Summary Summary of the range of pH values ​​when using polyaluminum chloride Table 1 Summary of polyaluminium chloride contents when adjusting pH values ​​Point polyaluminium chloride or polyaluminum chloride coagulant Suitable pH range, some raw water to achieve the best coagulation, the need to adjust the pH value. Usually CO2, HCL, H2SO4 to reduce the pH value, but should not be used more than cast aluminum salt or iron salt to reduce the pH value of the method. Some raw water has a low pH and alkalinity, and sodium hydroxide or lime is added to raise the pH to compensate for the acidity of the aluminum or iron salts. Reduce the PH value of raw water can improve the hydrolysis conditions of water purification agents, while reducing the negative potential of organic matter, algae in water. Reduce drug consumption to improve water quality. Under the condition of low pH, the amount of ionic organic compounds decreased while the number of molecular organic compounds increased and the solubility of organic compounds decreased. They were relatively easily adsorbed on a large amount of AL (OH) 3 and other particles for coprecipitation. At the same time, the coagulant hydrolyzate with positive Charge group charge increased, the positive charge density of the dissolved organic matter in water, the electrical neutralization adsorption more obvious. The effect of pH on coagulation performance is as follows. 1. Affect the scope of aluminum salt. Aluminum sulfate added to the water after the formation of colloidal aluminum hydroxide in order to play the role of coagulation, when the pH is less than 4, the aluminum hydroxide will dissolve in water is only aluminum ions, can not afford any coagulation, even dosing a lot of coagulation Agent, due to the pH value decreased, the effect may be getting worse; pH value of 6.5 to 7.5 when the formation of aluminum hydroxide gel solubility is small, coagulation effect is good; pH greater than 8.5 when the aluminum hydroxide will dissolve, Poor coagulation effect. 2. Some water in the organic matter, such as humic acid, fulvic acid, coagulation at different pH values ​​vary. The humus content of the raw water, the pH value of the water must be less than 6, plus aluminum salt, iron salt can precipitate it, or most of the humus is still dissolved, the color of water can not be removed. 3. Iron salt pH range is wider than the aluminum salt, generally suitable for pH6.0 ~ 8.4, otherwise the generated iron hydroxide will dissolve, will not generate hydroxide precipitation, can not afford coagulation. Poly aluminum ferric chloride for food, brewing, textile, paper and other industrial waste water, iron in water will directly affect the quality of products such as color, taste, it is not appropriate to use iron salt as flocculant, if used, the need to strictly control the water PH value, so that the residual iron content of water to meet the technical requirements.