Science of Water : Concepts and Applications

Water Chemistry 113
COAGULATION
Chemical coagulation conditions water for further treatment by the removal of
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Turbidity, color, and bacteria
Iron and manganese
Tastes, odors, and organic pollutants
In water treatment, normal sedimentation processes do not always settle out particles effi ciently.
This is especially the case when attempting to remove particles less than 50 µm in diameter.
In some instances, it is possible to agglomerate (to make or form into a rounded mass) particles
into masses or groups. These rounded masses are of increased size and therefore increased settling
velocities, in some instances. For colloidal-sized particles, however, agglomeration is diffi cult—as
colloidal particles are diffi cult to clarify without special treatment.
Chemical coagulation is usually accomplished by the addition of metallic salts such as aluminum
sulfate (alum) or ferric chloride. Alum is the most commonly used coagulant in water
treatment and is most effective between pH ranges of 5.0 and 7.5. Sometimes polymer is added to
alum to help form small fl oc for faster settling. Ferric chloride, effective down to a pH of 4.5, is
sometimes used.
In addition to pH, a variety of other factors infl uence the chemical coagulation process,
including
1. Temperature
2. Infl uent quality
3. Alkalinity
4. Type and amount of coagulant used
5. Type and length of fl occulation
6. Type and length of mixing
TASTE AND ODOR REMOVAL
Although odor can be a problem with wastewater treatment, the taste and odor parameter is only
associated with potable water. Either organic or inorganic materials may produce tastes and odors
in water. The perceptions of taste and odor are closely related and often confused by water practitioners
as well as by consumers. Thus, it is diffi cult to precisely measure either one. Experience has
shown that a substance that produces an odor in water almost invariably imparts a perception of
taste as well. This is not the case, however. Taste is generally attributed to mineral substances in the
water. Most of these minerals affect water taste but do not cause odors.
Along with the impact minerals can have on water taste, there are other substances or practices
that can affect both water taste and odor (e.g., metals, salts from the soil, constituents of wastewater,
and end products generated from biological reactions). When water has a distinct taste but no odor,
the taste might be the result of inorganic substances. Anyone who has tasted alkaline water has also
tasted its biting bitterness. Then there are the salts; they not only give water that salty taste but also
contribute to its bitter taste. Other than from natural causes, water can take a distinctive color or
taste, or both, from human contamination of the water.
Organic materials can produce both taste and odor in water. Petroleum-based products are
probably the prime contributors to both these problems in water.
Biological degradation or decomposition of organics in surface waters also contributes to both
taste and odor problems. Algae are another problem. Certain species of algae produce oily substances
that may result in both taste and odor. Synergy can also work to produce taste and odor
problems in water. Mixing water and chlorine is one example.