12.5.1 Metal Salts

Metal salts such as alum (aluminum sulfate) or ferric chloride are among the most widely used flocculants in wastewater treatment. When dissolved in water, alumi­num or ferric iron forms metal hydroxides that are positively charged and can induce flocculation by charge neutralization. At higher doses, the hydroxides form precipitates that cause flocculation through a sweeping mechanism (Wyatt et al.

2012) . Metal salts work well for harvesting microalgae, but the doses required for effective flocculation are very high (usually >100 mg L :). Even higher doses are required in seawater compared to freshwater (Sukenik et al. 1988). Another dis­advantage is that the use of metal salts results in contamination of the harvested biomass with metals.