Energy and carbon neutral urban water management

Urban water utilities are among the societal sectors most vulnerable to climate change, yet also have a critical role to play in mitigating it. Many utilities have pledged to achieve net-zero greenhouse gas (GHG) emissions (carbon neutrality) by 2050 or earlier. Amid shifting priorities in some regions that may lessen emphasis on science-based sustainability efforts, maintaining and amplifying these strong commitments to evidence-driven net-zero targets is all the more vital for climate mitigation and leadership. Direct (Scope 1) GHG emissions from water utilities include nitrous oxide (N 2 O), methane (CH 4 ), and fossil-CO 2 arising from oxidation of fossil fuel-derived organics. Major sources of Scope 1 emissions in urban water management include infrastructure such as water resource recovery facilities (WRRFs), sewer networks, and dams/ reservoirs, in addition to receiving water bodies. In contrast to many other sectors, the large majority of these fugitive emissions are microbially derived, and innovations in managing microbial communities is therefore essential to mitigation strategies. Water/wastewater treatment and conveyance are well recognized as energy-intensive activities, and urban water utilities therefore also contribute indirectly to GHG emissions via energy consumption (Scope 2). Urban water systems also influence GHG emissions from other upstream and downstream activities (Scope 3), including from materials (e.g., cement, steel) used in facility construction, chemicals (e.g., ferric, methanol, polymer, disinfectants) employed in operation, and transportation of materials (e.g., biosolids). Together, the three scopes define the overall carbon footprint of urban water systems, and position water utilities as pivotal actors in reducing GHGs across the broader urban water cycle. In recognition of this critical and rapidly evolving area, the goal of this special issue is to highlight the latest and most significant outcomes in energy and carbon-neutral urban water management for net-zero emissions. The papers in this special issue reflect the intense interest and activity in both academia and practice in monitoring, understanding, and ultimately mitigating water sector GHG emissions, with rapid innovation in sensing and measurement technologies, biological and physicochemical process development and engineering, elucidation of key drivers and controls on microbial GHG emissions, mechanistic and data-driven modeling, and decision support frameworks for actionable guidance to water utilities (Fig. 1). The special issue comprises 35 papers that can be broadly categorized into 6 themes delineated below. Together, these manuscripts provide unique perspectives and innovative approaches to achieving net-zero in the urban water sector.