How to use biochemical respiration lines to evaluate the degradability of substrates

Generally consider the B/C of wastewater. If it is above 0.3, it can be considered to be biologically treatable. If it is below 0.2, there is basically no need to consider biochemical treatment. Try to improve the B/C between 0.2 and 0.3. Bar - hydrolysis acidification, advanced oxidation, etc.

The biodegradability of wastewater (Biodegradability), also known as the biodegradability of wastewater, is the ease with which organic pollutants in wastewater are biodegraded. It is the One of the important characteristics.

　

The main reason for the difference in biodegradability of wastewater is that the organic matter contained in wastewater, in addition to some that are easily decomposed and utilized by microorganisms, also contains some that are not easily degraded by microorganisms and even It has an inhibitory effect on the growth of microorganisms

The biodegradation properties of these organic substances and their relative content in wastewater determine the feasibility of biological treatment of this kind of wastewater (usually referred to as aerobic biological treatment) and difficulty level. Under certain circumstances, the biodegradability of wastewater not only reflects whether and to what extent organic pollutants in wastewater can be utilized, but also reflects the rate at which organic pollutants are utilized by microorganisms during the treatment process. : Once the decomposition and utilization rate of microorganisms is too slow, the treatment process will take too long, which is difficult to achieve in actual wastewater projects. Therefore, it is generally believed that the biodegradability of this type of wastewater is not good. High[6].

Determining the biodegradability of wastewater to be treated is of great significance for the selection of wastewater treatment methods and the determination of important process parameters such as water inlet volume and organic load in the biochemical treatment section. Domestic and foreign methods for determining biodegradability can be roughly divided into aerobic respiration parameter methods, microbial physiological index methods, simulation experiment methods, and comprehensive model methods based on the judgment parameters used.

1 Aerobic respiration parameter method

During the aerobic degradation of organic pollutants by microorganisms, in addition to COD (ChemicalOxygenDemand) and BOD (BiologicalOxygenDemand) In addition to changes in water quality indicators, it is also accompanied by the consumption of O2 and the generation of CO2.

　

The aerobic respiration parameter method uses the above facts to measure the changes in water quality indicators such as COD and BOD, as well as the O2 or CO2 content (or consumption or generation) during the respiratory metabolism process. A method to determine the biodegradability of a certain organic pollutant (or wastewater) by changing the rate). According to the water quality indicators used, it can be mainly divided into: water quality indicator evaluation method, microbial respiration curve method, and CO2 production measurement method.

1.1 Water quality index evaluation method

The BOD5/CODCr ratio method is the most classic and currently the most commonly used water quality index evaluation method to evaluate the biodegradability of wastewater.

　

BOD refers to the amount of oxygen consumed by aerobic microorganisms in the metabolic process of decomposing and utilizing organic pollutants in wastewater under aerobic conditions. We usually refer to BOD5 (five-day biochemical Oxygen demand) directly represents the part of organic matter in wastewater that can be biodegraded. CODCr refers to the amount of oxygen consumed during the complete oxidation of organic pollutants in wastewater using chemical oxidant (K2Cr2O7). CODCr usually represents the total amount of organic pollutants in wastewater.

The traditional view is that BOD5/CODCr, that is, the B/C ratio reflects the proportion of biodegradable organic pollutants in wastewater to the total amount of organic pollutants, so it can

be used value to evaluate the microbial degradability of wastewater under aerobic conditions. It is generally believed that wastewater with BOD/COD<0.3 is difficult to biodegrade, and it is difficult to use aerobic biological treatment before necessary pretreatment; while wastewater with BOD/COD>0.3 is biodegradable. wastewater. The higher the ratio, the better the effect of aerobic biological treatment of wastewater.

Among various organic pollution indicators, total organic carbon (TOC), total oxygen demand (TOD) and other indicators can be measured by instruments more quickly than COD, and the measurement process is more reliable. , can more accurately reflect the content of organic pollutants in wastewater.

With the development and improvement of the above-mentioned indicator measurement methods in recent years, foreign countries often use the ratio of BOD/TOD and BOD/TOC as indicators to determine the biodegradability of wastewater, and provide a series of standards. But regardless of BOD/COD, BOD/TOD or BOD/TOC, the main principle of the method is to measure the proportion of biodegradable organic matter (BOD) to total organic matter (COD, TOD or TOC). To determine the biodegradability of wastewater.

The main advantage of this determination method is that the significance of water quality indicators such as BOD and COD has been widely understood and accepted, the measurement method is mature, and the required instruments are simple.

　

However, this judgment method also has obvious shortcomings, which leads to greater limitations in the application process of this method. First of all, BOD itself is an empirical parameter, and their reproducibility and comparability can only be compared under strict and consistent test conditions. Any deviation in the test conditions will lead to extremely unstable test results. The dilution process, the analyst's experience and the changes in the inoculation materials can lead to large errors in the BOD test. At the same time, it is difficult for us

Find a standard inoculation material to test how much error the inoculated microorganisms bring, and it is not known which measurement value is closer to the true value. In fact, the reproducibility of BOD test results of the same water sample from different laboratories is very poor. The reason may be errors caused by differences in the preparation process of dilution water or the specific operations of different laboratories; Secondly, domestic and foreign scholars have done a lot of measurement work on the BOD and COD values ​​of various types of industrial wastewater and urban sewage

, and determined the relationship expression that can represent the correlation between the two:

< p>COD＝a+bBOD(1)

In formula (1), a=CODnB, b=CODB/BOD

CODnB—COD of the part of organic matter that cannot be biodegraded value;

CODB—the COD value of that part of organic matter that can be biodegraded.

　

According to formula 1, it can be seen that the BOD/COD value cannot represent the ratio of biodegradable organic matter to all organic matter. Only when the a value is zero, the BOD/COD of the wastewater The ratio is a constant; finally, certain properties of wastewater

will also cause errors in using this method to determine the biodegradability of wastewater or even lead to the opposite conclusion. For example: BOD cannot reflect the harmful and toxic substances in wastewater. The inhibitory effect of microorganisms. When the wastewater contains suspended and colloidal pollutants that degrade slowly, there is no good correlation between BOD and COD.

1.2 Microbial respiration curve method

The microbial respiration curve is a curve obtained by plotting time as the abscissa and oxygen consumption during the biochemical reaction as the ordinate. Curve characteristics Mainly depends on the nature of organic matter in wastewater [14]. Instruments used to measure oxygen consumption rate include Warburg respirometer and electrode dissolved oxygen meter [15].

　

Microbial endogenous respiration curve: When microorganisms enter the endogenous respiration phase, the oxygen consumption rate is constant, and the oxygen consumption is proportional to time. It is shown on the microbial respiration curve as follows The slope of a straight line passing through the origin of the coordinates represents the rate of oxygen consumption during internal source respiration. As shown in Figure 1, comparing the microbial respiration curve and the endogenous respiration curve of microorganisms, curve a is located at the upper part of the endogenous respiration curve of microorganisms, indicating that organic pollutants in wastewater can be degraded by microorganisms, and the oxygen consumption rate is greater than The oxygen consumption rate during endogenous respiration, over a period of time, curve a is almost parallel to the endogenous respiration line, indicating that the biodegradation of the substrate has been basically completed, and the microorganism has entered the endogenous respiration stage; curve b is consistent with the endogenous respiration line of the microorganism

< p>The absorption curves coincide, indicating that the organic pollutants in the wastewater cannot be degraded by microorganisms, but they do not inhibit the microorganisms. The microorganisms maintain endogenous respiration. Curve c is located at the lower end of the endogenous respiration curve of microorganisms, and the oxygen consumption rate is small

The oxygen consumption rate during endogenous respiration shows that the organic pollutants in the wastewater cannot be degraded by microorganisms and have inhibitory or toxic effects on microorganisms. Once the microbial respiration curve coincides with the abscissa, it means that the respiration rate of the microorganisms< /p>

The breathing has stopped and death has occurred.

Changing the abscissa of the microbial respiration curve to substrate concentration becomes another biodegradability determination method - the oxygen consumption curve method. Although the meaning of the graph is different, it is the same as the principle of the microbial respiration curve method

It is consistent with the experimental method. Units with wastewater that need to be treated can also go to the Wastewater Bao project service platform to consult companies with similar wastewater treatment experience.