Encyclopedia of Environmental Science and Engineering, Volume I and II

1304 WATER: PROPERTIES, STRUCTURE, AND OCCURRENCE IN NATURE

ecosystem, that is the smaller the number of species (less diver-

sity), the more liable it is to perturbations and to catastrophe.

Most of the conscious and reasonably successful efforts

of environmental and sanitary engineers have been and may

well continue to be directed towards the removal of organic

carbon of biological wastes. For many natural waters, how-

ever, other pollutants are ecologically more harmful and may

become even more so in the future. As shown earlier, inor-

ganic fertilizing elements are often more serious pollutants

than biological wastes for most of our lakes, estuaries, and

littoral waters. While biological wastes and inorganic fertiliz-

ers cannot yet cause substantial alterations (besides local and

temporal transients) in the composition and biology of the

oceans, the invasion of the seas by man-made chemicals (e.g.,

persistent petrochemicals) may cause substantial modifica-

tion of ecological interrelations in the marine environment.

Conventional biological waste treatment expedites the

breakdown of those organic compounds that would eventu-

ally be degraded in receiving waters whose self-purification

capacities are not overloaded. Such treatment is relatively

inefficient with regard to many synthetic organic chemicals.

Thus, for many inland waters more emphasis upon the elimi-

nation of refractory carbons is necessary.

The inability of some rivers and lakes to readjust rapidly

to their initial conditions after even minor shock loadings

(e.g., of pesticides or excess quantities of organic wastes)

is frequently indicative of a loss of diversity (heterogeneity)

in the aquatic biological communities. While many refrac-

tory chemicals, at the low concentrations encountered, do

not impair the dissolved oxygen balance and do not interfere

with the apparent health and survival of fish, they may pre-

vent some fish from returning to their streams of birth for

breeding. Often an environmental stress and a decrease in

community diversity is signaled by the disappearance of a

species, for example, a salmon in a particular stream.

Because of the continuous increase in industrial energy

generation, heat will become one of the most significant

water pollutants of the future. Woodwell (1969), has shown

that many types of environmental disturbances (chronic irra-

diations, exposure to SO 2 , pollution by pesticides, hydro-

carbons, heavy metals, excess nutrients) cause similar and

roughly predictable gross changes in ecosystems. In each

case there is a shift away from complex arrangements of spe-

cialized species toward monotone, away from tight nutrient

cycles toward loose ones with terrestrial systems becoming

overloaded, away from stability toward instability.

Global Aspects of Water Pollution

A comparison of the fluxes of materials resulting from geo-

logic processes and weathering with those of substances

introduced by civilization (e.g., agricultural and industrial

wastes) shows the relative importance of the role of man as a

geological agent and manipulator.

The energy fixed by all plants on the surface of the

earth is approximately 8 
 10 17 kilocalories per year. Man’s

metabolism (3 
 10 15 kilocalories per year) plays a minor

role in the physiology of the biosphere. Man in his capacity to

manipulate nature dissipates from 10 to 20 times (in devel-

oped countries, 50 to 100 times) as much energy as he

requires for his metabolism. Much of the energy utilized by

our industrialized society causes localized simplification of

the ecosystem and tends to accelerate the cycling of individ-

ual elements.

Some of the fluxes caused by man’s manipulation are of

similar magnitude to those which do not involve man. The

combustion of fossil fuels may introduce some elements

into the atmosphere and subsequently into the hydrosphere

at rates comparable to those of natural processes.

Strong Acids in Rainwater

Rainwaters often contain strong acids. These acids are

formed from atmospheric pollutants (HCl, HNO 3 , H 2 SO 4 )

(Figure 14).

Chemical Management of Water and Waste

Water Quality

Assigning a level of quality to water is relative and depends

on the user of the water. For example, different quality crite-

ria apply to drinking waters, to waste water effluents and to

receiving waters. Table 8 summarizes some of the objectives of

chemical unit processes typically used in water and waste treat-

ment. Water treatment schemes often include several of these

processes since generally no single one can achieve the desired

water quality objectives. Pertinent chemical reactions in the

management of water and waste water are listed in Table 9.

Precipitation reactions are used to remove objectionable

ionic species from solution. Hard water containing calcium

and magnesium ions, which form insoluble films with soap,

is softened by precipitating the ions as calcium carbonate

and magnesium hydroxide. Iron and manganese which cause

undesired turbidity and stain surface are removed by oxida-

tion and precipitation. Phosphate, a critical aquatic nutrient,

can be precipitated as aluminium, iron or calcium solids.

Ion exchange involves the interchange of ions between

a solid polymer resin and solution and is used in water treat-

ment for softening and demineralization.

Coagulation is a physico-chemical process involving the

agglomeration of colloidal particles for subsequent removal

by sedimentation and filtration. The addition of chemical

coagulants enhances the mutual attachment of colloidal par-

ticles by reducing the repulsive forces acting between these

particles (destabilization). In water and waste water treatment

practice the hydrolyzed metal ions of Al and Fe(III) salts are

commonly used as coagulants. Initial particle contact due to

thermal motion (perikinetic coagulation) is normally aug-

mented by the introduction of velocity gradients in solution

by slow mixing (orthokinetic coagulation).

The coagulation process may be aided by the addition of

macromolecular polymers (polyelectrolytes) which bridge

dispersed colloidal particles. Such destabilization is fre-

quently called flocculation.

Gaseous chlorine is one of the economically most useful

disinfectants for municipal and industrial waters. During

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