Encyclopedia of Environmental Science and Engineering, Volume I and II

1312 WATER TREATMENT

section on wastewater treatment. It should, however, be real-

ized that there is a very fine line between treated wastewater

discharged into a water body and the use of that water body

as a source for drinking water and the treatment of the waste-

water before discharge. Clearly in those areas where wastes

are still not treated prior to release, a water treatment plant is

essentially dealing with the treatment of diluted wastewater.

We must therefore determine the significance of water

quality before we examine the types of treatment necessary

to achieve this quality. Water quality very much depends

upon the use for which the water was intended. For example,

industrial boiler feed water requires a very low hardness

because the hardness tends to deposit on the pipes in the

boiler system and reduces the efficiency of the heat transfer.

However, if the hardness of the boiler feed water is zero, the

water tends to be very corrosive and this of course is also

very undesirable for a boiler system.

If the water is to be used for a brewery or a distillery, a

number of other chemical parameters are important. If the

water is to be used for cooling then clearly the temperature

is one of the most important parameters.

In the past the methods for setting standards for water

supplies was very much a hit and miss affair and relied pretty

well upon the philosophy of “If no one complains, all is well.”

Clearly, that is not a very satisfactory criterion. There are a

number of drinking water standards or objectives published

by various nations of the world, such as the World Health

Organization International and European Drinking Water

Standards (1963 and 1961), the US Public Health Service

Drinking Water Standards (1962) and Objectives (1968). These

standards are established on the principle that water in a public

water supply system must be treated to the degree which is

suitable for the highest and best use. The highest and best use

for water of course is human consumption. This can frequently

be argued as a rather unnecessary quality when one considers

that much water which is processed in a municipal plant is used

for watering lawns, washing cars and windows. However, the

difficulty in ensuring that a second-class, perhaps unsafe water

supply is not used as a potable supply is extremely difficult. It

will be found that very few cities have a dual water supply rep-

resenting a drinking water system and a non-potable system.

A few large cities, particularly when they are adjacent

to large standing bodies of water, occasionally have a fire

water supply system where the water is taken untreated from

the lake or river and pumped under high pressure through a

system connected only to fire hydrants and sprinklers.

Thus, assuming that natural water requires some kind

of treatment in order to achieve certain predetermined stan-

dards, and the process of treating these waters can be subdi-

vided into physical and chemical processes, the remainder of

this section will deal with the physical and chemical meth-

ods of treating water for municipal or industrial use.

WATER SOURCES

The magnitude of the problem of supplying water to the major

cities of the world is in fact a huge engineering problem.

According to a US Department of Commerce estimate, the

cities of the United States in 1955 with a total population

of 110 million produced and distributed 17 billion gallons

of water daily to their domestic, commercial, and industrial

consumers. Of this, 12.88 billion gallons were from surface

water sources which usually, it will be seen, require more

elaborate treatment, whereas the remaining 4.12 billion gal-

lons came from groundwater sources—only a small propor-

tion of which would require treatment.^4 The most voluminous

source of water is the oceans. It is estimated that they contain

about 1060 trillion acre-feet.^5 Clearly this water is of little

value as a potable source, but it certainly remains the main

reservoir in the hydrologic cycle.

It can be seen from Figure 1 that evaporation is the first

step in the purification of ocean water, and this requires the

full energy of the sun in order to accomplish. Precipitation,

percolation, and runoff are all parts of the cycle of water

which is without a beginning or an ending. Of the water

which falls upon the earth, part of it directly runs off to the

nearest stream or lake, and part of it infiltrates down to the

groundwater table and percolates through the groundwater,

also into a stream or lake. Transpiration takes place through

the leaves of green plants, and evaporation takes place from

the groundwater, where it surfaces through swamps, lakes

or rivers, and of course from the ocean. Of the water that

soaks into the ground, part of it is retained in the capillary

voids near the surface. Thus it can be said that the poten-

tial sources of water for society consist of wells, which are

drilled or dug down to the groundwater table and withdraw

water from that level; springs, which are natural outcrop-

pings of groundwater table through rocks or ground; rivers,

where the groundwater table has naturally broken through

the ground and flown in a certain direction sufficiently to

gouge out a channel for the water to flow in; lakes, where

large bodies of water gather usually somewhere along a river

system; and finally the ocean, if not other sources are avail-

able and the ocean is close by. The benefit derived from the

costly treatment required to desalinate the ocean under these

circumstances is outweighed by the necessity of having a

fresh water source at any cost.

There are new water sources which exist deep in the earth’s

crust. These sources are rarely considered, due to the high salt

and sulphur content which is frequently found in them.

The recycling of used water of course is a further source

which may be tapped directly. It can be seen from the hydro-

logic cycle that all water is being continually reused, but the

direct recycling of municipal treated sewage into the potable

treatment plant is being considered in some water-scarce

areas.

Some of the advantages and disadvantages which might

be listed for the various sources of water are as follows:

1) Wells provide usually an extremely pure source

of potable water. Rarely is any treatment required

of this water, certainly before it is safe to drink,

although certain industrial uses may require

the removal of some of the soluble salts such

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