This study evaluates the biological water-gas shift reaction of syngas into hydrogen (H2) using natural microbial consortia from wastewater treatment facilities. Various inoculum pretreatment techniques to inhibit methanogenesis and different operational conditions such as temperature, pH, and inoculum concentrations were tested to improve H2 production. An inoculum heat pretreatment of 85 °C for 30 min followed by a fermentation temperature of 65 °C and an initial pH value of 9, resulted in the maximum CO-to-H2 conversion (2.35 ± 0.00 mmol) in which Firmicutes dominated with a total inhibition of methanogens, such as with the chemical inhibition treatment. Additionally, volatile fatty acids (VFA) production was observed, being influenced by the pre-treatment. This study highlights the potential of natural consortia for efficient, cost-effective and scalable syngas fermentation processes, offering new insights into the bioconversion of H2 without costly chemical inhibitors.

A major source of CO2 emissions is the flaring of steel mill gas. This work demonstrated the enrichment of carboxydotrophic bacteria for converting steel mill gas into volatile fatty acids and H2, via gas fermentation. Several combinations of pure and mixed anaerobic cultures were used as inoculum in 0.5-L reactors, operated at 30 and 60 °C. The process was then scaled up in a 4-L membrane bioreactor, operated for 20 days, at 48 °C. The results showed that the enriched microbiomes can oxidize CO completely to produce H2/H+ which is subsequently used to fix the CO2. At 30 °C, a mixture of acetate, isobutyrate and propionate was obtained while H2 and acetate were the main products at 60 °C. The highest CO conversion and H2 production rate observed in the membrane bioreactor were 29 and 28 mL/LR/h, respectively. The taxonomic diversity of the bacterial community increased and the dominant species was Pseudomonas.

Raw syngas contains tar contaminants including toluene and naphthalene, which inhibit its conversion to methane. Cell encasement in a hydrophilic reverse membrane bioreactor (RMBR) could protect the cells from hydrophobic contaminants. This study aimed to investigate the inhibition of toluene and naphthalene and the effect of using RMBR. In this work, toluene and naphthalene were added at concentrations of 0.5?1.0 and 0.1?0.2 g/L in batch operation. In continuous operation, concentration of 0?6.44 g/L for toluene and 0?1.28 g/L for naphthalene were studied. The results showed that no inhibition was observed in batch operation for toluene and naphthalene at concentrations up to 1 and 0.2 g/L, respectively. In continuous operation of free cell bioreactors (FCBRs), inhibition of toluene and naphthalene started at 2.05 and 0.63 g/L, respectively. When they were present simultaneously, inhibition of toluene and naphthalene occurred at concentrations of 3.14 and 0.63 g/L, respectively. In continuous RMBRs, no inhibition for toluene and less inhibition for naphthalene were observed, resulting in higher methane production from RMBR than that of FCBR. These results indicated that RMBR system gave a better protection effect against inhibitors compared with FCBR.

of waste treatment processes, such as the anaerobic digestion. This biochemical process converts organic substrates into biogas, with anaerobic microorganisms. However, some types of substrates have low bio-degradability due to its recalcitrance or the presence of inhibitors. This can be solved by the coupling of anaerobic digestion with gasification, a thermochemical process that can convert organic substrates into syngas (H2, CO, and CO2) regardless of the substrate´s degradability. Consequently, syngas can be converted into biogas and other fermentative products via anaerobic digestion, in a process known as syngas fermentation. In comparison to the catalytic conversion of syngas, syngas fermentation has several advantages such as lower sensitivity to CO/H2/CO2 ratio and to syngas contaminants as well as higher product specificity.

The main goal of this thesis was to improve the syngas conversion rate into CH4 and H2 by addressing the cell washout, the cell inhibition by syngas contaminants, and the low gas-to-liquid mass transfer, which are major challenges in syngas fermentation. For this purpose, a reverse membrane bioreactor, containing a mixed culture encased in membranes, was used in various set ups. The membranes were used in order to retain the cells inside the bioreactors, to protect the cells against inhibitors, and to improve the gas holdup and gas-to-cell contact by decreasing the rise velocity of syngas bubbles. As evident from the results, the cell washout was successfully tackled during a continuous experiment that lasted 154 days. In addition, membrane bioreactors fed with the syngas contaminants, toluene and naphthalene, achieved approximately 92% and 15% higher CH4 production rate, respectively, compared with the free cell bioreactors. In order to improve the gas holdup and consequently the gas-to-liquid mass transfer of syngas, a floating membrane bed bioreactor was set up. This bioreactor contained membrane sachets, filled with inoculum that formed a packed floating membrane bed and achieved an increase of 38% and 28% for the conversion rate of H2 and CO, respectively. Furthermore, the addition of a mixture of heavy metals improved the production rates and yields during the syngas conversion into fermentative H2.

The low gas-to-liquid mass transfer rate is one of the main challenges in syngas biomethanation. In this work, a new concept of the floating membrane system with high gas hold-up was introduced in order to enhance the mass transfer rate of the process. In addition, the effect of the inoculum-to-syngas ratio was investigated. The experiments were conducted at 55 °C with an anaerobic mixed culture in both batch and continuous modes. According to the results from the continuous experiments, the H₂ and CO conversion rates in the floating membrane bioreactor were approximately 38% and 28% higher in comparison to the free (suspended) cell bioreactors. The doubling of the thickness of the membrane bed resulted in an increase of the conversion rates of H₂ and CO by approximately 6% and 12%, respectively. The highest H₂ and CO consumption rates and CH₄ production rate recorded were approximately 22 mmol/(L·d), 50 mmol/(L·d), and 34.41 mmol/(L·d), respectively, obtained at the highest inoculum-to-syngas ratio of 0.2 g/mL. To conclude, the use of the floating membrane system enhanced the syngas biomethanation rates, while a thicker membrane bed resulted in even higher syngas conversion rates. Moreover, the increase of the inoculum-to-syngas ratio of up to 0.2 g/mL favored the syngas conversion.

Thermal processes of wastes lead to production of energy in form of electricity and/or heat. However, if the goal is to produce materials, thermochemical processes can be applied. These processes via e.g. gasification produce raw syngas that is a mixture of principally H₂, CO and CO₂, with some impurities. This raw syngas is traditionally cleaned and catalytically treated via chemical processes such as Fischer-Tropsch. However, as there is a variety of microorganisms that can assimilate syngas, this gas can be used as a substrate to produce different chemicals via biochemical routes. This chapter is dedicated to describe an efficient thermochemical-biochemical route of waste treatment. The gasification process, the design and the factors that affect the syngas composition are firstly described. Thereafter, the microbiology, biochemical reactions, metabolic pathways and process conditions toward production of several metabolic products from syngas such as carboxylic acids, ethanol, butanol, 2,3-butanediol, methane and biopolymers are presented. 

The effects of three heavy metals on hydrogen production via syngas fermentation were investigated within a metal concentration range of 0 to 1.5 mg Cu/L, 0 to 9 mg Zn/L, 0 to 42 mg Mn/L, in media with initial pH of 5, 6, and 7, at 55 degrees C. The results showed that at lower metal concentration, pH 6 was optimum while at higher metal concentrations, pH 5 stimulated the process. More specifically, the highest hydrogen production activity recorded was 155% +/- 12% at a metal concentration of 0.04 mg Cu/L, 0.25 mg Zn/L, and 1.06 mg Mn/L and an initial medium pH of 6. At higher metal concentration (0.625 mg Cu/L, 3.75 mg Zn/L, and 17.5 mg Mn/L), only pH 5 was stimulating for the cells. The results showed that the addition of heavy metals, contained in gasification-derived ash, can improve the production rate and yield of fermentative hydrogen. This could lead to lower costs in gasification process and fermentative hydrogen production and less demand for syngas cleaning before syngas fermentation.

The effects of three heavy metals on hydrogen production via syngas fermentation were investigated within a metal concentration range of 0-1.5 mg Cu/L, 0-9 mg Zn/L, 0-42 mg Mn/L, in media with initial pH of 5, 6 and 7, at 55 °C. The results showed that at lower metal concentration, pH 6 was optimum while at higher metal concentrations, pH 5 stimulated the process. More specifically, the highest hydrogen production activity recorded was 155.28% ± 12.02% at a metal concentration of 0.04 mg Cu/L, 0.25 mg Zn/L, and 1.06 mg Mn/L and an initial medium pH of 6. At higher metal concentration (0.625 mg Cu/L, 3.75 mg Zn/L, and 17.5 mg Mn/L), only pH 5 was stimulating for the cells. The results show that the addition of heavy metals, contained in gasification-derived ash, can improve the production rate and yield of fermentative hydrogen. This could lead in lower costs in gasification process and fermentative hydrogen production and less demand for syngas cleaning before syngas fermentation.

Biogas or biomethane is traditionally produced via anaerobic digestion, or recently bythermochemical or a combination of thermochemical and biological processes viasyngas (CO and H2) fermentation. However, many of the substrates feedstocks haverecalcitrant structure and difficult to digest (e.g., lignocelluloses or keratins), or theyhave toxic compounds (such as fruit flavors or high ammonia content), or not digestibleat all (e.g., plastics). To overcome these challenges, innovative strategies for enhancedand economically favorable biogas production were proposed in this review. Thestrategies considered are commonly known physical pretreatment, rapid decompression,autohydrolysis, acid- or alkali pretreatments, solvents (e.g. for lignin or cellulose)pretreatments or leaching, supercritical, oxidative or biological pretreatments, as well ascombined gasification and fermentation, integrated biogas production and