Sustainable and environmentally friendly waste-to-energy conversion technologies, such as anaerobic digestion (AD) and gasification, have received significant attention in recent energy research. These technologies have proven their ability to reduce reliance on fossil fuels and greenhouse gas emissions by converting organic waste into products and fuels with market value, such as biomass, biogas, and synthetic gas. Since the syngas produced by biomass gasification contains highly toxic CO and flammable H₂, converting syngas into renewable natural gas has recently gained a lot of interest. By coupling AD with syngas, microbial consortium in the AD reactor converts the syngas into methane through a process known as biomethanation. Feeding syngas into the AD reactor is a method that not only can enhance methane production by conversion of CO₂ to CH₄ during the AD process but also converts syngas into methane as pure energy. This study aims to assess and compare the effect of different syngas compositions on methane production and optimize the SB process by identifying the best syngas composition and gas-biomass ratio under mesophilic temperature conditions. The study was conducted using batch and semi-continuous reactors in a lab-scale setting. The results of this study can contribute to the development of more efficient and sustainable methods for SB. In phase I of this study, syngas biomethanation under different syngas compositions was conducted under three different gas-biomass ratios (0.5, 1 and 1.5) in bench-scale experiments to study the impact on CO and H₂ partial pressure and CO toxicity on operation parameters (e.g., pH and VFA) and syngas conversion efficiency. The results showed that the optimum syngas composition with the highest amount of CH₄ is H₂-rich syngas (CO₂:CO; H₂ 1:1:7) and syngas with stoichiometric ratios between H₂ and CO/CO₂ (CO:H₂ 1:3; CO₂:H₂ 1:4) because of the sufficient available amount of hydrogen in the headspace. Methane content in the produced biogas reached 80.0%, 63.6% and 57.7%, respectively, compared to the control sample with 30.2% methane in the headspace. In phase II, the optimum syngas compositions were selected for experimenting with semi-continuous mode to 1) investigate the effect of injecting syngas in several stages in increasing syngas conversion efficiency, 2) adapt microorganisms to hydrogen and enhance biohydrogen production, and 3) test higher stoichiometric ratio between H₂ and CO/CO₂ to enhance syngas biomethanation efficiency. The data indicated higher methane content and syngas conversion in a semi-continuous mode. The biogas had methane concentration of 82.3, 76.9, 73.8, 84.9 and 81.7% in samples CO₂:CO: H₂ (1:1:7), CO:H₂ (1:3), CO₂:H₂ (1:4), CO: H₂ (1:4) and CO₂:H₂ (1:5). By injecting gas into the biomass in several stages, methane levels in the produced biogas in each stage increased, demonstrating the adaptation of microorganisms to the injected hydrogen and carbon-sourced gases. A higher stoichiometric ratio of H₂ to CO/CO₂ promoted the growth and activity of methanogens, leading to increased methane production.

The goal of this work was to establish the suitable and limiting concentrations of Zn, Cu and Mn compounds during syngas fermentation. The results showed that cells encased in polyvinylidene difluoride (PVDF) membranes had a faster accumulation of methane in reactors containing fermentation medium dosed with 5 mg/L of each heavy metal compared to free cells. It was also revealed that total inhibition of biohydrogen production occurred in medium containing 5 mg/L Cu, 30 mg/L Zn and 140 mg/L Mn while the most suitable metal concentration level was 0.1 mg/L Cu, 0.6 mg/L and 2.8 mg/L Mn. In addition, a comparison test showed that for the most suitable metal concentration in the medium, rate of performance at pH 6 and 7 was higher than at pH 5.