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Chapter I Catalogue of Questions

一. Introduction

1. Why sewage treatment?

2. What is ISO14000 (Environmental Management Standard)?

3. How to implement scientific environmental protection management?

4. What is "Environmental Protection 110"?

5. What are the tasks of cleaner production management?

6. Why should COD and BOD be used frequently in wastewater analysis?

7. What is COD?

8. What is BOD5?

9. What is the relationship between COD and BOD5?

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

11. What is pH?

12. Why is the concentration unit of mg/L often used in wastewater analysis?

二. Pretreatment of Wastewater

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

14. What is the use of the wastewater catchment pool?

15. Why is it difficult for colloidal particles in wastewater to settle naturally?

16. How to precipitate colloidal particles?

17. What is cohesion?

18. What is flocculation?

19. Why should the wastewater be pretreated by flocculation and adsorption with polyferric?

20. What is coagulation?

21. What is adsorption?

22. What is iron-carbon treatment?

23. Why should the iron-carbon effluent be neutralized with lime powder?

24. How to estimate the production of chemical sludge?

Chapter II Question Answering

1. Why sewage treatment?

(This is the technical data of a company's wastewater treatment station)

The *** company is located in ***, which mainly produces **. The wastewater comes from the extraction process of Ginkgo biloba leaves. Ethanol is used as extractant in the extraction process. Many pollution factors in the wastewater seriously exceed the standard, which has long been one of the difficulties affecting and restricting the company's production and development. If these wastewater is not treated before discharge, it will seriously pollute the environment. Generally speaking, when the environment and resources are destroyed and the ecological balance is out of balance, it is difficult to recover without more than ten years, decades or even hundreds of years, and sometimes it is impossible to recover. Therefore, any enterprise attempting to sacrifice environment and resources for temporary economic development, not only the national law is not allowed, the people do not agree, but also the existence and development of enterprises themselves will be limited.

2. What is ISO14000 (Environmental Management Standard)?

ISO14000 series standards are environmental management standards formulated by the Technical Committee of Environmental Management of the International Organization for Standardization. Its guiding ideology is "comprehensive management, pollution prevention and continuous improvement", which is an innovation of ideas and methods of environmental management. ISO14000 has very strict standards and regulations. From the beginning of purchasing raw materials to the end of the factory, 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. Wastewater treatment is only one part of ISO14000 series standards. At present, ISO14000 series standards are being piloted and implemented in some big cities and large enterprises in China.

ISO14000 environmental quality certification is known as the "green passport" approved by the international market, who has passed the certification will undoubtedly get the "international passport". Many countries have announced that goods and products without environmental management certification will be limited in quantity and price when imported. Therefore, with the gradual integration with the international market, ISO14000 environmental quality certification has been widely implemented in all enterprises in China, just like ISO9000 (Quality Management Standard).

Therefore, from the perspective of environmental management standards, we should not only strive to do a good job in the treatment of waste water at the end of pollution sources, implement scientific environmental management to ensure that the treated effluent meets the discharge standards, but also vigorously focus on clean production management at the front end of pollution sources to prevent pollution and reduce pollution.

3. How to implement scientific environmental protection management?

Environmental protection has become the basic national policy for the sustainable development of China's economy. Therefore, wastewater treatment should conform to the environmental protection laws and policies formulated by China. In the planning and design of environmental protection, it is necessary to combine production ideas with ecological concepts and environmental protection, and to integrate wastewater treatment, production process improvement and cleaner production. Through systematic analysis and textual research, a more reasonable governance scheme is sought.

The main principles of environmental protection management can be summarized as follows:

(1) Eliminate unreasonable products

For some traditional waste products with low output value and great difficulty in wastewater treatment, we should make up our minds to replace them with products with high output value and high technology content. If the annual profit of a product is not equal to the annual cost of wastewater treatment, such a product should be determined to stop production and replace it with a product with less pollution and easy to control.

(2) Strengthen management and reduce pollution

Enterprise management is also an important factor in preventing and controlling pollution. For example, the running, running, dropping and leaking of equipment, the production accidents caused by non-compliance with operating rules or the waste products, etc. result in the production of a large number of high-concentration wastewater, the use of a large number of water to wash equipment and the ground, resulting in an increase in wastewater volume, and the failure of "turbidity-clearing diversion" between cooling water and production wastewater will increase the water volume and the difficulty of wastewater treatment.

(3) Establishment of regional small-scale sewage treatment plants

Where factories are concentrated, the principle of "who pollutes, who governs" should not be applied. Instead, the links among enterprises should be strengthened, and the Countermeasures for pollution control should be considered as a whole. If necessary and possible, wastewater from factories can be centralized, a unified sewage treatment plant can be established, and the treatment method of "who pollutes, who pays" should be implemented. Because each factory has different products, the quality of wastewater is not the same, for example, some factory wastewater is acidic, while some factory wastewater is alkaline, which can be treated together to reduce the cost of neutralizing agents; some factories discharge wastewater with high salinity and low COD, while some factories'wastewater is high concentration and biodegradable, if treated separately. If we put them together for biochemical treatment, the improvement of water quality can not only reduce the difficulty of wastewater treatment, but also improve the efficiency of treatment.

(4) Improving water recycling efficiency

In order to reduce the amount of wastewater, more articles should be made on the source of wastewater. If water recycling or multiple reuse can be considered, the water recycling efficiency can be improved and the drainage capacity can be minimized. Overseas, the water recycling efficiency of some advanced enterprises has reached 96%, while the water recycling efficiency of Shanghai enterprises is still at a low level of 20-30%, which still has great potential to be tapped. Improving the recycling efficiency of production water can not only alleviate environmental pollution, but also reduce the supplementary consumption of fresh water. To a certain extent, it can alleviate the increasingly tense water resources problem. In wastewater treatment, the recycling of treated effluent should also be considered as far 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. In particular, some chemical reactions in fine chemical production are often not very safe and the separation process of products is not very thorough. Therefore, a certain amount of useful substances are often contained in wastewater, especially in reaction mother liquor. Discharging these pollutants will pollute the environment and cause harm. However, if it is recycled or comprehensively utilized, it can turn waste into treasure and turn harm into benefit; or it can save the cost of water treatment by using waste to treat waste, learn from its strengths and make up for its weaknesses.

4. What is "Environmental Protection 110"?

In view of the current situation that environmental protection administrative law enforcement and environmental management are not compatible with the complaints of the masses, Shanghai has opened an emergency hotline for environmental protection 6286 3110, the so-called "environmental protection 110". In the future, the telephone number will be simplified to 63110 ("green 110" homophonic). This is the first "environmental 110" in the national environmental protection system. With the strengthening of environmental protection, emergency hotlines for environmental protection will be implemented throughout the country.

The responsibilities of the emergency hotline for environmental protection are: to receive and organize reports of major pollution accidents occurring in the whole city, such as illegal discharge of pollutants from pollutant discharging units, such as stolen discharge and direct discharge; to accept and deal with incidents that may cause social instability caused by environmental problems; to assist relevant departments in dealing with major incidents that may affect the environment; and other incidents that do not need to be on-site. To deal with environmental pollution, the environmental emergency hotline can receive complaints from the people of the whole city within 24 hours.

For pollution discharge units, the opening of environmental protection 110 is both pressure and power. Only by doing a good job in pollution management and control can we stand the test of supervision by environmental law enforcement agencies and the masses.

5. What are the tasks of cleaner production management?

Wastewater and its pollutants are the products of the production process, so reforming the production process and implementing cleaner production are the fundamental measures to eliminate or reduce the harm of wastewater. Through the reform of process and equipment, wastewater can be eliminated in the production process, which can not only improve the utilization of raw and auxiliary materials, but also reduce the cost of wastewater treatment. This work should be done jointly by production process engineers and environmental engineers. It should be recognized that environmental protection is not only the work of environmental engineers, but also the control from the source of pollution, so that wastewater can be really treated well. Therefore, in the process design and product trial production, we should consider the possible environmental pollution problems in the future. When choosing the synthetic route, we should try our best to adopt pollution-free and less-polluting production process. We should choose the route with the highest utilization rate of raw materials. We should not use or use less biodegradable substances or toxic and harmful substances, including raw and auxiliary materials and solvents, and strengthen the recovery and comprehensive utilization of solvents and by-products. Specific measures are as follows:

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

New process, new technology and new route are adopted. Firstly, the proportion of ingredients in the production process can be verified. The raw materials which are polluted heavily and exceed the theoretical ratio should be reduced to increase the utilization of raw materials and the disposability of wastewater.

In chemical production, new routes are sometimes adopted, which can not only improve the production level, but also solve the problem of wastewater treatment. For example, in the past, the raw material of anti-tuberculosis drugs, isonicotinic acid, was prepared by electrolytic oxidation with sulfuric acid as electrolyte. The amount of acid wastewater produced in the process was large and difficult to treat. At present, the new technology of air catalytic oxidation is used to react in fluidized bed. The amount of wastewater is also less, and the pollution problem is relatively easy to solve.

(2) Replacement of raw and auxiliary materials

This is a commonly used method, such as replacing highly toxic or highly toxic raw materials with non-toxic or low toxic raw materials, and replacing biodegradable materials with biodegradable materials. In addition, as far as possible, no and less restrictive substances should be used in the discharge standards, especially some strict substances, so as to reduce the burden of wastewater treatment. For example, there are stringent requirements for ammonia nitrogen concentration in wastewater, which requires that ammonia water or liquid ammonia be used as little as possible in production. For example, in the past, when adjusting the pH of wastewater, some treatment processes were regulated by ammonia water, the ammonia nitrogen in effluent would exceed the standard greatly, which also increased the difficulty of biochemical treatment of wastewater. In the same way, we should use less potassium dichromate as oxidant and less nitro compounds and chlorinated hydrocarbons as solvent.

The biodegradability and toxicity of solvents should be considered in the selection of solvents besides meeting the requirements of production technology.

For the above requirements, the order of preference is shown in the table below.

Solvent Selection Priority List

To make good use of first

Methanol, ethanol, isopropanol, acetone, acetic acid, ethyl acetate, glycerol and ethylene glycol can be used as

Benzene, toluene, dimethylformamide (DMF), formamide, xylene

Avoid or use

Tert-butanol, dimethyl sulfoxide (DMSO), triethylamine, dimethylaniline, chloroform*, carbon tetrachloride*, chlorobenzene, nitrobenzene*, pyridine*, morpholine, tetrahydrofuran

Note: * indicates toxicity or inhibition to microorganisms.

(3) Selecting new post-treatment process to reduce or eliminate pollution in production process

This method is very useful for the technicians engaged in chemical and chemical production. For example, in organic synthesis industry, the method of diluting reaction materials by adding water (water analysis) is often used to make reaction products precipitate from reaction organic solvents. The mother liquor produced by water precipitation is difficult to recover because of the large amount of water, and the organic solvents (such as methanol, ethanol and other water-soluble solvents) are brought into the wastewater stream to cause pollution. If most of the solvents are recovered by distillation before dilution, and then diluted by water, the content of organic matter in wastewater can be significantly reduced.

In order to ensure better quality of the product, the reaction products or intermediate products often need to be washed to remove the impurities entrained in the product. Whether the washing operation is reasonable or not has a considerable impact on the degree of wastewater pollution. However, if the new post-treatment technology is adopted, the washing wastewater can be completely eliminated in the process of operation and zero sewage discharge can be realized. Too high salt content in wastewater can inhibit the growth and reproduction of microorganisms and affect the effect of biochemical treatment. We can also adopt a new post-treatment process to solve this difficulty in wastewater treatment. For example, a factory will react nitrochlorobenzene with sodium hydroxide in methanol solvent to prepare p-nitroanisole. The original post-treatment process used water washing to remove NaCl salts from reaction materials. The result of the operation was that the amount of wastewater was large and the salt content in wastewater was high, which made the subsequent biochemical treatment difficult. Later, the plant improved the operation process of post-treatment. First, NaCl in the reactant material (organic phase) was filtered out, then p-nitroanisole was washed and precipitated with water. The improved operation process not only reduced the amount of wastewater by 50%, but also recovered 97.4% of salt in wastewater, reduced the organic load of wastewater by 58.7%, and greatly improved the biodegradability of wastewater.

(4) Strengthening solvent recovery

In most chemical raw material factories, the proportion of solvents used in raw materials and accessories is quite high. It can be said that the organic load in many production wastewater basically comes from solvents. Therefore, paying attention to and doing a good job in the recovery of solvents is not only an important measure to prevent pollution and reduce pollution, but also an important way to reduce costs, increase efficiency and increase profits. It has dual 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), which is a major polluter in the region. The environmental protection treatment of the plant starts with the recovery of solvents. The mother liquor wastewater containing the same solvents is collected and recycled. As a result, the total organic load of the wastewater is reduced from 8 tons to 3 tons per day, and the recovery of solvents exceeds the operation cost of the wastewater treatment station.

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

There are many organic substances in wastewater, including more than a dozen, dozens, even hundreds of organic substances, which are often encountered in wastewater. If the organic substances in wastewater are analyzed qualitatively and quantitatively, it will not only take time, but also consume drugs. So can we use only one pollution index to represent all organic substances and their quantities in wastewater? Environmental scientists have found that all organic substances have two commonalities: one is that they are at least composed of hydrocarbons; the other is that most organic substances can be chemically oxidized or oxidized by microorganisms, and their carbon and hydrogen form non-toxic and harmless carbon dioxide and water with oxygen, respectively. Organic substances in wastewater consume oxygen both in chemical oxidation and biological oxidation. The more organic substances in wastewater, the more oxygen they consume. There is a positive relationship between them. Therefore, environmental scientists call the amount of oxygen consumed when wastewater is oxidized by chemical agents COD, and the amount of oxygen consumed by microbial oxidation of wastewater BOD. Because COD and BOD can comprehensively reflect the amount of all organic substances in wastewater and their analysis is relatively simple, they are widely used in wastewater analysis and environmental engineering.

In fact, COD does not simply mean organic matter in water. It can also indicate inorganic substances with reducing properties in water, such as sulfide, ferrous ion, sodium sulfite, and even chloride ion. For example, if the ferrous ions in the effluent of iron-carbon pool can not be completely removed from the neutralization pool, the COD in the effluent of biochemical treatment may exceed the standard due to the presence of ferrous ions.

7. What is COD?

Chemical oxygen demand (COD) refers to the amount of oxygen required by oxidizable substances in wastewater when oxidized by chemical oxidants in milligrams per liter of oxygen. It is currently used to determine the content of organic matter in wastewater as one of the most commonly used means. The oxidants commonly used in COD analysis are potassium permanganate (CODMn by manganese method) and potassium dichromate (CODCr by chromium method). Now potassium dichromate method is commonly used. The wastewater is oxidized by boiling and reflux of strong acid. When silver sulfate is used as catalyst, the oxidation rate of most organic compounds can be increased to 85-95%. If the wastewater contains high concentration of chloride ion, mercury sulfate should be used to shield the chloride ion to reduce the interference of COD determination.

8. What is BOD5?

Biochemical oxygen demand (BOD5) can also indicate the degree of organic contamination of wastewater. The most commonly used five-day BOD5 represents the amount of oxygen required for biochemical degradation of wastewater in the presence of microorganisms within five days. In the future, we will often use biochemical oxygen demand for five days.

9. What is the relationship between COD and BOD5?

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

It is generally believed that COD basically represents all organic substances in water. BOD is a biodegradable organic matter in water, so the difference between COD and BOD can indicate the biodegradable organic matter in wastewater.

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

B/C is the abbreviation of BOD5/C OD ratio, which can indicate the biodegradability of wastewater. If CODNB represents the non-biodegradable part of COD, the proportion of organic matter in wastewater that can not be biodegraded by microorganisms can be expressed by CODNB/COD.

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

CODNB/COD 0.10.20.30.40.50.60.7 0.8

BOD5/COD 0.52 0.46 0.41 0.35 0.29 0.23 0.17 0.12

When BOD5/COD is more than 0.45, non-biodegradable organic matter only accounts for less than 20% of all organic matter, while when BOD5/COD is less than 0.2, non-biodegradable organic matter accounts for more than 60% of all organic matter.

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

BOD5/COD__0.45__biodegradable

BOD5/COD__0.30__biodegradable

BOD5/COD__0.30__is difficult to biodegrade.

BOD5/COD__0.20__is more difficult to biodegrade.

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

11. What is pH?

In fact, pH is a representation of acidity and alkalinity in aqueous solution. Usually, we are used to express the acidity and alkalinity of water solution by percentage concentration, such as 1% sulfuric acid solution or 1% alkali solution. But when the acidity and alkalinity of water solution is very small, it is too troublesome to express by percentage concentration again. At this time, it can be expressed by pH. The application range of pH ranges from 0 to 14, when pH=7, water is neutral; when pH<7, water is acidic, the smaller the pH, the greater the acidity of water; when pH>7, water is alkaline, and the greater the pH, the greater the alkalinity of water.

Water is indispensable to all living things in the world, but the range of pH value suitable for living things is often very narrow. Therefore, the National Environmental Protection Agency (NEPA) strictly stipulates the pH value of treated water between 6 and 9.

The pH test paper is often used to detect the pH value in water. It is also used by instruments, such as the pH meter.

12. Why is the concentration unit of mg/L often used in wastewater analysis?

Generally speaking, the content of organic and inorganic substances in wastewater is very small. It is too troublesome and inconvenient to express them by percentage concentration or other concentration. For example, a ton of wastewater often contains only a few grams, dozens of grams, hundreds or even several kilograms of pollutants, the unit of which is g/t. If the ton is converted into a liter, the unit is mg/L. When calculating, the following table can be used for conversion:

One milligram per liter

1000 mg/l/1/1000

10,000 mg/l_/1

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

The pretreatment 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 main treatment method of wastewater in *** company is biochemical method. However, the wastewater of *** Company contains some organic substances which are inhibiting and poisonous to microorganisms, so the wastewater must be pretreated before entering the biochemical pond. The purpose is to reduce or remove the substances which are inhibiting and poisonous to microorganisms as much as possible in order to ensure the normal operation of microorganisms in the biochemical pond.

There are two purposes of pretreatment: one is to reduce and remove as much as possible the substances that inhibit toxicity and inhibition to microorganisms in wastewater, or to convert them into harmless or beneficial substances to ensure the normal operation of microorganisms in biochemical ponds; the other is to reduce the COD load in the pretreatment process in order to reduce the operating burden of biochemical ponds.

The pretreatment process of *** Company is iron-carbon micro-electrolysis and Fe2+/Fe3+ reduction-oxidation process. Numerous small iron-carbon primary batteries are formed to facilitate the oxidation-reduction reaction, which can destroy and remove toxic and harmful substances from wastewater. In the process of neutralization and precipitation, organic substances in wastewater can be adsorbed by activated flocs formed by ferrous and trivalent iron in alkaline conditions to reduce COD. Load, ensure that the follow-up biochemical treatment system can run normally.

14. What is the use of the wastewater catchment pool?

The function of wastewater catchment pool is to collect, store and balance the quality and quantity of wastewater.

The quantity and quality of wastewater discharged from each workshop are generally unbalanced. There are wastewater in production, but there is no wastewater in production, and there may be great changes even within a day or between shifts. Especially in the fine chemical industry, if the turbid wastewater is not diverted, the quality and quantity of concentrated wastewater and light polluted wastewater will vary greatly. Variations are harmful to the normal operation and treatment effect of wastewater treatment facilities. Therefore, before entering the main wastewater treatment system, a certain volume of wastewater catchment pool should be set up to store wastewater and make it homogeneous, so as to ensure the normal operation of wastewater treatment equipment and facilities.

15. Why is it difficult for colloidal particles in wastewater to settle naturally?

Many impurity suspended substances, large particles and easily settled suspended substances with a proportion of more than 1 in wastewater can be removed by natural sedimentation and centrifugation.

However, it is difficult for suspended particulates whose specific gravity is less than 1, small or even invisible to the naked eye to settle naturally. For example, colloidal particles are 10-4-10-6 mm in size, which are very stable in water. Its sedimentation speed is very slow, and it takes 200 years to settle 1 m. There are two reasons for the slow settling. (1) Generally speaking, colloidal particles are negatively charged. Because of the same-sex repulsion, colloidal particles can not be bonded to each other and suspended in water. (2) The surface of colloidal particles is also surrounded by a layer of molecules. This layer of hydration also hinders and isolates the contact between colloidal particles. It can not be bonded to each other and suspended in water.

16. How to precipitate colloidal particles?

In order to precipitate colloidal particles, colloidal particles should be contacted to form large particles, that is to say, coagulation, and precipitation with a ratio of 1.

There are many methods adopted. The commonly used technologies in engineering are coagulation, flocculation and coagulation.

17. What is cohesion?

In wastewater, coagulants with positive ions are added, and a large number of positive ions exist between colloidal particles to eliminate the electrostatic repulsion between colloidal particles, so that particles can coalesce. The process of coagulation of colloidal particles is called coagulation by adding positive ion electrolyte. Commonly used coagulants are aluminium sulfate, ferrous sulfate, alum, ferric chloride and so on.

18. What is flocculation?

Flocculation is the addition of polymer coagulant agent in wastewater. When the polymer coagulant agent dissolves, it will form a polymer. The structure of this polymer is a linear structure. One end of the line pulls a tiny particle and the other pulls another tiny particle. It acts as a bridge between two particles far apart, making the particles gradually larger and eventually forming a large particle floc (commonly known as alum) to accelerate particle settling. The commonly used flocculants are polyacrylamide (PAM), polyiron (PE) and so on.

19. Why should the wastewater be pretreated by flocculation and adsorption with polyferric?

Polyferric hydroxide floc formed during coagulation process has a good ability to adsorb organic substances in wastewater. The experimental data show that the COD in wastewater can be removed by flocculating and adsorbing with polyferric oxide, which can greatly reduce the operating burden of biochemical ponds and is conducive to the discharge of wastewater up to the standard. In addition, coagulation pretreatment with Polyferric can remove trace substances which are toxic and inhibiting to microorganisms in wastewater, so as to ensure the normal operation of microorganisms in biochemical ponds. Among many coagulants, the price of Polyferric is relatively cheap (25-300 yuan/ton), so the cost of treatment is relatively low, which is more suitable for the pretreatment of process wastewater.

Polyferric is acidic and corrosive, so the treatment equipment should be well anticorrosive.

20. What is coagulation?

The process of coagulation combined with flocculation is coagulation. Coagulation is often used in experiments or engineering, such as adding ferrous sulfate and other agents in water to eliminate electrostatic repulsion between colloidal particles, and then adding polyacrylamide (PAM), so that the particles gradually become larger, forming visible alum, and finally producing sedimentation.

21. What is adsorption?

Porous solids (such as activated carbon) or flocs (such as polyferric) are used to adsorb toxic and harmful substances in wastewater onto the surface of solids or flocs or micro-pore to purify water quality. This treatment method is called adsorption treatment. The object of adsorption can be either insoluble solid substance or soluble substance. Adsorption treatment has high efficiency and good effluent quality, so it is often used as advanced treatment of wastewater. Adsorption treatment can also be introduced into the biochemical treatment unit to improve the efficiency of biochemical treatment (PACT method is one of them).

22. What is iron-carbon treatment?

Iron-carbon treatment, also known as iron-carbon micro-electrolysis or iron-carbon internal electrolysis, is an application form of iron-metal wastewater treatment technology. It has a unique effect to treat toxic, harmful and highly concentrated COD wastewater with iron-carbon pretreatment technology. At present, the mechanism of iron-carbon process is not completely clear. One of the most accepted explanations is that under acidic conditions, numerous micro-current reaction pools are formed between iron and carbon, and organic compounds are reduced and oxidized under the action of micro-current. Ferrocarbon effluent is neutralized by lime or lime milk, and the Fe(OH)2 colloidal flocculent has a strong flocculation and adsorption capacity for organic matter. Therefore, the iron-carbon process is a comprehensive application of iron reduction properties, iron-carbon electrochemical properties and flocculation and adsorption of iron ions. It is the combined effect of these three properties that the iron-carbon process has a good treatment effect.

The shortcomings of iron-carbon process are: (1) iron chips are easy to agglomerate after long-term immersion in acidic medium, resulting in blockage and formation of ditch flow, which makes operation difficult and treatment effect reduced; (2) iron dissolves in acidic conditions with a large amount of iron and produces more sludge after neutralization with alkali.

23. Why should the iron-carbon effluent be neutralized with lime powder?

When the wastewater is adjusted to pH 2 by sulfuric acid and treated with iron-carbon, sulfuric acid becomes ferrous sulfate. The pH value of wastewater increases from 2 to 5-6. Why should the iron-carbon effluent be neutralized with lime powder? Or is it possible to add less lime powder in neutralization treatment?

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

Is it possible to add less lime powder in neutralization treatment? We can do a comparative experiment in the laboratory. The same amount of iron-carbon water (pH 2) and iron-carbon water (pH 5-6) were placed in two beakers, and then added to the lime powder for neutralization and coagulation. When the pH value of the wastewater in the two beakers was adjusted to 9, we can find that the amount of lime powder added in the two beakers is the same. This is because iron is not a neutralizing agent. Ferrous sulfate converted from sulfuric acid is still an acidic substance. Lime powder used in the conversion of ferrous sulfate into ferrous hydroxide and calcium sulfate in the neutralization process is indispensable. Therefore, lime powder should not be added to iron-carbon effluent and treatment.

24. How to estimate the production of chemical sludge?

Sludge produced by chemical reactions (e.g. neutralization) and physicochemical treatments (e.g. chemical coagulation) is traditionally referred to as chemical sludge. The sludge formed from iron-carbon effluent after neutralization and coagulation is mainly composed of ferrous hydroxide and calcium sulfate. Sludge production can be calculated by adding sulphuric acid and lime powder. Experience can also be used for engineering estimation. Generally speaking, if the influent pH of iron and carbon is about 2, the chemical sludge produced per ton of wastewater after neutralization and coagulation (moisture content 80%) is about 50 kg.