Submitted to: Institute of Biological Engineering Meeting Proceedings
Publication Type: Abstract only
Publication Acceptance Date: 3/6/2010
Publication Date: N/A
Citation: Interpretive Summary:
Technical Abstract: Conversion of second-generation renewable energy sources to useful products is gaining attention as an alternative to traditional conversion of sugar and starch-based renewable energy crops. The natural recalcitrance of second-generation energy resources, such as (ligno)cellulosic feedstock, makes biological conversion exceptionally challenging. We studied conversion of fractionated corn fiber, which is a byproduct in the corn-to-ethanol industry, into n-butyrate, which can be utilized as a precursor for the biofuel butanol. The bioconversion process, which consists of anaerobic hydrolysis and fermentation, is performed with thermophilic (55 deg C) anaerobic systems with open, undefined mixed microbial cultures. These systems are less energy intensive than pure culture processes because intensive substrate pretreatments, such as complete saccharification and sterilization are unnecessary. However, undefined culture bioprocessing presents major hurdles to overcome because the fermentation end products are typically less concentrated and more diverse. To study product selectivity, we operated 3 identical 5-L anaerobic sequencing batch reactors (ASBRs) treating corn fiber for 275 days - each with a different substrate pretreatment. The 3 pretreatments were performed in fluidized sand bath reactors at 36% corn fiber w/v in water at 160 deg C for 20 min with the variations: 1. 0.5% w/v H2SO4 (acid); 2. 1:10 CaO to biomass ratio (base); or 3. hot water only. The environmental conditions of the reactors were selected to direct the flow of reducing equivalents toward n-butyrate production, while maximizing hydrolysis. Inhibition of methanogenesis was studied in batch tests and in the reactors, and we found that undissociated organic acids must be maintained at high concentrations. Inhibition of hydrogenotrophic methanogens caused reducing equivalents to divert toward n-butyrate and other reduced products because hydrogen levels made nicotinamide adenine dinucleotide hydride (NADH) oxidation by H+ unfavorable according to thermodynamic models. Even though elevated undissociated acid concentrations were valuable for complete methanogen inhibition, their toxicity limited total substrate conversion. Dilution of reactor product concentrations improved efficiency of substrate conversion to total acid and alcohol products from ~27 to 32%, ~21 to 26%, and ~25 to 29% for reactors treating acid, base, and hot water pretreated substrate, respectively (based on chemical oxygen demand). The dilutions also significantly increased the n-butyrate fraction of acid and alcohol products, indicating that n-butyrate production can be maximized when product inhibition is minimized. Additionally, thermodynamically feasible (negative delta G) secondary microbial conversion of products was identified. Strategies for controlling secondary conversions in a useful way were evaluated. Although 100% conversion of reducing equivalents to n-butyrate is unlikely, thermodynamic analysis and reducing equivalent balances elucidate strategies to improve performance.