The biodegradability of wastewater, that is, the difficulty of biodegradation of organic pollutants in wastewater, is one of the important characteristics of wastewater.

The main reason for the difference of biodegradability of wastewater is that besides some organic substances which are easy to be decomposed and utilized by microorganisms, there are also some organic substances which are not easy to be degraded by microorganisms and even inhibit the growth of microorganisms. The biodegradability of these organic substances and the relative content of these organic substances in Wastewater determine that the wastewater is treated by biological method (usually referred to as aerobic process). Feasibility and difficulty of material treatment. Under certain circumstances, the biodegradability of wastewater not only reflects whether organic pollutants in wastewater can be utilized and the degree of utilization, but also reflects the utilization speed of organic pollutants by microorganisms in the process of treatment: once the decomposition and utilization speed of microorganisms is too slow, the treatment process takes too long, and it is difficult to achieve in actual wastewater engineering, therefore, it is generally recognized that The biodegradability of the wastewater is not high.

The determination of biodegradability of wastewater is of great significance to the selection of wastewater treatment methods, the determination of influent volume and organic load of biochemical treatment section. The biodegradability judgment methods at home and abroad can be roughly divided into aerobic breathing parameter method, microbial physiological index method, simulation experiment method and comprehensive model method.

A. Aerobic Respiratory Parameter Method

In the process of aerobic degradation of organic pollutants by microorganisms, besides the changes of water quality indicators such as COD (Chemical Oxygen Demand chemical oxygen demand) and BOD (Biological Oxygen Demand biochemical oxygen demand), the consumption of oxygen dioxide and the formation of carbon dioxide are also accompanied.

Aerobic breathing parameter method is to determine the biodegradability of an organic pollutant (or wastewater) by measuring the changes of COD, BOD and other water quality indicators as well as the changes of oxygen or CO2 content (or consumption, production rate) in the process of respiratory metabolism. According to the water quality index adopted, it can be divided into water quality index evaluation method, microbial respiration curve method and CO2 production measurement method.

1. Water quality index evaluation method

BOD5/CODCr ratio method is the most classical and 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 process of decomposition and utilization of organic pollutants in wastewater for metabolism under aerobic conditions. We usually use BOD5 (five-day biochemical oxygen demand) to directly represent the biodegradable part of organic matter in wastewater. CODCr refers to the amount of oxygen consumed in the process of thorough oxidation of organic pollutants in wastewater by chemical oxidants (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 organic pollutants in wastewater, so it can be used to evaluate the microbial degradability of wastewater under aerobic conditions. In general, the greater the BOD5/COD value, the better the biodegradability of wastewater.

The main advantage of this method is that the significance of BOD, COD and other water quality indicators has been widely understood and accepted, and the determination method is mature, and the instrument needed is simple. However, there are obvious shortcomings in this method, which leads to the limitations of this method in the application process.

When using this method, we should pay attention to the following problems

1. Suspended organic solids in some wastewater are easily oxidized by potassium dichromate in the form of COD. However, due to the limitation of physical morphology in BOD reaction bottle, the BOD value is low, which leads to the decrease of BOD5/COD value. In fact, suspended organic solids can be removed by biological flocculation, and then can be hydrolyzed by extracellular enzymes and oxidized into cells. Although the BOD5/COD value is small, the bio-treatment ability is not poor.

2. The COD value includes the amount of oxygen consumed by some inorganic reducing substances (such as sulfide, sulfite, nitrite, ferrous ion, etc.) in wastewater. The BOD5 value also includes the amount of oxygen consumed by sulfide, sulfite and ferrous ion. However, because of the difference between the determination methods of COD and BOD5, the final concentration and state of these inorganic reducing substances are different. That is to say, the amount of oxygen consumed in the two methods is different, which directly affects the determination values of BOD5 and COD and their ratios.

3. Potassium dichromate has strong oxidation ability under acidic conditions. In most cases, COD value can approximately represent the content of all organic matter in wastewater. However, some compounds such as pyridine, which is not oxidized by potassium dichromate, can not show oxygen demand in the form of COD, but may be oxidized under the action of microorganisms and show oxygen demand in the form of BOD5, which has a great impact on BOD5/COD value.

In summary, the BOD5/COD value of wastewater can not be directly equal to the percentage of biodegradable organic matter in the total organic matter. Therefore, although it is convenient to use BOD5/COD value to evaluate the feasibility of biological treatment of wastewater, it is rough. In order to make accurate conclusions, model experiments of biological treatment should be supplemented.

2. Microbial Respiration Curve Method

Microbial respiration curve is a curve drawn by time as abscissa and oxygen consumption in biochemical reaction as ordinate. The characteristics of the curve mainly depend on the nature of organic matter in wastewater. The instruments for measuring oxygen consumption rate are Wabble respirator and electrodes dissolved oxygen meter.

Endogenous Respiration Curve of Microorganisms: When microorganisms enter the endogenous respiratory period, the rate of oxygen consumption is constant, and the oxygen consumption is proportional to time. On the microorganism respiratory curve, it shows a straight line over the origin of coordinates, and its slope is the rate of oxygen consumption during endogenous respiration.

Compared with other methods, this method has the advantages of simple operation and short experimental period, and can satisfy the needs of mass data determination. However, it must be pointed out that this method is not widely used in China because of its low popularity in China and strict regulations on the sources, concentration, domestication and concentration of organic pollutants and reaction time of microorganisms, etc. in evaluating the biodegradability of wastewater.

3. Measurement of CO2 generation

In the process of degradation of pollutants, microorganisms consume CO2 in wastewater and produce a corresponding amount of CO2. Therefore, the biodegradability of pollutants can be judged by measuring the production of CO2 in biochemical reaction process.

At present, the most commonly used method is the Stem method. The reaction time is 28 days. The actual and theoretical output of CO2 can be compared to determine the biodegradability of wastewater. The value of CO2/DOC can also be used to determine the biodegradability of wastewater. Because this kind of judgment experiment needs special instruments and methods, and its operation is complex, it is limited to laboratory research and use, and its application in actual production has not been reported.

B. Microbial Physiological Index Method

After microorganism contacts with wastewater, organic matter in wastewater is used as carbon source and energy for metabolism. Microbial physiological index method is to determine the biodegradability of wastewater by observing the changes of important physiological and biochemical indexes in the process of microbial metabolism. At present, the main physiological and biochemical indicators that can be used as a basis for determination are dehydrogenase activity and adenosine triphosphate (ATP).

1. Dehydrogenase activity index method

The oxidative decomposition of organic matter by microorganisms is accomplished with the participation of various enzymes, in which dehydrogenase plays an important role: catalyzing the transfer of hydrogen from the oxidized substance to another substance. Because dehydrogenase is very sensitive to toxicants, its activity (the ability to activate hydrogen per unit time) decreases in the presence of toxicants. Therefore, dehydrogenase activity can be used as an index to evaluate the ability of microorganisms to decompose pollutants: if the activity of microbial dehydrogenase growing in a culture medium based on some wastewater (organic pollutants) increases, it indicates that microorganisms can degrade the wastewater (organic pollutants).

2. Adenosine triphosphate (ATP) index method

The oxidative degradation of pollutants by microorganisms is actually a process of energy metabolism. The capacity of microorganisms directly reflects their activity. Adenosine triphosphate (ATP) is a substance that stores energy in microbial cells. Therefore, the activity of microorganisms can be reflected by measuring the level of ATP in microbial cells, and it can be used as an index to evaluate the ability of microorganisms to degrade organic pollutants. If the activity of microbial ATP growing in a culture solution based on wastewater (organic pollutants) increases, it shows that microorganisms can degrade such wastes. Water (organic pollutants).

In addition, the methods of microbial physiological indicators include bacterial standard plate counting, DNA assay, INT assay, luminescent bacteria light intensity assay and so on.

Although there are mature methods for the determination of dehydrogenase activity and ATP, the microbial physiological index method is mainly used to determine the biodegradability and ecological toxicity of a single organic pollutant because of the high requirement for instruments and drugs and the complex operation of these parameters.

C. Simulated experiment method

Simulated experiment is a method to judge the feasibility of wastewater biological treatment directly by simulating the actual wastewater treatment process. According to the approximation degree between the simulated process and the actual process, it can be roughly divided into culture medium determination method and simulated biochemical reactor method.

1. Culture medium determination method

The method of culture medium determination is also called shaking table test method. The specific operation method is: a series of triangular bottles are filled with a culture medium with a pollutant (or wastewater) as carbon source, adding appropriate nutrients such as N and P, adjusting the pH value, and then inoculated with one or more microorganisms (or domesticated activated sludge) into the bottle, shaking the triangular bottle on the shaking bed to simulate the actual aerobic treatment process. The physical appearance (concentration, color, odor, etc.) of the culture medium in the triangle bottle, the changes of microorganisms (bacteria, biomass and bioequivalence) and the changes of pH, COD or the concentration of a pollutant in the culture medium were monitored continuously in a certain stage.

2. Simulated biochemical reactor method

Simulated biochemical reactor method is carried out in model biochemical reactor (such as aeration tank model). Reaction conditions of actual sewage treatment facilities (such as aeration tank) are simulated in biochemical model, such as MLSS concentration, temperature, DO, F/M ratio, to predict the removal effect of various wastewater in sewage treatment facilities and the influence of various factors on biological treatment.

Because the microorganism and wastewater used in the simulation experiment are the same as the actual process, and the biochemical reaction conditions are close to the actual value, from the point of view of water treatment research, it is equivalent to the small-scale study of the actual treatment process. All kinds of actual influencing factors can be reflected in the experiment process, avoiding the errors of other determination methods in the experiment process, and because of the experimental strip. The simulation experiment method can explain the feasibility of wastewater biological treatment more accurately than the culture medium method because the reaction space is closer to the actual situation.

D. Comprehensive Model Method

The comprehensive model method is mainly aimed at the biodegradability determination of an organic pollutant. Through the correlation between the biodegradability and molecular structure of a large number of known pollutants, the computer simulation is used to predict the biodegradability of new organic compounds. The main models are BIODEG model, PLS model and so on.

The integrated model method relies on a large database of known pollutants (such as the EINECS database of EU), and the simulation process is complex and costly. It is mainly used to predict the biodegradability of new compounds and the degradation pathways after entering the environment.

In addition to the above biodegradability determination methods, many other methods have been developed in recent years. For example, the particle size distribution (PSD) and COD distribution of wastewater are obtained by multi-stage filtration and ultrafiltration as indicators to predict the biodegradability of wastewater, and the biodegradability of wastewater is evaluated by oxygen consumption, end products of biochemical reaction and biological activity value. Flow charts are used to predict the biodegradability of an organic pollutant.

In summary, at present, there are many different methods for judging the biodegradability of wastewater at home and abroad. In practice, the appropriate judging method should be selected according to the nature of wastewater and experimental conditions.