Nowadays, algae industry is mainly focused on high value specialty products related to nutritional industry, sold from $10,000 to $100,000 per tonne; however, com­modity products, as fuels, are usually sold for less than $1,000 per tonne. This means that, in order to obtain algae commodity products, the production costs of the current technologies have to be reduced of almost one order of magnitude. In particu­lar, operating open ponds and processes as the harvest and the dewatering of algal biomass have a large impact on overall costs and energy balance. Thus, the develop­ment of a method in which algal metabolites, as hydrocarbons, can be directly extracted from the algal culture without energy costly steps can make the process more beneficial, economical, and sustainable.

The extraction of B. braunii hydrocarbons through a “liquid/liquid” method has been already accomplished by using n-hexane [45]; in our work, the SPS DBU/octanol was used analogously (Fig. 8), by exploiting the fact that octanol is a water-immiscible alcohol and that DBU, in spite of its water solubility, can be shifted into the organic phase through an adjustment of the pH to alkaline conditions (Fig. 9) [25].

Analogously to the obstacle represented by the presence of free fatty acids which could react with DBU hindering the switch of the non-ionic form of the SPS into the ionic one, also residual water in the system can be a problem. Spectroscopic data for the reaction between DBU and CO2 in presence of water are in fact consistent with the formation of the salt DBU bicarbonate ([DBUH+][HCO3-]) [40].

In our liquid/liquid extraction system after the adjusting of the pH of the aqueous phase with KOH, the water content in the upper organic phase (DBU/octanol) was 7% [25]. However, the residual water could be easily removed from DBU/octanol by bubbling N2 in the system for 30 min at room temperature; after this time, the residual water was less than 0.1%, without any significant loss of the organic com­ponents (Fig. 10).

The extractions of B. braunii cultures were performed at room temperature, cen­trifuging the samples at different speeds and simulating a high (3,000 rpm) and a low energy (300 rpm) liquid/liquid extraction process from the growth medium (Table 4). Centrifuging was chosen as the best procedure to obtain a clearer separa­tion (useful for small samples operation) of the aqueous and organic phases. This method allows to avoid vigorous stirring that should end up into an untreatable foaming, still maintaining a good extraction efficiency.

Fig. 9 Liquid/liquid

extraction system of B. braunii culture with the SPS DBU/ octanol

Although the extraction process is somewhat sluggish, the SPS DBU/octanol after 24 h at 300 rpm gives 8.2% hydrocarbon yield, almost an half of the yield achieved with DBU/octanol on freeze-dried samples (16%). Moreover, at higher rate (3,000 rpm) the extraction results faster, obtaining in 4 h approximately the same yields obtained at 300 rpm in 24 h. This can be explained by the fact that by raising the centrifuge rate, less dense algae (with higher hydrocarbon content) move quickly to the top of the water phase and release the hydrocarbons in the upper organic layer by contact.