Photosynthetic Oxygenation and Nutrient Utilization by Chlorella vulgaris in a Hybrid Membrance Bioreactor and Algal Membrane Photobioreactor System

The imbalance between urban growth and freshwater is demanding sustainable wastewater treatment technologies. Aerobic activated sludge membrane bioreactors (AS-MBR) in municipal wastewater treatment are compact systems that can efficiently perform organic oxidation for pollutant removal. However, aerobic processes require high-mechanical aeration accounting for nearly 40% of total expenditure of the wastewater facility. There is also a global urgency for nutrient (Nitrogen/Phosphorus) removal strategies due to surges of eutrophication events, which requires more complex MBR configurations that further adds cost without advantages. The main motive behind this work was to develop an innovative cost-effective municipal wastewater treatment process with a dual income-stream: treated wastewater with superior quality and value-added microalgal biomass for several applications. The proposed treatment process investigated in this study involves an ultrafiltration AS-MBR for organic oxidation followed by a microalgal membrane photobioreactor (MPBR) to remove nutrients (N/P) through assimilation while simultaneously photosynthetically generating supersaturated dissolved oxygen effluent that is recirculated back into the AS-MBR, thereby reducing or even eliminating the need for mechanical aeration for the aerobic biological degradation. The study was conducted using small-scale pilot plant systems fed with a synthetic medium-strength municipal wastewater. The microalgal species C. vulgaris was initially tested in batch trials as a proof-of-concept study on its potential as a photosynthetic oxygenator for the AS-MBR and identify its nutrient utilization efficiencies, taking the research further by proposing potential configurations for the proposed setup. The MPBR and MBR were later constructed for continuous operation, with the aim to identify an optimal process configuration. The unit processes were subsequently isolated, where the AS-MBR was subjected to modeled algal effluent to assesses the impact of varying influent characteristics and effluent recycle rates. A microbial community analysis was performed by high-throughput sequencing and a statistical data-driven modeling approach to assess treatment performances. The MPBR stage was then subjected to the effluent achieved by the AS-MBR stage under varying operating conditions to assess its treatment performance and the resulting algal biomass biochemical compositions. The aim of this task was to identify the suitability of the algal biomass generated as a resource for high-value products such as bioethanol, biodiesel, or animal feed production. The findings of this study ultimately confirmed the ability of C vulgaris to support the AS-MBR for organic removal and fractional nutrient removal by supplying the oxygen demand, and further achieve an effluent polish stage for nutrient removal. The process configuration also demonstrated the ability to achieve a high microalgal biomass production with the potential of extracting valuable products as an added benefit of the wastewater treatment.