Did
you know?
Reverse osmosis is the finest water filtration method known. This process will
allow the removal of particles as small as ions from a solution. It is used
to purify water and remove salts and other impurities in order to improve the
color, taste or properties of the fluid. R.O. uses a membrane that is semi-permeable,
allowing the fluid that is being purified to pass through it, while rejecting
other ions and contaminants from passing. This technology uses a process
known as crossflow to allow the r.o. membrane to continually clean itself. This
is the reason of why an r.o. element can last many years before clogging or
need replacement. This
water purification process requires a driving force to push the fluid through
the membrane, and the most common force is household water pressure or pressure
from a booster pump. The higher the pressure, the larger the driving force and
efficiency.
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How
do water treatment plants work?
Water
treatment-or the purification and sanitation of water-varies as to the
source and kinds of water. Municipal waters, for example, consist of surface
water and ground water, and their treatment is to be distinguished from
that of industrial water supplies. Municipal water supplies are treated
by public or private water utilities to make the water potable (safe to
drink) and palatable (aesthetically pleasing) and to insure an adequate
supply of water to meet the needs of the community at a reasonable cost.
Except in exceedingly rare instances, the entire supply is treated to
drinking water quality for three reasons: it is generally not feasible
to supply water of more than one quality; it is difficult to control public
access to water not treated to drinking water quality; and a substantial
amount of treatment may be required even if the water is not intended
for human consumption.
Raw
(untreated) water is withdrawn from either a surface water supply (such
as a lake or stream) or from an underground aquifer (by means of wells).
The water flows or is pumped to a central treatment facility. Large municipalities
may utilize more than one source and may have more than one treatment
facility. The treated water is then pumped under pressure into a distribution
system, which typically consists of a network of pipes (water mains) interconnected
with ground level or elevated storage facilities (reservoirs). As it is
withdrawn from the source, surface water is usually screened through steel
bars, typically about 1 in (2.54 cm) thick and about 2 in (5.08 cm) apart,
to prevent large objects such as logs or fish from entering the treatment
facility. Finer screens are sometimes employed to remove leaves. If the
water is highly turbid (cloudy or muddy), it may be pretreated in a large
basin known as a pre-sedimentation basin to allow time for sand and larger
silt particles to settle out.
All
surface waters have the potential to carry pathogenic (disease-causing)
microorganisms and must be disinfected prior to human consumption. Since
the adequacy of disinfection cannot be assured in the presence of turbidity,
it is first necessary to remove the suspended solids causing the water
to be turbid. This is accomplished by a sequence of treatment processes
that typically includes coagulation, flocculation, sedimentation, and
filtration. Coagulation is accomplished by adding chemical coagulants,
usually aluminum or iron salts, to neutralize the negative charge on the
surfaces of the particles (suspended solids) present in the water, thereby
eliminating the repulsive forces between the particles and enabling them
to aggregate. Coagulants are usually dispersed in the water by rapid mixing.
Other
chemicals may be added at the same time, including powdered activated
carbon (to absorb taste- and odor causing chemicals or to remove synthetic
chemicals); chemical oxidants such as chlorine, ozone, chlorine dioxide,
or potassium permanganate (to initiate disinfection, to oxidize organic
contaminants, to control taste and odor, or to oxidize inorganic contaminants
such as iron, manganese, and sulfide); and acid or base (to control pH).
Coagulated particles are aggregated into large, rapidly settling "floc"
particles by flocculation, accomplished by gently stirring the water using
paddles, turbines, or impellers. This process typically takes 20 to 30
minutes. The flocculated water is then gently introduced into a sedimentation
basin, where the floc particles are given about two to four hours to settle
out. After sedimentation, the water is filtered, most commonly through
24-30 in (61-76 cm) of sand or anthracite having an effective diameter
of about 0.02 in (0.5 mm).
When
the raw water is low in turbidity, coagulated or flocculated water may
be taken directly to the filters, bypassing sedimentation; this practice
is referred to as direct filtration. Once the water has been filtered,
it can be satisfactorily disinfected. Disinfection is the elimination
of pathogenic microorganisms from the water. It does not render the water
completely sterile but does make it safe to drink from a microbial standpoint.
Most water treatment plants in the United States rely primarily on chlorine
for disinfection. Some utilities use ozone, chlorine dioxide, chloramines
(formed from chlorine and ammonia), or a combination of chemicals added
at different points during treatment. There are important advantages and
disadvantages associated with each of these chemicals, and the optimum
choice for a particular water requires careful study and expert advice.
Chemical
disinfectants react not only with microorganisms but also with naturally
occurring organic matter present in the water, producing trace amounts
of contaminants collectively referred to as disinfection byproducts (DBPs).
The most well-known DBPs are the trihalomethanes. Although DBPs are not
known to be toxic at the concentrations found in drinking water, some
are known to be toxic at much higher concentrations. Therefore, prudence
dictates that reasonable efforts be made to minimize their presence in
drinking water. The most effective strategy for minimizing DBP formation
is to avoid adding chemical disinfectants until the water has been filtered
and to add only the amount required to achieve adequate disinfection.
Some DBPs can be minimized by changing to another disinfectant, but all
chemical disinfectants form DBPs. Regardless of which chemical disinfectant
is used, great care must be exercised to ensure adequate disinfection,
since the health risks associated with pathogenic microorganisms greatly
outweigh those associated with DBPs.
There
are a number of other processes that may be employed to treat water, depending
on the quality of the source water and the desired quality of the treated
water. Processes that may be used to treat either surface water or groundwater
include: 1) lime softening, which involves the addition of lime during
rapid mixing to precipitate calcium and magnesium ions; 2) stabilization,
to prevent corrosion and scale formation, usually by adjusting the pH
or alkalinity of the water or by adding scale inhibitors; 3) activated
carbon adsorption, to remove taste- and odor-causing chemicals or synthetic
organic contaminants; and 4) fluoridation, to increase the concentration
of fluoride to the optimum level for the prevention of dental cavities.
Compared
to surface waters, groundwaters are relatively free of turbidity and pathogenic
microorganisms, but they are more likely to contain unacceptable levels
of dissolved gases (carbon dioxide, methane, and hydrogen sulfide), hardness,
iron and manganese, volatile organic compounds (VOCs) originating from
chemical spills or improper waste disposal practices, and dissolved solids
(salinity). High-quality groundwaters do not require filtration, but they
are usually disinfected to protect against contamination of the water
as it passes through the distribution system. Small systems are sometimes
exempted from disinfection requirements if they are able to meet a set
of strict criteria. Groundwaters withdrawn from shallow wells or along
riverbanks may be deemed to be "under the influence of surface water,"
in which case they are normally required by law to be filtered and disinfected.
Hard
groundwaters may be treated by lime softening, as are many hard surface
waters, or by ion exchange softening, in which calcium and magnesium ions
are exchanged for sodium ions as the water passes through a bed of ion-exchange
resin. Groundwaters having high levels of dissolved gases or VOCs are
commonly treated by air stripping, achieved by passing air over small
droplets of water to allow the gases to leave the water and enter the
air. Many groundwaters-approximately one quarter of those used for public
water supply in the United States-are contaminated with naturally occurring
iron and manganese, which tend to dissolve into groundwater in their chemically
reduced forms in the absence of oxygen.
Iron
and manganese are most commonly removed by oxidation (accomplished by
aeration or by adding a chemical oxidant, such as chlorine or potassium
permanganate) followed by sedimentation and filtration; by filtration
through an adsorptive media; or by lime softening. Groundwaters high in
dissolved solids may be treated using reverse osmosis, in which water
is forced through a membrane under high pressure, leaving the salt behind.
Membrane processes are rapidly evolving, and membranes suitable for removing
hardness, dissolved organic matter, and turbidity from both ground and
surface waters have recently been developed.
Healthy and Convenient!
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