Washington Aqueduct
Treatment Process
Raw (untreated) water contains suspended solids, sediment,
bacteria, and microorganisms that must be removed to produce drinking
water. These are removed by full conventional treatment, described below:
Screening - On its way from the river
to the Dalecarlia and McMillan treatment plants, raw water passes
through a series of screens designed to remove debris such as twigs
and leaves.
Pre-sedimentation – While the water
moves slowly through Dalecarlia Reservoir, much of the sand and silt
settles to the bottom.
Coagulation - A coagulant, aluminum sulfate
(alum), is added to the water as it flows to sedimentation basins.
Coagulants aid in the removal of suspended particles by causing them
to consolidate and settle. Alum contains positively charged atoms
called ions which attract the negatively charged particles suspended
in water causing them to gather into clumps of particles heavy enough
to settle.
Flocculation – The water is gently
stirred with large paddles to distribute the coagulant; this causes
particles to combine and grow large and heavy enough to settle. This
process takes approximately 25 minutes.
Sedimentation – The water flows into
quiet sedimentation basins where the flocculated particles settle
to the bottom. After about four hours, approximately 85 percent of
the suspended material settles.
Filtration – Water at the top of
the basins flows to large gravity filters, where the water flows down
through filter media consisting of layers of small pieces of hard
coal (anthracite), sand, and gravel placed in the bottom of deep,
concrete-walled boxes. Filtered water passes through to a collecting
system underneath.
Disinfection – Chlorine is added
with precision equipment to kill pathogenic microscopic life such
as bacteria or viruses. Ammonia is then added. The chlorine and ammonia
combine to form chloramine compounds. The concentration of chloramines
in the water is closely monitored from the time it is added at the
treatment plants to points near the furthest reaches of the distribution
systems. Disinfection is considered by many to be one of the most
important scientific advances of the 20th century.
Fluoride, in the form of hydrofluorosilicic acid, is added
to reduce tooth decay; this is especially beneficial for children.
Orthophosphate is added to control corrosion in pipes, service
lines, and household plumbing throughout the distribution system. It
works by building up a thin film of insoluble material in lead, copper,
and iron pipes and fixtures. This thin film acts a barrier to prevent
leaching of metals into the water. Calcium hydroxide (lime) is also
added to adjust the pH of the water to ensure optimal performance of
the orthophosphate.
Powdered activated carbon is occasionally used for taste
and odor control.
After the water has completed its path through the treatment
process, it is referred to as finished or potable water. Most people
simply call it drinking water.
Another site which provides information on treatment processes
and interesting links is the Environmental Protection Agency's How is Drinking Water Treated web site.
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