Fast-growing, oil-producing species of microalgae have become the focus of attention for both biomass and biodiesel biofuels, but questions remain about scalability, economics, and the competition between large-scale microalgae cultivation and agriculture, with regard to water, fertilizer, and land use. By cultivating microalgae on domestic wastewater, the water and fertilizer problems can be overcome, and by using algae for improved wastewater treatment, economic and environmental benefits can be realized. Land use for traditional large-scale algae cultivation systems, open ponds and closed photobioreactors (PBRs), continues to be a formidable challenge, however.
Assuming that algae production must be linked to existing wastewater treatment facilities and given that these facilities are deeply embedded in urban infrastructure, the integration of algae production into exiting urban infrastructure is both prohibitive in cost and unrealistic to implement. Alternatively, installing algae production in remote locations at great distances from the wastewater facilities requires significant investments in pipelines and transport infrastructures as well as in energy for pumping water and delivering materials. Could the solution for a practical and affordable large-scale algae production system, linked to wastewater treatment be located offshore, at least for coastal cities?
We propose a system called OMEGA (Offshore Membrane Enclosures for Growing Algae), consisting of floating photobioreactors (PBRs) made of flexible plastic sheets welded into a series of interconnected chambers. The modular PBRs are attached to each other to form a system that is attached to moorings or tethered to piers. If necessary, the system is protected by a breakwater.
The OMEGA PBRs are filled with secondary-treated wastewater redirected from established outfalls and inoculated with oil-producing freshwater algae. Nearby Power Plants or other sources of fossil fuel combustion provide CO2 to stimulate algae growth. Unlike land-based PBRs, which require significant energy input for mixing and temperature control, OMEGA uses surface waves for mixing and the heat capacity of the surrounding water for temperature control. The salinity difference between wastewater and seawater is used for forward osmosis, which 1) concentrates nutrients in the wastewater, stimulating algae growth; 2) dewaters the algae, facilitating harvesting; and 3) cleans the wastewater released into the surrounding environment. If the cultivated freshwater algae accidentally escape into the surrounding, seawater they pose no threat to the marine environment, as they cannot survive in saltwater.
While the proposed OMEGA system overcomes many of the difficulties inherent in existing land-based algae cultivation, there remain long-standing challenges in biology, engineering, environmental impact, and politics. Some of these challenges are associated with algae cultivation in general (e.g., growth control, grazers, pathogens, and dewatering) and others with OMEGA in particular (e.g., materials, permitting, fouling, marine mammals). These issues and others are under investigation as part of an OMEGA feasibility study supported by grants from NASA ARMD and the California Energy Commission. The purpose of this paper is to evaluate if indeed the future of algae biofuels is offshore?

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