Environmental Engineering Reference
In-Depth Information
is repeated; and so it goes, again and again. Up to half the water in American rivers is
recycled this way.
At its northern reach, the Mississippi supplies drinking water to St. Paul, Minnesota,
then carries the city's effluent downstream. By the time it has traveled over a thousand
miles south to New Orleans, the river has flowed through the “kidneys” of over a dozen
cities before emptying into the Gulf of Mexico. Likewise, the Colorado River is used
both for drinking water and to flush away treated sewage by more than two hundred
communities, including Las Vegas, Phoenix, and Los Angeles.
The effluent in a river such as the Mississippi or Colorado doesn't go directly into
people's taps, of course. Once it leaves a sewage plant, the treated sewage is swept along
by river currents and is heavily diluted with clean, naturally supplied water. Sunlight,
and time spent mixing and settling, degrade pollutants further. At the next town, treat-
ment plants filter and disinfect the water to a high quality and often mix it with other
supplies before piping it into people's homes. To ensure quality, the EPA requires utilit-
ies to monitor pathogens and report any abnormalities, and most do so with admirable
efficiency.
Though properly recycled water has proven safe for people to use, it doesn't make
sense to use water that has been laboriously and expensively cleaned to drinking water
standards to fill toilets or irrigate gardens and golf courses.
In the 1960s, concerned about dwindling water supplies, hydrologists in arid states
suggested using recycled water for those purposes. They separated treated water into
two classes: water clean enough to drink, and “gray” water clean enough that it could be
used for such things as irrigation. (Technically, gray water is household wastewater that
does not contain sewage, while blackwateris wastewater that contains sewage. Both can
be treated and reused.)
A gray water movement sprang up in dry and increasingly populated Southern Cali-
fornia. Since 1994, the state has been a leader in developing large-scale treatment sys-
example, rely on a townwide gray water system for nondrinking use. Frustrated by Cali-
fornia's byzantine plumbing codes, “graywater guerrillas” have created illegal but effi-
cient systems made of PVC pipes, buckets, gravel, and cattails that send sink and dish-
washer runoff into toilets and lawns.
“In a drought-prone region like ours, it doesn't make sense to use potable water to
irrigate,” explained
Michael Markus
, general manager of the Orange County Water Dis-
trict (OCWD). Recycled water helps to conserve drinking water and reduce energy use.
Orange County lies south of Los Angeles and gets only fourteen inches of rain a
year, on average. Though it once had plentiful groundwater, supplies began to drop in
the 1950s. The depletion of aquifers—originally by irrigators, and recently by urbaniz-
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