1, preface
Water treatment agent is an essential chemical in the process of industrial water, domestic water and wastewater treatment. By using these chemicals, water can meet certain quality requirements. Its main function is to control the formation of scale and sludge, reduce foam, reduce the corrosion of substances in contact with water, remove suspended solids and toxic substances in water, deodorize and decolorize, and soften water quality. At present, due to the rapid increase of water consumption in various countries in the world, the successive formulation of various environmental protection laws (water purification laws) and the increasingly strict requirements, various high-efficiency water treatment agents have grown rapidly. In China, the contradiction with the increasingly severe water crisis is that the production capacity of water treatment chemicals is very low and the quality can not be guaranteed. It is urgent to accelerate the development of water treatment chemicals, an environmentally friendly material industry.
Water treatment agent includes flocculant, corrosion inhibitor, scale inhibitor, bactericide, dispersant, cleaning agent, prefilming agent, defoaming agent, decoloring agent, chelating agent, deoxidizer and ion exchange resin. This paper will systematically introduce flocculants and biocides.
2. Coagulant
The key of flocculation technology is the choice of flocculant. Flocculants can be divided into inorganic, organic and microbial flocculants.
2. 1, inorganic flocculant
Inorganic low molecular weight flocculants include aluminum chloride, aluminum sulfate, ferric sulfate and ferric chloride. Its aggregation speed is slow, and the floc formed is small and corrosive, which has great problems in the process of water treatment and is gradually replaced by inorganic polymer flocculant.
Inorganic polymer flocculant is a new type of water treatment agent developed on the basis of traditional aluminum salt and iron salt, which has low price and good water purification effect.
Polyaluminum chloride (PAC) has good coagulation performance, large alum particles, low dosage, high efficiency, fast sedimentation and wide range of applicable water quality. Mainly used for 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, lead, fluorine and oil in water, so it can be used to treat various industrial wastewater.
Polyaluminum ferric chloride (PAFC) is a new type of inorganic polymer water purifier. The proportion of aluminum and iron in the product can be adjusted to meet the needs of different water quality, and it has been applied to wastewater purification in petrochemical, steel, coal and other industries. The results show that the agent is a new, efficient, stable, high quality and low price water purifier, and has broad application prospects. Some people come to the conclusion that the water purification effect of PAFC is slightly better than PAC, but the cost of adding PAFC is much less than PAC.
Polyferric sulfate has good flocculation and adsorption performance, and is widely used in the treatment of raw water, drinking water, tap water, industrial water, industrial wastewater and domestic sewage. Polyaluminum sulfate is one of the most widely used coagulants, which is mainly used to purify drinking water and industrial water.
Polysilicic acid is developed on the basis of polysilicic acid and traditional aluminum salt and iron salt. Highly polymerized silicic acid can produce good coagulation effect with metal ions. By combining the electric neutralization ability of metal ions with the adsorption bridging ability of polysilicic acid, the composite product has strong electric neutralization and adsorption bridging effect, and achieves better water purification effect. Their flocculation destabilization performance far exceeds that of polysilicic acid and polymetallic ions. Compared with polysilicic acid, they not only improve the stability, but also increase the electric neutralization ability. Compared with multi-metal ions, the bonding and bridging properties are enhanced. Composite inorganic polymer flocculants, such as aluminum polysilicate sulfate (PASS), aluminum polysilicate chloride (PASC) and ferrosilicon composite inorganic polymer flocculants, have been successfully applied in various processes of water supply, industrial wastewater and municipal sewage, and have now become mainstream flocculants.
However, the relative molecular weight, particle size and flocculation bridging ability of inorganic polymer flocculant are still much worse than those of organic flocculant, and the further hydrolysis reaction is unstable.
2.2 Organic polymer flocculant
Compared with inorganic flocculants, synthetic organic polymer flocculants have less dosage, faster flocculation speed, less influence by salt, medium pH and environmental temperature, and less sludge. Moreover, organic polymer flocculant molecules can carry electrophilic groups such as-Chief Operating Officer, -NH-, -SO3 and -OH, and have various structures such as chain and ring, which is beneficial for pollutants to enter flocs and has good decolorization effect. Generally speaking, the chroma removal rate of organic flocculants is about 20% higher than that of inorganic flocculants. Polyacrylamide is widely used at present. It can adapt to a variety of flocculation objects, with low dosage, high efficiency, less sludge and easy post-treatment. It is often mixed with other inorganic flocculants, such as aluminum chloride.
However, the monomers or hydrolysis degradation products of synthetic polymer flocculants are often toxic. For example, the monomers of polyacrylamide (PAM) are neurotoxic, teratogenic, carcinogenic and mutagenic.
2.3 Microbial flocculant
Microbial flocculant is a safe, efficient and naturally degradable new water treatment agent extracted and purified from microorganisms or their secretions by biotechnology. Up to now, more than 17 kinds of flocculating microorganisms have been found, including molds, bacteria, actinomycetes and yeasts. It is divided into:
(1) Flocculants from microbial cells, such as some bacteria, molds, actinomycetes and yeasts, are abundant in soil, activated sludge and sediments;
(2) Flocculants extracted from microbial cell walls, such as dextran, mannan, protein, N- acetylglucosamine and other components of yeast cell walls;
(3) Using flocculants of microbial cell metabolites, the metabolites secreted by microbial cells outside the cell are cell membranes and mucus. Besides water, its main components are polysaccharides and a small amount of peptides, protein, lipids and their complexes. Among them, polysaccharide can be used as flocculant to some extent.
So far, the best microbial flocculant is Nocardia rhodopsin NOC- 1. It can be used for the treatment of livestock and poultry wastewater, the settlement of expanded sludge and the decoloration of colored wastewater such as pulp wastewater (black liquor) and pigment wastewater, with remarkable effect.
Although the research of microbial flocculant is often reported in newspapers, most of them are in the laboratory research stage and have not been applied to industry. China started late in this field, and the current research is limited to strain screening.
Six strains of microbial flocculant-producing bacteria were initially isolated and screened by Chengdu Institute of Biology. Flocculation experiments were carried out by using the fermentation centrifugal supernatant of papermaking black liquor, leather wastewater, azo dye wastewater, sulfide dye wastewater, electroplating wastewater, color printing plate wastewater, petrochemical wastewater, coinage wastewater, blue black water and carbon ink. The results show that the solid-liquid separation effect of wastewater is good, the removal rate of COD is 55%-98%, and the removal rates of suspended solids, chromaticity and turbidity are all above 90%.
Three flocculant-producing strains were isolated from the return sludge and sediment of sewage treatment plant by the Department of Environmental Science of Shanghai University. The turbidity removal rate of soil suspension, COD removal rate of alkaline dye wastewater and chromaticity removal rate can reach over 99%, 70% and 92%, respectively.
At present, flocculants are developing in the direction of low cost, practicality, non-toxicity and high efficiency. Organic polymer flocculant will gradually replace inorganic flocculant which is widely used at present. On the other hand, microbial flocculant has the advantages of stability, safety, high efficiency and low consumption. It is one of the most promising flocculants. Therefore, the future development should not only develop new cheap and efficient microbial flocculants, but also study the synergistic use of microbial flocculants and other flocculants. The existing experiments show that the combination of the two can complement each other, which can not only improve the flocculation efficiency, but also reduce the dosage.
3. Insecticide
Biocide is a kind of water treatment agent, which is used in circulating cooling water system to kill microorganisms (bacteria and algae) and prevent them from multiplying in large quantities, causing corrosion and accidents of metal equipment in cooling water system and affecting normal operation. According to the killing mechanism, it can be divided into oxidizing biocides and non-oxidizing biocides.
3. 1 oxidative antimicrobial agent
Chlorine gas is a strong oxidizing biocide with strong bactericidal power, low price and simple use. It is one of the most widely used fungicides. But it is not suitable for alkaline water treatment. In addition, it may form carcinogen trihalomethane with organic matter in water, which limits its application. So bromine, ozone and chlorine dioxide have attracted attention one after another.
Bromine insecticides mainly include sodium bromide, hydantoin bromide, active bromine and propionamide bromide. Propylamide bromide is an oxidative biocide developed in recent years, among which 2,2-dibromo -3- azapropionamide is a very effective broad-spectrum biocide. With the increase of pH value and temperature of cooling water, the half-life becomes shorter rapidly, and the environmental pollution is less.
Ozone has excellent bactericidal activity, strong stripping and sliminess, and also has corrosion and scale inhibition effects. When used to treat circulating cooling water, the concentration multiple can reach 30 ~ 50 times. However, it has not been widely used at present because of its high cost.
Chlorine dioxide has a strong ability to adsorb and penetrate the cell wall, and has a good killing effect on the main harmful bacteria in cooling water, such as heterotrophic bacteria, iron bacteria, sulfate-reducing bacteria and so on. It is characterized by low dosage, high efficiency, quick response and long duration of drug effect. For example, 2mg/L chlorine dioxide can kill nearly 100% of microorganisms after 30s; After adding 0.5mg/L chlorine dioxide to the water with pH of 8.6 and the number of viable bacteria reaching 765438 0.000 /ml for 0.2 h, the killing rate of heterotrophic bacteria remained above 99%. In addition, it is not affected by pH value and does not react with ammonia, organic amines and phenols in water. It can not only kill microorganisms, but also decompose the residual cell structure, and has the function of killing spores and viruses; Suitable for alkaline water treatment, no threat to the environment. In China, its instability limits its application. In recent years, some manufacturers have produced stable chlorine dioxide in batches, and a company in Nanjing has also introduced a chemical chlorine dioxide generator with unique design, simple operation, safety and reliability. Using chlorine dioxide instead of chlorine as biocide for industrial circulating cooling water has many advantages, especially for cooling water systems in ammonia plants, chemical plants and refineries. Because of the high content of organic matter and ammonia in the system, the demand for chlorine gas is large, and the pH value is alkaline, replacing chlorine gas with chlorine dioxide can achieve better economic and environmental benefits.
3.2 Non-oxidizing bactericide
There are many kinds of non-oxidizing biocides, and chlorophenols used earlier are gradually abandoned because of their high toxicity and easy pollution to water bodies. Organic amines are rarely used.
Dithiocyanatomethane is an earlier organic sulfide fungicide. It is very effective in inhibiting algae, fungi and bacteria, especially sulfate-reducing bacteria. But it is not suitable for alkaline cooling water system.
Isothiazolinone is a relatively new organic sulfide fungicide. This biocide can kill bacteria by breaking the bond between bacteria and algae protein. When the concentration is 0.5mg/L, it can effectively inhibit algae, fungi and bacteria in the cooling water system, and has the advantages of broad spectrum and high efficiency, long action time (still effective after 5 weeks of use at 0.5mg/L), low toxicity, wide pH range, good compatibility and miscibility, no foaming, and prevention of slime formation. It has been widely used in cooling water treatment abroad.
Quaternary ammonium salt biocide has low cost, low toxicity and corrosion inhibition. Therefore, it is widely used, but there are still some problems in use, such as easy drug resistance, increased cost, foaming, increased corrosion and so on. In view of this, two newly synthesized dialkyl quaternary ammonium salts, hexadecyl octyl dimethyl ammonium bromide (168) and hexadecyl decyl dimethyl ammonium bromide (16 10), changed the surface activity and molecular stability of quaternary ammonium salts, produced less foams and improved bactericidal activity.
Glutaraldehyde has high-efficiency and broad-spectrum bactericidal and algae-killing effects, and also has a certain stripping effect on biological mucus. Union Carbide Company produced a series of glutaraldehyde biocides A5 15, A525, A530, etc. Experiments show that A5 15 has obvious killing effect on heterotrophic bacteria, and the efficacy lasts for a long time. After 72h, the killing rate is still above 90%. It is suitable for alkaline water treatment and has good compatibility with phosphorus chemicals. In recent years, a company in Wuhan introduced glutaraldehyde series for circulating cooling water system, and the effect was obvious. At the recommended cooling water concentration, glutaraldehyde is almost nontoxic, and its aqueous solution itself will be biodegradable. With the strengthening of social environmental awareness, glutaraldehyde biocide will have broad development prospects.
To develop new biocides, we should not only consider the price, toxicity, use safety, storage stability, microbial resistance and other factors, but also consider the synergistic effect between biocide combinations, which can not only enhance the killing ability, but also reduce the dosage.
4. Development direction of water treatment chemicals
4. 1 development of special water treatment agent
To meet the needs of different wastewater systems (such as papermaking wastewater, printing and dyeing wastewater, food processing wastewater, etc.). ), it is imperative to develop a variety of specific chemicals.
4.2 Development of multifunctional water treatment agent
Multifunctional water treatment agent is an important aspect of water treatment agent research. The appearance of this new water treatment technology will expand the production and application scope of water treatment agent and greatly promote the development of chemical treatment of industrial water.
The research in this field mainly includes: corrosion and scale inhibitor, flocculation-corrosion inhibitor, flocculation-bactericide, flocculation-sterilization-corrosion inhibitor, flocculation-corrosion and scale inhibitor and so on.
4.3 Development of Green Water Treatment Agent
In the green development of water treatment chemicals, non-toxic, harmless and biodegradable is the direction. The most typical green water treatment agent is the dispersed scale inhibitor polyaspartic acid (PASP) developed at home and abroad in recent years. PASP is a synthetic biopolymer. It has good biocompatibility and biodegradability. Toxicological studies show that polyaspartic acid (PASP) is non-toxic, insensitive or mutation-free.
4.4 Development of cost-effective water treatment chemicals
At present, the prices of high-performance chemicals are generally high. We can develop high-performance products by looking for cheap and easily available raw materials, and we can also strengthen the research on compound technology, that is, adding cheap additives to reduce the actual consumption of chemicals and maintain the efficiency of water purification, so as to achieve the purpose of reducing costs.