Strengthen the Application of Coagulation in Water Treatment Engineering

One of the most serious problems currently hindering further improvement in purified water quality is the control and removal of organic matter in water. For decades, water treatment professionals both domestically and internationally have conducted extensive research on organic matter removal, exploring various materials and methods for its removal. In recent years, the U.S. Environmental Protection Agency (USEPA) has recommended the following processes to achieve the first-stage control targets for drinking water disinfection/disinfection byproducts (D/DBP)—total trihalomethanes (THMs) ≤ 0.08 mg/L and haloacetic acids (HAAs) ≤ 0.06 mg/L: enhanced coagulation, granular activated carbon adsorption (GAC adsorption), and membrane filtration. Filtration), and enhanced coagulation is listed as the best method for controlling natural organic matter (NOM). Coagulation treatment is one of the most commonly used processes in conventional water treatment systems. Its main function is to remove suspended particles and colloidal particles from water, and it can also remove some organic matter, but the efficiency of organic matter removal is not high and fluctuates greatly, mainly due to the type and form of organic matter in the water. Currently, commonly used coagulants in water treatment processes are Al2(SO4)3, FeCl3, PFS (polyferric sulfate), and PAC (polyaluminum chloride). Since the pH value of water directly affects the hydrolysis form of coagulants and the surface characteristics of particles in water, thus affecting the coagulation effect, for most raw water, the optimal coagulation effect does not occur when the particle zeta potential is 0. In fact, when the coagulant dosage is low, the mechanism for achieving better coagulation is mainly charge neutralization and adsorption bridging; while when the coagulant dosage is high, the mechanism for achieving better coagulation is mainly adsorption bridging and trapping sedimentation.

Organic matter in natural water bodies (usually mainly humic acid-based organic matter, whose molecular structure often contains a large number of -COOH and -OH groups) can be classified into three types according to their existence in water: suspended (including isolated organic particles and organic matter adsorbed on the surface of microparticles in water), colloidal, and dissolved. Suspended and colloidal components are typically organic components with larger molecular weights and lower solubility. A considerable portion of the organic matter in natural water is adsorbed by tiny solid particles. Coagulation and clarification are the first treatment units in conventional water treatment systems. The properties of suspended and colloidal organic matter in natural water bodies are very similar to those of the microparticles present in the water, such as carrying a negative charge under normal conditions (organic matter tends to dissociate in water). Therefore, their removal mechanisms during coagulation treatment should be similar, namely, removal through charge neutralization, adsorption bridging, and sedimentation, with a high removal rate (up to 80%–90%).

Meanwhile, organic matter with smaller molecular weights and higher solubility in water (mainly fulvic acid and other humic acids) has a lower removal rate under general coagulation conditions. The low concentration of dissolved organic matter is mainly due to its good hydrophilicity, making it difficult for it to be adsorbed by the hydrolysis products of coagulants—metal hydroxides. However, by improving coagulation conditions—specifically, by using enhanced coagulation with low pH and high coagulant dosage to form a large amount of metal hydroxides, the morphology of the coagulant hydrolysis products can be improved, increasing their positive charge density. Simultaneously, the low pH conditions affect the degree of dissociation of organic matter and alter its form in the water, increasing the degree of organic matter atomization and decreasing its charge density, thereby reducing its solubility and hydrophilicity. This makes it more easily adsorbed, adsorbing onto the abundant metal hydroxide particles for co-precipitation. This improves the removal rate of dissolved organic matter in the water, thus increasing the overall removal rate of organic matter. Therefore, theoretically, improving coagulation conditions (enhanced coagulation) is a feasible and effective way to improve the removal rate of organic matter in water treatment projects.

Extensive research on enhanced coagulation treatment processes is primarily conducted in the United States, mainly in the drinking water treatment industry, with the primary goal of improving the removal rate of D/DBP precursors in drinking water. Thomas Studies by R. Hundt et al. have shown that fulvic acid (FA) compounds in water are mainly removed through charge neutralization precipitation and adsorption co-precipitation, and are primarily related to the hydrolysis forms of aluminum salts; under low pH conditions, polyaluminum chloride is more effective than AlCl3 in removing FA. Grozes et al.'s enhanced coagulation experiments on Sacramenta and other rivers found that pH control during coagulation was the decisive factor in achieving the maximum NOM removal rate. Under conditions of pH≈6, enhanced coagulation could increase the NOM removal rate by 65%. When adding similar doses of coagulant, ferric salts showed significantly better NOM removal than aluminum salts. The reasons are: ① Ferric salts have a stronger acidification ability than aluminum salts, so their coagulation pH is lower. Lower pH conditions increase the protonation degree of humic acids, increase the positive charge density on the coagulant hydrolysis products, reduce the amount of coagulant required, and facilitate the adsorption of organic matter onto metal hydroxides; ② Although the specific surface area of aluminum salt hydrolysis products [200–400 m²/g Al(OH)₃] is greater than that of ferric salt hydrolysis products [160–230 m²/g Fe(OH)₃], the amount of Fe(OH)₃ produced by the hydrolysis of similar doses of ferric salt is less than that of aluminum salt. The amount of Al(OH)3 produced by salt hydrolysis is 2.8 times that of organic matter. Furthermore, it was found that high molecular weight polymers, as coagulants, have poor removal efficiency for dissolved NOM. This is because they cannot produce hydrolysis products with good adsorption properties for organic matter, indicating that the coagulation removal mechanism of NOM in water mainly involves adsorption onto the hydrolysis products of the coagulant (metal hydroxides) and separation from the water. Studies by F. Julien et al. also showed that iron salts are more effective than aluminum salts in removing NOM. Organic matter with one or no functional groups cannot be simultaneously removed by typical coagulation-flocculation and adsorption onto metal hydroxide flocs. However, compounds containing at least two adjacent functional groups (-COOH and -OH) can be simultaneously removed by coagulation-flocculation and adsorption. The removal of organic matter depends on the number and molecular weight of the functional groups -COOH and -OH, and the maximum removal rate does not occur at a zeta potential of 0. Joseph When G. et al. conducted coagulation experiments on raw water with high humic acid content, they found a sudden change in the curve showing the relationship between coagulant dosage and TOC removal rate. In contrast, the curve for raw water with relatively low humic acid content was flatter, indicating that NOM removal in water mainly relies on precipitation and co-precipitation. Joseph G. et al. also compared the characteristics of several major organic matter removal processes (see Table 1) and concluded that enhanced coagulation is a more economical and practical process for removing organic matter from natural water. Eric M. Vrijenhoek et al. found that the optimal pH for removing THMS precursors was 5.5. The removal mechanism may be that at low coagulant dosages, metal-organic complexes are formed; while at higher coagulant dosages, organic matter is adsorbed onto metal hydroxides and removed. Raw water with high humic acid content showed higher NOM removal rates. The experiments also showed that the original turbidity removal effect could be maintained under low pH conditions.

Coagulation experiments conducted by Ding Huanru et al. on Huangpu River water using polyferric sulfate and polyaluminum chloride showed that, under optimal pH conditions (5.6 for polyferric sulfate and 5.5 for polyaluminum chloride), the removal rates of total, dissolved, and UV-absorbed organic matter in the water by polyferric sulfate and polyaluminum chloride coagulants reached 70%, 52%, and 55%, and 70%, 52%, and 33%, respectively. The optimal pH range for removing organic matter from this water body was 5.5–6.5. Ferric salts have a wider pH range than aluminum salts. Under low pH coagulation conditions, the removal rate of organic matter in the water body was significantly improved, and polyferric sulfate was superior to polyaluminum chloride. Enhanced coagulation experiments conducted by Huang Tinglin using Al2(SO4)3 as a coagulant on humic acid water samples and Xingqing Lake water samples showed that the mixing intensity affected the TOC removal effect. Under lower mixing intensity, a slightly better TOC removal effect could be obtained by extending the flocculation reaction time. However, the impact of mixing intensity on TOC removal is far less significant than that of pH and coagulant dosage. The optimal pH for TOC removal is around 5.5, with removal rates reaching over 80% and 70% respectively, and turbidity changes do not affect DBP precursor removal.

Domestic and international experimental studies have yielded several consistent conclusions: ① Organic matter in water is mainly removed through adsorption and co-precipitation; ② The optimal pH for removing organic matter in water is 5.5–6.5, and increasing the coagulant dosage is beneficial; ③ The main influencing factors for NOM removal under coagulation conditions are pH and coagulant; ④ Enhanced coagulation is particularly effective for removing humic acid-rich organic matter.

Enhanced coagulation is a treatment process developed based on conventional coagulation to remove organic matter in water, especially humic acid-rich organic matter. Compared to other treatment processes, it has lower costs and can be implemented with minor modifications to existing treatment equipment. Domestic and international experimental studies have shown that controlling the pH of coagulation treatment at around 6 and appropriately increasing the coagulant dosage has a good removal effect on most organic matter in source water; the key is pH adjustment. pH adjustment can be achieved by adding acid, using a strong acidifying coagulant (such as FeCl3), or adjusting the pH using acidic effluent from the cation exchange desalination system. Such process conditions are feasible in industrial water supply engineering, especially in power plant water treatment. Because most power plant water treatment systems have auxiliary acid-base systems for ion exchange desalination, pH adjustment for coagulation is easier to achieve. However, for systems containing reverse osmosis (RO) pre-desalination devices, to reduce the hydrolysis rate of the RO membrane and prevent scaling, the influent pH value is generally adjusted to an acidic range. For enhanced coagulation, this essentially means moving the pH adjustment point forward. Enhanced coagulation primarily increases the costs associated with corrosion prevention and pH adjustment for related equipment in water treatment systems. However, for source water with low organic matter content, it can eliminate the investment and operating costs of GAC adsorption equipment. For source water with high organic matter content, enhanced coagulation can further improve the operating efficiency of GAC adsorption equipment (lowering the influent pH value, which is more effective for GAC adsorption in removing NOM from the water). Ultimately, the goal is to achieve a significant improvement in water quality at a lower cost and by fully utilizing existing processes. Therefore, conducting enhanced coagulation experiments on source water to determine the optimal pH range and coagulant dosage for removing organic matter, and further studying the combined use of enhanced coagulation with other processes (such as GAC adsorption), is of great practical significance given the widespread micro-pollution of source water in my country. It is believed that enhanced coagulation technology also has significant application and promotion value in water treatment engineering.