COD is an indicator that reflects the amount of organic matter in wastewater and is one of the most closely monitored parameters when designing and operating a wastewater treatment system. Depending on the manufacturing industry, COD can originate from fats and oils, proteins, starches, chemicals, or persistent organic compounds. Therefore, no single technology is suitable for all types of wastewater. Choosing the right COD treatment method not only determines treatment efficiency but also directly impacts the investment and operating costs of the entire system.
If you are not yet familiar with what COD is and its role in wastewater treatment, you can refer to the article: What is the COD Index in Wastewater? for a more comprehensive overview.
Biological methods use microorganisms to decompose organic substances in wastewater into CO₂, water, and new biomass. This is currently the most widely applied technology due to its ability to effectively treat readily biodegradable organic compounds with relatively low operating costs.
Popular biological technologies today include AO, AAO, MBBR, MBR, and SBR. Depending on the wastewater characteristics and output requirements, each technology will have different advantages and scopes of application.
In practice, many seafood processing wastewater treatment systems with an influent COD of 2,000 – 3,000 mg/L can reduce it to under 100 mg/L after passing through the DAF stage and AO or MBBR biological tanks. This is also why biological technology is consistently considered a priority in most industrial wastewater treatment projects with high organic content. However, not all types of wastewater are suitable for biological treatment.
Coagulation-flocculation and Dissolved Air Flotation (DAF) are typically used in the pretreatment stage to remove suspended solids, fats and oils, and suspended organic matter before the wastewater enters the biological unit. Reducing the pollution load right from the start helps the system operate more stably and minimizes the risk of organic shock to the microorganisms.
DAF is particularly suitable for the food processing, seafood, slaughterhouse, and cooking oil production industries. Depending on the characteristics of the waste source, this technology can remove about 20 – 60% of the COD and plays a crucial role in enhancing the treatment efficiency of subsequent stages.
For wastewater containing persistent or non-biodegradable organic compounds, advanced oxidation technologies such as Fenton, Ozone, or UV/H₂O₂ are frequently used to break down the structure of pollutants prior to further treatment.
Depending on the wastewater characteristics, these technologies can reduce COD by 40 – 90% and improve the treatability of the downstream biological unit. This solution is commonly applied to textile and dyeing, chemical, paint, ink, or pharmaceutical wastewater when biological technology alone struggles to meet output requirements.
Each method has its own advantages and scope of application. However, actual treatment efficiency depends not only on the chosen technology but also on the wastewater characteristics and the system's operating conditions.
Treatment of COD in industrial wastewater
Biological technology can treat COD effectively with low operating costs, but its actual performance depends on various factors. Among them, the biodegradability of the organic matter is the most critical. Wastewater containing highly biodegradable organics—such as from food, seafood, beverages, or livestock—generally yields higher treatment efficiencies compared to wastewater with persistent organic compounds.
Some factors affecting biological COD treatment efficiency include:
When these conditions are maintained steadily, biological systems can achieve COD treatment efficiencies of 70 – 95% and operate sustainably over the long term.
Although considered the flagship technology in industrial wastewater treatment, the biological method is not always suitable for every waste source. For wastewater streams containing high levels of persistent organic compounds, the efficiency of microbial treatment is often limited.
Some types of wastewater that typically present challenges when relying solely on biological treatment include:
Additionally, excessively high influent COD or the presence of heavy metals, residual chlorine, and toxic chemicals can impair the activity of the microbial system. In these instances, businesses usually need to integrate additional stages such as coagulation-flocculation, DAF, Fenton, or Advanced Oxidation Processes (AOP) to enhance overall COD treatment efficiency.
| Method | COD Treatment Efficiency | Investment Cost | Operating Cost | Suitable For |
| Biological | 70 – 95% | Medium | Low | Food, seafood, livestock |
| Coagulation-Flocculation | 20 – 50% | Low | Medium | Wastewater with high TSS |
| DAF | 20 – 60% | Medium | Medium | Wastewater with fats and oils |
| Fenton/Ozone | 40 – 90% | Medium - High | High | Textile, chemical |
| UF/NF/RO | 80 – 99% | High | High | Water reuse |
Each method has its own advantages and specific applications. In reality, industrial wastewater treatment systems often combine multiple technologies to leverage the strengths of each method and achieve optimal COD treatment efficiency.
Choosing a COD treatment technology depends not only on the influent COD concentration but also on the specific wastewater characteristics of each industry. In practice, the treatment solution for the same COD level can be entirely different due to varying pollutant compositions and output water quality requirements.
| Influent COD | Commonly Applied Solution |
| Under 1,000 mg/L | Biological (AO, MBBR, SBR) |
| 1,000 – 3,000 mg/L | DAF + Biological |
| 3,000 – 8,000 mg/L | DAF + High-load Biological or Biogas |
| Over 8,000 mg/Ln | Physico-chemical + Biological + Advanced Oxidatio |
Note: These are reference values only. The choice of technology also relies on the BOD/COD ratio, pollutant composition, wastewater flow rate, and specific output requirements of each project.
Therefore, selecting a COD treatment technology must be based on actual wastewater analysis results rather than applying a universal formula to every case.
No single COD treatment technology is considered optimal for all types of wastewater. Each method—whether biological, DAF, Fenton, Ozone, or membrane technology—has its own advantages and specific applications.
In reality, combining multiple technologies often delivers higher treatment efficiency than utilizing a single method. To select the right solution, businesses must thoroughly evaluate the wastewater composition, influent COD concentration, and output water quality requirements before investing in or upgrading a treatment system.
