Dry powder polyacrylamide (dry PAM) is used as a water purification coagulant, and its input point is the key to determine the effect of coagulation and precipitation. The practical application results in water purification production show that the best coagulation and precipitation effect can be obtained by adding polyacrylamide between 1/2 and 2/3 of the total flocculation reaction time. The optimum dosage is between 0.03 and 0.4 mg/L. In the practical application of water purification production, it is also proved that its coagulation-assisting effect is not significant when the dosage is less than 0.03mg/L. When it exceeds 0.4mg/L, its coagulation-assisting effect is too fast, and the formed alum flower particles are very large, which is easy to cause a large amount of sludge to be deposited in the rear of the reaction tank and the front of the sedimentation tank, and the length and area of the sedimentation tank cannot be fully utilized and affect the reaction sedimentation effect.
If it is added at the same time as the coagulant, it will have no coagulation-aiding effect; if it is added before or after 1/3 of the total flocculation reaction time, its coagulation-aiding effect is not significant. If it is added too early, the small alum flowers are not formed, too late addition of its aggregation net capture effect time is not sufficient. Therefore, if pam flocculant is added too early or too late, its coagulation-aiding effect cannot be exerted.
Regarding the dosage of polyacrylamide, it should be determined through experiments according to the characteristics of different water sources and water purification processes. According to relevant information, the flocculation process of polymer flocculants may have the following rules:
(1) When the dosage of dry polymer polyacrylamide (dry PAM) flocculant is guaranteed to cover the surface of the solid particles that can be covered, the best conditions can be achieved.
(2) The surface of the particles is supersaturated with polymer molecules, which will lead to the deterioration of flocculation, because in this case, the free ends of the polymer can also be adsorbed on the same surface, forming a curved shape, and the number of bridging bonds between adjacent particles is thus reduced.
(3) When the agitation is strong enough to destroy the bond of the polymer, the flocculated particles will disperse. If the dosage of the polymer polyacrylamide is less than the optimum dosage, the bridging bond will be weaker.
(4) There is a linear relationship between the optimal dosage of polymer and the allowable adsorption area on the surface of dispersed phase particles. Nonionic polyacrylamide is relatively suitable for flocculation of raw water with serious pollution, a wide variety of pollutants, and low pH and alkalinity, while anionic polyacrylamide is slightly inferior, and cationic polyacrylamide is less effective. According to the flocculation theory, it may be that the turbidity of the raw water in the dry season with serious pollution is relatively low, and the particles in the water body that can be used for bridging are relatively small. Add liquid chlorine first for disinfection, and then add the flocculant, which may have a destructive effect on the structure of cationic flocculants, the water body that has been added with coagulants and disinfectants, the pH value is generally not higher than 7.0, the charge tendency of the water body is not obvious, and the anionic and cationic polyacrylamide shows no advantage.