What substances can increase the COD value?

In general, they are organic substances; additionally, chloride ions can also cause higher readings during testing

COD stands for Chemical Oxygen Demand; therefore, any substance that consumes oxygen can affect this value, not just organic substances. It is necessary to clarify the specific definitions and differences between COD and BOD. I am ignorant and inexperienced; these are merely my personal opinions, hoping they can be of some use! :$

Organic matter; chemical oxygen demand refers to the amount of oxygen consumed by activated sludge in decomposing organic matter

The standards make it clear that inorganic reducing substances such as nitrites, sulfides, ferrous ions, etc., will all affect the results positively. Additionally, Cl ions at levels above 1000 mg/L can also have a significant impact on the results.

It’s obviously an organic substance. COD represents the amount of oxidizing agent consumed when a water sample is treated with a certain strong oxidizer under specific conditions. It is an indicator of the amount of reducing substances in water. Reducing substances in water include various organic compounds, nitrites, sulfides, ferrous salts, etc. But the main component is organic matter. Therefore, Chemical Oxygen Demand (COD) is often used as an indicator to measure the amount of organic matter in water. The higher the chemical oxygen demand, the more severe the pollution of the water body by organic matter.

That’s absolutely right. But sometimes, for industrial wastewater, the proportion of inorganic substances that can be oxidized by potassium dichromate in COD is relatively high.

Without a doubt, organic substances make up the majority

Substances that can react with oxygen include organic compounds and inorganic substances with reducing properties.

The COD we usually refer to is the one determined by the potassium dichromate method. COD is the abbreviation for Chemical Oxygen Demand; it represents the amount of oxidizing agent consumed when a water sample is treated with a certain strong oxidizer under specific conditions. It is an indicator of the amount of reducing substances in water. Reducing substances in water include various organic compounds, nitrites, sulfides, ferrous salts, etc. But the main component is organic matter. Therefore, Chemical Oxygen Demand (COD) is often used as an indicator to measure the amount of organic matter in water. The higher the chemical oxygen demand, the more severe the pollution of the water body by organic matter. The determination of Chemical Oxygen Demand (COD) varies depending on the reductive substances present in the water sample being tested, as well as the testing method used. The most commonly used methods at present are the potassium permanganate oxidation in acidic medium and the potassium dichromate oxidation method. According to the method for measuring COD, any substance with an oxidizing power lower than that of chromium in the +7 valence state will result in a high COD value; thus, this figure indicates the degree of water pollution. However, it is not necessarily related to organic substances. In some wastewater samples, high COD values are caused by inorganic salt ions – for example, wastewater containing ferrous ions has a high COD value, but this is not due to organic substances. The standard answer! This post was last edited by hesonchang214 on 2009-3-15 14:07.]

Soluble organic matter is the cause of high COD levels in water. However, since the national standard method for testing COD is the potassium dichromate method, and potassium dichromate has oxidizing properties, inorganic reducing substances in water such as nitrites, sulfides, ferrous ions, etc., as well as chloride ions, will consume potassium dichromate during the testing process, resulting in a higher COD value. But this is an artificially inflated figure, and it can be easily resolved through biological treatment. Organic compounds, especially those with benzene rings, are more difficult to treat biologically; therefore, as long as the organic compounds in the water are removed and the levels of inorganic ions are not too high, the wastewater can meet the discharge standards after biological treatment. The standard answer! This post was last edited by hesonchang214 on 2009-3-15 14:10.]

The answers on floors 12–13 are already very comprehensive!