【Daily Question】Chemical Engineering Principles 290: Factors Affecting Coagulation Efficiency (January 10)

(1) Water temperature: Water temperature affects the hydrolysis of inorganic salts. At low water temperatures, the hydrolysis reaction is slow, the water viscosity is high, and it is difficult for flocs to form. (2) The pH value and alkalinity of water. At different pH values, the forms of the hydrolysis products of aluminum and iron coagulants vary, resulting in different coagulation effects. (3) Properties, composition, and concentration of impurities in water. Cations with a valence of two or more present in water are beneficial for compressing the natural water’s double layer ； A variety in the size distribution of impurity particles will be beneficial for coagulation ； Too low a concentration of impurities (too few particles) will hinder collisions between particles, thereby affecting agglomeration. (4) Hydraulic conditions. During the mixing stage of the coagulation process, it is necessary for the coagulant to mix rapidly and evenly with the turbid water ； During the reaction phase, the stirring intensity should be gradually reduced as the flocs grow larger.

1. Influence of water temperature 2. Influence of water pH value 3. Influence of water alkalinity 4. Influence of the concentration of turbid particles in water 5. Influence of organic pollutants in water 6. Influence of the type and dosage of coagulants 7. Influence of the method of adding coagulants 8. Influence of hydraulic conditions

What are the factors that affect the coagulation effect? Water temperature ； pH ； Impurities in water (including solid particles and organic pollutants) ； Hydraulic conditions

1. Water temperature 2. pH value 3. Dosage of chemicals 4. Contact medium 5. Hydraulic conditions 6. Quality of raw water

1. The effect of water temperature. 2. The influence of water pH. 3. The effect of water alkalinity. 4. Influence of the concentration of suspended particles in water. 5. The impact of organic pollutants in water. 6. Influence of coagulant type and dosage. 7. Influence of the dosing method of coagulants. 8. Influence of hydraulic conditions.

1. Water temperature 2. pH value 3. Composition, properties, and concentration of impurities in water 4. Hydraulic conditions

1. Effect of water temperature: Water temperature has a significant impact on the coagulation process; both excessively high and low temperatures are unfavorable for coagulation. The optimal water temperature for coagulation is between 20 and 30°C. At low water temperatures, flocs form slowly; the floc particles are small, resulting in poor coagulation efficiency. The reasons are as follows: ① Since the hydrolysis reaction of inorganic coagulants is endothermic, low water temperatures slow down this hydrolysis process, thereby affecting the destabilization of colloidal particles. ②When the water temperature is low, the viscosity of the water increases, which raises the resistance to the movement of colloidal particles, affecting effective collisions between them as well as flocculation. ③At low water temperatures, the Brownian motion of colloidal particles in the water is reduced, which hinders the heterogeneous flocculation of destabilized colloidal particles. When the water temperature is too high, the coagulation effect also deteriorates, mainly because the hydrolysis reaction of coagulants proceeds too rapidly at high temperatures, resulting in enhanced hydration of the formed flocs, which makes them loose and difficult to settle ； During wastewater treatment, the sludge generated is large in volume and high in moisture content, making it difficult to handle. 2. Impact of water pH: The pH value of water has a significant effect on the coagulation process, influencing it mainly in two ways. On the one hand, the pH value of water is directly related to the surface charge and potential of the colloidal particles in the water; at different pH values, the surface charge and potential of these colloidal particles vary, which in turn results in different amounts of coagulant being required ； On the other hand, the pH value of water has a significant impact on the hydrolysis of coagulants; different coagulants require different pH ranges for optimal hydrolysis. Therefore, the effect of water’s pH value on the coagulation effect also varies depending on the type of coagulant used. For our company, the optimal pH range for coagulation and turbidity removal using polyaluminum chloride is between 5 and 9. 3. Effect of water alkalinity: When coagulants are added to raw water, hydrolysis reactions occur, and this process consumes the alkalinity of the water; inorganic salt-based coagulants consume even more alkalinity. When the alkalinity in the raw water is very low, the addition of coagulants consumes the alkalinity present in the water, thereby lowering its pH value. If the pH value of the water falls outside the optimal range for coagulation by the coagulants, it will significantly affect the efficiency of coagulation. When the alkalinity of the raw water is low or a large amount of coagulant is used, it is usually necessary to add a certain amount of alkaline agents such as lime to improve the coagulation effect. 4. Effect of the concentration of suspended particles in water: The concentration of suspended particles in water has a significant impact on the coagulation process. When the particle concentration is too low, the chances of collisions between particles decrease, resulting in a poorer coagulation effect. If the value is too high, polymeric coagulants such as polyacrylamide must be used to reduce the turbidity of the raw water to a certain level, after which coagulants can be added for conventional treatment. 5. Effects of organic pollutants in water: Organic substances in water have a protective and stabilizing effect on colloids; that is, soluble organic molecules in water adsorb onto the surface of colloid particles, forming an organic coating that protects these particles and prevents collisions between them. It also hinders the destabilization and aggregation of colloid particles by coagulants. As a result, colloid particles are more difficult to destabilize in the presence of organic substances, requiring higher amounts of coagulant. Potassium permanganate, ozone, chlorine, and other pre-oxidizing agents can be used, but it is necessary to consider whether toxic by-products will be generated. 6. Influence of coagulant type and dosage: Due to the differences in hydrolysis characteristics and the water quality conditions under which different types of coagulants are used, it is necessary to select the appropriate type of coagulant based on the characteristics of the raw water. For inorganic salt coagulants, it is required that they take a form capable of effectively compressing the double electric layer or inducing strong electrostatic neutralization; for organic polymer coagulants, an appropriate amount of functional groups and a polymeric structure along with a high molecular weight are necessary. My treatment plant uses polyaluminum chloride as a coagulant and PAM as a flocculant aid. Under normal circumstances, the coagulation effect improves as the amount of coagulant used increases. However, once the dosage of the coagulant reaches a certain level, the coagulation effect reaches its peak; further increase in the coagulant dosage leads to a stabilization effect, and the coagulation effect actually declines. Theoretically, the optimal dosage is one that results in the lowest turbidity of the purified water after coagulation and sedimentation, with both the colloidal titratable charge and the zeta potential approaching 0. However, due to cost considerations, in actual production the optimal dosage of coagulant is usually chosen to ensure that the water quality meets **the standards while minimizing the amount of coagulant used. 7. Influence of the coagulant addition method: The influence of the coagulant addition method: Due to the impact of the solid coagulant addition method, there are two ways of adding coagulants: dry addition and wet addition. Among various liquid coagulants, or even among liquid coagulants at different concentrations, the hydrolyzed forms of those capable of compressing the double layer or exerting electroneutralization effects are not exactly the same; therefore, the coagulation effects produced after they are added to water also differ. If other coagulants are added in addition to the coagulant, the order in which these various agents are added also has a significant impact on the coagulation effect; it is necessary to determine the appropriate method and order of addition through simulation experiments and actual production practices. 8. Influence of hydraulic conditions: After coagulants are added, the coagulation process can be divided into two stages – rapid mixing and flocculation reaction. However, in actual water treatment processes, these two stages are continuous and inseparable, and hydraulic conditions also need to be continuous. Since the hydrolyzed form of the coagulant can change rapidly after it is added to water, the rapid mixing stage is necessary to ensure that the added coagulant disperses evenly throughout the raw water. This allows the coagulant to hydrolyze and polymerize uniformly in the water, thereby destabilizing and aggregating the colloidal particles. Rapid mixing requires intense hydraulic or mechanical stirring to be carried out in a short amount of time. In the flocculation reaction stage, the destabilized colloidal particles gradually grow into flocs with good sedimentation properties through counter-directional and co-directional flocculation. Therefore, during this stage, the stirring intensity and water flow velocity should be gradually reduced as the flocs grow in size, so as to prevent the already formed flocs from being broken apart, which would otherwise impair the coagulation and sedimentation efficiency. At the same time, since the flocculation reaction is a slow process of gradual growth of flocs, if it is necessary for the flocs to reach a sufficient particle size after coagulation so that they can be removed by sedimentation, a certain flocculation time must be ensured. If air flotation or direct filtration is used after coagulation, the reaction time can be **reduced**.

There are many factors that affect the coagulation effect. The main factors include the following. (1) Water temperature: Water temperature has a significant impact on the effectiveness of coagulation treatment. At low temperatures, it takes longer for flocs to form; these flocs contain more water, and their settling speed is slower, which hinders both the coagulation reaction and the settling of the flocs. When aluminum sulfate is used as a coagulant, the optimal water temperature is 25–30°C ； When iron salts are used as coagulants, water temperature has little impact on the coagulation effect. (2) pH value of water The impact of the water’s pH value on coagulation efficiency: When using iron salts as coagulants, it is also necessary to pay attention to controlling the pH value of the water. (3) The amount of coagulant used should be determined through testing, based on specific conditions, to find the optimal dosage. The data listed in Table 2–1 can be used for reference. (4) Contact medium: Practice has shown that maintaining a certain amount of sediment in water can increase the opportunities for collisions between colloids, and it also serves as a \"core\" for coagulants to initiate particle crystallization. This facilitates the formation and growth of flocs on the sediment particles, thereby aiding in the adsorption, sedimentation, and separation of impurities in the water. (5) Hydraulic conditions in the coagulation process From a hydraulic perspective, the coagulation process can be divided into two stages: mixing and reaction. ①Mixing process. Under intense stirring, the coagulant is rapidly and evenly dispersed, creating conditions for its hydrolysis, polycondensation reactions, and the destabilization of colloids. From the addition of the coagulant until the formation of flocs begins, this process generally takes no more than 2 minutes. ②Reaction process. The water that has been thoroughly mixed with the reagent begins to cause the colloidal particles in the water to continue colliding and adsorbing at a higher reaction flow rate ； At lower reaction flow rates, the destabilized colloids form flocs with better precipitation properties. As the reaction flow rate gradually decreases, the flocs also gradually grow larger, and their settling velocity increases accordingly. The duration of the reaction phase generally does not exceed 30 minutes. The water flow velocity during the reaction phase should not be too fast, otherwise it can easily break apart the already formed flocs. When a colloid is formed, its stability increases, but the coagulation effect deteriorates. To achieve a flow rate that is fast first and then slow ； Clarification equipment is usually designed with several reaction chambers to allow the water flow velocity to be adjusted according to the requirements of the reaction, thereby meeting the needs of the reaction for hydraulic agitation.

1) Water temperature 2) Water pH value 3) Water alkalinity 4) Concentration of suspended particles in water 5) Organic pollutants in water 6) Type and dosage of coagulant 7) Method of coagulant addition 8) Hydraulic conditions

The main factors are water temperature, water pH and alkalinity, and the concentration of suspended solids in the water