Numerical investigation of methanotrophs in a biofilter for methane emission mitigation

Methane is the second highest contributor to human greenhouse gas emissions. In Ireland, it represented 29% of the total CO₂ equivalent emissions in 2022. A drastic reduction methane emissions is thus crucial to meet the Paris Agreement commitment of 30% reduction of emissions between 2005 and 2030. It is, however, challenging as more than half of anthropogenic methane emissions are produced at concentrations below 5%. At such low concentrations, methane cannot be efficiently recovered or even flared as it is lower than its flammability level in air. In Ireland, this concerns mainly the agriculture and the waste sector. In terms of wastewater treatment, on-site domestic wastewater treatment systems serve approximately one third of the households in Ireland and so represent a significant source of methane coming from the anaerobic processes (mainly septic tanks).

A promising alternative way to treat methane is microbial oxidation by methanotrophs grown on a suitable porous media. Such a passive biofilter could be easily placed on top of septic tank gas vents, capturing the emitted methane before it is released into the atmosphere. The methane-oxidizing bacteria will then convert it into carbon dioxide, which is approximately thirty times less potent greenhouse gas in terms of global warming potential.

 

Multiple questions must be addressed to confirm the practical feasibility of this methane biofilter concept. The environmental conditions in the filter must allow the methanotrophs to thrive and outcompete other bacteria, thus ensuring an efficient methane oxidation, without obstructing the airflow. In addition to methane, an adequate supply of oxygen is necessary. This requires complex simulations of both the fluid dynamics involved as well as microbial growth and other kinetic dynamics. To investigate these different physical and biological aspects, a numerical study has been conducted combining computational fluid dynamics (CFD) modelling and multispecies biofilm modelling.

The CFD approach is carried on at the system level, producing velocity fields in the septic tank and the different pipes and vents. Knowledge of the gas flow in the full wastewater treatment system is essential to estimate the inlet flow conditions the biofilter will be subjected to depending on the wind weather. The information obtained on the gas phase, especially oxygen and methane levels, is then fed into a multispecies biofilm model. At this local level, we model the methanotrophs as well as the other bacteria expected to grow, compete for space and oxygen, and decay in the biofilter environment: heterotrophs, nitrifiers and Sulphate Reducing Bacteria. The results show that the methanotrophs should not be outcompeted. Moreover, the model enables the height of the filter to be estimated such that it should reach the target of 90% of methane consumption. Finally, transient simulations give insight into the expected time of usage of the filter before it needs to be regenerated.