Abstract
The transition towards sustainable industrial systems requires expanding the production of renewable energy carriers while enhancing their economic profile. Biomethane is a key energy vector potentially interlinking waste management systems, agricultural industries, renewable electricity storage, and existing energy distribution and utilization infrastructure. Coupling state-of-the-art anaerobic digestion (AD) with novel biomass thermochemical conversion processes is a possible approach to reduce the cost of biomethane production through the valorization of recalcitrant and non-recalcitrant biomasses in integrated processing lines. This thesis investigates the biological conversion of syngas (gaseous mix of H2, CO, CO2 produced from biomass gasification) to methane by co-digestion with organic waste streams. The focus lays on the methanation performance of lab-scale continuous bioreactorsand CO inhibitory effects on the microbial communities. The co-digestion of syngas and brewery spent yeast (BSY) was carried out in a lab-scale continuously stirred tank reactor (CSTR, Chapter 2). The CSTR was continuously fed with syngas (45% H2, 20% CO) and BSY (respectively, 1.13 L·L −1 · d −1 and 0.55 gVS · L −1 · d −1 , Hydraulic Retention Time: 15 days) resulting in increased biomethane productivity (233 mLCH4 · L −1 · d −1 ) compared to a control period in which syngas was used as sole carbon and energy source (150 mLCH4 ·L −1 · d −1 ). However, nutrient limitations decreased methanogenic activity after prolonged co-feeding operation (i.e., after 1.5 hydraulic retention time) pointing at the need for appropriate nutrient management practices, e.g., co-feeding of additional substrates, when co-digesting syngas with BSY. Chapter 3 focuses on a further experimental campaign on continuous syngas co-digestion in a CSTR at mesophilic conditions using cattle manure as co-substrate. In an initial control period in which the reactor was fed with manure only (2 gVS · L −1 · d −1 , Hydraulic Retention Time: 20 days) a steady biomethane production rate corresponding to a yield on volatile solids of 158 L · kg1 was achieved. The methane productivity was incremented through syngas (55% H2, 45% CO) feeding (0.43 L·L −1 · d −1 ) from 317 mL·L −1 · d −1 in the Control Stage to 477 mL·L −1 · d −1 in the final co-feeding stage. Furthermore, the reactor showed stable behavior encountering no nutrient limitations. Syngas conversion efficiency and methane productivity showed high sensitivity towards the gas-liquid mass transfer rate. Such experimental evidence can be used to validate kinetic models to simulate and optimize syngas co-digestion as a function of several operational variables (i.e., syngas loading rate, syngas composition, organic loading rate, gas-liquid mass transfer, substrate composition). As a fundamental step towards such optimization, Chapter 4 aims at assessing and quantifying the inhibitory effects of carbon monoxide on key anaerobic digestion steps, i.e., glucose acidogenic fermentation and aceticlastic methanogenesis. Aceticlastic methanogenic archaea (especially the genus Methanosarcina), were significantly inhibited at CO dissolved concentrations in equilibrium with partial pressure above 0.25 bar. Indeed, in mesophilic batch assays fed with acetate and subjected to the presence of carbon monoxide in the gas phase (0.25-1.00 bar), the methane produced after 9 days of culturing was only 2-20% of the methane produced by cultures not subjected to CO. In analogous batch experiments fed with glucose, the glucose-degrading community was overall less inhibited by CO compared to aceticlastic archaea. However, the microbial community shifted the metabolism from predominant acetate production towards propionate production thanks to higher resilience towards CO of propionate-producing species (e.g., Anaerotignum propionicum). Carbon monoxide inhibitory effects on glucose-degrading bacteria, homoacetogenic bacteria, and aceticlastic methanogenic archaea were modeled kinetically as non-competitive inhibition. The key conclusions that could be drawn from the thesis work include the fact that (1) CO inhibits significantly AD-involved microbial groups (aceticlastic methanogens, glucose degrading acidogens, homoacetogens), although such inhibitory effects are regulated by carboxydotrophic activity and mass transfer; 2) nutrient limitations are limiting factors to be carefully assessed in syngas co-digestion processes along with mass transfer limitations and CO inhibitory effects; 3) under further optimization of the unit operation focusing on mass mass transfer limitations, syngas biomethanation applied in organic waste co-digestion processes does represent a promising strategy for the reduction of biomethane unit costs through an enhancement in volumetric productivity. It can play a relevant role in exploiting more efficiently bioresources to renewable methane necessary for the energy transition. Future work will rely on simulations validated on the experimental data presented in Chapters 3 and 4 to analyze the energetic and economic performance of AD-gasification integrated process configurations under different energy management and cost scenarios. Possible process configurations are qualitatively discussed in Chapter 5.