Page 280 - Water Engineering Hydraulics, Distribution and Treatment

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Pumping, Storage, and Dual Water Systems
Chapter 8
can spend in the network. What is needed, argues Okun
(2007), is a switch to dual systems, with potable supplies
Such times make adequate chlorine residuals at the tap
unlikely.
provided through a smaller bore network using a material
such as stainless steel, allowing reclaimed water to be used
The inadequacy of disinfection, with the resulting risk
in existing networks.
of microbial exposure at the tap, is not the most troublesome
problem arising from ineffective disinfection. In attempting
to provide adequate disinfection despite the poor conditions
8.7.1 Background
in the pipelines, providers considerably increase the chlorine
dose, resulting in increased levels of disinfection by-products
In the eighteenth and early nineteenth centuries, the demand
(DBPs) through reactions with both chemical and microbial
for protection against fire and the great conflagrations and
loss of life that they brought predicated the provision of
contaminants in the water.
water distribution systems designed for fire protection. Only the state revealed residence times of more than 10 days.
Trihalomethanes (THMs) and haloacetic acids (HAAS)
later were these distribution systems put into service for are the only two DBPs that are being regulated by the US
commercial and then residential use, which led to the Environmental Protection Agency (U.S. EPA), but then with
subsequent development of the water closet and sewerage great difficulty. Their maximum contaminant levels (MCLs)
systems. were epidemiologically uncertain, as indicated by the arbi-
That our present distribution systems are delivering trary adoption of a THM level of a “round” 0.10 mg/L in
water of exceedingly poor quality today, almost without 1979. This figure was reduced recently, based in large part
regard to the water’s source, treatment, or distribution, has on the ability of utilities to reach a lower level.
been made manifest by the vast literature emerging from The DBP problem is much more difficult to manage than
every corner of the water supply scene. The American Water is evident from recent research. As shown later, many more
Works Association (AWWA) has shown considerable aware- contaminants are present in drinking water networks than
ness of the problems. Its annual water quality technology are recognized today as potential reactants with the chlorine
conferences have each had more than 100 papers identify- present in the water and therefore there are many more other
ing the problems. Some 40 classes of problems are set out DBPs that need to be regulated.
by AWWA, with recommendations for individual utilities Even more concern for the health effects of THMs has
to assess their own particular problems and find their own arisen because of a study carried out on 50 women in two very
answers. But relatively few utilities have the appropriate staff different locations (Cobb County, GA, and Corpus Christi,
or financial resources to undertake the required studies and TX), which have water supplies with very different THM
address each of the many problems. bromide concentrations and disinfectant types: chloroform
The recommended practice of frequent flushing of the in the former and brominated THMs in the latter.
distribution systems has been widely adopted, but it hardly Blood samples were taken from women and water sam-
addresses the problems. Flushing is costly in personnel and ples were taken from their showers in the early morning. It
extremely wasteful of treated drinking water, which is dis- was shown that the THMs in their blood samples rose sig-
charged to stormwater sewerage systems. In addition, fre- nificantly after showering. The types of THMs in their blood
quent flushing is not very effective in keeping the pipes free samples matched the type of THM in the water. THM stan-
from biofilm growths on pipe walls and maintaining hydraulic dards are based on lifetime exposures, but recent studies have
capacity. suggested that THMs pose possible reproductive problems
for women that would dictate more rigorous MCLs for DBPs
in the future.
8.7.2 The Nature of the Problems with Drinking
Water Quality
8.7.3 The Pipes in the Distribution Systems
The critical problem is that fire protection requires there to
be many hydrants throughout a city, which have to have Because the pipes in all urban water distribution systems
the capacity to deliver relatively high flow rates at all times currently need to be a minimum of 6 in. (150 mm) in diameter
and at all locations throughout the area. Pipe sizes were or larger, they are generally heavy cement-lined ductile iron
initially a minimum of 6 in. (150 mm), but today this has pipes, each section 16 ft (5.3 m) in length. These require
increased in many communities to a minimum of 8 in. some 350 joints per mile of pipe, including those needed for
(200 mm). fire hydrants. The pipes are laid on soil in trenches, and in
Because fires are infrequent, the velocity of the water time the joints leak and lose water. If the pipes are below
in the network is almost always slow, resulting in resi- the water table, any infiltration of contaminated groundwater
dence times of months between when the water is treated would pose a health risk.
and when it arrives at the taps of consumers in the outer Because the pipes are always under pressure, it had
regions of the service area. Recent tracer studies by the been believed that contamination from surrounding ground-
University of North Carolina in two of the larger cities in water would not be a problem. Recent studies, however, have