Volatile fatty acid and methane production from vinasse and microalgae using two-stage anaerobic co-digestion

The effects of adding vinasse (VIN) as a co-substrate on the stability and production of volatile fatty acids (VFA) and methane (CH₄) during the anaerobic digestion (AD) of microalgal biomass (MB) were evaluated. The AD system consisted of an acidogenic reactor (AR) followed by a methanogenic reactor (MR). The experiment was divided into phase I-start-up and AD of VIN; phase II-MB+VIN co-digestion (50:50 based on chemical oxygen demand (COD)); and phase III-co-digestion of pretreated MB and VIN (PTMB+VIN, 50:50). In phase I, the total amount of VFA in the AR increased from 240 to 2126 mg/L. In the MR, the conversion of VFA into CH₄ yielded an average of 71 ± 37 NmL CH₄/g COD_in. In phase II, the initial CH₄ production was 246 ± 31 mL CH₄/g COD_in but it decreased to 63 mL CH₄/g COD_in due to the accumulation of longer chain acids. More stable conditions were achieved after two hydraulic retention cycles and the average CH₄ yield in this phase was 183 mL CH₄/g COD_in. In phase III, when using PTMB, 197 ± 72 NmL CH₄/g COD_in were obtained, i.e., a 2.7- and 1.1-fold increases compared to phases I and II, respectively. The predominance of acetate producers and syntrophic organisms suggests acetoclastic methanogenesis, confirmed by the occurrence of Methanosaeta (10.5%).