Polymeric Flocculants for Wastewater and Industrial Effluent Treatment

The fact that water is the most important material used by mankind defines the importance of water purification. Wastewater and industrial effluent treatment require removal of suspended solids for purification and possible re-usage. The removal can be accomplished by gravitation (very slow), by coagulation (dependent on electric charge situation) and by flocculation (not dependent on electric charges and the fastest). Inorganic flocculants are used in very large quantities, they leave large amounts of sludge and are strongly affected by pH changes. We describe the mechanism of flocculation by polymers in ppm concentrations by strong solvation of the liquid by the flocculant, thus pushing the solid particles outside the solvated domains. The solid particles aggregate in much smaller regions then available to them. Polymeric flocculants cause formation of large cohesive aggregates (flocs) and are inert to pH changes. Both natural and synthetic polymers are used as flocculants. Natural polymers are biodegradable, are effective at large dosages and are shear stable. Synthetic polymers are highly effective flocculants at small dosages and have high tailorability but poor shear stability. Concern for environmental and ecological issues warrants the use of biodegradable flocculants. Natural polysaccharides such as starch, guar gum, alginate, glycogen or dextran can be used. Chemical modification of these polysaccharides to improve their flocculation efficacy is possible, including grafting polyacrylamide branches on polysaccharide backbones and insertion of cationic moieties on the backbones. We discuss flocculants based on polysaccharides more in detail since they provide both shear stability and biodegradability. Since flocculants exist as polymers in solution, their standard characterization is in terms of the radius of gyration; that radius is related to the flocculation efficacy. Drag reduction (DR) is also related to flocculation since the same polymeric agents cause both. DR is briefly described and the common mechanism of both phenomena is explained. Possibilities of using recycled industrial water after flocculation as agricultural water are pointed out. Such usage results in less contamination of the environment and also in saving fresh water. Brostow, Hagg Lobland, Pal and Singh Journal of Materials Education Vol. 31 (3-4) 158