Biological upgrading of biogas to biomethane: evolution of microbial community in response to hydrogen addition
Author(s)
Lembo, Giuseppe
Date Issued
July 16, 2021
Type
Doctoral Thesis
Abstract
Energy from biomass is considered throughout the world as a type of sustainable and renewable energy. In particular, the anaerobic digestion (AD) of biomass produces a biogas with a content of about 50-60% of CH4 and 40-50% of CO2. However, the upgrading of biogas with > 90% of CH4 content has a higher calorific value, and it can be directly injected into the national grid or directly used as fuel by methane-powered vehicles. Currently, physical-chemical methods for the upgrading of biogas to biomethane are already commercially used in the biogas industry. However these technologies increase biogas production costs by 20-72% due to high investment, electricity demand and chemical-water requirement. Moreover the CO2 is release into the atmosphere. An attractive and cost effective method to increase the CH4 content in biogas is based on the biological reduction of CO2 to CH4 by using H2 produced either biologically or through water electrolysis using off-peak electricity surplus from wind or photovoltaic plants. In the AD process this reaction is carried out by hydrogenotrophic methanogens. Currently two main process approaches are used for the biological methanation: the in situ methanation, which consists of the direct injection of H2 into the anaerobic digester, using internally produced CO2 and the ex situ methanation, realized into a separate reactor where H2 and CO2 or H2 and biogas are injected and containing enriched cultures of hydrogenotrophic methanogens. Both biological upgrading technologies are still under developing. In particular, in situ biomethantion research was developed with the aim to modify the existing fullscale biogas plants, reducing the initial investment cost and the plant operation complexity. Among biogas plant the continuously stirring reactor (CSTR) configuration is the most popular employed for anaerobic treatment of industrial, domestic and municipal wastes.
Several studies suggest that the H2 diffusion in the liquid phase is the limiting factor of the process since the low H2 solubility make it not bioavailable for microorganisms. In order to overcome this limit, several in situ strategies for CSTR reactor configuration have been proposed, mainly, introducing different H2 injection devices, all using different stirring speeds or biogas recirculation rates. Although the hallow fibre membrane plus stirring seems to be the most favorable candidate for in situ biomethanation, the biofilm formation on the membrane damages the permeability and the lifespan of the membrane itself and then the connected costs have to be considered for full-scale applications. To overcome this problem, the project PhD course proposes the immobilization strategy as a method to extending gas-liquid contact area as well as to increase microorganisms abundance. Then a new anaerobic hybrid reactor, defined as Gas Stirred Tank Reactor (GSTR) where about one third
of the reactor working volume is filled with polymeric supports (high-density polyethylene, HDPE) was developed. The biogas recirculation was also applied to ensuring proper mixing of substrate and nutrients within the reactor and to increasing the availability of H2 for the hydrogenotrophic community. Moreover to counteract the increase of pH due to removal of CO2, an easily fermentable substrate was chosen, such as second cheese whey, a by-product of the dairy industry, very abundant in Italy and in Europe. Nevertheless the direct injection of H2 into the reactor brings various implications that shape the microbial community and the stability of the entire AD process. The increase of dissolved H2 affects all the bacterial guilds that use hydrogen in their metabolic pathways. These microorganisms include hydrogen-producing guilds such as acid-forming and acetogenic bacteria, as well as bacteria that consume hydrogen, such as homoacetogenic and hydrogenotrophic methanogenic bacteria. In this way the entire AD trophic chain is exposed to altered conditions and the system is put under stress. It is therefore clear that a deeper knowledge and characterization of the structure of the microbial community involved in biogas upgrading systems will provide useful information for the optimization of the process. For this purpose, the community structure was monitored using Next Generation Sequencing (NGS) technique. The extracted DNA was sequenced on the Illumina platform. Further investigations on the structure of the microbial community were carried out using the FISH (Fluorescence In Situ Hybridization) technique in order to evaluate the relationship between the Bacteria and Archaea domains. Samples were collected inside the reactor to investigate the microbial community response to H2 addition on both immobilized area and effluent.
Finally, the potential of the GSTR configuration to enhance H2 consuming efficiency by increasing gas recirculation rates and CO2/H2 ratio was investigated. The response of microbial community structure was studied by NGS techniques on effluent.
Additional information
Dottorato di ricerca in Ecologia e gestione sostenibile delle risorse ambientali
File(s)![Thumbnail Image]()
Name
glembo_tesid.pdf
Size
3.42 MB
Format
Adobe PDF
Checksum (MD5)
245a9e1ac4d2b351742a945881c242ff