Value-Added By-Products During Dark Fermentation of Agro-Industrial Residual Biomass: Metabolic Pathway Analysis

Abstract

The Colombian agroindustry has high diversity and availability of residual biomass year after year. These wastes are susceptible to being recovered through biological processes to reducing production costs of some raw materials and the pollutant load to the environment. This study identified the value-added by-products during the use of pig manure (PM), coffee mucilage (CM), and cocoa (CoM) through a dark fermentation process in a battery of batch reactors (200 ml bottles). The organic load (2, 5, and 8 gCOD/l), the C/N ratio (25, 35, and 45), and the ratio of the CM: CoM (3:1, 1:1, and 1:3) were evaluated. The highest hydrogen production yield of 271.3 ml H2/gCOD was associated with the experimental conditions of mixture 2 (2 gCOD/l, 35 C/N ratio, and CM: CoM 1: 3). Acetate, propionate, butyrate, and lactate fermentations were associated with the metabolic pathways of the process and participated in the soluble COD with 29, 12, 46, and 13%, respectively. It was identified that the acetate-butyrate pathway was the main metabolic pathway, and the acetate/butyrate ratio was in the range of 1:1–5.9:1. The microbial population found in the co-digestion processes worked on was dominated by the genera Clostridium, Bacillus, and Tisierella. The complex substrates used in this co-digestion process can produce value-added by-products and energy in a range of theoretical stoichiometry.