Chitosan for Direct Bioflocculation Processes

Coagulation-flocculation is a major process allowing to remove suspended particles from municipal and industrial wastewater. This process commonly involves metal salts as coagulants and synthetic organic polymers as flocculants. Although those chemicals are cheap, efficient, available and easy to use, they have drawbacks such water pollution by metals, and production of large amounts of toxic sludges. Therefore, safer biocoagulants and bioflocculants of biological origin are currently developed. For instance, the direct flocculation process involves water-soluble, ionic organic polymers, and thus do not need the addition of metal coagulants. In particular, chitosan and byproducts have been recently designed as bioflocculants to remove particulate matter and dissolved pollutants. Chitosan is a partially deacetylated polysaccharide obtained from chitin, a biopolymer extracted from shellfish sources. Chitosan exhibits various physicochemical and functional properties of interest for many environmental applications. Key achievements of chitosan applications include the removal of more than 90% of solids and 95% of residual oil from palm oil mill effluents. Chitosan highly reduces the turbidity of agricultural wastewater and seawater. Comparison of raw chitosan with modified chitosan, such as 3-chloro-2-hydroxypropyl trimethylammonium chloride grafted onto carboxymethyl-chitosan, to treat a solution of high turbidity (400 mg/L kaolinite) and phosphate (25 mg/L), shows that the modified chitosan decreases the turbidity by 99% and the phosphate content by 97% at all pH, whereas those abatements are below 80% for the raw chitosan. Chitosan also removes toxic Microcystis aeruginosa cyanobacterial cells by 99% and microcystins by 50%. This chapter discusses advantages and drawbacks of using chitosan for direct flocculation for water and wastewater treatment, sludge dewatering, and post-treatment of sanitary landfill leachates.