Organic polymeric flocculants have been used in water purification for several decades as coagulant aids or floc builders, after the addition of inorganic coagulants like alum, iron salts or lime. The increased use of cationic polyelectrolytes as primary coagulants instead of inorganic salts, which has occurred in recent times, arises from their significant inherent advantages. The main ones are faster processing, a lower content of insoluble solids to handle, whether by sedimentation, filtration, flotation or in biological conversion, and a much smaller sludge volume. Polymers have often been used in chemically assisted sedimentation of sewage solids to enhance the removal of suspended matter. The concept is applicable as well to the primary coagulation of industrial wastewaters where the separation may be based on flotation, as in examples from the leather, steel, wool scouring, cosmetic, detergent, plastics, dyehouse, paper, food processing and brewing industries. A cationic polymer of particular charge density is optimal, and hydrophobically modified polymers have relevance in the case of oil and grease removal. The burden of solids which must be floated is much reduced relative to systems utilising inorganic coagulants, and the dosage of chemicals overall is lower. In some cases the addition of some inorganic coagulant is unavoidable, as in the case of highly coloured effluents; in others, an anionic surfactant is needed to facilitate flotation.

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