Non-destructive extraction of lipids from Botryococcus braunii and its potential to reduce pond area and nutrient costs

Abstract

The production of oil from microalgae is a process with a long way to go before reaching commercial viability. High costs associated with harvesting and solvent extraction make it difficult to compete with fossil fuel. The in-situ, non-destructive, repetitive extraction of lipids from Botryococcus braunii (milking) offers an alternative to the conventional microalgal-oil production process, with significantly reduced nutrient, dewatering and cultivation costs. Here, we compare low shear column solvent extraction with high shear stirred tank solvent extraction to ascertain which is better suited to a repetitive extraction process using B. braunii and carry out a preliminary economic analysis on the nutrient costs and raceway pond area requirements once scaled to industrial production. Stirred tank extraction with heptane resulted in five times higher productivities than that of column solvent extraction. Using this to generate an extraction model we found a lipid productivity of 104 mg L-1 day-1 was possible, six times greater than that of previous repetitive extraction studies. Thanks to the high lipid content of the B. braunii biomass generated in the recovery phase, when scaled to open raceway ponds, a lipid productivity of 13.4 g m-2 day-1 can be predicted. With this productivity, an estimated combined carbon, nitrogen and phosphorus cost of $2.52 L-1 of oil produced and a cultivation pond area of 3.53 km2 (assuming an annual oil production of 15,900 m3 year-1) is predicted, corresponding to reductions when compared to conventional extraction of 40% and 50% respectively. This repetitive extraction process provides an alternative method for the production of 3rd generation biofuels without the need to harvest and dewater algal biomass with significant reductions in nutrient and pond area requirements.