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Chemical phosphorus removal precipitates metal phosphate. Stoichiometry is one mole of metal per mole of phosphorus, but a molar ratio of 1.5-2.5 is needed in practice because the metal also hydrolyses and competes with other reactions. Each kilogram of phosphorus removed produces roughly 4 kilograms of extra sludge.
Metal dose
Metal (mg/L) = P to remove / 30.97 x molar ratio x metal atomic mass
Product dose
Product (mg/L) = metal dose / metal fraction in the product
Metal salt coagulants are corrosive and acidic, consuming alkalinity and depressing pH. Handling, containment and alkalinity control must be designed accordingly.
Metal ions also form hydroxide precipitates and react with alkalinity, so only part of the dose is available for phosphate. The excess ratio rises steeply as the target residual phosphorus falls below 0.5 mg/L.
Simultaneous precipitation into the aeration tank is most common; pre-precipitation into primary settlement removes more but wastes carbon; tertiary dosing achieves the lowest residuals but needs filtration.