Outdoor cultures of Scenedesmus vacuolatus: biomass and biodiesel production

Biofuels production coupled with carbon dioxide sequestration by means of photosynthetic microrganisms appeared a promising process since the end of the last century. Microalgae biomass may be processed to produce liquid fuels: bio-oil is extracted from microalgae and it may be either adopted as crude fuels or transesterified to biodiesel. A twofold advantage characterizes the potential success of microalgae as biofuel feedstocks. The first advantage: biodiesel-microalgae production rate may be 1–3 orders of magnitude larger than that from oil crops . The second advantage: microalgal biomass fixes a large amount of CO2 – 1.83 kg of CO2 per kg of dry microalgae – and strongly contributes to the reduction of greenhouse gas emissions. However, yield and productivity of biomass and total lipid content are strongly affected by nutrient and light supply strategy. The present work reports the results of outdoor cultures of Scenedesmus vacuolatus in 1.7 L inclined bubble column (IBC) photobioreactors characterized by 250 cm2 irradiated surface. IBC were operated according to Olivieri et al. (2012). The temperature was set at 23°C. A 2% CO2 gas stream (50 nL/h flow rate) was sparged at the bottom of the IBC. pH was kept constant at about 7.0 by the CO2 feeding. Bold Basal Medium (BBM) - NaNO3 as nitrogen source - was adopted. Cultures were carried out outdoor: i) a test campaign during the May-July period under shadow conditions, irradiance maximum of 450 microE/(m2 s); ii) a test campaign during the September-November period under direct sun light, irradiance maximum of 2000 microE/(m2 s). Cultures were operated under fed-batch or semi-continuous mode. 30% of suspension was withdrawn – and substituted with fresh medium - with a frequency of 1-2 times a week. Two procedures were adopted to induce nitrogen-starvation stress: I) the BBM feeding was stopped for two weeks and the nitrogen content was continuously monitored; II) the gas flow rate was stopped for few hours to separate the biomass by sedimentation and the clarified culture liquid was replaced with fresh BBM without nitrogen source.