Views: 913 Author: Site Editor Publish Time: 2024-09-21 Origin: Site
In wastewater treatment, total nitrogen and phosphorus are two key indicators, respectively, reflecting the nitrogen and phosphorus content in the water body. Excessive discharge of these elements will lead to eutrophication of water bodies and cause water pollution problems.
Table of contents（Click to go to where you want to see）
1. What is wastewater treatment flocculant
4. What factors should be considered when selecting a flocculant?
5. The application and mechanism of flocculants in dissolved air flotation (DAF) process are as follows
6.1 Difference Between Coagulation and Flocculation
Wastewater treatment flocculants are chemicals used in sewage treatment processes to promote the aggregation of suspended particles into larger clumps through chemical reactions. This facilitates sedimentation and filtration. The main mechanism of action of flocculants is to change the charge properties of suspended particles in water, causing them to attract each other and form larger flocs, which are easier to separate from the water.
One of the most economical and simple ways to improve water treatment is flocculation. In the water treatment process, flocculation has become an important field in the development of environmental protection technology due to its key role in determining the final water quality and production costs.
coagulation and flocculation process in water treatment
Flocculants are chemical agents destabilizing, coagulating, and precipitating colloids and suspended particles in wastewater. Different flocculants have different effects on the wastewater treatment process. The main mechanisms include double compression, electrical neutralization, adsorption bridging, and net capture.
There are many types of flocculants, up to two or three hundred. According to their chemical composition, flocculants can be divided into inorganic flocculants, organic flocculants, and microbial flocculants. In addition, there are mixed flocculants and composite flocculants, which are a mixture of inorganic coagulants and organic flocculants.
include aluminum and iron salts, such as aluminum sulfate, aluminum chloride, ferric sulfate, and ferric chloride. These inorganic flocculants generate positively charged ions through hydrolysis reactions, which can neutralize negatively charged colloidal particles, causing them to lose stability and thus condense into larger particles.
Due to their low cost, inorganic flocculants are currently the most widely used flocculants. According to their molecular weight, inorganic flocculants can be divided into two categories.
Inorganic low molecular weight flocculants can be divided into aluminum salts, iron salts, calcium salts, zinc salts, and composite metal salts according to metal salts. Among them, aluminum and iron salts are the most common and have a more comprehensive range of applications. They mainly include aluminum sulfate, aluminum chloride, ferric sulfate, and ferric chloride, primarily used to treat water supply, industrial wastewater, and municipal sewage.
Inorganic polymer flocculants have fast sedimentation speed, low dosage, good use effect, and wide application. They have been successfully applied in water supply, industrial wastewater, and urban sewage treatment and have gradually become mainstream flocculants.
Inorganic polymer flocculants can be classified according to the charge properties of different molecules into cations and anions. Cationic flocculants include polyaluminium chloride, polyaluminium sulfate, polyaluminium chloride iron, polyaluminium silicate, polyaluminium chloride polyphosphate, and polyferric phosphate sulfate. Anionic flocculants are primarily composed of polyaluminium silicate. Inorganic polymer flocculants require less dosage and have fast flocculation speed, so the market widely favors them. Inorganic polymer flocculants account for more than 80% of the market, and inorganic polymer flocculants have replaced the market share of inorganic low molecular weight flocculants.
Among them, polyaluminium chloride (PAC) has a higher charge than traditional inorganic low molecular weight coagulants, so it has stronger adsorption and neutralization capabilities. It also has a good flocculation effect in wastewater treatment. In addition, PAC has the advantages of low dosage, high turbidity removal rate, low sludge formation rate, and little impact on wastewater pH.
The advantages of organic flocculants are low dosage, wide applicable pH range, high product stability, low sludge production, good treatment effect, low treatment cost, etc. Therefore, organic polymer flocculants are widely used in sewage treatment. The relative molecular weight of organic polymer flocculants ranges from millions to tens of millions. It contains charged or neutral functional groups and exhibits electrolyte behavior after water dissolves. Organic flocculants can be divided into two categories according to their sources: natural organic polymers and synthetic organic polymers.
In wastewater treatment, total nitrogen and phosphorus are two key indicators, respectively, reflecting the nitrogen and phosphorus content in the water body. Excessive discharge of these elements will lead to eutrophication of water bodies and cause water pollution problems.
The commonly used natural organic polymer flocculants mainly include starch-based flocculants, chitosan-based flocculants, cellulose-based flocculants, etc. Natural organic flocculants have the advantages of being green, environmentally friendly, non-toxic, low-priced, and easy to biodegrade. They also have disadvantages such as low charge density, low relative molecular weight, and easy biodegradation and loss of activity. To overcome these shortcomings, chemical methods such as etherification, lipidation, and oxidation can modify natural organic polymer flocculants. However, the experimental modification steps are relatively cumbersome, and it isn't easy to achieve large-scale industrial production. With the development of the natural organic modified flocculant industry, natural polymer-modified flocculants will have a broader development prospect with their unique advantages.
Relevant researchers have synthesized a new type of modified chitosan flocculant (M-CTS) and compared and evaluated the oil removal and suspended solids removal effects of M-CTS, polyaluminum, and cationic polyacrylamide (CPAM) on alcohol-containing wastewater from a gas field. The results show that the treatment effect of M-CTS is significantly better than that of PAC and CPAM. Some researchers have modified cassava starch by grafting polydiallyl dimethyl ammonium chloride and evaluated the flocculation performance of simulated kaolin suspension when the grafting rates were 1.76%, 14.84%, and 21.98%, respectively. Compared with gelatinized starch, the modified flocculant improved the removal rate of turbidity and TSS, and the removal rates of turbidity and TSS increased with the increase of grafting rate and dosage.
Synthetic organic polymer flocculants have the advantages of small dosage, low sludge production, strong flocculation ability, fast sedimentation speed, and broad application prospects in water treatment. Synthetic organic polymer flocculants can be divided into four types according to the different charges after functional group dissociation: cationic, anionic, nonionic, and zwitterionic organic polymer flocculants.
Among them, cationic synthetic organic polymer flocculants mainly include polyacrylamide and its derivatives, homopolymers of dimethyl diallyl ammonium chloride, etc., which are the main direction of development of organic polymer flocculants at present and are mainly suitable for treating colloidal particles with negative charges on the surface of the wastewater.
Among cationic synthetic organic polymer flocculants, cationic polyacrylamide is the most commonly used, but its polymerized monomer acrylamide has substantial neurotoxicity. Therefore, how to reduce the monomer residue in the polymerization process is an urgent problem to be solved in the future.
Anionic synthetic organic polymer flocculants are mainly used to accelerate the sedimentation and suspension separation of inorganic suspensions, especially cationic-charged particles such as heavy metal hydroxides. Non-ionic organic polymer flocculants primarily generate short-term charges in aqueous solutions by protonation. The flocs formed are small and unstable. They are mainly used to accelerate the sedimentation of inorganic suspensions, and their flocculation effect is poor.
Zwitterionic synthetic organic polymer flocculants have both anionic and cationically charged functional groups and have the comprehensive properties of cationic and anionic synthetic organic polymer flocculants. They can treat wastewater systems where positive and negative charged particles coexist. This type of flocculant is currently rarely used in the field of water treatment.
Microbial flocculant is a new type of water treatment flocculant that is readily biodegradable, efficient, and non-toxic, obtained by separation and purification from microorganisms or their secretions, and the flocculation product has the characteristics of no secondary pollution. At the same time, it has a good coagulation effect and a unique decolorization effect. It is a non-tox, harmless, green, environmentally friendly product with many applications. Protein, polysaccharide, and nucleic acid biopolymer compounds are the main active ingredients.
Flocculants occupy an essential position in wastewater treatment. With the development of wastewater treatment, flocculants are gradually increasing. The most widely used inorganic salt flocculants are prone to secondary pollution, and polymerized AIC13 may cause Alzheimer's disease: polyacrylamide and its monomers have specific toxicity, and its application is subject to certain restrictions.
Microbial flocculants have advantages that other traditional flocculants cannot match, especially their biodegradability and environmental friendliness, and they are easy to produce in industry, so microbial flocculants will gradually replace most traditional flocculants. The wide application of microbial flocculants will become a development trend, and there is a broad application prospect in water treatment.
(1) Flocculants directly produced by microbial cells;
(2) Flocculants extracted from the components of microbial cell walls;
(3) Various flocculants produced by microbial cell metabolism.
The main components of microbial flocculants are polypeptides, proteins, mucopolysaccharides, esters, cellulose DNA, and the molecular weight is generally above 100,000.
The main components of microbial flocculants are proteins, DNA, polysaccharides, cellulose, etc.
There are two main structures:
(1) Fibrous：For example, the flocculant extracted from Nocardia Amara by Kazuo et al. has a fibrous structure like silk.
(2) Spherical：For example, Nakamura's flocculant obtained from the yeast Aspergillus sojae has a spherical structure.
Compared with other flocculants, microbial flocculants have the following advantages in sewage treatment:
Under the same usage amount, the efficiency of conventional flocculants is significantly lower than that of microbial flocculants.
Using microbial flocculants to treat various food wastewaters can reduce the amount of pollutants discharged and recover various valuable components, which will become a new method for wastewater treatment in the food industry.
Microbial flocculants are biodegradable; they can degrade by themselves, so flocculation will not cause secondary pollution to the water body.
Many microbial flocculants can achieve sound wastewater treatment effects.
The principle of water treatment with composite polymer flocculants is to produce a synergistic effect by mixing different flocculants, thereby achieving an enhanced flocculation effect. It is more widely used in water treatment, which can significantly save the flocculants used, produce less sludge, and directly reduce water treatment costs.
Many research results show that mixing or reacting two or more flocculants to obtain a composite flocculant can effectively improve the water treatment process, or using them together in a separate addition can achieve different effects.
Flocculants can be divided into three categories according to their components: inorganic-inorganic composite polymer flocculants, inorganic-organic composite polymer flocculants, and organic-organic composite polymer flocculants.
Among inorganic polymer flocculants, aluminum and iron salts are the most widely used in sewage treatment. These polymerized hydrolysis products have positive charges and can combine with negatively charged colloidal pollutants in water bodies through adsorption, electrolysis, and purification. However, they have little effect on the adsorption and bridging of colloidal substances and usually require higher dosages. Otherwise, the flocculation effect is challenging to achieve good results.
To further reduce the demand for inorganic flocculants and improve the treatment efficiency of inorganic flocculants, a large number of research results have shown that under certain conditions, the copolymerization of two or more separate inorganic flocculants can produce relatively high molecular weight inorganic-inorganic composite flocculants. Such flocculants mainly include polysilicates (polyaluminum silicate sulfate PASS, poly ferric silicate sulfate PFSS, poly ferric silicate chloride PFSC), aluminum-iron copolymers (poly aluminum ferric chloride PAFC, poly aluminum ferric silicate PSAF), etc.
Since the 1930s, polysilicate has been widely used in water treatment as an intermediate product of silicate polymerization to a certain extent and used as a coagulant. At present, its preparation method, polymerization mechanism and factors affecting the degree of polymerization have been thoroughly studied. Polysilicate will not have an electrical neutralization effect on the colloid particles in the water, because polysilicate is an anionic substance with a negative charge, and the surface of the colloid particles in the water usually also has a negative charge.
In addition, the performance of polysilicate is unstable in a neutral environment. During storage, polysilicate will polymerize itself to form a high molecular polymer that is insoluble in water, thereby losing its flocculation properties. Due to its poor stability, the scope of application of polysilicate has been greatly reduced, and its convenience of use has also been limited.
Studies have shown that in a strong acid or strong base environment, the stability of polysilicate will gradually improve, and the introduction of alkaline metal ions can prolong its coagulation time and improve the stability of the coagulant. Therefore, people began to study polymer silicate coagulants made of polysilicates injected with aluminum salts or iron salts. They can be hydroxylated and polymerized first and then mixed, or mixed first and then polymerized.
The flocculant combines polysilicate with aluminum or polyiron, making it have the advantages of all three. At the same time, it also has the functions of adsorption, bridging and electrical neutralization. It is much stronger than the flocculant and unstable performance of polysilicic acid or polymetallic ions alone. Because compared with polysilicic acid, while increasing stability, it also improves the electrical neutralization ability and the capture efficiency; and compared with the same polymetallic ions, it improves both the absorption capacity and the bridging ability.
A typical example of the continuous development of polyaluminum and polytron inorganic polymer coagulants into composite types is the aluminum-iron composite polymer, which has the common properties of aluminum salt flocculants and iron salt flocculants. Aluminum salt flocculants have good decolorization and large flocs, but the structure of the flocs is loose and easy to break, and the sedimentation rate is relatively slow. In contrast, although the iron salt flocculants have a fast sedimentation rate and dense flocs, they are small, resulting in poor net capture and sweeping effects. After treatment, the color of the water is relatively dark. Polyaluminum and polytron copolymers constitute a new type of polymer, which effectively integrates the advantages of aluminum salt flocculants and iron salt flocculants and enhances the flocculation effect of aluminum-iron composite polymers. Therefore, scholars are paying more and more attention to and studying this flocculant.
According to the different coexisting anions, it can be divided into several major categories, such as polyaluminum chloride iron, polyaluminum sulfate iron, and polysilicate iron. These types are collectively referred to as polyaluminum iron.
Inorganic-organic composite flocculant is a physical and chemical reaction that occurs under specific conditions, transforming its original components and forming a stable polymer structure. At the same time, all components work together to improve the flocculation effect. Compared with organic polymer flocculants, inorganic polymer flocculants have advantages such as a wide range of raw material sources, low prices, and a more extensive range of optimal dosages, but they also have some disadvantages, such as small molecular weight and particle size, low adsorption and bridging ability of aggregates, large dosage, large amount of sludge, and difficult post-treatment. However, the advantages of organic polymers are high molecular weight, good flocculation effect, strong adsorption capacity for bridging colloid substances, good product stability, low dosage, and wide application range, but the synthesis process is complex, the optimal dosage surface is narrow, and the water treatment cost is high.
Given that these two flocculants have advantages and disadvantages, performance and cost are highly complementary. This has prompted people to consider combining the two to make inorganic-organic composite coagulants to overcome their respective shortcomings, enhance the effect, and expand the application field. In production practice, various flocculants must be used so that their strengths can be used to avoid weaknesses and achieve complementary effects. The research and development of composite flocculants, driven by theory and practice, are rapidly carried out and are the focus of scientific research on flocculants.
The different inorganic components of inorganic-organic composite flocculants can be divided into aluminum-based, iron-based, and magnesium-iron-based composite flocculants. The organic substances involved in the composite are mainly polyacrylamide and its derivatives, dimethyl diallyl ammonium diacid (DMDAAC), and its copolymers, and natural polymer organic materials.
Organic-organic composite flocculants mainly include natural polymer compounds and their nonionic, cationic, anionic and amphoteric modified derivatives: such as starch, lignin, cellulose, plant rubber and copolyamide.
Synthetic polymer flocculants among organic polymer flocculants have shown remarkable effects in wastewater treatment, but their decomposition products are toxic. On the contrary, natural polymer flocculants are non-toxic and degrade quickly, but their relative molecular weight is small, resulting in poor flocculation effect.
Therefore, according to their respective advantages and disadvantages, synthetic polymer flocculants and natural polymer flocculants can be combined to improve the flocculation effect of the flocculants by relying on the synergy between different flocculants.
After the flocculant is added to the water, rapid stirring promotes complete contact and mixing between the flocculant and the suspended particles in the water.
The hydrolysis products of the flocculant neutralize the charges on the surface of the suspended particles, reducing the stability of the particles and causing them to attract each other.
During the stirring and standing process, the particles gradually form larger flocs through adsorption, bridging, and netting.
The formed flocs are easy to settle during the standing process due to their large surface area and density, thus being separated from the water.
When selecting a suitable flocculant, several key factors need to be considered comprehensively to ensure that the flocculant can effectively promote the coagulation and sedimentation of suspended particles. The following are detailed considerations based on reference materials:
pH value:
Different flocculants have different effects under different pH conditions. For example, aluminum salts work better at pH values of 5.5 to 8, while iron salts are effective over a wider pH range (4 to 11).
High turbidity water may require a stronger flocculant or increased dosage
Such as organic matter, heavy metals, types and contents of suspended matter
Water bodies with high organic matter content may require the selection of flocculants with stronger adsorption capacity.
Water with higher hardness may require the selection of flocculants that can effectively handle calcium and magnesium ions.
Charge characteristics:
cationic, anionic, or non-ionic to match the charge of pollutants in wastewater.
The molecular weight affects the flocculation effect and the size of the flocs formed. Flocculants with high molecular weight usually have a more substantial bridging effect.
determines its ability to form flocs in water.
Different functional groups affect flocculants' adsorption and bridging ability.
such as requirements for turbidity, color, COD, and other indicators.
including sludge production, dewatering performance, etc.
whether to remove suspended solids, reduce turbidity, remove organic matter, or remove specific pollutants. Different treatment purposes may require different flocculants.
Choose flocculants that are less toxic to humans and the environment.
Choose biodegradable flocculants to reduce environmental impact.
The price of flocculants is a crucial consideration when choosing.
Dosage:
affects operating costs.
such as sludge treatment, discharge costs, etc.
Compatibility with other water treatment chemicals: Ensure that there is no mutual interference or adverse reaction
Small and pilot tests: observe and test the treatment effect under different dosages and operating conditions to determine the most suitable flocculant type and dosage
Solubility:
Choosing a flocculant with good solubility can reduce dissolution time and energy consumption.
Dosage method:
Choosing a flocculant that is easy to add and operate can reduce the difficulty of operation and labor costs.
(9) Supplier Reputation and Service
Choose suppliers with stable product quality and a good reputation.
Ensure that necessary technical and after-sales services are available.
Flocculants mainly play the following roles in the dissolved air flotation process:
Flocculants reduce the repulsive force between particles by changing the charge properties on the surface of suspended particles, allowing the particles to approach each other and connect into larger flocs through physical or chemical interactions. Bubbles more readily adsorb this floc and float to the water surface, thereby improving the separation efficiency of air flotation.
The use of flocculants can significantly improve the effect of air flotation. Larger flocs can be formed by optimizing the flocculation process, which is more readily adsorbed by bubbles and floats to the water surface, thereby improving the separation efficiency of air flotation.
Through the use of flocculants, the effect of air flotation can be significantly improved, thereby reducing the burden of subsequent treatment steps. Using flocculants can remove more suspended solids and colloidal substances and enhance the quality of the effluent.
Precise flocculant addition can reduce chemical waste and reduce treatment costs. Using flocculants can significantly improve the effect of air flotation, thereby reducing the burden on subsequent treatment steps and operating costs.
Effective flocculation can remove more suspended solids and colloidal substances and improve the quality of the effluent. Using flocculants can significantly enhance the effect of air flotation, thereby improving the quality of effluent.
A good flocculation effect can reduce the production of sludge and the cost of sludge treatment and disposal. Using flocculants can significantly improve the impact of air flotation, thereby reducing sludge production.
By optimizing the flocculation process, the effect of air flotation can be significantly improved, thereby improving treatment efficiency. Using flocculants can dramatically enhance the impact of air flotation, thereby improving treatment efficiency.
The flocculant addition system can adapt to different types and concentrations of flocculants and the specific needs of various wastewater treatments. Using flocculants can significantly improve the effect of air flotation, thereby improving treatment efficiency.
Modern flocculant addition systems are usually equipped with automatic control units that can adjust the addition of flocculants based on preset programs or real-time monitoring data to achieve unattended automated operation. Using flocculants can significantly improve the effect of air flotation, thereby improving treatment efficiency.
The system is designed with ease of maintenance in mind. Key components such as metering pumps and mixing devices are easy to disassemble and clean, reducing maintenance time and costs. Using flocculants can significantly improve the effect of air flotation, thereby improving treatment efficiency.
The flocculant addition system was designed with environmental factors in mind, reducing the use of chemicals, lowering treatment costs, and reducing potential environmental impact. Using flocculants can significantly improve the effect of air flotation, thereby improving treatment efficiency.
In summary, the role of flocculants in the dissolved air flotation process is mainly reflected in promoting the accumulation of suspended particles, improving the flotation effect, reducing the burden of subsequent treatment steps, reducing operating costs, improving effluent quality, reducing sludge production, Improve processing efficiency, strong adaptability, automatic control, easy maintenance, and environmental friendliness.
Coagulation includes two processes: coagulation and flocculation. Coagulation in sewage treatment is a commonly used physical and chemical treatment method. By adding coagulants to sewage, suspended matter and colloidal particles are coagulated and precipitated, thereby purifying water quality.
The coagulation sedimentation method adds coagulants and coagulants to sewage to aggregate harmful substances such as suspended matter, chromaticity, and heavy metal ions in water to form larger particles, which are easy to separate from water.
The principle of coagulation is to add coagulants to sewage to destroy the stability of colloids so that fine suspended particles and colloidal particles aggregate into coarser particles and settle and separate from water so that wastewater is purified. Coagulants refer to chemical agents that can aggregate colloidal particles and suspended matter in water. Commonly used coagulants include aluminum salts, iron salts, polyacrylamide (PAM), etc. Coagulants refer to substances that can promote the aggregation of suspended matter and colloidal particles during coagulation, such as activated carbon, filter material, etc.
The coagulation sedimentation method has the advantages of good treatment effect, wide application range, and simple operation. Still, it also has disadvantages, such as adding chemical agents, an extended treatment cycle, and possibly generating suspended matter. Some improvement measures can be taken to overcome these disadvantages, such as optimizing the selection of chemical agents and improving treatment efficiency. In addition, the coagulation sedimentation method can also be used in combination with other treatment methods to achieve better treatment effects.
Coagulation includes two processes: coagulation and flocculation. Agents that can play a coagulation and flocculation role are collectively referred to as coagulants. Coagulation mainly refers to the process of colloid destabilization and the formation of micro-aggregates, while flocculation mainly refers to the process of destabilized colloids or micro-suspended matter agglomerating into large floccules.
Coagulation: Coagulation is a chemical process. By adding coagulants (such as aluminum salts, iron salts, etc.), the negative charges of colloid particles and micro-suspended matter in water are neutralized, destabilizing them to form micro-aggregates.
Coagulation process: Coagulants react with colloid particles and micro-suspended matter in water, neutralize their charges, destabilize them, and form micro-aggregates.
Flocculation: Flocculation is a physical process. By adding flocculants (such as high molecular polymers), bridges are built between destabilized colloid particles and micro-suspended matter, so that they aggregate into larger floccules.
Flocculation process: The polymer chains of flocculants bridge between destabilized colloid particles and micro-suspended matter, so that they aggregate into larger floccules.
Coagulation mechanism: Coagulants neutralize the negative charge of colloidal particles and tiny suspended matter, destabilizing them and forming tiny aggregates.
Coagulation chemical reaction: Coagulants react with colloidal particles and tiny suspended matter in water, neutralize their charge, destabilize them and form tiny aggregates.
Flocculation mechanism: The polymer chains of flocculants bridge between destabilized colloidal particles and tiny suspended matter, causing them to aggregate into larger flocs.
Flocculation physical process: The polymer chains of flocculants bridge between destabilized colloidal particles and tiny suspended matter, causing them to aggregate into larger flocs
