In the water treatment industry, TOD BOD COD is a professional term. Do you know what it means? If you don't know, it's embarrassing to say you know about water treatment~
There are two types of comprehensive indicators that indicate the content of organic matter in water. One is the indicator represented by oxygen demand (O2) equivalent to the amount of organic matter in water, such as biochemical oxygen demand (BOD), chemical oxygen demand (COD), and total oxygen demand (TOD); Another type of indicator is represented by carbon (c), such as total organic carbon (TOC). For the same type of wastewater, the values of these indicators are generally different, arranged in the order of TOD>COD>BOD5>TOC
1. 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, measured in mg/L. The TOD value can reflect the amount of oxygen required to convert almost all organic matter in water (including carbon C, hydrogen H, oxygen O, nitrogen N, phosphorus P, sulfur S, etc.) into CO2, H2O, NOx, SO2, etc. after combustion.
2. Total Organic Carbon (TOC) Total Organic Carbon (TOC) is a comprehensive indicator that indirectly represents the content of organic matter in water. The displayed data is the total carbon content of organic matter in wastewater, expressed in mg/L of carbon (c). The TOC of general urban sewage can reach 200mg/L, while the TOC range of industrial sewage is relatively wide, with the highest reaching tens of thousands of mg/L. After secondary biological treatment, the TOC of sewage is generally less than 50mg/L.
3. Biochemical Oxygen Demand (BOD), also known as Biochemical Oxygen Demand (BOD), refers to the dissolved oxygen consumed by aerobic microorganisms during the biochemical oxidation process of organic matter in water under aerobic conditions at a temperature of 20 ℃. It is the oxygen required for the stabilization of biodegradable organic matter in water, measured in mg/L. BOD not only includes the oxygen consumed by the growth, reproduction, or respiration of aerobic microorganisms in water, but also the oxygen consumed by reducing inorganic substances such as sulfides and ferrous ions, but the proportion of this part is usually very small. Under natural conditions of 20 ℃, the time required for organic matter oxidation to reach the nitrification stage, that is, to achieve complete decomposition stability, is over 100 days. However, in reality, the biochemical oxygen demand BOD20 of 20 days at 20 ℃ is commonly used to approximately represent the complete biochemical oxygen demand. In production applications, the time of 20 days is still considered too long. Generally, the biochemical oxygen demand BOD5 at 20 ℃ for 5 days is used as an indicator to measure the organic matter content in wastewater.
4. Chemical Oxygen Demand (COD) refers to the amount of oxidant consumed by the interaction between organic matter and strong oxidants in water under certain conditions, converted into oxygen, measured in mg/L. When potassium dichromate is used as an oxidant, almost all (90% to 95%) of the organic matter in water can be oxidized. At this time, the amount of oxidant consumed converted to oxygen is commonly referred to as chemical oxygen demand, commonly abbreviated as CODcr. The CODcr value of sewage not only includes the oxygen consumption of almost all organic matter oxidized in water, but also includes the oxygen consumption of reducing inorganic substances such as nitrite, ferrous salts, and sulfides oxidized in water.
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 wastewater. 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 Wangyi regulation. For specific sewage treatment plants, a correlation between BOD5 and COD can be established, and COD can be used to roughly estimate BOD5 values to guide the adjustment of treatment processes. Sometimes, due to the lack of conditions for microbial growth and reproduction in certain production wastewater (such as the presence of toxic organic compounds), it is difficult to accurately determine its BOD5 value. The chemical oxygen demand (COD) value of laboratory wastewater can accurately determine the organic content in water, but COD cannot distinguish between biodegradable organic matter and non biodegradable organic matter. People are accustomed to using the measurement of BOD5/COD in sewage to determine its biodegradability. It is generally believed that if the BOD5/COD of sewage is greater than 0.3, biodegradation can be used for treatment. If the BOD5/COD of sewage is less than 0.2, other methods can only be considered for treatment. The COD value of chemical oxygen demand is generally higher than the BOD5 value of biochemical oxygen demand, and 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 calculated from each other. In addition, the determination of COD takes less time. According to the national standard method of reflux for 2 hours, it only takes 3-4 hours from sampling to results, while the determination of BOD5 value takes 5 days. Therefore, in actual sewage treatment operation and management, COD is often used as a control indicator.
