Page 41 - Industrial Wastewater Treatment, Recycling and Reuse

24 Industrial Wastewater Treatment, Recycling, and Reuse

considered in primary treatment involve size based separation, by and large,
involving physical driving force for effecting separation. They mainly com-
prise screening, sedimentation, thickening, precipitations, centrifugation,
cyclone separations, and filtration.
In secondary and tertiary treatment stages, more advanced separation pro-
cesses are used with huge variation in the nature of process and equipment.
These processes include evaporation, distillation, absorption, extraction,
adsorption, ion exchange, crystallization, cavitation, biological processes,
and membrane separations. The separation processes employed here can be
classified on the basis of driving force such as thermal driving force (distillation,
evaporation, pervaporation) and pressure-driven processes such as membrane
separation-microfiltration (MF), ultrafiltration (UF), nano-filtration (NF), and
RO or electrical forces such as electrodialysis. Physico-chemical methods
form the most important class of separation processes that play a crucial role
in the area of wastewater treatment. These mainly include processes that
exploit both physical and chemical reactions/interactions for effecting desired
separation. This important class includes a wide variety of processes, such as
coagulation/flocculation, extractions, reactive separations, oxidations, and
cavitation. Separation processes such as adsorption and ion exchange also
come into physico-chemical methods of treatment employing both surface
forces and chemical/electrostatic attraction. Coagulation, adsorption, ion
exchange, and some of membrane separations (ion exchange membranes)
belong to the class of charge-based separations where separation is largely
effected by neutralizing the charges and is specifically applicable for the
removal of charged bodies/ions from the solution. Depending on the nature
of the effluent, one or more separation processes are employed for meeting
end objectives of discharge/water recycling/reuse.
Figure 1.10 shows an analysis of the trends in the application of different
separation processes in wastewater treatment, depicting the number of pub-
lications made over last 40 years. It is evident that biological methods (aerobic
and anaerobic-combined together) top the list, closely followed by individual
physico-chemical methods such as adsorption, oxidation, membrane separa-
tion, coagulation, ion exchange, extraction, and last, cavitation. From the
analysis of the plots, the spectrum can be easily divided into two zones.
Zone I: Corresponds to well-established processes in the area of waste-
water treatment and where there has been continued interest both in terms
of application and in process modification. This class incorporates processes
such as biological processes, adsorption, oxidation, membrane separation
processes, coagulation, and ion exchange.