Improving anaerobic digestion of brewery and distillery spent grains through aeration across a silicone membrane

Authors: Berry, Zachary; Loughrin, John; BURRIS, STUART - Western Kentucky University; CONTE, ERIC - Western Kentucky University; Lovanh, Nanh; Sistani, Karamat

Submitted to: Sustainability
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/24/2022
Publication Date: 2/26/2022
Citation: Berry, Z.P., Loughrin, J.H., Burris, S., Conte, E.D., Lovanh, N.C., Sistani, K.R. 2022. Improving anaerobic digestion of brewery and distillery spent grains through aeration across a silicone membrane. Sustainability. 14(5). Article 2755. https://doi.org/10.3390/su14052755.
DOI: https://doi.org/10.3390/su14052755

Interpretive Summary: An increase in the number of small breweries and distilleries has led to a large increase in the amount of used grains without adequate means of disposal. One means of disposal could be to use the grains as feedstock for anaerobic digesters. In anaerobic digesters, wastewater is broken down in the absence of oxygen to produce biogas which is largely a mixture of carbon dioxide and methane. The methane can be used as fuel for heating or to produce electricity. In this study, brewer's spent grain and distiller’s spent grain were used as the substrate for anaerobic digestion for 32 weeks. The digestate was treated by recirculation through a silicone hose located in an external tank which was filled with a salt solution. The hose served as a membrane allowing for the passage of gas depending on the pressure gradient across the membrane. The recirculation tanks were fitted with check valves to maintain three pressure treatments: 26 mm of Hg N2, 26 mm of Hg aeration or 100 mm of Hg aeration. A fourth control digester was operated with no recirculation. These treatments were chosen to determine if differences in digester stability, wastewater treatment efficiency, and biogas production could be detected. Animal manure was used to seed the digesters to provide microorganisms capable of methane production and provide buffering to resist acidification of the digesters. Initially, all four digesters had low buffering and, consequently low pH as organic acids such as acetic accumulated in the digestate. After six weeks, bicarbonate buffering and pH increased as organic acids decreased and methane production increased. The pH of the digestate treated by N2 recirculation was lower than the other digesters and decreased further after distiller’s spent grain was substituted for brewer’s spent grain. Aeration at a pressure of 100 mg Hg significantly increased biogas production compared to other treatments.

Technical Abstract: An increase in the number of independent breweries and distilleries has led to an increase in the amount of spent grains with inadequate means of disposal. One option for disposal is as feedstock for anaerobic digestion if digester stability is ensured. In this study, brewers’ spent grain and distillers’ spent grain were used as substrate for anaerobic digestion for 32 weeks. The digestate was treated by recirculation through a silicone hose located in an external tank filled with saline solution. The hose served as a permeable membrane allowing for the passage of gases. The recirculation tanks were fitted with check valves to maintain three pressure/gas regimes: 26 mm Hg N2, 26 mm Hg aeration or 100 mm Hg aeration. A fourth digester was operated with no recirculation as the control. These treatments were chosen to determine if differences in digester stability, wastewater treatment efficiency, and biogas production could be detected. A combination of dairy and swine manure was used as seeding to provide a methanogenic consortium and bicarbonate buffering. However, despite trying to provide for adequate initial bicarbonate buffering, all four digesters had low initial buffering and consequently low pH as short-chain fatty acids accumulated. After six weeks, bicarbonate buffering and pH increased as methane production increased, and short-chain fatty acids decreased. Later, despite the fluxes of O2 and N2 across the silicone membrane being very low, differences between the various treatments were noted. The pH of the digestate treated by N2 recirculation was lower than the other digesters and decreased further after distillers’ spent grain was substituted for brewers’ spent grain. Aeration at a pressure of 26 mm Hg and 100 mg Hg increased biogas production compared to other treatments but only significantly so at 100 mm Hg. These results suggest that partial purging of dissolved gases in anaerobic digestate by the small fluxes of N2 or O2 across a permeable membrane may affect digester performance.