The mechanism of enhanced coagulation is not much different from that of conventional coagulation, mainly including compression of the double electric layer, adsorption charge neutralization, adsorption-bridging, sweep flocculation by precipitates, and special coagulation effects, etc. [2]. After adding coagulant to polluted water, on the one hand, through compression of the double electric layer and adsorption charge neutralization, the colloidal diffusion layer is compressed, the ξ potential decreases, and the colloids are destabilized; on the other hand, through adsorption-bridging and sweep flocculation by precipitates, the destabilized colloids aggregate into large flocs and settle, ultimately achieving solid-liquid separation. The use of new polymer coagulants enhances the above effects. It not only has the physical adsorption effect of flocs adsorbing insoluble macromolecular organic pollutants in water, but also can produce strong chemical adsorption and strong oxidation and other purification effects on dissolved low-molecular organic matter in water, thereby improving the removal rate of pollutants. However, achieving good coagulation results is also related to many factors, including the type of coagulant, coagulant dosage, water quality, hydraulic conditions, water temperature, alkalinity, and pH, etc. Only by optimizing these reaction conditions and making the coagulant work under optimal conditions can the purpose of enhancing coagulation and improving conventional coagulation effects be achieved.

1　Application of enhanced coagulation technology at home and abroad 1.1 Application in domestic wastewater treatment

　　The United Kingdom [3] began applying coagulation technology as early as 1870, but it was quickly replaced by biological treatment. By the 1980s, with the continuous emergence of new high-efficiency coagulants and to further improve the removal rate of organic matter and phosphorus in wastewater, enhanced coagulation technology began to be applied in practical engineering.
The United States has conducted more research and applications of enhanced coagulation technology in water supply treatment [4], but there are also reports in municipal wastewater treatment [5]. The Hyperion Wastewater Treatment Plant in Los Angeles, USA, uses an anionic polymer (0.15 mg/L) combined with 10 mg/L FeCl3 to treat municipal wastewater. After 6 years of continuous operation, the primary treatment removal rates of SS and BOD5 remained stable at about 83% and 51%, respectively, and the removal effects on phosphorus and heavy metals were also good, while its capital and operating costs were only about 30% of those of secondary treatment plants. The four major wastewater treatment plants in Southern California improved the traditional primary treatment process by adding FeCl3 coagulant and some coagulant aids, and the treatment effect was greatly improved. After the improvement, the SS removal rate of the primary treatment process reached 85%, and the BOD5 removal rate increased to more than 50%. Mete et al. [6] believe that, economically and technically, enhanced coagulation is a simple and effective water treatment technology that can effectively remove dissolved organic matter, colloidal impurities, etc. from water.
In addition, Israel [7], Egypt [8], Japan [9], Norway [10] and other countries have many successful examples of research and application of enhanced coagulation. In recent years, with the strengthening of environmental protection, enhanced coagulation technology has also achieved certain development in China.
Before the construction of the largest CEPT wastewater treatment plant in Hong Kong, Harleman et al. [11] conducted comparative tests between the enhanced coagulation process and the conventional primary treatment process. The tests showed that 10 mg/L FeCl3 and 0.15 mg/L polymer could increase the SS removal rate from 71% to 91%, and the BOD5 removal rate from 42% to 80%, and could save 30% of the sedimentation tank volume.
Chen Chiuyang from Taiwan studied enhanced coagulation treatment of municipal wastewater before discharge into the sea. Aluminum sulfate and PAC were each added at 30 mg/L, and after 1 h of sedimentation, the removal rates of SS and BOD5 were 70% and 60%, respectively, which were 25 and 35 percentage points higher than before enhanced treatment.
Wang Donghai [12], Ren Jie et al. [13] used inorganic flocculants to treat low-concentration domestic wastewater. When the PAC dosage was 30-50 mg/L, the CODCr removal rate reached over 70%, meeting discharge standards.
There are many successful examples of enhanced coagulation treatment of domestic wastewater at home and abroad. Towns around large lakes in Northern Europe and towns along the Mediterranean coast in Southern Europe often use enhanced coagulation technology as a domestic wastewater treatment technology. It can be said that enhanced coagulation is the mainstream domestic wastewater treatment technology second only to biochemical treatment. In the research and application of enhanced coagulation technology, both domestic and foreign efforts focus on the combination or compounding of existing conventional coagulants and flocculants to achieve the unity of low cost and high removal rate. Compared with conventional biochemical treatment processes, enhanced coagulation technology can save engineering investment, reduce water treatment costs, and save land area. In particular, the removal rate of total phosphorus, one of the elements causing water eutrophication, can reach over 90%, which is unmatched by many conventional biological treatment technologies. Therefore, enhanced coagulation technology is one of the ways to solve the low wastewater treatment rate in Chinese towns caused by insufficient funds. The two super-large wastewater treatment plants under construction in Shanghai: Zhuyuan Wastewater Treatment Plant (Phase I) and Bailonggang Wastewater Treatment Plant (designed daily treatment capacity of 1.7 million m3 and 1.3 million m3, respectively) also adopt a treatment process mainly based on enhanced coagulation. With the popularization of enhanced coagulation technology in China, the national urban wastewater treatment plant discharge standard (GB189118-2002) issued in 2003 stipulated the discharge standards for this process technology.