Environmental Engineering Reference
In-Depth Information
Ozonation removed up to 54% of influent TKN (at the highest ozone dose), which exceeded
the proportion of COD removal. There was a statistically significant difference in TKN
removal between Treatments 1 and 3, suggesting that TKN removal rate depended on ozone
dose. Nitrogen-containing compounds are more prone to ozone-mediated destabilization
than many other organics, facilitating bonding with opposite electrical charges
(Razumovskii & Zaikov, 1984). In this case, N-containing compounds likely bonded directly
to charges at the surface of gas bubbles (fractionation effect) or with polyvalent ions, and
subsequently were removed with foam. Some of these molecules also were mineralized,
which is reflected in the ammonia increase during ozonation. In turn, NO
3
-
-N rose slightly
during ozonation as some ammonia was oxidized further due to favorable conditions in an
alkaline environment and pH above 8 (Lin & Wu 1996). Our results showed a generally
higher TKN removal than other studies. For example, Beltran et al. (2001) found a 26% TKN
removal at ozone doses between 40-60 mg/l on domestic wastewater that had been treated
biologically. Higher TKN removal in our study could be attributed to higher alkalinity in
BRA wastewater and to different composition of organics in the two wastewaters.
In the trickling filter, TKN was reduced by 15-31%. The percent removal did not appear to
depend on the ozone dose applied in the previous treatment step. Although an increase in
ozone dose should promote TKN removal (Beltran et al. 2001), our finding differed,
probably because a great part of TKN was in the form of ammonia after ozonation. In this
circumstance, TKN removal efficiency was rather dependent on the nitrification
performance of the trickling filter.
TKN removal by chemical flocculation ranged from 77-79%. Comparing the average of 1.7-2.0
mg/l for TKN after chemical flocculation to an average of 1.5-1.7 mg/l for TAN, it is clear that
the organic component of TKN was almost entirely removed by the treatment train.
3.6 TAN
Table 8 shows TAN dynamics through the treatment train. The average influent TAN
concentration ranged between 2.53 and 2.58 mg/l in all experimental treatments. These
values were higher than the average of 2.06 mg/l in the recirculating aquaculture systems,
with the increase likely due to bacterial activity in the storage tanks and sedimentation
basin. Ammonia is utilized preferentially as a nitrogen source by heterotrophic bacteria
(Grady et al. 1999), explaining the 48-50% reduction of TAN as the stream underwent
denitrification.
During ozonation, TAN concentration rose higher than influent levels by a treatment
average of 29-40%. These TAN concentrations were over twice those in the denitrification
reactor effluent in Treatments 2 and 3. The increase of TAN concentration during ozonation
exhibited a positive, linear relationship with ozone dose (slope = 0.012;
r
2
= 0.93). The
increase of TAN probably was due to amino acid and protein oxidation by ozone. Ammonia
is a byproduct of these reactions, especially when they are complete (i.e., mineralization).
The basic pH of the ozonation reactor influent appeared to promote partial oxidation of
ammonia to nitrate, because NO
3
-
-N increased by more than expected from influent NO
2
-
-N
oxidation. However, the oxidation reaction was insignificant and ammonia accumulation
predominated. Rosenthal & Otte (1979) and Wang & Pai (2001) also reported partial
oxidation of ammonia to NO
3
—
N during ozonation under alkaline conditions, with TAN
accumulating via oxidation of organic nitrogen.
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