A Perspective on the Future of Liquid Fuels
Big Oil, Algae Oil and Snake Oil
With the limits and liabilities of fossil oil becoming increasingly apparent, the need for a sustainable and environmentally compatible alternative liquid fuel is growing in importance. Of the proposed alternatives, microalgae with high growth rates and high yields of oil have become the focus of research on biofuels. Microalgae species that can be grown on domestic wastewater are of particular interest to avoid the criticism of other biofuels that compete with agriculture for water and fertilizer; i.e., the food vs. fuel critique.
On a practical level, microalgae, which are traditionally grown in open ponds or closed photobioreactors (PBRs), must be grown in close proximity to wastewater treatment plants to avoid the energy and economic costs of pumping water. Excavating ponds or installing PBRs near wastewater treatment plants is impossible in most cities however, because existing plants are surrounded by urban infrastructure or expensive land that would make algae production costs prohibitively expensive. Thus realistically, algae companies that claim to produce "biofuels" have business plans based on selling higher-value products.
There may be an effective pathway to algal biofuels however, at least for coastal cities with offshore wastewater outfalls, using a system we call “OMEGA” (Offshore Membrane Enclosures for Growing Algae). In the OMEGA system, freshwater microalgae are grown in flexible, clear plastic (LLDPE) photobioreactors (PBRs) attached to floating docks in naturally or artificially protected bays. Wastewater and CO2 from coastal facilities provide water, nutrients, and carbon. The surrounding seawater maintains the temperature inside the PBRs and provides environmental control of the cultivated algae that escape (i.e., if a PBR module accidentally leaks, the freshwater algae that grow in wastewater cannot compete in the marine environment).
The salt gradient between seawater and wastewater can be used for forward osmosis (FO) to concentrate nutrients and facilitate algae harvesting. Both the algae and FO clean the wastewater, removing nutrients as well as pharmaceuticals and personal care products. The economics of the OMEGA system are supported by biofuels production, wastewater treatment, carbon sequestration, and aquaculture, while the OMEGA system is powered by local renewable sources, including solar panels, wave generators, and wind turbines.
The feasibility of OMEGA was investigated over the last three years with support from the California Energy Commission ($0.8M) and NASA ($10M). Floating prototypes (110-l and 1600-l) were developed and tested for continuous algae growth, wastewater treatment, biofouling, and impact on marine animals. A techno-economic analysis indicated the requirements of a full-scale system. The plans for the next phase of OMEGA will be discussed.