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114 Industrial Wastewater Treatment, Recycling, and Reuse

as solvents. The separation of acid, phenol, chlorophenol, and dichlorophe-
nol was reported by various researchers (Anwar et al., 1971a,b, 1979, 1995;
Laddha and Sharma, 1978; Wadekar and Sharma, 1981a,b; Gaikar and
Sharma, 1985; Gaikar et al., 1989; Guy et al., 1994; Malmary et al.,
1995; Modak et al., 1999), and the separation of xylenols was reported by
Coleby (1971). In most studies, solvent selection was found to be crucial.
Jagirdar and Sharma (1980) explored the possibility of the recovery/separa-
tion of various carboxylic acids from dilute aqueous solutions using an
extracting agent such as tri-n-octylamine in various solvents. Extraction
studies in the presence of inorganic acids and inorganic salts for improving
the extraction efficiency have also been reported for the recovery of carbox-
ylic acids (Ingale and Mahajani, 1996). However, in most cases, separation
factors were not large. Solvent extraction, although it appears to be a poten-
tially attractive process for the recovery of acids, still has many unresolved
problems with respect to proper selection of a solvent and solvent recovery
due to high affinity of acids for water.
While methods such as adsorption/ion exchange and solvent extraction
can be used for removal and recovery of acids, destructive methods such as
oxidation and cavitation can be used to eliminate acids from dilute waste-
water solutions. Extreme conditions of cavitation can break down pollutants
and organic molecules to bring COD down to desired levels.

2.4 OTHER ADVANCED PHYSICO-CHEMICAL METHODS
OF TREATMENT
2.4.1 Membrane Separations

Membranes are extensively used in industrial wastewater treatment. Com-
pared to the adsorption and ion exchange processes, membrane separation is
commonly employed as a primary treatment as well as a fine polishing step in
tertiary treatment. In fact, it is one of the processes that has the explicit
capacity to yield pure water that may be recycled and reused.
Conventional membrane processes are pressure-driven processes that
require specific pressures to effect separation, depending on the type of
membrane. However, many a times, it is the combination of two or more
driving forces such as pressure, concentration and electrical potential that
may also be interdependent. Conventional membrane separations are essen-
tially physical processes that employ a sieving mechanism based on the size
of the pores. These physical separation membrane processes are classified
as microfiltration (<2 atm pressure, can separate suspended solids),