Page 45 - Industrial Wastewater Treatment, Recycling and Reuse

28 Industrial Wastewater Treatment, Recycling, and Reuse

their place in the industry. Thus, the difference between the physico-chemical
methods and biological methodsin terms of usageand interest was significantly
large. With improved biological processes and operations due to efficient
microorganisms and better understanding, the use of biological processes has
increasedsubstantiallyinindustrialwastewatertreatmentandcanbeconsidered
stable atpresent. However, although applicationofphysico-chemical methods
decreased corresponding to the increase in biological processes in the 1980s,
recent trends again show an upward rise in the use of physico-chemical
methods. This can be attributed mainly to the rise of refractory pollutants in
wastewaters that are difficult to degrade using biological methods.
With the addition of newer methodologies, such as cavitation, and also
with the development of newer materials for membranes (composite mem-
branes), adsorbents, ion exchange resins, and so on, and also in view of more
refractory chemicals being found in wastewaters that are difficult to degrade
biologically, the physico-chemical methods are likely to retain their high
position in the area of industrial wastewater treatment, recycling, and reuse.
The commonly used treatment methods are briefly discussed in the follow-
ing section.

1.4.1 Coagulation/Flocculation

Coagulation is a process in which destabilization of colloidal particles present
in the fluid is achieved by addition of salts, which reduce, neutralize, or
invert the electrical repulsion between particles. Coagulants can be broadly
classified as inorganic and organic. Coagulation is one of the most com-
monly employed methods in effluent treatment. However, use of coagulants
in wastewater treatment containing refractory pollutants is a complex prob-
lem, and no general solutions are available yet.

1.4.1.1 Commonly Used Coagulants
Inorganic
Aluminum salts (alum)
(Al 2 (SO 4 ) 3 14H 2 Oor Al 2 (SO 4 ) 3 18H 2 O (alum))
Ferric and ferrous salts
(FeCl 3 ,Fe 2 (SO 4 ) 3 , FeSO 4 7H 2 O)
Lime (Ca(OH) 2 )
Organic
Cationic polymers
Anionic and non-ionic polymers