Which Polymer Should You Use?
It’s not always easy to know which polymer to use. But for most water and wastewater streams you can start with a coagulant or a cationic flocculant when you do your jar testing. All particles carry an electrical charge on their surface, the sign and intensity of which depend on the nature of the surface and on the chemistry of the aqueous suspending medium. In general, aqueous suspensions at a pH of 3 to 4 and above carry a negative charge. Positively charged suspensions occur in particular in strong acids.
Cationic Polymers. Because most particles in water and wastewater have a negative charge, cationic polymers (positively charged) are most commonly used for settling solids, sludge thickening, and sludge dewatering.
The “work horse” backbone for sludge conditioning is polyacrylamide. Polyacrylamide, itself, has a nonionic charge. To be an effective flocculant, the polyacrylamide must be reacted to give it a positive charge. One such process is the Mannich reaction. In this process, the polyacrylamide is reacted with formaldehyde and dimethylamine to form an aminomethylated polyacrylamide.
Because of their tertiary nature, Mannich polymers lose their positive charge when used in sludges with a pH of approximately 7.2 or higher. A more recent development in Mannich technology has been in the polyacrylamide co-polymers. These co-polymers have the advantage over conventional Mannichs of being stable when used with high pH sludges.
Polyacrylamide is also the most common backbone used in emulsion, dry, and gel cationic polymers. Many of the dry and emulsion cationic polymers now being manufactured are polyacrylamide co-polymers. In addition to remaining stable at high pH levels, these products also show improved performance over standard polyacrylamides on waste activated sludges and various digested sludges.
Polyamines are low-to-medium molecular weight liquid cationic polymers. High in cationic charge, polyamines are considered to have 100% charge density. Because of their relatively low molecular weight , polyamines have limited use in sludge conditioning (sludge dewatering). They have been successfully used on high pH sludges and septic sludges.
Diallyldimethylammonium chloride (DADM) is considered a vinyl addition polymer. Like polyamines, DADMs are low to medium in molecular weight and are effective at high pH levels. Because of their relatively low molecular weight they are rarely used for sludge dewatering applications.
A few manufacturers now offer co-polymers of acrylamide with DADM. These products combine the acrylamide’s high molecular weight advantages with the pH stability of the DADM. DADM use in sludge conditioning other than as an acrylamide co-polymer is very limited.
Anionic Polymers. The particles in most water and wastewater streams have an anionic or negative surface charge. This fact limits the usefulness of anionic polymers. Anionic polymers are extremely effective when straight primary sludges or sludges high in iron or lime are to be conditioned. Anionic polymers have been used in two-polymer systems in conjunction with a cationic polymer or as part of a metal (iron or aluminum) coagulant combination.
The most widely used backbone for anionic polymer production is polyacrylamide. Again, polyacrylamide has neutral or essentially no charge; therefore, it must be reacted with another molecule to give it a negative charge. Anionic polyacrylamides are produced in one of two methods. The first method uses sodium carbonate, sodium hydroxide, or similar material to partially hydrolyze the acrylamide. This is followed by polymerization. The second method is similar, but an acrylic acid or an acrylate salt is used to co-polymerize with the acrylamide.
Anionic polyacrylamides are very high molecular weight polymers with low to medium charge densities. They are available in dry, emulsion, and gel forms.
Nonionic Polymers. Nonionic polymers have neutral or no charge. In the manufacturing process it is difficult to produce a product with no charge; therefore, most nonionic polymers have a very slight anionic charge.
Nonionic polymers, although extremely useful in many industrial wastewater and raw water clarification applications, have very limited usefulness in sludge conditioning. Nonionic polymers have been found to be effective in some gravity thickening and dewatering applications. Normally, however, if nonionic polymers work in these applications, they will be out-performed by either a cationic or anionic polymer.
As with anionic polymers, nonionic polymers are polyacrylamides. Nonionic polyacrylamides have very high molecular weights and are available in dry, emulsion, and gel forms.