Page 46 - Industrial Wastewater Treatment, Recycling and Reuse

Industrial Wastewater Treatment, Recycling, and Reuse: An Overview 29

In most cases coagulation has been effective in removing color, especially
from wastewaters containing dissolved solids and charged matter. However,
high chemical dosages are usually required, and large volumes of sludge must
be disposed of, in general, for inorganic coagulants, resulting in high cost of
sludge disposal. Alternatively, organic coagulants such as polydiallyldimethy-
lammonium chloride can be used and are known to enhance coagulation effi-
ciencies in some cases. A near-zero production of sludge in the case of organic
coagulants almost eliminates sludge disposal problems and significantly
reducestreatmentcosts.Thus,theformulationofinorganicandorganiccoag-
ulants can provide a better techno-economically feasible operation in waste-
water treatment. Typically, inorganic coagulants are aluminum sulfate,
aluminum chloride, ferric chloride/sulfate, calcium/magnesium oxide, and
PAC. In general, inorganic coagulants produce smaller and lighter flocs that
require more time to settle. This is reflected in the sludge volume, which is
always greater with inorganic coagulants. Another disadvantage with most
inorganic coagulants is that they are pH sensitive and therefore work only
in a narrow pH range. Some of the disadvantages of common inorganic coag-
ulants can be eliminated with the use of organic coagulants or formulations
of both inorganic and organic coagulants.
It is to be noted that coagulation alone is not generally a complete solu-
tion to wastewater treatment problems. It is again a charge-based separation
method that works on the principle of charge neutralization. However, in
the floc formation process, some of the uncharged particles and organics get
physically trapped and removed, consequently improving the process per-
formance. Recent trends in this area involve development of suitable coag-
ulant formulations for specific cases, development of coagulant aid/
flocculant aid, and newer polymeric coagulants with varying basicity for
wider applicability.

1.4.2 Adsorption
Solid adsorbents have been used since ancient times for various separation/
purification applications. Today, they find applications in drinking water
treatment, the food industry, the pharmaceutical industry, the chemical
and petrochemical industries, and wastewater treatment.
In the adsorption process, adsorption of organics/inorganics can take
place on the surface of the adsorbent, thereby effecting removal of pollut-
ants from the wastewaters. Adsorption in wastewater treatment is typically
considered as a physico-chemical process involving selective attachment of