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Title:Evaluation of anaerobic membrane bioreactors and hydrothermal catalytic gasification for enhanced conversion of organic wastes to renewable fuels
Author(s):Ong, Matthew
Advisor(s):Schideman, Lance C.
Department / Program:Engineering Administration
Discipline:Agricultural & Biological Engr
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Anaerobic digestion
Biogas production
Hydrothermal catalytic gasification
Membrane bioreactor
Renewable energy
Waste biomass
Abstract:Anaerobic membrane bioreactor (AnMBR) systems, merging anaerobic digestion with membrane filtration technology, have been studied as an alternative wastewater treatment process that can effectively generate energy while removing particulates and organics in a single process. AnMBR studies with municipal wastewater have observed high COD removal rates typically in the range of 72 – 94%, and methane yields higher than can be achieved with conventional anaerobic digesters. In this paper, routine bioaugmentation—the repeated addition of externally cultured microorganisms into the process stream—was tested as a means to increase hydrolysis and acid-production with the aim of thereby improving methane yields. The effects of bioaugmentation were tested in a pilot-scale continuous two-phase AnMBR system with a submerged (vacuum-driven) membrane in the methane-phase reactor. The acid-phase reactor was operated with a hydraulic retention time (HRT) and solids retention time (SRT) of 2.5 days, whereas the methane-phase reactor used an HRT between 15.2 – 18.3 days and an SRT between 79 – 92 days. With an organic loading rate (OLR) of 2.5 g-COD/L/day and bioculture addition to the acid-phase reactor at a dose of 3.9% of influent volatile solids, the acid-phase SCOD concentrations were increased by 56%, and total VFA concentrations were increased by 111%, indicating improved solubilization and acid-production. However, total methane yield was about 12% lower with bioaugmentation—423 ± 8 ml/g-VS without bioaugmentation and 372 ± 28 ml/g-VS with bioaugmentation. However, this difference was only statistically significant at a ≤ 78% confidence interval. The higher variability and minor decrease in methane production with bioaugmentation was correlated with higher production of hydrogen sulfide in the methane-phase, which could be explained by propionate and acetate accumulation in the acid-phase. The biological reactions of sulfate-reduction are more energetically favorable than those of acetogenesis and aceticlastic methanogenesis. Thus, sulfate-reducers can grow faster using these substrates than competing methanogens. After stopping bioaugmentation, the AnMBR system performance improved to achieve 98% of the theoretical maximum methane yield. It was found that bioaugmentation can have a neutral or slightly negative effect on an AnMBR system that is already achieving a high level of organics destruction without bioaugmentation. Both with and without bioaugmentation, the AnMBR system maintained > 99% COD removal while feeding a highly concentrated feedstock of 42 – 44 g-COD/L at a range of OLRs between 0.73 – 4 g-COD/L/day, resulting in an effluent COD concentration of 300 – 400 mg/L. High COD removal and methane yields (99.4% and 89.2%, respectively) were also achieved at a higher OLR of 5.1 g-COD/L/day, although membrane fouling inhibited consistent operation at this high OLR during this study. This document also investigates the application of subcritical hydrothermal catalytic gasification (HCG) as an alternative to anaerobic digestion. HCG of a lignocellulosic feedstock (newspaper) with Raney nickel (Ra-Ni) catalyst at 350°C for 30 minutes was capable of gasifying the feedstock into a combustible syngas, but the catalyst lifetime was far too low for economic feasibility. However, tests with routine addition of NaOH as a secondary catalyst showed high, consistent methane and energy yields by means of in-situ reactivation of Ra-Ni, which was designed upon the same principles of ex-situ Ra-Ni regeneration. At these yields, HCG was shown to potentially be economically viable. In-situ synthesis of Ra-Ni was also tested in order to lower operating costs, and increase safety by eliminating the need to store a pyrophoric material. Experiments showed that the activity of in-situ synthesized Ra-Ni catalyst was equivalent to or higher than that synthesized ex-situ. This could possibly be explained by the beneficial presence of bayerite during in-situ synthesis. Based upon the consistent energy yields achieved with routine NaOH addition, a comparison of HCG to literature reports of conventional AD was made. This analysis found that HCG could extract 40% of the newspaper’s energetic content while conventional AD could only achieve up to 18% (21% of theoretical methane). However, HCG’s higher heating requirements may lead to AD being more energetically favorable when processing organics with high moisture contents. Further analysis with newspaper feedstock, taking into account operational heating and catalyst embedded energy costs, showed that HCG is more favorable than conventional AD. At newspaper’s original moisture content of 8%, net energy production is roughly 2 times greater with HCG than AD. HCG is also advantageous because of its ability to process material on the timescale of 30 – 60 minutes while AD typically takes 20 – 40 days. Thus, HCG reactors are significantly smaller and can convert material that might otherwise remain unused for AD. However, the heat input for HCG is much higher than anaerobic digesters. As a result, when processing wet feedstocks, HCG would need to have a heat recovery system to be competitive with the net energy produced by anaerobic digestion. Overall, HCG with Ra-Ni and routine NaOH addition is a promising thermochemical alternative to biological anaerobic digestion processes, and has shown potential for economic viability and energy-positive operations. Further work should be conducted to minimize the amount of NaOH needed and maximize Ra-Ni lifetime, which are currently HCG’s greatest energy burdens.
Issue Date:2014-01-16
Rights Information:Copyright 2013 Matthew Ong
Date Available in IDEALS:2014-01-16
Date Deposited:2013-12

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