Environmental Engineering Reference
In-Depth Information
Y
COD
was obtained by the equation:
g
Y
=×
1.42
Y
(3)
COD
VSS
g
Additionally,
Y
NO3-
-N was determined from data collected from the denitrification reactor
using Equation 2.
3. Results and discussion
3.1 BRA waste stream characterization
Characteristics of BRA aquaculture effluent regarding alkalinity, pH, hardness, and
nitrogenous compounds are presented in Table 3.
Parameter
Average
1
Minimum
Maximum
Alkalinity (mg/l)
118
76
141
Hardness (mg/l)
150
139
170
pH
7.21
6.95
7.62
TKN (mg/l)
31.5
5.11
47.36
TAN (mg/l)
2.55
1.72
4.11
NO
3
-
-N (mg/l)
42.98
6.8
68.8
NO
2
-
-N (mg/l)
0.91
0.45
1.93
TAN (mg/l)
2.55
1.72
4.11
1
Average values represent non-flow-weighted averages (12% for samples from 2:00 p.m. and 44% each
for the other times).
Table 3. Waste stream characteristics for BRA effluent collected on different days at 6:00
a.m., 2:00 p.m. and 10:00 p.m. each day.
3.2 Alkalinity
Alkalinity is managed at BRA by addition of industrial-grade sodium bicarbonate, NaHCO
3
,
which buffers pH and replaces alkalinity lost by nitrification and water exchange. Data from
operation of the pilot station (Table 4) showed that alkalinity lost by water exchange was
recovered and, further, that treated effluent was enriched by up to one-third of the initial
amount of alkalinity. Alkalinity was generated in the denitrification reactor, increasing by
approximately 40% in all experimental treatments. Despite the significant increase in
percentage terms, the net production represented only about 2 mg alkalinity per mg of NO
3
-
-N reduced. Because the stoichiometry of denitrification reactions would lead us to expect a
ratio of about 3.6, we infer that some NO
3
-
-N must have been transformed into ammonia by
assimilative reduction and used in cell synthesis when ammonia was lacking (Grady et al.
1999). The inference was supported by the observation that the wastewater treated was low
in TAN, with a ratio of TAN: NO
3
-
-N of about 1:20.
The stream entering the ozonation reactor had an alkalinity of 175.5-187.5 mg/l. During
ozonation, between 6-12% of alkalinity was lost. Loss of alkalinity could be due to the
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