The aeration technique of water treatment is used for the removal of

Aeration bubbles air through water so dissolved volatile organic compounds escape into the gas phase and are stripped out.

Persistent organic pollutants are not volatile and resist removal by simple aeration.

Hardness comes from dissolved calcium and magnesium salts that aeration cannot remove.

Dissolved metals generally need chemical precipitation or filtration rather than aeration.

Volatile organic compounds, along with gases like carbon dioxide and hydrogen sulphide, are what aeration targets.

Aeration also raises dissolved oxygen and helps oxidise iron and manganese for later filtering.