Optimized conditions for methane production and energy valorization through co-digestion of solid and liquid wastes from coffee and beer industries using granular sludge and cattle manure

Abstract

Every year there is excess waste generated from coffee processing, such as wastewater and solid waste (pulp and husk), in addition to waste from the breweries’ production chain that causes environmental impacts due to inappropriate disposal. This study focuses on optimizing the interaction of these wastes for anaerobic co-digestion to obtain methane in batch reactors with mixed culture composed of cattle manure and granular sludge. Thus, the experimental planning via Rotational Central Composite Design (RCCD) with a complete factorial design of 23 was carried out to evaluate the influence of the concentration of coffee wastewater processing (5.2–17.2 g COD L−1), wastewater from brewery industry (0.5–1.2 g COD L−1) and coffee pulp and husk hydrothermally pretreated (0.5–1.2 g L−1). After validation, the optimal conditions for CH4 production were 10 g COD L−1 of coffee wastewater co-digested with 0.9 g COD L−1 of brewery wastewater and 1 g L−1 of coffee pulp and husk, obtaining a yield of 750.8 ± 18.7 mL CH4 g−1 TVS. The taxonomy and metabolic inference were carried out by sequencing of the 16 S rRNA gene on the Illumina HiSeq platform. Under validation assay conditions, producers of cellulolytic enzymes were inferred by Macellibacteroides, Christensenellaceae_R-7_group, Ruminococcus and Bacteroides, in addition to archaea Methanosaeta. With that, it was possible to infer that acetyl-CoA synthetase enzyme was inferred in higher proportion in validation assay, related to acetoclastic methanogenesis. Through the optimized condition and the energy balance, it was possible to predict the supplying of 91.1% of the farm's energy demand.