Encyclopedia of Environmental Science and Engineering, Volume I and II

1318 WATER TREATMENT

The filter medium is supported on a fine metal screen

or a porous material. There are three steps in the filtration

cycle. There are three steps in the filtration cycle. First of all,

the deposit of a pre-coat, which is a thin layer of diatomite

deposited on the filter element. The second step is the actual

filtration and the body feed addition. The reason why body

feed is continually added to the filter is to reduce the amount

of clogging that occurs at the surface. This also permits sig-

nificantly longer filter runs. The third step, when the pres-

sure drops or the filtration rate reaches such a low very thin

film over or under the source of irradiation. Commercial

equipment is currently being developed for the individual

water supply of the small household or institution, and is

gaining some acceptance in some quarters. The irradiation

of water by ultra-violet light of suitable wave-lengths for a

proper period of time will kill bacteria, spores, molds, and

viruses and in fact all microorganisms. The bactericidal

wave-lengths extend from about 2000 to 2950 Å (angstrom

units) with a maximum effect around 2540 Å.

CHEMICAL TREATMENT

The unit operations of chemical coagulation, precipitation,

ion exchange and stabilization all produce change in the

chemical quality of the water. Some of these are aimed at the

removal of the suspended and colloidal substances, others

are aimed at the removal of dissolved substances. Finally,

some chemicals are simply added for their own sake, but

these will not be discussed in this section.

To understand some of the basic chemistry of the treat-

ment processes, it is first of all essential to understand a

phenomenon known as chemical equilibrium and reaction

velocities. An analogy might be considered as the physical

equilibrium between ice and water.

Ice Water

Add Heat

Remove heat

If the ice-water system is maintained at 0°C, then molecules

of water are transferred from the solid to the liquid state

and back again at the same rate. The addition of heat or the

removal of heat from the system will result in the equilib-

rium moving in one direction or the other. The same princi-

ples might be applied to what is known as ionic equilibrium,

which, like molecular equilibria, are subject to a shift under

given stresses.

As an example, we might consider pure water

H 2 O U H^ 
^ 
 OH^ ^.

Certain stresses will give rise to an increase in hydrogen ion

concentration (H^ 
^ ). The expression of this shift is a reduction

in pH, whereas an increase in the OH^ ^ concentration brings

about an increase of pH. One of the most important equilib-

ria which exists in natural waters is the relationship between

carbon dioxide and carbonate ion, which is shown in the fol-

lowing four equilibrium expressions.

CO 2 (gas) U CO 2 (solution) (1)

CO 2 (solution) 
 H 2 O U H 2 CO 3 (2)

H 2 CO 2 U H^ 
^ 
 HCO 3 ^ (3)

HCO 3 ^ U H^ 
^ 
 CO 3 ^. (4)

FIGURE 11

Filter

Backwash High density

Small particles

Medium density

Low density

Large particles

Cosmlc Gamma Rays

Rays X Rays

Ultra Violet Visible Light

Infra Red Radio Waves

10 –4 10 –2 102 104 106 108 1010

Ultra Violet Visible

Bacteriacidal

Max. Bacteriacidal Violet Green Red

Infra

Red

0 1000 2000 3000 4000 5000 6000 7000 8000

ELECTROMAGNETIC SPECTRUM

Light

FIGURE 12

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