Page 141 - Industrial Wastewater Treatment, Recycling and Reuse

Advanced Physico-chemical Methods of Treatment for Industrial Wastewaters 115

ultrafiltration (pressure up to 10 atm, can remove organics of high molecular
weight), nanofiltration (pressures up to 20 atm, can remove many divalent/
polyvalent ions and organics of medium molecular weight), and finally
reverse osmosis (pressures as high as 50–70 atm, can remove practically all
pollutant species, yielding pure water). Thus, depending on the separation
requirement, a desirable membrane process is selected. Because reverse
osmosis can give practically pure water, it finds maximum use in the area
of desalination and drinking water and is less preferred in wastewater treat-
ment due to its high cost. For all practical purposes, industrial wastewater
treatment mostly uses microfiltration and ultrafiltration, while nanofiltration
is an emerging area in this field.
Although the conventional membrane separation principle indicates a
purely physical separation mechanism, many new developments in this area
have membranes suitably modified to enhance the performance in terms of
flux, reduced fouling, and higher selectivity. In this sense, membrane sepa-
rations are transformed as physico-chemical separation processes. Ion
exchange membranes and modified membrane forms such as facilitated
transport membranes represent this transformation from physical separation
to physico-chemical separation. The charge effects, apart from physical sep-
aration, have a direct impact on equilibria and transport in these membranes
and can be attributed to the ionization of functional groups present in the
membrane and/or to the adsorption of ions from solution when the mem-
brane is immersed (Mafe ´ et al., 1993). The research on modification of
membranes has been an important theme in the last few decades, and appli-
cations of ion exchange membranes for acid recovery and metal removal
from various wastewaters (e.g., pickling wastewater) have been highlighted
for a long time (Sata, 1991; Muthukrishnan and Guha, 2006; Navarro et al.,
2008; Wang et al., 2008). A modification of commercial composite mem-
brane by deposition of acrylic acid was shown to yield COD removal of 84%
ascompared to 67% before the modification (Cho and Ekengren, 1993).
However, whether physical or physico-chemical, membranes have occu-
pied a niche area in the world of wastewater treatment, and their role will
be increasingly important in future wastewater treatment technologies,
especially through process integration and intensification such as membrane
bioreactors (MBRs), membrane distillation, dialysis/electrodialysis, ELM
separation systems, and so on. A number of polymeric membranes have been
studied for the treatment of alcohol industry wastewaters, and nanofiltration
polyterphthalate NF45 membrane has shown high ability to remove organics
(98%) (Madaeni and Mansourpanah, 2006). A study on multiple membrane