Significant Adjustment

Record:

• date and time;
• reason for change;
• previous setting;
• new setting;
• expected effect;
• observed response.

Use Process Relationships

Useful relationships include:

• higher load can increase oxygen demand;
• higher solids inventory can increase oxygen demand;
• low SRT can impair nitrification;
• high clarifier loading can increase solids loss;
• high effluent TSS can increase effluent BOD.

Look for Cause-and-Effect Sequences

A useful troubleshooting timeline might show:

1. WAS pump output decreases;
2. MLSS begins rising;
3. clarifier blankets rise;
4. DO declines;
5. effluent TSS increases.

This sequence points toward a solids-control problem rather than five unrelated failures.

Common Troubleshooting Mistakes

• Reacting to one abnormal number without verifying it.
• Changing several process controls at once.
• Ignoring equipment failures while assuming the problem is biological.
• Looking only at concentrations instead of mass loading.
• Ignoring clarifier solids inventory.
• Using RAS and WAS as if they perform the same function.
• Ignoring process response time.
• Assuming every ammonia problem is caused by DO.
• Assuming every high BOD result means poor aeration.
• Failing to document changes and results.

A Practical Troubleshooting Sequence

1. Verify the abnormal result or observation.
2. Determine when the condition began.
3. Identify what changed first.
4. Review influent flow and loading.
5. Review DO and aeration equipment.
6. Review MLSS, RAS, and WAS.
7. Review settling and clarifier blankets.
8. Review effluent TSS, BOD, and ammonia.
9. Identify the most likely biological, hydraulic, mechanical, or measurement cause.
10. Make a controlled corrective change and trend the response.

A Practical Poor-Effluent Review

1. Verify laboratory results.
2. Determine which effluent parameters changed.
3. Compare BOD with TSS.
4. Review ammonia where nitrification is required.
5. Review influent loading.
6. Review DO.
7. Review solids inventory and SRT.
8. Review settling and clarifier operation.
9. Review equipment status.
10. Correct the identified cause rather than the most visible symptom.

A Practical Process-Upset Review

1. Preserve and review historical trend data.
2. Check for unusual industrial or hauled waste inputs.
3. Review pH, temperature, and alkalinity.
4. Review oxygen demand and DO response.
5. Check biomass concentration and settling.
6. Review nitrification performance.
7. Inspect equipment for simultaneous failures.
8. Make gradual adjustments appropriate to the cause.
9. Monitor recovery over the expected biological response period.

A Practical Daily Monitoring Review

1. Review influent flow and load.
2. Review DO profile and aeration operation.
3. Review MLSS and MLVSS.
4. Review RAS and WAS operation.
5. Review settling test and SVI.
6. Review secondary clarifier blankets.
7. Review effluent TSS and BOD.
8. Review ammonia and other process-specific parameters.
9. Compare current conditions with normal trends.
10. Document abnormalities before they become major upsets.

What to Remember for the Exam

• Activated-sludge troubleshooting should use trends and relationships rather than one isolated result.
• Verify unusual laboratory, instrument, and SCADA data before making major process changes.
• Mass loading depends on both flow and concentration.
• Load in lb/day can be calculated as MGD × mg/L × 8.34.
• Low DO can result from higher loading, higher MLSS, aeration failure, diffuser problems, or measurement error.
• RAS primarily moves solids between the clarifier and aeration basin.
• WAS controls long-term solids inventory and SRT.
• A rising sludge blanket can result from low RAS, poor settling, high loading, excess solids inventory, or mechanical failure.
• High effluent TSS usually points toward solids-separation problems.
• High effluent BOD can result from poor biological treatment, solids carryover, or both.
• Rising ammonia can be caused by low SRT, low DO, low temperature, insufficient alkalinity, high load, or toxicity.
• High DO can sometimes indicate reduced biological oxygen uptake, not simply excellent aeration.
• Hydraulic overload and organic overload are different conditions.
• Bulking, pin floc, and rising sludge are different settling problems and should not be treated as identical.
• Mechanical failures can create symptoms that resemble biological process problems.
• Airflow changes can affect DO quickly, while wasting changes affect biomass over a much longer period.
• Changing several controls simultaneously makes troubleshooting more difficult.
• Process changes should be documented with the reason, setting, expected response, and actual result.
• The best troubleshooting question is often: what changed first?
• Good activated-sludge troubleshooting combines influent, aeration, solids, settling, clarification, equipment, and effluent data into one process picture.