COD: Chemical Oxygen Demand

What Is In It?

by Rami Elias Kremesti M.Sc., Cci, CEnv, CWEM

Introduction

Few people understand what is in COD, that is why I decided to publish this page as a chemist who is interested in the composition of pollutants in water. Non-biodegradable Chemical Oxygen Demand (COD), measured as a lab test, is caused by soluble organic molecules that resist biological breakdown by natural biological processes (microorganisms) in a reasonable timeframe (5 days or measured as BOD5).

Examples of Organics Contributing to COD

Examples of non-biodegradable COD molecules found in industrial and municipal wastewater include:

Pharmaceuticals: Various pharmaceutical compounds, such as diclofenac, anti-biotics, paracetamol, and propofol, are persistent in the environment and contribute to non-biodegradable COD.
Pesticides and Insecticides: Many pesticides, including DDT (dichlorodiphenyltrichloroethane), are synthetic organic chemicals that are highly stable and resistant to microbial degradation.
Industrial Chemicals: This category includes persistent organic pollutants (POPs), certain solvents, and petrochemical by-products that pose significant challenges for conventional treatment.
Synthetic Dyes: The textile and dyeing industries often produce wastewater with non-biodegradable synthetic dyes and pigments.
Plastics and Polymers: While the bulk material is a solid waste issue, the chemical components of many synthetic polymers (like polyethylene and PVC) are non-biodegradable on a human timescale and contribute to the overall organic load when present as microplastics or dissolved components.
Humic Substances: These large molecular weight natural organic matters are produced in environments like forest areas and are a major cause of non-biodegradable organic accumulation in some water sources.
Fluorinated and Brominated Organic Compounds: These man-made molecules are often very stable and resistant to natural breakdown.
Cellulose: A Major Component of COD. In domestic wastewater, cellulose (primarily from toilet paper and undigested salads) is a major component of the total insoluble COD, accounting for a significant percentage (up to 25-30% in some cases).
PFAS: Studies have found significant correlations between per- and polyfluoroalkyl substances (PFAS) concentrations and Chemical Oxygen Demand (COD), particularly in certain environmental samples like landfill leachate. This indicates that PFAS can contribute to the measured COD in these specific contexts.
Surfactants: Let us look at them more closely below.

Surfactants

Surfactants are surface active organic compounds that are omniphilic, i.e. they have a water loving moiety and a fat loving or lipophilic moiety, mostly used as detergents, and because chemical oxygen demand (COD) is a measure of the total oxidizable organic matter in water, virtually all surfactants contribute to COD. Specific examples commonly found in industrial and municipal wastewater include:

Anionic Surfactants

Anionic surfactants are the most widely produced and consumed type, making up about 60% of the total market.
Linear alkylbenzene sulfonates (LAS) (e.g., sodium dodecylbenzene sulfonate): Widely used in laundry and dishwashing detergents.
Alkyl sulfates (AS) (e.g., sodium dodecyl sulfate, or SDS): Common in personal care products and detergents.
Alcohol ethoxysulfates (AES): Also prevalent in domestic and industrial cleaning agents.

Nonionic Surfactants

These are also significant contributors, particularly in industrial applications.
Alkylphenol ethoxylates (APEOs) (e.g., nonylphenol ethoxylate or NPEO): Used as detergents, emulsifiers, and wetting agents, though their toxic degradation products are a concern.
Alcohol ethoxylates (AEOs): Found in various domestic, cosmetic, and industrial applications.
Polysorbates (e.g., T80): Used as emulsifiers in food and pharmaceuticals, and can be used as a standard for COD assessment in lab settings.

Cationic Surfactants

These are often used for their disinfecting or fabric-softening properties.
Quaternary ammonium compounds (QACs) (e.g., benzalkonium chloride, didecyl dimethyl ammonium chloride): Used in fabric softeners, disinfectants, and hair conditioners.

These compounds, due to their organic nature, consume a significant amount of oxygen during chemical oxidation, resulting in high COD values in wastewater, often in the range of thousands of mg/L in industrial effluents.

Substances contributing to COD often pass through standard biological wastewater treatment processes largely unchanged, which is why a high COD to BOD (Biochemical Oxygen Demand) ratio is an indicator of significant non-biodegradable pollutants in the water. Specialized advanced chemical or physical treatment methods are needed for their removal.

How To Remove COD from Waste Water Effluent

Traditional coagulation and flocculation will remove a large percentage of COD but the rest requires specialized treatment.

Chemical Oxidation: Strong oxidizers (ozone, H₂O₂, chlorine, UV) break down non-biodegradable COD pollutants.

Electro-oxidation: Uses electrodes to generate .OH radicals that destroy pollutants.

Advanced & Polishing Steps

Adsorption (Activated Carbon): Effective for removing residual organics after primary treatment.

Advanced Oxidation Processes (AOPs): For tough, recalcitrant COD.

Advanced Non Thermal Plasmas

Ultra Filtration and MBR will remove a large proportion of COD since it consists of molecules that are large enough to be rejected by the small pore size of UF membranes.

Conclusion

Micro-pollutants that contribute to COD are recalcitrant organics that need to be removed from waste water effluent and the EU has updated its Urban Waste Water Treatment Directive to deal with these pollutants strategically in the next 5-10 years. The UK is still behind in addressing this environmental issue but is slowly catching up.

About the Author

Chemists are a unique breed of people and Rami Elias Kremesti is one of those chemists that deeply care about the environment. Ever since he was a kid, Rami Elias Kremesti was fascinated by nature and he used to go fishing and snorkelling in the Mediterranean in Beirut, Lebanon where he was born and grew up during the civil war. The Mediterranean in Lebanon is very polluted by sewage and industrial effluents and Rami once contracted a fungus while swimming in a polluted beach in Beirut. Kremesti studied chemistry in the prestigious American University of Beirut and pursued graduate studies in the USA also in chemistry. He resides in the UK where he works as a water treatment expert.

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Rami Elias Kremesti Portrait