Methane Emissions from Municipal Wastewater Collection and Treatment Systems

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.

Keywords: IPCC; Literature text mining; Methane; Mitigation strategies; Monitoring campaigns; Monte Carlo analysis; Wastewater treatment.

Figure 1

Typical wastewater collection and treatment trains with various biological treatment and sludge management processes.

Figure 2

CH₄ monitoring campaigns for sewer networks (A) and WRRFs (B–D). (A) Sewer networks are shown in different types of sewer systems grouped by hydraulic retention time (HRT). WRRF data are shown for (B) the year of publication and applied measurement techniques, (C) plant configurations differentiated by main biological treatment processes, and (D) spatial scales of the studies. TDM: tracer gas dispersion method; IR: optical gas imaging infrared video camera; MGD: million gallon per day (1 MGD = 3785 m³/day); pond refers stabilization pond; W, S, and AD are CH₄ emissions from wastewater line, sludge line (without anaerobic digestion), and anaerobic digestion, respectively.

Figure 3

A summary of plant-wide CH₄ emissions with respect to different biological treatment processes for WRRFs with and without anaerobic digestion. The inset in (B) highlights those with relatively low CH₄ emissions. Boxplots show 25th, 50th, and 75th percentiles and outlier-bounds are based on 1.5 × IQR (interquartile range, equaling 75th percentile minus 25th percentile) of CH₄ emissions. Red dots represent the arithmetic mean. n: the number of monitoring data.

Figure 4

Plant-wide CH₄ emissions with respect to measurement techniques (IR: optical gas imaging infrared video camera, TDM: tracer gas dispersion method), the length of the monitoring period (short: less than one month, intermediate: a few months without capturing the whole spectrum of temperature changes, long: at least one year), and monitoring mode of gaseous sampling for WRRFs with AD (A–C) and those without AD (D–F). Boxplots show 25th, 50th, and 75th percentiles and outlier-bounds are based on 1.5 × IQR of CH₄ emissions. Red dots represent the arithmetic mean. n: the number of monitoring data. Data in the shaded areas in panels A and D are used to further analyze impacts of duration and mode on CH₄ emissions to avoid bias caused by various measurement techniques.

Figure 5

Mean CH₄ emissions of different process units in WRRFs. Error bars indicate upper 95% CI values.

Figure 6

Dissolved CH₄ concentrations in different types of sewer networks with respect to wastewater temperature, geographical regions, and hydraulic retention time (HRT).

Figure 7

Nationwide CH₄ emissions from the U.S. wastewater sector. (A) Estimated annual mean (±s.d.) CH₄ flux of each group and (B) accumulative probability of CH₄ emissions from the whole wastewater sector.