Chemical Oxygen Demand (COD) is one of the most commonly used water quality indicators in the treatment of industrial and municipal wastewater. High COD levels typically indicate a significant presence of oxidizable organic matter in the water; consequently, reducing COD is a key objective for many wastewater treatment systems.
Activated carbon is widely used for advanced wastewater treatment and COD reduction due to its highly developed pore structure, large internal surface area, and capacity to adsorb a variety of organic pollutants. Particularly following processes such as biological treatment, coagulation-sedimentation, or other forms of pretreatment, activated carbon can further remove residual recalcitrant organics, color-causing substances, phenols, aromatic compounds, and other dissolved organic matter, thereby achieving a further reduction in effluent COD.
What Is COD in Wastewater?
In wastewater treatment, COD is commonly used to assess the total load of organic pollutants in the water. A higher COD value generally indicates the presence of a greater quantity of oxidizable substances requiring further treatment. However, COD is a composite indicator rather than a single, specific chemical substance.
For instance, the COD in wastewater may stem primarily from:
• low-molecular-weight organic acids;
• sugars and alcohols;
• phenols;
• aromatic organic compounds;
• dyes;
• surfactants;
• humic substances;
• petroleum-based organics;
and recalcitrant (non-biodegradable) organic compounds.
Therefore, when evaluating the COD removal performance of activated carbon, one cannot simply assume that a higher initial COD necessitates a higher dosage of activated carbon to achieve a greater reduction in COD. What truly needs to be understood is the specific composition of the organic matter contributing to the COD and what fraction of that matter can be effectively adsorbed by the activated carbon.
How Does Activated Carbon Remove COD?
The primary mechanism by which activated carbon removes COD is adsorption. Upon activation, activated carbon develops a structure comprising a vast number of micropores and mesopores, alongside some macropores. Once organic molecules from the water enter the activated carbon particles, they are retained within the material through surface interactions and the pore structure.
The overall process can be simplified as follows: organic pollutants in wastewater → activated carbon surface → interior of pores → adsorption → reduction of organic content in water → decrease in COD.
Second, in addition to reducing COD, activated carbon can simultaneously lower the levels of:
• color;
• phenol and phenolic compounds;
• aromatic organic compounds;
• certain surfactants;
• some refractory organic compounds;
• certain odor-causing substances;
and some trace organic pollutants.
Consequently, in the treatment of certain industrial wastewaters, the use of activated carbon is not solely aimed at reducing COD; rather, it serves the dual purpose of advanced COD removal alongside the removal of color and specific organic pollutants.
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PAC & GAC for COD Removal
The activated carbons used for COD removal primarily include PAC (Powdered Activated Carbon) and GAC (Granular Activated Carbon).
Powdered Activated Carbon for COD Removal
Due to its small particle size, PAC typically exhibits rapid mass transfer kinetics. For wastewater with significant COD fluctuations, the PAC dosage can be adjusted based on influent COD levels, pollutant composition, and experimental results.
PAC is particularly suitable for:
• Advanced treatment of industrial wastewater;
• Handling sudden spikes in COD;
• Addressing seasonal variations in water quality;
• Post-treatment following biological processes;
• Wastewater requiring rapid adsorption;
• Systems requiring simultaneous removal of color and organic pollutants.
However, since PAC must be separated from the water after use, it is usually combined with downstream processes such as coagulation, sedimentation, or filtration. Furthermore, increasing the PAC dosage does not always yield better results; as the dosage rises, the efficiency of COD removal typically experiences diminishing returns. If the COD level ceases to drop despite further increases in PAC dosage, it suggests that the wastewater may contain a high proportion of COD with low adsorbability, or that the system is constrained by other limiting factors.
| Parameter | Typical Value |
|---|---|
| Raw material | Coal-based (bituminous/lignite) |
| Iodine number | 800–1000 mg/g (typical 900) |
| Particle size | 200 mesh (≥90% through 75 μm); 325 mesh for faster kinetics |
| Ash content | ≤10–15% (coal-based) |
| Moisture | ≤5% |
| pH | 8–11 |
| Apparent density | 400–550 g/L |
| Surface area (BET) | 900–1100 m²/g |
| COD removal rate | 30–70% (typical 40–60%) |
Granular Activated Carbon for COD Removal
GAC is well-suited for use in fixed-bed adsorbers or activated carbon filters. As wastewater passes through the activated carbon bed, dissolved organic pollutants gradually transfer from the water into the interior of the activated carbon particles.
A key advantage of GAC is its capability for continuous operation, making it particularly suitable for process sequences such as: Biological Treatment → GAC Adsorption → Final Discharge/Water Reuse.
For industrial wastewater with relatively stable COD levels, GAC can serve as a final barrier for the advanced removal of organic matter. Additionally, fixed-bed systems allow for the determination of the activated carbon’s effective service life by monitoring influent and effluent COD to establish a breakthrough curve.
| Parameter | Typical Value |
|---|---|
| Raw material | Coal-based (bituminous) most common |
| Iodine number | 900–1100 mg/g |
| Particle size | 4×8, 6×12, 8×30 mesh |
| Effective size | 0.8–1.2 mm (coal-based) |
| Uniformity coefficient (UC) | ≤2.0 |
| Hardness (ball-pan) | ≥90–95% |
| Ash content | ≤8–12% (coal-based) |
| Moisture | ≤5% |
| Apparent density | 450–550 g/L |
| COD removal rate | 40–70% (typical 50–60%) |
Conclusion
In industrial wastewater treatment and water purification processes, activated carbon serves a critical function in water quality standard compliance. It removes refractory organic contaminants—including soluble organic pollutants, residual dyes, phenolic compounds, grease substances and biochemical degradation-resistant organics—from industrial wastewater, effectively improving effluent water quality and mitigating common operational issues such as excessive COD discharge and substandard wastewater treatment results triggered by persistent organic impurities.
If you are looking for activated carbon solutions for COD reduction in industrial wastewater treatment units, please contact our technical experts to develop a customized, cost-effective filtration and purification solution.
