Table 3.

Technologies for biomethane generation from wastewater

Technologies	Process	Typical Systems	Scale of the technology	Advantages	Limitations	Source
Anaerobic digestion	Biological process that converts organic matter in wastewater into biogas	anaerobic sequencing batch reactor, upflow anaerobic sludge blanket	Full scale	Can treat domestic sewage, industrial wastewater, and agricultural waste Can reduce the volume and mass of the organic waste Can reduce greenhouse gas emissions by capturing and utilising methane	Requires careful control of process conditions to maintain efficient operation, can generate odour, can produce a high concentration of residual sludge, and requires a long retention time	(Kumar et al. 2021)
Membrane bioreactors	Combined with anaerobic digestion	Consists of a bioreactor tank, a membrane filtration system, and a control system	Lab-scale and full-scale	High biogas production, reduced footprint, lower sludge production, lower odour emissions	High energy consumption, high capital costs, membrane fouling, sensitivity to shock loads	(Elmoutez et al. 2022)
Microbial Electrolysis Cells	microbes to catalyse the reduction of carbon dioxide (CO2) and other organic compounds to produce methane (CH4) through an electrochemical process	consists of an anode and a cathode separated by a membrane	Primarily lab-scale, some pilot-scale	High energy efficiency, reduced carbon footprint, flexibility	High cost, low methane yield, technical challenges	(Koul et al. 2022)
Upcycling (co-digestion)	Use other waste materials such as food waste or agricultural waste or algal biomass as feedstock	Involves collecting organic waste materials, and processing them through anaerobic digestion or microalgae cultivation on wastewater	From small to full	Waste reduction, local production, economic benefits	Technical challenges and high capital cost	(Tsapekos et al. 2021); (Vaz et al. 2023); (Deng et al. 2023)
Biogas Upgrading	Upgrade biogas to biomethane	Water scrubbing, pressure swing adsorption, membrane separation	Varying levels of maturity, with some being widely implemented and others still in developmental stages	Suitable for grid injection, flexible in handling various input gas compositions	Involve high capital and operational costs Chemical scrubbing can be energy-intensive	(Martín-Hernández et al. 2020); (Assunção et al. 2021)
Thermal hydrolysis	Pre-treatment process	Break down cell walls and complex organic molecules to make it easier for microbes to digest		Increase biogas production and reduce pathogen levels	Energy intensive and requires high capital investment	(Balasundaram et al. 2023)