Impacts of Carbohydrate-to-Protein Ratio on Fermentation Products of Chain Elongation

Organic waste streams, such as food and yard waste, are a largely untapped resource, and can be converted to valuable compounds using anaerobic digestion (AD). The production of volatile fatty acids (VFAs) and medium-chain fatty acids (MCFAs) in AD is a promising alternative to methane generation, with the potential to increase economic and environmental value. However, fatty acid production is not well studied, particularly in mixtures of complex macromolecules. In this experiment, synthetic wastewater with a varying mixture of carbohydrate (cellulose) and protein (α-lactalbumin) was fed into 1 L continuously stirred-tank reactors at a rate of 6.7 g COD d-. The bioreactors were controlled at a pH of 5.5 and solids retention time of six days to promote fatty acid production and inhibit methane generation. The reactors experienced an initial stabilization followed by four phases lasting 18 days each, with carbohydrate-to-protein ratios in the feedstock of 4:1, 3:2, 2:3, and 1:4, in phases 1-4, respectively. The microbial community was characterized and concentrations of VFAs, carbohydrates, proteins, chemical oxygen demand (COD), headspace gases, and ammoniacal nitrogen were monitored every six days. High degradation rates (>99%) of the influent carbohydrates and proteins were observed throughout, as well as a large consumption of COD (80-90%). Acetic acid and propionic acid were the most abundant VFAs produced. Acetic acid concentration was inconsistent and showed large variation during the experiment. The concentrations of 3- to 8-carbon VFAs increased slightly in both reactors from phase 1 to phase 4, particularly for the larger fatty acids. Therefore, high protein ratios in the feedstock may be favorable for the production of the more valuable, longer chain VFAs. The majority of the microbial community were from phyla Firmicutes and Bacteroidota in both bioreactors, however there was large variation in the genera present between the bioreactors and between sampling days. These data suggest that the microbial community in AD can adjust rapidly to changes in feedstock composition to maintain VFA production rates.