Methanotrophic bacterial biorefineries: resource recovery and GHG mitigation through the production of bacterial biopolymers

Wastewater treatment relies on the concerted activity of a diverse set of microbial populations which depending on the conditions, can produce harmful greenhouse gases such as carbon dioxide, methane, and nitrous oxide. Furthermore, conventional wastewater treatment produces high quantities of waste sludge that requires post-treatment, thus increasing the costs and, greenhouse gas emissions, and the carbon footprint. In addition, the lack of regulation on greenhouse gas mitigation results in nonquantified and nonregulated emissions to the atmosphere from wastewater. Anaerobic digestion systems are a useful tool for the production and recovery of energy from organic waste in the form of methane-rich gas mixtures, commonly referred to as biogas. Traditionally, the recovery of methane from anaerobic digestion has been utilized as a feedstock for the production of heat and electricity. However, this is only feasible in large sewage treatment facilities, and the recovery efficiency remains a challenge with secondary emissions from dissolved methane upon discharge. Furthermore, with the emerging rise of wind and solar power, the value of natural gas or biogas for electricity production is rendering the future of methane as an energy source, uncertain. With the current trends in the circular economy, waste management is shifting from the canonical view of waste neutralization into waste valorization and nutrient recovery, a concept known as bio-based circular economy. Methane produced in anaerobic digestion systems can serve as the energy source for a diverse array of microorganisms capable of consuming methane and producing valuable compounds. These microorganisms are called methanotrophs and they possess great potential for emerging applications to create novel and sustainable applications from methane through novel methane-driven biorefineries. Methanotrophic bacteria oxidize methane to methanol, which is an important chemical for a large number of industries globally. Furthermore, methanol is oxidized to formaldehyde and formate which are in turn integrated in biomass or accumulated in the form of bacterial polymers. Methane-derived bacterial biopolymers can be used as sustainable alternatives to chemical polymers in the plastic industry, or hydrocarbons in the fuel industry, and even contribute to the global protein demand. This chapter presents an overview of methane in wastewater treatment from formation, to environmental relevance, and summarizes the current state of three examples involving emerging methane-driven technologies intended for circularity through the production of biopolymers by methanotrophic bacteria: (1) methane to bioplastics through the production of polyhydroxy alkanoates, (2) methane to biodiesel through the hydrodeoxygenation of bacterial lipids from methanotrophs, and (3) the use of bacterial proteins from methanotrophs as an alternative protein source for animal and human consumption. Furthermore, this chapter analyzes the bottlenecks from a microbial and engineering perspective in order to streamline methane-driven bacterial biorefineries as a mainstream process. The potential to transform wastewater treatment plants into circular factories through the application of methanotrophs is an emerging field that could shift wastewater management paradigms into a “trash to treasure” bio-based circular economy.