Encyclopedia of Environmental Science and Engineering, Volume I and II

1314 WATER TREATMENT

desalting of sea water as a major water source has

a restricted application to small islands and those

coastal stretches of countries where fresh water

reserves are either not available or not reliable.

6) Recycled water. A considerable amount of research

has been undertaken in the United States and else-

where for the renovation of treated wastewater for

the purposes of returning it directly into the potable

supply. Some rather complex chemical and physi-

cal processes are required to make this a satisfactory

process, and the details of many of these processes

will be described subsequently in the next section

of this chapter.

PHYSICAL TREATMENT

The items of treatment described under this section will be

only those which alter the physical properties of the water

or represent a unit process which is physical in nature. All

of the processes described may be used individually, collec-

tively or in any combination, in order to accomplish a prede-

termined water quality.

Screens

Whatever the source of water, it is necessary to insert some

kind of screen in the system in order to prevent the passage

of solids into the subsequent steps of water treatment. If

the source of water is simply a well, the screens tend to be

simply designed to prevent the admission of sand from the

water-bearing strata into the pumping system. Where water

supply is drawn from rivers or lakes, the intakes usually have

to be screened and built of corrosion-resistant materials in

order to prevent the admission of fish or logs or any other

undesirable solids into the system. Intake screens are usually

provided with openings approximately equal to one and one-

half to two times the area of the intake pipe. The purpose of

this is to ensure that the velocity through the screens is suf-

ficiently low to prevent jamming of the screens. On occasion

other screens are required as a backup system within the

water treatment plant.

In some locations where it is found that seasonally algal

blooms become a nuisance, a new type of screening known

as microstraining has been introduced. Microstrainers are a

very fine weave of stainless steel wire with apertures suffi-

ciently small to prevent the passage of the microscopic algae

which is normally found in an algal bloom. Such a screen-

ing system is normally only required on a seasonal basis

and in certain locations where these problems are prevalent.

Microstraining is conducted at such a very small diameter

orifice that it is sometimes considered to be a part of a filtra-

tion process.

Coagulation

Although the basis of coagulation is in fact chemical treatment

and will be discussed in the next section, the coagulation

process itself (sometimes referred to as flocculation) is

accomplished by a physical process involving the gentle agi-

tation of the fluid which allows the small suspended particles

to collide and agglomerate into heavier particles or flocs and

settle out. Flocculation or coagulation is the principle used in

the removal of turbidity from water. It will be shown subse-

quently that colloidal or very finely divided material will not

settle very rapidly. Various processes have been employed

to accomplish flocculation. Some of these are; diffused air,

baffles, transverse or parallel shaft mixers, vertical turbine

mixers, to mention but a few.

The most common type of flocculator used today is the

paddle type, the other methods having shown some disad-

vantage such as being too severe for the fragile floc, or being

too inflexible, or being too costly to operate. Horizontally

mounted paddles, either located transverse or parallel to the

floor, consist of a shaft with a number of protruding arms

on which are mounted various blades. The shaft rotates at a

very slow rate of 60 to 100 rpm, causing a very gentle agita-

tion which results in the flocculation of the particles. The

time required for the flocculation process is very carefully

controlled and strongly related to the dosage of chemical

which is used. The chemicals used and the chemistry of this

process will be described later.

Prior to the flocculation step which has just been

described, occurs a flash mixing step when the chemicals

are added and mixed very rapidly at high speed to get uni-

form distribution of the chemical in the stream. A variety of

devices are used for this rapid mixing operation; frequently

one of the most common includes the low lift pumps which

are usually located adjacent to the intake where the water is

lifted up into the treatment plant. Here of course the chemi-

cals must be pumped into the pump casing at a higher pres-

sure than the pump is producing, and the mixing takes place

in the casing of the pump.

Other devices frequently used are venturi flumes, air jets,

paddles, turbines, propellers, the latter being one or the most

favored and most widely used of the rapid mixing devices.

It usually is composed of a vertical shaft driven by a motor

CASING

WET

WELL

SCREEN

INTAKE

SCREEN

SANDY STRATA

(I) WELL SCREEN (II) LAKE OR RIVER SCREEN

FIGURE 2

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