Dissolved methane in the influent of three Australian wastewater treatment plants fed by gravity sewers

Methane (CH₄) is an important anthropogenic greenhouse gas and a by-product of urban sewage management. In recent years and contrary to international (IPCC) consensus, pressurised (anaerobic) sewers were identified as important CH₄ sources, yet relatively little remains known regarding the role of gravity sewers in CH₄ production and conveyance. Here we provide the results of a nine month study assessing dissolved CH₄ levels in the raw influent of three large Australian wastewater treatment plants (WWTPs) fed by gravity sewers. Similar to recent international research and contrary to IPCC guidance, results show that gravity sewered wastewater contains moderate levels of CH₄ (≈1mgL^-1). Dissolved CH₄ concentration correlated negatively with daily sewage flow rate (i.e. inversely proportional to sewer hydraulic residence time), with daily CH₄ mass loads on average some two-fold greater under low flow (dry weather) conditions. Along with sewage hydraulic residence time, sewer sediments are thought to interact with sewage flow rate and are considered to play a key role in gravity sewer CH₄ production. A per capita load of 78gCH₄person^-1y^-1 is offered for gravity sewered wastewater entering WWTPs, with a corresponding emission estimate of up to 62gCH₄person^-1y^-1, assuming 80% water-to-air transfer of inflowing CH₄ in WWTPs with combined preliminary-primary plus secondary treatment. Results here support the emerging consensus view that hydraulic operation (i.e. gravity versus pressurised, sewage flow rate) is a key factor in determining sewer CH₄ production, with gravity sewer segments likely to play a dominant role in total CH₄ production potential for large metropolitan sewer networks. Further work is warranted to assess the scale and temporal dynamics of CH₄ production in gravity sewers elsewhere, with more work needed to adequately capture and assess the scale of diffuse sewer network CH₄ emissions from sprawling urban settlements globally.