The Metabolic Model And Process Control Of Mixed Culture Fermentation

Mixed culture fermentation (MCF) has been recognized as a promising approach for municipal and industrial waste treatment and realizing resources recovery and valuable chemicals production such as methane, hydrogen, acetate and biofuels. Since MCF is a complex process including several steps with different functional microorganisms involved:hydrolysis/acidogenesis, acetogenesis, and methanogenesis, this process is sensitive to environmental and operating conditions. The followed separation and upgrading technologies are also necessary for the utilization of metabolites. Though there have been lots of research works, more studies are still urgently needed to uncover MCF.For the control of mixed culture fermentation, the thesis focused on the metabolic model, hydrogen supersaturation, the control of microorganisms community and metabolic distribution, gas fermentation, and biogas upgrading:the modification of Rodriguez model was proposed in the viewpoint of biochemistry and physiology to improve the model results similar to the metabolic distributions under experimental conditions; the hydrogen supersaturateion was firstly demonstrated and explained in thermophilic conditions by the online determination of hydrogen concentration in liquid with membrane inlet mass spectrometry (MIMS); hydrogenotrophic microorganisms were selectively enriched in extreme-thermophilic CSTR and the main metabolites were only methane and acetate; the in-situ utilization of syngas to produce medium-chain fatty acids was realized in a hollow fiber membrane biofilm reactor; the alkali production from bipolar membrane electrodialysis powered by microbial fuel cell was proposed to remove CO2and upgrade biogas. The main research contents and results are as follows:i. Based on the work of Rodriguez, a modified metabolic model for mixed culture fermentation (MCF) is proposed with the consideration of an energy conserving electron bifurcation reaction and the transport energy of metabolites. The production of H2related to NADH/NAD+and Fdred/Fdox is proposed to be divided in three processes in view of energy conserving electron bifurcation reaction. Meanwhile, the metabolic pathways of butyrate and propionate were also modified according to biochemical researches. The modeling results for a glucose fermentation in CSTR show that the metabolite distribution is consistent with the literature:1) acetate, butyrate and ethanol are main products at acidic pH, while the production shifts to acetate and propionate at neutral and alkali pH;2) the main products acetate, ethanol and butyrate shift to ethanol at higher glucose concentration;3) the changes for acetate and butyrate are following an increasing hydrogen partial pressure. The findings demonstrate that our modified model is more realistic than previous proposed model concepts. Moreover, since our modified model illustrates an improved methodology for estimating the metabolic stoichiometry of glucose in mixed culture fermentation, it can be applied on other fermentation strategies, such as batch and UASB systems.ii. Hydrogen supersaturation in thermophilic MCF was investigated online by a membrane inlet mass spectrometry. The results showed that with the increase of glucose loading rate and the decrease of Reynolds number, the hydrogen supersaturation occurred and the supersaturation ratio was between1.7and3.0. Meanwhile, higher H2aq resulted in lower hydrogen yield, lower glucose degradation rate and higher mole ratio of ethanol/(acetate+butyrate). Thus, H2aq is more appropriate than PH2when discussing the H2role in MCF. Furthermore, the calculated KLa clearly illustrated that the required KLa values for maintaining low H2aq were order of magnitudes higher than the experimental ones. Therefore, hydrogen supersaturation is inevitable in practice and should be considered in MCF.iii. The simultaneous methane and acetate production in extreme-thermophilic (70℃) mixed culture fermentation was first investigated. Within100-day operation of a continuous stirred tank reactor, it was found that the system was quite stable, the yields of methane and acetate were0.89-1.22mol/mol glucose and1.45-1.60mol/mol glucose, respectively, which were close to the theoretical yields. Acetate accounted for more than90%of metabolites in liquid solutions. Batch experiments showed acetate could be produced up to34.4g/L, significantly higher than the one from hydrogen producing fermentation. The microbial community analysis found hydrogenotrophic methanogens (mainly Methanothermobacter thermautotrophicus and Methanobacterium thermoaggregans) dominated98%of methanogens, confirming that high temperature could select hydrogenotrophic methanogens and inhibit aceticlastic methanogens effectively.iv. Gasification of organic waste and biomass to syngas (H2/CO2) is seen as a promising route to a circular economy. Biological conversion of the gaseous compounds into a liquid fuel or chemical, preferably medium chain fatty acids (caproate and caprylate) is an attractive concept. This study for the first time demonstrated in-situ production of medium chain fatty acids from H2and CO2in a hollow-fiber membrane biofilm reactor by mixed microbial culture. The hydrogen was for100%utilized within the biofilms attached on the outer surface of membrane. The obtained concentrations of acetate, butyrate, caproate and caprylate were7.4,1.8,0.98and0.42g/L, respectively. The biomass specific production rate of caproate (31.4mmol-C/g-VSS/d) was similar to literature reports for suspended cell cultures while for caprylate the rate (19.1mmol-C/g-VSS/d) was more than6times higher. Microbial community analysis showed the biofilms were dominated by Clostridium ljungdahlii and Clostridium kluyveri. This study demonstrates a potential technology for syngas fermentation in membrane biofilm reactors.(?). The biogas upgrading is necessary for its application, and alkali CO2adsorption is one efficient method. In this study, a coupled system, bipolar membrane electrodialysis (BPMED)-microbial fuel cell (MFC), was proposed for alkali production, which could also realize electricity in situ utilization. It was found that the pH in the alkali production chamber was9.8. With higher NaCl concentration, bigger applied voltage and lower external resistance, the pH of the produced alkali solution also increased and the maximum value of which was11.6. Meanwhile, our system also performed desalination. Furthermore, the produced alkali solution was utilized for biogas upgrading. The CO2content decreased notably in headspace, which even reached0%at pH11.6of alkali solution. This study provides an elegant and sustainable way to extend BPMED and MFC application.