The flocculation of algae with synthetic polymeric flocculants

The flocculation of a pure algal culture of Chlorella ellipsoidia, was evaluated by means of measurements of filtration rate, light transmission and electrophoretic mobility, at algal concentrations of 50–3000mg l −1, polymer concentrations of 0.01–1000 mg 1 −1 at pH 4–7. Non-ionic and anionic polyacrylamides, and anionic polystyrene sulfonate with molecular weights of about 1,000,000 failed to show any flocculation at polymer concentrations from 0.01 to 200 mg 1 −1. Cationic polyethyleneimine was effective as a flocculant as shown by the results from the three methods of measurement. There was a significant increase in flocculation efficiency as the polymer molecular weight increased from 800 to about 2000 but little improvement above 2000. An effect of pH from 4 to 7 was absent. A variation in the magnesium ion concentration from 2.45 to 0.005 g 1 −1 in the nutrient did not produce any effect. Earlier results with crystalline silica and E. coli and these same polymers had shown a similar effect of molecular weight and an influence of pH. A brief discussion is presented on a comparison of results on algae with those of other colloids, it being especially significant that at the same concentration of biocolloid the algae need about 100 times higher concentration of polymer than do E. coli.