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Sequential temperature phased enhanced anaerobic digestion using bioaugmentation microbes to enhance biomethane production from alkali pretreated late cut grass and late cut grass silage
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Date
2024
Authors
Donkor, Kwame O.
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Publisher
University College Cork
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Abstract
Anaerobic digestion is a biomass and waste conversion technology that uses a consortium of microbes to convert the organic matter to biogas which is primarily made up of methane and carbon dioxide. The technology tackles environmental pollution by reducing the waste going to landfills causing greenhouse gas emissions. Apart from producing biomethane that can substitute for fossil derived natural gas, heat and electricity, the digestate from the anaerobic digestion process can serve as biofertilizer that can also replace fossil-based fertilizer usage. Anaerobic digestion in most commercial scale settings is applied as a standalone system relying on a consistent supply of feedstock including for lignocellulosic feedstock such as late cut grass and late cut grass silage. However, such lignocellulosic feedstock is relatively recalcitrant and suffers from lower biomass biodegradability and lower biomethane production compared to starch and sugar rich waste streams.
For the biogas plants to extract maximum biomethane potential from recalcitrant lignocellulosic biomass, further supporting technologies aimed at increasing the feedstock digestibility are required. Hence, an integrated anaerobic digestion concept was proposed which combined alkali pre-treatment, temperature phased anaerobic digestion and microbial bioaugmentation to improve biomethane production from lignocellulosic biomass. The proposed concept was known as Sequential Temperature-phased Enhanced Anaerobic Digestion using Microbes (STEADM). An assessment of the proposed and modelled STEADM concept indicated that biomethane production from lignocellulosic biomass such as late cut grass could increase by 47% as compared to the conventional standalone anaerobic digestion system. Consequently, this propelled the research thesis to develop the Sequential Temperature-phased Enhanced Anaerobic Digestion using Microbes (STEADM) concept.
The development of the STEADM concept started with initially investigating an optimum alkali pre-treatment that can enhance biomethane production from late cut grass and late cut grass silage. Alkali pre-treatment targets hemicellulose and lignin solubilization to improve microbial bioconversion of lignocellulosic biomass. The first experimental work examined the effectiveness of alkali pre-treatment of late cut grass and late cut grass silage in improving biomethane production by using a mixture of calcium, magnesium and potassium hydroxides. This involved an optimization batch study that determined the optimum conditions of mixed alkali pre-treatment of biomass to enhance biomethane production. The optimum conditions of the mixed alkali pre-treatment for anaerobic digestion of late cut grass silage were found at an alkali loading of 16 wt.% (16g/100 g dry matter) at 50 °C. This generated a 78% increase in biomethane yield compared to anaerobic digestion of untreated late cut grass silage. Also, the optimal conditions for late cut grass were 14.8 wt.% alkali loading at 36 °C, which delivered a 98% increase in biomethane yield as compared to anaerobic digestion of untreated late cut grass.
After optimization study, an energy and techno-economic assessment was conducted to assess the cost-effectiveness of the optimized mixed alkali pre-treatment anaerobic digestion system as compared to the conventional anaerobic digestion system. The essence of integrating the techno-economic analysis with the optimization batch study was to determine at an early stage whether the mixed alkali pre-treatment to improve grass biomethane was a potentially worthwhile route to take in developing the STEADM concept. The techno-economic analysis indicated that mixed alkali pre-treatment improved the process economics, as the levelized cost of biomethane energy production (LCOE) was 21% and 45% lower for late cut grass silage and late cut grass respectively as compared to the conventional anaerobic digestion system. The LCOE for late cut silage was determined to be 19.0 and 23.9 c€/kWh for the alkali pre-treatment AD system and conventional AD system, respectively. The LCOE for late cut grass was 23.2 and 42.0 c€/kWh for the alkali pre-treatment AD system and conventional AD system, respectively.
The mixed alkali pre-treatment proved to be cost-effective and thus the study progressed to develop the STEADM concept by performing continuous digestion experiments at increasing organic loading for various anaerobic digestion (AD) scenarios. The investigated AD scenarios were single stage and temperature phased anaerobic digestion of untreated late cut grass silage [S0 & T0-M0], single stage and temperature phased anaerobic digestion of pre-treated late cut grass silage [S1 & T1-M1] and single stage and temperature phased anaerobic digestion of untreated late cut grass silage enhanced with bioaugmentation [S2 & T2-M2]. Assessment of the various AD scenarios indicated that mixed alkali pre-treatment improved biodegradability and allowed for biomass feeding at high organic loading up to 4.0 g VS L-1 d-1. Furthermore, bioaugmentation increased biomass solubilization about 1.4 to 2.8 times in bioaugmented digesters. Phase separation in temperature phased anaerobic digestion additionally improved anaerobic digester stability by limiting FOS-TAC below 0.400, even at high organic loading of 4.0 g VS L-1 d-1.
The developed STEADM system was able to operate at a high organic loading rate of 4.0 g VS L-1 d-1 but was limited to a short term 30-day digestion because of mechanical mixing difficulties in the digester. This was not a biological operational issue and thus in a commercial scale setting, appropriately designed mixers for fibrous materials will be able to operate the STEADM process at organic loading rates of 4.0 g VS L-1 d-1. As a result of the mechanical mixing limitation in this study, long term 90-day operation of the developed STEADM concept was conducted at operationally feasible organic loading of 3.0 g VS L-1 d-1 and produce a stable biomethane yield of 339 ± 27 L CH4 kg VS-1. This was 97% methane increase as compared to the conventional single stage anaerobic digestion of untreated late cut grass silage which produced a daily average methane yield of 172 ± 46 L CH4 kg VS-1 at organic loading of 1.0 g VS L-1 d-1.
In conclusion, this thesis demonstrated that considerable improvement in biomethane production from recalcitrant lignocellulosic biomass such as late cut grass silage can be achieved by integrating alkali pre-treatment, temperature phased anaerobic digestion and microbial bioaugmentation. The results from this thesis can provide future pathways to developing improved versions of the STEADM concept to further enhance the anaerobic digestion of various biomass feedstock.
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Keywords
Lignocellulose biomass , Alkali pre-treatment , Temperature-phased anaerobic digestion , Bioaugmentation , Late cut grass , Late cut grass silage , Anaerobic digestion , Response surface methodology , Biomethane , Techno-economic analysis , Levelized cost of energy (LCOE)
Citation
Donkor, K. O 2024. TSequential temperature phased enhanced anaerobic digestion using bioaugmentation microbes to enhance biomethane production from alkali pretreated late cut grass and late cut grass silage. PhD Thesis, University College Cork.