Hoda Rahimi

ORCID Identifier(s)

0000-0002-2317-8317

Graduation Semester and Year

2019

Language

English

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Civil Engineering

Department

Civil Engineering

First Advisor

Melanie L Sattler

Second Advisor

Sahadat Hossain

Abstract

Methane from anaerobic processes is being increasingly utilized as an alternative energy source in developed countries, via large projects that extract methane from landfills or wastewater treatment plants. Anaerobic degradation of organic material (biomass) involves decomposition by bacteria under humid conditions without any oxygen. Organic carbon deposited in landfills is converted by microbes to carbon dioxide (CO2) and methane (CH4). If the methane is not captured, it contributes to climate change (28 times more effectively than CO2 on a per-mass basis). If the methane is captured, it can be used as a renewable energy resource. Lignocellulose comprises a significant portion of municipal solid waste (MSW) - 40-70% in developed countries, including paper, wood, yard waste, and textiles such as cotton fibers. Cellulose, the main biodegradable plant polymer, is often shielded by lignin, as well as hemicellulose. Lignin is unfortunately resistant to microbial degradation under anaerobic conditions that normally occur in MSW landfills. Lignin destruction can make cellulose and hemicelluloses available for anaerobic microbial conversion to biogas. This could potentially increase a landfill’s methane production by a factor of 2-3, depending on waste composition. Termite-associated Verrucomicrobia (TAV) is a gram-negative, coccoid-shaped, microaerophilic bacterium isolated from the hindgut of the Reticulitermes flavipes termite (the most common termite in North America). The TAV5 genome contains genes that code for enzymes which structurally modify lignin. The overall goal of this research is thus to increase the rate of degradation of MSW, and resulting methane generation, via use of TAV5 microorganisms. Specific objectives were: 1. To determine the optimum ratio of TAV5 to traditional methane-producing microorganisms for waste degradation and methane production, using batch reactor tests. 2. To compare the effectiveness of TAV5 cultured at 2% oxygen and 40°C, to TAV5 cultured at 20% oxygen and room temperature, in breaking down lignin-containing waste for methane production, using batch reactor tests. 3. To test the effectiveness of the optimum ratio of TAV5 in degrading lignocellulose and generating methane at 40°C in lab-scale landfill reactors for separated types of waste and mixed waste. To accomplish the objectives, three kinds of waste, including paper and cardboard, yard waste and wood, were collected. 72 batch scale reactors were prepared with selected MSW and different ratios of TAV5 to anaerobic digester microorganisms and incubated at 40º C. In the batch scale reactors, 3 different types of separated waste and 4 different ratios of mixed of waste were tested to see the effect of results of TAV5 ratios. Based on the preliminary studies on batch scale tests, five 6-gallon laboratory reactors were conducted for wood, mixed of waste with equal weight ratio and yard and incubated at 40º C, the leachate were recirculated, and the pH level were controlled in the recirculated leachate. For batch scale reactors (through day 150 of operation), the paper with 10 % TAV5 at 2% Oxygen and 40º C reactor had the highest rate of methane generation, followed by 70% TAV5 wood waste, and finally yard with 40% TAV5. For the mixed of waste batch scale reactors, in 2 cases 10% TAV5 produced the highest cumulative methane generation, and in 2 cases 40% TAV5 produced the highest. For batch scale reactors seeded with TAV5 at 20% Oxygen, the paper with 40% TAV5, yard with 40% TAV5, and wood with 30% TAV5 had the highest cumulative methane generation, and mixed waste (1:1:1) seeded with 10% TAV5 among the mixed waste ratios produced higher methane. Better results were obtained overall for reactors seeded with TAV5 cultured at 2% Oxygen compared to 20% Oxygen, although TAV5 cultured at 2% Oxygen performed almost as well in some cases. Additional testing of a fresh strain of TAV5 grown at 20% oxygen is recommended. For 6-gallon laboratory reactors, mixed waste, wood waste, and yard waste seeded with 10%, 40%, and 40% TAV5, respectively, generated 49%, 277%, and 34% more methane compared to reactors seeded with anaerobic digester microorganisms alone. CHN carbon results for the batch scale and 6-gallon reactors were consistent with the results of total cumulative methane generation, with reactors generating the greatest methane having the lowest CHN carbon values remaining in the solid phase at the end of the reactor runs. Soluble lignin and Klason lignin were lower for reactors seeded with TAV5, for both batch reactors and 6-gallon reactors, compared to reactors seeded only with anaerobic digester microorganisms. This suggests that addition of TAV5 was successful in breaking down lignin. Changes in lignin were greatest for paper, but still substantial for yard and wood wastes. Bacterial DNA analysis was done at the end to identify the type of bacteria and percent in solid samples extracted from batch reactors and 6-gallon reactors. Results showed the existence of TAV5 in all the samples.

Keywords

Lignin, Termite, Bioreactors, Landfill

Disciplines

Civil and Environmental Engineering | Civil Engineering | Engineering

Comments

Degree granted by The University of Texas at Arlington

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