Sulfidogenesis serves as the core principle driving THIOTEQ and SANI Process for applications in wastewater treatment. Their applications benefit from the development of a simple yet efficient bioreactor which can largely promote their volumetric activities and make the systems more compact. The overarching objective of this study is to develop a super high-rate system for the biotechnological applications of sulfidogenesis through three research parts. In part one, to enhance the biomass retention ability in bioreactor, sulfidogenic granule was first cultivated through gas-enhanced mixing. The average particle size and sludge settling performance were continuously monitored. The scanning electron microscope (SEM) imaging displayed that the granules tended to possess channel-like porous...[ Read more ]

Sulfidogenesis serves as the core principle driving THIOTEQ and SANI Process for applications in wastewater treatment. Their applications benefit from the development of a simple yet efficient bioreactor which can largely promote their volumetric activities and make the systems more compact. The overarching objective of this study is to develop a super high-rate system for the biotechnological applications of sulfidogenesis through three research parts. In part one, to enhance the biomass retention ability in bioreactor, sulfidogenic granule was first cultivated through gas-enhanced mixing. The average particle size and sludge settling performance were continuously monitored. The scanning electron microscope (SEM) imaging displayed that the granules tended to possess channel-like porous structures under the influence of gas mixing. Further, a high ratio of acidogens, i.e. Lactococcus, to sulfate-reducing bacteria (SRB) was observed in the microbial analysis. These findings suggest the use of gas mixing might not only contribute to the sludge granulation, but also be important in reactor operation in achieving an improved granule microstructure for mass transfer as well as an efficient organic degradation pathway by Lactococcus.

Sludge bed clogging is one of the major issues in granular sludge system operation, and it could lead to a disappointing reactor performance and/or unnecessary biomass flotation. In part two, to resolve this challenge, intermittent gas sparging was therefore proposed as a reactor operation strategy for regular disaggregation of clogged sludge while minimizing continuous hydrodynamic disturbance. Over a 196-day lab-scale trial, the sulfidogenic system achieved the highest organic loading rate (OLR) of 13.31 kg COD/m³∙day which is substantially greater than the typical loading rate of 2.0 to 3.5 kg COD/m³∙day in a conventional upflow anaerobic sludge bed (UASB) reactor. The average organic removal efficiency and total dissolved sulfide of this system reached 90 ± 4.2 % and 158 ± 28 mg S/L, while organic residual in the effluent was only 34 ± 14 mg COD/L. When sludge bed clogging occurred in the control stage (intermittent gas sparging stopped), it was revealed that the content of α & β-polysaccharides inside granules plunged due to the excessive biofilm overgrowth on granules’ surface, through relevant chemical measurements and confocal laser scanning microscopy (CLSM) analyses. To compare gas sparging with the conventional approach, i.e. effluent recirculation for the biofilm suppression, a three-dimensional computational fluid dynamics (CFD) modeling in combination with energy dissipation analysis was undertaken to investigate their effectiveness of liquid shear promotion. And it demonstrated that the gas sparging (at a superficial gas velocity of 0.8 m s^-1) creates a liquid shear 23 times greater as well as an enhanced particle attrition than that of applying a higher effluent recirculation (from 1.4 to 5). Over the stages of operation in the high-rate sulfidogenic system, the granules were successively transformed in terms of their morphological and microbiological structures.

In part three, analytical characterizations were carried out for investigating the reason for the transformations. Digital image processing was first applied for the quantification of their surface properties; Brunauer–Emmett–Teller specific surface area and Barrett-Joyner-Halenda pore volume were further used for the identification of their macroscopic microstructures. In correlation analyses, the loading ratio (the substrate surface loading to the upward velocity) was identified to be the main parameter controlling the granule transformations, and the surface structures were classified into three categories for further interpretation. Besides, through 16S rRNA sequencing, granules with higher porosity were found to have a more diversified microbial community. And as the major dominant genus, Lactococcus continued to enrich under the influence of intermittent gas sparging and accounted for up to 74.8% in relative abundance. In total, 22 genera in relation to sulfide formation were detected, among which five were complete organics-oxidizing SRB genera. Under high organic loading and in low shear environments, the microbial composition of the overgrowth biofilm was mostly found to be Trichococcus (29%), Synergistaceae (17%) and Bacteroidetes_VC2.1_Bac22 (6.94%), respectively, which are highly similar to that of the supernatant after gas sparging. Overall, in the high-rate operation of sulfidogenic system, the roles of intermittent gas sparging can be regarded for the improvement of granules surface structure and discharging the overgrowth biofilms as part of effluent in balancing the bioreactor’s microbial ecology.