Date of Award
8-2024
Document Type
Thesis
Degree Name
Master of Science (MS)
Department
Environmental Engineering and Science
Committee Chair/Advisor
Sudeep Popat
Committee Member
David Freedman
Committee Member
Kevin Finneran
Abstract
Anaerobic digestion is a technology that allows wastewater treatment plants to convert sludge to energy by recovering the biogas produced during the breakdown of proteins, carbohydrates, and lipids. Furthermore, adding fats, oils, and greases (FOG) through co-digestion with wastewater sludge can increase energy production as lipids have a higher methane yield than proteins and carbohydrates. However, adding FOG can also lead to operational problems in the digester due to the potential accumulation of certain long-chain fatty acids (LCFAs). Current research is limiting in the degradation pathways of prominent LCFAs in FOG and the microbial communities responsible for their degradation
The objective of this research is to investigate the degradation pathways and intermediates of five LCFAs prevalent in municipal wastewater sludge and analyze the microbial communities involved in their degradation.
This study was completed using batch assays. Each acid chosen was analyzed at 2, 4, and 6 g/L COD. The study employed the use of a gas chromatograph (GC) for the analysis of biogas produced in the assays, a GC flame ionizing detector for the analysis of LCFAs, a high-performance liquid chromatograph for the analysis of volatile fatty acids, a third party for sequencing data, and qiime2 for sequencing analysis.
The results of this study revealed a correlation between lag time in methane production and saturation as the unsaturated acids had much longer lag times when compared to the saturated acids. There was no apparent correlation between chain length of saturated acids and lag time. In the unsaturated assays and myristic 6 g/L COD, the pH dropped significantly due to acetate accumulation. The pH dropped below the optimum of 6.8 for methanogens, causing more acetate to accumulate. Syntrophomonas, the suspected beta oxidizer, has an optimal pH of 6.5. pH levels below 6.5 could have been the cause of palmitic acid accumulation in the unsaturated assays since some of these assays had pHs below this value.
For the saturated acids, the anticipated LCFA intermediates that should have formed through beta-oxidation were not observed and did not accumulate. Both unsaturated acids had palmitic acid accumulate at high concentrations and stearic acid accumulate at low concentrations. The degradation pathway for unsaturated acids can be concluded to be either hydrogenation followed by beta-oxidation or simultaneous hydrogenation and beta-oxidation. There was no inhibitor identified for the saturated acids. The inhibitor for the unsaturated acids was palmitic acid.
Based on the degradation kinetics calculations for the saturated acids, myristic acid degraded the fastest followed by palmitic and stearic acid. Of the unsaturated acids, oleic acid degraded faster than linoleic acid. Both unsaturated acids degraded faster than the three saturated acids.
Acetate was the only volatile fatty acids observed at a high concentration. Acetate accumulated in varying degrees in all assays except the stearic acid assays. Propionate and butyrate were present in most assays and controls, but production and degradation did not correspond to lag phases or periods of inhibition. In all instances of a low pH, the cause can be linked to acetate accumulation.
Based on sequencing data, different microbial communities were present in unsaturated acid assays when compared to saturated acid assays. Also, the unsaturated acid assays also had differing microbial communities when compared to each other. In the saturated acid assays, the microbial communities associated with each concentration of each saturated acid was similar. However, in the unsaturated acids, the microbial communities at each concentration of each acid varied significantly.
Recommended Citation
Funk, Claire, "Degradation Products and Microbial Communities Associated with the Conversion of Five Long-chain Fatty Acids Relevant for Anaerobic Co-digestion of FOG with Sludge" (2024). All Theses. 4352.
https://open.clemson.edu/all_theses/4352