There are two types of comprehensive indicators of organic matter content in sewage treatment:

● One type is an indicator expressed in terms of oxygen demand equivalent to the amount of organic matter in water, such as biochemical oxygen demand BOD, chemical oxygen demand COD and total oxygen demand TOD;

● One type is an indicator expressed in terms of carbon, such as total organic carbon TOC.

For the same type of sewage, the values ​​of these indicators are generally different, and the order of numerical size is: TOD>COD>BOD5>TOC

01 Total Oxygen Demand TOD

Total oxygen demand (TOD) refers to the amount of oxygen required when reducing substances in water are burned at high temperatures to become stable oxides, and the result is measured in mg/L. The TOD value can reflect the amount of oxygen required to burn almost all organic matter in water (including carbon C, hydrogen H, oxygen O, nitrogen N, phosphorus P, sulfur S and other components) to become CO2, H2O, NOx, SO2, etc. after combustion.

TOD is closer to the theoretical oxygen demand value than BOD, COD and permanganate index. The total oxygen demand (TOD) is determined by the amount of oxygen consumed by burning and oxidizing organic matter at 900°C in a special burner with platinum as a catalyst. The measurement result is closer to the theoretical oxygen demand than COD.

02 Total Organic Carbon TOC

Total organic carbon (TOC) is a comprehensive indicator that indirectly indicates the content of organic matter in water. The data it displays is the total carbon content of organic matter in sewage, expressed in mg/L of carbon (C). Generally, the TOC of urban sewage can reach 200 mg/L, and the TOC of industrial sewage has a wider range, reaching tens of thousands of mg/L. The TOC of sewage after secondary biological treatment is generally

The determination of total organic carbon (TOC) is similar to the determination of TOD. It is to oxidize and burn water samples at a high temperature of 900°C with platinum as a catalyst, and measure the increase of CO2 in the gas to determine the total carbon content in the water sample, which is a comprehensive indicator of the total amount of organic matter in the water sample.

Since the determination of TOC uses high-temperature combustion, it can oxidize all organic matter. It can directly indicate the total amount of organic matter more than BOD or COD. Therefore, it is often used to evaluate the degree of organic pollution in water bodies.

03 Chemical oxygen demand COD

Chemical oxygen demand (COD) refers to the amount of oxygen converted from the oxidant consumed by the reaction of organic matter in water with strong oxidants (such as potassium dichromate, potassium permanganate, etc.) under certain conditions, measured in mg/L of oxygen. When potassium dichromate is used as an oxidant, almost all (90%-95%) of the organic matter in the water can be oxidized. At this time, the amount of oxygen converted from the oxidant consumed is commonly known as chemical oxygen demand, often abbreviated as CODcr.

The CODcr value of sewage not only includes the oxygen consumption for the oxidation of almost all organic matter in the water, but also includes the oxygen consumption for the oxidation of reducing inorganic matter such as nitrite, ferrous salt, sulfide, etc. The chemical oxygen demand measured using potassium permanganate as an oxidant is called the potassium permanganate index or oxygen consumption, abbreviated as CODMn or OC in English, and the unit is mg/L.

Since the oxidizing ability of potassium permanganate is weaker than that of potassium dichromate, the specific value of the potassium permanganate index CODMn of the same water sample is generally lower than its CODCr value, that is, CODMn can only indicate the content of easily oxidizable organic or inorganic matter in the water.

04 Biochemical oxygen demand BOD

The full name of biochemical oxygen demand is biochemical oxygen demand, abbreviated as BOD. It indicates the amount of dissolved oxygen consumed in the biochemical oxidation process of aerobic microorganisms decomposing organic matter in water at a temperature of 20°C and under aerobic conditions, that is, the amount of oxygen required to stabilize biodegradable organic matter in water, and the unit is mg/L.

BOD not only includes the amount of oxygen consumed by the growth and reproduction of aerobic microorganisms or respiration in water, but also includes the amount of oxygen consumed by reducing inorganic substances such as sulfide and ferrous iron, but the proportion of this part is usually very small.

Under natural conditions at 20°C, it takes more than 100 days for organic matter to oxidize to the nitrification stage, that is, to achieve complete decomposition and stabilization. However, in fact, the biochemical oxygen demand BOD20 of 20 days at 20°C is often used to approximately represent the complete biochemical oxygen demand. In production applications, 20 days is still too long, and the biochemical oxygen demand BOD5 of 5 days at 20°C is generally used as an indicator to measure the content of organic matter in sewage.

05 Relationship between BOD5 and COD

BOD5 is not only an important water quality indicator, but also an extremely important control parameter in the biological treatment process of sewage. However, due to the long measurement time (5 days), it cannot reflect and guide the operation of sewage treatment equipment in a timely manner, and can only be used for process effect evaluation and long-term process regulation. For a specific sewage treatment plant, a correlation between BOD5 and COD can be established, and the BOD5 value can be roughly estimated using COD to guide the adjustment of the treatment process. Sometimes, because some production sewage does not have the conditions for microbial growth and reproduction (such as the presence of toxic organic matter), its BOD5 value cannot be accurately measured.

Testing the COD value of sewage can accurately determine the organic matter content in water, but COD cannot distinguish between biodegradable and non-biodegradable organic matter. People are accustomed to using the BOD5/COD of sewage to determine its biodegradability. It is generally believed that if the BOD5/COD of sewage is greater than 0.3, it can be treated by biodegradation. If the BOD5/COD of sewage is lower than 0.2, only other methods can be considered for treatment.

The COD value of chemical oxygen demand is generally higher than the BOD5 value of biochemical oxygen demand. The difference between them can roughly reflect the content of organic matter in sewage that cannot be degraded by microorganisms. For sewage with relatively fixed pollutant components, there is generally a certain proportional relationship between COD and BOD5, which can be inferred from each other.

TOD stands for the total amount of all organic matter in water, TOC stands for the total amount of all organic carbon in water, COD stands for the chemical oxidation demand of organic matter in water, and BOD stands for the oxygen demand for biological degradation of organic matter in water. They have different applications and meanings in water quality analysis and assessment.

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