Do you know how polyacrylamide and polyaluminum chloride can be used together?

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This post was last edited by slll611 on 2016-9-17 at 23:30. In the workplace, it is common to encounter issues such as small flocculation particles, weak floccules, leakage of the coagulant, and the floating of flocculation masses when using polyaluminum chloride. Such problems occur quite frequently; they may be caused by insufficient dosage of the coagulant or too long mixing time, but in many cases I’ve encountered them due to the inherent properties of the polyaluminum chloride product itself. Today, I am writing down what I know in the hope of helping more people. Since the advent of polyaluminum chloride in the 20th century, it has indeed attracted much attention and been widely used in various water treatment applications due to its efficient and excellent properties. Unfortunately, like other coagulants, polyaluminum chloride is not a solution that can cure all problems. During use, polyaluminum chloride encounters problems when treating certain types of wastewater. Due to inherent shortcomings such as small floccules and slow sedimentation, these issues arise not only in the winters of the northern regions but also in the southern areas, such as the Pearl River Delta, where problems like incomplete sedimentation and flocculent leakage occur even during the summer. This places additional strain on the filtration systems used in subsequent treatment processes, resulting in higher energy consumption and the need for large amounts of water for backwashing. Of course, these problems are particularly severe during the winters in the north. This drawback of PAC requires it to be used in combination with PAM, an organic polymer chemical, in order to improve efficiency while reducing costs. PAM is the most widely used synthetic organic polymer coagulant at present; its degree of polymerization can range from 20,000 to 90,000, resulting in corresponding molecular weights of 1.5 million to 23 million. Its coagulation effect arises from its strong adsorption capacity for the surfaces of colloids, which enables the formation of bridges between these colloidal particles. When PAC and PAM are used together, aluminum polychloride in water hydrolyzes to form polymeric metal complex ions, which undergo electroneutralization, adsorption, and sweeping actions with the suspended colloids in wastewater. Through the bridging and adsorptive trapping action of its chain-linked molecules, PAM can effectively increase the size of flocs; the diameter of these floc particles is 3-5 times larger than that of the flocs formed by the use of PAC alone. When PAC and PAM are used together, the dosage of the chemicals poses a problem. Generally speaking, as a coagulant aid, polyacrylamide should usually be added after aluminum chloride polymer is added. Of course, there are also practices in the industry where both are added simultaneously, or where PAM is added first followed by PAC. Personally, I believe that adding PAC first and then PAM yields better results. As for the time interval between their addition, it is best to leave about 30 seconds to 1 minute. Furthermore, the dosage of polyacrylamide is also an issue: too little will result in no coagulation enhancement, while too much will cause excessive coagulation, leading to large sediment particles that result in a rapid settling rate. This prevents the full utilization of the length and area of the sedimentation tank, thereby affecting the flocculation process. Generally, the dosage concentration of polyacrylamide is 0.3–0.6 mg/L; specific cases should be handled on a case-by-case basis. It should be noted that polyacrylamide cannot be added directly when in use; adding it directly will result in the formation of lumps, which do not dissolve even after prolonged stirring, thereby causing blockages in the dosing system. It is generally dissolved using a dispersion device before being added to the sedimentation tank via a metering pump. The stirring time needs to be adjusted appropriately – usually 30–40 minutes in summer and 1 hour in winter; the time in the south can be slightly shorter than that in the north. Polyacrylamide comes in different ion types, and the choice of which ion type to use depends on the quality of the water. It is best to consult the manufacturer to avoid wasting money without achieving the desired results.