Page 210 - Industrial Wastewater Treatment, Recycling and Reuse

184 Industrial Wastewater Treatment, Recycling, and Reuse

not aiding in BOD enhancement significantly. At higher pressure (13 bar),
the ratio is enhanced to a value of 0.29 at zero dilution and to 0.32 at a spent
wash concentration of 25%. As compared to lower pressure, higher pressure
yields a slightly better BOD/COD ratio on treatment. From the obtained
results, it can be inferred that lower inlet pressure (5 bar) is suitable for a
reduction in toxicity (COD/TOC reduction), whereas for enhanced biode-
gradability (higher BI), higher inlet pressure operation would be preferred.
Thus, it is shown that HC is capable of reducing the toxicity of distillery
wastewater, and pretreatment increases the biodegradability of the B-
DWW. Hence, depending on the final objective of the pretreatment,
HC can be effectively utilized for the treatment of complex wastewater
pollutants such as B-DWW.
The biodegradability of the cavitationally pretreated B-DWW was
further evaluated by subjecting it to the conventional biological treatment
process. The amount of methane generated and reduction in COD was
compared for the cavitationally pretreated B-DWW as against the untreated
B-DWW. It was observed that in the cavitationally pretreated B-DWW
(13 bar, 25% dilution, 50 min, BI: 0.32), 400 mL of gas volume was gener-
ated after a total duration of 40 days (including the lag period of 6 days),
along with a net 70% COD reduction, whereas in the untreated system
(BI: 0.168), the gas volume was observed to be only 60 mL with around
12% COD reduction under similar experimental conditions.
Hence, it can be concluded that HC is capable of enhancing the effi-
ciency of conventional biological processes in terms of a reduction in the
toxicity as well as an increase in biogas generation, along with a significantly
higher reduction in COD and color. Due to HC pretreatment, the effi-
ciency of the conventional biological process is increased by almost sixfold
in terms of COD removal and biogas formation.

3.7 SUMMARY
Different AOPs such as cavitation (acoustic and hydrodynamic), Fenton,
photocatalysis, and other combined processes have been tested on the lab-
oratory scale and proven to be effective in degrading various organic pollut-
ants that are biorefractory in nature. The efficacy of these processes depends
strongly on the rate of generation of free radicals along with the extent of
contact of the generated radicals and/or chemical oxidants with the contam-
inant molecules. An efficient design should aim at maximizing both these
attributes of the process. Although highly successful on a laboratory scale,