Date of Award

5-2022

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Environmental Engineering and Earth Science

Committee Chair/Advisor

Sudeep Popat

Committee Member

Kevin Finneran

Committee Member

David Freedman

Committee Member

Barbara Campbell

Abstract

Anaerobic wastewater treatment technologies produce renewable energy and recover resources through microbial conversion of organics. Anaerobic digestion is commonly utilized for municipal wastewater sludge treatment, and co-digestion with other organic-rich substrates like fats, oils, and grease (FOG) can improve methane production. Methane can be captured for combined heat and power production. Microbial electrochemical cells (MXCs) convert organic waste to electrical current to produce electricity or other valuable chemicals. Both these technologies would benefit from understanding the microbial conversion processes more thoroughly. FOG co-digestion sometimes inhibits rather than bolsters methane production. MXCs have potential for blackwater (urine and feces) treatment in remote locations. Optimizing microbial organic conversion through operational parameters, not addition of chemicals for pH or inhibition of unwanted microorganisms, is necessary.

FOG co-digestion with municipal sludge was studied in batch assays and semi-continuous digesters at 0%, 23%, and 43% volatile solids from FOG. For both studies, the FOG and waste activated, primary, and anaerobic digester inoculum sludges were obtained from Renewable Water Resources in Greenville, SC. Though FOG long chain fatty acid content was ~80% linoleic, oleic, and palmitic acids in approximately equal proportions, only palmitic, stearic, and myristic acids accumulated during methane inhibition in both batch assays and the semi-continuous fed co-digester. Palmitic acid accumulated to more than 15 mM while myristic and stearic acids accumulated to less than 5 mM. Acetate did not accumulate in inhibited batch assays, and consumption of amended acetate in addition to mcrA gene expression demonstrated sustained methanogen activity despite accumulated long chain fatty acids. Acetate accumulated up to 80 mM in the inhibited semi-continuous co-digester. Batch operation of the co-digester resulted in palmitic acid conversion first, followed by rapid acetate conversion corresponding to methane production once palmitic acid concentrations fell below 10 mM. Results from both studies suggested that palmitic acid was the primary bottle neck during FOG inhibition of methane production. The LCFA-oxidizer Syntrophomonas and hydrogenotrophic methanogen Methanoculleus were associated with batch assays that successfully overcame FOG inhibition and the co-digester after recovering methane production.

Operational strategies may improve microbial conversion of organics to electrical current in MXCs treating blackwater. MXCs may be useful for decentralized blackwater treatment in remote locations, such as space, that have limited access to other treatment options. Batch MXC experiments assessed the effect of source separation (treating the feces fraction without urine) on chemical oxygen demand (COD) conversion and microbial communities. Source separation had no effect; source separated and combined conditions achieved more than 20% COD conversion to electrical current and < 1% conversion to methane. A decrease in hydraulic retention time (HRT) from 12-days to 3-days in semi-continuous MXCs fed synthetic primary sludge-based blackwater demonstrated a decrease in COD conversion to electrical current from more than 20% to less than 10%. During the batch operation study and at longer HRT, a robust biofilm dominated by anode respiring Geobacteraceae and suspension communities dominated by hydrolytic and fermentative bacteria, not methanogens, aided conversion of organics to electrical current. However, during semi-continuous operation residual volatile fatty acids accounted for more than 20% of the influent COD demonstrating a need to cultivate an acid tolerant organic acid consuming microbial community for MXC operation in remote locations.

Author ORCID Identifier

0000-0001-5031-5658

Available for download on Saturday, April 15, 2023

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