Indicators in Industrial Wastewater

Indicators in Industrial Wastewater

There are two kinds of comprehensive indicators indicating the content of organic matter in water. One is the indicator expressed by the 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; The other is the indicator expressed by 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 values is TOD>COD>BOD5>TOC.

1. Total Oxygen Demand 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 temperature, and the result is calculated in mg/L. TOD value can reflect the amount of oxygen consumed when almost all organic substances in water (including carbon C, hydrogen H, oxygen O, nitrogen N, phosphorus P, sulfur S and other components) are converted 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 data displayed is the total carbon content of organic matter in sewage, expressed in mg/L of carbon (c). The TOC of general urban sewage can reach 200mg/L, the TOC of industrial sewage can reach tens of thousands of mg/L, and the TOC of sewage after secondary biological treatment is generally<50mg/L.

3. Biochemical Oxygen Demand BOD

Biochemical oxygen demand is called biochemical oxygen demand, abbreviated as BOD. It represents the dissolved oxygen consumed in the biochemical oxidation process of aerobic microorganisms decomposing organic substances in water at 20 ℃ and aerobic conditions, that is, the oxygen required for the stabilization of biodegradable organic substances in water, in mg/L. BOD includes not only the amount of oxygen consumed by the growth and reproduction or respiration of aerobic microorganisms in water, but also the amount of oxygen consumed by sulfide, ferrous and other reducing inorganic substances, but the proportion of this part is usually very small.

Under the natural conditions of 20 ℃, the time required for the oxidation of organic matter to the nitrification stage, that is, to achieve complete decomposition and stability, is more than 100 days, but in fact, the biochemical oxygen demand (BOD20) of 20 days at 20 ℃ is commonly used to approximate the complete biochemical oxygen demand. In production and application, it is still too long for 20 days. Generally, BOD5, a biochemical oxygen demand of 5 days at 20 ℃, is used as an indicator to measure the content of organic matter in sewage.

4. Chemical Oxygen Demand COD

Chemical oxygen demand (COD) refers to the amount of oxidant consumed by the action of organic matter and strong oxidant in water converted into oxygen under certain conditions, calculated in mg/L. When potassium dichromate is used as oxidant, almost all (90% - 95%) of the organic matter in the water can be oxidized. At this time, the amount of oxidant consumed converted into oxygen is the commonly known chemical oxygen demand, often abbreviated as CODcr. The CODcr value of sewage includes not only the oxygen consumption of almost all organic substances in the water to be oxidized, but also the oxygen consumption of nitrite, ferrous salt, sulfide and other reducing inorganic substances in the water to be oxidized.

5. Relationship between BOD5 and COD

BOD5 is not only an important water quality index, but also an extremely important control parameter in the process of sewage biological treatment. However, due to the long measurement time (5d), it can not timely reflect and guide the operation of the sewage treatment plant, and can only be used for process effect evaluation and long-term technique regulation. For a specific sewage treatment plant, the correlation between BOD5 and COD can be established, and the BOD5 value can be roughly estimated by 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 substances), it is impossible to accurately determine its BOD5 value.

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

COD value of chemical oxygen demand is generally higher than BOD5 value of biochemical oxygen demand, and the difference between them can roughly reflect the content of organic substances in sewage that cannot be degraded by microorganisms. For sewage with relatively fixed pollutant composition, COD and BOD5 generally have a certain proportion relationship, which can be calculated from each other. In addition, the determination of COD takes less time. According to the national standard method of refluxing for 2 hours, it only takes 3 to 4 hours to get the results from sampling, but it takes 5 days to determine the BOD5 value. Therefore, COD is often used as the control index in the actual sewage treatment operation management.

In order to guide the production and operation as soon as possible, some sewage treatment plants have also formulated the enterprise standard for measuring COD by reflux for 5 minutes. Although the measured results have certain errors with the national standard method, the errors are systematic errors and the results of continuous monitoring can correctly reflect the water quality. However, the measurement time can be reduced to less than 1 hour, which provides a time guarantee for timely adjustment of sewage treatment operating parameters and preventing water quality mutations from impacting the sewage treatment system.