Flocculant

Introduction

Flocculation is a common treatment method used in current water treatment processes and has been widely used in wastewater treatment, sludge dewatering treatment and etc. In the process of flocculation, flocculant is the basis and core. Flocculant refers to the chemical or substance added to a suspension to accelerate the rate of flocculation or to strengthen the flocs formed during flocculation. As the water pollution becomes increasingly serious, water purification treatment becomes increasingly important. Although there are many methods of water purification treatment, such as biochemical treatment, ion exchange, adsorption, chemical oxidation, electrodialysis, etc., flocculation has been generally considered as a relatively effective treatment method with the advantages of low cost, efficiency and simple operation. Therefore, flocculant is indispensable in water purification and widely used in chemical, mining, environmental protection, municipal and many other fields.

Fig. 1. The result of flocculation-sedimentation by flocculants.

Classification

According to chemical composition, flocculant can be mainly divided into inorganic flocculant, synthetic polymeric flocculant, natural polymeric flocculant, bioflocculant and compound flocculant.

• Inorganic flocculant: Inorganic flocculant mainly includes the two types of iron and aluminum-based flocculant and can be further divided into inorganic small-molecule flocculant and inorganic polymeric flocculant. Inorganic small-molecule flocculant includes aluminum chloride, aluminum sulfate, alum, ferrous sulfate, ferric chloride and so on. Inorganic polymeric flocculant is the polymer of inorganic small-molecule flocculant, including polyaluminum chloride (PAC), polyferric sulfate (PFS) and polysilicic acid, etc. Inorganic polymer flocculant is mainly commercial used inorganic flocculant at present due to their strong flocculation ability, good flocculation effect and low price.
• Synthetic polymeric flocculant: Synthetic polymeric flocculant is usually formed by the addition polymerization of one or more monomers through the solution polymerization, reverse phase emulsion polymerization and reverse phase suspension polymerization, which has the characteristics of high molecular weight, long molecular chain and many active functional groups. According to different charge, synthetic polymeric flocculant can be divided into cationic, anionic, nonionic and amphoteric polymeric flocculants. Among them, cationic, anionic and nonionic organic polymeric flocculants are most frequently used in water treatment. The commonly used synthetic polymeric flocculants include polyacrylamide (PAM), polydimethyldiallylammonium chloride (PDADMAC), sulfonated polyethylene benzene, polyethylene ether and others, among which the polyacrylamide series is the most widely used.

Fig. 2. Typical structures of various synthetic polymeric flocculants ^[1].

• Natural polymeric flocculant: Natural polymeric flocculant has attracted significant attention as environmentally friendly, biodegradable, and nontoxic, and they include chitosan, starch, lignin, alginate and so on. However, they also have many drawbacks in practical application, including small molecular weight, low charge density, and poor water solubility. To overcome these disadvantages, various chemical modification methods are used to graft special groups onto their molecular chains to obtain the efficient and environmentally friendly flocculant ^[1].

Fig. 3. Preparation process of modified starch flocculant.

• Bioflocculant: Bioflocculant refers to macromolecular substances with flocculating effect obtained from microorganisms or microbial secretions, including polysaccharide, protein, cellulose, glycoprotein and DNA, etc. Bioflocculant belongs to green products with the advantages of good flocculation effect, unique decolorization effect, wide application range, biodegradation, safety and reliable. Bioflocculant can be produced by the microorganisms of bacteria, molds, actinomyces, yeasts, algae and others. These microorganisms are widely found in nature, such as soil, sludge and wastewater.
• Compound flocculant: With the development of economy, the pollutants in water are increasingly complex and diverse. A single flocculant can't meet the requirements of water treatment. So, the compound flocculant made by chemical or physical methods has become the trend. Compound flocculant can exert the advantages of different flocculants and make them with synergistic effect. At present, the research mainly focuses on the composite of inorganic flocculants such as PAC and PFS with organic macromolecule flocculants such as PAM and PDMDAAC, and the obtained compound flocculant has better flocculation effect than single flocculant.

Alfa Chemistry can offer various flocculants. If you can't find the desired product in our product catalog, we also offer you with custom synthesis service. Please contact us, if you have any needs.

Reference

• Wei H., et al. Coagulation/flocculation in dewatering of sludge: a review[J]. Water Research, 2018, 143: 608-631.