(19d) Microalgae Cultivation in Membrane Photobioreactors for Tertiary Wastewater Treatment

Biological wastewater treatment using activated sludge is one of the best treatment methods for municipal wastewater. While the organic content of wastewater can be sufficiently reduced in this aerobic process, the remove of inorganic pollutants such as nitrogen (N) and phosphorus (P) is not adequate. The discharge of effluent with high P and N in natural water streams may cause eutrophication – a process of rapid plant/algae growth which adversely affects water quality and aquatic life. One approach to remove N and P from wastewater is through bioremediation using autotrophic microalgae. In this process, microalgae can assimilate these pollutants as fertilizers for their growth and these compounds can be recovered as microalgal biomass, which is a coveted feedstock for the production of a variety of useful chemicals, including biofuels. However, the low growth rate of microalgae as compared to bacteria makes it difficult to treat tertiary wastewater in a photobioreactor without significant reduction in the hydraulic retention time (HRT).

One approach to obviate cell growth rate limitation on HRT is the use of membrane bioreactors, wherein the effluent stream is filtered through a polymeric membrane and the microorganisms are completely retained inside the bioreactor. These bioreactors also maintain a large biomass concentration in the bioreactor, which accelerates the removal of pollutants. In addition to biomass retention, new membrane separation processes such as forward osmosis (FO) can also provide high N and P rejection due to very small pore size, which may result in better effluent quality.

In this research, Chlorella vulgaris was cultivated in an osmotic membrane photobioreactor (OMPBR) for N and P removal from tertiary wastewater. The bioreactor was sparged with 5% CO₂-enriched air and illuminated with fluorescent lights for photosynthesis, and the operation was continued for over 6 months with regular monitoring of NH₄⁺, PO₄^3- and NO₃^- concentrations at HRTs ranging from 1-4.5 days. Under best set of operating conditions, the removal efficiency of NH⁺₄, PO^3-₄ and NO^-₃ were 85.9%, 99.5% and 96.6%, respectively. It was also found that most of the removal was through microalgae assimilation, but the rejection of these pollutants during osmotic filtration also contributed to the excellent effluent quality. During the prolonged operation, the changes in filtration performance due to microalgal biofilms were investigated and the cost of bioreactor operation was estimated. The results suggest that autotrophic microalgae can be used for effectively treating and recycling the inorganic pollutants in tertiary wastewater.