Strengthening the Connotation and Development of Coagulation Technology

The main targets of conventional drinking water treatment processes are suspended solids, colloidal impurities, and bacteria in source water. These purification technologies emerged during the fight against pollution and have been continuously developed, improved, and perfected. With rapid industrial development, water pollution has become increasingly serious, with harmful substances in water increasing year by year. Simultaneously, with the gradual improvement of water quality analysis technology, the types of trace pollutants detectable in source water and drinking water are constantly increasing. As people's living standards continue to improve, their requirements for water quality are also rising. To meet these needs and align with international standards, on June 7, 2001, the Ministry of Health of China formulated the "Standards for Drinking Water Quality." The new standards increased the number of water quality indicators from 35 in the original "Standards for Drinking Water Quality" (GB5749-85) to 96, and for the first time, standards were set for the organic matter content in drinking water. Furthermore, the standards address changes in source water quality (in addition to the original sediment, colloidal substances, and pathogenic microorganisms, there are also organic pollutants, high ammonia nitrogen, disinfection byproducts, and changes in water biological stability). Conventional drinking water treatment processes (which can only remove 20%-30% of organic matter in water, and due to the presence of dissolved organic matter, which is not conducive to the destruction of colloid stability, the turbidity removal effect of conventional processes is also significantly reduced to only 50-60%), which are proving inadequate. Experiments show that coagulation and sedimentation can remove more than 80% of organic matter with a molecular weight of 10,000-100,000, and about 50% of organic matter with a molecular weight of 3,000-10,000. According to experiments conducted by Tianjin Water Company, treating Luanhe River water to a turbidity of less than 0.5 NTU reduced the total area of organic matter peaks measured by gas chromatography by 80%. Therefore, it is imperative to develop new water treatment technologies and processes based on existing conventional treatment technologies and processes. Since the 1970s, after decades of effort, water treatment workers at home and abroad have researched and developed many new water treatment technologies and processes, and many engineering applications have achieved good purification results.

1. Strengthening the Connotation and Development of Coagulation There are several methods for modifying conventional water purification processes: adding advanced treatment structures, such as activated carbon adsorption technology; adding pretreatment structures, such as biological pretreatment (contact oxidation tanks or biological filters); and modifying existing processes without adding pre- or post-processing purification structures, such as enhanced coagulation, enhanced filtration, and optimized disinfection. The basic characteristics of each method are as follows: Enhanced coagulation (EC) refers to adding excessive amounts of coagulant, novel coagulant, or coagulation aids during coagulation treatment in the conventional treatment process. By using coagulants or other agents and controlling a certain pH value, and enhancing coagulation and flocculation, the removal efficiency of natural organic matter (NOM) in conventional treatment is improved, and the precursors of disinfection byproducts (DBPFP) are removed to the maximum extent, ensuring that the disinfection byproducts of drinking water meet drinking water standards. Compared with advanced treatment and biological pretreatment, enhanced conventional water treatment has advantages such as lower investment, no need to build new structures, no land occupation, and low operating costs, making it more suitable for the transformation of existing systems.

2. EC Mechanism and Common Methods

Enhanced coagulation mainly expands and improves the removal range and rate of organic matter by improving coagulation conditions. Its mechanism is explained as follows: The increase in colloidal stability is due to the formation of a protective layer on the surface of inorganic colloidal particles by large molecular natural organic matter, causing steric hindrance or double-layer repulsion. The main function of coagulation is to remove suspended particles and colloidal particles in water. It is generally believed that the coagulation process involves the hydrolysis products of the coagulant neutralizing the charge of colloids in the water, destabilizing them, forming fine particles, which then flocculate into large and dense flocs, and bridging through adsorption. Alternatively, the entrapment effect can cause destabilized colloids to form larger flocs, which are then separated and removed by sedimentation and filtration. However, organic matter with small molecular weight and high solubility in water (mainly fulvic acid and other humic acids) has a very low removal rate under normal coagulation conditions. This is mainly because its good hydrophilicity makes it difficult for it to be adsorbed by the hydrolysis products of coagulants—metal hydroxides. Organic matter not only increases the surface charge of colloids but also causes steric hindrance. However, if the coagulation treatment conditions are improved, i.e., at low pH and high coagulant dosage... Under enhanced coagulation conditions, a large amount of metal hydroxides are formed, improving the morphology of coagulant hydrolysis products and increasing their positive charge density. Simultaneously, low pH conditions affect the degree of dissociation of organic matter and alter its form in water. Increased ionicity and decreased charge density of organic matter reduce its solubility and hydrophilicity, making it more readily adsorbed. Randtke believes that the mechanism of enhanced coagulation in removing organic matter mainly includes the charge neutralization of colloidal natural organic matter (NOM), the precipitation of humic acid and fulvic acid polymers, and the adsorption of metal hydroxides. This refers to co-precipitation on the surface of hydroxides. For dissolved organic matter in water, removal relies on the latter effect, namely adsorption onto the metal precipitate of the coagulant. For example, production practice at Beijing No. 9 Water Plant shows that, under the same dosage, mechanically stirred clarifiers are more effective than reaction-sedimentation tanks in removing organic matter and disinfection byproduct precursors. This is because the large amount of floc maintained in mechanically stirred clarifiers can fully exert its adsorption effect, while in the reaction-sedimentation process, the floc is precipitated and removed as soon as it forms, and its adsorption potential is not fully realized.