3.1. Flocculation mechanism and efficiency
Chitosan was found to be the most effective flocculant for N. oleoabundans ( Fig. 2). The optimal concentration of chitosan was found to be 100 mg/L. At this dose, dense flocs formed during the 1-min fast mixing at 350 rpm and then quickly precipitated during the settling phase of the test. The supernatant was almost clear; the removal rate reached 95%. Higher doses of chitosan were not beneficial; at a dose of 150 mg/L the turbidity of the algae samples was increased. Multivalent metal salts were of low effectiveness. In the case of ferric sulfate, decrease in turbidity was relatively low at all doses tested. Alum, which has proven to be an efficient flocculant for other species of algae ( Clasen et al., 2000), instead increased the turbidity of N. oleoabundans samples. As alum was found to be essentially ineffective, it was omitted from the remainder of the analysis. Attempts to flocculate the algae solely through pH adjustment did not affect the turbidity removal rate, showing that charge neutralization alone is not sufficient for colloidal destabilization of this species of algae.