Environmental Engineering Reference
In-Depth Information
and other DCs (1) where separate trenches are not feasible (due to physical
constraints or right-of-way constraints), and (2) in wider streets where the
savings would result in enhancing the overall health protection obtainable
with limited funds.
Reuse of Treated Wastewater
Situation in ICs
Ever since human activities reached levels that caused signif-
icant pollution of streams, urban water supply system (UWSSs) have, in effect,
been producing the water supply delivered to the distribution system by treating
polluted stream waters — representing a kind of reuse. In the 1940s, USPHS stud-
ies concluded the stream waters, to be suitable as raw water to be treated, must not
be polluted beyond specified limits, but since then these limits have steadily risen
as the removals achieved by rapid sand filtration progressively improved with the
development of superior additives for improving the coagulation/flocculation/
filtration processes included in rapid sand filtration plants — so much so that
even badly polluted raw waters can be utilized. Despite this, U.S. public health
officials have persisted in not being willing to authorize direct reuse of sewage
for urban drinking water supply purposes (the treated sewage could be utilized
for irrigation of selected types of crops). For urban water supply purposes, first
the sewage must be released into a stream, then the raw water removed at a
downstream location. As a result, the only systems acceptable to U.S. health
authorities have been those where the wastewater, after treatment (conventional
complete treatment followed by reverse osmosis if salinity must be reduced) is
infiltrated into the ground to become part of the natural groundwater supply,
then can be pumped out for acceptable use as raw water supply. This system
was pioneered in Southern California in the 1950s to 1960s by the Los Angeles
County Sanitation Districts and the Eastern Riverside Municipal Water District
34
and since has been utilized in many other communities in Southern California
including the Orange County Water District
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. However, the City of San Diego's
Water District has since implemented partial direct reuse, approved by the health
authorities, by which treated wastewater is mixed with natural fresh water in an
impounding reservoir used for raw water supply.
Situation in DCs
The current situation in the DCs resembles that in the United
States prior to the interest in projects for direct reuse beginning in the 1950s.
However, some initial studies are being made in recent years for exploring the
potentials for recharging groundwater in desert type regions, as in Pakistan
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and
in regions with ample water in the rainy season but with negligible rainfall for
the remainder of the year (prolonged dry season) — for example, in the Bangkok
region of Thailand
138
.
IAA-Sponsored Community Water Supply Programs
WHO/UN Program
A primary effort of WHO has been the development
of the UN's “Water and Sanitation Decade” program and follow-up efforts
