01. What kinds of chemicals are commonly used in wastewater treatment?

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

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

(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 excess sludge before dehydration, and its varieties include some of the above flocculants and coagulant aids.

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

Defoamer: it is mainly used to eliminate a lot of bubbles in the process of aeration or stirring.

(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 after wastewater treatment and before discharge or reuse.

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

02. What is flocculant? What is its function?

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

In the application of traditional flocculant, coagulant aid can be used to enhance the flocculation effect. For example, activated silicic acid can be used as coagulant aid of inorganic flocculants such as ferrous sulfate and aluminum sulfate, which can achieve good flocculation by adding them in sequence. Therefore, generally speaking, the inorganic polymer flocculant IPF is actually prepared by combining the coagulant aid and flocculant, and then adding them together to simplify the user's operation.

Coagulation treatment is usually placed in front of the solid-liquid separation facility, and combined with the separation facility, the particle size of raw water can be effectively removed from 1nm to 100nm μ It can be used in the pretreatment and advanced treatment of wastewater treatment process, and can also be used in the treatment of excess sludge. Coagulation treatment can also effectively remove microorganisms and pathogenic bacteria in water, as well as emulsified oil, chroma, heavy metal ions and other pollutants in sewage. The removal rate of phosphorus contained in sewage can be as high as 90-95% by coagulation precipitation treatment, which is the cheapest and most efficient method of phosphorus removal.

03. What is the action mechanism of flocculant?

The colloidal particles in water are small, surface hydrated and charged, which make them stable. After adding flocculant into water, they are hydrolyzed into charged colloids and the ions around them form micelles with electric double layer structure. The collision chance and times of colloidal impurity particles in water and the micelle formed by flocculant hydrolysis were promoted by rapid stirring after dosing. Under the action of flocculant, the impurity particles in water first lose their stability, then agglomerate into larger particles, and then precipitate or float in the separation facility.

The product gt of the velocity gradient g and the stirring time t can indirectly represent the total number of particle collisions in the whole reaction time, and the coagulation effect can be controlled by changing the GT value. Generally, the GT value is controlled between 104 and 105. Considering the influence of the concentration of impurity particles on the 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 the sewage, and the GTC value is recommended to be about 100.

The process of promoting the rapid diffusion of flocculant into water and mixing with all wastewater is mixing. The process that the impurity particles in water react with the flocculant, lose or reduce the stability by compressing the electric double layer and electric neutralization, and form micro floccules is called coagulation. Under the agitation of bridging material and water flow, the micro flocs grow into large flocs through adsorption bridging and sediment net catching mechanism, which is called flocculation. The combination of mixing, coagulation and flocculation is called coagulation. The mixing process is generally completed in the mixing tank, and coagulation and flocculation are carried out in the reaction tank.

04. What are the types of flocculants?

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

Inorganic flocculants include aluminum salts, iron salts and their polymers. Organic flocculants can be divided into anionic, cationic, nonionic and amphoteric types according to the charge properties of charged groups of polymerized monomers. According to their sources, organic flocculants can be divided into synthetic and natural polymer flocculants. In practical application, according to the different properties of inorganic flocculant and organic flocculant, they are combined to make inorganic organic composite flocculant. Microbial flocculant is the product of the combination of modern biology and water treatment technology, which is an important direction of the research and application of flocculant.

05. What are the types of inorganic flocculants?

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

Generally speaking, inorganic flocculants are widely used in water treatment due to their easy availability of raw materials, simple preparation, low price and moderate treatment effect.

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

Since the end of the 19th century, the United States first used aluminum sulfate in water treatment and obtained a patent, aluminum sulfate has been widely used for its excellent coagulation and sedimentation performance. Aluminum sulfate is the most widely used flocculant in the world. The annual output of aluminum sulfate in the world is about 5 million tons, nearly half of which is used in the field of water treatment. Aluminum sulfate on the market has two forms: solid and liquid. The solid one can be divided into refined one and crude one according to the content of insoluble matter. Alum, a solid product commonly used in drinking water purification in China, is the double salt of aluminum sulfate and potassium sulfate, but it is rarely used in industrial water and wastewater treatment.

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

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

Ferric chloride is another common inorganic low molecular coagulant. The product has black brown crystal of solid and high concentration of liquid. It has the advantages of easy to dissolve in water, large and heavy alum, good sedimentation performance and wide adaptability to temperature, water quality and pH. The suitable pH range of ferric chloride is 9-11, the floc density is high, it is easy to precipitate, and the effect is still good at low temperature or high turbidity. The solid ferric chloride has strong water absorption, strong corrosiveness and easy to corrode the equipment. It has high anti-corrosion requirements for dissolving and dosing equipment, pungent odor and poor operating conditions.

The mechanism of ferric chloride is to flocculate the impurity particles in water by using various hydroxyl iron ions generated by the step-by-step hydrolysis of ferric ions. 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 jarosite, forms fast and stable flocs with short settling time. It is suitable for the situation of high alkalinity and turbidity, but its chroma is not easy to remove and its corrosiveness is also strong.

08. What are the types of inorganic polymer flocculants?

Inorganic polymer flocculant (IPF) is a new type of flocculant developed since 1960s. At present, the production and application of IPF have made rapid progress all over the world. The inorganic polymer flocculants of aluminum, iron and silicon are actually the intermediate products of their hydrolysis, sol-gel and precipitation processes, namely, the hydroxy and oxy polymers of Al (Ⅲ), Fe (Ⅲ) and Si (Ⅳ). Aluminum and iron are positively charged with cations, while silicon is negatively charged with anions. Their unit molecular weight in water-soluble state is about hundreds to thousands, and they can combine with each other to form aggregates with fractal structure. Their coagulation flocculation process is the comprehensive embodiment of the electric neutralization and adhesion bridging of particles in water. The particle size of suspended particles in water ranges from nanometer to micron, and most of them are negatively charged. Therefore, the main factors that determine the flocculation effect are the positive and negative charge of flocculant and its morphology, the strength of electric property, the molecular weight and the particle size of aggregates. At present, there are dozens of kinds of inorganic polymer flocculants (see table 8-1 for main varieties), and the output of the flocculants reaches 30% - 60% of the total output, among which polyaluminum chloride is widely used.

09. What are the characteristics of inorganic polymer flocculant?

Al (Ⅲ), Fe (Ⅲ) and Si (Ⅳ) hydroxyl and oxygen based polymers will be further combined into aggregates. Under certain conditions, they will remain in the aqueous solution, and their particle size is about in the range of nanometer. In order to play the role of coagulation flocculation, low dosage and high effect will be obtained. If we compare the polymerization rate, the trend from Al → Fe → Si is strong, and the trend from hydroxyl bridging to oxygen bridging is also in 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 advantages of IPF are that it is more effective than traditional flocculants such as aluminum sulfate and ferric chloride, and cheaper than organic polymer flocculants (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 gradually becomes the mainstream flocculant. However, in terms of morphology, degree of polymerization and coagulation flocculation effect, inorganic polymer flocculants are still in the position between traditional metal salt flocculants and organic polymer flocculants. Its molecular weight, particle size and flocculation bridging ability are still much worse than those of organic flocculants, and there is also instability for further hydrolysis reaction. These weaknesses of IPF promote the research and development of various composite inorganic polymer flocculants.

10. What are the characteristics of polyaluminum chloride?

Polyaluminum chloride (PAC), also known as basic aluminum chloride, chemical formula is AlN (OH) mcl3n-m. PAC is a kind of multivalent electrolyte, which can significantly reduce the colloidal charge of clay impurities in water. Because of the high molecular weight, strong adsorption capacity and large flocs, the flocculation and sedimentation performance is better than other flocculants.

PAC has a high degree of polymerization, which can greatly shorten the formation time of flocs by rapid stirring after dosing. PAC is less affected by water temperature, and it works well at low water temperature. It can be used in a wide range of pH (pH = 5-9), so it can be used without adding alkali agent. PAC has less dosage, less sludge production, convenient use, management and operation, and less corrosion to equipment and pipeline. Therefore, PAC has a tendency to gradually replace aluminum sulfate in the field of water treatment, and its disadvantage is its high price.

In addition, from the point of view 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. The addition of 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. The structure and morphology distribution of the polymer can be changed to a certain extent by using polymerization, so as to improve the stability and efficacy of PAC; If other cationic components such as Fe3 + are introduced in the process of PAC production, the composite flocculant poly (aluminum iron) can be prepared by cross hydrolysis polymerization of Al3 + and Fe3 +.

Generally speaking, the higher the density of flocculant product, the higher the content of aluminum oxide. Generally speaking, the higher the alkalization degree, the better the adsorption bridging ability of polyaluminum chloride, but it is easy to produce precipitation due to its proximity to [al (OH) 3] n, so its stability is also poor.

11. What is the alkalinity of PAC?

Polyaluminum chloride can be regarded as the intermediate product in the process of gradual hydrolysis of AlCl3 to Al (OH) 3, that is, all kinds of products in which Cl - is gradually replaced by hydroxyl Oh -. The degree of hydroxylation in a certain form of polyaluminum chloride is the degree of alkalization, which is the ratio of hydroxyl equivalent and aluminum equivalent in polyaluminum chloride.

The practice shows that the degree of alkalization is one of the most important indexes of polyaluminum chloride. The degree of alkalization is closely related to the degree of polymerization, the amount of charge, the coagulation effect, the pH value of the finished product, the dilution rate in use and the stability of storage. The alkalinity of commonly used polyaluminium chloride is 50% ～ 80%.

12. What are the characteristics of the composite flocculant and the precautions for its use?

The main raw materials are aluminum salt, iron salt and silicate. In terms of manufacturing process, they can be hydroxylated and polymerized separately and then mixed, or they can be mixed first and then hydroxylated and polymerized, but in the end, the inorganic polymer form with higher degree of polymerization must be formed to achieve excellent flocculation effect. Each component of the complex agent will play a certain role in the overall structure and coagulation flocculation process, but in different aspects, there may be positive or negative effects.

IPF products usually need to consider three factors: stability, electric neutralization ability and adsorption bridging ability. The weakness of polyaluminium and Polyferric flocculants is that the molecular weight and particle size are not high enough, and the adhesion and bridging ability of aggregates is not strong enough, so it is necessary to add larger particle size silicon polymer to enhance the flocculation performance. However, the total charge will decrease after adding anionic silicon polymer, which weakens the ability of electric neutralization.

Therefore, compared with polyaluminum, the effect of the present composite flocculant can only be increased by 10-30% even if its quality is excellent. 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 suitable flocculant and adding process operation procedure, only according to the characteristics of wastewater quality, careful analysis and judgment can obtain the best treatment effect.

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

Synthetic organic polymer flocculants are mostly polypropylene and polyethylene, such as polyacrylamide and polyethylenimine. These flocculants are water-soluble linear polymer materials, each macromolecule is composed of many repetitive units containing charged groups, so it is also called polyelectrolyte. The polyelectrolyte containing positive group is cationic polyelectrolyte, and the polyelectrolyte containing negative group is anionic polyelectrolyte, which contains both positive group and negative group, so it is called non-ionic polyelectrolyte.

At present, anionic polymer flocculants are widely used, and they can only play a coagulant aid role on negative colloidal impurities in water. Often can not be used alone, but with aluminum salt, iron salt use. Cationic flocculant can play the role of coagulation and flocculation at the same time and be used alone, so it has been developed rapidly.

At present, polyacrylamide nonionic polymers are widely used in China, which are often used with iron and aluminum salts. By using the electric neutralization effect of iron and aluminum salts on colloidal particles and the excellent flocculation function of polymer flocculant, satisfactory treatment effect can be obtained. Polyacrylamide has the characteristics of small dosage, fast coagulation speed, large floc size and strong toughness. At present, 80% of the synthetic organic polymer flocculants produced in China are of this kind.

14. What are the characteristics of polyacrylamide flocculants?

Polyacrylamide (PAM) is one of the most widely used synthetic organic polymer flocculants, sometimes used as coagulant aid. The raw material of polyacrylamide is polyacrylonitrile (CH2 = CHCN). Under certain conditions, acrylonitrile is hydrolyzed to produce acrylamide, and then acrylamide is obtained by suspension polymerization. Polyacrylamide is a kind of water-soluble resin. Its products are granular solid and viscous aqueous solution of 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 curls into a coil, the bridging range is small. After the two amide groups are formed, it is equivalent to counteracting each other and losing two adsorption sites. In addition, part of the amide groups are embedded in the inner structure of the coil and can not contact and adsorb with the impurity particles in water, so its adsorption capacity can not be fully exerted.

In order to separate the amide groups and expose the amide groups, people try to extend the random coil properly, even try to add some cationic or anionic groups on the long molecular chain, and improve the ability of adsorption bridging and the effect of electric neutralization and compression of electric double layer. In this way, a series of polyacrylamide flocculants or coagulant aids with different properties were derived from PAM.

For example, alkali is added to polyacrylamide solution to convert the amide group on some chain segments into sodium carboxylate, which is easy to dissociate sodium ions in water, leaving coo group on the branched chain, thus forming partially hydrolyzed anionic polyacrylamide. The COO group in the molecular structure of anionic polyacrylamide makes the molecular chain have negative charge, which repels each other. It pulls apart the amide group that was originally formed together, and makes the molecular chain gradually extend from the coil shape to the chain shape, so as to expand the bridging range and improve the flocculation ability. 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 "degree of hydrolysis" will increase the production cost.

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

The "degree of hydrolysis" of anionic polyacrylamide is the percentage of amide group converted to carboxyl group in PAM molecule during hydrolysis. However, because it is difficult to determine the carboxyl number, the "hydrolysis ratio" is commonly used in practical application, that is, the weight ratio of sodium hydroxide dosage and PAM dosage during hydrolysis.

If the hydrolysis ratio is too large, the cost of adding alkali will be 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.

16. What are the factors affecting the use of flocculants?

(1) pH value of water

The pH value of water has a great influence on the use effect of inorganic flocculants. The pH value is related to the type of flocculants, dosage and coagulation sedimentation effect. H + and oh - in water participate in the hydrolysis reaction of flocculant. Therefore, pH value strongly affects the hydrolysis rate of flocculant, the existing form and performance of hydrolysate. For example, when the pH value is less than 4, Al3 + can not be hydrolyzed into Al (OH) 3, mainly in the form of Al3 + ions, and the coagulation effect is very poor. When pH value is between 6.5 and 7.5, Al3 + is hydrolyzed and polymerized to form neutral colloid al Al (OH) 3 with high degree of polymerization, and the coagulation effect is better. When pH value is higher than 8, Al3 + is hydrolyzed to AlO2 -, and the coagulation effect becomes very poor.

The alkalinity of water has buffering effect on pH value. When the alkalinity is not enough, lime and other chemicals should be added to supplement. When the pH value of water is high, acid should be added to adjust the pH value to neutral. In contrast, the effect of pH value on polymer flocculant is small.

(2) water temperature

Water temperature affects the hydrolysis rate of flocculant and the formation rate and structure of alum. When the water temperature is low, the hydrolysis rate is slow and incomplete. At low temperature, the viscosity of water is high, Brownian motion is weakened, the number of collisions between flocculant colloidal particles and impurity particles in water is reduced, and the shear force of water is increased, 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, the particles are small and difficult to remove. The effect of low temperature on polymer flocculant is small. However, it should be noted that when using organic polymer flocculant, the water temperature should not be too high, and the high temperature is easy to cause the organic polymer flocculant to age or even decompose into insoluble substances, thus reducing the coagulation effect.

(3) impurities in water

The uneven size of impurity particles in water is good for coagulation, and the small and uniform size will lead to poor coagulation effect. Too low concentration of impurity particles is often unfavorable to coagulation. At this time, reflux sediment or adding 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, and it is necessary to increase the dosage or add oxidants and other coagulant aids. Calcium and magnesium ions, sulfides and phosphates in water are generally beneficial to coagulation, while some anions and surfactants have adverse effects on coagulation.

(4) types of flocculant

The selection of flocculant mainly depends on the properties and concentration of colloid and suspended solids in water. If the pollutants in water are mainly in colloidal state, inorganic flocculant should be preferred to make it destabilize and coagulate. If the floc is small, polymer flocculant or activated silica gel and other coagulant aids should be added. In many cases, the combination of inorganic flocculant and polymer flocculant can significantly improve the coagulation effect and expand the scope of application. For polymer, the larger the amount of charge on the chain molecule, the higher the charge density, the more fully the chain can be extended, the larger the scope of adsorption bridging, and the better the coagulation effect.

(5) dosage of flocculant

When using coagulation to treat any wastewater, there are 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 common iron salt and aluminum salt is 10-100mg / L, the dosage of polymerized salt is 1 / 2-1 / 3 of that of common salt, and the dosage range of organic polymer flocculant is 1-5mg / L.

(6) flocculant dosing sequence

When a variety of flocculants are used, it is necessary to determine the optimal dosing sequence through experiments. Generally speaking, when inorganic flocculant and organic flocculant are used together, inorganic flocculant should be added first, and then organic flocculant should be added. The particle size of the treated impurity is 50 μ When it is more than 10 m, the organic flocculant is often added to adsorb the bridge, and then the inorganic flocculant is added to compress the double electric layer to make the colloid destabilize.

(7) hydraulic conditions

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

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

The application of natural organic polymer flocculant in water treatment has a long history. Up to now, natural polymer compound is still an important kind of flocculant, but its usage 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 easy to biodegrade and lose its flocculating 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 good 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, which can be divided into chitosan flocculants, modified starch flocculants, modified cellulose flocculants, lignin flocculants, gum flocculants, alginate flocculants, animal glue and gelatin flocculants according to their main natural components (including the matrix components used for modification). Most of these natural polymers have polysaccharide structure, in which there is only one monosaccharide structure in the main starch chain, which belongs to the same polysaccharide; Chitosan, gum, alginate and so on contain a variety of monosaccharide structures, belonging to heteropolysaccharide; Lignin is a special aromatic natural polymer; Animal glue and gelatin belong to proteins.

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

Organic polymer flocculant is a long chain macromolecule with coil structure, which must undergo a swelling process in water. Solid products or high concentration liquid products must be prepared into aqueous solution before being used, and then added to the water to be treated. The chemical dissolving tank for preparing water solution must be equipped with mechanical stirring equipment, and the continuous stirring time of the chemical dissolving tank should be controlled at more than 30 minutes. The concentration of water solution is generally about 0.1%. If the concentration is higher, the viscosity of solution will increase, and it is difficult to add. If the concentration is lower, the volume of solution pool will be too large. The water used for dissolving drugs should avoid containing a large amount of suspended solids as far as possible, so as to avoid the formation of alum by flocculation reaction between organic polymer flocculant and these suspended solids, which will affect the use effect after dosing.

When dissolving the solid organic polymer flocculant, the dosing point of the solid particles must be at the place where the flow turbulence is the strongest, and at the same time, the minimum dosing amount must be slowly injected into the dissolving tank to disperse the solid particles into the water, so as to prevent the solid particles from dispersing too fast in the water and forming agglomerates with each other Once formed, it often takes a long time to dissolve the hydrolysate evenly into the water, even for several days in the continuous dissolving tank.

The feeding point of solid particles must be far away from the mixing shaft of mechanical stirrer, because the mixing shaft is usually the place with the worst turbulence in the chemical dissolving tank. The organic polymer flocculant that is not fully dissolved often adheres to the shaft and accumulates day by day. Sometimes it can form quite large agglomerates. If it is not carefully cleaned in time, the agglomerates will become larger and larger, The scope of influence will be larger and larger.

As a coagulant aid, inorganic flocculant is usually added to the treated water to compress the electric double layer, and then organic polymer flocculant is added to achieve bridging effect. Under the condition of sufficient inorganic flocculant dosage, the coagulant 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 deliquescence into blocks, so waterproof packaging must be used, and the storage place must be dry to avoid open storage.

19. 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 and metabolites. The former is the main aspect of Microbial Flocculant Research, so far there are more than 17 kinds of flocculating microorganisms, including mold, bacteria, actinomycetes and yeast. The latter is similar to organic flocculant. Microbial flocculants have many advantages over traditional inorganic or organic flocculants, such as no secondary pollution and low production cost.

The flocculation performance of bioflocculant is affected by many factors. The internal factors include the inheritance and expression of flocculation genes, while the external factors include the composition of microbial culture medium, the change of cell surface hydrophobicity, and the existence of divalent metal ions in the environment. At present, there are good microbial flocculants 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 the production cost. The development of microbial flocculant in China is moving forward in this direction, but there is still a certain distance from industrial production.

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

The selection and dosage of flocculant should be determined according to the operation experience of waterworks under similar conditions or the results of raw water coagulation and sedimentation test, combined with the local reagent supply and through technical and economic comparison. The principle of selection is low price, easy to get, good water purification effect, easy to use, dense floc, fast sedimentation, easy to separate from water, etc.

The purpose of coagulation is to produce larger flocs. Due to many influencing factors, the corresponding data are usually obtained by coagulation beaker stirring test. The coagulation test is carried out in a beaker, including three steps: fast mixing, slow mixing and static sedimentation. The flocculent was rapidly dispersed and contacted with the colloidal particles in the water sample after rapid stirring, and the colloidal particles began to agglomerate and produce micro flocs; By slow stirring, the micro flocs further contact with each other and grow into larger particles; After stopping stirring, the formed colloidal aggregates naturally settle to the bottom of the beaker by gravity. Through the comprehensive evaluation of coagulation effect, such as flocculant sedimentation, supernatant turbidity, chroma, pH value, oxygen consumption and so on, the appropriate flocculant species and its optimal dosage were determined.

A six unit mixer was used in the experiment. It has six vertical moving shafts, and its bottom position is equipped with mixing blades. The blade size is 6cm × 2cm。 The rotation speed and time of the rotating shaft can be set in advance and can work automatically. In general, the experiment was carried out by stirring for 2 min, n = 300 R / min; Stirring slowly for 3 min, n = 60 R / min. In the test, 1 l raw water was added into six 1 000 ml beakers and placed directly under six rotating shafts to move the shaft down to the bottom; Then, in six small glass beakers connected to a horizontal rotating shaft, different amounts of liquid medicine are added in turn. 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 agitation stops automatically, slowly pull the blade out of the beaker and let it stand for 20 minutes. Use a pipette to draw 25 ml of water sample from about 10 cm below the water surface, and use a turbidimeter to measure the turbidity of the supernatant. Taking the dosage as the abscissa and the residual turbidity of the supernatant as the ordinate, the curve is drawn to compare the effects of different flocculants. According to the turbidity removal effect and comprehensive technical and economic factors, the flocculant for treating this kind of wastewater is selected.

The method of beaker stirring test can be divided into single factor test and multi factor test. The raw water used in the test should be identical with the actual water quality, and the type, dosage and order of flocculant should be determined according to the pH value and impurity properties of the water. The test should be the simulation of the actual process, and the hydraulic conditions (mainly GT value) of the two must be the same or close.

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

In the coagulation treatment of wastewater, sometimes the use of a single flocculant can not achieve good coagulation effect, often need to add some auxiliary agents to improve the coagulation effect, this auxiliary agent is called coagulant aid. Commonly used coagulant aids include chlorine, lime, activated silicic acid, bone glue, 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 the coagulation conditions. Some coagulant aids are involved in the formation of flocs, improving the structure of flocs, which can make the small loose flocs produced by inorganic flocculants into thick and tight alum.

22. What kinds of coagulant aids are commonly used?

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

(1) agents for adjusting or improving coagulation conditions

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

For the wastewater with high content of dissolved organic matter, oxidants such as Cl2 can be used to destroy the 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 +) to high valent iron (Fe3 +) to improve the coagulation effect.

The above alkali agents, sulfuric acid, CO2 and chlorine do not act as coagulants but as auxiliary coagulants.

(2) to increase the particle size, density and firmness of alum

The result of coagulation requires the formation of alum flower with large particle size, high density and solid, which is not only conducive to precipitation, but also not easy to break. In order to obtain this result, combined with the characteristics of water quality, sometimes it is necessary to add some substance or agent into the water. For example, the addition of coarse particles such as silica, activated carbon and clay or the return part of precipitated sludge to the low turbidity wastewater containing light impurities that are not suitable for sedimentation can aggravate and increase the alum bloom; When aluminum salt and iron salt are used as flocculants, only fine and loose flocs can be produced. Polyacrylamide, activated silicic acid, bone glue and other polymer coagulants can be added to make use of their strong adsorption bridging effect to make the fine and loose flocs coarse and dense.

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

Adding flocculant in wastewater treatment can accelerate the aggregation and sedimentation of solid particles in wastewater, and also can remove part of dissolved organic matter. This method has the advantages of less investment, simple operation and flexibility. It is especially suitable for the treatment of wastewater with small amount of water and high content of suspended impurities. When using inorganic flocculant, because the dosage is large, the amount of sludge is also large, so in practical application, the synthetic organic polymer flocculant OPF is mainly used, or the combination of inorganic flocculant and OPF is used.

According to relevant reports, 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. The 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 wastewater.

In the primary precipitation treatment of wastewater, the mixed use of organic polymer polyelectrolyte and inorganic flocculant is better than their separate use. Because the concentration, size distribution and species of suspended particles in the wastewater can change at any time, the optimal dosage of flocculant is sometimes difficult to control. In this case, if the inorganic flocculant is added excessively and the suspended impurities are precipitated and removed by the roll sweep mechanism, the method is feasible, but its disadvantages are also very prominent. First, the action time is relatively long (15-30min), and then the flocs formed are easy to break. If inorganic flocculant is added and a certain amount of organic polymer polyelectrolyte is added at the same time, the flocculation time can be reduced to 2-5min, and the formed flocs are relatively strong.

A double electrolyte system can be used to remove colored organic colloidal impurities in water by precipitation. First, the organic colloids were destabilized by cationic polyelectrolytes with high positive charges, and then the destabilized organic colloids were flocculated into flocs with high molecular weight nonionic or anionic polyelectrolytes.

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

In addition, coagulation treatment can also remove phosphate and heavy metal ions in 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 the same effect can be achieved by using cationic polymer instead of inorganic flocculant on the premise that the removal rate of phosphate is not reduced, which indicates that the polymer participates in the adsorption of anionic phosphate. For example, in the coagulation 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 23mg / L ferric sulfate. The removal rate of BOD5 in the effluent increased from 30% to 55% with the same phosphorus removal rate. At the same time, after coagulation treatment, the inorganic components in activated sludge can be reduced, and the biodegradation function of activated sludge can be improved.

In the process of filtration and flotation used in wastewater treatment, the quality of effluent can be improved by using inorganic flocculant and polyelectrolyte coagulant aid. Combined with the characteristics of wastewater quality, flocculant can be used alone, or combined with multiple flocculants, or combined with one main flocculant and one auxiliary flocculant. The selection of flocculant can be preliminarily screened by beaker static test, and then verified on the production device.

24. What kinds of sludge conditioners are commonly used?

Conditioner, also known as dehydrating agent, can be divided into inorganic conditioner and organic conditioner. Inorganic conditioners are generally suitable for vacuum filtration and plate and frame filtration of sludge, while organic conditioners are suitable for centrifugal dewatering and belt filter pressing dewatering of sludge.

(1) inorganic conditioner

The most effective, cheapest and most commonly used inorganic conditioners are mainly iron salts and aluminum salts. The iron salt conditioner mainly includes ferric chloride (FeCl3 · 6H2O), ferric sulfate (Fe2 (SO4) 3 · 4H2O), ferrous sulfate (FeSO4 · 7H2O) and polymerized ferric sulfate (PFS) ([Fe2 (OH) n (SO4) 3-N / 2] m), and the aluminum salt conditioner mainly includes aluminum sulfate (Al2 (SO4) 3 · 18H2O), aluminum trichloride (AlCl3), basic aluminum chloride (Al (OH) 2Cl), aluminum chloride (Al (OH) 2Cl), aluminum chloride (Al (OH) 2Cl), aluminum chloride (Al (OH) 2Cl), aluminum chloride (Al (oh) 2Cl), aluminum chloride (Al (OH) 2Cl), aluminum chloride (Al (OH) 2C Polyaluminum chloride (PAC) ([Al2 (OH) n · cl6-n] m), etc.

After adding inorganic conditioner, the thickening process of sludge can be greatly accelerated and the effect of filtration and dehydration can be improved. Moreover, the combination of ferric salt and lime can further improve the conditioning effect. One of the disadvantages 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 ferric salt), and the dosing system should have anti-corrosion performance. It should be noted that when ferric chloride is used as conditioning agent, it will increase the corrosiveness to the metal components of dewatered sludge treatment equipment, so the anti-corrosion grade of dewatered sludge treatment equipment should be appropriately improved.

(2) organic conditioner

There are many kinds of organic synthetic polymer conditioners. According to the degree of polymerization, they can be divided into two kinds: low degree of polymerization (molecular weight is about 10-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, nonionic type, anionic and anionic type, etc. Compared with inorganic conditioner, the dosage of organic conditioner is less, generally 0.1% ~ 0.5% of dry solid weight of sludge, and it is not corrosive.

Organic conditioners used for sludge conditioning are mainly high polymerization degree polyacrylamide series flocculant products, including cationic polyacrylamide, anionic polyacrylamide and non-ionic polyacrylamide. Among them, cationic polyacrylamide can neutralize the negative charge on the surface of sludge particles and produce bridging effect between particles, which shows strong cohesion and significant conditioning effect, but the cost is high. In order to reduce the cost, a cheaper anionic polyacrylamide lime method can be used. The Ca (OH) 2 flocs with positive charge can be used to adsorb the negatively charged flocculant and sludge particles together to form a composite coagulation system.

25. What factors should be considered in selecting sludge conditioner?

(1) variety characteristics of conditioner

As far as the commonly used inorganic conditioners of aluminum salt and iron salt are concerned, when aluminum salt is used, the dosage of reagent is large, the density of flocs formed is small, the conditioning effect is poor, and the filter cloth will be blocked in the dehydration process. Therefore, iron salt should be used as far as possible in the selection of inorganic conditioner; 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 are smaller, but the floc strength is higher.

Therefore, when using vacuum filter and plate and frame filter press to dewater sludge, inorganic conditioner can be considered. Compared with the inorganic conditioner, the dosage of organic conditioner is smaller, and the floc is bulky, but the floc strength is lower, which is easier to break than the inorganic conditioner. And once the flocs are destroyed, it is not easy to return to the original state whether using inorganic or organic conditioners.

Therefore, when using centrifugal dehydrator and belt filter press to dehydrate sludge, organic conditioner can be considered. When one of the inorganic or organic conditioners is difficult to achieve the ideal conditioning effect, the combination of inorganic and organic conditioners can be considered, 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 great differences in the types and dosage of conditioners for different types of sludge. For the sludge with high organic matter content, the more effective conditioner is cationic organic polymer conditioner, and the higher the organic matter content is, the more suitable cationic organic polymer conditioner with higher degree of polymerization is to be selected.

For the inorganic sludge, anionic organic polymer can be used. The different properties of sludge directly affect the conditioning effect: sludge in primary sedimentation tank is easy to be dehydrated, while scum and excess activated sludge are difficult to be dehydrated, and the dewatering performance of mixed sludge is between the two. In order to achieve a certain conditioning effect, the amount of conditioner needed is significantly different. 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 are, which will lead to the increase of the consumption of conditioning agent. The high content of organic matter and alkalinity in the sludge will also lead to the increase of the dosage of conditioning agent. In addition, the solid content of sludge also affects the dosage of conditioner. Generally, the higher the solid content of sludge is, the greater the dosage of conditioner is.

(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, and the conditioning effect can be improved by appropriately extending the conditioning time. When organic polymer conditioner is used, 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 the uniform dilution, conditioning and mixing, thus affecting the sludge conditioning effect and dewatering effect.

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

(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 different. The hydrolysis reaction of aluminum salt is greatly affected by pH value, and the optimum pH value range of condensation reaction is 5-7. When the pH value is higher than 8 or lower than 4, it is difficult to form flocs, that is to say, it loses the function of conditioning. No matter the sludge is acidic or alkaline, it can form Fe (OH) 3 flocs, and the optimal pH 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, the specific pH value of dewatered sludge should be considered first. If the pH value deviates from the optimal range of coagulation reaction, it is better to use another conditioner instead. Otherwise, it is necessary to consider adding acid or alkali to adjust the pH value of sludge before conditioning. Generally, this measure is not taken.

The pH value of sludge affects the ionization, charge state and molecular shape of conditioner molecules. The cationic polyelectrolyte has higher ionization degree in the acid sludge with low pH value, and the molecular shape tends to stretch; However, in alkaline sludge with high pH value, the ionization degree is small and the molecular shape tends to curl. In contrast to cationic polyelectrolyte, anionic polyelectrolyte has smaller ionization degree and more curly molecular shape in low pH acid sludge; However, in alkaline sludge with high pH value, the ionization degree is larger and the molecular shape tends to stretch. 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 a kind of excess charge on the molecules, which makes the molecules tend to stretch due to mutual repulsion.

(5) preparation concentration

The preparation concentration of the conditioner not only affects the conditioning effect, but also affects the consumption of the reagent and the yield of the mud cake. Generally speaking, the lower the concentration of organic polymer conditioner, the less the consumption of the agent, 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 molecular chain is extended, the better the bridging and condensation function is played, and the better the conditioning effect is of course. However, if the concentration is too high or too low, the cake yield will be reduced. However, the conditioning effect of inorganic polymer conditioner is almost not affected by the preparation concentration. Experience and related research show that the concentration of organic polymer conditioner is 0.05% ～ 0.1%, the concentration of ferric chloride is 10%, and the concentration of aluminum salt is 4% ～ 5%.

(6) dosing sequence

When more than one conditioner is used, the order of conditioner dosing will also affect the conditioning effect. When ferric salt and lime are used as conditioning agents, ferric salt is usually added first, and then lime is added. The flocs formed in this way are easier to separate from water, and the total consumption of conditioning agent is also less. When inorganic conditioner and organic polymer conditioner are used to treat sludge, the better effect can be obtained by adding inorganic conditioner first and then organic polymer conditioner.

(7) mixed reaction conditions

In order to achieve the best conditioning effect, it is very necessary to achieve full mixing of sludge and conditioner. However, it is worth noting that the flocs formed by the mixed reaction of sludge and conditioner can never be damaged again, because once the flocs are damaged, it is difficult to return to the original state.

The experience shows that only when the intensity and time of mixed reaction are within a certain range, can better conditioning effect be achieved by using a certain conditioner for a certain sludge, 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.

26. How to determine the dosage of conditioner?

There is no fixed standard for the consumption of sludge conditioning agent. According to the specific properties of sludge, such as sludge type, digestion degree, solid concentration and so on, the dosage will be different. Therefore, most of them are directly tested in the laboratory or in the field to determine the type and specific dosage of conditioner.

Generally speaking, the dosage of ferric chloride, ferrous sulfate, hydrated lime, polyaluminum chloride and polyferric sulfate is 5% - 10%, 10% - 15%, 20% - 40%, 1% - 3% and 0.1% - 0.3%, respectively, according to the percentage of dry solid weight of sludge. According to relevant information, due to the high price of commonly used polyacrylamide series organic synthetic polymer conditioner (some varieties are more than ten or twenty times of ordinary inorganic conditioner), although the dosage is small, the cost of organic synthetic polymer conditioner is still high, which is equivalent to the cost of conditioning per ton of sludge. The common practice is to select the best inorganic conditioner. When the function of inorganic conditioner is poor and it is difficult to achieve the ideal conditioning effect, the organic synthetic polymer conditioner or the combination of inorganic and organic conditioner should be considered.

27. What are the precautions for using conditioner?

In order to better use the conditioner, the following matters should be paid attention to: ① fully understand and master the properties (concentration, composition, etc.) of the sludge to be treated; ② test to determine the type of conditioner suitable for the properties of sludge and dehydrator; ③ test to determine the injection point, reaction conditions, dosage, etc. of the conditioner; ④ according to the properties of the conditioner to determine the dissolving rate of the conditioner Storage, etc.

Generally speaking, inorganic conditioners are suitable for vacuum filtration dehydration and plate and frame pressure filtration dehydration, while organic conditioners are more suitable for centrifugal dehydration and belt pressure filtration dehydration. When using centrifugal dehydrator and belt pressure dehydrator, in order to form coarse flocs that are not easy to break, cationic series polymer conditioners with molecular weight of 100000 or even more than 1 million are generally used.

At the same time, it should be noted that since the centrifugal dehydrator is used for solid-liquid separation under the high centrifugal force of 2000-3000g, the use of macromolecular conditioner with larger molecular weight makes it easier to form solid flocculate, which is more conducive 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 the poor peeling of the filter cake on the filter cloth. For the same sludge, compared with centrifugal dehydrator, belt pressure dehydrator requires higher cationic degree and less dosage.

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 properties of excess sludge produced by biological treatment of wastewater are the same as that of return sludge. The main component of excess sludge is microbial flocculate, which generally has negative charge. Therefore, in order to coagulate excess sludge, it is best to use cationic conditioner. At present, the most commonly used cationic modifiers are copolymers of poly (Propionamide) or amino methylated modifiers. By adjusting the conditions of cationic modification, the modifiers with different cationic degrees can be obtained. According to the different cationic degree (which can be determined by colloidal titration), cationic conditioners can be divided into high, medium and low cationic degree conditioners.

28. What is the relationship between dehydrator, conditioner, flocculant and coagulant aid?

Dewatering agent is the agent added before sludge dewatering, which is also 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 of water to be treated in unit volume.

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 flocculant auxiliary in water treatment field. The same coagulant aid is generally not called coagulant aid in excess sludge treatment, but collectively referred to as conditioning agent or dehydrating agent.

When using flocculants, due to the limited amount of suspended solids in water, in order to achieve full contact between flocculants and suspended particles, mixing and reaction facilities should be equipped, and they should have enough time. For example, mixing takes tens of seconds to several minutes, and reaction takes 15 to 30 minutes. In sludge dewatering, the time from adding conditioner to sludge entering dehydrator is usually only tens of seconds, which is equivalent to 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.

29. What are the types of defoamers?

The effect of defoamer is related to the type of foaming liquid, that is, some defoamers have obvious effect on some foaming liquid, but not obvious or even no effect on other foaming liquid. The commonly used defoamers can be divided into silicone (resin), surfactant, alkanes and mineral oil.

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

Surface active agents: 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 hydrocarbon: alkane hydrocarbon defoamer is a defoamer made by emulsifying alkane wax or its derivatives with emulsifier. Its use is similar to that of emulsified defoamer of surfactant.

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

30. What are the common pH regulators?

When the pH value of acidic wastewater is raised, alkali or alkaline oxide is used as neutralizer, while when the pH value of alkaline wastewater is lowered, acid or acidic oxide is used as neutralizer. Lime, limestone, dolomite, sodium hydroxide, sodium carbonate and so on are often used as neutralizers when adjusting the pH value of acidic wastewater, while sulfuric acid and hydrochloric acid are generally used when adjusting the pH value of alkaline wastewater.

When neutralizing acid containing wastewater, some alkaline industrial wastes can also be used nearby, such as calcium carbonate waste discharged from chemical water softening station, calcium carbide waste discharged from organic chemical plant or acetylene generating station (the main component is calcium hydroxide), waste ash screened from steel plant or calcium carbide plant, furnace ash from thermal power plant and boron sludge from boric acid plant. In the treatment of alkaline wastewater, flue gas can also be used to neutralize alkali in wastewater by using acidic gases such as CO2 and SO2.

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

31. What factors should be considered in the selection of disinfectants?

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

At present, it is well known that chlorine disinfection can produce harmful substances and affect human health. This is because chlorine reacts with organics in water, and has oxidation and substitution. 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 THMs in the United States is 100 μ G / L, Germany, Canada and Japan are also 25 μ g/L、350 μ g/L、100 μ The upper limit of chloroform is 60 g / L, which is also stipulated in the 1985 edition of drinking water hygienic standard μ g/L。 In view of this, the first is to control the appropriate dosage of wastewater disinfection, and the second is to use other disinfectants instead, such as chlorine dioxide, ozone, ultraviolet radiation, etc., in order to reduce the generation of harmful substances.

The advantages, disadvantages and applicable conditions of various disinfectants are shown in table 8-2. Referring to this table, we can preliminarily determine the disinfectant that should be used.

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

Commonly used disinfectants include hypochlorite, chlorine dioxide, ozone, ultraviolet radiation, etc. Hypochlorite disinfectants have the forms of liquid chlorine, bleaching powder, bleaching powder concentrate, chlorine tablets, sodium hypochlorite and so on, mainly through HOCl. The weakness of hypochlorite disinfectants is that it is easy to form chlorinated hydrocarbon 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.

Hypochlorite disinfectant dust and chlorine gas have strong irritating effect on human respiratory tract, eyes and skin. If it accidentally splashes into eyes or touches skin, wash with plenty of water immediately. The storage environment should be cool, ventilated and dry, far away from heat sources and kindling. It should not be stored and transported together with organic matter, acids and reducing agents. In the process of transportation, it should be prevented from rain and sunlight exposure, and the loading and unloading should be light to avoid collision and rolling.

When hypochlorite disinfectants are used for disinfection, the substitution reaction often occurs, which is also the fundamental reason why hypochlorite disinfectants will produce chlorinated hydrocarbons. When ozone and chlorine dioxide are used for disinfection, the pure oxidation reaction occurs, which can destroy the structure of organic matter, improve the biodegradability of wastewater (BOD5 / CODcr value) and remove part of CODCr in water. Compared with ozone or ultraviolet disinfection, chlorine dioxide disinfection has lower one-time investment and higher 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 total number of bacteria and total coliform in the effluent or reuse water of the secondary sedimentation tank after ozone disinfection and ultraviolet disinfection can meet the requirements, but their disadvantages are instantaneous reaction, unable to maintain the effect, and resistant to the breeding and reproduction of microorganisms in the pipeline, Therefore, when the reuse water system is disinfected by these two methods, it is often necessary to add 0.05-0.1mg/l chlorine dioxide or 0.3-0.5mg/l chlorine to the effluent to keep enough residual chlorine at the end of the pipe network.

33. What are the physical and chemical properties of chlorine?

Chlorine is a yellow green gas at atmospheric pressure, and its density is 3.2mg/ml at 0oC and one atmospheric pressure, which is about 2.5 times of the weight of air. In general, chlorine is prepared by electrolysis of salt aqueous solution, and then liquid chlorine is obtained by pressure cooling. Liquid chlorine is easily gasified, and its boiling point is - 34.5oc. The liquid chlorine under pressure becomes 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. Its solubility decreases with the increase of water temperature. Chlorine is a strong irritating asphyxiating gas, which can cause damage 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 it is mixed with other flammable gases in sunlight, it will burn and explode, and can react with most substances.

Chlorine is a kind of strong oxidant, which has the advantages of strong bactericidal ability, low price and convenient source. It is the disinfectant with the longest application history in water treatment industry. The mechanism of chlorine disinfection depends on the diffusion of hypochlorous acid (HOCl) generated by hydrolysis into the cell wall of microorganisms and the formation of N-Cl bond with excellent chemical stability by reaction with cell proteins. In addition, chlorine can oxidize some active substances of microorganisms, inhibit and kill microorganisms.

34. How to prevent chlorine poisoning?

Although the maximum allowable concentration in the air is 1ml / m3, long-term working in the environment below this value will also lead to chronic poisoning, such as chronic bronchitis, chronic gastroenteritis, gingivitis, stomatitis, skin itching and other diseases. Short time exposure to high chlorine environment will lead to acute poisoning. Mild acute poisoning showed dry throat, chest tightness, pulse acceleration and other symptoms. Severe acute poisoning showed bronchospasm and edema, even coma or shock. Measures to prevent chlorine poisoning can be summarized as follows:

(1) workers who are often exposed to chlorine will be less sensitive to the smell of chlorine, and people will be injured by chlorine when they can't smell it. Therefore, the operator's duty room should be set up separately from the chlorination room, and monitoring and alarm devices should 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 heat-insulating, well ventilated, the door shall be opened outside, and shall be strictly separated from other working rooms without any direct connection. Because chlorine is heavier than air, when chlorine leaks in the room, it will crowd out the air, accumulate in the lower part of the closed room and gradually spread upward. Therefore, forced exhaust facilities must be installed at the bottom of the chlorination room, and the air inlet should be set at a high place.

(3) maintenance tools, gas masks and rescue appliances shall be reserved outside the chlorination room. The switches of lighting and ventilation equipment shall also be set outside. Ventilation shall be carried out before entering the chlorination room. The penstock leading to the chlorination room must ensure uninterrupted water supply, keep the water pressure stable, and take measures to deal with sudden water cut-off. The alkali pool shall be set in the chlorination room, and regular inspection shall be conducted to ensure that the alkali 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 lye pool.

(4) when someone is found to have acute chlorine poisoning at the scene, 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. 2% sodium bicarbonate solution can be used to wash their eyes, nose, mouth and other parts, and atomized 5% sodium bicarbonate solution can also be allowed to inhale.

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

Liquid chlorine is the most widely used disinfectant at home and abroad. Besides disinfection, it also has oxidation effect. Liquid chlorine is usually stored and transported in steel cylinders. When used, liquid chlorine is converted into chlorine gas and added into water.

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

(2) it needs to absorb heat when liquid chlorine turns into chlorine. It needs about 289kj heat when 1kg liquid chlorine turns into 1kg chlorine. When the air temperature is low, the heat absorbed by the chlorine bottle from the air is limited, and the quantity of liquid chlorine gasification is limited, so it is necessary to heat the chlorine bottle. However, the chlorine bottle should not be heated directly by open fire or steam, and the temperature of the chlorine bottle should not be increased too much or too fast. Generally, the chlorine bottle can be heated by continuous sprinkling of 15-25oc warm water.

(3) always use 10% ammonia water to check whether there is leakage at the connection between the chlorination machine and the chlorine bottle. If the chlorine pipe of the chlorination machine is blocked, do not flush it with water, dredge it with steel wire, and then blow off the sundries with air pump or compressed air.

(4) before opening, check whether the position of the chlorine bottle is correct, and make sure that the outlet is upward, that is to say, chlorine is released instead of liquid chlorine. When opening the main valve of chlorine bottle, first slowly open half a circle, then check whether there is air leakage with 10% ammonia, and then gradually open it after everything is normal. If the valve is difficult to open, it can not be knocked with a hammer, nor can it be pulled hard by a long spanner to prevent the valve stem from being broken.

What are the precautions when using sodium hypochlorite for disinfection?

Solid sodium hypochlorite NaClO is white powder with pungent odor. It is very unstable in 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 gas is introduced into 30% sodium hydroxide solution for reaction. Commercial solid sodium hypochlorite is easy to use, but its disinfection effect is worse than that of chlorine, and its cost is higher than that of chlorine.

As sodium hypochlorite is easy to decompose due to the effect of sunlight and temperature, sodium hypochlorite generator is usually used to prepare sodium hypochlorite on site and then add it immediately. The sodium hypochlorite solution obtained by electrolysis of salt water with titanium anode (seawater can be used as salt solution in coastal areas) is a light yellow transparent liquid, containing 6 g / L ~ 11 g / L available chlorine. In general, the consumption of salt is about 3-4.5kg and the power consumption is 5-10kwh per 1kg of available chlorine, which is lower than that of bleaching powder.

Sodium hypochlorite solid or solution should not be stored for a long time, and must be stored in dark and low temperature environment. The best way to produce sodium hypochlorite solution by electrolysis is to use and produce it 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 is higher than 30oC, the loss of available chlorine can reach 0.3-0.7mg/l per day. Therefore, in order to have a certain amount of reserve, the storage time should not exceed one day in summer and seven days in winter.

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

The bleaching powder CaCl2 · Ca (OCL) 2 · 2H2O is white powder with chlorine smell, containing 20% ～ 25% available chlorine. Bleaching powder is easy to absorb moisture, and its properties are extremely unstable. Sunlight and heat can make it deteriorate and reduce the effective chlorine content. Mixed with organic matter and flammable liquid, it can generate heat and spontaneous combustion, and explosion will occur under high heat. Chlorine tablets are made of bleaching powder 3CA (OCL) 2 · 2ca (OH) 2 · 2H2O, containing 60% ～ 70% available chlorine. The stability of chlorine tablets and bleach powder is higher than that of bleach powder, which can be stored at room temperature for more than 200 days without decomposition. The disinfection effect of the two is the same as that of chlorine, and the chlorine dosage and contact time calculated by effective chlorine can refer to liquid chlorine.

When bleaching powder is used as disinfectant, it needs to be filled with solution, and a mixing tank is usually set up. Each 50kg package of bleaching powder is first mixed with 400-500kg water to form 10% - 15% solution, and then added with water to form 1% - 2% solution. There are two types of mixing tank: diaphragm type and blast type. When chlorine tablets are used for disinfection, the waste water flows into a special chlorine tablet disinfector, infiltrates and dissolves the chlorine tablets, mixes with them, and then enters the contact pool.

37. What are the physical and chemical properties of chlorine dioxide?

Chlorine dioxide (ClO2) is a kind of yellow green gas, which is very unstable. It has the same pungent odor as chlorine. Its toxicity is greater than chlorine, and its 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 temperature rises, when exposed to light or when contacting with some organic substances. Moreover, liquid chlorine dioxide is easier to explode than gas chlorine dioxide. Explosion occurs when the volume concentration of chlorine dioxide in air exceeds 10% or in water exceeds 30%. Chlorine dioxide is easy to dissolve in water, and its solubility in water is 5 times that of chlorine. However, ClO2 does not react with water, and it is very volatile in water, so it is easy to photochemical decompose under light irradiation. When ClO2 solution is stored in an open container, the content of ClO2 will decrease rapidly. Therefore, chlorine dioxide is not suitable for storage, it is best to make and use on site.

Most of the commercial stable chlorine dioxide solutions sold on the market are colorless or yellowish green transparent liquid. The chlorine dioxide content is generally about 2%, and a certain amount of special stabilizers (such as sodium carbonate, sodium borate and perchloride aqueous solution or diethylenetriamine pentamethylene phosphonic acid, etc.) should be added. However, high temperature and strong light should be avoided during transportation and storage. Therefore, when chlorine dioxide is used for disinfection, it is best to use at the same time of production. Chlorine dioxide can produce non-toxic substances after sterilization, which has no pollution to environmental water.

38. 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 and application of raw water. When only used as disinfectant, the dosage range is 0.1-1.3mg/l; When used as deodorizer, the dosage range is 0.6-1.3mg/l; When used as oxidant to remove iron, manganese and organic matter, the dosage range is 1-1.5mg/l.

(2) chlorine dioxide is a kind of strong oxidant, its transportation and storage should use anti-corrosion, anti-oxidation inert materials, to avoid contact with reducing agent, so as to avoid explosion.

(3) when using the method of preparing chlorine dioxide on site, it is necessary to prevent the accumulation of chlorine dioxide in the air from causing explosion due to too high concentration. Generally, it is necessary to provide measures to collect and neutralize the gas released or leaked during the preparation of chlorine dioxide.

(4) in the working area and finished product storage room, there should be ventilation devices and monitoring and alarm devices, and protective equipment should be equipped outside the door.

(5) the stable chlorine dioxide solution itself is not toxic, and chlorine dioxide can be released only after activation. Therefore, the reaction intensity should be well controlled during activation, so as to avoid excessive concentration of chlorine dioxide accumulated in the air and cause explosion.

(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 try to be stable.

39. What are the preparation methods of chlorine dioxide?

There are many ways to prepare chlorine dioxide. In the water treatment industry, chlorine, hydrochloric acid or dilute sulfuric acid are generally used to react with sodium chlorite or sodium chlorate, and sodium hypochlorite is used to acidify with sodium chlorite to synthesize chlorine dioxide. The reaction equations 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, the chlorine water solution with pH < 2.5 is prepared first, and then enters the reaction chamber together with a certain amount of 10% sodium chlorite solution, which is fully mixed and reacted in the reaction chamber to generate chlorine dioxide. Theoretically, 7.5G chlorine dioxide can be produced by adding 3.9G chlorine to 10g sodium chlorite. In order to prevent the 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 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 excess 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 equipments for producing chlorine dioxide by electrolysis in the domestic market, but in fact, the so-called chlorine dioxide produced by these equipments is at most a compound of chlorine dioxide and chlorine. It is impossible to completely solve the problem that chlorine disinfectants will produce chlorinated hydrocarbons, and there have been cases of explosion when compound chlorine dioxide is used.

40. What are the common oxidants and reducing agents in wastewater treatment?

Theoretically, the two substances with different redox potential can relatively become oxidants or reducers, but the oxidants or reducers that can be used in wastewater treatment practice must meet the following requirements:

① It has good oxidation or reduction effect on the pollutants in wastewater

② The substance produced after reaction should be harmless to avoid secondary pollution

③ Cheap and reliable source

④ It can react quickly at room temperature without heating

⑤ The pH value needed for the reaction should be neutral, not too high or too low.

Oxidants commonly used in wastewater treatment are as follows:

① After receiving electrons, they are reduced to neutral atoms with negative charge ions, such as O2, Cl2, O3, etc;

② For example, under alkaline conditions, Cl + in OCL - and CL4 + in ClO2 are reduced to Cl -;

③ For example, Fe3 + in ferric chloride and mn7 + in potassium permanganate are reduced to Fe2 + and Mn2 + after receiving electrons.

Reducing agents commonly used in wastewater treatment are as follows:

① After the electrons are given, they are oxidized into neutral atoms with positive charge, such as iron filings, zinc powder, etc;

② The negatively charged atoms are oxidized to positively charged atoms after giving electrons. For example, the boron element in sodium borohydride has negative 5 valence, which can reduce mercury ions to metal mercury under alkaline conditions, and at the same time, it is oxidized to positive 3 valence.

③ A positively charged atom of metal or nonmetal, given an electron, is oxidized to a more positively charged atom. For example, Fe2 + in ferrous sulfate and ferrous chloride is oxidized to Fe3 + after giving an electron; After giving two electrons, sulfur dioxide SO2 and tetravalent sulfur in sulfite SO32 - are oxidized to hexavalent sulfur to form SO42 -.

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