Environmental Engineering Reference
In-Depth Information
SB
DR
OR
TF
Total increase 2
Treatment
mg/l
mg/l (%)
mg/l (%)
mg/l (%)
(%)
1
175.5 (-6.6) a
163.6 (-6.8) a
30.0 a
114.5
192.1 (+40.4) a
2
179.9 (-6.4) a
169.3 (-5.9) a
32.4 a
3
187.5 (-7.1) a
179.5 (-4.2) a
32.2 a
121.7
201.8 (+39.7) a
4
177.8 (-11.9) a
164.3 (-7.5) a
26.0 a
1 Abbreviations: SB = sedimentation basin, DR = denitrification reactor, OR = ozonation reactor, and TF
= trickling filter.
2 Alkalinity increase between sedimentation basin and trickling filter effluent concentrations.
Table 4. Dynamics of alkalinity through the pilot plant for all treatments (unit outlet values) 1 .
Means in a column with the same superscript are not significantly different ( p >0.05).
scavenging effect of its ions on ozone. HCO 3 - and CO 3 2- ions compete in wastewater with
organic matter for reaction with the OH o radical, and high alkalinity can impair the reaction
of ozone with targeted organics (Wang & Pai 2001); alkalinity depletion increases with
ozone dose due to increased probability of OH o radical formation due to faster organics
removal. The relatively low removal of alkalinity that we observed may have been limited
by the pH of slightly over 8.0, not high enough for carbonate ion formation, and with ozone
being less reactive with bicarbonate ion. Buxton et al. (1988) found the reaction rate
constants for reaction of hydroxyl ion to be 39x10 7 l/mol-s for CO 3 -2 ions and 0.85x10 7 l/mol-
s for HCO 3 .
More alkalinity was removed in the tricking filter, probably due to nitrification. This
observation was not surprising, because stoichiometrically 1g of TAN can destroy 8.62 g of
HCO 3 - during oxidation to NO 3 - -N (Grady et al., 1999). However, because TAN
concentration was relatively low and because the nitrification was not complete (i.e., NO 2 - -N
was produced in the biofilter), the final effluent still had 26.0 - 32.6% more alkalinity than
the stream entering the wastewater treatment train. Hence, reuse of this treated effluent
could result in savings regarding supplemental alkalinity addition to the aquaculture
system.
3.3 pH
The pH of the aquaculture effluent was neutral or slightly basic (Table 5), close to that of
water in the fish production tanks. Neutral pH in wastewater can be due to the presence of
inorganic salts (Millamena 1992) or to the heterogeneous composition of its organic matter
(Medley & Stover 1983); BRA effluent exhibited both of these characteristics. After entering
the treatment train, pH increased slightly in the storage tanks and settling basin, and then
increased more significantly during denitrification, reaching values between 8.22 and 8.26 in
denitrification reactor effluent. The pH increase was probably due to intense biological
activity in these units, especially in the denitrification reactor, where pH increase was
promoted by alkalinity generation. During ozonation, pH decreased, probably because
some alkalinity was lost to attack by ozone-derived radicals. Kirk et al. (1975) found that
whether ozonation feed water is acidic or basic, the product water always shifts toward
neutrality, and that the pH change is greater for higher-COD feedwaters. Further, Wang &
Pai (2001) suggested that the greatest organics removal by ozonation is obtained at low pH,
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