Environmental Engineering Reference
In-Depth Information
SB
DR
OR
TF
Total removal
3
Treatment
mg/l
mg/l (%)
mg/l (%)
mg/l (%)
2
(%)
1
0.45 (+55.6)
a
44.4
a
0.96
0.23 (76.0)
0.00 (24.0)
2
0.54 (+52.9)
a
47.1
a
3
0.61 (+58.7)
a
41.3
a
0.92
0.26 (71.7)
0.00 (28.3)
4
0.51 (+63.8)
a
36.2
a
1
Abbreviations: SB = sedimentation basin, TF = trickling filter, DR = denitrification reactor, and OR =
ozonation reactor.
2
Percentages for the trickling filter express the mass generated as a fraction of the treatment train
influent concentrations.
3
Total percent decrease of NO
2
-
-N concentration relative to initial TAN concentration in the
sedimentation basin.
Table 9. Nitrite-nitrogen (NO
2
-
-N) dynamics through the pilot plant
1
for all treatments (unit
outlet values). Means in a column with the same superscript are not significantly different
(
p
>0.05).
denitrification reactor as a positive outcome from application of excess methanol.
Although the excess methanol could impose a higher dissolved organics load on the
ozone reactor, this outcome may be preferred to poorer removal of nitrogenous
compounds.
The remaining NO
2
-
-N then was oxidized totally to NO
3
-
-N in the ozone reactor, regardless
of the ozone dose applied. Rosenthal & Otte (1979) also found that even with light
ozonation, NO
2
-
-N in aquaculture wastewaters can be oxidized efficiently to NO
3
-
-N.
Although the stream entering the trickling filter had little or no NO
2
-
-N, the effluent had an
average of 0.45-0.61 mg/l NO
2
-
-N, varying among experimental treatments. This
concentration represented 53-64% of the treatment train influent concentration. The
generation of NO
2
-
-N in the trickling filter probably was due to incomplete nitrification of
ammonia. One of the causes could be the lack of NO
2
-
-N itself as substrate in the influent,
which did not support the growth of bacteria converting NO
2
-
-N to NO
3
-
-N (i.e.,
Nitrobacter
sp
.). Summerfelt (2003) suggested that lack of these species in nitrification biofilters can be a
drawback of integrating an ozonation step in a treatment loop in a recirculating aquaculture
system, although the decrease of nitrite levels is a substantial benefit. Another cause of
nitrite generation could be suppressed growth of
Nitrobacter sp
. by faster-growing
heterotrophs under conditions of abundant of organic material (Parker & Richards 1986).
Nitrobacter sp
. are the slowest-growing nitrifiers and are the first to be eliminated by
heterotrophs in a biofilter when competing for space (Grady et al. 1999). Considering nitrite
and organic concentrations coming into the trickling filter in our study, both mechanisms
appear plausible explanations for nitrite accumulation.
The presence of nitrite is undesirable in waters used for exchange in recirculating
aquaculture systems because of its toxicity to fish, although the concentrations in our final
effluent did not present a threat to fish. Further, the rotating biological contactors in the BRA
fish production systems would be able to remove the amounts of NO
2
-
-N returned with
exchange water.
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