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Phenolic Wastewater Treatment: Development and Applications of New Adsorbent Materials 327

remove 4-NP (Yi et al., 2006). Typically, phenols are biodegradable pro-
vided that the concentration is below the toxic level. Chlorinated phenols
are not amenable to biodegradation except at low temperatures or specific
conditions. pH can have a significant and adverse impact on the process. The
biological method of wastewater treatment is generally adopted at concen-
1
tration levels in the range 5–500 mg L .

8.1.1.7 Reverse Osmosis
Reverse osmosis in the form of membrane technology is being used for
the large-scale reclamation of wastewater, although the technique depends
on proper pretreatment, chemical control, and use of reverse osmosis mem-
branes that are resistant to fouling. It is also an energy-intensive operation.
Other disadvantages include risk of bacterial contamination of the mem-
brane, cost of pretreatment of the wastewater to avoid membrane fouling,
and sensitivity of the membrane toward chemicals.

8.1.1.8 Solvent Extraction
This method involves separation of the organic phase using solvents that are
immiscible with water. Phenol is then extracted by treating the organic layer
with alkali or distilling the phenol or the solvent in case there is a large
difference between the boiling temperatures of phenol and the extracting
agent. Some of the organic solvents that have been used include n-hexane
and cyclohexane, benzene, toluene, ethyl benzene, cumene, acetate esters,
and methyl-isobutyl ketone (Greminger et al., 1982). Because of the forma-
tion of a third phase, solvent extraction is accompanied by solvent loss in
wastewater that inevitably creates secondary pollution (Shen et al., 2006;
Yang et al., 2006). This secondary contamination of wastewater with sol-
vents is the main drawback of this method over and above the requirement
of high energy input and high operating cost. This method is also less
efficient for the removal of trace levels of pollutants (Goto et al., 1984).

8.1.1.9 Adsorption
Adsorption is more suitable for treatment of phenolic wastewaters, especially
in view of low concentrations and superior process efficiencies compared to
many of the conventional treatment methodologies. The advantages of the
adsorption technique over several conventional methods of wastewater
treatment include:
1. Less space requirement, particularly in comparison to biological treatment.
2. Flexibility in design and operation.