Page 124 - Industrial Wastewater Treatment, Recycling and Reuse

98 Industrial Wastewater Treatment, Recycling, and Reuse

8. Ease of regeneration, important for techno-economic feasibility.
9. Low cost is most crucial aspect in wastewater treatment, dictating con-
stant search for newer materials.
Activated carbons are probably the most used adsorbent materials, derived
from a range of raw materials, available in variety of forms ranging from
powdered to nanofiber type, and sold as inexpensive materials to highly
expensive adsorbents depending on the type. They are most widely used
for adsorption of organic pollutants, metal removal, and for color removal
applications. It is reported that the adsorption capacity of carbon per cycle
usually ranges from 0.25 to 0.87 kg COD removed per kg of carbon applied
(Guyer, 2011). Regeneration is usually carried out using thermal, steam, or
solvent extraction, acid or base treatment, or chemical oxidation. For
adsorptive wastewater treatment, thermal regeneration is the preferred
route. Sometimes, reactivation using high temperatures is required, increas-
ing cost of treatment (usually to reopen pores that are blocked due to adsorp-
tion of pollutant species). Activated carbons can be a very good adsorbent,
especially for adsorption of pollutants from pesticides, herbicides, aromatics,
chlorinated aromatics, phenolics, chlorinated solvents, high-molecular-
weight organics, amines, esters, ethers, alcohols, surfactants, and soluble
organic dyes, practically covering majority of industrial wastewater treat-
ment requirements, although there is some limitation in removal for low
molecular weight or high polarity compounds (Cecen, 2011). Thus, adsorp-
tion finds widespread use in the treatment of wastewaters of most industries,
for example, petrochemicals, chemicals, pesticides, dyes and textiles, phar-
maceuticals, food, and inorganic mineral processing. In view of the wide
range of pollutants that can be removed by activated carbons, adsorption
using these materials is very frequently used as a process in secondary treat-
ment to remove pollutants that are toxic to microorganisms of biological
treatment—a process integration strategy—or as an independent tertiary
treatment process for the removal of pollutants to achieve desired pollution
control limits. The other adsorbents, such as inorganic adsorbents and poly-
meric adsorbents, are also gaining importance. A recent review on inorganic
adsorbents has suggested detailed studies to explore the feasibility of repla-
cing activated carbon with low-cost inorganic adsorbents, such as modified
clays and zeolites for water and wastewater treatment that can be used for
removal of metals and trace organics (Jiang and Ashekuzzaman, 2012).
Recent developments in the area of nanoadsorbents and applications in
the removal of inorganic and organic pollutants have been discussed by
Ali (2012), indicating advantages in terms of low dose and high rate for
nanomaterials as adsorbents. However, both these recent reviews have