Autoflocculation of algae has been observed during some studies (Golueke and Oswald, 1965; Hill et al., 1977; McGriff and McKinney, 1971; McKinney, 1971). Chlorella was the predominant alga occurring in most of the cultures. Laboratory-scale continuous experiments with mixtures of activated sludge and algae have produced large bacteria-algae flocs with good settling characteristics (Hill et al., 1977; Hill and Shindala, 1977). Floating algae blankets have been reported in some cases in the presence of chemical coagulants (Shindala and Stewart, 1971; van Vuuren and van Duuren, 1965). The phenomenon may be caused by the entrapment of gas bubbles produced during metabolism or by the fact that in a particular physiological state the algae have a neutral buoyancy. In a 3000-gph (11,355 L/hr) pilot plant (combined flocculation and sedimentation), a floating algal blanket occurred with alum doses of 125 to 170 mg/L. About 50% of the algae removed were skimmed from the surface (van Vuuren and van Duuren, 1965). Because of the infrequent occurrence of conditions necessary for autof-locculation, it is not a viable alternative for removing algae from wastewater stabilization ponds. Phase isolation experiments to remove algae from lagoon effluents are based on this concept, and some success has been reported; however, full-scale operation of a phase isolation system did not produce consistent results (McGriff, 1981).
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