1310 WATER REUSE
membrane. Nanofiltration rejects dissolved ionic contami-
nants, but to a lesser degree than in reverse osmosis.
In electrodialysis, direct current is applied to a series of
alternating anionic and cationic membranes. Anions pass
through anion-permeable membranes but are prevented
from migrating through cationic permeable membranes.
Electrodialysis deals only with ionic species, but reverse
osmosis can be shared with electrodialysis.
Vapor-compression evaporation and waste-heat evapora-
tion remove water from contaminants instead of removing
contaminants from water. Application of these processes
is most economical when waste heat is readily available,
such as industrial installations. Water thus produced is of
high quality. However, organic substances that steam-distill
or form azeotropes may appear as contaminants. A distinct
advantage is concentration of residue. Transportation costs
are thus reduced. Use of excess heat for these processes can
reduce the amount of cooling water required.
Oxidation ponds are shallow ponds into which waste water
is led. Also called sewage lagoons, these ponds have long
been in use in Europe but are not popular in North America.
Public resistance has been on aesthetic grounds. A second-
ary effluent can be further purified in these ponds, and the
water produced is suitable for many purposes. Algal action
and surface aeration make this an aerobic process. Depth
is an important consideration. The pond must be shallow
enough to allow penetration of sunlight, but it must be deep
enough to keep from being choked by aquatic weeds. Four
feet is a common depth. Some installations have aerators at
the pond center.Water reuse is an art as well as a science. The processes pre-
sented here are commonly used in preparation of waste water for
immediate reuse. However, the particular situation under study
must dictate the economics of reuse. The ultimate level of reuse
is the dominant factor in deciding if reuse is practical.REFERENCES^Cartwright, P.S., Membranes for process water reuse. Chemical Engineering,
pp. 38–42. June 2004.
Cecil, L.K., Water reuse. The encyclopedia of environmental science and
engineering. 2nd Ed., Vol. 3. Gordon and Breach. New York. 1983.
Cooper, W.J., Ed. Chemistry in water reuse. Ann Arbor Science. Woburn,
MA.^2 1981.
Dean, R.B. and E. Lund, Water reuse: problems and solutions. Academic
Press. New York. 1981.
Delyannis, E., Desalination. The Encyclopedia of Environmental Science
and Engineering. 4th Ed., Vol. 1. Gordon and Breach. New York.
1998.
Guidelines for Water Reuse. U.S. Environmental Protection Agency.
Washington, D.C. 1992.
Hairston, D., Desalting process water. Chemical Engineering, pp. 27–30.
November 2004.
Impelluso, A. and J.R. Pfafflin, Water reuse. The Encyclopedia of Environ-
mental Science and Engineering. 4th Ed., Vol. 2. Gordon and Breach.
New York. 1998.
Pfafflin, J.R., Water reuse. The Encyclopedia of Chemical Technology. Vol. 26.
Wiley-Interscience. New York. 1983.
Reynolds, T.D., Unit Operations and Processes in Environmental Engi-
neering. Wadsworth. Belmont, CA. 1982.
Wolman, A., Sewage Works J. 20 (1), 1. 1948.^PRASANNA RATNAWEER
Open University
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