下水道部門は、従来考えられていたよりも約2倍のメタンを排出している(Wastewater sector emits nearly twice as much methane as previously thought)

2023-02-28 プリンストン大学

＜関連情報＞

• https://engineering.princeton.edu/news/2023/02/28/wastewater-sector-emits-nearly-twice-much-methane-previously-thought
• https://pubs.acs.org/doi/10.1021/acs.est.2c05373
• https://pubs.acs.org/doi/10.1021/acs.est.2c04388

米国の廃水処理によるセクター全体のメタン排出量の過小評価について Underestimation of Sector-Wide Methane Emissions from United States Wastewater Treatment

Daniel P. Moore, Nathan P. Li, Lars P. Wendt, Sierra R. Castañeda, Mark M. Falinski, Jun-Jie Zhu, Cuihong Song, Zhiyong Jason Ren, and Mark A. Zondlo
Environmental Science & Technology  Published:February 27, 2023
DOI:https://doi.org/10.1021/acs.est.2c05373

Abstract

An increasing percentage of US waste methane (CH₄) emissions come from wastewater treatment (10% in 1990 to 14% in 2019), although there are limited measurements across the sector, leading to large uncertainties in current inventories. We conducted the largest study of CH₄ emissions from US wastewater treatment, measuring 63 plants with average daily flows ranging from 4.2 × 10^–4 to 8.5 m³ s^–1 (<0.1 to 193 MGD), totaling 2% of the 62.5 billion gallons treated, nationally. We employed Bayesian inference to quantify facility-integrated emission rates with a mobile laboratory approach (1165 cross-plume transects). The median plant-averaged emission rate was 1.1 g CH₄ s^–1 (0.1–21.6 g CH₄ s^–1; 10th/90th percentiles; mean 7.9 g CH₄ s^–1), and the median emission factor was 3.4 × 10^–2 g CH₄ (g influent 5 day biochemical oxygen demand; BOD₅)⁻¹ [0.6–9.9 × 10^–2 g CH₄ (g BOD₅)⁻¹; 10th/90th percentiles; mean 5.7 × 10^–2 g CH₄ (g BOD₅)⁻¹]. Using a Monte Carlo-based scaling of measured emission factors, emissions from US centrally treated domestic wastewater are 1.9 (95% CI: 1.5–2.4) times greater than the current US EPA inventory (bias of 5.4 MMT CO₂-eq). With increasing urbanization and centralized treatment, efforts to identify and mitigate CH₄ emissions are needed.

自治体の廃水収集・処理システムからのメタン排出量 Methane Emissions from Municipal Wastewater Collection and Treatment Systems

Cuihong Song, Jun-Jie Zhu, John L. Willis, Daniel P. Moore, Mark A. Zondlo, and Zhiyong Jason Ren
Environmental Science & Technology  Published:February 3, 2023
DOI:https://doi.org/10.1021/acs.est.2c04388

Abstract

Municipal wastewater collection and treatment systems are critical infrastructures, and they are also identified as major sources of anthropogenic CH₄ emissions that contribute to climate change. The actual CH₄ emissions at the plant- or regional level vary greatly due to site-specific conditions as well as high seasonal and diurnal variations. Here, we conducted the first quantitative analysis of CH₄ emissions from different types of sewers and water resource recovery facilities (WRRFs). We examined variations in CH₄ emissions associated with methods applied in different monitoring campaigns, and identified main CH₄ sources and sinks to facilitate carbon emission reduction efforts in the wastewater sector. We found plant-wide CH₄ emissions vary by orders of magnitude, from 0.01 to 110 g CH₄/m³ with high emissions associated with plants equipped with anaerobic digestion or stabilization ponds. Rising mains show higher dissolved CH₄ concentrations than gravity sewers when transporting similar raw sewage under similar environmental conditions, but the latter dominates most collection systems around the world. Using the updated data sets, we estimated annual CH₄ emission from the U.S. centralized, municipal wastewater treatment to be approximately 10.9 ± 7.0 MMT CO₂-eq/year, which is about twice as the IPCC (2019) Tier 2 estimates (4.3–6.1 MMT CO₂-eq/year). Given CH₄ emission control will play a crucial role in achieving net zero carbon goals by the midcentury, more studies are needed to profile and mitigate CH₄ emissions from the wastewater sector.