Inorganic low molecular flocculants include aluminum chloride, aluminum sulfate, ferric sulfate, ferric chloride, etc. Its aggregation speed is slow, the flocs formed are small, and it is highly corrosive. There are major problems in the water treatment process, and it is gradually being replaced by inorganic polymer flocculants.
Inorganic polymer flocculant is a new type of water treatment agent developed on the basis of traditional aluminum salts and iron salts. It is relatively cheap and has good water purification effect.
Polyaluminum chloride (PAC) has good coagulation performance, produces large alum flowers, small dosage, has high efficiency, fast settling, and is suitable for a wide range of water quality. Mainly used for the purification of drinking water and industrial water supply. At the same time, it can also be used to remove heavy metals such as iron, manganese, chromium, and lead contained in water, as well as fluoride and oil in water, so it can be used to treat a variety of industrial wastewater.
Polyaluminum ferric chloride (PAFC) is a new type of inorganic polymer water purifier. The ratio of aluminum and iron in the product is adjustable to meet the needs of different water qualities. It has been used in the purification treatment of wastewater in the petrochemical, steel, and coal industries. The results show that the agent is of high quality and low price. It is a new, efficient, and stable water purification agent with broad application prospects. Some people have concluded through experimental comparison that the water purification effect of PAFC is slightly better than that of PAC, but the cost of adding PAFC is much less than that of PAC.
Polymeric ferric sulfate has good flocculation and adsorption effects and is widely used in the treatment of raw water, drinking water, tap water, industrial water, industrial wastewater, and domestic sewage.
Polyaluminum sulfate (PAS) is the most widely used coagulant, mainly used for the purification of drinking water and industrial water.
Polysilicate is developed on the basis of polysilicate and traditional aluminum salts and iron salts. Highly polymerized silicic acid, together with metal ions, produces good coagulation effects. By combining the electrical neutralization ability of metal ions with the adsorption and bridging ability of polysilicic acid, the composite product has strong electrical neutralization and adsorption bridging effects, achieving better water purification effects.
Their flocculation and destabilization performance far exceeds that of polysilicic acid and polymetal ions. Compared with polysilicic acid, it not only improves the stability but also increases the electrical neutralization ability; compared with polymetal ions, it enhances the bonding and bridging performance.
Composite inorganic polymer flocculants, represented by polymeric aluminum silicate sulfate (PASS), polysilicoaluminum chloride (PASC), and ferrosilicon composite inorganic polymer flocculants, have been successfully used in various processes of water supply, industrial wastewater, and urban sewage, and have now become mainstream flocculants. However, the relative molecular weight, particle size, and flocculation bridging ability of inorganic polymer flocculants are still much worse than those of organic flocculants, and there is the problem of instability due to further hydrolysis reactions.
Organic polymer flocculant
Compared with inorganic flocculants, synthetic organic polymer flocculants use less amount, have a faster flocculation speed, are less affected by coexisting salts, medium pH, and environmental temperature, and generate less sludge. Moreover, organic polymer flocculant molecules can carry electrophilic groups such as -COO, -NH-, SO3, -OH, and can have chain, ring, and other structures, which facilitates the entry of pollutants into the floc and has good decolorization. Generally, the color removal of organic flocculants is about 20% higher than that of inorganic flocculants. Currently, polyacrylamide is widely used. It can adapt to a variety of flocculation objects, with low dosage, high efficiency, less sludge generated, and easy post-processing. It is often used in combination with other inorganic flocculants, such as aluminum chloride.
However, the monomers or hydrolysis and degradation products of synthetic polymer flocculants are often toxic. For example, the monomer of polyacrylamide (PAM) has neurotoxicity, teratogenesis, carcinogenesis, and mutagenesis.
Microbial flocculant
Microbial flocculant is a safe, efficient, naturally degradable new water treatment agent obtained by extracting and purifying microorganisms or their secretions using biotechnology. So far, more than 17 types of microorganisms with flocculating properties have been found, including molds, bacteria, actinomycetes, and yeasts. It is divided into:
(1) Flocculants that directly utilize microbial cells, such as certain bacteria, molds, actinomycetes, and yeasts, which are abundant in soil, activated sludge, and sediments;
(2) Flocculants using microbial cell wall extracts, such as glucan, mannan, protein, and N-acetylglucosamine of yeast cell wall;
(3) Use the flocculant of microbial cell metabolites. The metabolites secreted out of the cells by microbial cells are the cell capsule and mucus. In addition to water, its main components are polysaccharides and a small amount of polypeptides, proteins, lipids, and their complexes. Polysaccharides can be used as flocculants to some extent.
So far, the microbial flocculant with the best flocculation effect found is Nocardia rubrum NOC-1. It can be used for the treatment of livestock wastewater, the settlement of expanded sludge, and the decolorization of colored wastewater such as pulp wastewater (black liquor) and pigment wastewater, with remarkable effects.
At present, flocculants are being developed in the direction of being cheap, practical, non-toxic, and efficient. Organic polymer flocculants will gradually replace the currently widely used inorganic flocculants. On the other hand, microbial flocculants have the characteristics of stability, safety, high efficiency, and low consumption. It is one of the most promising flocculation technologies today. Therefore, future development must not only develop new cheap and efficient microbial flocculants, but also study the use of microbial flocculants in combination with other flocculants. Existing tests have shown that the two can complement each other when used together, which can not only improve the flocculation efficiency, but also reduce the dosage.