BOD and COD indicators in sewage

BOD and COD indicators in sewage

In the sewage treatment process, in order to achieve the discharge of the treated water, the water quality monitoring equipment will be used to detect the water quality in each link of the sewage treatment. According to the data measured by the water quality monitoring equipment, the corresponding treatment method is adopted to make this link The water quality index meets the requirements, and then enters the next treatment link. Among these water quality monitoring indicators, the two most-heard and most important indicators are COD and BOD. So what is the difference and connection between the two? Henan Saifu Trading Co., Ltd. introduces to you why COD and BOD are commonly used in water pollution indicators, and the difference and connection between COD and BOD.

What is COD?

   COD (Chemical Oxygen Demand): It is the amount of oxidant consumed when a certain strong oxidant is used to treat water samples under certain conditions. It reflects the degree of pollution in the water. The greater the chemical oxygen demand, the more serious the pollution of organic matter in the water. COD is expressed in mg/L. The value of COD detected by water quality monitoring equipment can be divided into five categories. Among them, the first and second categories have COD≤15mg/L, which can basically meet the drinking water standard, and the value is greater than the second category. Those that cannot be used as drinking water, of which three types of COD≤20mg/L, four types of COD≤30mg/L, and five types of COD≤40mg/L belong to polluted water quality. The higher the COD value, the more serious the pollution.

What is BOD?

   BOD (Biochemical Oxygen Demand): It refers to the mass concentration of dissolved oxygen required in the biochemical process of microorganisms in water to decompose organic matter under aerobic conditions. In order to make the measured values of BOD comparable, a time period is generally specified, and the dissolved oxygen consumption in the water is measured. Generally, five days are used, called the five-day biochemical oxygen demand, which is recorded as BOD5, and the five-day biochemical oxygen demand is often used. . The larger the BOD value, the more organic matter contained in the water, and therefore the more serious the pollution.

   BOD is an environmental monitoring indicator used to monitor the pollution of organic matter in water. Organic matter can be decomposed by microorganisms. Oxygen is consumed in this process. If the dissolved oxygen in water is not enough to supply the needs of microorganisms, the water body will treat the pollution state.

What is the relationship between COD and BOD? 

   In the sewage treatment process, there are hundreds of organic substances, and analyzing these organic substances one by one takes time and medicine. After research, it is found that all organic substances have two things in common. One is that they are all composed of hydrocarbons. The other is that most organic substances can be chemically oxidized or oxidized by microorganisms. Their carbon and hydrogen form non-toxic with oxygen. Harmless carbon dioxide and water. The organic matter in sewage consumes oxygen in both the chemical oxidation process and the biological oxidation process. The more organic matter in the wastewater, the more oxygen is consumed. There is a direct proportional relationship between the two. Therefore, the amount of oxygen consumed by chemical oxidation of sewage is called COD (Chemical Oxygen Demand), and the amount of oxygen consumed by the oxidation of microorganisms in sewage is called BOD (Oxygen Demand for Gas).

   Since COD (Chemical Oxygen Demand) and BOD (Oxygen Demand for Gas) can comprehensively reflect the quantity of all organic substances in the water, there are also many such detection instruments, the detection method is simple, and the detection results can be obtained in a short time. Therefore, it has been widely used in water quality detection and analysis, and it has become an important indicator of water quality monitoring and an important basis for environmental monitoring of water bodies. We all hear more about sewage treatment.

   In fact, COD (Chemical Oxygen Demand) not only reacts with organic substances in water, but also represents inorganic substances with reducing properties in water, such as sulfides, ferrous ions, and sodium sulfite. For example, if the ferrous ions in the sewage are not completely removed in the neutralization tank, there will be ferrous ions in the effluent of the biochemical treatment, and the COD (chemical oxygen demand) of the effluent may exceed the standard.

   Some of the organic matter in sewage can be biologically oxidized (such as glucose and ethanol), and some can only be partially degraded by biological oxidation (such as methanol), and some of the organic matter cannot be degraded by biological oxidation, and some Toxicity (certain surfactants). In this way, the organic matter in the sewage can be divided into two parts, biodegradable and non-biodegradable organic matter. Traditionally, COD (chemical oxygen demand) basically means all the organic matter in sewage, BOD (oxygen demand for gasification) is the organic matter in sewage that can be biodegraded, so the difference between COD and BOD can indicate that the sewage cannot be biodegradable Of organic matter.

Reduce BOD and COD indicators

   Usually in the process of water treatment, when the content of organic pollutants is not high, it is not necessary to carry out "biological treatment" (that is, secondary treatment), but it is necessary to remove part of the organic pollutants in the sewage treatment system; or in the urban sewage treatment In the process, in order to reduce the “operating load” of the secondary treatment and thus reduce the overall operating cost of the sewage treatment plant, we can consider

   The above two purposes can be achieved by strengthening the removal of BOD and COD in the primary treatment.

   Sometimes the average content of suspended solids (SS) in the sewage ditch is relatively high, but the content of organic pollutants is low. The average indicators of BOD and COD meet the requirements of the national secondary emission standards. However, in order to ensure that all the control indicators of the sewage after the primary treatment can meet the requirements of the national discharge standard, we consider the selected coagulant (flocculant) which can effectively remove the suspended solids in the sewage, but also To be able to remove BOD and COD in sewage to the maximum.

   BOD and COD removal rate experiment:

   Take an appropriate amount of experimental stock solution, measure the current BOD and COD indicators, configure the coagulant polyaluminum chloride solution, polyaluminum ferric chloride solution and flocculant polyacrylamide solution, add different amounts of pharmaceutical solutions, and then measure them separately The BOD and COD indicators after 15 minutes, 30 minutes and 45 minutes of rest:

   Through experimental analysis, we know that "BOD 5, COD cr removal rate" is related to the following factors:

1. Related to the "coagulant" selected, different coagulants have different removal rates of BOD 5 and CODcr under the same conditions.

2. It is related to the "concentration" of the coagulant used. The same coagulant uses different concentrations, and the removal rate of BOD 5 and COD cr is different under the same conditions.

3. It is related to the "sedimentation time", the same coagulant, the same concentration, under the same conditions, after different settling times, the removal rates of BOD 5 and COD cr are also different.

   In addition, whether the temperature, PH value and the existence of various salts of sewage affect the removal rate of BOD and COD,

   As well as the above-mentioned "concentration", "sedimentation time" and "removal rate" whether there is a "proportional relationship" and many other issues, it remains to be tested, researched and discussed. But there is no doubt that the amount of enhanced removal of BOD and COD in the primary treatment is not very large (the removal rate is about 30%), because the coagulation enhancement in the primary treatment is all suspended Sexual and colloidal organic matter, soluble BOD and COD are difficult to remove. They can only be effectively removed in the subsequent biochemical treatment.