Environmental Engineering Reference
In-Depth Information
Small amounts of hardness were lost between the settling basin and denitrification reactor
effluents, suggesting that it was removed by microscreen filtration after bonding with
dissolved and colloidal organics, or was assimilated during bacterial growth in the
denitrification reactor.
Hardness decreased further during ozonation, with the decrease proportional to the amount
of ozone applied (e.g., 4.1% removal in Treatment 1 and 7.8% removal in Treatment 3, a
statistically significant difference). Ozone-induced hardness removal was probably because
increased carboxyl acid concentration due to ozonation led to greater magnesium and
calcium association, resulting in precipitation of metal-humate complexes (Grasso & Weber
1988). Higher hardness removal at higher ozone doses was attributable to more destabilized
carboxyl acids binding with a larger amount of hardness ion species. Noting that increasing
hardness to 150 mg/L CaCO
3
in ozonated water improved removal of TSS (Rueter &
Johnson 1995), BRA effluent exhibited favorable conditions for TSS removal. After being
formed in the aqueous medium, TSS complexes likely were caught on bubble surfaces and
buoyed to the top as foam, explaining the high TSS removal efficiency during ozonation
(Sandu et al. 2008). Once in the foam, however, the association of hardness with TSS seems
to have been attacked by ozone, which broke and dissolved TSS particles.
Small amounts of hardness were lost in the trickling filter, probably due to bacterial
assimilation and bonding into solids.
Although hardness was not greatly reduced during treatment, it could become depleted
below critical limits after repeated treatments, and hence may require periodic adjustment.
3.5 TKN
In raw influent, TKN ranged between 41.8-42.3 mg/l, mostly as organic nitrogen. Table 7
shows TKN dynamics through the entire treatment train. TKN was removed in
approximately the same proportion as COD during sedimentation, indicating that nitrogen-
containing organics were distributed similarly between solid and dissolved forms. A slight
decrease in TKN was observed in the denitrification reactor, which could be attributed to
ammonia consumption in this reactor.
RI
SB
OR
CF
Total removal
2
Treatment
mg/l
mg/l (%)
mg/l (%)
mg/l (%)
(%)
1
9.2 (45.6)
a
2.0 (78.7)
a
95.2
a
41.8
16.9 (59.6)
2
8.2 (51.5)
ab
1.9 (76.8)
a
95.5
a
3
7.4 (54.3)
b
1.7 (77.0)
a
96.0
a
42.3
16.2 (61.7)
4
8.2 (49.4)
ab
1.9 (76.8)
a
95.5
a
1
Abbreviations: RI = raw influent, SB = sedimentation basin, OR = ozonation reactor, and CF = chemical
flocculation.
2
Decline of TKN between raw influent and trickling filter effluent concentrations.
Table 7. Total Kjeldall nitrogen (TKN) dynamics through the pilot plant
1
for each treatment
(unit outlet values). Means in a column with the same superscript are not significantly
different (
p
>0.05).
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