Prashant Praveen and Kai-Chee Loh
Department of Chemical & Biomolecular Engineering, National University of Singapore, Singapore
Microalgae are one of the most promising and sustainable feedstock for the production of biofuels because of their high growth rate, efficient CO2 fixation and high lipid content. However, despite making significant progress, production of biodiesel from microalgae remains an expensive process. One approach to reduce the cost of microalgae cultivation is the use of urban wastewater for microalgae growth. Wastewater can not only be a cheap and readily available source of water, it can also provide fertilizers for microalgae growth.
Another challenge in microalgae cultivation is the low biomass concentration, which increases the harvesting costs. This can be alleviated by cultivating the microalgae in a osmotic membrane photobioreactor (OMPBR). While the OMPBR will allow continuous supply of wastewater to the microalgae without any constraint regarding the hydraulic retention time, the microalgal biomass will be completely retained in the bioreactor, resulting in high biomass concentration.
In this research, an OMPBR was designed and operated for continuous cultivation of microalgae in tertiary wastewater containing NH+4, PO3-4 and NO-3. Sparging the bioreactor with 5% CO2-enriched air, and in the presence of light at an intensity of 200 μmol photo/m2-s, the microalgae exhibited exponential growth rate and the biomass concentration increased from 21 mg/L to 190 mg/L in 22 days. The removal efficiency of NH+4-N, PO3-4-P and NO-3-N were 85.9%, 99.5% and 96.6%, respectively. The salt concentration profiles in the bioreactor and in the effluent were different, suggesting that membrane rejection of the pollutants played a key role in improving the effluent quality. The permeate flux through the membranes remained stable, indicating that membrane biofouling was negligible during the operating period. These results suggest that the use of OMPBR can be quite promising in tertiary wastewater treatment and in microalgae cultivation.