Arrested Methanogenesis of Organic Waste: A Sustainable Source of Short-Chain Volatile Fatty Acids

Volatile Fatty Acids (VFAs) are high-value short-chain organic acids with applications in biofuels, industrial chemicals, and materials. However, commercial VFAs are traditionally produced from fossil fuels, contributing to greenhouse gas emissions and climate change.^[1] In a circular economy, using waste streams to produce valuable fatty acids allows for a sustainable and cost-effective source of these products. This project aims to produce FAs through arrested methanogenesis of organic waste at pilot-scale digesters (50 liter). Volatile fatty acids are an intermediate product in anaerobic digestion, a natural biological process in which microorganisms break down organic matter in the absence of oxygen. The typical products of anaerobic digestion are methane and carbon dioxide. However, if the last step of anaerobic digestion, methanogenesis, is stopped, then VFAs can be obtained as final products. This is favorable over methane as VFAs are more versatile and have a higher market value.^[2] In this project, cheese whey, a by-product of cheese production, was used as the feedstock. Methanogenesis was arrested by controlling experimental conditions including pH (~6), organic loading rate (OLR), and hydraulic retention time (HRT). Various operating conditions, including OLR and HRT, were evaluated to assess their impact on carbon conversion, VFA yield, productivity, and titer, and the concentration of different VFA products during the process scale up .

Over three different operation periods, the OLR was increased while keeping the HRT constant (4.8 days). The first condition had the lowest OLR (13.3 gCOD/L·day) and produced the lowest concentration of VFAs (29.20 g/L) and had the lowest productivity (6.08 g/L_liq·day) but had the highest conversion (0.97 gCOD_digested/gCOD_fed) and yield (0.76 gCOD_acid/gCOD_digested). The second condition had an intermediate OLR (16.7 gCOD/ L·day) produced an intermediate VFA concentration (31 g/L) and had an intermediate productivity (6.52 g/Lliq·day), but had the lowest conversion (0.94 gCOD_digested/gCOD_fed) and yield (0.71 gCOD_acid/gCOD_digested). The third condition had the highest OLR (20.1 gCOD/L·day) and produced the highest VFA concentration (34.6 g/L) as well as the highest productivity (7.24 g/L_liq·day), but had a slightly lower conversion than condition 1 (0.96 gCOD_digested/gCOD_fed) and slightly higher yield than condition 2 (0.72 gCOD_acid/gCOD_digested). In addition, higher OLR resulted in significantly longer system adjustment time (28 days more) to reach steady state than other lower OLR conditions, and had a higher amount (21.55 wt%) of lactic acid present than condition 1 (2.09 wt%) or condition 2 (6.51 wt%). Overall, this project was able to successfully arrest methanogenesis, shown by a lack of methane produced in the bioreactor, and produce the desired product, VFAs.

References:

[1] Holtzapple, M. T., Wu, H., Weimer, P. J., Dalke, R., Granda, C. B., Mai, J., & Urgun-Demirtas, M. (2022). Microbial communities for valorizing biomass using the carboxylate platform to produce volatile fatty acids: A Review. Bioresource Technology, 344, 126253. https://doi.org/10.1016/j.biortech.2021.126253

[2] Wu, H., Scheve, T., Dalke, R., Holtzapple, M., & Urgun-Demirtas, M. (2023). Scaling up carboxylic acid production from cheese whey and brewery wastewater via methane-arrested anaerobic digestion. Chemical Engineering Journal, 459, 140080. https://doi.org/10.1016/j.cej.2022.140080