Environmental Engineering Reference
In-Depth Information
Demineralization
Demineralization
refers to the removal of dissolved solids (inorganic mineral
substances) from water. Dissolved solids contain both cations and anions
and therefore require two types of ion exchange resins. Cation exchange res-
ins used for demineralization purposes have hydrogen exchange sites and
are divided into strong acid and weak acid classes. The anion exchange resin
commonly used contain hydroxide ions and are divided into strong and
weak base classes.
Demineralization is commonly used in industry in waste treatment for
removal of arsenic, barium, cadmium, chromium, fluoride, sulfate, and zinc.
Some general advantages of using ion exchange to remove these contami-
nants are the low capital investment required and the mechanical simplicity
of the process. In addition, the ion-exchange process can be used to recover
valuable chemicals for reuse, or harmful ones for disposal. For example, it is
often used to recover chromic acid from metal finishing waste for reuse in
chrome-plating baths. It also has some application in the removal of radioac-
tivity. The major disadvantages are the high chemical requirements needed
to regenerate the resins and to dispose of chemical wastes from the regen-
eration process. These factors make ion exchange more suitable for small
systems than for large ones.
Membrane Processes
Membrane processes
used in water treatment are primarily demineralization
processes. Demineralization of water can be accomplished using thin, micro-
porous membranes. Electrodialysis and reverse osmosis are the most com-
mon membrane processes. Before we briefly discuss these two membrane
processes, you need a basic understanding of osmosis. During osmosis, two
solutions containing different concentrations of minerals are separated by a
semipermeable membrane. Water tends to migrate through the membrane
from the side of the more dilute solution to the side of the more concentrated
solution. This is
osmosis
, and it continues until the build-up of hydrostatic
pressure on the more concentrated solution is sufficient to stop the net flow.
In
reverse osmosis
, the flow of water through the semipermeable membrane
is reversed by applying external pressure to offset the hydrostatic pressure.
This results in a concentration of minerals on one side of the membrane and
pure water on the other side. Reverse osmosis can treat for a wide variety
of health and aesthetic contaminants in water. Effectively designed, reverse
osmosis equipment can treat aesthetic contaminants that cause unpleas-
ant taste, color, and odor problems, such as a salty or soda taste caused by
chlorides or sulfates. Reverse osmosis can also be effective for treating arse-
nic, asbestos, atrazine, fluoride, lead, mercury, nitrate, and radium. When
used with appropriate carbon prefiltering, additional treatment can also be
provided for such “volatile” contaminants as benzene, trichloroethylene,
