Encyclopedia of Environmental Science and Engineering, Volume I and II

WATER: PROPERTIES, STRUCTURE, AND OCCURRENCE IN NATURE 1305

the chlorination process diatomic chlorine gas hydrolyses in

solution to form hypochlorous acid and hydrochloric acid.

Hypochlorous acid is a stronger disinfectant in its undissoci-

ated form (HOCI). Aqueous chlorine, a powerful oxidizing

agent, will react with reduced compounds by the formation

of chloramines by reaction with aqueous ammonia. (Also see

Water Chemistry.)

Water pollution control and improvement of water qual-

ity cannot solely depend on water purification and waste

treatment. Various physical and biological means of stream

management must be directed towards restoring an ecological

balance. Because unlimited growth in energy dissipation is

incompatible with maintenance of ecological stability and

high quality of life, the ecological constraints demand the

GENESIS OF AN ACID RAINWATER

Strong acids from atmospheric pollutants interact in the atmosphere with bases

HCI HNO 3 H 2 SO 4

MgCO 3

(s) (s) NH^3

NaCl+

KCI

CaSO 4 S 1 O 2

(s) Al–Silicate

+

+

=

CaCO 3

ACIDS

introduced into atmosphere

BASES

introduced into atmosphere

DUST

(aerosols)

RAINWATER (dissolved components)

Cl– NO 3 – SO2– 4

H+ (strong acids) Na

+

K+

Mg2+ Ca2+ NH 4 +

pH = 4.3

Scale 10 20 30 40 50 micromol charges/l

1mg/l sulfuric acid corresponds 20 micromol charges/1

ANIONS

CATIONS incl H+

(strong acids)

+

FIGURE 14 Strong acids in rainwater. The acid-base reaction involved in the genesis of a typical acid rainwater. Acids formed from

atmospheric pollutant react in the atmosphere with bases and dust particles. The resulting rainwater contains an excess of strong acids. H 2 SO 4

originates mostly from S in fossil fuels; after combustion the SO 2 formed is oxidised to SO 3 which gives, with H 2 O, H 2 SO 4 ; HNO 3 originates

from NO and NO 2. These molecules are formed in the combustion of fossil fuels and to a large extent in the combustion of the automobile

engine. For each molecule of NO one of HNO 3 is formed, for example, NO  O 3 →NO 2  O 2 ; 3NO 3  H 2 O→2HNO 3  NO. HCl may

largely originate from the combustion of Cl-bearing polymers, for example, polyvinyl chloride, in refuse incinerations. Most bases in the

atmosphere are often of natural origin. Atmospheric dust may contain carbonates (calcite and dolomite). NH 3 is released from many soils

(together with urea). (From Stumm and Morgan, 1981.)

TABLE 8

Chemical unit processes in water and waste water treatment

Substances to be removed Precipitation

Filtration

(chemically aided)

Coagulation

flocculation

Disinfection by

chlorination Ion exchange

Activated carbon

adsorption Aeration

Algae        

Bacteria       

Colloids turbidity        

Natural color       

Odor, taste    

Organic macromolecules      

Organic solutes   

Ca^2 , Mg^2        

Phosphate    

Inorganic nitrogen

compounds



Heavy metal ions      

Dissolved Fe, Mn        

Effectiveness: + + + very effective, + + effective, + marginally effective, − ineffective.

C023_004_r03.indd 1305C023_004_r03.indd 1305 11/18/2005 11:12:34 AM11/18/2005 11:12:34 AM