Page 181 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

WATER TREATMENT AND PURIFICATION

WATER TREATMENT AND PURIFICATION 4.17

retention times are necessary. This unit is also free from cold weather problems and can be
used for chemical mixing.

CLARIFICATION

Clarification is a process to reduce or remove turbidity, silt, and sediment present in the raw
water supply. The water can be treated with chemicals or filtered, depending on the amount
of impurities present and the volume of water to be treated. If treated with chemicals, time
is required for the solids to settle out of suspension. The chemical treatment process is usu-
ally reserved for large volumes of water.
Suspended and colloidal particles will normally be in the range of 0.1 to 10 mm in size.
They stay apart because negatively charged ionized matter is absorbed on their surfaces and
thus they repel each other. A measure of the charge that surrounds the colloid is known as the
“zeta potential.” These negative charges are reduced by the use of positively charged chemi-
cals called coagulants. Coagulants, through a chemical reaction, reduce the zeta potential and
allow the colloids to cluster to form a larger, jellylike mass called floc. In a process called floc-
culation, colloidal particles, bacteria, and organic matter are mechanically brought together
into larger and heavier particles. These particles are now too heavy to remain suspended, and
so, they settle out of the water to the bottom. This process is called sedimentation.
Clarification of large volumes of water is done in a basin. First, the coagulant(s) must
be introduced with a strong action in order to completely disperse and mix the chemicals
with the incoming water. Often, some of the sludge produced by flocculation is mixed
with the coagulant. After coagulation is accomplished, the mixture is then gently mixed in
order to allow the now larger particles to settle rapidly to the bottom of the tank. This set-
tling process is called sedimentation. The particles and chemicals that settle out are called
sludge, which must be removed from the tank and disposed of. Most clarifier designs do
both processes in the same operating unit.
The chemical coagulant used most frequently is aluminum sulfate, called alum or filter
alum. Other coagulants used often are sodium aluminate and potash alum. Iron coagulants
include ferric sulfate, ferris sulfate, and ferric chloride. Organic polyelectrolytes, which
are high-molecular-weight polymers, are also employed in low dosages to increase the
effectiveness of treatment.
It is usually a matter of trial and error to find the correct amount of alum. This is because
the amount depends on the size and quantity of suspended solids of the raw water, reten-
tion time prior to flocculation, water temperature, and the amount of mixing. All of these
conditions change from day to day.
As with almost every process, there is more than one method appropriate to achieve
a desired goal when the demand for water is not very large. One alternative flocculation
method is to produce a finer floc which can be removed by an appropriate filter system. By
eliminating a large settling basin, a high quality water can be produced more quickly with
smaller equipment.

DEAERATION

Dissolved gases in the water supply can produce corrosion and pitting. They must be
removed prior to using the water for most purposes. These gases are oxygen, carbon diox-
ide, hydrogen sulfide, and ammonia.
Although carbon dioxide and hydrogen sulfide can be removed by aeration, in many
cases it is easier and less expensive to remove them by deaeration. This process operates
on the principle of raising the water temperature to the saturation point for the existing

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