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Author ORCID Identifier


Open Access Dissertation

Document Type


Degree Name

Doctor of Philosophy (PhD)

Degree Program

Civil and Environmental Engineering

Year Degree Awarded


Month Degree Awarded


First Advisor

Chul Park

Second Advisor

Caitlyn S. Butler

Third Advisor

John E. Tobiason

Fourth Advisor

Klaus R. Nüsslein

Fifth Advisor

Khalid M. El-Moselhy

Subject Categories

Biotechnology | Civil and Environmental Engineering | Environmental Engineering | Environmental Microbiology and Microbial Ecology | Microbial Physiology | Microbiology


In recent years, the oxygenic photogranule (OPG) process has gained increasing interest because of its potential to treat wastewater without supplemental aeration. Oxygenic photogranules (OPGs) are dense spherical aggregates comprised of phototrophic and nonphototrophic microorganisms. In OPG wastewater treatment reactors, photogranules grow in number as well as in size. The primary goal of this dissertation was to investigate how OPGs grow in size and how the growth affects their structure and functions. We found that OPGs undergo structural changes as they grow bigger in size. As OPGs grow larger, filamentous cyanobacteria become enriched while other phototrophic microbes diminish significantly. OPGs larger than 3 mm in diameter developed a layered structure in which a concentric filamentous cyanobacterial layer encloses noncyanobacterial aggregates. We found that the photogranules’ capability of producing oxygen, the key element in OPG wastewater treatment, is size-dependent. The results also show that the availability of iron strongly influences the growth and aggregation of filamentous cyanobacteria and thus the size-growth of photogranules in bioreactors. The selection of filamentous cyanobacteria during the size evolution of OPGs was linked with their ability to utilize the EPS as well as the Fe bound with EPS for their growth. We observed that the aggregation of filamentous cyanobacteria was promoted as both EPS and Fe bound with EPS became limited. The final section of this dissertation focused on the role of shear force in photogranulation. We found that the size of OPGs in reactors is inversely related to hydrodynamic shear. Compared with the OPGs developed at a high shear stress, OPGs produced at low and medium hydrodynamic shear stresses were bigger in size, more spherical, and less hairy. The variations in the particle-size-distribution of OPG biomass in reactors because of shear conditions resulted in significant differences in organic matter and nitrogen removals. Overall, these research findings are expected to enhance the fundamental knowledge on photogranulation phenomenon. Furthermore, engineering the OPG system based on a better understanding of the growth and function of photogranules is expected to advance the development of a new granular technology, which has the potential to treat wastewater without energy-intensive aeration.


Creative Commons License

Creative Commons Attribution 4.0 License
This work is licensed under a Creative Commons Attribution 4.0 License.