When receiving wastewater analysis results, many businesses often ask: What COD level is acceptable? Does the current COD value meet environmental technical regulations? In reality, each type of wastewater is subject to different COD limits depending on the corresponding technical regulations. Understanding these limits clearly not only helps businesses comply with legal regulations but also supports the evaluation of the wastewater treatment system's operational efficiency.
In a wastewater treatment system, COD is measured at various locations to assess the pollution level and treatment efficiency. Among these, the two most concerning values are influent COD and effluent COD.
| Criteria | Influent COD | Effluent COD |
| Sampling Location | At the collection pit, collection tank, or system inlet | At the treated water storage tank or the environmental discharge point |
| Time of Measurement | Before wastewater enters the WWTP (Wastewater Treatment Plant) | After wastewater passes through the entire WWTP |
| Reflects | Initial pollution level of the wastewater | Post-treatment water quality |
| Purpose | Calculating pollution load, selecting technology, and system capacity | Evaluating treatment efficiency and the ability to meet QCVN (National Technical Regulations) |
| Target Audience | System design and operation units | Businesses, monitoring units, and environmental management agencies |
Example: At a seafood processing plant, the influent COD can range from 2,500 – 3,000 mg/L because the wastewater contains a lot of protein, grease, and organic matter from the production process. After being treated through stages such as DAF flotation, AO biological tank, and settling tank, the effluent COD can drop below 65 mg/L. This shows that the system has removed most of the pollution load and met current discharge requirements.
In the operation of a wastewater treatment system, monitoring both influent COD and effluent COD simultaneously is essential.
When influent COD spikes or treatment efficiency drops abnormally, businesses can easily detect issues such as system overload, lack of oxygen, microbial shock loads, or unstable equipment operation to take timely corrective measures.
QCVN 14:2025/BTNMT applies to wastewater generated from residential areas, apartment buildings, hotels, schools, office buildings, and similar service facilities.
Applicable Subjects:
Domestic wastewater typically contains organic matter from eating, drinking, sanitation, and daily living activities. Although the pollution level is lower than many types of industrial wastewater, it still needs to be treated before being discharged into the environment.
Example: The influent COD of a domestic wastewater treatment system usually ranges from 300 – 800 mg/L. To meet discharge requirements, the system typically must remove 70 – 90% of the COD load, depending on the influent water quality.
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COD limits in wastewater treatment - must be noted before discharging into the environment
QCVN 40:2025/BTNMT is currently the technical regulation of greatest interest to many manufacturing enterprises. This regulation applies to most types of industrial wastewater generated from factories, production facilities, and industrial parks.
Depending on the protection purpose of the receiving source, COD limits are divided into different levels. According to the current technical regulation:
Applicable Subjects:
| Receiving Source Zoning | Maximum COD |
| Level A | ≤ 65 mg/L |
| Level B | ≤90 mg/L |
| Level C | ≤ 130 mg/L |
These values are used to assess the quality of treated wastewater before being discharged into the environment.
Example: Wastewater from a seafood processing plant usually has an influent COD of 1,500 – 4,000 mg/L. If applying Level A according to QCVN 40:2025, the treatment system must reduce the COD to below 65 mg/L, equivalent to a treatment efficiency of over 95%.
This is why factories often have to combine multiple treatment stages such as equalization, biological, physicochemical, and disinfection to ensure the effluent wastewater meets regulations.
Livestock wastewater is currently one of the wastewater types with the highest organic pollution load. Besides COD, this type of wastewater also contains large amounts of Nitrogen, Phosphorus, and suspended solids.
Therefore, QCVN 62:2025/BTNMT issues strict control requirements to limit impacts on the environment and receiving water sources.
Applicable Subjects:
Example: Many pig farms have an influent COD from 3,000 – 8,000 mg/L, or even higher. In cases where only biogas digesters or simple biological ponds are used, meeting the effluent regulations is often very difficult without adding appropriate treatment stages.
Before assessing whether a COD value is low or high, businesses need to clearly understand the nature of this indicator. If you do not yet grasp the concept of COD and its significance in wastewater treatment, you can refer to the article: What is the COD index in wastewater?
This is a question many businesses care about. However, it should be noted that this is only a reference assessment level in the design and operation of wastewater treatment systems, not a legal limit.
| COD (mg/L) | Assessment |
| < 250 | Low |
| 250 – 500 | Medium |
| 500 – 2,000 | High |
| > 2,000 | Very High |
The actual level of impact also depends on the type of wastewater and the treatment technology being used.
Is a Lower COD Always Better?
Many businesses believe that the lower the effluent COD, the more efficient the treatment system. However, this is not entirely accurate.
The goal of a wastewater treatment system is to bring the effluent water quality to meet applicable regulations and the requirements in the environmental permit. In many cases, trying to reduce COD to an overly low level can significantly increase electricity, chemical, and operational costs.
Example: A system that has fully met discharge requirements but still increases the amount of chemicals or extends aeration time to reduce a few more mg/L of COD can increase operating costs without bringing much practical benefit.
Not all businesses have to upgrade their wastewater treatment systems when new technical regulations are issued. However, re-evaluating the current system status is necessary in some cases such as:
For systems that frequently experience high effluent COD or declining treatment efficiency, businesses should review the treatment technology and have an appropriate renovation plan. There is no universal COD level applicable to all types of wastewater. Assessing whether COD is acceptable or not must be based on the wastewater type and the corresponding technical regulations. Businesses need to regularly monitor effluent COD, review system operation efficiency, and update new regulations to ensure compliance with environmental laws.
Dai Nam specializes in consulting on environmental dossiers, designing, renovating, and operating wastewater treatment systems for factories, industrial parks, hospitals, buildings, and residential areas. With experience implementing many practical projects, Dai Nam is ready to support businesses in evaluating their ability to meet QCVN 2025 and proposing optimal solutions for each type of wastewater.
