Demonstration of the feasibility of milking lipids from algae for biodiesel production

Abstract

A major challenge to the development of industrial-scale biodiesel production from cultured algae is the identification of energy efficient and cost effective methods of harvesting/dewatering algal cells. Producing 1 gallon of biodiesel from algae, even if grown in highly concentrated suspensions, requires processing thousands of gallons of water. An alternative to growth and dewatering methods is milking of algal cells whereby lipids are continually extracted without requiring the sacrificing of cells. Growth and extraction are combined by contact between cells and a biocompatible solvent. Algal cells can be maintained in a stationary phase for extended periods and with nitrogen-limiting conditions which favor lipid accumulation in the cells. With continual extraction of lipids, cells in the stationary phase would direct photosynthetic activity more towards lipid production instead of new cellular growth. Essential nutrients such as nitrogen and phosphorous would also be retained in algal suspensions because milking selectively extracts lipids (compounds of C, H, and O) which are produced from the carbon dioxide, water, and light supplied to the bioreactor, thus diminishing the need for expensive fertilizer use. Batch studies showed that n-dodecane and biodiesel were biocompatible solvents with the green algae Ankistrodesmus falcatus and Chlorella kessleri for 4 days of contact time. Long term (28 day) milking experiments were performed in 2 L photobioreactors with continuous n-dodecane solvent recirculation. The results showed that Chlorella maintained a viable population with a half-life of 49 days, but Ankistrodesmus populations became non-viable after 6-26 days of solvent contact. Lipid analysis by HPLC demonstrated successful milking. The total lipid concentrations reached up to 4,720 mg/L in the solvent phase, equaling a lipid milking productivity of 16.1 mg/L-d, and representing up to 86% extraction of total lipids in the system. Milking favored extraction of monoglycerides and free fatty acids from algal cells. Nutrients were largely retained in algal suspensions; 55-85% of total nitrogen and 66-98% of total phosphorous were retained at the end of experiments. Overall, successful milking of Chlorella with n-dodecane was demonstrated. Possibilities for scale up of the process, critical analysis of energy balances, and lipid purification methods should be investigated.