A method of treating an algal containing aqueous medium comprises adding an effective amount of the treatment composition to the aqueous medium wherein the treatment composition comprises 1) a) a water soluble cationic quaternary ammonium starch or b) a water soluble quaternary ammonium starch/gum blend or c) a water soluble modified tannin and 2) a metal containing inorganic coagulant. In certain aspects of the invention, the so-treated algal containing aqueous medium is filtered such as by microfiltration and/or ultrafiltration to result in potable water. In another aspect of the invention, the algal containing aqueous medium is an agglomerated mass of algae with water dispersed throughout the mass. The method comprises a step of separating the algae from the water, thereby harvesting the algae for further processing such as may ultimately lead to the production of biodiesel fuel.
