The role of hydrodynamic shear in the oxygenic photogranule
(OPG)
wastewater treatment process was investigated in three sequencing
batch reactors (SBRs) operated under different shear conditions for
250 days. Mixing in Reactor 1, Reactor 2, and Reactor 3 was set at
50, 100, and 250 rpm, which induced theoretical shear stresses (τ)
of 0.015, 0.04, and 0.14 N/m2, respectively. As shear increased,
we observed significant increases in the relative abundance of filamentous
cyanobacteria, the key microbial group for OPG granulation. Additionally,
increasing hydrodynamic shear force in OPG reactors stimulated linearly
increased production of extracellular polymeric substances (EPSs).
Compared to high shear, photogranules produced at low shear were more
spherical and much larger in size and, thus, contributed to higher
total nitrogen removal through denitrification, a potential legacy
of limited oxygen transfer in their structure. On the contrary, the
smaller photogranules produced under high shear promoted better oxidation
processes and, thus, higher removal of tCOD and NH4
+ due to their higher oxygen production capabilities compared
to the larger photogranules. Hydrodynamic shear can be manipulated
to drive photogranulation toward desired size distribution and, thus,
achieve specific treatment goals in the OPG wastewater treatment process.
Cyanobacteria occasionally self-immobilize and form spherical
aggregates.
This photogranulation phenomenon is central for oxygenic photogranules,
which present potential for aeration-free and net-autotrophic wastewater
treatment. Light and iron are tightly coupled via photochemical cycling
of Fe, suggesting that phototrophic systems continually respond to
their combined effects. Thus far, photogranulation has not been investigated
from this important aspect. Here, we studied the effects of light
intensity on the fate of Fe and their combined effects on the photogranulation
process. Photogranules were batch-cultivated with the activated sludge
inoculum under three photosynthetic photon flux densities: 27, 180,
and 450 μmol/m2·s. Photogranules were formed
within a week under 450 μmol/m2·s compared to
2–3 and 4–5 weeks under 180 and 27 μmol/m2·s, respectively. Batches under 450 μmol/m2·s showed faster but lower quantity of Fe(II) release
into bulk liquids compared to the other two sets. However, when ferrozine
was added, this set showed substantially more Fe(II), indicating that
Fe(II) released by photoreduction undergoes fast turnover. Fe linked
with extracellular polymeric substances (EPS), FeEPS, diminished
significantly faster under 450 μmol/m2·s, while
the granular shape in all three batches appeared along with the depletion
of this FeEPS pool. We conclude that light intensity has
a major influence on the availability of Fe, and light and Fe together
impact the speed and characteristics of photogranulation.
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