Page 108 - Industrial Wastewater Treatment, Recycling and Reuse

82 Industrial Wastewater Treatment, Recycling, and Reuse

removing pollutants from the effluent. These processes are sometimes able
to accomplish the desired reduction in the pollutant levels. Occasionally,
adsorption, ion exchange, or other processes also form part of the secondary
treatment, depending on the nature and concentration of the pollutants. The
tertiary treatment mainly involves polishing to remove the final traces of the
pollutants; this typically uses only physico-chemical methods such as adsorp-
tion, ion exchange, and membranes as processing options. The tertiary treat-
ment methods are important from the point of view of water recycling and
reuse because they can achieve the final reusable water quality.

2.1.1 Selection of Method
If the problem of wastewater treatment is well defined, the selection of
physico-chemical or biological methods can be straightforward. For exam-
ple, the nature of the pollutants, their concentrations, and the ratio of
biological oxygen demand (BOD) to COD primarily dictate the selection
strategy. However, many times, the strategy can involve changes in the
manufacturing process itself, consequently redefining the wastewater treat-
ment problem because the nature and concentration of the pollutants are
drastically altered in such cases because of changes in solvent, catalyst, or
raw material. It is therefore instructive to evaluate the overall processing
of the chemical plant (shown schematically in Figure 2.1) in the context
of wastewater treatment.
As seen in Figure 2.1, the central theme of any chemical plant is a reac-
tion process that has inputs in the form of reactants, solvent/catalyst (if any),
and utilities such as cooling water, steam, and inert gases. The central part is
surrounded by separation from all the sides. The reaction process may
require separation in the form of removal of impurities from the raw mate-
rials (pretreatment). After the reaction is over, the product mass again
requires separation to get desired product with required specification apart
from separation required for treatment of gaseous/liquid effluent (post-
treatment). The reaction component of the process therefore deals with
selection of suitable raw materials, reaction process, and reactors; the final
objective is to have a most efficient and environmentally friendly reaction
process. Similarly, the separation component of the process involves selec-
tion of suitable separation methods and equipment with the final objective of
achieving the most efficient and environmentally friendly process. The
separation and reaction parts here are not independent; any change in
the reaction zone automatically alters the separation zone and vice versa.