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Advanced Physico-chemical Methods of Treatment for Industrial Wastewaters 117

The process calculations for membrane separations are discussed widely in
the literature and in standard textbooks (see, e.g., Noble and Stern,
1995). From a practical application point of view, the most important
parameters include membrane selection and type, trans-membrane pressure
drop, operating flux, and fouling characteristics for getting desired water
quality at the required rate. Fouling of membranes is a serious problem in
wastewater treatment and dictates pretreatment needs, separation behavior,
and the cost of operation apart from impacting the engineering design of the
system.

2.4.1.1 Membrane Variants in Wastewater Treatment
There are some membrane separation types that differ significantly from the
conventional membrane separations. These mainly include:
• Membrane distillation: This is similar to conventional distillation. The
main difference is that the feed and the product are separated by a porous
hydrophobic membrane, either at different temperatures or at different
compositions. Because of the vapor pressure difference, water evaporates
on one surface of the membrane and passes through the membrane in the
form of vapor and gets condensed on the other side of the membrane.
Membrane distillation is commercially used for recovery of hydrochloric
acid from waste pickling liquors containing sulfuric acid and other
ionic species such as chromium, nickel, cobalt, and zinc (Lawson and
Lloyd, 1997).
• Pervaporation: This is an energy-efficient combination of membrane per-
meation and evaporation. It is commonly used for removal of organics
from aqueous streams.
• ELM separation systems: These comprise an emerging separation technol-
ogy with potential applications in wastewater treatment. Chemicals that
can be removed/recovered from industrial streams using ELM separation
systems are organic acids, phenols, cresols, and amines, as well as metallic
ions such as lead, copper, cadmium, and mercury (Lee and Hyun, 2010).
• Membrane contactors: These are used in solvent extraction. Here, the
membrane acts as a barrier between the feed and the stripping solution.
• MBRs: These may be considered to involve a physico-biological process
and not a physico-chemical process in the conventional sense (combina-
tion of a bioreactor+ inline membrane separation step). MBRs combine
two familiar technologies: activated sludge and membrane filtration. Sig-
nificant engineering expertise can be applied to MBR design and oper-
ation and principles underlying MBRs are familiar enough to ensure