where the percentage of COD removal
has been plotted as a function of pH after 60 min of treatment.
This lack of pH influence on removal efficiency indicates that
there is no need for additives for pH control in the investigated
pH range. It can also be observed that a maximum of 45% COD
removal can be expected when the reacting solution is treated
solely by ozone.
Figures 3 and 4 show the color and turbidity reduction in
wastewater as a function of treatment time and pH. It is interesting
to note that color is being reduced as time increases and that the
trends are very similar to those observed for COD removal.
Actually, it is worth mentioning that both, CODand color profiles,
reach a plateau approximately at the same time (25 min). This
behavior and correspondence between COD and color suggests
that the coloring species are stable and are the ones to be blamed
for the lack of further COD removal. Hence, these substances are
probably amines or melanoidins, which have been reported to
hardly be degraded by ozone.26 Besides, both type of compounds
are also likely to be in the mixture due to the type of
industry whose effluent has been the subject of this research. As
observed for COD removal, there is no evidence of color
removal dependence on pH.