There is limited literature on algae sedimentation in ponds without any flocculation process. Isolation of facultative oxidation pond from inflow feed to promote water clarifica­tion was investigated (Koopman et al., 1978). Operations involving fill-and-draw cycles for secondary ponds gave rise to significant removal of algae from facultative oxidation pond effluent (Benemann et al., 1980).

Similar secondary ponds were used for algae settling from high rate oxidation pond effluent (Adan and Lee, 1980; Benemann et al., 1980). Well-clarified effluent and algae slurry of up to 3% solids content were achieved at the secondary ponds attributable to algae autoflocculation, which enhanced the settling. The autoflocculation phenomenon is distinctly different from the coprecipitative autoflocculation suggested by Sukenik and Shelef (1984), as discussed earlier. The autoflocculation mechanism involved remained unclear (Eisenberg et al., 1981).

Coagulant dosing to a settling tube to promote algae sedimentation was looked into by Mohn (1980). The batched operation achieved an algal concentration of 1.5% solids content. Algae separation by sedimentation tanks or tubes is considered a simple and inexpensive process. Its concentrating reliability is low without coagulant dosing. Algae autoflocculation may be used as an inexpensive reliable algae separation method. However, the natural flocculation processes should be closely studied and well understood before it can be incor­porated for primary concentration.