Difference between Soluble and Insoluble Chemical Oxygen Demand (COD)

Difference between Soluble and Insoluble Chemical Oxygen Demand (COD)

 COD stands for Chemical Oxygen Demand, which is a measure of the amount of oxygen required to oxidize organic and inorganic substances in water. It is commonly used as an indicator of water pollution and the presence of organic contaminants.

Soluble COD refers to the portion of COD that is dissolved in water and can be easily measured using standard analytical methods. These are typically organic compounds that are in solution and can be readily oxidized. Soluble COD is often associated with easily biodegradable organic matter and is more readily treated or removed through biological processes, such as activated sludge treatment or aerobic digestion.

Insoluble COD, on the other hand, refers to the portion of COD that is not dissolved in water and exists as suspended solids or particulate matter. These are typically larger organic particles or compounds that are not easily oxidized and may require additional treatment processes, such as physical separation or chemical oxidation, to remove or reduce their concentration. Insoluble COD is often associated with more complex organic matter, such as fats, oils, and greases, or certain types of industrial waste.

Both soluble and insoluble COD contribute to the overall COD value of a water sample, but they represent different forms of organic pollution. Understanding the distribution of soluble and insoluble COD can help in designing appropriate treatment strategies to effectively remove or reduce organic contaminants in wastewater or other water sources.


The treatment of insoluble COD (Chemical Oxygen Demand) typically involves physical or chemical processes to separate or transform the particulate or suspended organic matter into a form that can be easily treated or removed. Here are some common methods used for the treatment of insoluble COD:

  1. Screening: Screening is a physical process that involves the use of screens or sieves to remove large solid particles or debris from the wastewater. It helps in reducing the load of insoluble COD by removing the larger particles before further treatment.

  2. Sedimentation: Sedimentation or settling is a process where gravity is used to allow the heavier suspended solids to settle at the bottom of a settling tank or clarifier. This can help in separating and removing a significant portion of the insoluble COD.

  3. Filtration: Filtration involves passing the wastewater through a filter media, such as sand or activated carbon, to remove suspended solids. It can be effective in reducing insoluble COD by capturing and retaining the solid particles in the filter media.

  4. Flotation: Flotation is a separation technique that utilizes the attachment of air bubbles to the suspended solids to float them to the surface, where they can be skimmed off. This process, known as dissolved air flotation (DAF), is effective in removing insoluble COD, particularly in the presence of fats, oils, and greases.

  5. Coagulation and Flocculation: Coagulation and flocculation processes involve the addition of chemicals, such as coagulants and flocculants, to promote the aggregation of fine suspended particles into larger flocs that can settle or be filtered more easily. This can aid in the removal of insoluble COD by facilitating the separation of suspended solids.

  6. Chemical Oxidation: Chemical oxidation processes, such as the use of oxidizing agents like chlorine, ozone, or hydrogen peroxide, can be employed to convert insoluble organic compounds into more soluble forms or break them down into smaller, more easily degradable substances. This can enhance the subsequent treatment of insoluble COD using biological methods.

  7. Biological Treatment: While insoluble COD is not directly biodegradable, after the application of physical or chemical processes, the residual organic matter may become more accessible to biological treatment. Therefore, biological treatment processes, such as activated sludge systems or aerobic digestion, can be employed to further degrade the remaining soluble COD fraction after the pre-treatment of insoluble COD.

The specific treatment methods used for insoluble COD will depend on the characteristics of the wastewater, the concentration and nature of the insoluble compounds, and the desired effluent quality. It is often necessary to combine multiple treatment techniques to achieve effective removal of insoluble COD and ensure compliance with regulatory standards.

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