ORCID Identifier(s)

0000-0001-5718-8479

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

Victoria Chen

Third Advisor

Hyeok Choi

Fourth Advisor

Sahadat Hossain

Abstract

Methane (CH4) is one of the major greenhouse gases (GHG) generated in landfills and has a global warming effect 28 times more than carbon dioxide (CO2). Therefore, decreasing methane emissions into the atmosphere from landfills is critically important. In the upper portions of a landfill cover, methane is exposed to oxygen and oxidized aerobically to carbon dioxide while passing through the cover soil; this lowers the overall contribution of the landfill to climate change. However, because of the low permeability of the landfill cover, no aerobic oxidation occurs in the bottom of the cover because oxygen cannot penetrate to those depths. One possibility for increasing the overall oxidation of methane through landfill covers is to increase anaerobic oxidation of methane (AOM) in the lower depths. Although AOM has been studied by previous researchers in fresh water, sea water, and peat soil, no previous study has focused on AOM in landfill cover soil. In this study, anaerobic oxidation of methane (AOM) in the landfill cover soil was studied. Specific objectives were: 1. To evaluate the ability of alternate electron acceptors (besides oxygen) to facilitate anaerobic methane oxidation in clay soil, using batch tests. Different concentrations of the electron acceptors such as sulfate, nitrate, and iron were evaluated. 2. To study the effect of environmental conditions such as different moisture contents, nutrients, and methane concentrations on anaerobic oxidation of methane through batch tests, as well as the effect of methane generation inhibitor. 3. Using the most promising electron acceptor concentrations determined from Objective 1, to measure rates of anaerobic oxidation of methane in clay landfill covers via column tests, which includes realistic conditions of gas flow, cover thickness, and cover compaction. Compaction, permeability, sieve, hydrometer, liquid limit, plastic limit, and electron spectroscopy for chemical analysis tests were conducted to characterize the soil. Batch tests were conducted in 125 mL glass Wheaton bottles with 17 g soil. Electron acceptors (red mud-containing iron, iron chloride, iron oxide, hematite, sodium nitrate, potassium nitrite, sodium sulfate, manganese oxide, and ammonium chloride) were added to the soil, along with water (20% or 47% moisture content), nutrient solution, and/or methane generation inhibitor, as appropriate. After flushing the reactors with nitrogen gas, landfill gas (LFG) (50% methane, 50% carbon dioxide) was injected. Methane concentration in the headspace of the reactors was measured over time using a gas chromatograph. Maximum oxidation rate was also calculated using Michaelis-Menten kinetics. Batch tests results showed that sulfate, nitrate, and a combination of sulfate+iron could remove more methane compared to the control test over the long-term and had higher maximum oxidation rates. Hence, they were chosen for testing in columns. Moreover, according to the batch tests, methane removal decreased in the reactors with no added nutrients, lower moisture content, and low initial concentration of methane. The results also showed that adding inhibitor increased methane removal in some reactors while it lowered AOM in other reactors. In columns, the soil was compacted to create a 2-foot layer of cover soil. Methane entered the column at a flux of 179.4 gCH4 m-2 day-1 from the bottom and passed through the cover. Oxidation rate was obtained by measuring methane concentration at the port, where gas entered the column, and at the end of the anoxic zone. The results of column tests showed that at a higher landfill gas flow rate, there was no significant difference in methane removal in the anoxic zone of the columns; however, at a lower flow rate, methane removal in the column amended with sulfate + iron had the highest (around 10%) removal of methane in the anoxic zone, followed by the column that contained sulfate. The results showed H2S gas at the headspace of these two columns, which indicated that sulfate-reducing bacteria were likely responsible for methane removal in the anoxic zone of the columns.

Keywords

Anaerobic oxidation of methane, Eelectron acceptor, Landfill cover soil, Soil tests, Batch reactors, Column reactors

Disciplines

Civil and Environmental Engineering | Civil Engineering | Engineering

Comments

Degree granted by The University of Texas at Arlington

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