Mechanisms, source, and factors that affect methane emissions

Abstract

Urban wastewater systems (UWSs) are contributing to global greenhouse gas (GHG) increasing concentrations with direct emissions of methane (CHinf4/inf). Despite its lower contribution when compared to other sources of anthropogenic methane such as extraction-delivery of fossil fuels and animal-agricultural practices, methane emitted from UWSs can be up to 5% of the total global CHinf4/inf emissions. This chapter provides an overview of the contribution of UWSs to anthropogenic methane emissions. CHinf4/inf formation in UWSs occurs through anaerobic digestion (AD) of organics contained in sewage. AD consists of a sequence of concomitant reactions by which a consortium of microorganisms, in the absence of oxygen, break down biodegradable carbon material producing biogas, a mixture of methane, carbon dioxide and traces of Hinf2/infS. In UWSs, those reactions can occur naturally in sewer systems depleted of oxygen or be artificially promoted in wastewater treatment plants to capture and recover the energy contained in molecules of methane. Specifics of methane generation in sewer biofilms, sewer sediments, anaerobic wastewater treatment and sludge disposal of wastewater treatment plants are presented in this chapter. Identification of CHinf4/inf emission spots in urban wastewater engineered systems represents the initial step for reliable quantification of GHG, to later develop and establish effective mitigation strategies.