The aeration basin influent ...

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The aeration basin influent is a better location than headworks to inject iron salts.   Peter Naylor is correct about the competing reaction with sulfide formation.  You may be able to implement a cost saving option at this time.  Ferrous chloride is generally much cheaper than ferric chloride.  The oxygen of the aeration basin will oxidize the ferrous (2+) ion to ferric (+3) ion which will then react with phosphate (-3) ion.    Ferrous chloride can be stored and pumped with the same equipment as ferric chloride.  Safety considerations are the same.