Microalgae biodiesel and by-products must be separated for increasing the biodiesel production. The main separation processes used hot water (50?C) (Li et al., 2007), organic solvents such as hexane (Halim et al., 2010; Wiltshire et al., 2000) and water-organic solvent for a liquid-liquid separation (Couto et al., 2010; Lewis et al., 2000; Samorì et al., 2010). When using a non-polar co-solvent for transesterification of lipids, only water is added to separate biodiesel from the by-products (M. B. Johnson & Wen, 2009).
To our present knowledge, there is no study on the purification of biodiesel from microalgae. Based on 1st generation biodiesel (Leung et al., 2010), three mains means of purification on biodiesel could be applied to microalgae biodiesel purification: “1-water washing 2-dry washing 3-membrane extraction.” Based on vegetable oil biodiesel production (Berriosa & Skelton, 2008) and microalgae composition, the main by-products could be unreacted lipids, water, alcohol, chlorophyll, metals and glycerol.
Among the by-products obtained from the biodiesel production, glycerol is the most interesting. Glycerol worldwide consumption remains relatively constant, in recent years, with a consumption of 600 kton/year. Twenty-six percent of glycerol consumption was associated with pharmaceutical, cosmetic and soap industries (Bondioli, 2003). From 2004 to 2011, massive biofuel production created a problem of overproduced glycerol and the price of crude glycerol (80% pure) decreased from 110 to 7.5 $US/ton (The Jacobsen, 2011; Yazdani & Gonzalez, 2007). Gained glycerol can be transformed into added-value products using many paths including chemical, thermochemical or biological conversion.
Among the chemical added-value products, glycerol can be oxidized or reduced to many compounds like propylene glycol, propionic acid, acrylic acid, propanol, isopropanol, allyl alcohol and acrolein but only some of these products are interesting in terms of market or profitability (D. T. Johnson & Taconi, 2007).
Glycerol can be also converted into Fischer-Tropsch fuel at low temperature (225-300?C) by catalytic processes (Soares et al., 2006) or transformed into hydrogen (H2) by catalytic (generally nickel, platinum or ruthenium) or non-catalytic reforming (Vaidya & Rodrigues, 2009). Biological conversion of glycerol includes fermentation into alcohols (ethanol, butanol, 1,3- propanediol) and other products like H2, formate, propionic or succinic acid (Yazdani &Gonzalez, 2007).
Anaerobic digestion of by-products is another possible way to make biodiesel from microalgae cost effective if the lipid content of the microalgae does not exceed 40% (g lipid/g dry weight) (Sialve et al., 2009). For example, Ehimen et al. (2010) used anaerobic digestion of microalgae residues issued from a direct transesterification with a constant loading rate of 5 kg volatile solids (VS)/m3, temperatures and carbon-to-nitrogen (C/N) mass ratio varying from 25 to 40?C ant 5.4 to 24, respectively. For a temperature of 40?C and a C/N mass ratio of 8.53, a maximum methane (CH4) concentration of 69% (v/v) with a specific CH4 yield of 0.308 m3 CH4/kg VS was obtained.