Why Is BOD Increasing in Your ETP? A Practical Troubleshooting Guide

Why Is BOD Increasing in Your ETP? This is one of the most common questions faced by industries operating Effluent Treatment Plants (ETPs). A sudden or gradual increase in Biological Oxygen Demand (BOD) in treated effluent can indicate problems with biological treatment, aeration, sludge management, chemical dosing, hydraulic loading, or even upstream process changes.

It can also indicate that the ETP is not operating efficiently, resulting in higher operating costs, unstable treatment performance, odour problems, excess sludge generation, and difficulty maintaining discharge standards.

The good news is that increasing BOD can usually be traced back to a few key operational factors.

In this practical troubleshooting guide, we explain the major reasons why BOD increases in an ETP and what you can check to restore stable treatment performance.

What Is BOD and Why Does It Matter?

Biochemical Oxygen Demand (BOD) is the amount of dissolved oxygen required by microorganisms to biologically break down biodegradable organic matter present in wastewater.

In simple terms, BOD indicates the amount of biodegradable organic pollution in the wastewater.

A high BOD value means the wastewater contains a higher concentration of biodegradable organic matter. If this organic matter is not adequately removed inside the ETP, the treated effluent may show elevated BOD levels.

BOD is therefore an important parameter for evaluating the performance of biological wastewater treatment systems.

Depending on the industry and treatment process, BOD can be affected by:

• Raw material and production changes
• Organic loading
• Aeration efficiency
• Microbial activity
• MLSS and MLVSS
• Sludge age
• Hydraulic loading
• Nutrient availability
• pH and temperature
• Toxic or inhibitory substances
• Sludge settling and recycle problems
• Poor operation and maintenance

Understanding the root cause is more effective than simply increasing aeration or chemical dosing.

10 Common Reasons Why BOD Is Increasing in Your ETP

1. Sudden Increase in Organic Load

One of the first things to investigate is whether the ETP is receiving a higher organic load than it was designed to handle.

Production increases, changes in raw materials, batch processing, or discharge of concentrated streams can significantly increase the organic load entering the ETP.

For example, if production increases by 30–40% but the ETP continues to operate at its previous capacity, the biological system may become overloaded.

What to check:

• Daily production quantity
• Raw wastewater flow
• Influent BOD and COD
• BOD/COD load in kg/day
• Changes in production processes
• High-strength wastewater streams
• Batch or intermittent discharge patterns

Practical tip: Don’t look only at BOD concentration. Calculate the actual organic load:

Organic Load (kg/day) = Flow (m³/day) × BOD (mg/L) ÷ 1,000

This provides a much better picture of what your biological system is actually receiving.

2. Insufficient Aeration

Aeration is one of the most critical components of an aerobic biological treatment system.

If microorganisms do not receive sufficient oxygen, their ability to degrade organic matter decreases. This can result in increased BOD in the treated effluent.

Low dissolved oxygen (DO) can occur because of:

• Blower problems
• Damaged diffusers
• Blocked diffusers
• Insufficient air supply
• Incorrect blower operation
• High MLSS
• Poor mixing
• Increased organic loading

What to check:

Measure DO at different locations in the biological tank rather than relying on a single reading.

Also inspect:

• Blower pressure
• Airflow
• Diffuser condition
• Air distribution
• Tank mixing
• DO trends during different operating periods

Simply increasing blower capacity may not solve the problem if the actual issue is poor air distribution or clogged diffusers.

3. Incorrect MLSS or Biomass Concentration

Mixed Liquor Suspended Solids (MLSS) represents the concentration of suspended solids in the biological reactor.

If MLSS is too low, there may not be enough active biomass to treat the incoming organic load.

If MLSS is excessively high, oxygen transfer and mixing can become difficult, potentially affecting biological treatment efficiency.

Therefore, maintaining the correct biomass concentration is important.

Operators should monitor:

• MLSS
• MLVSS
• MLVSS/MLSS ratio
• SVI
• Sludge age
• Return activated sludge (RAS)
• Waste activated sludge (WAS)

The optimum operating range depends on the specific ETP process, wastewater characteristics, and design.

4. Poor Sludge Age or SRT Control

Solids Retention Time (SRT) or sludge age is another important parameter in biological treatment.

If sludge is wasted too aggressively, the system may lose active microorganisms faster than they can reproduce.

This can result in:

• Low biomass concentration
• Poor organic removal
• Increasing BOD
• Biological instability

On the other hand, excessively long sludge age can also create operational problems, including poor settling characteristics and changes in microbial populations.

Therefore, sludge wasting should be based on actual process requirements rather than a fixed routine alone.

5. Nutrient Deficiency

Microorganisms require more than carbon-based organic matter to grow.

They also require nutrients such as:

• Nitrogen
• Phosphorus
• Trace elements

Certain industrial wastewaters may have a high carbon content but insufficient nitrogen or phosphorus.

When nutrients are deficient, microbial growth and biological activity can be affected, resulting in poor BOD removal.

What to check:

• Influent nitrogen
• Influent phosphorus
• Nutrient dosing system
• Chemical dosing pumps
• Nutrient solution concentration
• Changes in wastewater composition

Nutrient dosing should be based on wastewater analysis and process requirements rather than guesswork.

6. pH and Temperature Problems

Biological treatment depends heavily on maintaining suitable environmental conditions for microorganisms.

Extreme or rapidly changing pH can inhibit microbial activity.

Similarly, significant temperature changes can affect biological reaction rates.

Industrial wastewater may also contain chemicals that cause sudden changes in pH or temperature.

Monitor:

• pH at ETP inlet
• pH in biological tank
• Outlet pH
• Wastewater temperature
• Sudden process discharge events

If pH fluctuations are occurring, investigate the source rather than continuously compensating with chemicals.

7. Toxic or Inhibitory Chemicals Entering the ETP

A sudden increase in BOD can sometimes occur because microorganisms have been affected by toxic substances.

Industrial processes may introduce:

• Solvents
• Heavy metals
• Disinfectants
• Cleaning chemicals
• High concentrations of acids or alkalis
• Biocides
• Process-specific chemicals

A shock load can reduce biological activity and cause deterioration in treatment performance.

Warning signs may include:

• Sudden increase in treated BOD
• Change in sludge colour
• Poor settling
• Reduced oxygen uptake
• Foaming
• Unusual odour
• Sudden drop in biological activity

If a toxic shock is suspected, review production and cleaning activities around the time the problem started.

8. Hydraulic Overloading

ETP performance depends not only on organic loading but also on hydraulic loading.

If wastewater flow suddenly increases, the effective Hydraulic Retention Time (HRT) can decrease.

This means wastewater may move through the treatment system too quickly for adequate biological treatment.

Hydraulic overloading can be caused by:

• Increased production
• Excess process water
• Stormwater entering the system
• Cooling water entering the ETP
• RO reject or other streams being redirected
• Leakage or cross-connections

What to check:

Compare actual daily flow with the ETP’s design capacity.

Also investigate unusual flow peaks rather than looking only at daily average flow.

9. Poor Secondary Clarifier Performance

Even when the biological reactor is performing well, poor clarification can increase BOD in the final treated effluent.

If sludge is not settling properly, suspended solids can escape with the treated water.

These solids contain organic matter and can contribute to higher measured BOD.

Common causes include:

• Poor sludge settling
• High SVI
• Bulking sludge
• Excessive hydraulic loading
• Poor RAS control
• Short-circuiting
• Clarifier overload
• Mechanical problems

Therefore, if BOD suddenly increases, check both the biological reactor and secondary clarifier.

10. Sampling and Testing Errors

Not every increase in BOD is necessarily caused by an ETP process failure.

Sampling and laboratory practices can also influence results.

Check:

• Sampling location
• Sampling method
• Sample preservation
• Time between collection and analysis
• Laboratory procedures
• Sample contamination
• Testing frequency
• Changes in laboratory or testing methodology

If the BOD result appears unusually high compared with COD, TSS, and previous trends, verify the result before making major process changes.

A Practical BOD Troubleshooting Checklist

When treated BOD starts increasing, operators should follow a systematic approach.

Step 1: Compare Current and Historical Data

Review at least the previous few weeks of:

• Influent BOD
• Outlet BOD
• COD
• TSS
• Flow
• pH
• DO
• MLSS
• MLVSS
• SVI

Look for when the deviation started.

Step 2: Check Influent Conditions

Ask:

Has anything changed upstream?

Check production, raw materials, cleaning schedules, chemical usage, wastewater flow, and batch discharges.

Step 3: Check Biological Treatment

Evaluate:

• DO
• MLSS
• MLVSS
• SRT
• Sludge wasting
• Nutrient availability
• pH
• Temperature

Step 4: Inspect Aeration Equipment

Check blowers, diffusers, air headers, valves, airflow and mixing.

Step 5: Check Clarifier Performance

Look for sludge carryover, poor settling, floating sludge, short-circuiting and abnormal sludge blanket levels.

Step 6: Verify Laboratory Results

Repeat testing when necessary and compare the result with other process parameters.

Don’t Treat High BOD With Guesswork

One of the most common mistakes in ETP operation is changing multiple parameters simultaneously without identifying the root cause.

For example, increasing aeration, adding chemicals, changing sludge wasting and increasing nutrient dosing at the same time may temporarily change the result but make it difficult to understand what actually solved the problem.

A better approach is to:

Measure → Diagnose → Correct → Monitor

Start with data.

Identify the deviation.

Find the likely root cause.

Make a controlled correction.

Then monitor the trend.

How CIEMS Can Help Improve ETP Performance

Effluent treatment is not simply about operating equipment. Consistent performance requires understanding wastewater characteristics, biological processes, chemical treatment, hydraulic loading and plant operating conditions.

CIEMS provides environmental and wastewater management solutions to help industries improve their treatment processes and maintain reliable ETP performance.

Our approach focuses on identifying the actual cause of treatment problems rather than applying temporary fixes.

From wastewater assessment and ETP performance evaluation to process optimisation and environmental compliance support, the right technical assessment can help industries improve treatment efficiency and reduce operational challenges.

Conclusion

Increasing BOD in an ETP is a symptom, not a diagnosis.

The root cause could be increased organic loading, insufficient aeration, incorrect MLSS, poor sludge age control, nutrient deficiency, pH fluctuations, toxic shock loads, hydraulic overloading, clarifier problems or even sampling issues.

The most effective way to resolve high BOD is to evaluate the entire treatment process systematically.

If your ETP BOD is increasing repeatedly or the plant is struggling to meet treated effluent standards, a detailed technical assessment can help identify the underlying problem and improve treatment performance.

Need help troubleshooting your ETP? Contact CIEMS for professional environmental and wastewater treatment support.