(642a) An Innovative Microalgae-Bacteria Symbiotic Process for in-Situ Secondary and Tertiary Treatment of Wastewater

Activated sludge process (ASP) is one of the aerobic bioprocesses widely used in secondary wastewater treatment. Despite substantially reducing the organic content of wastewater, the process cannot remove nitrogen and phosphorus significantly. Hence, a tertiary treatment needs to be implemented in addition to ASP for enhancing the effluent quality. Existing secondary and tertiary wastewater treatment methods are limited by high energy demand and waste generation. One of the approaches to alleviate these limitations is through co-culture of heterotrophic bacteria and autotrophic microalgae. Microalgae furnish oxygen through photosynthesis, which can be consumed by bacteria to oxidize the organic matter and produce carbon dioxide (facilitating further photosynthesis). Therefore, the co-cultivation of microalgae and bacteria obviates the need for mechanical aeration as oxygen produced by symbiotic interactions supports the organic carbon removal. Furthermore, microalgae assimilate nitrogen and phosphorus and subsequently enhance the effluent quality, without additional tertiary treatment. Carbon dioxide utilization during this process also assists in reducing the carbon footprint. While co-cultivation of microalgae and bacteria seems an attractive solution for wastewater treatment, its implementation faces several challenges. Bioreactors with mixed microalgal-bacterial consortium generally operate at longer hydraulic retention times (HRT) given the slower growth rate of microalgae. Besides, light availability for photosynthesis may also get impeded if bacteria outgrows the microalgae during co-cultivation.

In this research, microalgae and bacteria were cultivated in two separate bioreactors and a membrane-mediated symbiotic interaction was established between them. The gas transfer was achieved by recirculating oxygen and carbon dioxide-rich streams through submerged membranes. Effect of different recirculation rates on the organic carbon removal was investigated. At the optimum recirculation rate, complete removal of organic carbon was observed, without any external aeration. Nitrogen and phosphorus removal was optimized by varying operating parameters of the microalgae bioreactor. Microalgal biomass accumulation during the operation was monitored. It was observed that even in the absence of an external carbon source, high-density microalgae culture was obtained. The chemical composition of microalgal biomass was studied under different operating conditions to evaluate its potential application.

Overall, this research is first to show that collaboration between physically segregated microalgae and bacteria enables an efficient symbiotic process for removal of organic and inorganic pollutants from wastewater.