Views: 5 Author: Ryde Environment Publish Time: 2019-05-20 Origin: Ryde Environment
Flocculants are most commonly used in wastewater treatment and sludge treatment. In the process of sludge concentrating and dewatering from secondary clarifier, flocculant should be added to mix with sludge evenly and then enter other equipment such as multi disc screw press or belt filter press. This can improve the sludge dewatering effect and enhance the efficiency of sludge dewatering.
Flocculant is a kind of substance which can reduce or eliminate the sedimentation stability and polymerization stability of dispersed particles in water and make dispersed particles coagulate and flocculate into aggregates. According to chemical composition, flocculants can be divided into inorganic flocculants, organic flocculants and microbial flocculants.
Inorganic flocculants include aluminium 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 polymeric monomers. According to their sources, organic flocculants can be divided into synthetic and natural polymer flocculants.
In practical application, inorganic-organic composite flocculants are often made by combining inorganic flocculants and organic flocculants according to their different properties.
What kinds of inorganic flocculants are there?
Traditional inorganic flocculants are low molecular aluminium salts and ferric salts. Aluminum salts mainly include aluminium sulfate (AL2 (SO4) 3•18H2O), alum (Al2(SO4) 3•K2SO4• 24H2O), sodium aluminate (NaALO3), ferric chloride (FeCL3•6H20), ferrous sulfate (FeSO4•6H2O) and ferric sulfate(Fe2(SO4)3•H2O).
Generally speaking, inorganic flocculants are widely used in water treatment because of their easy availability of raw materials, easy preparation, low cost and moderate treatment effect.
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. Aluminum, iron and silicon inorganic macromolecule flocculants are actually the intermediates of hydrolysis, sol and precipitation, namely hydroxyl and oxygen polymers of Al (III), Fe (III) and Si (IV).
Aluminum and iron are cationic positive charges, while silicon is anionic negative charges. Their molecular weight in water-soluble state is about hundreds to thousands, and they can be combined to form aggregates with fractal structure.
Their coagulation-flocculation process is a comprehensive reflection of the electroneutralization and bridging of particles in water. The particle size of suspended particles in water ranges from nanometer to micrometer, and most of them have negative charges. Therefore, the main factors that determine the flocculation effect are the positive and negative charges, the electrical strength and molecular weight of flocculants and the size of aggregates.
At present, there are dozens of kinds of inorganic macromolecule flocculants, and the output of inorganic macromolecule flocculants reaches 30%-60% of the total output of flocculants. Among them, polyaluminium chloride is widely used.
Synthetic organic macromolecule flocculants are polypropylene and polyethylene, such as polyacrylamide and polyethylenimide. These flocculants are water-soluble linear macromolecule substances, each macromolecule consists of many repetitive units containing charged groups, so they are also called polyelectrolytes. Cationic polyelectrolyte containing positive group and anionic polyelectrolyte containing negative group are called nonionic polyelectrolyte, which includes both positive group and negative group.
At present, most of the polymer flocculants used are anionic, and they can only play a coagulant role in the negative colloidal impurities in water. Often can not be used alone, but with aluminium salt, iron salt use. Cationic flocculants can play the role of both coagulation and flocculation and can be used alone, so they have been developed rapidly.
At present, polyacrylamide nonionic polymers are widely used in China, which are often combined with iron and aluminium salts. Satisfactory treatment results were obtained by utilizing the electric neutralization effect of iron and aluminium salts on colloidal particles and the excellent flocculating function of macromolecule flocculants. Polyacrylamide has the characteristics of low dosage, fast coagulation speed and large strength and toughness of flocculent particles in use. At present, 80% of synthetic organic polymer flocculants produced in China are made of this kind of product.
Natural organic macromolecule flocculants have a long history of application in water treatment. Until now, natural macromolecule compounds are still an important kind of flocculants, but their usage is much lower than synthetic macromolecule flocculants. The reason is that natural macromolecule flocculants have low charge density, low molecular weight and are prone to biodegradation and lose flocculating activity.
Compared with synthetic flocculants, natural organic macromolecule flocculants have less toxicity and simple extraction process. They are in good harmony with nature both in chemical composition and production process. Therefore, the research and utilization of these natural resources as water treatment agents has become a current hot spot, which is inseparable from the situation that the world attaches great importance to rational utilization of resources, protection and improvement of the environment. 。
At present, there are many kinds of natural macromolecule flocculants, which can be divided into chitosan flocculants, modified starch flocculants, modified cellulose flocculants, lignin flocculants, tree gum flocculants, alginate flocculants, animal gum 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 starch main chain contains only one monosaccharide structure, belonging to the same polysaccharide; chitosan, gum, alginate and other monosaccharide structures, belonging to heteropolysaccharide; lignin is a special aromatic natural polymer; animal gum and gelatin belong to protein substances.
Microbial flocculants are significantly different from traditional inorganic or organic flocculants. They either use microbial cells directly, or use microbial cell wall extracts, metabolites and so on. The former is the main aspect of microbial flocculant research. Up to now, more than 17 kinds of microorganisms with flocculant properties have been found, including fungi, bacteria, actinomycetes and yeasts. The latter is similar to organic flocculants. Microbial flocculants have many advantages that traditional inorganic or organic flocculants can not match, such as no secondary pollution and low production cost.
The flocculation performance of microbial flocculants is influenced by many factors, including the inheritance and expression of flocculant genes, the composition of microbial culture medium, the change of hydrophobicity of cell surface, the existence of divalent metal ions in the environment and so on. 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 reduce production cost. The research and development of microbial flocculants in China is moving in this direction, but there is still a certain distance from industrial production.
Usually, flocculants are in a solid state to facilitate transportation and sales. In use, it is necessary to dissolve the solid flocculant into a certain concentration of viscous water solution.
The function of Polymer preparation unit is to make the powder or liquid flocculants dissolve or dilute into flocculants solution. It is usually divided into manual dosing device and the integrated of dosing device. The integrated of dosing device is fully automatic operating including automatic dosing, dissolving and feeding.