Integration of subcritical water pretreatment and anaerobic digestion technologies for valorization of açai processing industries residues

Abstract The objective of this work was to study the valorization of residual biomass and wastewater from acai fruit processing to obtain energy products and higher added value from such residues, through the integration between subcritical waterpretreatment and anaerobic digestion technologies. Subcritical watertechnology was performed to pretreat the residual biomass. For this process, the reactorwas feed with 5 g of acai residue and a solvent to biomass mass ratio of 80 g of water per gram of acai residue was used. The process was carried out at 170 and 200 °C, during 20 min, using water flow of 20 mL min −1 (15 Mpa system). Then, the residual biomass was submitted to the anaerobic digestion process using a reactor with pretreatment (integration) and a control reactor, both in semi-continuous regime and under mesophilic conditions (35 °C). The acai stone had a high carbohydrate concentration including 43.81% cellulose and 25.89% hemicellulose. For the evaluated conditions, subcritical water pretreatment at 200 °C gave the bestresultsforyield of reducing sugars, total reducing sugars and monosaccharide concentration. Anaerobic digestion experiments resulted in analkalinity between 150 and 1537 mgCaCO3.L −1 for the control reactor and between 249 and 491 mgCaCO3.L −1 for the reactor integration.Acetic acid was the main volatile fatty acid produced in both reactors, but butyric acid also had a very high concentration in the control reactor. The methane composition in the biogas for the integration reactor in the stabilization phase was between 60 and 80%, whilefor the control reactor it was in the range between 40 and 60%.The accumulated biogas yield was 7.79 and 791.81 L.KgTVS −1 for the control and integration reactor, respectively. Anaerobic digestion using the acai hydrolysate obtained by subcritical water pretreatment showed a significant advantage over the control digestion, resulting in30% more methane in the biogas and 100 times higheryield.