UC Merced

Offshore Membrane Enclosures for Growing Algae (OMEGA) cultivate microalgae using wastewater contained in floating photobioreactors (PBRs) deployed in marine environments; thereby eliminating competition with agriculture for water, fertilizer, and land. The offshore placement in protected bays near coastal cities co-locates OMEGA with wastewater outfalls and sources of CO₂-rich flue gas on shore, while the seawater supports the PBRs, regulates temperature and can drive forward osmosis to concentrate nutrients and facilitate microalgal dewatering. To evaluate the feasibility of OMEGA, microalgae were grown on secondary-treated wastewater and simulated flue gas (8.5% CO₂ V/V) in a 110-liter prototype system tested in a seawater tank. The flow-through system consisted of tubular PBRs made of transparent linear low-density polyethylene, a gas exchange and harvesting column (GEHC), two pumps, and a custom supervisory control and data acquisition (SCADA) system. The PBRs contained regularly spaced swirl vanes to impart a helical flow and improve mixing of the circulating culture. About 5% of the culture volume was diverted through the GEHC to remove dissolved oxygen (DO), provide supplemental CO₂, and harvest microalgae in a settling chamber. The SCADA system controlled CO₂ injection and recorded DO levels, totalized CO₂ flow, temperature, circulation rates, photosynthetic active radiation, and photosynthetic efficiency as determined by fast repetition rate fluorometry. In two experimental trials, totaling 23 days in April and May 2012, microalgal productivity averaged 14.1 <±> 1.3 g m^-2 day^-1 (n = 16), supplemental CO₂ was converted to biomass with >50% efficiency, and >90% of the ammonia-nitrogen was recovered from secondary effluent.

Experimental data collected during prototype evaluation clearly demonstrated that the accumulation of marine biofouling on the PBR tubes strongly suppressed rates of microalgal photosynthesis, as biofouled PBRs consumed less CO₂ than clean PBRs. These results suggest that any OMEGA deployment must have means to remove or prevent biofouling from accumulating on the surface of PBRs. This work also presents preliminary data regarding the use of energy-efficient electrochemical harvesting processes appropriate for the OMEGA configuration presented here. If OMEGA can be optimized for energy efficiency and scaled-up economically, it has the potential to contribute significantly to biofuels production and wastewater treatment.