ANAEROBIC CO-DIGESTION OF MICROALGAE AND RESIDUAL GLYCEROL RECOVERED FROM BIODIESEL: EVALUATION OF PRETREATMENT AND COD/N RATIO

Abstract

Anaerobic digestion can be a viable alternative for the destination of effluents from stabilization ponds rich in naturally produced microalgae, without the need for concentration processes, however, there are challenges related to the rigid cell wall of microalgae and the low carbon content in its composition, to be overcome. To improve these parameters, this study comparatively evaluated hydrolytic pretreatments in microalgae from effluents from stabilization ponds, aiming to hydrolyze the cell wall of these microorganisms, for co-digestion with residual glycerol from biodiesel. In this case, glycerol is a by-product with limited applicability, acting in this scenario as a carbon supplier, improving the C/N ratio, microalgae biodegradation and biomethane production. Effluents with microalgae submitted to thermal and ultrasonic hydrolysis (for 30 and 90 minutes) were tested to assess their potential in the production of methane-rich biogas, monitored by gauge measurements and gas chromatography, respectively, in co-digestion with residual glycerol from biodiesel. The heat treatment for 30 minutes showed more satisfactory results and was replicated in a benchtop anaerobic reactor (R2), in parallel with a reactor operating untreated microalgae (R1), in a continuous feed system. The effects of pretreatment and COD/N ratio were evaluated on organic matter removal and biomethane production. R2 showed the most satisfactory effect on COD removal, resulting in up to 90% COD removed, with a theoretical biogas production of 0.52 L g-1 COD removed. As for the methane content contained in biogas, R1 reached percentages of up to 84% against 73% in R2.