Based on a large number of literatures, the concept, mechanism and influencing factors of enhanced coagulation are summarized. The application of enhanced coagulation technology at home and abroad is introduced. The research progress of enhanced coagulation technology and coagulant is reviewed. The problems to be solved in the research and application of enhanced coagulation technology and coagulant are put forward for future research reference.
Enhanced coagulation is a water treatment process developed by developing a new coagulant on the basis of conventional coagulation, which can effectively remove suspended particles, colloidal impurities, total phosphorus and algae in polluted water. There are many methods of enhanced coagulation, such as enhanced coagulation, chemical enhanced primary treatment, enhanced flocculation, etc. This paper is collectively called enhanced coagulation. The concept of enhanced coagulation technology has not yet formed an authoritative explanation. The author thinks that enhanced coagulation technology is to strengthen and optimize any one or more links in conventional coagulation, so as to further improve the purification effect of pollutants in water, including low molecular weight soluble pollutants.
The mechanism of enhanced coagulation is not much different from that of conventional coagulation, mainly including compression of electric double layer, adsorption neutralization, adsorption-bridging, precipitation net capture and special coagulation. After coagulant is added to the polluted water, on the one hand, the colloid diffusion layer is compressed, the zeta potential is reduced and the colloid is unstable through the action of compressing the electric double layer and neutralizing by adsorption; On the other hand, through adsorption-bridging and precipitation net capture, unstable colloids aggregate into large flocs and precipitate, and finally carry out solid-liquid separation. The use of new polymer coagulant strengthens the above functions, which not only has the physical adsorption effect of adsorbing insoluble macromolecular organic pollutants in water with floc; It can also produce strong chemical adsorption and strong oxidation on soluble low molecular weight organic matter in water, thus improving the removal rate of pollutants. However, to achieve good coagulation effect is also related to many factors, including coagulant varieties, coagulant dosage, water quality, hydraulic conditions, water temperature, alkalinity and pH value. Only by optimizing these reaction conditions and making the coagulant work under the best conditions can the purpose of strengthening coagulation and improving the effect of conventional coagulation be achieved.
1 application of enhanced coagulation technology at home and abroad
Application of 1. 1 in domestic sewage treatment
Coagulation technology was applied in Britain as early as 1870, but it was quickly replaced by biological treatment. In 1980s, with the continuous advent of new efficient coagulants, in order to further improve the removal rate of organic matter and phosphorus in sewage, enhanced coagulation technology began to be applied to practical projects.
Enhanced coagulation technology has been studied and applied in water supply treatment in the United States, but it has also been reported in urban sewage treatment. Hyperion Sewage Treatment Plant in Los Angeles, USA, used an anionic polymer (0. 15mg/L) and 10mg/L FeCl3 _ 3 to treat municipal sewage. After 6 years' continuous operation, the removal rates of SS and BOD5 in primary treatment are stable at about 83% and 5 1%, and the removal effect of phosphorus and heavy metals is also very good, and its capital construction cost is high. Four sewage treatment plants in southern California have improved the traditional primary treatment process by adding ferric chloride coagulant and some coagulant AIDS, and the treatment effect has been greatly improved. After the improved primary treatment process, the removal rate of ss reached 85%, and the removal rate of BOD5 increased to more than 50%. Mete and others think that enhanced coagulation is a simple and effective water treatment technology, which can effectively remove dissolved organic matter and colloidal impurities in water.
In addition, there are many successful examples in the research and application of enhanced coagulation in Israel, Egypt, Japan and Norway. In recent years, with the strengthening of environmental protection, enhanced coagulation technology has also been developed in China.
Before the completion of the largest CEPT sewage treatment plant in Hong Kong, Hallermann and others had done a comparative test between the enhanced coagulation process and the conventional primary treatment process. Experiments show that 10mg/L FeCl3 and 0. 15mg/L polymers can improve the removal rate of SS from 7 1% to 9 1%, and the removal rate of BOD5 from 42% to 80%, and can save 30% of the sedimentation tank volume.
ChenChiuyang of Taiwan Province Province studied the enhanced coagulation treatment of municipal sewage before it was discharged into the sea. When adding 30 mg/L aluminum sulfate and 30mg/L polyaluminum chloride, the precipitation time is 65438 0 h, and the removal rates of SS and BOD5 are 70% and 60% respectively, which are 25% and 35% higher than those before intensive treatment.
Wang Donghai, Vicky and others used inorganic flocculants to treat low-concentration domestic sewage. When the dosage of PAC is 30 ~ 50mg/L, the removal rate of CODCr is above 70%, reaching the standard.
There are many successful examples of domestic sewage treatment by enhanced coagulation at home and abroad. Enhanced coagulation technology is often used as domestic sewage treatment technology in towns around large lakes in northern Europe and towns along the Mediterranean coast in southern Europe. It can be said that enhanced coagulation is the mainstream technology of domestic sewage treatment after biochemical treatment. In the research and application of enhanced coagulation technology, both at home and abroad pay attention to the combination or compounding of existing conventional coagulants and flocculants in order to achieve the unity of low cost and high removal rate. Compared with conventional biochemical treatment process, enhanced coagulation technology can save engineering investment, reduce water treatment cost and save land area, especially the removal rate of total phosphorus, one of the factors leading to eutrophication of water body, can reach over 90%, which is incomparable to many conventional biological treatment technologies. Therefore, enhanced coagulation technology is one of the ways to solve the low sewage treatment rate caused by insufficient funds in cities and towns in China. Two super-large sewage treatment plants under construction in Shanghai: Zhuyuan Sewage Treatment Plant (Phase I) and Bailonggang Sewage Treatment Plant (designed daily treatment capacity of 6,543.8+700,000 m3 and 6,543.8+300,000 m3 respectively) also adopt enhanced coagulation as the main treatment process. With the popularization of enhanced coagulation technology in China, the national Discharge Standard for Urban Sewage Treatment Plants (GB189118-2002) promulgated in 2003 stipulated the discharge standard of this technology.
Application of 1.2 in industrial wastewater treatment
Enhanced coagulation technology is widely used in the pretreatment of industrial wastewater, especially chemical wastewater, printing and dyeing wastewater and papermaking wastewater. Ruan Xiangyuan and others used PAC and PAM to pretreat dyeing and finishing wastewater rich in organic dyes, and combined with oxidation flocculation bed, the effluent could meet the discharge standard of industrial wastewater. Hong Chu's research shows that the new flocculant polyphosphate ferric sulfate is a more effective flocculant for treating printing and dyeing wastewater. In addition, enhanced coagulation is widely used in decolorization treatment of dyeing and finishing wastewater, and Li Chunhua and others have made a detailed review on it.
In addition, enhanced coagulation is widely used in the treatment of papermaking wastewater. Li Furen adopts PAC and PAM composite pretreatment air flotation process to treat high-concentration CTMP pulping and papermaking wastewater, which has high treatment efficiency and stable effluent quality, and can be directly discharged into the municipal sewage treatment plant for centralized treatment. Zhang Xuehong and others compared the treatment of papermaking wastewater with various coagulants, and found that PAC was the most suitable, without adjusting pH, and the effluent reached the national wastewater discharge standard.
The application of enhanced coagulation in other industrial wastewater treatment is also often reported in China. Yao's research shows that flocculants such as polyaluminum chloride, chitosan, bentonite and polyacrylamide have good flocculation effect on the centrifugal waste liquid of alcohol tank, and the removal rate of SS is 86.57% ~ 89.62%, and the removal rate of CODCr is 58.2% ~ 59.2%. The phase wave pretreatment of copper phthalocyanine wastewater with Na2S, FeCl3 _ 3 and PAM, combined with anoxic-aerobic biological contact oxidation process, has achieved good results, and all indicators have reached the national first-class discharge standard. Wu Dunhu's research shows that the removal rate of pharmaceutical wastewater with COD of 1000 ~ 4000 mg/L can reach 80% by using polyaluminum chloride and polyaluminum ferric chloride coagulant.
Compared with the enhanced coagulation technology of domestic sewage, the research of industrial wastewater enhanced coagulation technology pays more attention to the development of new coagulants or composite coagulants containing new coagulants with better treatment effect for different kinds of wastewater or pollutants, and the joint use of enhanced coagulation and other processes, while the economic requirements are relatively loose. This is because some industrial wastewater contains toxic and harmful substances and cannot be directly treated biologically. Therefore, studying more and more effective new coagulants will promote the application of enhanced coagulation technology in industrial wastewater treatment, and it is also one of the effective methods to control industrial wastewater pollution.
1.3 Experiment on Treating Polluted Surface Water
In recent years, the application of enhanced coagulation in the treatment of polluted surface water has attracted more and more attention. Wang Shuguang, China Academy of Sciences, etc. The experimental study on enhanced coagulation treatment of Longgang River, Guanlan River, Yanchuan River and Damao River in Shenzhen was carried out with polyferric chloride (PFC) as coagulant. The results show that the removal rates of CODcr, turbidity, TP and TN in Guanlan River (raw water CODcr = 48.0 mg/L) are above 70%, 965, 438+0%, 95% and 465, 438+0%, respectively. The removal rates of CODcr, turbidity, TP and TN in Damaohe River (raw water CODcr = 84.0 mg/L) were above 50%, 78%, 96.5% and 465, 438+0.6%, respectively, which also had a certain removal effect on heavy metals. The treated water quality reaches or approaches the surface water quality standard.
Sun Congjun and others used a variety of coagulants to study the enhanced coagulation of several seriously polluted Suzhou River tributaries in the laboratory. The results show that diatomite has better effect. When the optimal dosage is 200mg/L, the removal rate of CODCr is 43% ~ 59%, and the removal rate of P is 92% ~100%, but NH3-N is hardly removed.
Cheng Wenpo and others used coagulant Al2(SO4)3, PAC, FeCl3 and PFS to treat reservoir water. The results show that compared with ferric chloride, PFS has better removal rate of dissolved organic matter (DOC) and less iron residue. Al2(SO4)3 has the best removal effect on turbidity, chromaticity and bacteria, but the removal effect on DOC is not ideal. When PFS and Al2(SO4)3 are used together, the treatment effect is the best, and DOC, turbidity and chromaticity can be removed well.
Polluted surface water is the part of water between sewage and clean surface water, especially small closed water bodies, including polluted urban landscape water bodies. The treatment of this part of water is a new field of enhanced coagulation technology, which has been studied in China. Because of its low pollutant concentration and low relative removal rate, its phosphorus removal is considerable, which can effectively prevent eutrophication of water body and has broad application prospects. Usually, the enhanced coagulation treatment of sewage can be realized by building structures or direct throwing. Shanghai Foxinhe Company used coagulants to inhibit algae flooding and achieved good results. However, the safety of some coagulants is worrying, especially the application of some new efficient coagulants and biological coagulants. While considering its therapeutic effect and cost, we should also consider its safety.
2 New progress in enhanced coagulation technology
2. 1 New development of coagulant.
2. 1. 1 inorganic polymer coagulant
Inorganic polymer flocculant (IPF) has attracted more and more attention because of its low dosage, low toxicity or low toxicity, low price and good treatment effect. It has gradually become the mainstream coagulant for water supply, industrial wastewater and urban sewage treatment, and is called the second generation coagulant. At present, polymeric aluminum and polymeric iron series are widely used, such as polymeric aluminum chloride and PAFC, but new types of polysilicate, polyphosphate and polysulfide are also coming out, showing extraordinary coagulation effects, such as polymeric aluminum silicate and polymeric iron phosphate. Therefore, inorganic polymer coagulants show the development trend of multi-variety, multi-component and multi-function, but there are many varieties and the product quality is not stable enough. In the future research and application, the preparation process of coagulant should be optimized to improve the performance and stability of the product, and the corresponding coagulant varieties and formulas should be developed according to the specific water quality components, and the coagulation effect should be further improved by combining the efficient mixing reactor and intelligent dosing monitoring technology.
2. 1.2 organic polymer flocculant
Organic polymer coagulant plays a role mainly through the adsorption-bridging effect of its chain molecules, and its application can effectively improve the particle size of floc, which is 3 ~ 5 times larger than that formed by adding PAC alone, so it is widely used in enhanced coagulation.
Organic polymer flocculants can be divided into natural and synthetic categories. Synthetic organic polymer flocculant has an absolute advantage in the market because of its large molecular weight and many functional groups in the molecular chain. Among them, polyacrylamide series is the most widely used, and its residual monomer is toxic, which limits its development in some water treatment fields. Natural organic polymer flocculant shows a good application prospect because of its wide source of raw materials, low price, non-toxicity and easy biodegradation. However, due to its low charge density, small molecular weight and easy biological reaction, its dosage is far less than that of organic synthetic polymer flocculant. Modified natural polymer flocculant can overcome the above shortcomings, which has attracted special attention. Among them, the research and development of starch modified flocculant is particularly eye-catching. Therefore, it is the future development direction to develop efficient, safe and biodegradable organic polymer flocculant.
2. 1.3 Other coagulants
Besides inorganic polymer coagulant and organic polymer flocculant, microbial flocculant (microbial flocculant) has attracted great attention of researchers in recent years. It is a safe, efficient and naturally degradable new flocculant for water treatment, which is extracted and purified from microorganisms or their secretions by biotechnology. MBF can overcome the inherent safety and environmental pollution defects of inorganic polymer and synthetic organic polymer flocculant, and is easy to biodegrade without secondary pollution. At present, it has been applied to the treatment of pulp wastewater, dye wastewater, sludge dehydration, fermentation bacteria removal and other fields, and achieved good flocculation effect. However, at present, the domestic research is mostly limited to its practical application, and the basic research on its mechanism is less, which needs to be further strengthened. Yu et al. pointed out that due to the rapid development of biotechnology, people's understanding and control of microbial cell genes are becoming more and more handy, and targeted and efficient MBF can be developed according to different wastewater quality, which can not only greatly reduce the dosage of flocculant, but also reduce the treatment cost.
In addition, mineral coagulants have also developed in recent years, and coagulants made of fly ash, diatomite, zeolite powder, bentonite and other minerals have also begun to be used in water treatment. It is reported that the coagulation effect of fly ash coagulant prepared by Huang Caihai and Yu is better than the traditional single aluminum-iron coagulant, and it can be used for the treatment of various industrial wastewater.
2. Modification and compounding of1.4 coagulant
Modification and compounding of coagulant can optimize the performance of coagulant and improve coagulation effect. Jiang et al.' s experimental study on enhanced coagulation treatment of Shanghai sewage phase II project shows that the combination of polyaluminium ferric acid and organic polymer flocculant is economical and effective. Pei zod, Lear, etc. Similar research has been done, which shows that two or more coagulants are better than a single coagulant in treating wastewater, and the combination of organic and inorganic coagulants is better, which has broad engineering application prospects.
2.2 New progress in the study of enhanced coagulation mechanism
2.2. Application of1surface complexation principle and its quantitative calculation model in enhanced coagulation.
In the early 1970s, Stumn and others first proposed that the exclusive adsorption of metal ions in hydrated oxide dispersion system should be treated by coordination chemistry. They think that the combination of particles with H, OH- and metal ions at the interface is a complex chemical reaction, and the adsorption capacity at this time can be discussed in a way similar to the complex equilibrium in solution according to the law of mass action. Schindler and others further elaborated this concept, and later called Stumn-Schindle composite model, which has been widely used in the study of solid-liquid interface reaction mechanism in recent years. Because the calculation of surface complexation model is quite complicated, the computer module is mainly used for complex calculation of multi-component and multi-phase. At present, the main computer programs are REDE-QL, MINEQL, MICROQL, SUREQL, HYDRAQL, FITEQL and so on. They can be used to calculate equilibrium constants and component concentrations in various chemical equilibria and surface complexation reactions. For example, MICROQL can calculate the speciation distribution and surface equilibrium constant of aluminum in saturated Al(OH)3 solution. Wang et al. calculated the surface complexation constants of kaolin and silica by Stumn-Schindle complexation model, and the calculated results were in good agreement with the experimental data.
2.2.2 Application of Fractal Theory in Enhanced Coagulation
Fractal theory is also an effective new means to study coagulation. The structure and properties of floc have always played a very important role in coagulation research. Its size, strength, density and permeability are very important for sludge disposal and effluent quality, and its formation often has fractal characteristics. Through fractal structure analysis, a non-integer dimension is used to describe the degree of irregularity in irregular bodies, which provides a mathematical framework for quantitative description of these seemingly complex irregular forms, and the most important characteristic parameter in fractal structure analysis is fractal dimension. Generally speaking, different fractal dimensions reflect the openness of aggregate structure corresponding to the irregularity and complexity of fractal body or the degree of space filling, and the application of fractal dimension in coagulation research can more accurately describe the floc structure formed under different conditions. Wang Dongsheng et al. discussed the fractal theory and research method and its application in enhanced coagulation in detail.
2.2.3 Coagulation mechanism research gradually developed from semi-quantitative to quantitative.
The introduction of surface complexation theory and fractal theory has promoted the semi-quantitative and quantitative process of coagulation research, and developed a variety of calculation models and software, but most of them are limited to traditional coagulants. The coagulation process of new polymer coagulant is still difficult to calculate and needs further study. Taking the interaction of typical IPF- particle-aqueous solution system as an example, Wang Dongsheng and others improved Dentel's adsorption-precipitation-neutralization model (PCNM), which can well predict the coagulation characteristics of polyaluminum, and the experimental results are basically consistent with the predicted values of the model.
2.3 New progress in other aspects of research
2.3. 1 On-line control of coagulation process
Due to the application of flowing current principle and its detection technology in coagulation, the on-line control of coagulation process is realized and the optimal dosage of coagulant is ensured. It is also reported that the on-line monitoring of coagulation process can be well realized by using the scattering effect of particles in water on light. According to this principle, a spectrophotometric analyzer (PDA) developed by Jin et al. monitors the coagulation process of humus online, and analyzes the characteristic parameters of the obtained FI (Flocculation Index) curve. It is found that the FI curve and its characteristic parameters are greatly influenced by the dosage of coagulant, and its change has a good correlation with colloid stability (zeta potential) and coagulation effect (TOC removal rate), which is shown online.
2.3.2 Development of enhanced coagulation equipment
Agitator is the most important coagulation equipment, and its main function is to mix drugs with water as soon as possible. Commonly used mixing equipment includes water pump mixing, pipeline mixing, pressure orifice mixing, mechanical stirring mixing, vortex mixing and jet mixing, among which jet mixing is a new development of mixing technology, with the advantages of fast mixing speed, low power loss and high flocculation efficiency. The specific process is to inject the flocculant into the entrance of the reaction tank with an injection pipe. There are several small holes around the side of the injection pipe, through which the coagulant enters at a great speed. The turbulent intensity of water flow is the largest at the position perpendicular to the central axis of raw water pipeline, and coagulant jet is the easiest to mix with raw water.
3 Conclusion
Enhanced coagulation technology has developed rapidly in recent years and made great progress in research and application. Due to the introduction of some new theories and methods, the study of enhanced coagulation has been deepened, especially the study of some basic mechanisms has been paid more and more attention. However, as enhanced coagulation is a rather complicated process, many problems need further in-depth study, especially the following aspects should be strengthened:
(1) Continue to develop efficient coagulants and coagulation equipment to improve their coagulation effect and reduce their production costs;
(2) Strengthen the study of enhanced coagulation mechanism, and find effective methods to study enhanced coagulation, such as the identification and quantitative analysis methods of the best forms in inorganic polymer flocculants, so as to maximize the content and stability of the best forms;
(3) Strengthen the study of enhanced coagulation dynamics, combine chemical reaction dynamics with mixed fluid dynamics, and comprehensively describe the morphological changes of flocculants and the instability model of pollutants after they are put into water, so as to predict and control enhanced coagulation and finally serve engineering practice.
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