Environmental Engineering Reference
In-Depth Information
are exceptions — for example, in Washington, D.C. In some Western Europe
countries (e.g., Sweden), such dual use of the trench is permitted under specified
conditions because of the resulting considerable savings in total trending costs.
Situation in DCs
The DCs have followed the WHO guidelines, which do not
permit dual use of the same trench following general U.S. practice. However,
a study sponsored by USAID completed in 1984 at the request of the Jordan
government evaluated dual use considering both economic and public health
parameters
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. The conclusions of this study (endorsed by Dr. Abel Wolman, then
a chief consultant to WHO), are summarized as follows:
A review of UWSS experience in the United States and elsewhere, which
included consultation with the U.S. and international agencies carrying out
evaluations and research on water supply contamination hazards, has clearly
indicated that dual use of the trenches is regarded as an acceptable practice
in many localities, without the use of special encasement of the sewer pipe
and/or the use of pressure pipe for the sewer, provided (1) the sewer pipe
is of first-class quality (can withstand low head pressures with rubber ring
joints or the equivalent); (2) the vertical separation between the two pipes
is at least 18 inches; and (3) the water main is located on a side-bench so
that the sewer main may be repaired without disturbing the water main. This
practice is commonly used in Western Europe, and in the United States, it is
generally allowed whenever the physical or right-of-way constraints make
it necessary to utilize a single trench for both mains. It is commonly used
in Washington, D.C.
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In Amman, dual use of the trenches has resulted in a reduction in the cost
of the water supply system of about 15 percent. Where excreta disposal
depends on the use of subsurface leaching areas, as is the case in the urban
areas of many developing countries, dual use on wide as well as narrow
streets, would be a relatively cheap way of providing health protection. The
alternative (i.e., the use of separate trenches) or of dual use with rigid sewer
requirements (i.e., the use of pressure pipe and/or encasement), will often
be too expensive for developing countries to afford. However, such dual
use assumes that the quality of construction for both water and sewer pipes
will meet minimum international standards, and the construction records of
municipal agencies in many developing countries may show that they do
not meet this requirement.
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This is one of the main reasons for recommending that the DC/USSS should
establish a Division of Water Quality (DWQ), one of whose functions would
be to monitor and exercise surveillance over the planning, design, and con-
struction of future water supply and sewer mains, including dual use, to
ensure that construction practice do in fact meet the minimum international
standards. The DWQ would, of course, work closely with the Ministry of
Health to keep the Ministry of Health appraised of its activities and progress.
Assuming that this is done, it would be good practice to allow use in Jordan
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