Various species of algae are known to produce valuable products ranging from food to fertilizer to biofuels. Methods have been developed for producing biofuels by processing algae and other micro-organisms that grow in aquatic environments, but the largescale commercial production of these algae, particularly for commodity products like biofuels, has been limited by the unfavorable economics of the current cultivation and harvesting methods.
A new cultivation and harvesting method was developed using flexible plastic photobioreactor (PBR) tubes with forward osmosis membranes. These PBRs are filled with nutrient-rich water (e.g., wastewater) and float in seawater. Forward osmosis, driven by the salt gradient between wastewater and the surrounding seawater, removes water and concentrates the algae for harvesting. The concentrated algae are processed, and the PBRs are cleaned and reused. The forward osmosis membranes also clean the water released into the seawater, which has a beneficial environmental impact.
This system is used in calm aquatic environments where the water provides infrastructural support through floatation and temperature regulation. The water motion from waves provides some mixing of the algae with the PBRs.
The size and plumbing of the PBRs in the overall system are critical to its function, which depends on light, temperature, and gas exchange. The forward osmosis membranes are used to remove water and concentrate the algae, and other types of semi-permeable membranes, such as gas-permeable and nutrient-permeable membranes, may also be incorporated. The different membranes influence the growth of algae, the harvesting, and the quality of water released into the environment.