Types of wastewater treatment agents

1. What kinds of agents are commonly used in wastewater treatment?

In order to meet the discharge standard or reuse the wastewater after treatment, a variety of chemicals need to be used in the treatment process. According to different uses, these agents can be divided into the following categories:

(1) Flocculant: sometimes called coagulant, it can be used as a means to strengthen solid-liquid separation and is used in process links such as primary sedimentation tank, secondary sedimentation tank, flotation tank, tertiary treatment or advanced treatment.

(2) Coagulant aid: auxiliary flocculant plays a role to strengthen the coagulation effect.

(3) Conditioning agent: also known as dehydrating agent, it is used for conditioning the residual sludge before dehydration. Its varieties include some of the above flocculants and coagulant aids.

(4) Demulsifier: sometimes called destabilizing agent, it is mainly used for the pretreatment of oily waste water containing emulsified oil before air flotation. Its varieties include some of the above flocculants and coagulant aids.

(5) defoamer: mainly used to eliminate a large number of bubbles during aeration or agitation.

(6) PH regulator: used to adjust the pH value of acidic wastewater and alkaline wastewater to neutral.

(7) Redox agent: used for the treatment of industrial wastewater containing oxidizing or reducing substances.

(8) Disinfectant: used for disinfection treatment after wastewater treatment and before discharge or reuse.

Although there are many kinds of the above medicaments, a medicament will have different names if it is used in different occasions and plays different roles. For example, Cl2 is called coagulant aid when it is used to enhance the coagulation treatment effect of sewage, oxidant when it is used to oxidize cyanide or organic matter in wastewater, and disinfectant when it is used for disinfection treatment.

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flocculant

1. What is flocculant? What is its function?

As a means of strengthening solid-liquid separation in the field of sewage treatment, flocculant can be used to strengthen the primary precipitation, flotation treatment and secondary precipitation after activated sludge process, as well as tertiary treatment or advanced treatment of sewage. When used for conditioning before dewatering of excess sludge, flocculant and coagulant aid become sludge conditioner or dehydrating agent.

In the application of traditional flocculant, the method of adding coagulant aid can be used to strengthen the flocculation effect. For example, good flocculation can be achieved by using activated silicic acid as coagulant aid of inorganic flocculants such as ferrous sulfate and aluminum sulfate and adding them in sequence. Therefore, generally speaking, inorganic polymer flocculant IPF is actually prepared by combining coagulant aid and flocculant, and then combined dosing to simplify the operation of users.

Coagulation treatment is usually placed in front of solid-liquid separation facilities and combined with separation facilities to effectively remove the particle size in raw water, which is 1nm ~ 100 μ M suspended solids and colloidal substances, reduce effluent turbidity and CODCr, and can be used in pretreatment and advanced treatment of sewage treatment process, as well as residual sludge treatment. Coagulation treatment can also effectively remove microorganisms and pathogens in water, and remove emulsified oil, color, heavy metal ions and other pollutants in sewage. When using coagulation precipitation to treat phosphorus contained in sewage, the removal rate can be as high as 90 ~ 95%, which is the cheapest and efficient phosphorus removal method.

2. What is the action mechanism of flocculant?

The colloidal particles in water are small, the surface hydration and electrification make them stable. After the flocculant is added to the water, it is hydrolyzed into charged colloid and its surrounding ions to form a gel mass with double-layer structure.

The method of rapid stirring after dosing is adopted to promote the collision opportunities and times between colloidal impurity particles in water and micelles hydrolyzed by flocculant. Under the action of flocculant, the impurity particles in water first lose stability, then agglomerate into larger particles, and then settle down or float up in the separation facilities.

The product gt of the velocity gradient g produced by stirring and the stirring time t can indirectly represent the total number of particle collisions in the whole reaction time. The coagulation reaction effect can be controlled by changing the GT value. Generally, the GT value is controlled between 104 and 105. Considering the impact of impurity particle concentration on collision, the GTC value can be used as the control parameter to characterize the coagulation effect, where C represents the mass concentration of impurity particles in sewage, and it is recommended that the GTC value is about 100.

The process of promoting the rapid diffusion of flocculant into water and mixing evenly with all wastewater is mixing. The process in which the impurity particles in the water act with the flocculant, lose or reduce the stability and generate micro floc particles through the mechanism of compressing the electric double layer and electric neutralization is called coagulation. Flocculation is the process in which micro flocs are formed by coagulation and grow into large flocs through adsorption, bridging and sediment net capture under the stirring of bridging materials and water flow. Mixing, coagulation and flocculation are called coagulation. The mixing process is generally completed in the mixing tank, and coagulation and flocculation are carried out in the reaction tank.

3. What are the types of flocculants?

Flocculant is a kind of substance that can reduce or eliminate the precipitation stability and polymerization stability of dispersed particles in water, and make dispersed particles agglomerate and flocculate into aggregates. According to chemical composition, flocculants can be divided into inorganic flocculants, organic flocculants and microbial flocculants.

Inorganic flocculants include aluminum salts, iron salts and their polymers.

According to the charge properties of charged groups of polymeric monomers, organic flocculants can be divided into anionic, cationic, non-ionic and amphoteric types. According to their sources, organic flocculants can be divided into synthetic and natural polymer flocculants.

In practical application, inorganic flocculants and organic flocculants are often compounded according to their different properties to make inorganic organic composite flocculants. Microbial flocculant is the product of the combination of modern biology and water treatment technology. It is an important direction of Flocculant Research, development and application.

4. What are the types of inorganic flocculants?

The traditional inorganic flocculants are low molecular aluminum and iron salts. Aluminum salts mainly include aluminum sulfate (Al2 (SO4) 3 ? 18H2O), alum (Al2 (SO4) 3 ? K2SO4 ? 24h2o), sodium aluminate (naalo3), and iron salts mainly include ferric chloride (FeCl3 ? 6H2O), ferrous sulfate (FeSO4 ? 6h20) and iron sulfate (Fe2 (SO4) 3 ? 2h20).

Generally speaking, inorganic flocculant has the characteristics of easy availability of raw materials, simple preparation, low price and moderate treatment effect, so it is widely used in water treatment.

5. What are the characteristics of inorganic flocculant aluminum sulfate?

Since the United States first used aluminum sulfate for water treatment and obtained a patent at the end of the 19th century, aluminum sulfate has been widely used for its excellent coagulation and sedimentation performance. Aluminum sulfate is the most used flocculant in the world. The annual output of aluminum sulfate is about 5 million tons in the world, nearly half of which is used in the field of water treatment.

Commercial aluminum sulfate has two forms: solid and liquid. The solid one is divided into refined and crude according to the content of insoluble matter. The solid product alum commonly used in drinking water purification in China is the double salt of aluminum sulfate and potassium sulfate, but it is not widely used in industrial water and wastewater treatment.

The applicable pH range of aluminum sulfate is related to the hardness of raw water. When treating soft water, the appropriate pH value is 5 ~ 6.6, when treating medium hard water, the appropriate pH value is 6.6 ~ 7.2, and when treating high hard water, the appropriate pH value is 7.2 ~ 7.8. The applicable water temperature range of aluminum sulfate is 20oC ~ 40oC. When it is lower than 10oC, the coagulation effect is very poor. Aluminum sulfate is less corrosive and easy to use, but the hydrolysis reaction is slow and needs to consume a certain amount of alkali.

6. What are the characteristics of inorganic flocculant ferric chloride?

Ferric chloride is another commonly used inorganic low molecular coagulant. The products include solid dark brown crystal and high concentration liquid. It has the advantages of easy solubility in water, large and heavy alum, good precipitation performance, wide adaptability to temperature, water quality and pH, etc.

The applicable pH value range of ferric chloride is 9 ~ 11. The floc formed has high density and is easy to precipitate. The effect is still good at low temperature or high turbidity. Solid ferric chloride has strong water absorption, strong corrosivity and is easy to corrode equipment. It has high anti-corrosion requirements for dissolution and dosing equipment, pungent smell and poor operating conditions.

The action mechanism of ferric chloride is to use various hydroxyl iron ions generated by the step-by-step hydrolysis of ferric ions to flocculate impurity particles in water. The formation of hydroxyl iron ions requires a large amount of hydroxyl in water, so a large amount of alkali will be consumed in the process of use. When the alkalinity of raw water is not enough, lime and other alkali sources need to be supplemented.

Ferrous sulfate, commonly known as green alum, forms flocs quickly and stably with short precipitation time. It is suitable for the situation of high alkalinity and turbidity, but the chromaticity is not easy to remove and the corrosivity is strong.

7. What are the types of inorganic polymer flocculants?

Inorganic polymer flocculant (IPF) is a new flocculant developed since 1960s. At present, the production and application of IPF have made rapid progress all over the world.

Aluminum, iron and silicon inorganic polymer flocculants are actually the intermediate products of their hydrolysis, sol and precipitation processes, namely, hydroxyl and oxygen polymers of Al (Ⅲ), Fe (Ⅲ) and Si (Ⅳ). Aluminum and iron are cationic positively charged and silicon is anionic negatively charged. Their unit molecular weight in water-soluble state is about hundreds to thousands, which can be combined with each other to form aggregates with fractal structure.

Their coagulation flocculation process is a comprehensive embodiment of the electric neutralization and adhesion bridging of particles in water. The particle size of suspended particles in water ranges from nano to micron, and most of them are negatively charged. Therefore, the positive and negative charge, electrical strength, molecular weight and particle size of aggregates of flocculants and their forms are the main factors determining their flocculation effect. At present, there are dozens of inorganic polymer flocculants (see table 8-1 for the main varieties), and the output also reaches 30% ~ 60% of the total output of flocculants, among which polyaluminium chloride is widely used.

8. What are the characteristics of inorganic polymer flocculant?

The hydroxyl and oxy polymers of Al (Ⅲ), Fe (Ⅲ) and Si (Ⅳ) will be further combined into aggregates, which will be maintained in aqueous solution under certain conditions, and their particle size is roughly in the nanometer range. In order to play the role of coagulation flocculation, the result of low dosage and high effect will be obtained.

If comparing their reaction polymerization rates, the trend from Al → Fe → Si tends to be strong, and the trend from hydroxyl bridging to oxygen bridging also follows this order. Therefore, aluminum polymers react more slowly and form more stable. The hydrolyzed polymers of iron react rapidly, and tend to lose stability and precipitate. Silicon polymers tend to form sol and gel particles.

The advantage of IPF is that it has better efficiency than traditional flocculants such as aluminum sulfate and ferric chloride, and is cheaper than organic polymer flocculant (OPF). Now it has been successfully applied in various treatment processes of water supply, industrial wastewater and urban sewage, including pretreatment, intermediate treatment and advanced treatment, and has gradually become the mainstream flocculant. However, in terms of morphology, degree of polymerization and corresponding coagulation flocculation effect, inorganic polymer flocculant is still between traditional metal salt flocculant and organic polymer flocculant.

Its molecular weight, particle size and flocculating bridging capacity are still much worse than those of organic flocculants, and there is also the instability of further hydrolysis reaction. These weaknesses of IPF promote the research and development of various composite inorganic polymer flocculants.

9. What are the characteristics of polyaluminium chloride?

Polyaluminium chloride (PAC), also known as basic aluminum chloride, has the chemical formula AlN (OH) mcl3n-m. PAC is a multivalent electrolyte, which can significantly reduce the colloidal charge of clay impurities (mostly negatively charged) in water. Due to the large relative molecular weight and strong adsorption capacity, the formed flocculate is large, and the flocculation and sedimentation performance is better than other flocculants.

PAC has a high degree of polymerization, and rapid stirring after dosing can greatly shorten the formation time of flocculate. PAC is less affected by water temperature and works well at low water temperature. It reduces the pH value of water less and is applicable to a wide pH range (it can be used in the range of pH = 5 ~ 9), so it can not add alkali agent. The dosage of PAC is small, the sludge production is also small, the use, management and operation are convenient, and the corrosion to equipment and pipelines is also small. Therefore, PAC tends to gradually replace aluminum sulfate in the field of water treatment, with the disadvantage of high price.

In addition, from the perspective of solution chemistry, PAC is the kinetic intermediate product of aluminum salt hydrolysis polymerization precipitation reaction process, which is thermodynamically unstable. Generally, liquid PAC products should be used within half a year. Adding some inorganic salts (such as CaCl2, MnCl2, etc.) or polymers (such as polyvinyl alcohol, polyacrylamide, etc.) can improve the stability of PAC and increase the coagulation capacity.

In terms of production process, one or several different anions (such as SO42 -, PO43 -) are introduced into the manufacturing process of PAC, and the structure and morphological distribution of the polymer can be changed to a certain extent by using the polymerization effect, so as to improve the stability and efficacy of PAC; If other cationic components, such as Fe3 +, are introduced into the manufacturing process of PAC to make the cross hydrolysis polymerization of Al3 + and Fe3 +, the composite flocculant polyaluminium iron can be prepared.

Aluminum oxide content is a measure of the effective component of polyaluminium chloride. Generally speaking, the higher the flocculant product density, the higher the aluminum oxide content. Generally speaking, polyaluminium chloride with higher alkalinity has better adsorption and bridging capacity, but it is easy to precipitate due to its proximity to [al (OH) 3] n, so its stability is also poor.

10. What is the alkalinity of PAC?

Polyaluminium chloride can be regarded as an intermediate product in the process of gradual hydrolysis of AlCl3 to Al (OH) 3, that is, various products in which Cl - is gradually replaced by hydroxyl Oh -. In some form of polyaluminium chloride, the degree of hydroxylation is the degree of alkalization, which is the ratio of hydroxyl equivalent to aluminum equivalent in Polyaluminium chloride.

Practice shows that alkalinity is one of the most important indexes of polyaluminium chloride. The degree of polymerization, charge, coagulation effect, pH value of finished products, dilution rate during use and storage stability of polyaluminium chloride are closely related to alkalinity. The alkalinity of commonly used polyaluminium chloride is mostly 50% ~ 80%.

11. What are the characteristics and precautions of composite flocculant?

The composite flocculant has various components, and its main raw materials are aluminum salt, iron salt and silicate. In terms of manufacturing process, they can be hydroxylated and polymerized respectively in advance and then mixed, or mixed and then hydroxylated and polymerized, but in the end, they always have to form the inorganic polymer form with higher hydroxylation degree in order to achieve excellent flocculation effect.

Each component in the composite agent will play a certain role in the overall structure and coagulation flocculation process, but it may have positive or negative effects in different aspects.

IPF products usually need to comprehensively consider three factors: stability, electric neutralization capacity and adsorption bridging capacity. The weakness of polyaluminium and polymerized iron flocculants is that the molecular weight and particle size are not high enough, and the adhesion bridging ability of aggregates is not strong enough. Therefore, silicon polymers with large particle size need to be added to enhance the flocculation performance. However, after adding anionic silicon polymer, the overall charge will be reduced, which weakens the electric neutralization ability.

Its molecular weight, particle size and flocculating bridging capacity are still much worse than those of organic flocculants, and there is also the instability of further hydrolysis reaction. These weaknesses of IPF promote the research and development of various composite inorganic polymer flocculants.

9. What are the characteristics of polyaluminium chloride?

Polyaluminium chloride (PAC), also known as basic aluminum chloride, has the chemical formula AlN (OH) mcl3n-m. PAC is a multivalent electrolyte, which can significantly reduce the colloidal charge of clay impurities (mostly negatively charged) in water. Due to the large relative molecular weight and strong adsorption capacity, the formed flocculate is large, and the flocculation and sedimentation performance is better than other flocculants.

PAC has a high degree of polymerization, and rapid stirring after dosing can greatly shorten the formation time of flocculate. PAC is less affected by water temperature and works well at low water temperature. It reduces the pH value of water less and is applicable to a wide pH range (it can be used in the range of pH = 5 ~ 9), so it can not add alkali agent. The dosage of PAC is small, the sludge production is also small, the use, management and operation are convenient, and the corrosion to equipment and pipelines is also small. Therefore, PAC tends to gradually replace aluminum sulfate in the field of water treatment, with the disadvantage of high price.

In addition, from the perspective of solution chemistry, PAC is the kinetic intermediate product of aluminum salt hydrolysis polymerization precipitation reaction process, which is thermodynamically unstable. Generally, liquid PAC products should be used within half a year. Adding some inorganic salts (such as CaCl2, MnCl2, etc.) or polymers (such as polyvinyl alcohol, polyacrylamide, etc.) can improve the stability of PAC and increase the coagulation capacity.

In terms of production process, one or several different anions (such as SO42 -, PO43 -) are introduced into the manufacturing process of PAC, and the structure and morphological distribution of the polymer can be changed to a certain extent by using the polymerization effect, so as to improve the stability and efficacy of PAC; If other cationic components, such as Fe3 +, are introduced into the manufacturing process of PAC to make the cross hydrolysis polymerization of Al3 + and Fe3 +, the composite flocculant polyaluminium iron can be prepared.

Aluminum oxide content is a measure of the effective component of polyaluminium chloride. Generally speaking, the higher the flocculant product density, the higher the aluminum oxide content. Generally speaking, polyaluminium chloride with higher alkalinity has better adsorption and bridging capacity, but it is easy to precipitate due to its proximity to [al (OH) 3] n, so its stability is also poor.

10. What is the alkalinity of PAC?

Polyaluminium chloride can be regarded as an intermediate product in the process of gradual hydrolysis of AlCl3 to Al (OH) 3, that is, various products in which Cl - is gradually replaced by hydroxyl Oh -. In some form of polyaluminium chloride, the degree of hydroxylation is the degree of alkalization, which is the ratio of hydroxyl equivalent to aluminum equivalent in Polyaluminium chloride.

Practice shows that alkalinity is one of the most important indexes of polyaluminium chloride. The degree of polymerization, charge, coagulation effect, pH value of finished products, dilution rate during use and storage stability of polyaluminium chloride are closely related to alkalinity. The alkalinity of commonly used polyaluminium chloride is mostly 50% ~ 80%.

11. What are the characteristics and precautions of composite flocculant?

The composite flocculant has various components, and its main raw materials are aluminum salt, iron salt and silicate. In terms of manufacturing process, they can be hydroxylated and polymerized respectively in advance and then mixed, or mixed and then hydroxylated and polymerized, but in the end, they always have to form the inorganic polymer form with higher hydroxylation degree in order to achieve excellent flocculation effect.

Each component in the composite agent will play a certain role in the overall structure and coagulation flocculation process, but it may have positive or negative effects in different aspects.

IPF products usually need to comprehensively consider three factors: stability, electric neutralization capacity and adsorption bridging capacity. The weakness of polyaluminium and polymerized iron flocculants is that the molecular weight and particle size are not high enough, and the adhesion bridging ability of aggregates is not strong enough. Therefore, silicon polymers with large particle size need to be added to enhance the flocculation performance. However, after adding anionic silicon polymer, the overall charge will be reduced, which weakens the electric neutralization ability.

Therefore, even if the manufacturing quality of the current composite flocculant is excellent, its effect can only be improved by 10 ~ 30% compared with polyaluminum. As a wastewater treatment technician using IPF, it is equally important to understand the characteristics, adaptability, advantages and disadvantages of different kinds of composite flocculants. When selecting the most appropriate flocculant and adding process operation procedures, the best treatment effect can be obtained only by careful analysis and judgment according to the characteristics of wastewater quality.

12. What are the types of synthetic organic polymer flocculants?

Synthetic organic polymer flocculants are mostly polypropylene and polyethylene substances, such as polyacrylamide, polyethyleneimine, etc. These flocculants are water-soluble linear macromolecules. Each macromolecule is composed of many repeating units containing charged groups, so they are also called polyelectrolytes. Cationic polyelectrolytes contain positively charged groups and anionic polyelectrolytes contain negatively charged groups. They contain both positively charged and negatively charged groups, which are called non-ionic polyelectrolytes.

At present, the most commonly used polymer flocculants are anionic, and they can only play a role in coagulating negative colloidal impurities in water. It can not be used alone, but with aluminum salt and iron salt. Cationic flocculant can play the role of coagulation and flocculation at the same time and can be used alone, so it has developed rapidly.

At present, polyacrylamide non-ionic polymers are widely used in China, which are often used with iron and aluminum salts. Using the electric neutralization of iron and aluminum salts to colloidal particles and the excellent flocculation function of polymer flocculant, a satisfactory treatment effect is obtained. Polyacrylamide has the characteristics of small dosage, fast coagulation speed, large floc particles and strong toughness. At present, 80% of the synthetic organic polymer flocculants produced in China are this product.

13. What are the characteristics of polyacrylamide flocculants?

Polyacrylamide PAM is one of the most widely used synthetic organic polymer flocculants, and sometimes it is also used as coagulant aid. The raw material for the production of polyacrylamide is polyacrylonitrile CH2 = CHCN. Under certain conditions, acrylonitrile is hydrolyzed to produce acrylamide, which is then obtained by suspension polymerization. Polyacrylamide is a water-soluble resin. Its products include granular solid and viscous aqueous solution with a certain concentration.

The actual existing form of Polyacrylamide in water is random coil. Because the random coil has a certain particle size and some amide groups on its surface, it can play a corresponding bridging and adsorption capacity, that is, it has a certain flocculation capacity.

However, because the long chain of polyacrylamide is curled into a thread group, the bridging range is small. After the two amide groups are concluded, it is equivalent to the mutual offset and loss of two adsorption sites. In addition, some amide groups are trapped inside the thread group structure and can not contact and adsorb with impurity particles in water, so its adsorption capacity can not be brought into full play.

In order to make the amide groups bound together separate again and the inner amide groups can also be exposed to the outside, people try to extend the random coil appropriately, and even try to add some groups with cations or anions to the long molecular chain, so as to improve the adsorption bridging ability and the effect of neutralization and compression of the electric double layer. In this way, a series of polyacrylamide flocculants or coagulant aids with different properties were derived on the basis of PAM.

For example, adding alkali to the polyacrylamide solution can convert the amide group on some chain links into sodium carboxylate, which is easy to dissociate sodium ions in water, leaving the COO group on the branch chain, so as to generate partially hydrolyzed anionic polyacrylamide.

The COO group in the molecular structure of anionic polyacrylamide makes the molecular chain negatively charged, repels each other, pulls the amide group originally bound together, and promotes the molecular chain to gradually extend from the thread to the chain, so as to expand the bridging range and improve the flocculation capacity. As a coagulant aid, it has better advantages.

The use effect of anionic polyacrylamide is related to its "degree of hydrolysis". Too small "degree of hydrolysis" will lead to poor coagulation or coagulation aid effect, and too high "degree of hydrolysis" will increase the production cost.

14. What is the degree of hydrolysis of anionic polyacrylamide?

The "degree of hydrolysis" of anionic polyacrylamide is the percentage of amide group in PAM molecule converted to carboxyl group during hydrolysis. However, due to the difficulty in determining the carboxyl number, the "hydrolysis ratio" is often used in practical application, that is, the weight ratio of sodium hydroxide dosage to PAM dosage during hydrolysis.

If the hydrolysis ratio is too large, the cost of adding alkali is high, and if the hydrolysis ratio is too small, the reaction will be insufficient and the coagulation or coagulation aid effect of anionic polyacrylamide will be poor. Generally, the hydrolysis ratio is controlled at about 20% and the hydrolysis time is controlled at 2 ~ 4H.

15. What are the factors affecting the use of flocculant?

(1) PH value of water

The pH value of water has a great influence on the use effect of inorganic flocculant. The pH value is related to the type, dosage and coagulation sedimentation effect of flocculant. H + and oh - in water participate in the hydrolysis reaction of flocculant. Therefore, pH value strongly affects the hydrolysis speed of flocculant, the existing form and performance of hydrolysate.

Taking the aluminum salt that realizes coagulation by generating Al (OH) 3 charged colloid as an example, when pH value is less than 4, Al3 + can not be hydrolyzed into Al (OH) 3 in a large amount, mainly exists in the form of Al3 + ions, and the coagulation effect is very poor. When the pH value is between 6.5 and 7.5, Al3 + is hydrolyzed and polymerized into Al (OH) 3 neutral colloid with high degree of polymerization, and the coagulation effect is good. When pH > 8, Al3 + is hydrolyzed to AlO2 -, and the coagulation effect becomes very poor.

The alkalinity of water can buffer the pH value. When the alkalinity is not enough, lime and other agents should be added to supplement it. When the pH value of water is high, add acid to adjust the pH value to neutral. In contrast, the polymer flocculant is less affected by pH value.

(2) Water temperature

Water temperature affects the hydrolysis rate of flocculant and the rate and structure of alum formation. The hydrolysis of coagulation is mostly endothermic reaction. When the water temperature is low, the hydrolysis speed is slow and incomplete.

At low temperature, the viscosity of water is large, the Brownian motion is weakened, the collision times between flocculant colloidal particles and impurity particles in water are reduced, and the shear force of water increases, which hinders the mutual adhesion of coagulation flocs; Therefore, although the dosage of flocculant is increased, the formation of flocs is still very slow, and the structure is loose and the particles are small, which is difficult to remove.

Low temperature has little effect on polymer flocculant. However, it should be noted that when using organic polymer flocculant, the water temperature should not be too high. High temperature is easy to aging and even decompose the organic polymer flocculant into insoluble substances, so as to reduce the coagulation effect.

(3) Impurity composition in water

The uneven size of impurity particles in water is beneficial to coagulation, and small and uniform will lead to poor coagulation effect. Low impurity particle concentration is often unfavorable to coagulation. At this time, reflux sediment or coagulant aid can improve the coagulation effect. When the impurity particles in the water contain a large amount of organic matter, the coagulation effect will become worse. It is necessary to increase the dosage or add oxidant and other agents that play the role of coagulation aid. Calcium and magnesium ions, sulfides and phosphates in water are generally beneficial to coagulation, while some anions and surfactant have adverse effects on coagulation.

(4) Flocculant type

The choice of flocculant mainly depends on the properties and concentration of colloid and suspended solids in water. If the pollutants in the water are mainly in colloidal state, inorganic flocculants should be preferred to destabilize and coagulate them. If the flocs are small, polymer flocculants or coagulant aids such as activated silica gel need to be added.

In many cases, the combination of inorganic flocculant and polymer flocculant can significantly improve the coagulation effect and expand the application range. For polymers, the greater the amount of charge on chain molecules, the higher the charge density, the more fully extended the chain, the greater the scope of adsorption bridging, and the better the coagulation effect.

(5) Flocculant dosage

When using coagulation method to treat any wastewater, there is the best flocculant and the best dosage, which are usually determined by experiments. Excessive dosage may cause colloid re stabilization. Generally, the dosage range of ordinary iron salt and aluminum salt is 10 ~ 100mg / L, the dosage of polymeric salt is 1 / 2 ~ 1 / 3 of that of ordinary salt, and the dosage range of organic polymer flocculant is 1 ~ 5mg / L.

(6) Flocculant dosing sequence

When a variety of flocculants are used, the best dosing sequence needs to be determined through experiments. Generally speaking, when inorganic flocculant and organic flocculant are used together, inorganic flocculant shall be added first, and then organic flocculant shall be added.

The particle size of treated impurities is 50 μ When it is more than m, organic flocculant is often added first for adsorption and bridging, and then inorganic flocculant is added to compress the electric double layer to destabilize the colloid.

(7) Hydraulic conditions

In the mixing stage, the flocculant and water are required to be mixed quickly and evenly. In the reaction stage, it is necessary to create sufficient collision opportunities and good adsorption conditions to allow sufficient growth opportunities for the flocs, and prevent the generated small flocs from being broken. Therefore, the stirring intensity should be gradually reduced and the reaction time should be long enough.

16. What are the types of natural organic polymer flocculants?

The application of natural organic polymer flocculant in water treatment has a long history. Until today, natural polymer compound is still an important flocculant, but the use amount is far lower than that of synthetic polymer flocculant. The reason is that the charge density of natural polymer flocculant is small, the molecular weight is low, and it is prone to biodegradation and lose flocculation activity.

Compared with synthetic flocculants, natural organic polymer flocculants have less toxicity and simple extraction process. Both chemical composition and production process can be in harmony with nature. Therefore, the research and utilization of these natural resources as water treatment agents has become a hot spot, which is consistent with the global emphasis on rational utilization of resources, The situation of protecting and improving the environment is inseparable.

At present, there are many kinds of natural polymer flocculants. According to their main natural components (including matrix components used for modification), they can be divided into chitosan flocculants, modified starch flocculants, modified cellulose flocculants, lignin flocculants, gum flocculants, alginate flocculants, animal glue and gelatin flocculants.

Most of these natural polymers have polysaccharide structure, in which there is only one monosaccharide structure in the starch main chain, which belongs to the same polysaccharide; Chitosan, gum and algin contain a variety of monosaccharide structures and belong to heteropolysaccharides; Lignin is a special aromatic natural polymer; Animal glue and gelatin belong to protein substances.

17. What should be paid attention to when using polymer organic flocculant?

Organic polymer flocculants are long-chain macromolecules with coil structure. They must undergo a swelling process in water. Solid products or high concentration liquid products must be prepared into aqueous solution before use and then added to the water to be treated.

The chemical dissolving tank for preparing aqueous solution must be equipped with mechanical mixing equipment, and the continuous mixing time of chemical dissolving shall be controlled above 30min. The concentration of aqueous solution is generally about 0.1%. If it is higher, the viscosity of solution will increase, and it is difficult to add. If it is lower, the volume of solution pool will be too large. The water used for drug dissolution should avoid containing a large amount of suspended solids as far as possible, so as to avoid the flocculation reaction between organic polymer flocculant and these suspended solids to form alum, which will affect the use effect after dosing.

When dissolving solid organic polymer flocculant, the dosing point of solid particles must be at the place with the strongest turbulence of water flow. At the same time, it must be slowly injected into the dissolving tank with the minimum dosage to disperse the solid particles into the water, so as to prevent the solid dosage from dispersing too quickly in the water and bonding with each other to form agglomerates. The structure of agglomerates is that there are solid particles inside Some hydrolysates are surrounded by the outside. Once such a mass is formed, it often takes a long time to dissolve evenly into the water. It can even exist for several days in the continuous drug dissolving tank.

The dosing point of solid particles must be far away from the mixing shaft of the mechanical agitator, because the mixing shaft is usually the place with the worst turbulence of water flow in the drug dissolving tank. The organic polymer flocculant with insufficient dissolution will often adhere to the shaft and accumulate day by day, sometimes forming a considerable agglomeration. If it is not cleaned in time and carefully, the agglomeration will become larger and larger, The scope of influence is becoming larger and larger.

As a coagulant aid, generally, inorganic flocculant shall be added to the treated water for compression and electric double layer destabilization, and then organic polymer flocculant shall be added to realize bridging. Under the condition of sufficient dosage of inorganic flocculant, the coagulation aid effect of organic polymer flocculant will not be greatly different due to the difference of dosage. Therefore, as a coagulant aid, the dosage of organic polymer flocculant is generally 0.1mg/l.

Solid organic polymer flocculant is easy to absorb water and deliquesce into blocks. It must be packaged with waterproof packaging, and the storage place must be dry to avoid outdoor storage.

18. What are the types of microbial flocculants?

Microbial flocculants are significantly different from traditional inorganic or organic flocculants. They either directly use microbial cells, or use microbial cell wall extracts, metabolites and so on.

The former is the main aspect of Microbial Flocculant Research. So far, more than 17 kinds of microorganisms with flocculation performance have been found, including mold, bacteria, actinomycetes and yeast. The latter is similar to organic flocculant. Microbial flocculant has many advantages over traditional inorganic or organic flocculants, such as no secondary pollution and low production cost.

The flocculation performance of microbial flocculant is affected by many factors. The internal factors include the inheritance and expression of flocculation gene, while the external factors include the composition of microbial culture medium, the change of cell surface hydrophobicity, the existence of divalent metal ions in the environment and so on.

At present, there are microbial flocculants with good performance abroad, such as noc-1 produced in Japan. The key problem of microbial flocculant from research to production is to develop mature microbial breeding technology and strive to reduce production cost. The development of microbial flocculant in China is moving in this direction, but there is still a certain distance from industrial production.

19. How to determine the type and dosage of flocculant?

The selection and dosage of flocculant shall be determined through technical and economic comparison according to the operation experience of water plants under similar conditions or the results of raw water coagulation and sedimentation test, combined with the local reagent supply. The principle of selection is that the price is cheap and easy to obtain, the water purification effect is good, the use is convenient, the generated floc is dense, the sedimentation is fast, and it is easy to separate from water.

The purpose of coagulation is to generate large flocs. Due to many influencing factors, the corresponding data are generally obtained through coagulation beaker stirring test. The coagulation test is carried out in a beaker, including three steps: rapid mixing, slow mixing and static settlement.

After rapid stirring, the flocculant dispersed rapidly and contacted with the colloidal particles in the water sample, and the colloidal particles began to agglomerate and produce micro flocs; Through slow stirring, the micro flocs further contact each other and grow into larger particles; After stopping stirring, the formed colloidal particle aggregate naturally settles to the bottom of the beaker by gravity. Through the comprehensive evaluation of coagulation effect, such as flocculant sedimentation, supernatant turbidity, chromaticity, pH value, oxygen consumption, etc., determine the appropriate flocculant variety and its optimal dosage.

The six unit mixer is used for the test. It has six vertically movable rotating shafts, and its bottom position is equipped with mixing blades with a blade size of 6cm × 2cm。

The rotation speed and rotation time of the rotating shaft can be preset and can work automatically. The general test shall be carried out by rapid stirring for 2min, n = 300r / min; Slow stirring for 3min, n = 60R / min. During the test, 1L raw water was added into 6 1000ml beakers, placed directly under the six rotating shafts, and the rotating shaft was moved down to the bottom; Then, different quantities of liquid medicine are added into six small glass beakers connected to a horizontal rotating shaft. When the horizontal shaft is rotated, the liquid medicine in the small tube is poured into the corresponding raw water at the same time. Then start the agitator to make it work automatically.

After the stirring stops automatically, slowly pull up the blade from the beaker and stand for 20min. Use a pipette to suck 25ml of water sample from about 10cm below the water surface, and measure the turbidity of the supernatant with a turbidimeter. Taking the dosage as the abscissa and the residual turbidity of the supernatant as the ordinate, draw a curve to compare the effects of different flocculants. According to the turbidity removal effect and comprehensive technical and economic factors, the flocculant for treating this wastewater is selected and determined.

The beaker stirring test method can be divided into single factor test and multi factor test. During the test, the raw water used shall be exactly the same as the actual water quality. At the same time, the type, dosage and dosing sequence of flocculant shall be determined according to the pH value and impurity properties of the water. Moreover, the test shall be the simulation of the actual process, and the hydraulic conditions (mainly GT value) of the two must be the same or close.

three

Coagulant aid

1. What is coagulant aid? What is its function?

In the coagulation treatment of wastewater, sometimes using a single flocculant can not achieve good coagulation effect. It is often necessary to add some auxiliary agents to improve the coagulation effect. This auxiliary agent is called coagulant aid. Common coagulant aids include chlorine, lime, activated silicic acid, bone glue and sodium alginate, activated carbon and various clays.

Some coagulant aids do not play the role of coagulation, but play the role of auxiliary flocculant to produce coagulation effect by adjusting and improving coagulation conditions. Some coagulant aids participate in the formation of flocs, improve the structure of flocs, and turn the small loose flocs produced by inorganic flocculants into coarse and compact alum.

2. What are the types of common coagulant aids?

There are many kinds of coagulant aids, but according to their role in the coagulation process, they can be roughly divided into the following two categories:

(1) Agent for regulating or improving coagulation conditions

The coagulation process should be carried out within a certain pH value range. If the pH value of raw water cannot meet this requirement, the pH value of raw water should be adjusted. This kind of coagulant aid includes acid and alkali. When the pH value of raw water is low and the alkalinity is insufficient, which makes it difficult to hydrolyze the flocculant, alkaline substances such as Cao, Ca (OH) 2, Na2CO3 and NaHCO3 can be added (lime is commonly used); When the pH value is high, sulfuric acid or CO2 is often used to reduce the pH value of raw water.

For wastewater with large content of dissolved organic matter, oxidants such as Cl2 can be used to destroy organic matter and improve the removal effect of dissolved organic matter. In addition, when ferrous salt is used as flocculant, chlorine can be used to oxidize ferrous (Fe2 +) into high valent iron (Fe3 +) to improve the coagulation effect.

The above alkali agent, sulfuric acid, CO2 and chlorine do not play the role of coagulation, but only play the role of auxiliary coagulation.

(2) Coagulant aid for increasing alum particle size, density and firmness

The result of coagulation requires the generation of alum flowers with large particle size, high density and firmness, which is not only conducive to sedimentation, but also not easy to break. In order to obtain this result, combined with the characteristics of water quality, sometimes a substance or agent must be added to the water.

For example, in the low turbidity wastewater containing light impurities that are not suitable for sedimentation, adding coarse particles such as silica, activated carbon and clay or returning some precipitated sludge can aggravate and increase alum; When aluminum salt and iron salt are used as flocculants, only small and loose flocs can be produced, high molecular coagulant aids such as polyacrylamide, activated silicic acid and bone glue can be added, and their strong adsorption and bridging effect can be used to make the small and loose flocs coarse and dense.

3. What are the applications of flocculant and coagulant aid in enhanced wastewater treatment?

Adding flocculant in wastewater treatment can not only accelerate the aggregation and sedimentation of solid particles in wastewater, but also remove part of dissolved organic matter.

This method has the advantages of less investment, simple operation and flexibility. It is especially suitable for treating wastewater with small water volume and large suspended impurity content. When inorganic flocculant is used, because the dosage is large and the amount of sludge is large, synthetic organic polymer flocculant OPF or the combination of inorganic flocculant and OPF is mainly used in practical application.

It is reported that anionic hydrolyzed polyacrylamide is often used to remove suspended impurities in wastewater in primary sedimentation tank, but the effect of non-ionic polyacrylamide (PAM) is not good. Experience shows that adding 1mg / L hydrolyzed polyacrylamide in the primary sedimentation tank can remove more than 50% of suspended particles and more than 40% of BOD5 in the incoming wastewater.

In the primary precipitation treatment of wastewater, the mixed use of organic polymer polyelectrolyte and inorganic flocculant has better effect than their separate use. Because the concentration, particle size distribution and types of suspended particles in the incoming wastewater will change at any time, the optimal dosage of flocculant is sometimes difficult to control.

At this time, if excessive inorganic flocculant is added and the coil sweeping mechanism is used to precipitate and remove suspended impurities, although the method is feasible, its disadvantages are also very prominent. First, the action time is relatively long (15 ~ 30min), and then the formed flocs are easy to break. If a certain amount of organic polymer polyelectrolyte is added at the same time of adding inorganic flocculant, the flocculation time can be reduced to 2 ~ 5min, and the flocs formed are relatively strong.

When removing colored organic colloidal impurities in water by precipitation method, double electrolyte system can be used. Firstly, cationic polyelectrolytes with high positive charge are used to destabilize these organic colloids, and then large molecular weight non-ionic or anionic polyelectrolytes are used to flocculate the destabilized organic colloids into flocs that are easy to precipitate.

Cationic polyelectrolyte is often used as flocculant in secondary sedimentation tank, such as polydimethylhexylene ammonium chloride or polyaminomethyldimethylhexylene ammonium chloride, but its dosage is less than that in primary sedimentation tank. The reason is that part of the anionic polyelectrolyte added in the primary sedimentation tank continues to play a role after entering the secondary sedimentation tank, and the polyelectrolyte added in the secondary sedimentation tank can be reused in sludge reflux.

In addition, coagulation treatment can also remove phosphate and heavy metal ions from wastewater. For a long time, people have been using the method of adding metal salt inorganic flocculant to remove part of phosphate in wastewater.

However, experiments show that under the premise of ensuring that the removal rate of phosphate is not reduced, the same phosphorus removal effect can be achieved by using cationic polymer instead of inorganic flocculant, which shows that the polymer is involved in the adsorption of anionic phosphate.

For example, in the coagulation treatment process of a wastewater treatment plant, 12mg / L ferric sulfate, 3mg / L cationic polymer with high charge density and 0.2mg/l anionic polymer with high molecular weight were used to replace the original 23mg / L ferric sulfate. Under the condition that the phosphorus removal rate remained unchanged, the BOD5 removal rate of effluent increased from 30% to 55%.

At the same time, after coagulation treatment, the inorganic components in the sludge produced in the activated sludge stage can be reduced and the biodegradation function of activated sludge can be improved.

In the filtration, flotation and other treatment processes used in wastewater treatment, the effluent quality can be improved by using inorganic flocculant and polyelectrolyte coagulant aid. Combined with the characteristics of wastewater quality, the flocculant can be used alone, a variety of flocculants can be used in combination or one main and one auxiliary (the auxiliary is used as the coagulant aid). The selection of flocculant can be preliminarily screened through beaker static test, and then verified and determined on the production unit.

four

Sludge conditioner

1. What are the types of commonly used sludge conditioner?

Conditioning agent, also known as dehydrating agent, can be divided into inorganic conditioning agent and organic conditioning agent. Inorganic conditioner is generally suitable for vacuum filtration and plate frame filtration of sludge, while organic conditioner is suitable for centrifugal dewatering and belt pressure filtration dewatering of sludge.

(1) Inorganic conditioner

The most effective, cheapest and most commonly used inorganic conditioner mainly includes iron salt and aluminum salt. Iron salt conditioner mainly includes ferric chloride (FeCl3 ? 6H2O), ferric sulfate (Fe2 (SO4) 3 ? 4H2O), ferrous sulfate (FeSO4 ? 7H2O) and polymeric ferric sulfate (PFS) ([Fe2 (OH) n (SO4) 3-N / 2] m). Aluminum salt conditioner mainly includes aluminum sulfate (Al2 (SO4) 3 ? 18H2O), aluminum trichloride (AlCl3), basic aluminum chloride (Al (OH) 2Cl), polyaluminium chloride (PAC) ([Al2 (OH) n ? cl6-n] m) Wait.

Adding inorganic conditioner can greatly accelerate the sludge concentration process and improve the filtration and dehydration effect. Moreover, the combination of iron salt and lime can further improve the conditioning effect. The disadvantage of adding inorganic conditioner is that the dosage is large. Generally speaking, the dosage should reach 5% ~ 20% of the dry solid weight of sludge, resulting in the increase of filter cake volume; Second, the inorganic conditioner itself is corrosive (especially iron salt), and the dosing system should have anti-corrosion performance.

It should be noted that when ferric chloride is used as the conditioner, it will increase the corrosivity to the metal components of the dewatered sludge treatment equipment. Therefore, the anti-corrosion grade of the equipped dewatered sludge treatment equipment should be appropriately improved.

(2) Organic conditioner

There are many kinds of organic synthetic polymer modifiers. According to the degree of polymerization, they can be divided into oligomeric degree (molecular weight is about 1000 ~ tens of thousands) and high degree of polymerization (molecular weight is about hundreds of thousands ~ millions); According to the ionic type, it can be divided into cationic type, anionic type, non-ionic type, anionic type, etc. Compared with inorganic conditioner, the dosage of organic conditioner is less, generally 0.1% ~ 0.5% of the dry solid weight of sludge, and it is not corrosive.

The organic conditioner used for sludge conditioning is mainly the flocculant products of polyacrylamide series with high polymerization degree, mainly including cationic polyacrylamide, anionic polyacrylamide and non-ionic polyacrylamide.

Cationic polyacrylamide can neutralize the negative charge on the surface of sludge particles and bridge between particles, showing strong cohesion. The conditioning effect is remarkable, but the cost is high. In order to reduce the cost, the cheaper anionic polyacrylamide lime combination method can be used to adsorb the negatively charged flocculant and sludge particles together with positively charged Ca (OH) 2 flocs to form a composite coagulation system.

2. What factors should be considered when selecting sludge conditioner?

(1) Variety characteristics of conditioner

As for the commonly used aluminum salt and iron salt inorganic conditioner, when using aluminum salt, the dosage of agent is large, the floc density is small, the conditioning effect is poor, and the filter cloth will be blocked in the dehydration process.

Therefore, when selecting inorganic conditioner, iron salt should be used as much as possible; When the use of iron salt will bring many problems, consider the use of aluminum salt. Compared with organic conditioner, the dosage of inorganic conditioner is larger, the floc particles formed are small, but the floc strength is higher. Therefore, when using vacuum filter and plate and frame filter press to dehydrate sludge, inorganic conditioner can be considered.

Compared with inorganic conditioner, the dosage of organic conditioner is small and the flocs formed are thick, but the flocs have low strength and are easier to break than those formed by inorganic conditioner. Moreover, once the floc is destroyed, it is not easy to restore to the original state whether inorganic conditioner or organic conditioner is used.

Therefore, when using centrifugal dehydrator and belt filter press to dehydrate sludge, organic conditioner can be considered. When it is difficult to achieve the ideal conditioning effect by using one of the inorganic conditioner or organic conditioner, the combination of inorganic and organic conditioner can be considered, and sometimes better conditioning effect can be achieved. For example, the combined use of lime and ferric chloride can not only adjust the pH value, but also increase the porosity of sludge and promote the separation of sludge and water.

(2) Sludge properties

There are also great differences in the type and dosage of conditioner for sludge with different properties. For sludge with high organic content, the more effective conditioner is cationic organic polymer conditioner, and the higher the organic content, the more suitable cationic organic polymer conditioner with higher degree of polymerization.

For sludge dominated by inorganic matter, anionic organic polymer conditioner can be considered. Different sludge properties directly affect the conditioning effect: the sludge in the primary sedimentation tank is easy to dehydrate, while the scum and residual activated sludge are difficult to dehydrate, and the dehydration performance of mixed sludge is between the two.

In order to achieve a certain conditioning effect, there are significant differences in the amount of conditioning agents required. Generally speaking, the more difficult the sludge is to be dewatered, the larger the dosage of conditioning agent is, and the smaller the sludge particles will lead to the increase of conditioner consumption. The high content of organic matter and alkalinity in the sludge will also lead to the increase of conditioner consumption. In addition, the sludge solid content also affects the dosage of conditioner. Generally, the higher the sludge solid content, the greater the dosage of conditioner.

(3) Temperature

The temperature of sludge directly affects the hydrolysis of inorganic salt conditioner. When the temperature is low, the hydrolysis will slow down. If the temperature is lower than 10oC, the conditioning effect will be significantly worse. The conditioning effect can be improved by appropriately prolonging the conditioning time.

When using organic polymer conditioner, if the temperature of mother liquor or tap water or sludge is too low, the dynamic viscosity of water and the viscosity of polymer conditioner solution itself will become larger, which is not conducive to uniform dilution and conditioning mixing, and then affect the sludge conditioning effect and dehydration effect.

Therefore, when the temperature is low in winter, attention should be paid to the thermal insulation link of the sludge conveying system (the temperature of sludge discharged from the sewage treatment system is generally not lower than 15oC) to minimize the heat loss in the process of sludge conveying. If necessary, the dissolution conditions can be improved by heating the organic polymer conditioner dilution tank or appropriately prolonging the mixing and dissolution time and increasing the stirring strength.

(4) PH value

The pH value of sludge determines the hydrolysate form of inorganic salt conditioner, and the conditioning effect of the same conditioner on sludge with different pH values is also very different. The hydrolysis reaction of aluminum salt is greatly affected by pH value, and the optimum pH value range of coagulation reaction is 5 ~ 7. When the pH value is greater than 8 or less than 4, it is difficult to form flocs, that is, it loses the role of conditioning.

The ferrate conditioner is less affected by pH value. Whether the sludge is acidic or alkaline, it can form Fe (OH) 3 flocs of hydrolysate, and the optimal pH value range is 6 ~ 11. In the sludge with pH value of 8 ~ 10, the hydrolysate with high solubility of ferrous salt can be oxidized into Fe (OH) 3 flocs with low solubility.

Therefore, when selecting inorganic salt conditioner, first consider the specific pH value of dewatered sludge. If the pH value deviates from the optimal range of coagulation reaction, it is best to replace another conditioner. Otherwise, it is necessary to consider adding acid or alkali to adjust the pH value of sludge before conditioning the sludge. Generally, such measures are not taken.

PH value also affects the conditioning effect of polyelectrolyte. The pH value of sludge affects the ionization, charging state and molecular shape of conditioner molecules.

The ionization degree of cationic polyelectrolyte in acid sludge with low pH value is large, and the molecular shape tends to stretch; In the alkaline sludge with high pH value, the ionization degree is small and the molecular shape tends to curl. Contrary to the properties of cationic polyelectrolytes, anionic polyelectrolytes have smaller ionization degree and molecular shape tends to curl in acid sludge with low pH value; In the alkaline sludge with high pH value, the ionization degree is large and the molecular shape tends to stretch.

The situation of anionic and anionic polyelectrolytes is slightly different. At the isoelectric point, the whole molecule is neutral, and the positive and negative charges attract each other, so the molecules are tightly curled into clusters. On both sides of the isoelectric point, there will be an excess charge on the molecule, which tends to stretch due to mutual repulsion.

(5) Preparation concentration

The preparation concentration of conditioner not only affects the conditioning effect, but also affects the drug consumption and mud cake yield, especially the organic polymer conditioner. Generally speaking, the lower the preparation concentration of organic polymer conditioner, the less the reagent consumption, and the better the conditioning effect.

This is because the lower the preparation concentration of organic polymer conditioner, the easier it is to mix evenly, the better the extension of molecular chain, the better the bridging and coagulation, and the better the conditioning effect. However, too high or too low preparation concentration will reduce the yield of mud cake.

The conditioning effect of inorganic polymer conditioner is hardly affected by the preparation concentration. Experience and relevant research show that the preparation concentration of organic polymer conditioner is between 0.05% ~ 0.1%, the preparation concentration of ferric chloride is 10%, and the preparation concentration of aluminum salt is 4% ~ 5%.

(6) Dosing sequence

When more than one conditioner is used, the order of adding the conditioner will also affect the conditioning effect. When iron salt and lime are used as conditioner, iron salt is generally added first and then lime. In this way, the flocs formed are easier to separate from water, and the total consumption of conditioner is less.

When inorganic conditioner and organic polymer conditioner are used to regulate sludge, inorganic conditioner is added first, and then organic polymer conditioner is added, which can generally achieve better conditioning effect.

(7) Mixed reaction conditions

In order to achieve the best conditioning effect, it is very necessary to fully mix sludge and conditioner. However, it is worth noting that the flocs formed by the mixed reaction of sludge and conditioner must not be damaged again, because once the flocs are damaged, it is difficult to return to the original state.

Experience shows that for a certain sludge, using a certain conditioner, only when the intensity and time of mixed reaction are within a certain range, can a better conditioning effect be obtained, and the conditioning effect will decrease with the increase of residence time.

That is to say, after determining the conditioning time and intensity through the test, it must be strictly observed in the actual operation. On the one hand, it is not allowed to extend or shorten the mixing reaction time at will. On the other hand, it is necessary to make the conditioned sludge enter the dehydrator as soon as possible.

3. How to determine the dosage of conditioner?

There is no fixed standard for the reagent consumption of sludge conditioning. The dosage will vary according to the specific properties such as sludge variety, digestion degree and solid concentration. Therefore, the type and specific dosage of conditioner are mostly determined by direct test in the laboratory or on site.

Generally speaking, according to the percentage of dry solid weight of sludge, the dosage of ferric chloride is 5% ~ 10%, ferrous sulfate is about 10% ~ 15%, hydrated lime is 20% ~ 40%, polyaluminium chloride and polyferric sulfate are about 1% ~ 3%, and cationic polyacrylamide is 0.1% ~ 0.3%.

According to relevant information, because the price of commonly used polyacrylamide series organic synthetic polymer conditioner is relatively expensive (some varieties are more than ten or even more than 20 times that of ordinary inorganic conditioner), although its dosage is small, it is equivalent to the cost of conditioning per ton of sludge, so the cost of using organic synthetic polymer conditioner is still high.

The common practice is to select inorganic conditioner. When the effect of inorganic conditioner is poor and it is difficult to achieve the ideal conditioning effect, consider using organic synthetic polymer conditioner or combining inorganic and organic conditioner.

4. What are the precautions for using conditioner?

In order to better use the conditioner, the following matters should be paid attention to:

Fully understand and master the properties (concentration, composition, etc.) of the treated sludge,

Test and determine the type of conditioner suitable for sludge properties and dehydrator properties,

Test and determine the injection point, reaction conditions and dosage of conditioner,

Determine the dissolution, storage and other use methods of the conditioner according to the properties of the conditioner.

Generally speaking, inorganic conditioner is suitable for vacuum filtration dehydration and plate and frame pressure filtration dehydration, while organic conditioner is more suitable for centrifugal dehydration and belt pressure filtration dehydration. When centrifugal dehydrators and belt pressure dehydrators are used, cationic series polymer conditioners with molecular weight of 100000 or even more are generally used in order to form coarse flocs that are not easy to be broken.

At the same time, it should be noted that since the centrifugal dehydrator performs solid-liquid separation under the high centrifugal force of 2000 ~ 3000g, the greater the molecular weight of polymer conditioner, the easier it is to form solid flocs and the more conducive it is to dehydration; For the belt filter press dehydrator, when the molecular weight is too high, part of the viscosity of the conditioner will remain on the flocs, resulting in poor stripping of the filter cake on the filter cloth. As for cationic conditioner, for the same sludge, compared with centrifugal dehydrator, belt pressure dehydrator requires higher cationic degree and less dosage of conditioner.

Generally speaking, when the sludge concentration is high, the effect of using high molecular weight conditioner is better, while when the sludge concentration is low, the effect of using low molecular weight conditioner is better.

The residual sludge produced by wastewater biological treatment has the same properties as the reflux sludge. Its main component is microbial flocculate, which generally has negative charge. Therefore, in order to agglomerate the residual sludge, it is best to use cationic conditioner.

At present, the most commonly used cationic modifier is the copolymer of polypropylamide or amino methylated denaturant. By adjusting the cationic denaturation conditions, the modifiers with different cationic degrees can be obtained. According to the different degree of cation (measured by colloidal titration), cationic modifiers can be divided into high, medium and low cationic modifiers.

5. What is the relationship between dehydrating agent, conditioner, flocculant and coagulant aid?

Dewatering agent is the agent added before sludge dewatering, that is, the sludge conditioner. Therefore, the meaning of dewatering agent and conditioner is the same. The dosage of dehydrating agent or conditioner is generally calculated as the percentage of dry solid weight of sludge.

Flocculant is an important agent in the field of water treatment. The dosage of flocculant is generally expressed by the dosage per unit volume of water to be treated.

The dosage of dehydrating agent (conditioner), flocculant and coagulant aid can be called dosage. The same agent can be used not only as flocculant in sewage treatment, but also as conditioner or dehydrating agent in excess sludge treatment.

Coagulant aid is called coagulant aid when it is used as an auxiliary agent of flocculant in the field of water treatment. The same coagulant aid is generally not called coagulant aid in excess sludge treatment, but collectively referred to as conditioner or dehydrating agent.

When using flocculant, due to the limited amount of suspended solids in the water, in order to achieve full contact between flocculant and suspended particles, mixing and reaction facilities need to be equipped with sufficient time. For example, mixing takes tens of seconds to minutes, and reaction takes 15 ~ 30min.

During sludge dewatering, the time from adding conditioner to sludge entering the dehydrator is often only tens of seconds, that is, only the mixing process of flocculant and no reaction time. Moreover, experience also shows that the conditioning effect will decrease with the extension of residence time.

five

Defoamer

1. What are the types of defoamers?

The effect of defoamer is related to the type of foaming solution, that is, some defoamers have a significant effect on some foaming solutions, but have no effect on other foaming solutions. Common defoamers can be divided into silicone (resin), surfactant, alkane hydrocarbon and mineral oil according to different components.

(1) Silicone (resin): silicone defoamer, also known as emulsion defoamer, is used by emulsifying and dispersing silicone resin with emulsifier (surfactant) in water and then adding it to wastewater. Silica fine powder is another silicon defoamer with good defoaming effect.

(2) surfactants: such defoamer is actually an emulsifier, that is, the dispersing effect of surfactants, so that the foam forming material can be dispersed in the stable emulsion state, so as to avoid foaming.

(3) Alkane: alkane defoamer is a defoamer made by emulsifying and dispersing alkane wax or its derivatives with emulsifier. Its purpose is similar to that of surfactant emulsified defoamer.

(4) Mineral oil: mineral oil is the main defoaming component. In order to improve the effect, it is sometimes mixed with metal soap, silicone oil, silica and other substances. In addition, various surfactants can be added sometimes in order to make the mineral oil easy to diffuse to the surface of the foaming liquid, or make the metal soap evenly dispersed in the mineral oil.

six

PH regulator

1. What are the common pH regulators?

When the pH value of acid containing wastewater is adjusted up, alkali or alkaline oxide is used as neutralizer, while when the pH value of alkaline wastewater is adjusted down, acid or acidic oxide is used as neutralizer. Neutralizing agents often used to adjust the pH value of acidic wastewater include lime, limestone, dolomite, sodium hydroxide, sodium carbonate, etc. sulfuric acid and hydrochloric acid are generally used to adjust the pH value of alkaline wastewater.

When neutralizing acid containing wastewater, some alkaline industrial waste residues can also be used nearby, such as calcium carbonate waste residue discharged from chemical soft water station, calcium carbide waste residue (mainly composed of calcium hydroxide) discharged from organic chemical plant or acetylene generation station, waste stone ash screened from steel plant or calcium carbide plant, furnace ash of thermal power plant and boron mud of boric acid plant.

When treating alkaline wastewater, flue gas can also be used to neutralize the alkali in wastewater by using CO2, SO2 and other acid gases.

When the pH value of wastewater is too large or too small, 40% NaOH and 98% H2SO4 can be used as pH value regulators for acid containing wastewater and alkali containing wastewater respectively in order to reduce the volume of drug dissolving tank and reagent tank required for pH value adjustment and realize the automatic control of pH value adjustment. At the same time, it can avoid the sludge problem caused by the use of lime alkali agent and reduce the opportunity of secondary pollution.

seven

disinfectant

1. What factors should be considered in the selection of disinfectants

After primary or secondary treatment, the water quality is improved and the bacterial content is greatly reduced, but its absolute value is still considerable and there is the possibility of pathogenic bacteria. Therefore, the wastewater should be disinfected before being discharged into the water body.

At present, it is well known that disinfection by chlorination can produce harmful substances and affect human health. This is because chlorine acts with organics in water and has oxidation and substitution at the same time. Oxidation can promote the removal of organics, while substitution is the combination of chlorine and organics to form halides with mutagenic or carcinogenic activity.

The maximum concentration of trihalomethane (THMs) in the United States is 100 μ G / L, which is also specified as 25 in Germany, Canada and Japan μ g/L、350 μ g/L、100 μ G / L, the upper limit of chloroform is also specified as 60 in the hygienic standard for drinking water in 1985 μ g/L。

In view of this, the first is to control the appropriate dosage of wastewater disinfection, and the second is to replace it with other disinfectants, such as chlorine dioxide, ozone and ultraviolet radiation, so as to reduce the generation of harmful substances.

See table 8-2 for the advantages, disadvantages and applicable conditions of various disinfectants. Referring to this table, the disinfectant that should be used can be preliminarily determined.

2. What are the types of disinfectants? What are their characteristics?

Commonly used disinfectants include hypochlorous acid, chlorine dioxide, ozone, ultraviolet radiation, etc. Hypochlorous acid disinfectants have the forms of liquid chlorine, bleaching powder, bleaching powder, chlorine tablets, sodium hypochlorite and so on. They mainly disinfect through HOCl.

The weakness of hypochlorous acid disinfectants is that they are easy to form chlorinated hydrocarbons with organic matter in water, which has been confirmed to be extremely harmful to human health. At the same time, the treated water will have some unpleasant smell.

Hypochlorous acid disinfectant dust and chlorine released have strong irritating effects on human respiratory tract, eyes and skin. If it accidentally splashes into eyes or touches skin, wash it with a large amount of clean water immediately. The storage environment shall be cool, ventilated and dry, away from heat source and fire, and shall not be stored and transported together with organic matter, acids and reducing agents. Rain and sunlight exposure shall be prevented during transportation, and the action shall be light during loading and unloading to avoid collision and rolling.

The substitution reaction often occurs in the disinfection of hypochlorous acid disinfectants, which is also the fundamental reason for the generation of chlorinated hydrocarbons when using hypochlorous acid disinfectants. The pure oxidation reaction occurs in the disinfection of ozone and chlorine dioxide, which can destroy the structure of organic matter, improve the biodegradability of wastewater (BOD5 / CODcr value) and remove part of CODCr in water while sterilization.

Compared with ozone or ultraviolet disinfection, chlorine dioxide disinfection has low one-time investment and high operation cost (about 0.1 yuan / m3); The latter has high one-time investment and low operation cost (about 0.02 yuan / m3).

Ozone disinfection and ultraviolet disinfection can achieve the disinfection effect in a very short time. The microbial indicators such as the total number of bacteria and total coliform group in the effluent or reuse water of the secondary sedimentation tank after ozone disinfection and ultraviolet disinfection can meet the requirements, but their disadvantage is instantaneous reaction, unable to maintain the effect and resist the breeding and reproduction of microorganisms in the pipeline, Therefore, when these two methods are used for disinfection in the reuse water system, it is often necessary to add 0.05 ~ 0.1mg/l chlorine dioxide or 0.3 ~ 0.5mg/l chlorine to the outlet water to maintain sufficient residual chlorine at the end of the pipe network.

3. What are the physicochemical properties of chlorine?

Chlorine is a yellow green gas under atmospheric pressure. At 0oC and one atmospheric pressure, the density is 3.2mg/ml, which is about 2.5 times the weight of air, and has strong irritation. Generally, chlorine is prepared by electrolysis of salt aqueous solution, and then the chlorine is pressurized and cooled to produce liquid chlorine. The liquid chlorine is very easy to gasify, and the boiling point is - 34.5oc.

The pressurized liquid chlorine becomes a yellow green transparent liquid, and the volume of 1kg liquid chlorine can be changed to 300L after gasification. Chlorine is very active and soluble in water. The solubility decreases with the increase of water temperature. Chlorine is a suffocating gas with strong irritation, which can cause harm to human respiratory system, eyes and skin. The maximum allowable concentration in the air is 1ml / m3.

Although it is not spontaneous combustion, it can support combustion. When mixed with other flammable gases in the sun, it will burn and explode, and can react with most substances.

Chlorine is a strong oxidant. It has the advantages of strong bactericidal ability, low price and convenient source. It is the disinfectant with the longest application history in the water treatment industry.

The mechanism of chlorine disinfection is that hypochlorite HOCl generated by hydrolysis diffuses into the cell wall of microorganisms and reacts with cell proteins to form N-Cl bonds with excellent chemical stability. In addition, chlorine can oxidize some active substances of microorganisms, inhibit and kill microorganisms.

4. How to prevent chlorine poisoning?

Although the maximum allowable concentration in the air is 1ml / m3, working in an environment below this value for a long time will also lead to chronic poisoning, manifested as chronic bronchitis, chronic gastroenteritis, gingivitis, stomatitis, skin pruritus and other diseases.

Short time exposure to high chlorine environment will lead to acute poisoning. Mild acute poisoning shows symptoms such as throat trunk chest tightness and accelerated pulse. Severe acute poisoning shows bronchospasm and edema, and even coma or shock. The measures to prevent chlorine poisoning can be summarized as follows:

(1) workers who are often exposed to chlorine will be less sensitive to chlorine smell, and people will be hurt by chlorine when they can't smell chlorine smell. Therefore, the duty room of the operator shall be set separately from the chlorination room, and a monitoring and alarm device shall be installed in the chlorination room to detect the chlorine concentration at any time.

(2) the chlorination room shall be close to the chlorination point, and the distance between them shall not exceed 30m. The building of chlorination room shall be solid, fire-proof, freeze-resistant and thermal insulation, well ventilated, the door shall be opened outside, and shall be strictly separated from other workshops without any direct connection. Because chlorine is heavier than air, when chlorine leaks indoors, it will crowd out the air, accumulate in the lower part of the closed room and gradually diffuse upward. Therefore, forced exhaust facilities must be installed at the bottom of the chlorination room, and the air inlet shall be set at a high place.

(3) maintenance tools, gas masks and rescue appliances shall be standby outside the chlorination room. The switches of lighting and ventilation equipment shall also be set outdoors. Ventilation shall be carried out before entering the chlorination room. The penstock leading to the chlorination room must ensure uninterrupted water supply, maintain stable water pressure, and take measures to deal with sudden water cut-off. An alkali liquor tank shall be set in the chlorination room and inspected regularly to ensure that the alkali liquor is effective at any time. When serious leakage is found in the chlorine bottle, wear a gas mask, and then quickly move the chlorine bottle into the alkali liquor pool.

(4) when acute chlorine poisoning is found on the site, try to quickly transfer the poisoned person to a place with fresh air. For those who have difficulty breathing, they should be allowed to breathe oxygen. Artificial respiration is strictly prohibited. They can wash their eyes, nose, mouth and other parts with 2% sodium bicarbonate solution, and they can also breathe atomized 5% sodium bicarbonate solution.

5. What are the precautions when using liquid chlorine bottle?

Liquid chlorine is the most widely used disinfectant at home and abroad. In addition to disinfection, it also has oxidation. Liquid chlorine is usually stored and transported in steel cylinders. When in use, liquid chlorine is transformed into chlorine and added to water.

(1) the pressure in the chlorine bottle is generally 0.6 ~ 0.8MPa, so it is not allowed to be exposed to the sun or close to the furnace fire or other high-temperature heat sources to avoid explosion due to too high pressure during gasification. Liquid chlorine and dry chlorine are not corrosive to metals such as copper, iron and steel, but when exposed to water or moisture, their chemical activity increases and can corrode most metals. Therefore, the chlorine storage cylinder must maintain a residual pressure of 0.05 ~ 0.1MPa and cannot be completely empty to avoid water inflow.

(2) heat is absorbed when liquid chlorine is changed into chlorine, and about 289kj heat is required to change 1kg liquid chlorine into 1kg chlorine. When the temperature is low, the heat absorbed by the chlorine bottle from the air is limited, and the amount of liquid chlorine gasification is limited, the chlorine bottle needs to be heated. However, do not directly heat the chlorine bottle with open fire and steam, and do not raise the temperature of the chlorine bottle too much or too fast. Generally, the chlorine bottle can be heated by continuously sprinkling the chlorine bottle with warm water of 15 ~ 25oC.

(3) check whether there is leakage at the connection between the chlorinator and the chlorine bottle with 10% ammonia frequently. If the chlorine gas pipe of the chlorinator is found to be blocked, it must not be washed with water. It can be dredged with steel wire, and then blow away the sundries with an air pump or compressed air.

(4) before opening, check whether the chlorine bottle is placed correctly, and ensure that the outlet faces upward, that is, chlorine gas rather than liquid chlorine is released. When opening the main valve of chlorine bottle, open it slowly for half a turn, then check whether there is air leakage with 10% ammonia, and then open it gradually after everything is normal. If the valve is difficult to open, do not knock it with a hammer or pull it hard with a long wrench to prevent the valve stem from being broken.

6. What are the precautions when using sodium hypochlorite for disinfection?

Solid sodium hypochlorite NaClO is a white powder with pungent smell. It is extremely unstable in the air and decomposes rapidly after heating. The available chlorine of commercial solid sodium hypochlorite is generally 10% ~ 12%. The common preparation method is liquid alkali chlorination, that is, chlorine is introduced into sodium hydroxide solution below 30% for reaction. Commercial solid sodium hypochlorite is easy to use, but the disinfection effect is worse than chlorine, and the cost is also higher than chlorine disinfection.

Since sodium hypochlorite is easy to decompose due to the action of sunlight and temperature, it is generally added immediately after on-site preparation with sodium hypochlorite generator. Salt water is electrolyzed by titanium anode (seawater can be used as salt solution in coastal areas). The obtained sodium hypochlorite solution is a light yellow transparent liquid containing 6G / L ~ 11g / L effective chlorine. Generally, for every 1kg of available chlorine produced, the consumption of table salt is about 3 ~ 4.5kg and the power consumption is 5 ~ 10kwh. Generally, the consumption is lower than that of disinfection with bleaching powder.

Sodium hypochlorite solid or solution should not be stored for a long time, and must be stored in a dark and low temperature environment. The best way to produce sodium hypochlorite solution by electrolysis is how much to use and how much to produce at any time. When the temperature is lower than 30oC, the loss of available chlorine is 0.1 ~ 0.15mg/l per day. If the temperature exceeds 30oC, the loss of available chlorine can reach 0.3 ~ 0.7mg/l per day. Therefore, in order to have a certain reserve for standby, the storage time is generally no more than one day in summer and no more than 7 days in winter.

7. What are the precautions when using bleaching powder for disinfection?

Bleaching powder CaCl2 ? Ca (OCL) 2 ? 2H2O is white powder with chlorine smell and contains 20% ~ 25% of available chlorine. Bleach is easy to absorb moisture and its properties are extremely unstable. Sunlight and heat can deteriorate it and reduce the effective chlorine content.

Mixing with organic matter and flammable liquid can generate heat and spontaneous combustion, and explosion will occur under high heat. Chlorine tablets are tablets processed with bleaching powder 3CA (OCL) 2 ? 2ca (OH) 2 ? 2H2O, containing 60% ~ 70% of available chlorine. Chlorine tablets and bleaching powder are more stable than bleaching powder, and can be stored at room temperature for more than 200 days without decomposition. The disinfection effect of both is the same as that of chlorine. The chlorine dosage, contact time and other parameters calculated by effective chlorine can refer to liquid chlorine.

When using bleaching powder as disinfectant, it needs to be filled with solution, and generally a mixing tank needs to be set. Add 400 ~ 500kg water to each pack of 50kg bleaching powder, stir it into a 10% ~ 15% solution, and then add water to make a 1% ~ 2% solution. There are usually two types of mixing tank: diaphragm type and blast type. When disinfecting with chlorine tablets, the waste water flows into a special chlorine tablet sterilizer, infiltrates and dissolves the chlorine tablets, mixes with them, and then enters the contact tank.

8. What are the physicochemical properties of chlorine dioxide?

Chlorine dioxide (ClO2) is a yellow green gas with extremely unstable properties. It has the same pungent smell as chlorine. It is more toxic than chlorine, and the relative density is 2.4. Chlorine dioxide can be compressed into liquid at room temperature and is easy to volatilize.

Chlorine dioxide is easy to explode. It may explode when the temperature rises, exposed to light or in contact with some organic substances, and liquid chlorine dioxide is more explosive than gas chlorine dioxide. Explosion will occur when the volume concentration in the air exceeds 10% or the concentration of chlorine dioxide in the water exceeds 30%.

Chlorine dioxide is easily soluble in water, and its solubility in water is 5 times that of chlorine, but ClO2 does not react chemically with water, is very volatile in water, and is prone to photochemical decomposition under light irradiation. The ClO2 content of ClO2 aqueous solution stored in an open container will decrease rapidly. Therefore, chlorine dioxide should not be stored, and it is best to prepare and use it on site.

The commercial stable chlorine dioxide solution sold on the market is mostly colorless or slightly yellow green transparent liquid. The chlorine dioxide content is generally about 2%, and a certain amount of special stabilizer (such as aqueous solution of sodium carbonate, sodium borate and perchloride or diethylenetriamine pentamethylene phosphonic acid, etc.) should be added. However, attention should be paid to avoiding high temperature and strong light during transportation and storage. Therefore, when chlorine dioxide is used for disinfection, it is best to use it while producing on site. Chlorine dioxide generates non-toxic substances after sterilization, which does not pollute the environmental water body.

9. What are the precautions when using chlorine dioxide?

(1) in water treatment, the dosage of chlorine dioxide is generally 0.1 ~ 1.5mg/l, and the specific dosage depends on the nature of raw water and the purpose of dosing. When only used as disinfectant, the dosage range is 0.1 ~ 1.3mg/l; When it is also used as deodorizer, the dosage range is 0.6 ~ 1.3mg/l; When it is also used as oxidant to remove iron, manganese and organic matter, the dosage range is 1 ~ 1.5mg/l.

(2) chlorine dioxide is a strong oxidant. It shall be transported and stored with anti-corrosion and anti-oxidation inert materials. It shall avoid contact with reducing agent to avoid explosion.

(3) when adopting the method of preparing chlorine dioxide on site, it is necessary to prevent the explosion caused by the excessive accumulation concentration of chlorine dioxide in the air. Generally, measures shall be taken to collect and neutralize the gas precipitated or leaked during the preparation of chlorine dioxide.

(4) ventilation devices, monitoring and alarm devices shall be provided in the work area and finished product storage room, and protective articles shall be provided outside the door.

(5) the stable chlorine dioxide solution itself has no toxicity, and chlorine dioxide can be released only after activation. Therefore, the reaction intensity shall be controlled during activation to avoid explosion caused by excessive accumulation concentration of chlorine dioxide in the air.

(6) chlorine dioxide solution shall be sealed in dark plastic barrels and stored in a cool and ventilated place to avoid direct sunlight and contact with air. During transportation, attention shall be paid to avoid high temperature and strong light environment and be as stable as possible.

10. What are the preparation methods of chlorine dioxide?

There are many preparation methods of chlorine dioxide. In the water treatment industry, chlorine, hydrochloric acid or dilute sulfuric acid are generally produced by reacting with sodium chlorite or sodium chlorate, and sodium hypochlorite is used to synthesize chlorine dioxide with sodium chlorite after acidification. The reaction formulas are as follows:

2NaClO3+2NaCl+2H2SO4→2ClO2+Cl2+2Na2SO4+2H2O

Cl2+2NaClO2→2ClO2+2NaCl

5NaClO2+4HCl→4ClO2+5NaCl+2H2O

10NaClO2+5H2SO4→8ClO2+5Na2SO4+4H2O+2HCl

NaClO+2HCl+2NaClO2→2ClO2+3NaCl+H2O

When using chlorine and sodium chlorite to synthesize chlorine dioxide, first prepare the chlorine aqueous solution with pH < 2.5, then enter the reaction chamber together with a certain amount of 10% sodium chlorite solution, and fully mix and react in the reaction chamber to generate chlorine dioxide. In theory, 7.5G chlorine dioxide can be generated by adding 3.9G chlorine to 10g sodium chlorite. In order to prevent unreacted sodium chlorite from being brought into the water, more excess chlorine than the theoretical value is usually added.

The operation of other methods is basically similar to that of the above methods. In order to ensure the safety of the reaction process, acid and sodium chlorate or sodium hypochlorite are mixed into aqueous solution, and excessive acid is added to improve the conversion rate of sodium chlorate or sodium hypochlorite. The generated ClO2 solution can be directly added to water for disinfection according to the appropriate dosage.

There are many equipment for producing chlorine dioxide by electrolysis in the domestic market, but in fact, the so-called chlorine dioxide produced by these equipment is at most a compound of chlorine dioxide and chlorine, which can not completely solve the problem that chlorine disinfectants will produce chlorinated hydrocarbons, and there have been cases of explosion when using compound chlorine dioxide.

eight

Oxidant and reductant

1. What oxidants and reducing agents are commonly used in wastewater treatment?

(1) Theoretically, two substances with different redox potentials can become oxidants or reductants relatively, but the oxidants or reductants that can be used in wastewater treatment practice must meet the following requirements:

It has good oxidation or reduction effect on the pollutants expected to be removed from wastewater

The substances generated after the reaction shall be harmless to avoid secondary pollution

Cheap price and reliable source

It can react quickly at room temperature without heating

The pH value required for the reaction is preferably neutral and cannot be too high or too low.

(2) Oxidants commonly used in wastewater treatment include:

After receiving electrons, it is reduced to neutral atoms with negatively charged ions, such as O2, Cl2, O3, etc;

Positively charged atoms are reduced to negatively charged ions after receiving electrons. For example, under alkaline conditions, Cl + in hypochlorite OCL - and CL4 + in chlorine dioxide in bleach, sodium hypochlorite and other agents are reduced to Cl -;

Atoms with high valence positive charge are reduced to atoms with low valence positive charge after receiving electrons. For example, Fe3 + in ferric chloride and mn7 + in potassium permanganate are reduced to Fe2 + and Mn2 + after receiving electrons.

(3) Common reducing agents in wastewater treatment include:

After giving electrons, they are oxidized into positively charged neutral atoms, such as iron filings, zinc powder, etc;

After giving electrons, negatively charged atoms are oxidized to positively charged atoms. For example, boron in sodium borohydride has a negative 5 valence. Under alkaline conditions, mercury ions can be reduced to metal mercury, and at the same time, they are oxidized to positive 3 valence.

A positively charged atom of metal or nonmetal that is oxidized to an atom with a higher positive charge after giving electrons. For example, the divalent iron ion Fe2 + in ferrous sulfate and ferrous chloride is oxidized to trivalent iron ion Fe3 + after giving an electron; After giving two electrons, the tetravalent sulfur in sulfur dioxide SO2 and sulfite SO32 - is oxidized to hexavalent sulfur to form SO42 -.