Sludge flotation and the consequent biomass loss is a long-standing issue that hinders not
only the widespread implementation of the upflow anaerobic sludge bed (UASB)-type
bioreactors, but also the development of anaerobic treatment biotechnologies such as the
traditional methanogenesis and the emerging anammox and sulfidogenesis in wastewater
treatment. To date, a fundamental understanding of anaerobic sludge flotation and feasible
control strategies remain elusive. In particular, little information about sulfidogenic sludge
flotation can be found in the literature. This PhD study investigates the underlying causes of
and devises effective control strategies for the sludge flotation problem in Sulfate-Reducing
Upflow Sludge Bed (SRUSB) reactors.
Three lab-scale SRUSB reactors...[
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Sludge flotation and the consequent biomass loss is a long-standing issue that hinders not
only the widespread implementation of the upflow anaerobic sludge bed (UASB)-type
bioreactors, but also the development of anaerobic treatment biotechnologies such as the
traditional methanogenesis and the emerging anammox and sulfidogenesis in wastewater
treatment. To date, a fundamental understanding of anaerobic sludge flotation and feasible
control strategies remain elusive. In particular, little information about sulfidogenic sludge
flotation can be found in the literature. This PhD study investigates the underlying causes of
and devises effective control strategies for the sludge flotation problem in Sulfate-Reducing
Upflow Sludge Bed (SRUSB) reactors.
Three lab-scale SRUSB reactors were operated in parallel with different modes of
mixing (hydraulic, mechanical, and pneumatic) at various mixing intensity shear rates (γ)
ranging from 0.7 to 6.6 s
-1. The sludge flotation potential (SFP) and its relationships with
reactor mixing, reactor performance, and sludge properties were also studied. The intrinsic
causality of sulfidogenic sludge flotation was elucidated as follows: improper reactor mixing
(i.e. ineffective mode and/or insufficient intensity) altered the sludge extracellular polymeric
substances (EPS) to be bulky in structure (i.e. indicated by a high loosely bound (LB)-EPS/tightly bound (TB)-EPS ratio) and poor in chemical composition (indicated by a high
proteins (PN)/polysaccharides (PS) ratio). These changes in EPS successively induced the
transitions of sludge properties to be highly adhesive to gas bubbles — that is, high
hydrophobicity, high viscosity and weak negative surface charge. Consequently, these factors
combined to promote gas entrapment in sludge and decrease the sludge density to less than
that of water, finally triggering sludge flotation.
The control of sulfidogenic sludge flotation was achieved by manipulating reactor
mixing in this study. The continuous gas recirculation (CGR) with γ = 4.2 s
-1, which achieved the lowest SFP (11 ± 2%, lower than the critical safe value of 20%) in the comparison, was
proposed as an optimal mixing strategy for controlling sludge flotation in the SRUSB
reactors. The optimal mixing strategy was then applied in a SRUSB reactor operation for 150
days. The outcomes demonstrated that i) satisfactory reactor hydrodynamics (indicated by a
low short-circuiting flow fraction of 1.3 ± 0.1% and a low dead zone fraction of 0.2 ± 0.01%)
were achieved in the CGR-SRUSB; ii) the energy consumption of the CGR-SRUSB was only
one-third of that in the traditional hydraulic-mixing SRUSB; and iii) the CGR-SRUSB sludge
was transformed into micro-granules (300–350 μm) with high sulfidogenic activity (0.62 ±
0.05 g COD/(g MLVSS·day)), low sludge flotation potential (< 20%), and high settleability
(SVI
5/SVI
30 < 1.3) within the initial 65 days, which was stably maintained for the rest of the
150 days. The CGR mixing successfully optimized the SRUSB through maintaining small-size
granules, enhancing mass transfer efficiency, and sustaining low gas entrapment
potential.
The operational performance of two granular sludge CGR-SRUSB reactors was
further examined for 30 days under two challenging conditions: the complete stoppage of
wastewater (called complete food starvation) and a decrease of the influent sulfate
concentration to a very low level (called sulfate starvation). The results show that i) 61% and
65% reductions in the specific removal rate of COD and 45% and 61% reductions in the
specific sulfidogenic activity were caused by complete food starvation and sulfate starvation,
respectively, in the CGR-SRUSB reactors; ii) the SFP of the complete-food-starved and
sulfate-starved granular sludge increased from 14% to 58% and from 10% to 35%,
respectively; and these performance deteriorations were found to be highly related to iii)
starvation-induced unfavorable transitions in the granular sludge characteristics, including the
formation of inner cavities, decreased mass permeability, decreased porosity, and increased
surface adhesion. Nevertheless, the reactor restoration was quickly achieved after 10 to 15
days of reactivation operation in the two starved CGR-SRUSB reactors.
The in-depth understanding of the sludge flotation problem and the proposed effective
control strategy presented in this study can not only improve the stability and efficiency of
SRUSB reactors but also can shed light on the optimization of other emerging UASB-type
anaerobic biotechnologies.
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