Bioflocculation occurs in high-rate ponds. This process involves removing algae from the paddlewheel-mixed ponds and placing them in a quiescent container, where they would spontaneously flocculate and rapidly settle. There are several apparently distinct mechanisms by which algae flocculate and then settle, including “autofloc­culation,” which is induced by high pH in the presence of phosphate and divalent cations (Mg+2 and Ca+2), and flocculation induced by N limitation. Bioflocculation refers to the tendency of normally repulsive microalgae to aggregate in large flocs, which then exhibit a rather high sedimentation velocity. The mechanisms of biofloc­culation involve extracellular polymers excreted by algae, but the details remain to be investigated (Sheehan et al. 1998).

Organic flocculants at about 2 to 6 g/kg and FeCl3 at about 15 to 200 g/kg of algal biomass are required to remove 90% or more of the algal cells. Because of the high cost of the organic flocculants, costs are comparable for both flocculants tested. The polymers can be used in very small amounts without contributing a major cost to the overall process (Sheehan et al. 1998).

Macroalgae have long been used for the production of phycocolloids such as al­ginates, carrageenans, or agars. They make up the major industrial products derived from algae (Radmer 1996; Pulz and Gross 2004). These polymers are located ei­ther in cell walls or within the cells serving as storage materials. A characteristic of marine algae is the abundance of sulfated polysaccharides in their cell walls.