The application of flocculants in wastewater treatment

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The Application of Flocculants in Wastewater Treatment by Beijing En Nader Technology Co., Ltd. The flocculation process is one of the most widely used and common unit operations in various wastewater treatment processes both domestically and internationally; it represents a crucial element in wastewater treatment. The quality of flocculation often determines the operation of subsequent processes, the quality of the final wastewater, as well as the associated costs. Choosing the right flocculant holds significant technical and economic value in terms of improving wastewater quality and reducing water treatment costs.            Based on their chemical composition, flocculants can be classified into inorganic salt flocculants, organic polymer flocculants, and microbial flocculants. There are few types of inorganic salt flocculants; they mainly include low-molecular-weight salts such as aluminum salts, iron salts, and hydrolyzed polymers, as well as inorganic polymer-based flocculants. Organic polymer flocculants mainly include synthetic organic polymer flocculants and naturally modified organic polymer flocculants.            1 Inorganic salt flocculants           1.1 Inorganic low-molecular-weight flocculants           Inorganic low-molecular-weight flocculants include aluminum sulfate, aluminum chloride, iron sulfate, iron chloride, etc. Among these, aluminum sulfate was developed in the United States and remains an important inorganic flocculant that is still in use to this day. Common aluminum salts include aluminum sulfate AL2(SO4)3·18H2O and alum AL2(SO4)3·K2SO4·24H2O. Another category consists of iron salts, such as hydrated ferric chloride FeCl3·6H2O, hydrated ferrous sulfate FeSO4·17H2O, and ferric sulfate. The advantages of inorganic flocculants are their low cost and simple usage ； However, it requires a large amount, has a low flocculation effect, and is also plagued by high costs and strong corrosivity.            1.2 Inorganic polymeric flocculants           Inorganic polymeric flocculants are a new type of wastewater treatment agent that were developed in the late 1960s. Compared to traditional flocculants, it can double the efficiency while being cheaper, which gives it a trend of gradually becoming the mainstream agent. At present, the production of such flocculants in Japan, Russia, Western Europe, and our country has reached an industrialized, scaled-up level with automated processes; coupled with stable product quality, the production of inorganic polymeric flocculants accounts for 30% to 60% of the total flocculant output.            1.2.1 Simple inorganic polymer flocculants     These inorganic polymer flocculants are mainly polymers of aluminum salts and iron salts. Such as polyaluminum chloride (PAC), polyaluminum sulfate (PAS), polyferric chloride (PFC), and polyferric sulfate (PFS), etc. The reason why inorganic polymer flocculants are more effective than other inorganic flocculants is that they can provide a large number of complexing ions, and they are capable of strongly adsorbing colloidal particles; through adsorption, bridging, and cross-linking, they facilitate the coagulation of these particles. At the same time, physicochemical changes occur, neutralizing the charges on the surfaces of colloidal particles and suspended solids, thereby reducing the δ potential. This causes the colloidal particles to shift from repelling each other to attracting one another, disrupting the stability of the colloidal aggregates and leading to collisions between these particles, which results in the formation of flocculent coagulated precipitates. The surface area of such precipitates can reach (200–1000) m2/g, giving them a high adsorption capacity.            1.2.2 Modified monocationic polymeric flocculants In addition to the commonly used polyalums and polyferrics, there are also polyactive silicas and their modified versions, such as polyalumina (iron) and polyphosphalumina (iron), which are modified by introducing certain highly charged ions to enhance the charge-neutralizing capacity ； Agents such as polyaluminum silicate sulfate (PASS) and polysilicate flocculants (PSAA) introduce hydroxyl groups, phosphate groups, etc., to enhance their coordination ability and thereby alter the flocculation effect. The possible reasons are: certain cations or anions can alter the morphological structure and distribution of the polymer, or there is a synergistic effect between two or more polymers. Studies on polysilicic acid flocculants containing aluminum ions (PSAA) have shown that PSAA possesses a stronger oil-removal capacity than PACS (modified polyaluminum chloride containing sulfate groups) in the treatment of heavy oil production water from oil fields; when used to treat wastewater from coal mines, it can achieve an COD removal rate of 98.2% and a removal rate of suspended solids of 99.4%. The preparation method of PASS is simple, the raw materials are readily available, and the cost is low, giving it great potential for development and wide application prospects. Research on the polyferric silicate sulfate (PFSS) flocculant has shown that highly polymerized silicic acid, in combination with metal ions, can produce excellent coagulation effects; therefore, it is possible to use it as a substitute for synthetically produced polymer flocculants in wastewater treatment in order to eliminate toxicity. Moreover, by adjusting the Fe/SiO2 molar ratio, the formulation of PFSS can be modified to achieve good flocculation results depending on the specific wastewater to be treated.            1.2.3 Polycationic inorganic polymeric flocculants           Polyaluminum-iron composite flocculants are complex inorganic polymeric flocculants that contain polyaluminum, polyiron, as well as polymeric complexes of chloride and sulfate ions; they are attracting increasing attention due to their combination of the excellent properties of polyaluminum and polyiron. Polyaluminum ferric chloride sulfate (PAFCS) is one of them; its effective iron-aluminum content (AL2O3+Fe2O3) is greater than 22%, and the product has a high hygroscopicity. Studies show that when the effective aluminum content in polyaluminum chloride (PAC) is higher than that in PAFCS, PAFCS yields better results in wastewater treatment compared to alum ； In oily wastewater as well as printing and dyeing wastewater, PAFCS performs better than PAC, and it also has a stronger decolorization capacity. The flocs have a high specific gravity, rapid flocculation speed, are easy to filter, and yield a high water recovery rate. Their raw materials are derived from industrial waste residues, resulting in low costs, making them suitable for wastewater treatment.            Polyferric silicate flocculants are also one of such agents. Song Zhiwei and others have used them to treat domestic wastewater, and the treatment efficiency as well as the COD removal rate achieved with these agents were superior to those of polyferric compounds; the turbidity removal rate exceeded 99%, the color removal rate was 65%–70%, and the COD removal rate reached 70%. Additionally, these agents can remove most of the ammonia nitrogen and all of the phosphorus present in domestic wastewater.            Aluminum-iron copolymer composite flocculants also belong to this category. Their raw materials, aluminum chloride and iron chloride, are both inexpensive traditional inorganic flocculants that are readily available and require simple production processes, making them suitable for development and utilization. The copolymer of aluminum salts and iron salts is different from a mixture of the two salts; it is a flocculant that more effectively combines the advantages of PAC and FeCL3, thereby enhancing its turbidity-removing effect. Among them, the influence of the iron content and its distribution pattern in the aluminum-iron copolymer composite flocculant on the flocculation properties requires further study. The pH value of the copolymer is determined by the hydrolysis capacity of the PAC and FeCl3 solutions; the pH value of these solutions lies between those of their respective parent solutions, depending on the amount of aluminum salt or iron salt present.            1.2.4 Boron sludge composite flocculant The boron sludge composite flocculant is a type of flocculant that contains polymers of water-soluble inorganic salts such as magnesium, iron, and aluminum. The main components of boron sludge are mixtures containing magnesium, aluminum, iron, silicon, boron, and calcium; it contains no chemically toxic substances to humans, and can be used as a raw material for wastewater treatment. Using boron sludge and pickling waste liquid as raw materials not only reduces the discharge of waste residues and waste liquids, but also enables the transformation of these wastes into useful resources. The coagulation mechanism of the boron sludge composite flocculant includes compression of the double electric layer, electrostatic neutralization through adsorption, adsorption and bridging, as well as precipitation and trapping. It combines effective components such as magnesium, aluminum, iron, and active functional group components, thereby enabling their synergistic effect during the coagulation process; effective coagulation can occur across various pH ranges. According to available information, the YJ-1807# composite wastewater treatment agent, which is now in mass production, is a flocculant synthesized from boron sludge and pickling waste liquid. This flocculant has functions such as demulsification and flocculation, removal of suspended solids, decolorization, reduction of COD, and elimination of various toxic substances.            2 Organic flocculants Compared with inorganic polymer flocculants, organic polymer flocculants require less dosage, achieve faster flocculation, are less affected by the presence of salts, pH levels, and temperature, result in less sludge formation, and are easier to handle; as a result, they have broad application prospects. The organic polymer flocculants currently in use mainly fall into two categories: naturally modified polymer flocculants and synthetic polymer flocculants.            2.1 Natural organic polymer flocculants The amount of natural polymer flocculants used is much smaller than that of synthetic organic polymer flocculants, due to their low charge density, low molecular weight, and tendency to undergo biodegradation, which results in the loss of their flocculating properties. Since the 1970s, many researchers have begun to focus on the development of chemically modified organic polymer flocculants. These natural polymer compounds contain various reactive functional groups, such as hydroxyl groups and phenolic hydroxyl groups, which endow them with relatively active chemical properties. Through chemical modifications such as esterification, etherification, oxidation, cross-linking, and graft copolymerization of hydroxyl groups, its active functional groups **increase**. The branched structure of polymers disperses the flocculating groups, thereby enhancing the ability to capture and aggregate particles in suspension systems. To improve the flocculation efficiency of such substances, extensive research has been conducted on their modification. Compared with synthetic organic polymer flocculants, modified natural polymer flocculants offer advantages such as greater selectivity, non-toxicity, and lower cost. Depending on their raw material sources, such flocculants can be broadly classified into starch derivatives, cellulose derivatives, chitin derivatives, modified plant gum products, and modified polysaccharide-protein products.            2.1.1 Starch derivatives Among the various natural modified polymer flocculants, those based on starch have attracted particular attention in terms of research and development, as starch is readily available, inexpensive, and its derivatives are fully biodegradable, enabling a sustainable cycle in nature. Starch is a substance composed of many dehydrated glucose units linked together by glycosidic bonds; each dehydrated glucose unit has an alcohol hydroxyl group at the 2, 3, and 6 positions, which means that starch molecules contain numerous reactive groups. Starch derivatives are produced by the reaction of the hydroxyl groups on the glucose units in their molecules with certain chemical reagents under specific conditions. It is worth noting that in recent years, research on the graft copolymerization of various starches with acrylamide, acrylic acid, acrylates, acrylonitrile, and similar substances, as well as the development and application of related products, have been carried out on a wide scale. Compared with polyacrylamide, it features strong stability, a wide range of applicability, and strong flocculation ability.            2.1.2 Lignin derivatives           Lignin is an aromatic polymer found in plant cellulose, and it is a major component in the papermaking pulp process. Due to the massive discharge of papermaking wastewater containing large amounts of lignin, which not only severely pollutes the environment but also leads to significant waste of material resources, research on using lignin as a raw material to produce various chemical products, including treatment agents, is attracting increasing attention.    In the mid-to-late 1970s, Rachor and Dilling synthesized quaternary amine-type cationic surfactants using lignin as a raw material, and achieved good flocculation effects when using these surfactants to treat dye wastewater. The lignin quaternary ammonium salt flocculant synthesized by Wu Bingyan and others in our country possesses excellent flocculation properties, and it shows good decolorization effects when used to treat acid-contaminated wastewater with high concentrations and high color intensity. Some people have also used lignin from papermaking steam waste liquids to synthesize lignin cationic surfactants, which are used to treat wastewater containing cationic dyes, direct dyes, and acid dyes. Experiments have shown that these agents possess excellent flocculation properties, with a decolorization rate of over 90% for various types of dyes. Lignin-modified products can also be used as flocculants for protein-containing wastewater, due to their non-toxicity, and the recovered protein can be used as feed.            2.1.3 Chitin Derivatives           Chitin is the second most abundant natural organic polymer after cellulose in nature, and it constitutes the main component of the exoskeletons of crustaceans (shrimp, crabs) and insects. Research on chitin is very active in many areas, and progress has been made. By appropriately modifying chitin and removing the acetyl groups, chitosan is obtained, which is a flocculant with excellent properties. Since the molecules of such substances contain amide groups, amino groups, and hydroxyl groups, they possess functions such as flocculation and adsorption. In recent years, significant progress has been made in research on the application of chitin in wastewater treatment, with many results having reached the practical stage or been commercialized. Japan uses about 500 tons of chitin for water treatment each year.            2.2 Synthetic Polymer Flocculants           Among synthetic polymer flocculants, polyacrylamide (PAM) is the most widely used. Polyacrylamide comes in three types: non-ionic, cationic, and anionic, with their relative molecular weights all ranging from 1.5 million to 8 million. Polyacrylamide adsorbs particles suspended in water, causing cross-linking between ions, thereby facilitating their flocculation and sedimentation. Polyacrylamide has a significant effect on wastewater treatment; it is widely used in the treatment of industrial wastewater and is an important and commonly employed polymer flocculant. However, due to the presence of a certain amount of residual monomer acrylamide in such flocculants, which inevitably leads to toxicity, their application is limited. Currently, research on the modification of polyacrylamide is also an important area of study. The amide groups in polyacrylamide are acyl derivatives of nitrogen or amines. Since the unshared electron pair of the nitrogen atom in the amide group forms a conjugated system with the π electrons in the hydroxyl double bond, the electron density of the nitrogen atom decreases, and the hydrogen atom attached to it becomes more reactive and is more prone to protonation. Therefore, under certain conditions, amine molecules are introduced onto polyacrylamide via the Mannich reaction to generate quaternary ammonium cations. Polyacrylamide cationic flocculants not only exert a bridging effect on the flocs but also an enveloping effect. Polymers that undergo bridging and encapsulation can also *interact to form three-dimensional network structures, which facilitates sedimentation and separation.            Polydimethyl diallyl ammonium chloride (PDADMA) and the dimethyl diallyl ammonium chloride-acrylamide copolymer (PDADMA-AM) are cationic polymers that possess advantages such as a positive charge, high density, good water solubility, easy control of molecular weight, high efficiency, low toxicity, and low cost. Therefore, they are widely used in industries such as oil extraction, papermaking, wastewater treatment, pharmaceuticals, textiles, and the food industry. When applied to wastewater treatment, it achieves better treatment results than the currently commonly used inorganic polymer flocculants and organic polymer flocculant PAM. It can be used alone or in combination with inorganic flocculants.            Synthetic polymer flocculants have been widely studied and applied at home and abroad, but they have drawbacks such as toxicity, difficulty in biodegradation, and high costs. In an era where environmental protection is receiving increasing attention, they are not favored by people.            2.3 Water-soluble amphoteric polymer flocculants           Water-soluble amphoteric polymers are those that contain both positive and negative charge groups on their polymer chains; compared to water-soluble anionic or cationic polymers that possess only one type of charge, they exhibit unique properties. As a flocculant, it can not only remove suspended solids and colloids from wastewater, but also eliminate dissolved substances in the wastewater that conventional flocculants cannot handle—such as colored substances and surfactants. Practice in using amphoteric polymer flocculants for sludge dewatering shows that sludge treated with these flocculants exhibits good settling properties and a low moisture content in the sludge cake. Furthermore, since the acidic and basic groups within amphoteric polymers can form chelate bonds with metal ions, which can then be released at the isoelectric point, this property can be utilized to separate and recover metal ions. Amphiphilic polymers can also be reused, which will play a positive role in the treatment of heavy metal pollution. Therefore, water-soluble amphoteric polymer flocculants have broad application prospects in wastewater treatment. Although there are reports in China on gendered polymer flocculants, these are limited to laboratory synthesis and preliminary studies on their properties; there is no market available with mature products that possess good performance.            3 Microbial flocculants           Microbial flocculants are the third type of flocculants that were developed in the late 1980s. They are metabolic products produced by microorganisms and possess flocculating properties; these include glycoproteins, polysaccharides, proteins, cellulose, DNA, as well as bacterial cells with flocculating capabilities. This flocculant is a new type of efficient and inexpensive water treatment agent obtained through biotechnology, via microbial fermentation, extraction, and purification; it is a non-toxic biological polymer compound. Reports on microbial flocculants abroad mainly include the AJ7002 microbial flocculant, the PF101 flocculant, and the NOC-1 flocculant, among others. Compared to the relatively well-understood flocculation mechanisms of classical gel-based flocculants, those of biological flocculants are not yet fully understood. Some of the representative flocculation mechanisms include the extracellular polymer bridging theory, electrical neutralization theory, in vitro cellulose fiber theory, capsule theory, and hydrophobicity theory. It is generally believed that biological polymer flocculants mainly cause the aggregation of particles and cells through bridging and electroneutralization effects; other flocculation mechanisms such as the meshing theory and the granular theory can explain some flocculation phenomena. In fact, flocculation is a complex process, as factors such as the type and concentration of flocculants, molecular structure, molecular weight, properties of the colloid surface, and pH can all affect its flocculating efficiency. Microbial flocculants possess advantages such as a wide flocculation range, high flocculation activity, safety, harmlessness, no pollution, and a unique decolorization effect. Additionally, the organisms that produce these flocculants are diverse, grow rapidly, and are easy to utilize on an industrial scale; as a result, research on microbial flocculants has become an important topic in the field of flocculants worldwide.            4 Development trends of flocculants At present, research on flocculants focuses mainly on polymeric flocculants. However, with further investigation into microbial flocculants, it is becoming a trend for these to replace some of the traditional inorganic polymeric flocculants and synthetic organic polymeric flocculants. Research on microbial flocculants abroad has been extensive, while research in China is still at the stage of strain screening. The main drawbacks include high costs, limited treatment capabilities, difficulties in maintaining activity, and challenges in achieving industrial application. Therefore, the future directions for research should be: ① Studies on the flocculation mechanism, kinetics, and the physicochemical properties of flocculants.            ②Search for inexpensive and efficient carbon and nitrogen sources to prepare cost-effective and high-performance media ； Optimize production conditions to reduce costs, explore the development of new technologies and processes, and select high-efficiency microbial strains.            ③Expand the application scope of flocculants by using genetic engineering techniques to introduce pollutant-degrading plasmids into microbial strains, thereby integrating flocculation, sedimentation, and degradation into one process.            ④Develop composite products of microbial flocculants and other flocculants to leverage their complementary advantages and enhance efficacy.            ⑤Process study on the preparation of microbial flocculants using high-concentration nitrogen-containing organic wastewater and inexpensive raw materials. In short, the development of new, efficient, safe, and economical flocculants is an inevitable trend in the advancement of flocculant production.