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8.7 Dual Water Supply Systems
revealed that sudden changes in the velocity of the water are
created by the opening and closing of valves in the lines and
The professionals engaged in providing drinking water to
the starting and stopping of pumps, events that occur sev-
the public make great efforts to ensure water of high quality,
eral times a day. This causes negative pressure transients that
which requires considerable investments in treating the water.
result in the infiltration of groundwater from the soil in the
However, in the last step, distribution of the water to the
vicinity of the pipes.
consumer, the water is permitted to be seriously degraded.
Such transients would, of course, also occur in small
We should be embarrassed by the tremendous efforts
pipes, but stainless steel pipes used for distribution systems
and funds we invest in providing high-quality water only to
that only carry drinking water would not leak because these
allow it to be subjected to the many problems enumerated
pipes do not have open joints. In the very small sizes, they
above, which result in increasing health risks to the public.
can be laid from spools and in the larger sizes the pipe can
In our efforts to address each of the problems, we oblige the
be welded. Stainless steel is already widely used in Japan 8.7.5 The Proposed System 259
water utilities to undertake studies and remedies far beyond
for water distribution systems. These pipes have an added
the capacities of all but the very large utilities.
advantage over cement-lined pipes because they are not prone
Just because we have inherited distribution systems cre-
to heavy growths of biofilms due to their smooth interior
ated for fighting fires over two centuries, a practice that is
walls.
responsible for all of the problems, that is no reason for
The fact that leakage from distribution systems increases
continuing the practice into the future, especially when the
with time is well recognized, but these losses are generally
only solution offered is frequent flushing of all the pipelines.
not considered important. However, recent studies show that
Flushing is costly in execution, relatively ineffective, and
leaks have much more serious consequences. The negative
wasteful of the treated drinking water.
pressures that have been found to occur regularly in pipelines
A reasonable solution is available: distribution systems
encourage infiltration of the water in the soil surrounding the
designed for drinking water alone. This option has many
pipes. This is possible because heavy pipes laid on soil tend
advantages. The pipe diameters for most of the distribution
to subside over time, which opens the joints sufficiently to
lengths would be much smaller than the current minimum
create two-way leakage.
sizes. Materials such as stainless steel can largely eliminate
the bane of our present systems: leaking joints.
8.7.4 Biofilms and the Problems They Cause One very great advantage is that the size of community
water treatment works would be a small fraction of what they
The poor water quality found in distribution systems today
now need to be, encouraging the use of membrane treatment,
results above all from the considerable growth of biofilms that
which improves the drinking water quality beyond what is
are attracted to the insides of the pipes because of the pipe
now available in most communities. The result would be
materials chosen and the long residence times of the water.
purer water at a lower cost. Figure 8.13 illustrates what a
The many joints, hydrants, valves, and other appurtenances,
new community might do to initiate such an approach to
along with the cement and other conventional linings, present
conserving its drinking water, affording high-quality treat-
attractive surfaces for the growth of biota, which deplete the
ment, and avoiding degradation in the distribution system.
disinfectants.
What makes this approach reasonable today is that dual
The heavy growths in turn shield the disinfectant from
systems on a large scale began to cover the United States
the biota. The biofilms grow thick because of the very slow
some 40 years ago. Some 2,000 communities in the United
velocities of the water and the long residence times in the
States and many abroad, both the largest and the smallest
pipes. They build up to the point that, with products of corro-
sionandtuberculation, theysignificantlyreducethehydraulic
capacity of the pipes. If water consumers were able to see the Reclaimed water tank
Residential
insides of most distribution systems, they would be encour-
aged to buy bottled water, now a fast-growing trend. Commercial Potable
Studies in Britain have compared conventional cement- and industrial water
tank
lined pipe with stainless steel pipes of various compositions
to assess their comparative rates of growth of biofilms. All
of the stainless steel pipes were found to be far better than Lakes, rivers,
and wells
the cement-lined pipe in terms of the rates of biofilm growth.
The stainless steel pipes that are available in very small sizes
are provided from spools that carry long lengths of product,
Water treatment plant
sufficient to serve the outlying residential areas of larger
cities without the problems that are now encountered. Larger
Water reclamation plant
stainless steel pipes are available in considerable lengths that
can be welded, thus avoiding joints. Figure 8.13 Dual water supply systems.