Environmental Engineering Reference
In-Depth Information
3.8 NO
3
-N
Nitrate was the most abundant nitrogenous waster in BRA effluent, resulting from nitrification
and accumulation in the system. Treatment averages were between 42.8 and 43.2 mg/l NO
3
-
-N
in the influent (Table 10), although large diurnal variations were observed due to different
representations of wastewaters from greenhouses and grow-out systems within the overall
BRA operation. On average, the denitrification reactor removed 96-97% of NO
3
-
-N among
experimental treatments, suggesting that the biofilter adapted rapidly to nitrate fluctuations.
This observation agrees with Jeris & Owens' (1975) suggestion that under conditions of nitrate
variation, it is sufficient to supply the right amount of carbon source at any time in order to
obtain satisfactory denitrification. Nitrate removal performance appeared to be independent of
the recycled stream fraction among different treatments.
SB
DR
TF
Total removal
2
Treatment
mg/l
mg/l (%)
mg/l
(%)
1
2.6
a
94.0
43.2
1.8 (95.8)
a
2
2.7
a
93.8
3
2.0
a
95.4
42.8
1.3 (97.0)
a
4
2.4
a
94.4
1
Abbreviations: SB = sedimentation basin, TF = trickling filter, and DR = denitrification reactor.
2
Total percent decrease of NO
3
-
-N concentration relative to initial TAN concentration in the
sedimentation basin.
Table 10. Nitrate-nitrogen (NO
3
-
-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).
The nitrate removal efficiency of the treatment train overall was slightly lower than that of
the denitrification reactor unit, as NO
3
-
-N was produced during ozonation and by
nitrification in the trickling filter. However, the final concentration was generally less than 3
mg/l, posing no issue for reuse of the recovered wastewater for fish production.
3.9 Cell yields and the effect of nitrate fed on denitrification
From tests on the batch reactor, cell yield,
Y
NO3
-N, was 0.69 g VSS cells produced/g NO
3
-
-N
consumed. Our
Y
b
value agrees with those reported from tests with similar NO
3
-
-N
concentrations (Table 11). However, Moore & Schroeder (1971) showed that under steady-
state flow-through conditions and NO
3
-
-N feed variation,
Y
NO3
-N decreases linearly with
increasing NO
3
-
-N concentration to about 35 mg/l NO
3
-
-N, and remains constant thereafter.
They attributed this relationship to a saturation effect, because some species of bacteria
synthesize polysaccharide storage materials under nitrogen-limited conditions.
Consequently, the process slows as more NO
3
-
-N is utilized, resulting in
Y
NO3-N
decreasing
until NO
3
-
-N reaches the saturation level. Above 35 mg/l NO
3
-
-N,
Y
NO3-N
was found to be
around 0.60 g VSS cells produced/g NO
3
-
-N consumed (Moore & Schroeder, 1971), which is
similar to the value we found. Hence, we infer that tests in our study were conducted under
saturation conditions. Indeed, batch tests started from a NO
3
-
-N concentration of 207 mg/l,
and were interrupted close to the putative saturation limit.
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