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The
Stages of
 Water Treatment |
Preliminary
Treatment
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Preliminary treatment or pretreatment is any physical,
chemical or mechanical process used on water before it undergoes the main treatment
process. During preliminary treatment:
- screens may be used to remove rocks, sticks, leaves and other debris;
- chemicals may be added to control the growth of algae; and
- a presedimentation stage can settle out sand, grit and gravel from raw water.
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Coagulation
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After preliminary treatment, the next step is coagulation.
Coagulation removes small particles that are made up of microbes, silt and other
suspended material in the water. Treatment chemicals such as alum are added to the
water and mixed rapidly in a large basin. The chemicals cause small particles to clump
together (coagulate). Gentle mixing brings smaller clumps of particles together to form
larger groups called "floc". Some of the floc begins to settle during this
stage.
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Flocculation
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During the flocculation stage, the heavy, dense floc settles
to the bottom of the water in large tanks. As you can imagine, this can be a slow process!
Once the floc settles, the water is ready for the next stage of treatment.
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Clarification
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Clarification occurs in a large basin where water is again
allowed to flow very slowly. Sludge, a residue of solids and water, accumulates at the
basin's bottom and is pumped or scraped out for eventual disposal. Clarification is also
sometimes called sedimentation.
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Softening
and
Stabilization
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When water is too "hard" (i.e. contains too much
calcium, magnesium or other minerals), it forms scale and causes a variety of problems in
pipes. Hard water can also result in laundering and washing problems, because it reduces
the effectiveness of soaps and detergents. Conversely, when too many of these minerals
are removed, water can become too "soft". Soft water can cause corrosion
in pipes.
Drinking water plants attempt to maintain a desirable balance between hardness and
softness. This is accomplished by adding minerals to soft water and removing them from
hard water. (Measure hardness as CaCO3 with a Hach titration procedure based on
the USEPA-approved method.)
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Filtration
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Turbidity is a physical characteristic that makes water
appear cloudy when suspended matter is present. The filtration process removes suspended
matter, which can consist of floc, microorganisms (including protozoan cysts such as Giardia
and Cyrptosporidium), algae, silt, iron, and manganese precipitates from
ground-water sources, as well as precipitants which remain after the softening process. These
suspended materials are filtered out when water passes through beds of granular material,
usually composed of layers of sand, gravel, coal, garnet, or related substances. (Measure
turbidity with a Hach turbidimeter.)
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Fluoridation
& Disinfection
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Fluoride (F-) is added to water to reduce tooth
decay. Fluoridation is an effective, economical process endorsed by many public health
groups worldwide. Fluoride is fed into the water system as either a dry powder or in
solution. (A Hach fluoride test detects fluoride levels in the water.) During
disinfection, disease-causing organisms are destroyed or disabled. Chlorine (Cl2)
is the most common disinfectant used in the United States because it is practical,
effective and economical.
Because chlorine dissipates rapidly, it is important to add the right amount of
chlorine at the water treatment plant to make sure disinfection continues while the water
is flowing through the distribution system. (Use a USEPA-approved Hach method to measure
chlorine in water.)
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Holding Tanks
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Finished water (the term water treatment professionals use)
is stored in holding tanks. The tanks provide a water reserve to meet the changing
water demands of the communities they serve.
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