Page 329 - Fair, Geyer, and Okun's Water and wastewater engineering : water supply and wastewater removal

JWCL344_ch08_265-296.qxd 8/2/10 9:53 PM Page 289

8.7 Dual Water Supply Systems 289
Even more concern for the health effects of THMs has arisen because of a study car-
ried out on 50 women in two very different locations (Cobb County, Georgia, and
Corpus Christi, Texas), which have water supplies with very different THM bromide
concentrations and disinfectant types: chloroform in the former and brominated THMs in
the latter.
Blood samples were taken from women and water samples were taken from their
showers in the early morning. It was shown that the THMs in their blood samples rose sig-
nificantly after showering. The types of THMs in their blood samples matched the type of
THM in the water. THM standards are based on lifetime exposures, but recent studies have
suggested that THMs pose possible reproductive problems for women that would dictate
more rigorous MCLs for DBPs in the future.

8.7.3 The Pipes in the Distribution Systems

Because the pipes in all urban water distribution systems currently need to be a minimum
of 6 in. (150 mm) in diameter or larger, they are generally heavy cement-lined ductile iron
pipes, each section 16 ft (5.3 m) in length. These require some 350 joints per mile of pipe,
including those needed for fire hydrants. The pipes are laid on soil in trenches, and in time
the joints leak and lose water. If the pipes are below the water table, any infiltration of con-
taminated groundwater would pose a health risk.
Because the pipes are always under pressure, it had been believed that contamination
from surrounding groundwater would not be a problem. Recent studies, however, have re-
vealed that sudden changes in the velocity of the water are created by the opening and clos-
ing of valves in the lines and the starting and stopping of pumps, events that occur several
times a day. This causes negative pressure transients that result in the infiltration of
groundwater from the soil in the vicinity of the pipes.
Such transients would, of course, also occur in small pipes, but stainless steel pipes
used for distribution systems that only carry drinking water would not leak because
these pipes do not have open joints. In the very small sizes, they can be laid from spools
and in the larger sizes the pipe can be welded. Stainless steel is already widely used in
Japan for water distribution systems. These pipes have an added advantage over ce-
ment-lined pipes because they are not prone to heavy growths of biofilms due to their
smooth interior walls.
The fact that leakage from distribution systems increases with time is well recognized,
but these losses are generally not considered important. However, recent studies show that
leaks have much more serious consequences. The negative pressures that have been found
to occur regularly in pipelines encourage infiltration of the water in the soil surrounding
the pipes. This is possible because heavy pipes laid on soil tend to subside over time,
which opens the joints sufficiently to create two-way leakage.

8.7.4 Biofilms and the Problems They Cause

The poor water quality found in distribution systems today results above all from the
considerable growth of biofilms that are attracted to the insides of the pipes because of
the pipe materials chosen and the long residence times of the water. The many joints,
hydrants, valves, and other appurtenances, along with the cement and other conven-
tional linings, present attractive surfaces for the growth of biota, which deplete the
disinfectants.
The heavy growths in turn shield the disinfectant from the biota. The biofilms grow
thick because of the very slow velocities of the water and the long residence times in the