Page 281 - Water Engineering Hydraulics, Distribution and Treatment
P. 281

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.
   276   277   278   279   280   281   282   283   284   285   286