Sewage from residents and industries is collected and transported to wastewater treatment plants (WWTPs) with sewer networks. The operation of WWTPs results in emissions of greenhouse gases, such as methane (CH4), mostly due to sludge anaerobic digestion. Amounts of emissions depend on the source of influent, i.e. municipal and industrial wastewater as well as sewer systems (gravity and rising). Wastewater is the fifth-largest source of anthropogenic CH4 emissions in the world and represents 7-9% of total global CH4 emissions into the atmosphere. Global wastewater CH4 emission grew by approximately 20% from 2005 to 2020 and is expected to grow by 8% between 2020 and 2030, which makes wastewater an important CH4 emitter worldwide. This review initially considers the emission of CH4 from WWTPs and sewer networks. In the second part, biotechniques available for biodegradation of low CH4 concentrations (<5% v/v) encountered in WWTPs have been studied. The paper reviews major bioreactor configurations for the treatment of polluted air, i.e. biotrickling filters, bioscrubbers, two-liquid phase bioreactors, biofilters, and hybrid reactor configurations, after which it focuses on CH4 biofiltration systems. Biofiltration represents a simple and efficient approach to bio-oxidize CH4 in waste gases from WWTPs. Major factors influencing a biofilter's performance along with knowledge gaps in relation to its application for treating gaseous emissions from WWTPs are discussed.
Keywords: Global warming; biofilter; biological treatment; biotrickling filter; greenhouse gas; methane; sewage; two-liquid phase bioreactor; waste gas; wastewater.