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Question 1 of yesterday’s Daily Question: 1. Which factor does not affect the coagulation effect? ( ) A. Hydraulic conditions B. pH value C. Acidity D. Alkalinity. The correct answer is C, acidity. Many people chose A, hydraulic conditions; I’d like to address this question by sharing a piece of academic material with you: The main factors affecting coagulation efficiency 1. Influence of water temperature: Water temperature has a significant impact on coagulation efficiency; both excessively high and low temperatures are unfavorable for coagulation. The most suitable water temperature for coagulation is between 20–30°C. When the water temperature is low, flocculation occurs slowly, the flocculation particles are small, and the coagulation effect is poor. The reasons are as follows: ① Since the hydrolysis reaction of inorganic salt coagulants is an endothermic process, at low water temperatures the hydrolysis of these coagulants proceeds slowly, which hinders 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 and hinders effective collisions and flocculation between them. ③At low water temperatures, the Brownian motion of colloidal particles in the water is reduced, which hinders the heterogeneous flocculation of the destabilized colloidal particles. When the water temperature is too high, the coagulation effect also deteriorates, mainly because the hydrolysis reaction of the coagulant proceeds too rapidly at high temperatures, resulting in enhanced hydration of the formed flocs, which makes them loose and difficult to settle ； In wastewater treatment, the sludge generated is large in volume and high in moisture content, making it difficult to handle. 2. Influence of water pH: The pH value of water has a significant impact on the coagulation process, affecting it in two main 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 efficiency also varies depending on the type of coagulant used. The optimal pH range for coagulation and turbidity removal using polyaluminum chloride in our company is between 5 and 9. 3. Effect of water alkalinity: When coagulants are added to raw water, hydrolysis reactions occur, and these reactions consume the alkalinity of the water; this is especially true for inorganic salt-based coagulants, which 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 falls outside the optimal range for coagulation by these 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. Influence of the concentration of suspended particles in water: The concentration of suspended particles in water has a significant impact on the coagulation effect. When the concentration of these particles is too low, the chances of collisions between them decrease, resulting in a poorer coagulation effect. If the value is too high, polymeric flocculants such as polyacrylamide must be added to reduce the turbidity of the raw water to a certain level, after which coagulants can be used 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, in the presence of organic substances, it is more difficult to destabilize colloid particles compared to when there are no organic substances, and therefore a larger amount of coagulant is required. 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: Since different types of coagulants have varying hydrolysis characteristics and are suitable for different water quality conditions, 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 flocculants, 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 conditions, the coagulation effect improves as the amount of coagulant used increases. However, once the amount of coagulant reaches a certain level, the coagulation effect reaches its peak; further increase in the amount of coagulant 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 simultaneously minimizing the amount of coagulant used. 7. Influence of coagulant addition method: There are two methods for adding coagulants, dry addition and wet addition. Since the hydrolyzed forms of coagulants that can compress the double electric layer or possess electro-neutralization capabilities differ among solid coagulants and liquid coagulants, as well as between liquid coagulants of different concentrations, the coagulation effects produced when these are added to water also vary. If other coagulation aids are added in addition to the coagulant, the order in which these various agents are added 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 disperse the added coagulant evenly throughout the raw water quickly, so that it can hydrolyze and polymerize uniformly in the water and cause the colloidal particles to become unstable and aggregate. Rapid mixing requires fast and intense hydraulic or mechanical stirring, and this process must be completed in a short time. The flocculation phase begins, during which the destabilized colloidal particles need to gradually grow into flocs with good sedimentation properties through counter-current and co-current flocculation. Therefore, the mixing intensity and flow rate during this phase should be reduced as the flocs grow, in order to prevent the already formed flocs from breaking apart and compromising the efficiency of coagulation and sedimentation. 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 large enough particle size after the coagulation reaction so that they can be removed through sedimentation, a sufficient flocculation time must be ensured. If air flotation or direct filtration processes are used after coagulation, the reaction time can be **reduced**. The concept of acidity: In chemistry, the acid value (also known as the neutralization value, acidity value) refers to the number of milligrams of potassium hydroxide (KOH) required to neutralize 1 gram of a chemical substance. The acid value is a measure of the number of free carboxyl groups in a compound (such as fatty acids) or mixture. The typical measurement procedure involves dissolving a sample of known amount in an organic solvent, titrating it with a potassium hydroxide solution of known concentration, using phenolphthalein solution as a color indicator. The acid value can be used as an indicator of the degree of deterioration of oils and fats. Unit of acid value: (KOH)/(mg/g). Acidity refers to the total amount of substances in water that can undergo neutralization with strong bases, including inorganic acids, organic acids, and salts of strong acids and weak bases. The higher the acidity value, the stronger the acidity of the solution. The concept of alkalinity: Alkalinity refers to the ability of water to absorb protons; it is usually determined by the total amount of substances in water that can react quantitatively with strong acids. Alkalinity in water arises primarily from the presence of bicarbonates, carbonates, and hydroxides, while borates, phosphates, and silicates also contribute to alkalinity. Wastewater and waters in other complex systems also contain organic bases, metal hydrolyzable salts, etc., all of which contribute to alkalinity. In these cases, alkalinity becomes a comprehensive indicator of water, representing the total amount of substances that can be titrated by strong acids.