Formation of Floc by Ferric Coagulents

Ferrous sulfate (oxidized by atmospheric air in the presence of alkali), ferric chloride, chlorinated copperas, and ferric sulfate are used as coagulants in water purification. Natural waters contain varying quantities of sulfate ion, chloride ion, sodium ion, calcium ion, etc. An experimental determination of the action of these ions on the formation of floe with ferric salts at different pH-values shows the following results: (1) On the acid side the sulfate ion has a much greater effect on coagulation than the chloride ion; (2) using from 25 to 250 ppm of sulfate ion, there is little change in the effect produced; (3) between a pH-value of 6.5 and a pH-value of 8.5, there is a zone in which ferric floe forms slowly or not at all; and (4) in and beyond this zone, sodium and calcium ions are most effective in coagulation.