Understanding the roles and characterizing the intrinsic properties of synthetic vs. natural polymers to improve clarification through interparticle Bridging: A review

Abstract

Flocculation and settling performance (kinetic, robustness, upflow velocity, and overall solids removal) have been significantly improved over the past forty years due to the development of very high molecular weight polymers. Intrinsic flocculant characteristics such as the molecular weight, charge density, and spatial molecular configuration are of importance during water flocculation by interparticle bridging. In this review, synthetic vs. natural flocculants are compared and the polymers’ respective advantages/disadvantages are listed. The different functional groups involved during initial polymer chain adsorption are also enumerated for each discussed polymer. A flocculant should principally be selected on the basis of its aggregation performance, solubility and ease of implementation, cost, stability, (bio)degradability, impact on downstream processes, commercial availability and aquatic/human toxicity. It is generally the rule that organic synthetic polymers are associated with better solids removal than natural ones during clarification. On the other hand, natural polymers require higher dosage but offer other advantages such as higher biodegradability and lower toxicity. Given the different benefits offered by each type of flocculant, promising and alternative natural-graft-synthetic polymers are presented as a compromise in terms of flocculation effectiveness vs. biodegradation and toxicity. Some starch-graft-polyacrylamide and carboxymethyl cellulose-graft-polyacrylamide exhibit similar flocculation performance to conventional acrylamide-acrylic acid copolymers but are more biodegradable and less toxic due to the nature of their backbone. Finally, floc monitoring techniques as well as polymer molecular weight and charge density analysis methods are reviewed and discussed.