Aeration provides the oxygen ...

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Aeration provides the oxygen necessary to change Fe++ into Fe+++ which is colloidal. The filtration (sand filters) after that removes the colloidal Fe+++. The best efficiency is achieved by spraying the water into a drum full of air (contact surface is enhanced). Some working pressure above atmospheric improves the solubility of air and the efficiency. The consumption of air is rather limited and therefore the compressed air make-up is not costly, but some air blow-down is recommended because the solubility of O2 is higher than N2 and the gas composition may become enriched by N2 after the preferential dissolution of O2 into the liquid phase