Define Alum (aluminum sulfate) Caogulation

• Purchased as dry or liquid alum { Al₂(SO₄)₃,14 H₂O}, MW 594 (other formulas with varying number of water molecules also used)
• 48.8% alum and rest water
• When alum is added to water containing alkalinity (pH 5.5 to 8.)

• Each mole of alum uses 6 moles of alkalinity (HCO^-₃) and produces 2 moles of aluminum hydroxide (solid precipitate) and 6 moles of CO₂ which decreases the ph
• However due to the presence of alkalinity pH won't lower drastically If there is not enough alkalinity, lime (either as CaO or Ca(OH)₂ ) or soda ash (Na₂CO₃) can be added to prevent pH from lowering due to the coagulation reaction.