Braden Myers and Jaron Hansen, Department of Chemistry and Biochemistry
Project Purpose
This project will demonstrate the technical feasibility of microbial pretreatment of lignocellulose for enhanced production of biogas when coupled with secondary anaerobic digestion as well as optimize the entire process.
Project Importance
Before anaerobic digestion of waste can reach its energy production potential, economically sound methods for pretreatments that allow increased digestion of waste materials must be developed. Many types of waste cannot be broken down sufficiently by anaerobic digestion alone. Despite the abundance of these types of waste material, anaerobic digestion of these potential energy sources remains inefficient because the anaerobic bacteria used in anaerobic digestion cannot readily consume the polymeric materials contained therein. These microorganisms are unable to break down long-chain molecules containing lignin. The main purpose of pretreatment of these waste materials is to alter the substrate composition in such a way that it is more suitable for anaerobic digestion. Therefore, pretreatment methods are attracting considerable attention for their potential ability to alter the structure and composition of the biomass and hence enhance (its) anaerobic digestion. Overcoming this hurdle through successful feedstock pretreatment will unlock a vast potential supply of renewable energy feedstock materials. While physical, chemical, and enzymatic (biological) pretreatment methods have been thoroughly investigated, microbial conversion provides another possible solution and is the focus of the proposed project.
Project Profile Body
Many wastes such as harvest residues, paper and lumber mill waste, and short rotation woody crops are available on a broad scale. Municipal solid waste (MSW) contains a large percentage of construction, landscaping, and yard waste and is available on a localized basis. The energy potential of this material is significant, primarily due to its abundance and availability across most geography. Despite this, its value has not been sufficiently captured for two reasons. First, there is an inability to efficiently convert lignocellulose biomass to fuel. Second, prominent existing systems (like gasification and pyrolysis) require large quantities of waste to make economic sense. Therefore, transportation and logistics often overwhelm the financial benefits of the system. Anaerobic digestion offers a readily scalable and well-understood mechanism for utilizing waste as an energy precursor. Anaerobic digestion is not only feasible in large-scale industrial installations, but can also be applied on a small scale. This observation specifically provides opportunities for anaerobic digestion in developing countries and rural areas, where energy supply is limited or unavailable. On the other end of the spectrum, large-scale municipal waste treatment centers in the U.S. and Europe often employ this waste mitigation technology as well. Simply stated, anaerobic digestion systems may be constructed and operated where waste is available. More importantly, this may be accomplished at minimal cost relative to competing technologies. Therefore, if pretreatment facilitates enhanced lignocellulosic feedstock utility in anaerobic digestion processes, it represents a major advancement in renewable energy generation.
Various pretreatment methods for lignocellulosic biomass have been attempted. These pretreatments can be classified into physical (e.g., mechanical pulverization and limited pyrolysis), physicochemical (e.g., steam explosion and ammonia fiber explosion), chemical (e.g., acid hydrolysis, alkaline hydrolysis, high temperature organic solvent pretreatment and oxidative delignification), biological (e.g., lignin degradation by white- and soft-rot fungi), and electrical (PEF) methods and various combinations thereof.
While the benefits of efficient pretreatment are overwhelming, none of the previously described techniques has effectively surmounted the technical barriers or minimize the tradeoff between benefit and cost. Operational requirements, lack of process control, and expense ultimately diminish the value of currently available methods. Therefore, it appears that the best option for conversion of the majority of carbon in these lignocellulosic feedstocks to a usable fuel is to employ microbial pretreatment of lignocellulosic anaerobic digestion feedstocks. The finding of simultaneous lignin and carbohydrate solubilization (facilitated by Caldicellulosiruptor bescii, C. bescii) bodes well for industrial conversion by extremely thermophilic microbes of biomass that requires limited or, more importantly, no chemical pretreatment (Kateva, et al. 2013). Past trials have yielded 85% lignin and cellulose solubilization rates when this bacterium is utilized. Therefore, this result will greatly improve down line methane production when pretreated feedstocks are introduced into secondary AD systems. Direct AD processing of untreated waste lignocellulose degrades only ~30 % of cellulose fed into a high-rate AD system and virtually none of the lignin constituent.
Because metabolic heat rate is directly proportional to the growth rate, and thus, optimal conditions of temperature, pH, and substrate composition for growth of microorganisms will be identified with isothermal biocalorimetry methods previously used in this lab to optimize anaerobic digestion for gas production (Cline, et al. 2010). The same methods will be used to rapidly screen the most suitable extremophiles for growth rate on lignocellulose. Isothermal biocalorimetry, specifically an 8-channel TAMAir (TA Instruments) with 20 mL vessels, was chosen for this part of the work because it is capable of rapidly measuring growth rates of both anaerobic and aerobic organisms in complex liquid cultures over a temperature range from 15 to 80⁰C under similar conditions to those used in anaerobic digestion tanks. Pressure sensors are incorporated into the sealed calorimeter ampules to determine if gases are produced by the bacteria and if so, to measure gas production rates. This calorimeter can thus provide growth rates for 8 different media at a given temperature. Also, the volume is sufficient to provide material for chemical analysis of the reaction products. At the end of a calorimetric run, gases will be analyzed, lignin degradates separated from saccharides, and degradation products identified and quantified with standard analytical methods such as chromatography, mass spectrometry, and spectrophotometry. Figure 1 shows the design of the new sample vials for simultaneous measurement of the heat rates and gas production.
The proposed project is designed to 1) establish the technical feasibility of microbial pretreatment of lignocellulosic feedstocks in preparation for anaerobic digestion to produce biogas, and 2) optimize the entire process on a bench/prototype scale. The collected data will be used to determine the optimum circumstances for the pretreatment of feedstocks.
Anticipated Academic Outcome
I plan to prepare a manuscript presenting the results of this project, which will be submitted to the journal Bioresource Technology.
Qualifications
Dr. Hansen has 15 years experience working with the production of biofuels. He has published numerous papers on the subject and is considered a pioneer in Biocalorimetry.
I am currently a junior, and have taken general chemistry and organic chemistry, as well as analytical chemistry and am currently taking physical chemistry as well as advanced organic synthesis. I have also been volunteering in Dr. Hansen’s research lab for over 14 months.
Project Timetable
January-February 2014 – Collect Data
February-March 2014 – Analyze data
April 2014 – Write and submit manuscript to Bioresource Technology