technologies would be inappropriate because
metal ions are not biodegradable; however,
both chemical precipitation, a chemical treatment
technology, and ion exchange, a physical
treatment technology, should work well,
based on the fundamental properties of the
substances to be removed (dissolved inorganic
cations and anions). The question then
reduces to a comparison between the advantages
and disadvantages of these two technologies,
and experience provides much of the
information appropriate to this evaluation.
For example, experience has shown that, for
most metal-plating wastewaters:
1. Chemical precipitation is far less costly
than ion exchange; however,
2. Chemical precipitation is not reliably
capable of reducing metal concentrations
to less than 5 or so mg/L.
3. A principal reason for the previous point is
that the process of removing precipitated
metals by settling in a clarifier typically
does not remove the very small particles of
precipitate.
4. Sand (or other) filtration effectively
removes most of those particles of metal
precipitate that will not settle.
5. The concentrations of dissolved metals
even after chemical precipitation and sand
filtration are still no lower than 1 to 2 mg/
L, at best. Furthermore,
6. Ion exchange can “polish” the effluent
from chemical precipitation and sand filtration
to very low concentrations (20 to
50 ppb).
7. Ion exchange could do the entire job of
removing metals from industrial wastewater
to very low concentrations without
being preceded by chemical precipitation
and sand filtration, but, usually, at a cost