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The biodegradability (degradability) of wastewater, which refers to the difficulty of biodegradation of organic pollutants in wastewater, is one of the important characteristics of wastewater.
The main reason for the difference in biodegradability of wastewater is that in addition to some organic substances that are easily decomposed and utilized by microorganisms, wastewater also contains some organic substances that are not easily degraded by microorganisms and even have inhibitory effects on their growth. The biodegradability properties and relative content of these organic substances in wastewater determine the feasibility and difficulty of using biological methods (usually aerobic biological treatment) to treat this type of wastewater. In specific situations, the biodegradability of wastewater not only reflects whether organic pollutants in the wastewater can be utilized and to what extent they can be utilized, but also reflects the speed at which microorganisms utilize organic pollutants during the treatment process. Once the decomposition and utilization speed of microorganisms is too slow, it leads to a long processing time, which is difficult to achieve in actual wastewater engineering. Therefore, it is generally believed that the biodegradability of this type of wastewater is not high.
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The determination of the biodegradability of the treated wastewater is of great significance for the selection of wastewater treatment methods, the determination of important process parameters such as influent volume and organic load in the biochemical treatment section. The methods for determining biodegradability at home and abroad can be roughly divided into aerobic respiration parameter method, microbial physiological index method, simulation experiment method, and comprehensive model method based on the adopted determination parameters.
A. Aerobic respiration parameter method
In the aerobic degradation process of organic pollutants by microorganisms, in addition to changes in water quality indicators such as COD (Chemical Oxygen Demand) and BOD (Biological Oxygen Demand), there is also consumption of O2 and generation of CO2.
The aerobic respiration parameter method is a method of determining the biodegradability of a certain organic pollutant (or wastewater) by measuring changes in water quality indicators such as COD and BOD, as well as changes in O2 or CO2 content (or consumption, generation rate) during respiratory metabolism, based on the above facts. According to the adopted water quality indicators, it can be mainly divided into: water quality indicator evaluation method, microbial respiration curve method, and CO2 production measurement method.
1. Evaluation method for water quality indicators
The BOD5/CODCr ratio method is the most classic and commonly used water quality indicator evaluation method for assessing the biodegradability of wastewater.
BOD refers to the amount of oxygen consumed by aerobic microorganisms during the metabolism of organic pollutants in wastewater under aerobic conditions. We usually use BOD5 (five-day biochemical oxygen demand) directly to represent the portion of biodegradable organic matter in wastewater. CODCr refers to the amount of oxygen consumed during the complete oxidation of organic pollutants in wastewater using a chemical oxidant (K2Cr2O7). CODCr is usually used to represent the total amount of organic pollutants in wastewater.
The traditional view is that BOD5/CODCr, or B/C ratio, reflects the proportion of biodegradable organic pollutants in the total amount of organic pollutants in wastewater, and can be used to evaluate the microbial degradability of wastewater under aerobic conditions. In general, the higher the BOD5/COD value, the better the biodegradability of the wastewater.
The main advantages of this judgment method are that the significance of water quality indicators such as BOD and COD has been widely understood and accepted, and the measurement method is mature with simple instruments required. However, this judgment method also has obvious shortcomings, resulting in significant limitations in its application process.
When using this method, the following issues should be noted
1. Some suspended organic solids contained in wastewater are easily oxidized by potassium dichromate in COD determination and manifested in the form of COD. However, due to physical limitations in the BOD reaction bottle, the BOD value is relatively low, resulting in a decrease in BOD5/COD value. In fact, suspended organic solids can be removed through biological flocculation and then hydrolyzed by extracellular enzymes to enter the cell for oxidation. Although their BOD5/COD value is small, their biological treatment ability is not poor.
The COD measurement value includes the oxygen consumption of certain inorganic reducing substances (such as sulfides, sulfites, nitrites, ferrous ions, etc.) in the wastewater, and the BOD5 measurement value also includes the oxygen consumption of sulfides, sulfites, and ferrous ions. However, due to the different determination methods of COD and BOD5, the final concentration and state of these inorganic reducing substances during the determination are not the same, that is, the amount of oxygen consumed in the two determination methods is different, which directly affects the determination values and ratios of BOD5 and COD.
Potassium dichromate has strong oxidation ability under acidic conditions, and in most cases, the COD value can approximately represent the content of all organic matter in the wastewater. However, some compounds such as pyridine are not oxidized by potassium dichromate and cannot exhibit oxygen demand in the form of COD. However, they may be oxidized by microorganisms and exhibit oxygen demand in the form of BOD5, thus greatly affecting the BOD5/COD value.
In summary, the BOD5/COD value of wastewater cannot directly equal the percentage of biodegradable organic matter in the total organic matter. Therefore, although using BOD5/COD value to evaluate the feasibility of biological treatment of wastewater is convenient, it is relatively rough. To make accurate conclusions, model experiments of biological treatment should also be supplemented.
2. Microbial respiration curve method
The microbial respiration curve is a curve obtained by plotting time as the horizontal axis and oxygen consumption during biochemical reactions as the vertical axis. The characteristics of the curve mainly depend on the properties of organic matter in the wastewater. The instruments used to measure oxygen consumption rate include the Weber breathalyzer and the electrode based dissolved oxygen analyzer.
Microbial endogenous respiration curve: When microorganisms enter the endogenous respiration period, the oxygen consumption rate is constant, and the oxygen consumption is proportional to time. It is represented as a straight line passing through the coordinate origin on the microbial respiration curve, and its slope represents the oxygen consumption rate during endogenous respiration.
Compared with other methods, this judgment method is simple to operate, has a short experimental period, and can meet the determination of large amounts of data. However, it must be pointed out that using this method to evaluate the biodegradability of wastewater requires strict regulations on the source, concentration, domestication of microorganisms, concentration of organic pollutants, and reaction time. In addition, the instruments required for measurement are not widely used in China, so their application is not widespread.
3. CO2 generation measurement method
Microorganisms generate a corresponding amount of CO2 while consuming O2 in wastewater during the degradation of pollutants. Therefore, by measuring the amount of CO2 generated during the biochemical reaction process, the biodegradability of pollutants can be determined.
The most commonly used method currently is the Sturm assay, with a reaction time of 28 days. It can compare the actual and theoretical production of CO2 to determine the biodegradability of wastewater, and can also use CO2/DOC values to determine the biodegradability of wastewater. Due to the need for special instruments and methods, the operation is complex and limited to laboratory research. Its practical application in production has not been reported yet.
B. Microbial physiological index method
After microorganisms come into contact with wastewater, they use the organic matter in the wastewater as a carbon source and energy source for metabolism. The microbial physiological index method determines the biodegradability of the wastewater by observing the changes in important physiological and biochemical indicators during the microbial metabolism process. At present, the physiological and biochemical indicators that can be used as criteria for judgment mainly include dehydrogenase activity and adenosine triphosphate (ATP).
1. Dehydrogenase activity index method
The oxidation and decomposition of organic matter by microorganisms are completed with the participation of various enzymes, among which dehydrogenases play an important role: catalyzing the transfer of hydrogen from the oxidized substance to another substance. Due to the sensitivity of dehydrogenases to toxins, their activity (the ability to activate hydrogen per unit time) decreases when toxins are present. Therefore, dehydrogenase activity can be used as an indicator to evaluate the ability of microorganisms to decompose pollutants: if the dehydrogenase activity of microorganisms grown in a culture medium with a certain wastewater (organic pollutant) as the substrate increases, it indicates that the microorganisms can degrade that wastewater (organic pollutant).
2. Adenosine triphosphate (ATP) index method
The oxidation and degradation process of pollutants by microorganisms is actually an energy metabolism process, and the capacity of microbial production directly reflects its activity level. Adenosine triphosphate (ATP) is a substance that stores energy in microbial cells. Therefore, the activity level of microorganisms can be reflected by measuring the ATP level in cells, and it can be used as an indicator to evaluate the ability of microorganisms to degrade organic pollutants. If the ATP activity of microorganisms grown in a culture medium with a certain wastewater (organic pollutant) as the substrate increases, it indicates that microorganisms can degrade that wastewater (organic pollutant).
In addition, microbial physiological index methods include bacterial standard plate counting, DNA assay, INT assay, luminescent bacterial light intensity assay, etc.
Although there are currently mature methods for measuring dehydrogenase activity, ATP, and other parameters, the microbial physiological index method is mainly used to determine the biodegradability and ecological toxicity of single organic pollutants due to the high requirements for instruments and drugs, as well as the complexity of operation.
C. Simulation experiment method
Simulation experiment method refers to the method of directly determining the feasibility of biological treatment of wastewater by simulating the actual wastewater treatment process. According to the degree of approximation between the simulation process and the actual process, it can be roughly divided into culture medium measurement method and simulated biochemical reactor method.
1. Culture medium determination method
The culture medium determination method, also known as the shaking table test method, involves filling a series of triangular bottles with a culture medium containing a certain pollutant (or wastewater) as a carbon source, adding appropriate nutrients such as N and P, adjusting the pH value, and then inoculating one or more microorganisms (or domesticated activated sludge) into the bottles. The triangular bottles are placed on a shaking table for oscillation, simulating the actual aerobic treatment process. During a certain stage, the physical appearance (concentration, color, odor, etc.) of the culture medium in the triangular bottles is continuously monitored, as well as changes in microorganisms (strains, biomass, and biological phases, etc.) and various indicators of the culture medium, such as pH, COD, or concentration of a certain pollutant.
2. Simulated Biochemical Reactor Method
The simulated biochemical reactor method is carried out in a model biochemical reactor (such as an aeration tank model), by simulating the reaction conditions of actual sewage treatment facilities (such as aeration tanks) in the biochemical model, such as MLSS concentration, temperature, DO, F/M ratio, etc., to predict the removal efficiency of various wastewater in sewage treatment facilities and the impact of various factors on biological treatment.
Due to the fact that the microorganisms and wastewater used in the simulation experiment method are the same as the actual process, and the biochemical reaction conditions are also close to the actual values, from the perspective of water treatment research, it is equivalent to a small-scale study of the actual treatment process. Various actual influencing factors can be reflected in the experimental process, avoiding errors that may occur in other judgment methods during the experimental process. Moreover, because the experimental conditions and reaction space are closer to the actual situation, the simulation experiment method can more accurately demonstrate the feasibility of wastewater biological treatment compared to the culture medium measurement method.
D. Integrated Model Method
The comprehensive model method is mainly aimed at determining the biodegradability of a certain organic pollutant. By using computer simulation to predict the biodegradability of new organic compounds based on the correlation between the biodegradability and molecular structure of a large number of known pollutants, the main models include BIODEG model, PLS model, etc.
The comprehensive modeling method relies on a large database of known pollutants' biodegradability (such as the EU's EINECS database), and the simulation process is complex and costly. It is mainly used to predict the biodegradability of new compounds and their degradation pathways after entering the environment.
In addition to the above-mentioned methods for determining biodegradability, many other methods have been developed in recent years, such as using multi-stage filtration and ultrafiltration to obtain the particle size distribution (PSD) and COD distribution of wastewater as indicators for predicting its biodegradability; Evaluate the biodegradability of wastewater by combining oxygen consumption, end products of biochemical reactions, and biological activity values; Using an empirical flowchart to predict the biodegradability of a certain organic pollutant.
In summary, there are various methods for determining the biodegradability of wastewater both domestically and internationally. In practical operations, the appropriate determination method should be selected based on the nature of the wastewater and experimental conditions.
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