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For
thousands of years people have treated their drinking water to make it look
cleaner and taste better. Medical lore from ancient India to Egypt advised
that water should be filtered through sand and coarse gravel. In Greece,
Hippocrates, the Father of Medicine, recommended boiling water and straining
it through a cloth to remove particles.
Early
in the 1800s, scientists began to suspect that diseases could be transmitted
by drinking water. Since the discovery of bacteria in the 1870s, treatment
to eliminate disease-causing microorganisms has dramatically reduced the
incidence of diseases transmitted through water.
Water
treatment facilities process water through many steps before it reaches your
tap as drinking water for your family. The overall process generally begins
with intake at the source, followed by pretreatment, mixing, coagulation and
flocculation, sedimentation, filtration, disinfection, and distribution to
the tap. See Figure 1. Other steps may be necessary depending on the quality
of the water supply.
Preliminary Treatment Processes Preliminary treatment processes to purify
water depend on the nature of the water supply and the type and the amount
of contaminants. Quality may vary seasonally, thus requiring more or less
treatment during certain water-flow conditions. Most groundwater in Alabama
receives only the disinfection treatment process. All surface water and
water from wells susceptible to contamination from surface water receive
full treatment through a filter plant. The various preliminary treatment
processes may include the following.
Screening. Large objects such as logs, sticks, fish, and plants are usually
screened out at the intake or as the water is drawn into the treatment plant
from a river, lake, or other surface water source. If the source is
groundwater, the screening is done by nature as the water travels under the
surface of the earth.
Pre-sedimentation. Gravel, sand, some silt, and other gritty materials
may be removed by fine screening.
Micro-straining. In some cases, algae, aquatic plants, and other very
small debris may be removed by still finer screening.
Chemical Pretreatment. The water is conditioned for removal of natural
organics, primarily algae and other aquatic microorganisms, as well as their
by- products. This may precede micro-straining.
Main
Treatment Processes Chemical Feed And Rapid Mix. Chlorine and other
chemicals, such as alum or lime, are added to the water to help remove
impurities, destroy any taste or odor, raise pH, disinfect, and sometimes
remove excess minerals such as iron that may cause rust or staining
problems. The water is then mixed rapidly to distribute the chemicals
evenly.
Since
the early 1900s, chlorine (as a solid, liquid, or gas) has been the primary
disinfectant used in the United States because it is effective and
inexpensive and can provide a disinfectant residual in the distribution
system. Ozone and ultraviolet radiation can also be used as primary
disinfectants, but chlorine or an appropriate substitute must also be used
as a secondary disinfectant after the main treatment processes to prevent
re-growth of microorganisms in the distribution system.
Chemicals may be added to oxidize ferrous iron (Fe++), which is relatively
high in some groundwater, to the ferric state (Fe+++). If pH of the water is
above 7 (either naturally or by adding lime), the insoluble compound of
ferric hydroxide is precipitated.
Softening. Sometimes chemicals are included to reduce the “hardness” or
mineral content of drinking water. This usually involves the exchange of
sodium for calcium and magnesium and, sometimes, the removal of iron and
manganese. However, softening is not as popular as it once was for several
reasons. The increased sodium in softened water is unhealthy for people with
high blood pressure. In soft water lead is more easily leached from
plumbing. Finally, detergents that clean favorably in hard water are readily
available. There are no softening plants in Alabama since total dissolved
solids are not excessive.
Coagulation And Flocculation. The water is sent into large basins where
the alum clings to other chemicals and impurities in the water
(coagulation), causing them to form larger, heavier particles called floc.
Gravity causes these larger particles to settle to the bottom.
Sedimentation. The water is allowed to sit undisturbed long enough so
that solid particles completely settle to the bottom. This process removes
chemical precipitates as well as extremely fine clay and organic particles,
including dead microorganisms.
Filtration. After flocs (large, heavy particles) settle to the bottom,
the water continues on its trip through filters. Layers of sand, gravel, and
sometimes hard coal are used to remove any other impurities that are left in
the water. Filtration helps to control biological contamination and
turbidity. (Turbidity is a measure of the cloudiness of water caused by the
presence of suspended matter.) Turbidity can shelter harmful microorganisms
and reduce the effectiveness of disinfection. Removing organics prior to
final chlorination of drinking water supplies is important.
Disinfection. After most impurities have been removed from the water, a
small amount of chlorine is added to keep the water from developing bacteria
as it travels throughout the distribution pipes. The amount of chlorine
(usually no more than 3 parts per million) is carefully measured to be the
lowest possible amount needed to keep the water free of germs. Residual
chlorine at the tap should be near 0.5 parts per million.
A
primary health concern with cholorination is the formation of disinfection
by-products. When chlorine combines with organic matter in water, such as
decaying plants or animals, it forms substances called trihalomethanes (THMs).
These have been shown to cause cancer in laboratory animals. Chloroform is a
common THM which has been linked to bladder cancer in those who drink from
treated public water supplies. There is a drinking water standard for total
trihalomethanes (TTHMs) of 0.10 milligrams per liter, but it applies only to
those systems that serve more than 10,000 people.
Fluoridation. In some places fluoride at concentrations up to 1 part per
million is also added to help prevent tooth decay. This is not as common as
it once was because of the health concern for excess fluoride. EPA has
established a maximum contaminant level (MCL) of 4 milligrams per liter (4
parts per million) for fluoride in drinking water.
If
conventional steps in the main treatment process are not adequate, certain
inorganic and organic contaminants must be removed by other methods.
Treatment To Remove Inorganic Contaminants
Reverse osmosis or ion
exchange are used to remove nonmetal inorganic contaminants. Nitrate and
fluoride are the nonmetals of greatest health concern in drinking water.
Nitrate is frequently found in ground- water supplies in high-density
agricultural areas. Likely sources of groundwater nitrate are nitrogen
fertilizers as well as decomposing plant and animal wastes, including human
waste from septic systems.
Coagulation and filtration, reverse osmosis, ion exchange, or activated
alumina are used to remove metal inorganic contaminants. The metals of
greatest health concern in drinking water include mercury, cadmium,
selenium, lead, arsenic, chromium, and barium. Industrial sources can
contribute rare metals and toxic heavy metals to surface waters.
Controlling corrosion of inorganic chemicals from the distribution or
plumbing system is another treatment alternative. Corrosion of plumbing
by-products such as copper and lead at the point of use (the consumer’s tap)
can usually be indirectly eliminated by controlling pH and water hardness.
Lead
contamination is the most serious threat from corrosion. Lead usually enters
the water from private plumbing where it is found in solder used to connect
copper pipes. Lead can also be corroded from public distribution system
pipes and joints.
Aeration effectively strips radon gas from source waters. Oxidation and
aeration will remove hydrogen sulfide gas.
Treatment To Remove Organic Contaminants
Methods to remove organic contaminants include activated carbon filtration
and aeration. Special filters may be used at water treatment plants to
remove many toxic organic substances such as pesticides and solvents;
however, these filter systems are expensive to build and maintain, and they
slow down the treatment process. As water passes through carbon filters,
organic impurities are trapped inside the filter material. This is called
adsorption.
Sources of organic compounds include storm-water runoff and leaching from
improperly disposed wastes, accidental spills, leaking fuel storage tanks
and pipelines, pesticides from agricultural areas, and industrial effluents.
Testing -
Water samples are taken regularly at many points in the treatment process
for laboratory testing. These tests let water plant personnel know whether
the primary and secondary drinking water standards set by the EPA are being
met. The laboratory equipment being used is so sensitive that it can measure
sub- stances in parts per million, parts per billion, or even parts per
trillion in some cases. |
