1. Why do COD and BOD are often used in wastewater analysis?

There are many organic substances in the wastewater, including more than ten, dozens, and even hundreds of organic substances, which can be often encountered. If the qualitative and quantitative analysis of organic substances in the wastewater is carried out, it will not only take time but also consume drugs. Can we use only one pollution index to represent all organic substances in wastewater and their quantity? After the research of environmental scientists, all organic substances have two common characteristics: one is that they are composed of hydrocarbons at least; Second, most organic substances can be chemically oxidized or oxidized by microorganisms. Their carbon and hydrogen form non-toxic and harmless carbon dioxide and water with oxygen respectively. The organic matter in wastewater is consumed in the process of chemical oxidation or biological oxidation. The more organic matter in the wastewater, the more oxygen consumption is, the positive proportion relationship between them. So environmental scientists call the oxygen demand, COD, which is consumed in the oxidation of wastewater with chemical agents; The oxygen consumption of wastewater by microbial oxidation is called biological oxygen demand, that is, BOD. Cod and BOD can comprehensively reflect the quantity of all organic substances in wastewater, and the analysis is simple, so they are widely used in wastewater analysis and environmental engineering.

In fact, cod is not only an organic substance in water, but also an inorganic substance with reductive properties in water, such as sulfide, ferrous ion, sodium sulfite, and even chloride ion. For example, if the ferrous ions in the effluent of the iron carbon pool can not be completely removed in the neutralization tank, the COD of effluent may exceed the standard due to the presence of ferrous ions in the effluent of biochemical treatment.

2. What is cod?

Cod refers to the oxygen required by the oxidizer when the substance in wastewater can be oxidized, in mg / L of oxygen. It is a most commonly used method to determine the organic content in wastewater. The common oxidants in COD analysis are potassium permanganate (manganese CODMn) and potassium dichromate (CODcr method). Now, potassium dichromate method is commonly used. The oxidation rate of most organic compounds can be increased to 85-95% when silver sulfate is used as catalyst. If the wastewater contains high concentration of chloride ion, mercury sulfate should be used to shield the chloride ion, so as to reduce the interference of COD determination.

3. What is BOD5?

Biochemical oxygen demand can also indicate the degree of wastewater being polluted by organic matter. The most commonly used one is five-day biochemical oxygen demand. BOD5 is used to indicate the amount of oxygen required for the wastewater to be biochemical degraded in the presence of microorganisms within five days. We will use biochemical oxygen demand for five days in the future.

4. What is the relationship between COD and BOD5?

Some organic substances can be bioxidized (such as glucose and ethanol), some can only be partially biodegraded (such as methanol), while others cannot be biodegraded and toxic (such as ginkgo, ginkgolic acid, some surface active agents). Therefore, we can divide the organic matter in water into two parts, namely, the organic matter which can be biodegraded and the organic matter which can not be biodegraded.

Cod is generally considered to represent basically all organic matter in water. BOD is an organic substance that can be biodegraded in water. Therefore, the difference between COD and BOD can indicate the organic matter of the non degradable part of wastewater.

5. What is b/c? What does b/c mean?

B/c is the abbreviation of BOD5 and COD ratio, which can indicate the biodegradability of wastewater. If codnb is the non biodegradable part of COD, the proportion of organic matter that cannot be biodegraded by microorganisms in wastewater can be expressed by codnb/cod.

The relationship between bod5/cod and codnb/cod is as follows:

CODNB/COD 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8  

BOD5/COD 0.52 0.46 0.41 0.35 0.29 0.23 0.17 0.12

When bod5/cod ≥ 0.45, the non biodegradable organic matter accounts for less than 20% of all organic substances, while when bod5/cod ≤ 0.2, the non biodegradable organic matter accounts for more than 60% of all organic substances.

Therefore, bod5/cod value is often used as an evaluation index of biodegradability of organic matter.

Bod5/cod 0.45 is easy to biodegradation  

Bod5/cod 0.30 biodegradability  

Bod5/cod 0.30 is difficult to biodegradation  

Bod5/cod 0.20 is more difficult to biodegradation

B / C is of great importance and practical significance in environmental engineering.

6. What is pH?

PH is actually a method of representing pH in aqueous solution. Usually we are used to using the percentage concentration to express the pH of the water solution, such as 1% sulfuric acid solution or 1% alkali solution. However, when the pH of the water solution is very small and very small, it is too troublesome to use the percentage concentration to express it. At this time, it can be expressed by pH. The application range of pH is between 0-14, and the water is neutral when pH = 7; When pH is less than 7, the water is acidic, the smaller the pH is, the greater the acid of water is; When pH > 7, water is alkaline, the greater the pH is, the greater the alkalinity of water.

All the organisms in the world can not leave water, but the range of pH value suitable for biological survival is often very narrow. Therefore, the National Environmental Protection Bureau strictly regulates the pH value of treated effluent between 6-9.

PH value in water is often measured by pH test paper, and also used by instruments, such as pH meter.

7. Why should the concentration unit of mg / l be used frequently in wastewater analysis?

Generally speaking, the content of organic and inorganic substances in wastewater is very small and small. If the concentration of percent or other concentration is used to express it, it is too inconvenient. For example, there are only a few grams, dozens of grams, hundreds of grams or even thousands of grams of pollutants in a ton of wastewater, and the unit is g / T, if the ton is converted into liter, it is mg / L. The calculation can be made according to the following table:

1 mg / L 1 / M  

1000 mg / L 1 / 1000  

10000 mg / L 1% of

8. What is wastewater pretreatment? What are the objectives of pretreatment?

The treatment before biochemical treatment is usually called pretreatment. Because the cost of biochemical treatment is relatively low and the operation is relatively stable, the general industrial wastewater is treated by biochemical method, and the treatment of wastewater of * * * company also takes biochemical method as the main treatment method. However, the wastewater of * * * company contains some organic substances which have inhibition and toxicity to microorganisms. Therefore, the wastewater must be pre-treatment before entering the biochemical tank, so as to reduce or remove the substances containing inhibition and toxicity to microorganisms in the wastewater to ensure the normal operation of microorganisms in the biochemical tank.

The purpose of pretreatment is to reduce and remove or convert the substances in wastewater that have toxic and inhibitory effects on microorganisms as much as possible to ensure the normal operation of microorganisms in biochemical pool; The second is to reduce COD load in the pretreatment process to reduce the operation burden of biochemical tank.

***The pretreatment process of the company is the micro electrolysis of iron carbon and Fe2 + / fe3+ reduction oxidation. Numerous small iron carbon original batteries formed are conducive to the redox reaction, and the toxic and harmful substances in the wastewater can be destroyed, In the process of neutralization and precipitation, the activated flocs formed by the Fe 2 and Fe 3 can absorb the organic matter in the wastewater to reduce the COD load, so as to ensure the normal operation of the subsequent biochemical treatment system.

9. What is the use of the wastewater collection pool?

The function of wastewater collection pool is to collect, store and balance the water quality and quantity of wastewater. The wastewater discharged from each workshop is generally unbalanced in water quantity and water quality. There is wastewater in production, there is no wastewater when it is not produced, and even there may be great changes in one day or between shift production. Especially in the fine chemical industry, if the turbid wastewater is not separated, the water quality and quantity of the process concentrated wastewater and light pollution wastewater vary greatly, This change is very unfavorable to the normal operation and treatment effect of the wastewater treatment facilities and equipment, and even harmful. Therefore, before entering the main sewage treatment system, a wastewater collection tank with a certain volume shall be set up to store the wastewater and make it homogeneous and equal to ensure the normal operation of wastewater treatment equipment and facilities.

10. Why is the colloidal particles in wastewater not easy to settle naturally?

Many impurities, large particles and easy to settle suspended substances with a specific gravity greater than 1 in wastewater can be removed by natural sedimentation and centrifugation.

However, the particles with a specific gravity less than 1, small or even invisible to the naked eye are difficult to settle naturally. For example, colloidal particles are 10-4-10-6mm particles, which are very stable in water, and their settlement speed is very slow, and the settlement of 1m needs 200 years of cultivation. There are two reasons for slow settlement, (1) generally speaking, colloidal particles have negative charge. Because of the same-sex repulsion, colloidal particles can not be bonded to each other and suspended in water（ 2) There is a layer of molecules tightly surrounded on the surface of colloidal particles, which also obstructs and isolates the contact between colloidal particles, and cannot be bonded to each other and suspended in water.

11. How to precipitate colloidal particles?

To make colloidal particles precipitate, they should be contacted with each other, and make them become large particles, that is, to condense, and make them precipitate with a specific gravity greater than 1.

There are many methods used, and the common technologies in engineering are coagulation, flocculation and coagulation.

12. What is cohesion?

The coagulant with positive ions is added to the wastewater, and a large number of positive ions exist between the colloidal particles to eliminate the electrostatic exclusion between the colloidal particles, so as to make the particles agglomerate. This method of adding positive ion electrolyte makes the colloidal particles agglomerate together, which is called the condensation. Commonly used coagulants include aluminum sulfate, ferrous sulfate, alum, iron chloride, etc.

13. What is flocculation?

Flocculation is to add polymer coagulant to wastewater, and polymer polymer will be formed after dissolving. The structure of this polymer is linear structure. One end of the line is a small particle and the other is pulled by another. It plays a role of bridging between two particles far away, which makes the particles become larger gradually, and finally forms the flocculate (commonly known as alum flower) of large particles, accelerating the particle settlement. The common flocculent is PAM and PE.

14. Why should the wastewater be treated by flocculation adsorption with poly iron?

The results show that the wastewater can remove about 10% to 20% of COD after it is adsorbed by the flocculation of poly iron, which can greatly reduce the operation burden of biochemical tank and help to discharge the wastewater to the standard. In addition, the coagulation pretreatment with poly iron can remove the micro substances in wastewater which have toxic and inhibitory effect on microorganisms, so as to ensure the normal operation of microorganisms in biochemical tank. Among many coagulants, the price of poly iron is relatively cheap (25-300 yuan / ton), so the treatment cost is relatively low, which is more suitable for pretreatment of process wastewater.

Polyiron is an acid material with strong corrosiveness, so the treatment equipment should be well treated with anti-corrosion.

15. What is coagulation?

The process of coagulation and flocculation is coagulation. Coagulation is often used in experiments or engineering. For example, the addition of ferrous sulfate in water to eliminate electrostatic rejection between colloidal particles, and then adding PAM, the particles gradually become larger, forming alum flowers visible to the naked eye, and finally settling.

16. What is adsorption?

The porous solid (such as activated carbon) or floc (such as Polyferric) is used to adsorb toxic and harmful substances in wastewater on the surface of solid or floc or microporous to purify water quality. This treatment method is called adsorption treatment. The object of adsorption can be insoluble solid matter or soluble substance. The adsorption treatment is efficient and the effluent quality is good, so it is often used as the wastewater treatment in depth. Adsorption treatment can also be introduced into biochemical treatment unit to improve the efficiency of biochemical treatment (such as pact is one of them).

17. What is iron carbon treatment?

Iron carbon treatment is also called micro electro solution or iron carbon electrolysis method. It is an application form of metal iron treatment wastewater technology. It has a unique effect to treat toxic and harmful and high concentration cod wastewater by using iron carbon method as pretreatment technology. The mechanism of the treatment of iron carbon method is not fully understood. Now, it is more agreed that under the acidic condition, there are numerous micro current reaction pools between iron and carbon, and the organic matter is reduced and oxidized under the action of micro current. The effluent of iron carbon is neutralized with lime or lime milk, and the Fe (OH) 2 colloidal floc has strong flocculation adsorption capacity for organic matter. Therefore, the iron carbon method is a comprehensive application of iron reduction properties, electrochemical properties of iron carbon and flocculation adsorption of iron ions, which is the joint action of these three properties. The use of iron carbon method has a good treatment effect.

The disadvantages of the iron carbon method are as follows: (1) iron chips are easy to block after long-term immersion in acid medium, which causes blockage and formation of gully flow, which makes operation difficult and treatment effect reduced（ 2) Iron dissolved in acid condition has a large amount of iron, and sludge produced after alkali neutralization is more.

18. Why should iron carbon effluent be neutralized with lime powder?

After the wastewater is treated with iron carbon, sulfuric acid becomes ferrous sulfate, and the pH value of the wastewater rises from 2 to 5-6. Why should iron carbon effluent be neutralized with lime powder? Or can we add less lime powder during neutralization?

Iron carbon effluent contains a large amount of ferrous sulfate, if not removed, it will affect the growth and reproduction of microorganisms in the subsequent biochemical tank. Therefore, we must use lime to increase the pH value of wastewater from 5-6 to above 9, so that the water-soluble ferrous sulfate can be converted into insoluble ferrous hydroxide and calcium sulfate, and then they will be precipitated by coagulation sedimentation, To ensure that the wastewater entering the biochemical tank does not contain ferrous sulfate.

Can we add less lime powder during neutralization treatment? We can do a comparative experiment in the laboratory. The same amount of iron carbon water (pH is about 2) and iron carbon effluent (pH is 5-6) are placed in two beakers respectively, and then lime powder is added to the two beakers for neutralization and coagulation respectively. When the pH value of the wastewater in the two beakers is adjusted to 9, we can find that the quantity of lime powder added in the two beakers is the same. This is because iron is not a neutralizing agent, and the ferrous sulfate converted into sulfuric acid is an acid substance. The lime powder used in the neutralization process of ferrous sulfate is not less than that of ferrous hydroxide and calcium sulfate. Therefore, it is not allowed to add less lime powder in neutralization treatment of iron carbon effluent.

19. How to estimate the production of chemical sludge?

Sludge produced by chemical reaction (such as neutralization) and physical and chemical treatment (such as dosing coagulation) is generally referred to as chemical sludge. The sludge formed after neutralization and coagulation treatment of the effluent from iron carbon is mainly composed of ferrous hydroxide and calcium sulfate. The sludge production can be calculated by the amount of sulfuric acid and lime powder added. Experience can also be used in engineering to estimate. Generally speaking, if the pH of iron carbon water is about 2, the chemical sludge (water content 80%) produced by each ton of wastewater after neutralization and coagulation is about 50 kg.

20. What is biochemical treatment of wastewater?

Biochemical treatment of wastewater is one of the most important processes in wastewater treatment system, which is called biochemical treatment. Biochemical treatment is to effectively remove the soluble organic matter and some insoluble organic matter in wastewater by the process of microbial life activities, so as to purify the water. In fact, we are not very unfamiliar with biochemical treatment. There is a food chain in natural water body, that is, big fish eat small fish, small fish eat shrimp, shrimp eats small insects, small insects eat microorganisms, microorganisms eat sewage. If there is no such food chain, nature will be in disorder. In natural rivers, there are a large number of microorganisms that depend on organic substances. They oxidize or reduce the organic matter (such as industrial wastewater, pesticide fertilizer, feces and other organic substances) discharged into the river day and night, and finally convert them into inorganic substances. If there is no microorganism, the rivers around us will be less than a few months, one or two years more, It will become a stinky River, just because the microbes are too small and scattered that people can't see it with the naked eye. The biochemical treatment of wastewater is to strengthen this process under artificial conditions. People concentrate countless microorganisms in a pool to create a very suitable environment for microbial reproduction and growth (such as temperature, pH value, oxygen, nitrogen, phosphorus and other nutrients), so as to make microorganisms proliferate in large quantities and improve the speed and efficiency of organic matter decomposition. Then, the wastewater is pumped into the tank, so that the organic matter in the wastewater can be oxidized and degraded in the process of microbial life activities, so that the wastewater can be purified and treated. Compared with other treatment methods, biochemical method has the characteristics of low energy consumption, no medicine, good treatment effect and low treatment cost.

21. How does microorganism decompose and remove organic pollutants from wastewater?

Because of the existence of carbohydrate, fat, protein and other organic substances in the wastewater, these inanimate organic substances are the food of microorganisms, one part of which is degraded and synthesized into cell substances (combined metabolites), the other part is degraded to water, carbon dioxide, etc. (decomposition metabolites). In this process, the organic pollutants in the wastewater are removed by microorganism degradation.

22. What factors are microorganisms related to?

Besides nutrition, microorganisms need suitable environmental factors, such as temperature, pH, dissolved oxygen, osmotic pressure, etc. If the environment is abnormal, it will affect the life activities of microorganisms, even mutate or die.

23. What temperature range is the best temperature for microorganisms to grow and reproduce?

In the biological treatment of wastewater, the most suitable temperature range of microorganisms is 16-30 ℃, the highest temperature is 37-43 ℃, and when the temperature is lower than 10 ℃, the microorganism will no longer grow.   In the suitable temperature range, the metabolic rate of microorganisms will be increased correspondingly for every 10 ℃ increase of temperature, and the removal rate of COD will also be increased by about 10%; On the contrary, the removal rate of COD will be reduced by 10% every 10 ℃ when the temperature is reduced. Therefore, in winter, the biochemical removal rate of COD will be significantly lower than that of other seasons.

24. What range should the most suitable pH conditions of microorganisms be?

The life activities and metabolism of microorganisms are closely related to pH value. Most microorganisms can adapt to pH in the range of 4.5-9, while the most suitable pH range is 6.5-7.5. When pH is lower than 6.5, fungi begin to compete with bacteria. When pH reaches 4.5, fungi will occupy a full advantage in biochemical pool, and the result is that the sludge sedimentation is seriously affected; When pH is over 9, the metabolic rate of microorganisms will be hindered.

The requirements of pH range of different microorganisms are different. In aerobic biological treatment, pH can vary from 6.5 to 8.5; In anaerobic biological treatment, the requirements of pH are strict, and the pH should be between 6.7-7.4.

25. What is dissolved oxygen? How is the relationship between dissolved oxygen and microorganism?

Dissolved oxygen in water is called dissolved oxygen. The living and aerobic microorganisms in water, the oxygen they rely on to survive is dissolved oxygen. Different microorganisms have different requirements for dissolved oxygen. Aerobic microorganisms need to supply sufficient dissolved oxygen. Generally speaking, it is advisable to maintain dissolved oxygen at 3mg/l, and the minimum value should not be less than 2mg/l; The range of dissolved oxygen is 0.2-2.0mg/l; But anaerobic microorganism requires that the range of dissolved oxygen is below 0.2mg/l.

26. Why is the high concentration of salt containing wastewater having a great influence on microorganisms?

Let's describe an experiment of osmotic pressure: separate two different concentrations of salt solutions with a semi permeable film, and the water molecules of low concentration salt solution will enter into high concentration salt solution through semi permeable film, while the water molecules of high concentration salt solution will enter into low concentration salt solution through semi permeable film, but the quantity is less, so the liquid level on one side of high concentration salt solution will rise, When the difference of liquid level on both sides produces enough pressure to prevent water from flowing again, the permeation will stop. At this time, the pressure generated by the height difference of both sides of liquid level is the osmotic pressure. Generally speaking, the higher the salt concentration, the greater the osmotic pressure.

The situation of microorganism in brine solution is similar to the experiment of osmotic pressure. The unit structure of microorganism is cell, cell wall is equivalent to semi permeable membrane. When the concentration of chloride is less than or equal to 2000mg/l, the osmotic pressure that cell wall can bear is 0.5-1.0 atmospheric pressure. Even if the cell wall and cell membrane have certain tenacity and elasticity, the osmotic pressure that cell wall can bear will not be greater than 5-6 atmospheric pressure. But when the concentration of chloride in water solution is above 5000mg/l, the osmotic pressure will increase to 10-30 atmospheric pressure. Under such a pressure, the water molecules in the microorganism will penetrate into the in vitro solution in large quantities, resulting in the cell losing water and the separation of the quality wall, which will cause the death of the micro organism. In daily life, people use salt (sodium chloride) to marinate vegetables and fish meat, sterilize and protect food, which is the use of this principle. The results of engineering experience show that when the concentration of chloride in wastewater is more than 2000mg/l, the activity of microorganism will be restrained and COD removal rate will decrease obviously; When the concentration of chloride ion in wastewater is greater than 8000mg/L, the volume of sludge will expand, and a lot of bubbles will appear on the water surface, and microorganisms will die one after another.

However, after long-term domestication, microorganisms will gradually adapt to the growth and reproduction in high concentration brine. At present, some people have domesticated microorganisms that can adapt to the concentration of chloride ion or sulfate above 10000mg/l. However, the principle of osmotic pressure tells us that the microorganism that has been adapted to grow and propagate in high concentration brine has a high salt concentration in the cell liquid. Once the salt concentration in the wastewater is low or low, the water molecules in the wastewater will penetrate into the microbial body in large quantities, which will cause the microbial cells to expand and the serious ones will break down and die. Therefore, after long-term domestication and can gradually adapt to the growth and reproduction of microorganisms in high concentration brine, the requirements of salt concentration in biochemical water inflow should always be kept at a very high level, and it can not be suddenly high or low, otherwise the microorganism will die in large quantities.

27. What is aerobic biochemical treatment? What is facultative oxygen biochemical treatment? What is the difference between them?

According to the different requirements of microbial growth on oxygen environment, biochemical treatment can be divided into two categories: aerobic biochemical treatment and anoxic biochemical treatment. The treatment can also be divided into facultative oxygen biochemical treatment and anaerobic biochemical treatment. In the process of aerobic biochemical treatment, aerobic microorganisms must grow and propagate in the presence of large amount of oxygen and reduce the organic matter in wastewater; In the process of aerobic biochemical treatment, only a small amount of oxygen is needed to grow and propagate and degrade the organic matter in wastewater. If there is too much oxygen in water, the aerobic microorganism will not grow well, which will affect the efficiency of organic matter treatment.

The aerobic microorganism can adapt to the wastewater with high COD concentration, and the COD concentration of the influent can be increased to over 2000mg/l, and the COD removal rate is generally 50-80%; Aerobic microorganisms can only adapt to wastewater with low COD concentration. The COD concentration of influent is generally controlled below 1000-1500mg/l, COD removal rate is generally 50-80%, and the time of aerobic biochemical treatment and aerobic biochemical treatment is not too long, generally 12-24 hours. People combine facultative oxygen biochemical treatment and aerobic biochemical treatment with the same advantages and differences between aerobic biochemistry and aerobic biochemistry. Wastewater with higher COD concentration will be treated with facultative oxygen biochemical treatment first, and then the treated water of the facultative oxygen tank will be used as the inflow of aerobic pool. Such combined treatment can reduce the volume of biochemical pool, It not only saves environmental investment but also reduces the daily operation cost.

The principle and function of anaerobic biochemical treatment and facultative biochemical treatment are the same. The difference between anaerobic biochemical treatment and facultative biochemical treatment is that no oxygen is needed in the process of anaerobic microorganism propagation and growth and degradation of organic matter, and anaerobic microorganism can adapt to wastewater with higher COD concentration (4000-10000mg/l). The disadvantage of anaerobic biochemical treatment is that the biochemical treatment time is very long, and the retention time of wastewater in anaerobic biochemical tank generally takes more than 40 hours.

28. What are the applications of biological treatment in wastewater treatment project?

There are two kinds of technologies that are widely used in wastewater treatment engineering: one is activated sludge method and the other is called biofilm method.

Activated sludge method is a kind of aerobic wastewater treatment form by biochemical metabolism of suspended biological group. In the process of growth and reproduction, microorganisms can form a large surface area of bacterial micelles, which can flocculate and absorb suspended colloidal or dissolved pollutants of wastewater, and absorb these substances into cells. With the participation of oxygen, these substances will be completely oxidized to release energy, CO2 and H2O. The sludge concentration of activated sludge method is usually 4G / L.

In biofilm, microorganisms attach to the surface of the filler to form colloidal biofilm. Biofilm is generally fluffy flocculent structure, with many micropores and large surface area, which has strong adsorption effect, which is conducive to the further decomposition and utilization of these adsorbed organic substances. During the process of treatment, the water flow and air agitation make the surface of biofilm contact with water continuously. The organic pollutants and dissolved oxygen in wastewater are adsorbed by biofilm, and the microorganisms on the biofilm decompose these organic substances constantly. While the organic substances are decomposed by oxidation, the biofilm itself is constantly metabolized, The aged biofilm is detached from the biological treatment facility and separated from the water in the sedimentation tank. The sludge concentration of biofilm method is usually 6-8g/l.

In order to improve the sludge concentration and improve the treatment efficiency, the activated sludge method and biofilm method can be combined, namely, adding filler to the activated sludge pool. This bioreactor with both suspended and suspended microorganisms is called the compound bioreactor, which has a high sludge concentration, generally around 14g / L.

29. What are the similarities and differences between biofilm and activated sludge?

Biofilm and activated sludge are different reactors for biochemical treatment. The main difference from appearance is that the former microorganism does not need packing carrier, and the biological sludge is suspended, and the latter is fixed on the packing. However, the mechanism of their treatment of wastewater and purified water quality is the same. In addition, both biological sludge are aerobic activated sludge, and the composition of sludge has certain similarity. In addition, the microorganism in biofilm method can form a relatively stable ecosystem because it is fixed on the filler. Its living energy and energy consumption are not as large as the microorganism in the activated sludge method, so the residual sludge of biofilm method is less than that of activated sludge method. The contact oxidation tank of Shanghai Xinyi BaiLuDa Pharmaceutical Co., Ltd. adopts biofilm method, while SBR biochemical tank adopts activated sludge method.

30. What is activated sludge?

From the perspective of microorganism, sludge in biochemical pool is a biological group composed of various bioactive microorganisms. If the sludge particles are observed under microscope, there are many kinds of microorganisms in it - bacteria, molds, protozoa and postnatal animals (such as rotifers, insect larvae and worms), which form a food chain. Bacteria and molds can decompose complex organic compounds, obtain the necessary energy for their own activities and construct themselves. Protozoa feeds on bacteria and molds, and is consumed by posterity. Postnatal animals can also live directly on bacteria. This kind of flocculent sludge with microbial and organic degradation capability is called activated sludge.

Besides the composition of microorganisms, activated sludge also contains some inorganic substances and organic substances (i.e. the metabolic residue of microorganisms) which can no longer be biodegraded on the activated sludge. The moisture content of activated sludge is 98-99%.

Activated sludge, like alum flower, has a large surface area, so it has strong adsorption capacity and oxidation decomposition ability.

31. How to evaluate activated sludge in activated sludge process and biofilm process?

The discrimination and evaluation of activated sludge growth in activated sludge process and biofilm process are different.

In the biofilm process, the growth of activated sludge is mainly evaluated by microscope.

In the activated sludge process, in addition to observing the biofacies directly by microscope, the commonly used evaluation indexes are: mixed liquid suspended solids (MLSS), mixed liquid volatile suspended solids (MLVSS), sludge settling ratio (SV), sludge settling index (SVI), etc.

32. What kind of microorganism directly indicates that the biochemical treatment effect is good when observing the biofacies with microscope?

The appearance of micro metazoa (such as rotifers, nematodes, etc.) indicates that the microbial community grows well, the ecosystem of activated sludge is relatively stable, and the biochemical treatment effect is the best at this time, which is just like the situation that small fish and shrimp grow well in the river where large fish can be captured frequently.

38. What is MLSS?

MLSS, also known as sludge concentration, refers to the weight of dry sludge contained in the mixed liquid of biochemical tank per unit volume, in mg / L, which is used to characterize the activated sludge concentration. It includes two parts: organic and inorganic. Generally speaking, the MLSS value in SBR biochemical pool should be controlled at about 2000-4000 mg / L.

33. What is MLVSS?

Mixed liquid volatile suspended solids (MLVSS) refers to the weight of volatile substances in dry sludge contained in the mixed liquid of biochemical tank per unit volume, and the unit is also mg / L. since it does not include inorganic substances in activated sludge, it can accurately represent the number of microorganisms in activated sludge.

34. Sludge settling ratio (SV)?

Sludge settling ratio (SV) refers to the volume ratio (%) of the precipitated sludge to the mixed liquid in the aeration tank after 30 minutes of static sedimentation in a 100 ml measuring cylinder. Therefore, SV30 is sometimes used to express it. Generally speaking, SV in biochemical pool is between 20-40%. The determination of sludge settling ratio is relatively simple, and it is one of the important indexes to evaluate activated sludge. It is often used to control the discharge of excess sludge and reverse sludge bulking in time. Obviously, SV is also related to sludge concentration.

35. Sludge index (SVI)?

Sludge index (SVI) is the full name of sludge volume index, which is the milliliter volume of 1G dry sludge in wet state. The calculation formula is as follows:

SVI＝SV*10/MLSS

SVI eliminates the influence of sludge concentration and can better reflect the coagulation and sedimentation of activated sludge

When 60 ＜ SVI ＜ 100, sludge settling performance is good  

When 100 ＜ SVI ＜ 200, sludge settling performance is general  

When 200 ＜ SVI ＜ 300, the trend of sludge bulking increased  

When SVI > 300, the sludge has expanded

36. What does do mean?

Dissolved oxygen (do) is the dissolved amount of oxygen in water, expressed in mg / L. Different biochemical treatment methods have different requirements for do. In facultative biochemical process, do in water is generally between 0.2-2.0mg/l, while in SBR aerobic biochemical process, do in water is generally between 2.0-8.0mg/l. Therefore, the aeration rate and aeration time of facultative tank should be small; In the SBR aerobic tank operation, the aeration rate and aeration time are much larger and longer, and we use contact oxidation, the dissolved oxygen is controlled at 2.0-4.0mg/l.

37. Which factors are related to the content of dissolved oxygen in wastewater?

The concentration of dissolved oxygen in water can be expressed by Henry's law

C=KH*P

Where: C is the solubility of oxygen in water at dissolution equilibrium; P is the partial pressure of oxygen in gas phase; KH is Henry coefficient, which is related to temperature; Increase aeration to make oxygen dissolution close to equilibrium, while activated sludge will consume oxygen in water. Therefore, the actual dissolved oxygen in wastewater is related to water temperature, effective water depth (affecting pressure), aeration rate, sludge concentration, salinity and other factors.

38. Who provides the oxygen needed by microorganisms in biochemical process?

The oxygen needed by microorganism in biochemical process is mainly supplied by roots blower.

39. Why do we need to replenish nutrients in wastewater during biochemical process?

The main way to remove pollutants by biochemical process is to use the metabolic process of microorganisms. The cell synthesis and other life processes of microorganisms need sufficient amount and variety of nutrients (including trace elements). For chemical wastewater, due to the singleness of production products, the composition of wastewater quality is also relatively single, lacking the necessary nutrients of microorganisms. For example, there is only carbon and nitrogen but no phosphorus in the production wastewater of the company, which can not meet the needs of microbial metabolism. Therefore, phosphorus in the wastewater must be added to improve the process of microbial metabolism and promote the synthesis of microbial cells. This is like eating rice, flour at the same time, but also intake enough vitamins.

40. What's the ratio of nutrient elements needed by microorganisms in wastewater?

Like animals and plants, microorganisms also need the necessary nutrients to grow and reproduce. The nutrients needed by microorganisms mainly refer to carbon (c), nitrogen (n), and phosphorus (P). The composition ratio of the main nutrient elements in wastewater has certain requirements. For aerobic biochemistry, it is generally C: n: P = 100:5:1 (weight ratio).

41. Why is excess sludge produced?

In the process of biochemical treatment, microorganisms in activated sludge constantly consume the organic matter in wastewater. In the consumed organic matter, some organic matter is oxidized to provide energy for microbial life activities, and the other part is used by microorganisms to synthesize new cytoplasm, so that microorganisms reproduce. While microorganisms metabolize, some old microorganisms die, resulting in excess sludge.

42. How to estimate the amount of excess sludge?

In the process of microbial metabolism, some organic substances (BOD) are used by microorganisms to synthesize new cytoplasm to replace dead microorganisms. Therefore, the amount of excess sludge is related to the amount of BOD decomposed, and there is a correlation between them.

In engineering design, it is generally considered that 0.6-0.8 kg of excess sludge (100%) will be produced for each kilogram of BOD5 treated, and 3-4 kg of dry sludge with water content of 80% will be converted.