The seven experiments of BTCW ozonation were performed corresponding
to seven different contact times from 20 min to 80 min.
The optimal ozone dose (0.3 O3(applied)/COD0) was applied with the
nature pH (7.5) in all cases. To obtain excellent ozonation, the feed
flow rate of ozone content was regulated to adapt the duration of
the contact time during each experiment with the optimal ozone
dose. Fig. 4 shows variations of yield of O3(applied), COD removal rate
and BOD5/COD at a series of different contact times.
During the ozonation, the COD removal rate increased in a
greater extent during the first 50 min. For longer times, the slowly
increasing removal percentages were presented. For instance, the
COD removal rates increased from 12.4% for 20 min to 23.8% for
50 min. Although the COD removal rate reached about 27.7% corresponding
to the 80 min, the option was not always optimal during
ozonation experiment. This is because that ozone is primarily used
for the BTCW biodegradability increment instead of mineralization
of organic compounds. Therefore, the 50 min contact time may be
appropriate.
On the other hand, the O3(applied) yield with reaction time also
displayed a decline which was initially slow but quick finally. It
is not hard to find that the yield of O3(applied) (corresponding to
43.7% for 80 min)decays with reaction time and was below 50%
after 70 min reaction which limits the practicability of ozonation