|
Items in cart [4] |
Questions? Call 888-624-8373 |
PAPERBACK PDF BOOK Free Resources |
||||||||||||||||
|
||||||||||||||||
|
|
|||||||||||||||
| ||||||||||||||||||||||||||||||
|
||||||||||||||||||||||||||||||
| Copyright © 2009. National Academy of Sciences. All rights reserved. Terms of Use and Privacy Statement |
Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter.
Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 9
11
Elements of
Public Water Supplies
Fundamentally, a water supply system may be described as consisting of
three basic components: the source of supply, the processing or treatment
of the water, and the distribution of water to the users. Water from the
source is conveyed to the treatment plant by conduits or aqueducts, either
by pressure or open-channel flow. Following treatment, the water enters
the distribution system directly or is transported to it via supply conduits.
SOURCE
For a public water supply, the raw water source must provide a quantity
sufficient to meet all municipal, institutional, and industrial uses as well
as the fire-fighting demand. Either surface water or groundwater may be
used. Although most water systems are supplied from only one source,
there are instances when both surface water and groundwater sources are
utilized.
Surface water is drawn from large rivers or lakes. Even a small stream
may be suitable if it is impounded by a dam. Groundwater is normally ob-
tained by sinking wells into the saturated zone located beneath the water
table.
RAW WATER QUALITY AND TREATMENT
The quality of surface waters varies. Characteristically, such waters con-
tain microorganisms as well as inorganic and organic particulate matter
9
OCR for page 10
10 DRINKING WATER AND HEALTH
and dissolved solids. They may also have an undesirable color' taste, and
odor. Surface waters are subject to contamination by sewage from cities,
industrial wastes, agricultural runoff, and waste from animals and birds.
The temperature of surface waters fluctuates with climatic variations.
Although groundwaters are also subject to contamination as a result of
human activities, they are often clear, colorless, and possess lower concen-
trations of organic matter and microorganisms than does surface water
because of the natural filtration effected by the percolation of water
through soil, sand, or gravel. Conversely, the mineral content, including
calcium and magnesium ions the main contributors to "water
hardness" may be higher in groundwater than in the nearby surface
waters. In general, the mineral content of groundwaters reflects the
mineral characteristics of the soil in the area. Over time, the quality of
groundwaters is usually more constant than that of surface waters. The
temperatures of groundwaters are also more constant, normally ap-
proaching the average annual temperature of the region instead of the
constant fluctuations reflected in the temperatures of surface waters.
To be made acceptable for a public water supply, groundwater may re-
quire only disinfection to ensure adequate health protection. On the other
hand, it may be necessary to remove certain objectionable constituents in
the water and/or to reduce others to tolerable limits, depending upon the
type of contamination, applicable criteria or standards, and/or the desire
of the users. Surface waters normally require more extensive treatment
than do groundwaters. Treatment of raw water might include coagula-
tion, sedimentation, filtration, softening, and iron removal in addition to
disinfection.
The corrosiveness of surface waters and groundwaters varies widely,
depending on their pH, hardness, and other characteristics. Some waters
may also contain dissolved minerals, which deposit on the inside of
pipelines, resulting in scale formation. Highly corrosive raw waters may be
treated to reduce this property in conjunction with other types of treat-
ment required. The temperature of treated water is generally the same as
that of raw water. Slight changes may be produced by ambient air
temperature during the detention time in the treatment plant. High water
temperatures accelerate corrosive action and decrease the viscosity of
water.
DISTRIBUTION OF WATER
That portion of a public water system transporting water from the treat-
ment plant to the users is called the distribution system. Physical aspects
OCR for page 11
Elements of Public Water Supplies 11
such as the design, construction. and operation of such systems can have
important impacts on water quality. The complexity of and demands on
these systems make them the most costly single element in the water sup-
ply system.
To avoid possible contamination and because it is delivered to the con-
sumers under pressure. treated or finished Hater is transported in con-
duits or pipes rather than by open channels. In addition to a network of
interconnecting mains or pipes, water distribution systems normally in
clude storage facilities, valves, fire hydrants, service connections to user
facilities, and perhaps pumping facilities. The ability of a distribution
system to deliver an adequate quantity of water to meet the present and
projected demands of the domestic, commercial, and industrial users and
to provide the necessary flow for fire protection depends upon the carrying
capacity of the system's network of pipes. In all but the largest systems,
the flow that is necessary to combat a major fire is usually the major factor
determining requirements for the amount of water to be stored, the size of
mains in the system, and the pressure to be maintained. Fire flow stan-
dards require a minimum residual water pressure of 20 pounds per square
inch gauge (psi") during flow. It is common practice to maintain pressures
of 60 to 75 psig in industrial and commercial areas and 30 to 50 psig in
residential areas. Distribution system mains and pipes must be designed
to withstand these pressures.
The flow of water distribution systems may be controlled either by
gravity or by pressure (pumping). Often, public water supply systems use
some combination of both. In gravity systems, water is impounded at
strategic locations sufficiently elevated to create the working pressure re-
quired to move the water to the points of demand. When elevated im-
poundment or storage is impractical, the required working pressure is
provided by pumps within the system. In these pressure systems, the
pumps are normally located at the treatment plant and perhaps within the
distribution system. In combined systems, facilities for water storage are
often provided along with provision for pumping. This type of system pro-
vides for the storage of water during times of least demand while assuring
that a sufficient quantity is available to meet the peak demand. Typically,
water is pumped directly into the distribution system. The quantity of
water exceeding the demand automatically feeds into a storage facility or
reservoir. A system may also be designed so that pumps supply the water
storage facility directly; the water, in turn, might flow into the distribution
system by gravity.
Reservoirs may be located at the beginning of a distribution system,
i.e., immediately following water treatment, or at an intermediate point in
the system. The stored water may be used to meet fluctuating demands or
OCR for page 12
12 DRINKING WATER AND HEALTH
to equalize rates of flow or operating pressures on the system. The reser-
voirs may be classified as underground, ground level, elevated, or stand-
pipe. An underground reservoir or basin, either open or covered, may be
at or below grade level and formed either by excavation or embankment.
It is customary to line such reservoirs with concrete, Gunite, asphalt or an
asphalt membrane, or butyl rubber. A standpipe consists of a cylindrical
shell with a flat bottom resting on a foundation at ground level. An
elevated reservoir is a tank supported above ground by a structural
framework. Steel and wood have been used in the construction of stand-
pipes and elevated tanks, which are normally covered. It is preferable to
use covered reservoirs for treated water because water in open reservoirs is
subject to falling dust, dust-borne microorganisms, and soot; to con-
tamination by animals, including birds and human beings; and to algal
growth. It may be necessary to control algae and microbial slime growths
in open distribution reservoirs by adding copper sulfate and/or chlorine to
the water. Furthermore, to ensure adequate disinfection, it is generally
believed that there should be a sufficient chlorine residual throughout the
distribution system. In a large distribution system, dechlorination of the
water may be required. This is often accomplished at the distribution
reservoirs.
The detailed layout of a distribution system and its flow characteristics
depend upon the area to be served and its topography, the street plan, the
location of the source of supply, and other variables. Regardless of the
type of system, there is usually at least one primal feeder line or
transmission main that transports a large quantity of finished water from
the treatment plant and/or pumping facility to a specific location within
the system. If the distribution system is large, there may be more than one
transmission main, each serving a specific geographical area within the
overall system. This flow is then distributed locally to the users through a
series of progressively smaller pipes or mains. The buildings being served
are connected to the mains by small pipes called service lines or connec-
tions.
This network of various sizes of interconnecting pipes is normally
designed as a grid with a series of loops to avoid dead ends. The result is a
circulating system capable of supplying water to all points within the
system, sustaining service even if a section must be removed for
maintenance and repair or if a portion of the system must be taken out of
service because of contamination. To accomplish this, all distribution
systems should have sufficient numbers, types, and sizes of valves so that
different sections can be isolated.
Mains are usually made of cast iron, ductile iron, steel, reinforced con-
crete, plastic, or asbestos-cement. The type of pipe used is dictated by cost
OCR for page 13
Elements of Public Water Supplies 13
considerations, local conditions, and the size pipe required. The piping
material for service lines, i.e., household connections, may be galvanized
wrought iron, lead, galvanized steel, copper, plastic, cast iron, or ductile
iron. Of these, copper appears to be the most widely used. Lead, copper,
zinc, aluminum, and such alloys as brass, bronze, and stainless steel may
also be used in addition to ferrous metals in pumps, small pipes, valves,
and other appurtenances. Linings may be used to prevent corrosion
and/or to reduce the roughness of pipes. For example, iron and steel pipes
and fittings are often lined with cement mortar and/or with bituminous
material. Plastic piping may also be used in water distribution systems,
especially in household connections. Thermoplastic material used in
plastic pipe include polyvinyl chloride (PVC), polyethylene (PE),
ac~ylonitrile-butadiene-styrene (ABS), polybutylene (PB) plastic, and
fiberglass-reinforced plastic (FRP).
Piping used in the distribution system is manufactured in different
lengths, depending upon the material and size, and must be joined.
Several types of joints are used with pipes of a given material. The joints
for cast-iron or ductile-iron pipes may be either bell-and-spigot,
mechanical, flanged, threaded, or push-on (rubber gasket). With many
joints, such as the bell-and-spigot, it is necessary to fill the space created
in the joining of two ends of pipe. With cast-iron pipe, for example, the
space may be packed or caulked with hemp or jute, and then lead poured
into the joint to complete the seal. Thus, materials used for joints include
lead and lead substitutes, sulfur compounds, cement mortar mixtures,
and rubber together with asbestos, hemp, jute, and other substances ap-
plied as packing. Sections of steel pipe may be joined by welding, rubber
gasket seals, threading, or mechanical coupling. Sections of asbestos-
cement pipe are usually coupled with a push-on joint and a rubber ring.
Plastic mains usually have push-on or roll-on joints, while flared, com-
pression, clamp, or solvent joints are used with service lines.
The carrying capacity of mains and smaller pipes is a function of their
size and length, the pressure, and the resistance to flow, i.e., internal fric-
tion, bends, or turns in the pipe, joints, control valves, and other devices.
The internal surface of pipe, regardless of the material from which it is
made, offers resistance to water flow. For example, new steel and unlined
cast-iron or ductile-iron pipe have approximately equal resistance, while
that associated with cement-lined cast-iron or ductile-iron, asbestos-
cement, and plastic pipe is somewhat less. Encrustation caused by tuber-
culation, rust, and sedimentary deposits of various salts, such as iron and
manganese precipitates, will also increase the resistance of water flow.
Tuberculation is believed to result from the corrosive action of water on
metal pipes. The tubercles formed by the accumulation of corrosion prod
OCR for page 14
14 DRINKING WATER AND HEALTH
ucts often resemble barnacles. Microorganisms, through their bio-
chemical reactions, are also involved in corrosion and the formation of
tubercles. Sulfate-reducing bacteria may be involved in the latter process.
The growth of other microorganisms, including the iron bacteria. causes
the build-up of biological slimes, which also contributes to frictional
resistance. In a distribution system, these events may lead to a deteriora-
tion in the quality of water delivered to the users.
It is beyond the scope of this report to review established engineering
practice as it relates to the proper design, construction, and operation of
distribution systems. It should be recognized, however, that random or
accidental events, such as pipe breakage, or situations leading to cross-
connections or back-siphonage, may severely affect the chemical or
bacteriological quality of water in distribution systems and, thus, the
water delivered to the users. Table II-1 lists several recent incidents of this
type and their consequences.
The quality of water in distribution systems may also be affected by
cross-connections, which can be any direct or indirect physical connection
TABLE II-1 Some Recent Incidents Caused by Hydraulic Problems
Within Distribution Systems and Resulting Effectsa
Incident
Result
Extermination contractor caused 3 gallons
(11.4 liters) of chlordane to be back-
siphoned into the distribution system.
Wastewater from a meat processing plant
contaminated plant's potable water.
Water from fire-protection system in a
steam electricity-generating plant
contaminated plant's potable water.
Ethylene glycol from fire-protection
system contaminated potable water sup-
ply at two Air Force bases.
Boiler in a school was receiving a heavy
dose of chemical at the same time the
water supply to the school was shut off.
Loss of pressure resulted in back-
siphonage of treated boiler water into
drinking water.
Contamination of drinking water. No im-
mediate effects reported.
U.S. Department of Agriculture destroyed
approximately 2.9 million pounds (1.3
million kg) of pork.
Gastroenteritis suffered by 31 of 160 em-
ployees. No hospitalizations.
"Temporary" illnesses reported.
Two students were hospitalized, and 11
were treated and released.
a From Springer. 1980.
OCR for page 15
Elements of Public Water Supplies 15
or structural arrangement that permits nonpotable water or water of ques-
tionable or known poor quality to enter or back-flow into a potable water
supply (Angele, 19741. An arrangement whereby a safe water system- is
physically joined to a system containing unsafe water or wastewater is con-
sidered a direct connection. If the arrangement is such that unsafe water
may be blown, sucked, or otherwise diverted into a safe system, the con-
nection is considered indirect. Contaminated water may enter a potable
supply either through the distribution system or through a defect in the
user's plumbing system. Cross-connections, together with back-flow or
back-siphonage, are the most critical factors in protecting a distribution
system from contamination. Back-siphonage occurs when a potable or
safe system is at a pressure less than atmospheric; in this situation, the at-
mospheric pressure on the unsafe system will force the flow toward the
partial vacuum associated with the safe system. Prevention of back-flow
may be accomplished by using vacuum breakers designed to admit air to
break any vacuum in a water main or pipe, swing connections that permit
a connection either to a potable water supply or to another source of water
but not to both simultaneously, air gaps, and reduced pressure back-flow
preventers, i.e., a device with at least two independently acting check
valves separated by an automatic pressure differential relief valve.
APPROACH TO THE STUDY
Although the quality of water in a public water supply may be acceptable
immediately after treatment, it may deteriorate before it reaches the
users. This may result from either chemical or biological transformations.
Public water supplies are disinfected to inactivate infectious agents, to
protect the users against possible recontamination, and to control subse-
quent microbial growths that might alter the quality of the water. For
these reasons, it is normal practice to add chlorine to a water supply to
provide a residual concentration that will persist until the water reaches
the user. However, small quantities of chlorine or the loss of chlorine
residual in a distribution system may lead to microbial regrowth s and/or
the development of slime growth, which may in turn affect the turbidity of
the water or cause taste and odor problems. For example, the depletion of
dissolved oxygen resulting from microbial activity may promote the pro-
duction of hydrogen sulfide by sulfate-reducing bacteria. Furthermore,
the microbial production or release of metabolic products, e.g., endotox-
ins or extracellular products of algae, may affect the health of the users
directly. There is evidence that the corrosion of unlined steel, cast-iron,
OCR for page 16
16 DRINKING WATER "D HEALTH
and ductile-iron mains may be significantly influenced by microbial activ-
ity. Thus, it is possible for microorganisms to alter the quality of water in
distribution systems before it reaches the users.
The corrosion of metals may not only change the surface properties of
pipe but also produce soluble corrosion products, which in turn may affect
the quality of the water. There is also the possibility that certain consti-
tuents in cast-iron, asbestos-cement, concrete, plastic, and other pipe
materials may be leached into the water. The formation of scale and
deposits on the wall of pipes during periods of low-velocity flow may lead
to the release or resuspension of associated materials when the velocity of
the water is increased.
This report reviews the factors or potential conditions associated with
water distribution systems and their effects on water quality, with par-
ticular attention to their possible impact on the health of users of public
water supplies. The discussions focus on finished water, i.e., quality
changes occurring between the time the water leaves a treatment plant
and the time it reaches the users. Chemical control at the treatment plant
is considered only as it affects changes of quality in the finished water in
the distribution system. Having reviewed and evaluated those conditions
or factors influencing the deterioration of water quality in distribution
systems and, in a sense, determined what is known and unknown, the
committee was able to make recommendations regarding control pro-
cedures and to identify existing research needs.
It is beyond the scope of this report to consider in depth the physical
reliability or integrity of a public water supply system. However, * is im-
portant to recognize that the quality of water in distribution systems can
be adversely affected if the system is not designed, constructed, and main-
tained according to accepted engineering practice. For example, a cross-
connection in the system, together with back-flow or back-siphoning,
which permits an unsafe water or even wastewater to enter the system,
represents a most serious source of potential contamination. This altera-
tion in water quality can impose a direct health risk upon the users or
make the water aesthetically unacceptable. Leaks or mechanical breaks in
the piping of the distribution system can have the same effect. In this
sense, attention must be given to proper repair procedures or, for that
matter, to the installation of piping, e.g., the need to adequately disinfect
new or repaired distributional pipe before placing it in service. Further,
pumping equipment must be adequate to meet the demands of the system,
and reserve units, ready to be placed in service immediately when needed,
must be available. Low-velocity flow, whereby the water has extended con-
tact with pipes, can also cause a deterioration in water quality. Dead ends
in the distribution system or the oversizing of pipes or mains can con
OCR for page 17
Elements of Public Water Supplies 17
tribute to this stagnation. Finally, it should be pointed out that open
distribution or service reservoirs in the system can also lead to contamina-
tion of the water before it reaches the users. Again, the importance of hav-
ing a properly engineered and operated distribution system as it relates to
reliability cannot be overemphasized. Water supply personnel must be
constantly vigilant of defects and problems associated with distribution
systems as they might impact water quality. For example, each public
water system needs a continuous cross-connection control program. In
considering the reliability of a distribution system, it would seem ap-
propr~ate to point out that water purveyors are legally responsible if any
illness or death results from a system defect.
REFERENCES
Angele, G. J., Sr. 1974. Cross Connections and Backflow Prevention, 2nd Ed. American
Water Works Association, Denver, Colorado.
Springer, E. R. 1980. A sip could be fatal. Pp. 81-88 in Proceedings, American Water
Works Association Distribution System Symposium. American Water Works Association,
Denver, Colorado.
Representative terms from entire chapter:
water supply
