Selective removal of heavy metals

When, after a waste water treatment plant, concentration of heavy metals are revealed near or above law limits it is of course necessary to improve treatment cycle.
But, sometimes, or because the allowed concentrations in the waste are extremely low, or because in the waste are present some complex builders that react with metals, it is not possible to reach law limits with conventional treatments.
It is then necessary to insert, after the conventional treatment system, a polisher unit with Chelating Ion exchange resins.
This kind of resin reacts only with metals leaving the unpolluting salts freely pass through the ion exchange beds.
When the capacity of the resin has been used a double regeneration with acids and alcalies remove metals and make the columns ready or the following operation cycle.
In order to minimize the leakage from the columns it is normal to connect two or even more columns in series.