High COD BOD Effluent: Common Causes and How Factories Can Control It

jones chee • 14 September 2026

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Why Are COD and BOD Levels High?

High Chemical Oxygen Demand (COD) and Biochemical Oxygen Demand (BOD) usually indicate excessive organic pollution entering the wastewater treatment system or poor treatment performance. Common causes include increased production loading, ineffective equalisation, biological process instability, inadequate aeration, incorrect chemical dosing, equipment failure, and poor sludge management. Identifying the root cause is essential before implementing corrective actions.

What Do High COD and BOD Mean?

Organic pollution. Treatment performance. Environmental compliance.


COD and BOD are two of the most important indicators used to assess industrial wastewater quality.



Although they measure different characteristics, both provide valuable information about the amount of organic matter entering or leaving a treatment plant.

Parameter

What It Measures

Typical Concern

COD

BOD

Total oxidisable organic matter

Biodegradable organic matter

Overall organic loading

Overall organic loading

When either parameter remains consistently high, the treatment system may be overloaded or operating inefficiently.

Common Causes of High COD and BOD

Most compliance failures are caused by operational changes rather than sudden equipment failures.

1. Increased Production Loading

One of the most common causes.



Examples include:

  • Production expansion
  • New product lines
  • Seasonal demand
  • Longer operating hours


When wastewater loading exceeds the original plant design, treatment efficiency gradually declines.

2. Poor Equalisation

An undersized or poorly operated Equalisation Tank allows shock loads to reach downstream treatment processes.



Symptoms include:

  • Sudden COD spikes
  • Variable pH
  • Inconsistent flow
  • Biological instability


3. Biological Process Instability

The biological reactor removes most biodegradable organics.


Performance may decline because of:

  • Low dissolved oxygen
  • Toxic chemicals
  • Nutrient imbalance
  • Incorrect sludge age
  • Temperature changes
  • Hydraulic shock loading


Signs include:

  • Rising effluent COD
  • High BOD
  • Foaming
  • Sludge bulking
  • Poor settling


4. Inadequate Aeration

Without sufficient oxygen, microorganisms cannot effectively degrade organic matter.



Common causes:

  • Blower failure
  • Dirty diffusers
  • Incorrect airflow
  • Excessive MLSS
  • Poor mixing


5. Incorrect Chemical Dosing

Chemical treatment should support and not replace the treatment process.


Common issues include:

  • Under-dosing
  • Over-dosing
  • Incorrect pH adjustment
  • Poor coagulation
  • Ineffective floc formation


Jar testing should be carried out periodically to optimise chemical dosage.

6. Equipment Problems

Mechanical failures often reduce treatment efficiency.


Equipment to inspect:

  • Pumps
  • Blowers
  • Mixers
  • DAF systems
  • Chemical dosing pumps
  • Clarifier mechanisms
  • Instrumentation


Preventive maintenance reduces the likelihood of unexpected process failures.

7. Poor Sludge Management

Sludge is often overlooked until performance begins to decline.


Common issues:

  • Excess sludge accumulation
  • Sludge carryover
  • Low sludge age
  • High sludge age
  • Poor settling


Stable sludge management supports consistent biological treatment.

Root Cause vs Symptom

One of the biggest mistakes is treating symptoms instead of identifying the underlying cause.

Symptom

Likely Root Cause

High COD

High BOD

High TSS

High Oil & Grease

Variable Results

Increased influent loading, poor biological treatment

Biological instability, insufficient aeration

Clarifier carryover, sludge bulking

Ineffective DAF or oil separation

Poor equalisation or inconsistent operation

Addressing the root cause produces more sustainable improvements than repeatedly adjusting chemical dosage.

Engineering Troubleshooting Checklist

When COD or BOD begins increasing, review the following operating conditions before making major process changes. Tick each item that has already been checked.

Checklist Progress 0 / 10 checked

Diagnostic Status

Start reviewing the checklist to assess how thoroughly the potential causes of high COD or BOD have been investigated.

This checklist is a preliminary troubleshooting aid only. High COD or BOD may result from multiple interacting causes, so process data, laboratory results, operating history, and plant design should be reviewed together before major adjustments are made.

Recommended Treatment Technologies

Different problems require different solutions.

Find the Right Wastewater Treatment Technology

Select the main wastewater treatment challenge at your facility to see technologies commonly considered for that application.

Recommended Technology

Select a wastewater challenge

Best Suited For

Select a treatment challenge above.

How It Helps

The recommended treatment approach will appear here.

Typical Treatment Approach:
Engineering Note: Technology selection should not be based on one wastewater parameter alone. Influent characteristics, flowrate, pollutant loading, discharge standard, available footprint, operating cost and existing treatment infrastructure should be evaluated together.
Request a Free Wastewater System Audit →

This interactive tool provides general engineering guidance only. The most appropriate treatment technology should be determined through wastewater characterisation, process assessment and engineering design.

Selecting the correct treatment technology depends on wastewater characteristics, discharge standards, available space, and long-term operating objectives.

Signs Your Plant Needs Immediate Technical Review

Consider a comprehensive engineering assessment if you experience:

  • Repeated high COD results
  • Increasing BOD trends
  • Rising chemical consumption
  • Higher sludge production
  • Frequent DAF adjustments
  • Low dissolved oxygen
  • Inconsistent effluent quality
  • Repeated DOE non-compliance


Early intervention is generally less costly than emergency corrective action.

Final Thoughts on Wastewater Treatment for High COD and BOD

High COD and BOD should be viewed as indicators of process performance rather than isolated laboratory results.


The most effective wastewater treatment plants identify the underlying causes of rising organic loading, optimize treatment processes, and monitor performance trends before compliance issues develop.


A structured engineering approach consistently delivers better long-term results than reactive adjustments after laboratory reports indicate failure.

Need Help Troubleshooting High COD or BOD?

If your wastewater treatment plant is experiencing recurring COD or BOD compliance issues, Cheme Advance Services Sdn. Bhd. can help identify the root cause and develop practical solutions.



Our engineering services include:

  • Industrial Effluent Treatment System (IETS) audits
  • Root cause analysis
  • Biological process optimisation
  • Chemical dosing optimisation
  • DAF optimisation
  • Wastewater treatment troubleshooting
  • Plant upgrading
  • Operation & Maintenance (O&M)


Book a Wastewater System Audit


Our technical team can assess your treatment plant, identify operational bottlenecks, and recommend practical engineering improvements to restore treatment stability and support long-term DOE compliance.

Frequently Asked Questions

Why is COD increasing even though production has not changed?

Changes in raw materials, cleaning procedures, wastewater segregation, equipment performance, or biological treatment conditions can all increase COD without a significant change in production volume.

Can high COD be reduced by adding more chemicals?

Sometimes, but not always. If the root cause is biological instability or excessive organic loading, increasing chemical dosage may provide only temporary improvement.

What is the first parameter operators should check?

Start with influent flow, pH, Dissolved Oxygen, MLSS, sludge settling characteristics, and recent production changes before making major process adjustments.

Is high BOD always caused by poor biological treatment?

Not necessarily. High BOD may also result from sudden increases in biodegradable wastewater entering the treatment plant.

About the Author

Jones Chee
Marketing Engineer | Cheme Advance Services Sdn. Bhd.


Jones is a Marketing Engineer at Cheme Advance Services Sdn. Bhd. and holds a Bachelor of Mechanical Engineering (Honours) from Monash University Malaysia. Drawing on over five years of experience in environmental engineering and working closely with Cheme Advance's technical specialists, he develops technically reviewed content on industrial wastewater treatment, plant optimisation, and environmental compliance.


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