Exploring Oxygenic Photogranules (OPGs) for Enhanced Nutrient Removal in Secondary Effluent Wastewater

About This Grant

This project explores an affordable and energy-efficient technology to remove inorganic nutrients discharged from wastewater treatment plants. Nitrogen and phosphorus discharges are a main cause for harmful algal blooms. This technology uses oxygenic photogranules (OPGs), which are dense, naturally forming clusters of algae and bacteria. The OPGs grow in sunlight to remove nutrients from wastewater. OPGs settle naturally due to their compact structure, eliminating the need for costly separation technologies. OPGs can operate in simple tank systems with low energy and space requirements. This project will test and optimize the use of OPGs for removing low levels of nutrients in wastewater after traditional treatments. The project will investigate how OPGs grow under these conditions, how the microbes inside them respond to stress, and how reactor design choices like light intensity, mixing patterns, and retention time affect their ability to remove nitrogen and phosphorus. This technology could significantly lower operational costs of wastewater treatment and reduce water bills for communities and households. The project will also provide hands-on research opportunities for students and support workforce development. Nutrient pollution from municipal wastewater effluent continues to degrade surface water quality across the United States, driving eutrophication, harmful algal blooms, and ecological decline in rivers, lakes, and estuaries. While conventional secondary treatment processes — such as activated sludge, aerated lagoons, and membrane bioreactors — reduce organic matter and nutrients, they often cannot meet increasingly stringent nitrogen (N) and phosphorus (P) discharge limits, particularly in ecologically sensitive watersheds. Advanced treatment options like chemical precipitation, enhanced biological nutrient removal (BNR), and photobioreactor systems provide improved nutrient removal but require high capital investment, intensive energy use, and operational complexity. This project will develop and optimize a low-energy, phototrophic wastewater treatment system based on oxygenic photogranules (OPGs) — dense microbial aggregates composed of cyanobacteria, algae, and heterotrophic bacteria that naturally self-immobilize and remove nutrients via photosynthesis and microbial metabolism. OPGs settle passively, eliminating the need for mechanical or chemical separation, and can be operated in tank-based systems with low light and mixing requirements. Unlike suspended algal systems, OPGs form compact, structurally stable granules that enable high-density biomass retention and simplified harvesting, significantly reducing the energy and space footprint of treatment infrastructure. The project will investigate the performance and resilience of OPGs in post-secondary applications using sequencing batch reactors (SBRs) treating nutrient-limited secondary effluent. The project has three specific aims: (1) demonstrate the structural integrity and stable growth of photogranules under low-nutrient conditions by evaluating granule formation, size distribution, density, porosity, and shear resistance; (2) assess microbial physiological responses through extracellular polymeric substance (EPS) profiling, enzymatic activity assays, and RT-qPCR to evaluate expression of key genes involved in ammonia oxidation, denitrification, and phosphorus uptake; and (3) optimize key reactor parameters, including light intensity, mixing patterns, hydraulic retention time (HRT), solids retention time (SRT), and shear stress, to enhance nutrient removal kinetics. The anticipated outcomes include a deeper mechanistic understanding of OPG function under nutrient-limited conditions and the development of a scalable, cost-effective post-secondary treatment process that achieves high nitrogen and phosphorus removal with minimal energy and chemical input. This work contributes to environmental engineering by advancing knowledge of phototrophic biofilm systems, biogranule formation, and low-energy nutrient recovery. It aligns with national priorities in sustainable infrastructure and water quality protection. The project will support hands-on training for graduate and undergraduate students, collaboration with municipal wastewater operators, and engagement with students and global partners through Engineers Without Borders. Together, these efforts aim to promote innovation, knowledge transfer, and improved access to sustainable water technologies. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.