Chemical oxygen demand, also known as chemical oxygen consumption, or COD for short, uses chemical oxidants (such as potassium dichromate) to oxidize and decompose oxidizable substances (such as organic matter, nitrite, ferrous salts, sulfides, etc.) in water, and then Oxygen consumption is calculated based on the amount of residual oxidant. Like biochemical oxygen demand (BOD), it is an important indicator of the degree of water pollution. The unit of COD is ppm or mg/L. The smaller the value, the lower the degree of water pollution. In the study of river pollution and industrial wastewater properties, as well as in the operation and management of wastewater treatment plants, it is an important and quickly measured COD pollution parameter.
Chemical oxygen demand (COD) is often used as an important indicator to measure the content of organic matter in water. The greater the chemical oxygen demand, the more serious the water body is polluted by organic matter. For the measurement of chemical oxygen demand (COD), the measured values vary depending on the reducing substances in the water sample and the measurement methods. The most commonly used determination methods at present are the acidic potassium permanganate oxidation method and the potassium dichromate oxidation method.
Organic matter is very harmful to industrial water systems. Strictly speaking, chemical oxygen demand also includes inorganic reducing substances present in water. Normally, since the amount of organic matter in wastewater is much greater than the amount of inorganic matter, chemical oxygen demand is generally used to represent the total amount of organic matter in wastewater. Under the measurement conditions, organic matter that does not contain nitrogen in the water is easily oxidized by potassium permanganate, while organic matter that contains nitrogen is more difficult to decompose. Therefore, oxygen consumption is suitable for measuring natural water or general wastewater containing easily oxidized organic matter, while organic industrial wastewater with more complex components is often used for measuring chemical oxygen demand.
The impact of COD on water treatment systems
When water containing a large amount of organic matter passes through the desalination system, it will contaminate the ion exchange resin. Among them, it is especially easy to contaminate the anion exchange resin, thereby reducing the resin exchange capacity. Organic matter can be reduced by about 50% during pretreatment (coagulation, clarification and filtration), but organic matter cannot be effectively removed in the desalination system. Therefore, make-up water is often brought into the boiler to reduce the pH value of the boiler water. , causing system corrosion; sometimes organic matter may be brought into the steam system and condensate water, reducing the pH value, which can also cause system corrosion.
In addition, excessive organic matter content in the circulating water system will promote microbial reproduction. Therefore, regardless of desalination, boiler water or circulating water systems, the lower the COD, the better, but there is currently no unified numerical index.
Note: In the circulating cooling water system, when COD (KMnO4 method) is >5mg/L, the water quality has begun to deteriorate.
The impact of COD on ecology
High COD content means that the water contains a large amount of reducing substances, mainly organic pollutants. The higher the COD, the more serious the organic pollution in the river water. The sources of these organic pollution are generally pesticides, chemical plants, organic fertilizers, etc. If not treated in time, many organic pollutants can be adsorbed by the sediment on the river bottom and deposited, causing lasting poisoning to aquatic life in the next few years.
After a large number of aquatic life dies, the ecosystem in the river will gradually be destroyed. If people feed on such organisms in the water, they will absorb a large amount of toxins from these organisms and accumulate them in the body. These toxins are often carcinogenic, deformational, and mutagenic, and are extremely detrimental to human health. In addition, if polluted river water is used for irrigation, plants and crops will also be affected and grow poorly. These polluted crops cannot be eaten by humans.
However, high chemical oxygen demand does not necessarily mean that there will be the above-mentioned hazards, and the final conclusion can only be reached through detailed analysis. For example, analyze the types of organic matter, what impact these organic matter have on water quality and ecology, and whether they are harmful to the human body. If detailed analysis is not possible, you can also measure the chemical oxygen demand of the water sample again after a few days. If the value drops a lot compared with the previous value, it means that the reducing substances contained in the water are mainly easily degradable organic matter. Such organic matter is harmful to the human body and Biological hazards are relatively minor.
Common methods for COD wastewater degradation
At present, adsorption method, chemical coagulation method, electrochemical method, ozone oxidation method, biological method, micro-electrolysis, etc. are common methods for COD wastewater degradation.
COD detection method
Rapid digestion spectrophotometry, the COD detection method of Lianhua Company, can obtain accurate results of COD after adding reagents and digesting the sample at 165 degrees for 10 minutes. It is simple to operate, has low reagent dosage, low pollution, and low energy consumption.
Post time: Feb-22-2024
