Graduation Year

2023

Document Type

Thesis

Degree

M.S.E.V.

Degree Name

MS in Environmental Engr. (M.S.E.V.)

Degree Granting Department

Civil and Environmental Engineering

Major Professor

Sarina Ergas, Ph.D.

Committee Member

Qiong Zhang, Ph.D.

Committee Member

Paul Kirchman, Ph.D.

Keywords

Antimicrobial, Biomethane, Energy Offset, Inhibition, Waste Management

Abstract

Florida is home to more than 300 craft breweries, that brew over a million barrels of beer annually. Brewing is an energy intensive process and produces large quantities of high strength waste including spent grains, yeast, hops, and high strength wastewater. Brewery wastewater, and spent yeast have high Chemical Oxygen Demand (COD) concentrations between 5000 mg/L – 10,000 mg/L, and 200,000 mg/L – 300,000 mg/L respectively. Brewery effluent wastewater is sent to Publicly Owned Treatment Works (POTWs) which implement high surcharges based on the strength of brewery effluent (COD concentration). While some of the spent yeast and hops can be diverted as animal feed along with spent grains, brewers are still left with huge quantities of yeast and hops to dispose. Furthermore, brewing requires large energy inputs in the form of heating fuel, and electricity.

Anaerobic digestion is a process in which microbes biodegrade organic material in the absence of oxygen. The byproduct of this process is digestate, and biogas, which predominantly consists of methane, and carbon dioxide. Methane can be harnessed as an energy source to offset energy requirements in breweries, and the digestate can be used as an organic fertilizer. Anaerobic digestion has been used by industrial breweries for onsite treatment of high strength effluent wastewater, and energy generation. However, the implementation of anaerobic digestion in smaller breweries faces obstacles including long pay back periods, and high capital costs. While anaerobic co-digestion of spent yeast, and brewery wastewater has yielded promising results, literature on mono-digestion of yeast waste, and co-digestion of yeast, and hops is sparse. Furthermore, spent hop metabolites including alpha and beta acids have antimicrobial properties that can inhibit methane production from anaerobic microbes found in rumens of ruminants. However, the effects of hops on anaerobic digestion of brewery waste are not explored. This study seeks to evaluate anaerobic mono-digestion of spent yeast, and co-digestion of spent yeast and hops.

In this study, Biomethane Potential Assays (BMPs) were set up using yeast and hops at varying substrate to inoculum ratios of 2.5 (Phase 1), and 1.7 (Phase 2). Both phases were set up with varying hop dosages of 0%, 20 % hops, and 40 % hops. Cumulative methane yields of 0.16 ml CH4/mg COD, and 0.15 ml CH4/mg COD were obtained for 20 % hops, and 40 % hops during Phase 1. Cumulative methane yields of 0.17 ml CH4/mg COD, 0.15 ml CH4/mg COD, and 0.11 ml CH4/mg COD were obtained from yeast only (0% hop dosage), 20 % hops, and 40 % hops respectively during Phase 2. In both phases, lower Gompertz methane rate constants were obtained for higher hop dosages.

Anaerobic Sequencing Batch Reactor (ASBR) studies were conducted in two phases. Duplicate ASBRs were operated at varying Organic Loading Rates (OLR) between 500 mg COD/L/day– 950 mg COD/L/day, and Solids Retention Times (SRTs) of 90 days, and 190-days during Phase 1 to evaluate mono-digestion of yeast. Average methane yields of 0.25 ml CH4/mg COD, and 90 % Total Chemical Oxygen Demand (TCOD) degradation was obtained during Phase 1. Phase 2 of the ASBR studies evaluated co-digestion of spent yeast, and hops with ASBRs operated at 0 % (yeast only), and 20 % hop dosages at OLRs between 700 COD/L/day to 900 mg COD/L/day, and SRT of 190 days. The digesters with 20 % hops provided lower methane yields, and COD degradation than the yeast only (0 % hops) reactor.

Results showed that addition of 20% hops dosage did not produce significant inhibitory effect in the BMP assays, whereas 40% hops dosage resulted in lower methane yields, and lower Gompertz rate constant. During the ASBR studies, mono-digestion of spent yeast produced methane yields comparable to yields obtained in literature for co-digestion of spent yeast with brewery effluent. Addition of 20 % hops dosage to the ASBR reactor resulted in lower methane yields, and lower TCOD degradation, however, long term studies are required to investigate this further.

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