Answers

1. Why is wastewater treatment necessary?

***Company is located in ***, mainly producing ****. Wastewater comes from the ginkgo leaf extraction process. Ethanol is used as an extractant in the extraction process. There are many pollutants in the wastewater that seriously exceed the standard. For a long time, it has been one of the difficulties that affect and restrict the company's production and development. If these wastewaters are not treated before discharge, they will seriously pollute the environment. Generally speaking, when the environment and resources are destroyed and the ecological balance is disrupted, it will take ten or twenty years, or even hundreds of years, to recover, and sometimes it is impossible to recover. Therefore, any enterprise that attempts to exchange the temporary development of the economy at the cost of sacrificing the environment and resources is not only intolerable by national laws and the people, but also the existence and development of the enterprise itself will be restricted.

2. What is ISO14000 (Environmental Management Standard)?

The ISO14000 series of standards are environmental management standards formulated by the Environmental Management Technology Committee of the International Organization for Standardization. Its guiding ideology is "comprehensive management, pollution prevention, and continuous improvement", which is an innovation in environmental management ideas and methods.

ISO14000 has very strict standards and regulations. From the purchase of raw materials to the completion of the product, each production process and management link has corresponding verification standards. It strictly prevents the generation of pollutants in the production process and ensures the effective treatment of pollutants from the system. Wastewater treatment is only part of the ISO14000 series of standards. At present, the ISO14000 series of standards are being piloted and implemented in some large cities and large enterprises in China.

ISO14000 environmental quality certification is known as the internationally recognized "green passport" in the international market. Those who pass the certification undoubtedly obtain an "international通行证". Many countries have announced that goods and products without environmental management certification will be subject to quantity and price restrictions upon import. Therefore, with the gradual integration with the international market, ISO14000 environmental quality certification has been comprehensively promoted and implemented in all enterprises in China, similar to ISO9000 (quality management standard).

Therefore, from the perspective of environmental management standards, we should not only strive to do a good job in wastewater treatment at the end of pollution sources, implement scientific environmental management, and ensure that treated wastewater meets the standards for discharge; we should also make great efforts to strictly control clean production management at the front end of pollution sources, prevent pollution, and reduce pollution.

3. How to implement scientific environmental management?

Environmental protection has become a fundamental national policy for the sustained economic development of our country. Therefore, wastewater treatment should comply with the environmental protection laws and regulations and policies formulated by our country. In environmental planning and design, production concepts and ecological concepts, and environmental protection must be considered comprehensively, and wastewater treatment and improvement of production processes, and the implementation of clean production must be considered comprehensively. Through systematic analysis and verification, a relatively reasonable governance plan should be sought. The main principles of environmental management are summarized as follows:

(1) Eliminate unreasonable products

For some traditional, low-value, and extremely difficult-to-treat waste products, we should resolutely replace them with high-value, high-tech products. If the annual profit of a product does not even cover the annual cost of wastewater treatment, the production of such products should be resolutely stopped, and replaced with products that produce less pollution and are easier to treat to meet standards.

(2) Strengthen management and reduce pollution

Enterprise management is also an important factor in pollution prevention and control. For example, the leakage of equipment; production accidents or product scrap caused by failure to follow operating procedures, resulting in the generation of a large amount of high-concentration wastewater; using a large amount of water to wash equipment and the ground, increasing the amount of wastewater; and the failure to separate cooling water and production wastewater, will all increase the amount of wastewater and the difficulty of wastewater treatment.

(3) Establish regional small wastewater treatment plants

For areas with a relatively high concentration of factories, it is not necessary to apply the principle of "who pollutes, who governs". Instead, we should strengthen the connection between enterprises, comprehensively consider pollution control strategies, and if necessary and possible, centralize the wastewater treatment of various factories and establish a unified wastewater treatment plant, implementing the "who pollutes, who pays" governance method. Because different factories have different products, the water quality of wastewater is also different. For example, the wastewater of some factories is acidic, while the wastewater of other factories is alkaline. Treating them together can reduce the cost of neutralizing agents; some factories discharge high-salt, low-COD wastewater, while other factories discharge high-concentration, easily biodegradable wastewater. If treated separately, both are very difficult to treat, but if treated together through biological treatment, due to the improvement of water quality conditions, not only can the difficulty of wastewater treatment be reduced, but also the treatment efficiency can be improved.

(4) Improve water recycling rate

In order to reduce the amount of wastewater, we should first focus on the source of wastewater generation. For example, we can consider water recycling or multiple reuse, improve the water recycling rate, and minimize the amount of external drainage. Abroad, the water recycling rate of some advanced enterprises has reached more than 96%, while the water recycling rate of Shanghai's manufacturing enterprises is still at a low level of 20-30%, and there is still great potential to be tapped. Improving the recycling rate of production water can not only reduce environmental pollution but also reduce the amount of fresh water supplementation, which can alleviate the increasingly tense water resources problem to a certain extent. During wastewater treatment, the recycling of treated water should also be considered as much as possible.

(5) Recycling and comprehensive utilization

Pollutants in wastewater are raw materials, semi-finished products, finished products, and reaction media (such as solvents) that enter the water during the production process, especially in the fine chemical industry. Some chemical reactions are often not very safe, and the product separation process cannot be completely thorough. Therefore, wastewater, especially reaction mother liquor, often contains a certain amount of useful substances. Discharging these pollutants will pollute the environment and cause harm. However, if they are recycled or comprehensively utilized, they can be transformed from waste into treasure, turning harm into benefit; or using waste to treat waste, taking advantage of each other's strengths to make up for each other's weaknesses, and comprehensive governance can save water treatment costs.

4. What is "Environmental Protection 110"?

In response to the current situation where environmental administrative law enforcement and environmental management are not in line with public complaints, Shanghai has opened an environmental emergency hotline 62863110, the so-called "Environmental Protection 110". The phone number will be simplified to 63110 (a homophone of "Green 110"). This is the first "Environmental Protection 110" in the national environmental protection system. With the strengthening of environmental protection efforts, environmental emergency hotlines will be successively implemented across the country.

The responsibilities of the environmental emergency hotline are: to accept and organize the handling of major pollution accidents that occur throughout the city; to accept reports of illegal discharge by polluting units, such as secret discharge and direct discharge; to accept and handle incidents that may cause social instability due to environmental problems; to assist relevant departments in handling major incidents that may affect the environment; for other environmental pollution problems that do not require on-site handling, the environmental emergency hotline can accept complaints from the public throughout the city 24 hours a day within the above scope.

For pollution discharge units, the opening of Environmental Protection 110 is both pressure and motivation. Only by seriously doing a good job in pollution management and control can we withstand the supervision and inspection of environmental law enforcement agencies and the public.

5. What are the tasks included in clean production management?

Wastewater and the pollutants it contains are products of the production process. Therefore, reforming production processes and implementing clean production are fundamental measures to eliminate or reduce the harm of wastewater. Through the reform of processes and equipment, wastewater can be eliminated during the production process, which can improve the utilization rate of raw and auxiliary materials and reduce wastewater treatment costs. This work should be jointly completed by production process engineers and environmental engineers. It should be recognized that environmental protection is not just the work of environmental engineers, but requires control from the source of pollution, so that wastewater can be truly treated well. Therefore, during process design and product trial production, potential environmental pollution problems should be considered. When selecting synthesis routes, try to adopt pollution-free or less polluting production processes, select routes with the highest raw material utilization rate, do not use or use less biologically non-degradable or toxic and harmful substances in the production process, including raw and auxiliary materials and solvents, and strengthen the recycling and comprehensive utilization of solvents and by-products. The specific methods are roughly as follows:

(1) Adopt new processes, new technologies, and new routes

Adopt new processes, new technologies, and new routes. First, the material ratio in the production process can be verified. The raw materials with greater pollution and exceeding the theoretical ratio should be reduced to increase the utilization rate of raw materials and the treatability of wastewater.

In chemical production, sometimes adopting a new route can not only improve production levels but also solve wastewater treatment problems. For example, in the past, isonicotinic acid, a raw material for anti-tuberculosis drugs, was prepared by electrolytic oxidation using sulfuric acid as the electrolyte. The process produced a large amount of acidic wastewater that was difficult to treat. Now, a new air catalytic oxidation technology is used, and the reaction is carried out in a fluidized bed, resulting in less wastewater and easier pollution control.

(2) Replace raw and auxiliary materials

Common methods include replacing highly toxic or extremely toxic raw materials with non-toxic or low-toxicity alternatives, and replacing non-biodegradable substances with biodegradable ones. Furthermore, the use of restricted substances specified in emission standards should be minimized, especially those with stringent requirements, thereby reducing the burden on wastewater treatment. For example, there are now stricter requirements for ammonia nitrogen concentration in wastewater, so the use of ammonia water or liquid ammonia in production should be minimized. For instance, in the past, some treatment processes used ammonia water to adjust the pH of wastewater, resulting in significantly exceeding the ammonia nitrogen limit in the effluent and increasing the difficulty of wastewater biotreatment. Similarly, the use of potassium dichromate as an oxidant, and nitro compounds and chlorinated hydrocarbons as solvents should be minimized.

When selecting solvents, in addition to meeting the requirements of the production process, the biodegradability and toxicity of the solvent must also be considered.

Based on the above requirements, the priority order for selection is shown in the table below.

Solvent Selection Priority Order Table

Preferred Use

Methanol, Ethanol, Isopropanol, Acetone, Acetic Acid, Ethyl Acetate, Glycerol, Ethylene Glycol

Acceptable Use

Benzene, Toluene, Dimethylformamide (DMF), Formamide, Xylene
Avoid Use
tert-Butanol, Dimethyl Sulfoxide (DMSO), Triethylamine, Dimethylaniline, Chloroform*, Carbon Tetrachloride*, Chlorobenzene, Nitrobenzene*, Pyridine*, Morpholine, Tetrahydrofuran

Note: * indicates toxicity or inhibition to microorganisms.

(3) Adopt new post-treatment processes to mitigate or eliminate pollution during the production process.

This method is very useful for technical personnel engaged in chemical and chemical engineering production. For example, in the organic synthesis industry, the method of diluting reaction materials with water (water separation) is often used to precipitate the reaction product from the reaction organic solvent. The mother liquor produced by water separation, due to the large amount of water, makes it difficult to recover the organic solvent (such as water-soluble solvents such as methanol and ethanol), causing pollution in the wastewater stream. If most of the solvent is recovered by distillation before dilution, and then diluted with water, the content of organic matter in the wastewater can be significantly reduced.

In order to ensure the quality of the product, the reaction product or intermediate product often needs to be washed to remove impurities. The rationality of the washing operation has a considerable impact on the degree of wastewater pollution. However, if new post-treatment technology is adopted, the washing wastewater can be completely eliminated during the process operation, achieving zero discharge. Too high a salt content in wastewater will inhibit the growth and reproduction of microorganisms, affecting the effect of biotreatment. We can also use new post-treatment processes to solve this difficulty in wastewater treatment. For example, a certain factory prepares p-nitroanisole by reacting p-chloronitrobenzene with sodium hydroxide in a methanol solvent. The original post-treatment process was to wash with water to remove the NaCl salt from the reaction materials. The result of this operation was a large amount of wastewater with a high salt content, leading to difficulties in subsequent biotreatment. Later, the factory improved the post-treatment process, first filtering out the NaCl from the reaction material (organic phase), and then washing with water and precipitating p-nitroanisole. The improved process not only reduced the wastewater volume by 50%, but also recovered 97.4% of the salt in the wastewater, reducing the organic load of the wastewater by 58.7%, and greatly improving the biodegradability of the wastewater.

(4) Strengthen solvent recovery work.

In most chemical raw material production plants, the proportion of solvent used in raw and auxiliary materials is quite high. It can be said that the organic load in many production wastewaters basically comes from solvents. Therefore, attaching importance to and doing a good job in solvent recovery is not only an important measure for pollution prevention and reduction, but also an important way to reduce costs and increase efficiency and improve profits, with both environmental and economic benefits. For example, a pharmaceutical factory in Shanghai that produces hormones has a daily total discharge of 8 tons of organic load (COD), making it a major polluter in the region. The factory's environmental protection work began with solvent recovery, concentrating wastewater containing the same solvent for recovery. As a result, the daily total discharge of organic load in the wastewater was reduced from 8 tons to 3 tons, and the revenue from solvent recovery exceeded the operating costs of the wastewater treatment plant.

6. Why are COD and BOD often used as pollution indicators in wastewater analysis?

Wastewater contains many organic substances, and wastewater containing dozens, or even hundreds of organic substances is also frequently encountered. If the organic substances in wastewater are qualitatively and quantitatively analyzed one by one, it is both time-consuming and requires many reagents. So, is it possible to use only one pollution indicator to represent all the organic substances in wastewater and their quantities? Environmental scientists have found through research that all organic substances have two common characteristics: firstly, they are all composed of carbon and hydrogen; secondly, the vast majority of organic substances can be chemically oxidized or bio-oxidized, and their carbon and hydrogen form harmless carbon dioxide and water with oxygen, respectively. Whether in the chemical oxidation process or the biological oxidation process, the organic substances in wastewater consume oxygen. The more organic substances in the wastewater, the more oxygen is consumed, and the two are positively correlated. Therefore, environmental scientists call the amount of oxygen consumed when wastewater is oxidized with chemical reagents the chemical oxygen demand, or COD; and the amount of oxygen consumed by microbial oxidation of wastewater is called the biological oxygen demand, or BOD. Because COD and BOD can comprehensively reflect the amount of all organic substances in wastewater, and the analysis is relatively simple, they are widely used in wastewater analysis and environmental engineering.

In fact, COD does not only represent organic substances in water; it can also represent inorganic substances with reducing properties in water, such as sulfides, ferrous ions, sodium sulfite, and even chloride ions. For example, if the ferrous ions in the effluent of the iron-carbon pool are not completely removed in the neutralization pool, the COD of the effluent from the biotreatment may exceed the standard due to the presence of ferrous ions.

7. What is COD (Chemical Oxygen Demand)?

Chemical Oxygen Demand (COD) refers to the amount of oxygen required to oxidize the oxidizable substances in wastewater using a chemical oxidant, measured in milligrams of oxygen per liter. It is currently the most commonly used method for determining the organic matter content in wastewater. Commonly used oxidants in COD analysis include potassium permanganate (manganese method CODMn) and potassium dichromate (chromium method CODCr), with potassium dichromate being commonly used now. Wastewater is oxidized under strong acid heating, boiling, and reflux conditions. Using silver sulfate as a catalyst can increase the oxidation rate of most organic matter to 85-95%. If the wastewater contains a high concentration of chloride ions, mercury sulfate should be used to mask the chloride ions to reduce interference with COD determination.

8. What is BOD5 (Biochemical Oxygen Demand)?

Biochemical Oxygen Demand (BOD) can also characterize the degree of organic pollution in wastewater. The most commonly used is the five-day biochemical oxygen demand, denoted as BOD5, which represents the amount of oxygen required for the biochemical degradation of wastewater in the presence of microorganisms within five days. We will frequently use the five-day biochemical oxygen demand in the future.

9. What is the relationship between COD and BOD5?

Some organic matter can be biodegraded (such as glucose and ethanol), some can only be partially biodegraded (such as methanol), while others cannot be biodegraded and are also toxic (such as ginkgo phenol, ginkgo acid, and some surfactants). Therefore, we can divide the organic matter in water into two parts: biodegradable and non-biodegradable organic matter.

It is generally believed that COD essentially represents all organic matter in water, while BOD represents the biodegradable organic matter in water. Therefore, the difference between COD and BOD can represent the non-biodegradable organic matter in wastewater.

10. What is B/C? What does B/C represent?

B/C is the abbreviation for the ratio of BOD5 to COD, which can represent the biodegradability of wastewater. If CODNB represents the non-biodegradable part of COD, then the proportion of organic matter in wastewater that cannot be biodegraded by microorganisms can be expressed as CODNB/COD.

The relationship between BOD5/COD and CODNB/COD is shown in the table below:

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 the total organic matter, while when BOD5/COD ≤ 0.2, the non-biodegradable organic matter accounts for more than 60% of the total organic matter.

Therefore, the BOD5/COD value is often used as an evaluation index for the biodegradability of organic matter.

BOD5/COD 0.45 Easily biodegradable
BOD5/COD 0.30 Biodegradable
BOD5/COD 0.30 Relatively difficult to biodegrade
BOD5/COD 0.20 Very difficult to biodegrade

B/C has a very important and practical significance in environmental engineering.

11. What is pH?

pH is actually a way to express the acidity and alkalinity of an aqueous solution. We usually use percentage concentration to express the acidity and alkalinity of an aqueous solution, such as a 1% sulfuric acid solution or a 1% alkaline solution. However, when the acidity and alkalinity of an aqueous solution are very small, it is too troublesome to use percentage concentration to express it. In this case, pH can be used. The application range of pH is between 0-14. When pH = 7, water is neutral; when pH < 7, water is acidic, and the smaller the pH, the stronger the acidity; when pH > 7, water is alkaline, and the larger the pH, the stronger the alkalinity.

All living things in the world cannot live without water, but the pH range suitable for the survival of organisms is often very narrow. Therefore, the National Environmental Protection Agency strictly regulates the pH value of treated water between 6-9.

pH value in water is often detected using pH test paper, and it can also be measured using instruments, such as a pH meter.

12. Why is the concentration unit milligrams per liter (mg/L) frequently used in wastewater analysis?

Generally speaking, the content of organic and inorganic substances in wastewater is very small. If percentage concentration or other concentrations are used, it is too troublesome and inconvenient. For example, there are often only a few grams, tens of grams, hundreds of grams, or even thousands of grams of pollutants in one ton of wastewater, and the unit is grams per ton (g/T). If tons are converted to liters, it becomes milligrams per liter (mg/L). Refer to the following table for calculations:

1 milligram per liter One part per million
1000 milligrams per liter One part per thousand
10000 milligrams per liter One percent

13. What is pretreatment of wastewater? What are the purposes of pretreatment?

The treatment before biological treatment is generally called pretreatment. Because biological treatment is relatively low in cost and stable in operation, general industrial wastewater uses biological treatment. [Company Name]'s wastewater treatment also uses biological treatment as the main treatment method. However, [Company Name]'s wastewater contains certain organic substances that inhibit and are toxic to microorganisms. Therefore, before the wastewater enters the bioreactor, necessary pretreatment must be carried out to reduce or remove substances that inhibit and are toxic to microorganisms as much as possible, to ensure that the microorganisms in the bioreactor can operate normally.

There are two purposes of pretreatment: First, to reduce or remove substances that inhibit or are toxic to microorganisms, or to convert them into substances that are harmless or beneficial to microorganisms, to ensure that the microorganisms in the bioreactor can operate normally; second, to reduce the COD load during pretreatment to reduce the operating burden of the bioreactor.

The company's pretreatment process uses iron-carbon microelectrolysis and Fe2+/Fe3+ reduction-oxidation methods. The numerous tiny iron-carbon micro-batteries formed facilitate redox reactions, destroying and removing toxic and harmful substances in wastewater. During the neutralization and precipitation process, the active flocs formed by ferrous and ferric iron under alkaline conditions can also adsorb organic matter in wastewater to reduce the COD load, ensuring the normal operation of the subsequent biological treatment system.

14. What is the purpose of a wastewater collection pool?

The role of the wastewater collection pool is to collect, store, and balance the water quality and quantity of wastewater. The production wastewater from various workshops generally has unbalanced discharge volume and water quality. Wastewater is produced during production, but not during non-production periods. There may be significant variations even within a day or between shifts, especially for wastewater in the fine chemical industry. If clear and turbid wastewater is not separated, the water quality and quantity of process-concentrated wastewater and lightly polluted wastewater will vary greatly. Such variations are very unfavorable, even harmful, to the normal operation and treatment effect of wastewater treatment facilities and equipment. Therefore, before entering the main sewage treatment system, a wastewater collection pool with a certain volume is set up to store the wastewater and make it homogeneous in quality and quantity, ensuring the normal operation of wastewater treatment equipment and facilities.

15. Why are colloidal particles in wastewater not easily naturally settled?

Many impurities with a specific gravity greater than 1, large particles, and easily settleable suspended solids in wastewater can be removed by natural sedimentation, centrifugation, etc.

However, suspended particles with a specific gravity less than 1, tiny or even invisible to the naked eye, are difficult to naturally settle, such as colloidal particles, which are microparticles of 10-4-10-6 mm in size, very stable in water, and have an extremely slow sedimentation rate. It takes 200 years to settle 1m. There are two reasons for the slow sedimentation: (1) Generally speaking, colloidal particles carry negative charges, and due to the repulsion of the same kind, they prevent contact between colloidal particles, preventing them from being bonded to each other and remaining suspended in water. (2) There is a layer of molecules tightly surrounding the surface of the colloidal particles. This hydration layer also hinders and isolates the contact between colloidal particles, preventing them from being bonded to each other and remaining suspended in water.

16. How to make colloidal particles precipitate?

To make colloidal particles precipitate, it is necessary to promote contact between colloidal particles, making them larger particles, that is, coagulating them, so that their specific gravity is greater than 1 and they precipitate.

There are many methods used. Commonly used technologies in engineering include: coagulation, flocculation, and coagulation.

17. What is coagulation?

Adding coagulants with positive ions to the wastewater, a large number of positive ions exist between the colloidal particles to eliminate the electrostatic repulsion between the colloidal particles, thus causing the particles to aggregate. This process of adding positive ion electrolytes to cause colloidal particles to aggregate is called coagulation. Commonly used coagulants include aluminum sulfate, ferrous sulfate, alum, ferric chloride, etc.

18. What is flocculation?

Flocculation is the addition of polymeric coagulants to wastewater. After dissolution, polymeric coagulants form polymeric polymers. The structure of this polymer is linear, with one end of the line pulling a tiny particle and the other end pulling another tiny particle, playing a bonding bridging role between two particles that are relatively far apart, causing the particles to gradually become larger and eventually forming large flocculated particles (commonly known as alum flowers), accelerating particle sedimentation. Commonly used flocculants include polyacrylamide (PAM) and polyferric (PE).

19. Why is polyferric used for flocculation and adsorption pretreatment of wastewater?

Polyferric forms ferric hydroxide flocs during coagulation, which have a good ability to adsorb organic matter in wastewater. Experimental data show that after flocculation and adsorption with polyferric, 10%-20% of COD in wastewater can be removed, which can greatly reduce the operating burden of the bioreactor and is conducive to the discharge of treated wastewater that meets standards. In addition, coagulation pretreatment with polyferric can remove trace substances that are toxic and inhibitory to microorganisms in wastewater, ensuring the normal operation of microorganisms in the bioreactor. Among many coagulants, polyferric is relatively inexpensive (25-300 yuan/ton), so the treatment cost is relatively low, making it suitable for pretreatment of process wastewater.

Polyferric is an acidic substance with strong corrosiveness, so the treatment equipment should be treated with anti-corrosion measures.

20. What is coagulation?

The process of combining coagulation and flocculation is called coagulation. Coagulation is frequently used in experiments or engineering, such as first adding coagulants such as ferrous sulfate to the water to eliminate the electrostatic repulsion between colloidal particles, and then adding polyacrylamide (PAM) to make the particles gradually larger, forming visible alum flowers, and finally producing sedimentation.

21. What is adsorption?

Using porous solids (such as activated carbon) or flocculent substances (such as polyferric) to adsorb toxic and harmful substances in wastewater onto the surface or micropores of solids or flocs to achieve the purpose of purifying water quality. This treatment method is called adsorption treatment. The objects of adsorption can be insoluble solid substances or soluble substances. Adsorption treatment has high efficiency and good effluent water quality, so it is often used as deep treatment of wastewater. Adsorption treatment can also be introduced into the biological treatment unit to improve the efficiency of biological treatment (such as the PACT method).

22. What is the iron-carbon treatment method?

The iron-carbon treatment method, also known as the iron-carbon microelectrolysis method or iron-carbon internal electrolysis method, is an application of metal iron wastewater treatment technology. Using the iron-carbon method as a pretreatment technology to treat toxic and harmful wastewater with high COD has a unique effect. The treatment mechanism of the iron-carbon method is not yet fully understood. A currently accepted explanation is that under acidic conditions, countless micro-current reaction cells are formed between iron and carbon, and organic matter is reduced and oxidized under the action of micro-current. The iron-carbon effluent is then neutralized with lime or lime milk, and the generated Fe(OH)2 colloidal flocculent has a strong flocculation and adsorption capacity for organic matter. Therefore, the iron-carbon method comprehensively applies the reducing properties of iron, the electrochemical properties of iron-carbon, and the flocculation and adsorption of iron ions. It is the combined action of these three properties that makes the iron-carbon method have a good treatment effect.

The disadvantages of the iron-carbon method are: (1) After long-term immersion in an acidic medium, iron filings are prone to agglomeration, causing blockage and channeling, making operation difficult and reducing treatment efficiency; (2) The amount of iron dissolved in acidic conditions is large, and the amount of sludge produced after neutralization with alkali is large.

23. Why does iron-carbon effluent need to be neutralized with lime powder?

After wastewater with a pH of 2 adjusted by sulfuric acid is treated with iron-carbon, sulfuric acid becomes ferrous sulfate, and the pH of the wastewater increases from 2 to 5-6. Why does the iron-carbon effluent still need to be neutralized with lime powder? Or can less lime powder be added during neutralization?

Iron-carbon wastewater contains a large amount of ferrous sulfate. If not removed, it will affect the growth and reproduction of microorganisms in the subsequent bioreactor. Therefore, we must use lime to increase the pH of the wastewater from 5-6 to above 9, converting the water-soluble ferrous sulfate into insoluble ferrous hydroxide and calcium sulfate. Then, through coagulation and sedimentation, these substances are precipitated to ensure that the wastewater entering the bioreactor does not contain ferrous sulfate.

Can we reduce the amount of lime powder during neutralization? We can conduct a comparative experiment in the laboratory. Take the same amount of iron-carbon influent (pH around 2) and iron-carbon effluent (pH 5-6) and place them in two beakers. Then, add lime powder for neutralization and coagulation. When the pH of the wastewater in both beakers is adjusted to 9, we can find that the amount of lime powder added to the two beakers is the same. This is because iron is not a neutralizing agent, and the ferrous sulfate formed from sulfuric acid is still an acidic substance. The lime powder consumed when ferrous sulfate is converted into ferrous hydroxide and calcium sulfate during neutralization cannot be reduced. Therefore, lime powder cannot be reduced during the neutralization treatment of iron-carbon effluent.

24. How to estimate the production of chemical sludge?

Sludge produced by chemical reactions (such as neutralization) and physicochemical treatments (such as chemical coagulation) is usually called chemical sludge. The sludge formed after neutralization and coagulation of iron-carbon effluent is mainly composed of ferrous hydroxide and calcium sulfate. The amount of sludge produced can be calculated based on the amount of sulfuric acid and lime powder added. It can also be estimated using experience in engineering. Generally, if the pH of iron-carbon influent is around 2, the amount of chemical sludge produced per ton of wastewater (water content 80%) after neutralization and coagulation is about 50 kg.

What is wastewater biotreatment?

Biochemical treatment of wastewater is one of the most important processes in wastewater treatment systems, referred to as biotreatment. Biotreatment utilizes the life activities of microorganisms to effectively remove soluble and some insoluble organic matter in wastewater, purifying the water. In fact, we are not unfamiliar with biotreatment. A food chain exists in natural water bodies: big fish eat small fish, small fish eat shrimp, shrimp eat insects, insects eat microorganisms, and microorganisms eat wastewater. Without this food chain, nature would be in chaos. In natural rivers, there are a large number of microorganisms that rely on organic matter for survival. Day and night, they oxidize or reduce the organic matter discharged into rivers by humans (such as industrial wastewater, pesticides, fertilizers, and feces), ultimately converting them into inorganic matter. Without microorganisms, the rivers around us would become foul-smelling rivers in a few months or a year or two. It's just that microorganisms are too small and dispersed to be seen with the naked eye. Wastewater biotreatment engineering is an intensification of this process under artificial conditions. Countless microorganisms are concentrated in a pool, creating an environment very suitable for microbial reproduction and growth (such as temperature, pH, oxygen, nitrogen, phosphorus, and other nutrients), allowing microorganisms to proliferate to improve the speed and efficiency of organic matter decomposition. Then, wastewater is pumped into the pool, and the organic matter in the wastewater is oxidized and degraded during the life activities of microorganisms, purifying and treating the wastewater. Compared with other treatment methods, the biotreatment method has the advantages of low energy consumption, no need for chemicals, good treatment effect, and low treatment cost.

26. How do microorganisms decompose and remove organic pollutants in wastewater?

Due to the presence of carbohydrates, fats, and proteins in wastewater, these lifeless organic substances are food for microorganisms. Part of them are degraded and synthesized into cell substances (combination metabolic products), and the other part is degraded and oxidized into water and carbon dioxide (decomposition metabolic products). During this process, the organic pollutants in the wastewater are degraded and removed by microorganisms.

27. What factors are microorganisms related to?

In addition to nutrients, microorganisms also need suitable environmental factors such as temperature, pH, dissolved oxygen, and osmotic pressure to survive. Abnormal environmental conditions will affect the life activities of microorganisms, and even cause mutations or death.

28. In what temperature range do microorganisms grow and reproduce most suitably?

In wastewater biotreatment, the most suitable temperature range for microorganisms is generally 16-30℃, and the highest temperature is 37-43℃. When the temperature is below 10℃, microorganisms will no longer grow. Within the suitable temperature range, for every 10℃ increase in temperature, the metabolic rate of microorganisms will increase accordingly, and the COD removal rate will also increase by about 10%; conversely, for every 10℃ decrease in temperature, the COD removal rate will decrease by 10%, so the biochemical removal rate of COD in winter will be significantly lower than in other seasons.

29. What is the most suitable pH range for microorganisms?

The life activities and material metabolism of microorganisms are closely related to pH. Most microorganisms adapt to a pH range of 4.5-9, while the most suitable pH range is 6.5-7.5. When the pH is below 6.5, fungi begin to compete with bacteria. When the pH reaches 4.5, fungi will completely dominate the bioreactor, resulting in serious effects on sludge sedimentation. When the pH exceeds 9, the metabolic rate of microorganisms will be hindered.

Different microorganisms have different requirements for the suitable pH range. In aerobic biotreatment, the pH can vary between 6.5-8.5; in anaerobic biotreatment, microorganisms have stricter pH requirements, and the pH should be between 6.7-7.4.

30. What is dissolved oxygen? What is the relationship between dissolved oxygen and microorganisms?

Oxygen dissolved in water is called dissolved oxygen. Aquatic organisms and aerobic microorganisms rely on dissolved oxygen for survival. Different microorganisms have different requirements for dissolved oxygen. Aerobic microorganisms require sufficient dissolved oxygen; generally, dissolved oxygen should be maintained at 3 mg/L, and the minimum should not be lower than 2 mg/L; facultative anaerobic microorganisms require dissolved oxygen in the range of 0.2-2.0 mg/L; while anaerobic microorganisms require dissolved oxygen below 0.2 mg/L.

31. Why does high-concentration saline wastewater have a particularly large impact on microorganisms?

Let's describe an osmotic pressure experiment: Two salt solutions with different concentrations are separated by a semipermeable membrane. Water molecules in the low-concentration salt solution will pass through the semipermeable membrane into the high-concentration salt solution, and water molecules in the high-concentration salt solution will also pass through the semipermeable membrane into the low-concentration salt solution, but the amount is less, so the liquid level on the high-concentration salt solution side will rise. When the height difference between the two sides produces enough pressure to prevent water from flowing, osmosis will stop. At this time, the pressure generated by the height difference between the two sides is the osmotic pressure. Generally, the higher the salt concentration, the greater the osmotic pressure.

The behavior of microorganisms in saline solutions is similar to osmosis experiments. The basic unit of microorganisms is the cell, and the cell wall acts as a semipermeable membrane. When the chloride ion concentration is less than or equal to 2000 mg/L, the osmotic pressure that the cell wall can withstand is 0.5-1.0 atmospheres. Even with the certain toughness and elasticity of the cell wall and cell membrane, the osmotic pressure the cell wall can withstand will not exceed 5-6 atmospheres. However, when the chloride ion concentration in the aqueous solution is above 5000 mg/L, the osmotic pressure will increase to approximately 10-30 atmospheres. Under such high osmotic pressure, a large amount of water molecules inside the microorganisms will permeate to the external solution, causing cell dehydration and plasmolysis, and severe cases can lead to microbial death. In daily life, people use table salt (sodium chloride) to pickle vegetables and fish, sterilizing and preserving food, which utilizes this principle. Engineering experience data shows that when the chloride ion concentration in wastewater is greater than 2000 mg/L, the activity of microorganisms will be inhibited, and the COD removal rate will decrease significantly; when the chloride ion concentration in wastewater is greater than 8000 mg/L, it will cause sludge volume expansion, a large amount of foam on the water surface, and microorganisms will die successively.

However, after long-term domestication, microorganisms will gradually adapt to growth and reproduction in high-concentration saline solutions. Microorganisms that can adapt to chloride or sulfate concentrations above 10000 mg/L have been domesticated. However, the principle of osmotic pressure tells us that microorganisms that have adapted to growth and reproduction in high-concentration saline solutions have a high salt concentration in their cell fluids. Once the salt concentration in the wastewater is low or very low, a large amount of water molecules in the wastewater will enter the microorganisms, causing the microbial cells to swell, and severe cases can lead to rupture and death. Therefore, microorganisms that have been domesticated for a long time and can gradually adapt to growth and reproduction in high-concentration saline solutions require the salt concentration in the biochemical influent to remain at a relatively high level, and it cannot fluctuate greatly, otherwise, a large number of microorganisms will die.

32. What is aerobic biological treatment? What is anoxic biological treatment? What are the differences between the two?

Biochemical treatment can be divided into two major categories according to the different oxygen requirements of microbial growth: aerobic biochemical treatment and anoxic biochemical treatment. Anoxic biochemical treatment can be further divided into facultative anaerobic biochemical treatment and anaerobic biochemical treatment. In the aerobic biochemical treatment process, aerobic microorganisms must grow and reproduce in the presence of a large amount of oxygen and reduce organic matter in wastewater; while in the facultative anaerobic biochemical treatment process, facultative anaerobic microorganisms only need a small amount of oxygen to grow and reproduce and degrade organic matter in wastewater. If there is too much oxygen in the water, facultative anaerobic microorganisms will not grow well, thus affecting their efficiency in treating organic matter.

Facultative anaerobic microorganisms can adapt to wastewater with higher COD concentrations, and the influent COD concentration can be increased to more than 2000 mg/L, with a COD removal rate generally between 50-80%; while aerobic microorganisms can only adapt to wastewater with lower COD concentrations, and the influent COD concentration is generally controlled below 1000-1500 mg/L, with a COD removal rate generally between 50-80%. The time for facultative anaerobic biochemical treatment and aerobic biochemical treatment is not long, generally between 12-24 hours. People utilize the differences and advantages between facultative anaerobic biochemical treatment and aerobic biochemical treatment, combining facultative anaerobic biochemical treatment and aerobic biochemical treatment. Wastewater with higher COD concentration is first subjected to facultative anaerobic biochemical treatment, and then the effluent from the facultative anaerobic pool is used as the influent of the aerobic pool. This combined treatment can reduce the volume of the biochemical pool, saving environmental protection investment and reducing daily operating costs.

The principle and function of anaerobic biochemical treatment are the same as those of facultative anaerobic biochemical treatment. The difference between anaerobic biochemical treatment and facultative anaerobic biochemical treatment is that anaerobic microorganisms do not require any oxygen in the process of reproduction, growth, and degradation of organic matter, and anaerobic microorganisms can adapt to wastewater with higher COD concentrations (4000-10000 mg/L). The disadvantage of anaerobic biochemical treatment is that the biochemical treatment time is long, and the residence time of wastewater in the anaerobic biochemical pool generally needs to be more than 40 hours.

33. What are the applications of biological treatment in wastewater treatment engineering?

There are two major categories of biological treatment technologies that are most widely and practically used in wastewater treatment engineering: one is called the activated sludge method, and the other is called the biofilm method.

The activated sludge method is a form of aerobic wastewater treatment using the biochemical metabolism of suspended microbial populations. During growth and reproduction, microorganisms can form flocs with a large surface area, which can flocculate and adsorb a large amount of suspended colloidal or dissolved pollutants in wastewater, and absorb these substances into the cells. With the participation of oxygen, these substances are completely oxidized to release energy, CO2, and H2O. The sludge concentration of the activated sludge method is generally 4 g/L.

In the biofilm method, microorganisms adhere to the surface of the filler to form a biofilm connected by a gel-like substance. The biofilm generally has a loose flocculent structure with many micropores and a large surface area, which has a strong adsorption capacity, which is conducive to the further decomposition and utilization of these adsorbed organic substances by microorganisms. During the treatment process, the flow of water and the stirring of air keep the biofilm surface and water in constant contact. The organic pollutants and dissolved oxygen in the wastewater are adsorbed by the biofilm, and the microorganisms on the biofilm continuously decompose these organic substances. While oxidizing and decomposing organic substances, the biofilm itself also continuously metabolizes, and the aging biofilm sloughs off and is carried out of the biological treatment facility with the treated water and separated from the water in the sedimentation tank. The sludge concentration of the biofilm method is generally 6-8 g/L.

In order to increase the sludge concentration and thus improve the treatment efficiency, the activated sludge method and the biofilm method can be combined, that is, adding fillers to the activated sludge pool. This bioreactor with both attached-growth microorganisms and suspended microorganisms is called a composite bioreactor, which has a high sludge concentration, generally around 14 g/L.

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

The biofilm method and the activated sludge method are different reactor forms of biochemical treatment. From the appearance, the main difference is that the former does not require a filler carrier for microorganisms, and the biological sludge is suspended, while the latter's microorganisms are fixed on the filler. However, their mechanisms for treating wastewater and purifying water quality are the same. In addition, the biological sludge of both is aerobic activated sludge, and the composition of the sludge is also similar. Furthermore, the microorganisms in the biofilm method, because they are fixed on the filler, can form a relatively stable ecosystem, and their energy consumption is not as large as that of microorganisms in the activated sludge method. Therefore, the excess sludge of the biofilm method is less than that of the activated sludge method. The contact oxidation pool of Shanghai Xinyi Bailuda Pharmaceutical Co., Ltd. uses the biofilm method, while the SBR biochemical pool uses the activated sludge method.

What is activated sludge?

From a microbiological perspective, the sludge in a bioreactor is a biological community composed of a wide variety of living microorganisms. If sludge particles are observed under a microscope, various microorganisms can be seen—bacteria, fungi, protozoa, and metazoa (such as rotifers, insect larvae, and worms, etc.). They form a food chain; bacteria and fungi decompose complex organic compounds, obtaining the energy and building blocks necessary for their own activities. Protozoa feed on bacteria and fungi, which are then consumed by metazoa; metazoa can also directly rely on bacteria for sustenance. This flocculent sludge particle, full of microorganisms and capable of degrading organic matter, is called activated sludge.

In addition to microorganisms, activated sludge also contains some inorganic substances and organic matter adsorbed onto the activated sludge that can no longer be biodegraded (i.e., metabolic residues of microorganisms). The water content of activated sludge is generally 98-99%.

Activated sludge, like floc, has a large surface area and therefore has a strong adsorption capacity and the ability to oxidatively decompose organic matter.

36. How to evaluate activated sludge in activated sludge method and biofilm method?

The judgment and evaluation of activated sludge growth in activated sludge processes and biofilm processes are different.

In biofilm processes, the evaluation of activated sludge growth mainly uses direct microscopic observation of the biological community.

In activated sludge processes, in addition to direct microscopic observation of the biological community, commonly used evaluation indicators for activated sludge growth include: mixed liquor suspended solids (MLSS), mixed liquor volatile suspended solids (MLVSS), sludge settling ratio (SV), and sludge volume index (SVI), etc.

37. When observing the biological phase with a microscope, which type of microorganisms directly indicates good biochemical treatment effect?

The appearance of microscopic metazoa (such as rotifers and nematodes) indicates that the microbial community is growing well and the activated sludge ecosystem is relatively stable. At this time, the biochemical treatment effect is optimal, similar to the situation where small fish and shrimp grow well in rivers where large fish are frequently caught.

38. What is mixed liquor suspended solids (MLSS)?

Mixed liquor suspended solids (MLSS), also known as sludge concentration, refers to the weight of dry sludge contained in a unit volume of bioreactor mixed liquor, with units of mg/L, used to characterize the activated sludge concentration. It includes both organic and inorganic matter. Generally, the MLSS value in an SBR bioreactor should be controlled at around 2000-4000 mg/L.

39. What is mixed liquor volatile suspended solids (MLVSS)?

Mixed liquor volatile suspended solids (MLVSS) refers to the weight of volatile matter in the dry sludge contained in a unit volume of bioreactor mixed liquor, also with units of mg/L. Since it does not include the inorganic matter in the activated sludge, it can more accurately represent the number of microorganisms in the activated sludge.

40. Sludge settling ratio (SV)?

The sludge settling ratio (SV) refers to the volume ratio (%) of settled sludge to mixed liquor after settling for 30 minutes in a 100 ml cylinder in the aeration tank; therefore, it is sometimes expressed as SV30. Generally, the SV in the bioreactor is between 20-40%. The determination of the sludge settling ratio is relatively simple and is one of the important indicators for evaluating activated sludge. It is often used to control the discharge of excess sludge and timely respond to abnormal phenomena such as sludge bulking. Obviously, SV is also related to sludge concentration.

41. Sludge volume index (SVI)?

The sludge volume index (SVI), also known as the sludge volume index, is the number of milliliters occupied by 1 gram of dry sludge in a wet state. The calculation formula is as follows:

SVI = SV * 10 / MLSS

SVI eliminates the influence of sludge concentration factors and better reflects the flocculation and settling properties of activated sludge. It is generally believed that:

When 60 < SVI < 100, the sludge settling performance is good
When 100 < SVI < 200, the sludge settling performance is average
When 200 < SVI < 300, the sludge has a tendency to bulk
When SVI > 300, the sludge has bulked

42. What does dissolved oxygen (DO) represent?

Dissolved oxygen (DO) represents the amount of oxygen dissolved in water, with units of mg/L. Different biochemical treatment methods have different requirements for dissolved oxygen. In anaerobic biochemical processes, the dissolved oxygen in water is generally between 0.2-2.0 mg/L, while in SBR aerobic biochemical processes, the dissolved oxygen in water is generally between 2.0-8.0 mg/L. Therefore, during anaerobic tank operation, the aeration volume should be small and the aeration time should be short; while during SBR aerobic tank operation, the aeration volume and aeration time should be much larger and longer. We use contact oxidation, and the dissolved oxygen is controlled at 2.0-4.0 mg/L.

43. What factors are the dissolved oxygen content in wastewater related to?

The concentration of dissolved oxygen in water can be expressed by Henry's law: when the dissolution equilibrium is reached:

C = KH * P

Where: C is the solubility of oxygen in water at dissolution equilibrium; P is the partial pressure of oxygen in the gas phase; KH is the Henry's law constant, which is related to temperature; increasing aeration efforts makes the oxygen dissolution approach equilibrium, while at the same time, activated sludge will also consume the oxygen in the water. Therefore, the actual dissolved oxygen in wastewater is related to water temperature, effective water depth (affecting pressure), aeration volume, sludge concentration, salinity, etc.

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

The oxygen needed by microorganisms in the biochemical process is mainly provided by the Roots blower.

45. Why is it necessary to frequently supplement nutrients in wastewater during the biological process?

The method of removing pollutants using biochemical processes mainly utilizes the metabolic processes of microorganisms, and the cell synthesis and other life processes of microorganisms all require sufficient amounts and types of nutrients (including trace elements). For chemical wastewater, due to the single nature of the products produced, the composition of the wastewater is also relatively simple, lacking the necessary nutrients for microorganisms. For example, the production wastewater of *** company only contains carbon and nitrogen but not phosphorus. This wastewater cannot meet the metabolic needs of microorganisms, so phosphorus must be added to the wastewater to improve the metabolic process of microorganisms and promote the synthesis of microbial cells. This is like a person eating rice and flour while also ingesting sufficient amounts of vitamins.

46. What is the ratio of various nutrient elements required by microorganisms in wastewater?

Like animals and plants, microorganisms also need necessary nutrients to grow and reproduce. The nutrients required 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 biochemical processes, it is generally C:N:P = 100:5:1 (weight ratio).

47. Why is there excess sludge?

During biological treatment, microorganisms in activated sludge continuously consume organic matter in wastewater. A portion of the consumed organic matter is oxidized to provide energy for microbial life processes, while another portion is used by microorganisms to synthesize new cytoplasm, allowing for microbial growth and reproduction. Simultaneously, some older microorganisms die during metabolism, resulting in excess sludge.

48. How to estimate the production of excess sludge?

During microbial metabolism, some organic matter (BOD) is used by microorganisms to synthesize new cytoplasm to replace dead microorganisms. Therefore, the amount of excess sludge produced is related to the amount of BOD decomposed; there is a correlation between the two.

In engineering design, it is generally considered that 0.6-0.8 kg of excess sludge (100%) is produced for every kilogram of BOD5 treated. This translates to 3-4 kg of dry sludge with a water content of 80%.