Recovery of Crude Glycerol and Citrus Vinasse in Anaerobic Digestion in Two Stages for Lactic Acid and Biomethane Production

Abstract

Global biodiesel production has increased, and 10 kg of crude glycerol (CG) is being generated as a coproduct for each 100 kg of biodiesel produced. Brazil is the main citrus-producing country in the world, generating highly fermentable carbohydrates that are used for the second-generation ethanol production with citrus vinasse (CV) as waste product. The goal of this study was to evaluate the use of CG, the coproduct from transesterification of waste cooking oil, co-digested in CV through a biosystem in two-sequential stages: the first one acidogenic for lactic acid generation and the second one methanogenic for CH4 production. For the first stage, with an acid pre-treated consortium, the maximum lactic acid generated was 2.31 g L-1 in anaerobic batch reactors (500 mL) containing 400 mL of medium composed of 10 g COD L-1 CG with 10 g COD L-1 CV, pH 6.0, headspace with N2, at 37°C. The liquid fraction generated in the first stage (200 mL) was used as substrate by an anaerobic consortium in natura in anaerobic methanogenic reactors (500 mL), headspace with N2, pH 7.0, at 37°C, generating a maximum of 318.65 mmol CH4 L-1 in 815 h of operation. In addition, 85.65% of the COD was removed and all methanol from the CG was consumed. Integrated systems correspond to major measures for the removal of CG in obtaining value-added products such as lactic acid, in addition to the generation of biogas as CH4.