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90 Industrial Wastewater Treatment, Recycling, and Reuse

coagulant dosage after the minimum/optimum dosage. In adsorption/
charge neutralization, it is found that excessive dosage results in charge
reversal and a negative effect on process performance. For organic coagu-
lants, the coagulation mainly takes place due to the adsorption/charge neu-
tralization mechanism through interactions of charged/ionogenic sites of
polymers that have capacity to adsorb or cover colloidal pollutant species.
For effective coagulation, coagulant dosage, pH of solution, and mixing
are critical process parameters.
Since a priori selection of coagulant for any particular wastewater is not
possible on the basis of theoretical analysis, it is imperative that selection be
made using an empirical approach through a set of experiments performed
on various coagulants and under different processing conditions. This is con-
ventionally carried out using “jar tests” (Figure 2.3). The jar test simulates
the coagulation/flocculation process in a batch mode, and a series of batch
tests are carried out using different process parameters such as pH, coagulant
type, and dosage (with/without coagulant aid/flocculant) to get the most
suitable performance in terms of coagulant dosage, pH, best settling, sludge
properties (minimum sludge volume), and so on. Finally, an economic anal-
ysis needs to be performed to select suitable coagulant and optimum proces-
sing parameters. Figure 2.3 demonstrates performance of coagulation
(visually) in dye wastewater treatment for different coagulant doses showing
clearly stepwise reduction in color and sludge.

Figure 2.3 Jar test apparatus for coagulation experiments.