What does TOD, TOC, BOD, COD mean in industrial wastewater treatment? What is the connection between them?

What does TOD, TOC, BOD, COD mean in industrial wastewater treatment? What is the connection between them?

There are two types of comprehensive indicators for the content of organic matter in water. One is an indicator that is equivalent to the amount of organic matter in water (O2), such as BOD, chemical oxygen demand (COD), and total oxygen demand (TOD). ; The other is an indicator expressed in carbon (c), such as total organic carbon TOC. For the same kind of sewage, the values ​​of these indicators are generally different, and the order of the values ​​is TOD> COD> BOD5> TOC.

 

1. TOD

 

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

 

2. TOC

 

Total organic carbon TOC is a comprehensive indicator that indirectly expresses the content of organic matter in water. The data it shows is the total carbon content of organic matter in sewage. The unit is expressed in mg / L of carbon (c). The TOC of general urban sewage can reach 200mg / L, the TOC range of industrial sewage is wide, the highest can reach tens of thousands of mg / L, and the TOC of sewage after secondary biological treatment-generally <50mg / L.

 

3． BOD BOD

 

Biochemical oxygen demand is called biochemical oxygen demand, which is abbreviated as BOD. It means the amount of dissolved oxygen consumed during the biochemical oxidation process of aerobic microorganisms to decompose organic matter in water at a temperature of 20 ° C and aerobic conditions. The amount of oxygen required to stabilize biodegradable organics in water, the unit is mg / L. BOD includes not only the amount of oxygen consumed by the growth and reproduction of aerobic microorganisms in the water, but also the amount of oxygen consumed by reducing inorganic substances such as sulfides and ferrous iron, but the proportion of this part is usually small.

 

Under natural conditions at 20 ° C, the time required for the organic matter to oxidize to the nitration stage, that is, to achieve complete decomposition stability, is more than 100d. However, BOD20, which is commonly used at 20 ° C for 20d, approximately represents the complete BOD. It is still considered that the time of 20d is too long in production and application. Generally, the BOD5 of BOD5 of 5d at 20 ℃ is used as an index to measure the content of organic matter in sewage.

 

4. COD

 

Chemical oxygen demand (COD) refers to the amount of oxygen converted into oxygen by the consumption of organic substances in water and strong oxidants under certain conditions, in mg / L of oxygen. When potassium dichromate is used as the oxidant, almost all of the organic matter in the water (90% to 95%) can be oxidized. At this time, the amount of oxygen consumed by the oxidant converted into oxygen is generally called chemical oxygen demand, which is often abbreviated as CODcr. The CODcr value of wastewater includes not only the oxygen consumption of almost all organic substances in the water, but also the oxygen consumption of reducing inorganic substances such as nitrites, ferrous salts, and sulfides in the water.

 

5. The 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 (5d), it cannot reflect and guide the operation of the sewage treatment device in time, and can only be used for process effect evaluation and long-term Wang Yi regulation. For a specific sewage treatment plant, the correlation between BOD5 and COD can be established, and a rough estimate of the COD5 value of COD is used to guide the adjustment of the treatment process. Sometimes, because some production wastewater does not have the conditions for the growth and reproduction of microorganisms (such as the presence of toxic organic substances), its BOD5 value cannot be accurately determined.

 

The chemical oxygen demand (COD) value of sewage can be used to accurately determine the content of organic matter in water, but COD cannot distinguish between biodegradable organic matter and non-biodegradable organic matter. People are used to measuring the BOD5 / COD of sewage to determine its biodegradability. It is generally believed that sewage can be treated by biodegradation if its BOD5 / COD is greater than 0.3. If the BOD5 / COD of sewage is lower than 0.2. Only other methods can be considered for processing.

 

The COD value of the chemical oxygen demand is generally higher than the BOD5 value of the biochemical oxygen demand, and the difference between them can roughly reflect the organic matter content in the 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 calculated from each other. In addition, the measurement of COD takes less time, and the national standard method for 2 hours of reflux is used to perform the test. It takes only 3 to 4 hours from the sampling to the result, and the BOD5 value takes 5 days. COD is often used as a control indicator.

 

In order to guide the production and operation as soon as possible, some sewage treatment plants have also established enterprise standards for measuring COD for 5 minutes at reflux. Although the measured results are somewhat different from the national standard method, the errors are systematic errors and the results of continuous monitoring can accurately reflect the water quality. The actual change trend of the measurement time can be reduced to less than 1h, which provides a time guarantee for timely adjustment of sewage treatment operating parameters and prevention of sudden changes in water quality to the sewage treatment system.