Water Treatment - The Drinking Water - page 253

trihalomethanes, and radon. Some reverse osmosis equipment is also capa-
ble of treating for Cryptosporidium . Reverse osmosis can be expected to play a
major role in water treatment for years to come.
Reverse osmosis (also called ultrafiltration) is the most common process
for reducing the salinity of brackish groundwater. In operation, a semiper-
meable membrane (the most essential element in the reverse osmosis method
of demineralization) separates salty water of two different concentrations.
Concentrations have a natural tendency to become equalized by a flow of
water from the dilute side to the concentrated side (osmosis). But high pres-
sure applied to the high concentration side of the membrane can reverse this
direction of flow. Freshwater diffuses through the membrane, leaving a more
concentrated salt solution behind. The performance of reverse osmosis units
is highly dependent on a number of water quality parameters. Suspended
solids, dissolved organics, hydrogen sulfide, iron, and strong oxidizing
agents (chlorine, ozone, and permanganate) are harmful to membranes.
Electrodialysis is the demineralization of water using the principles of
osmosis—but it uses ion-selective membranes and an electric field to sep-
arate anions and cations in solution. In the past, electrodialysis was most
often used for purifying brackish water, but it is now finding a role in indus-
trial waste treatment as well. For example, metals salts from plating rinses
are sometimes removed in this way.
Summary
These processes, from preliminary screening to filtration and disinfection,
as well as the advanced processes needed for specialized water problems,
are in place to serve one primary, essential purpose: to supply the consumer
with safe potable water. The consumer may put this water to a wide variety
of uses, from drinking to watering the lawn, but those uses are in many ways
beside the point, as safe, potable water is essential for human life.
References and Recommended Reading
AWWA. (1995). Water Treatment: Principles and Practices of Water Supply Operations .
Denver, CO: American Water Works Association.
AWWA and ASCE. (1990). Water Treatment Plant Design . New York: McGraw-Hill.
De Zuane, J. (1997). Handbook of Drinking Water Quality , 2nd ed. New York: John Wiley
& Sons.
Fox, J.C. (1999). Stakeholder collaboration key to new safe drinking water rules. Water
Environment and Technology , February.