Centralized wastewater treatment is crucial for population growth and urbanization, but the latest comprehensive assessment of emissions from water resource recovery facilities (WRRFs) in the USA revealed that this the sector represents a significantly underestimated source of greenhouse gases and ammonia.

Wastewater is a major urban source of methane (CH₄) and nitrous oxide (N₂O). These two gases are the most significant anthropogenic greenhouse gases (GHGs) after carbon dioxide (CO₂) and together are responsible for 22 % warming since 1850.

Dramatic underestimation of emissions

Study, which represents the most comprehensive set of measured data to date in terms of number of facilities and time periods, it included measurements of CH₄, N₂O, and NH₃ emissions in more than 96 US WRRFs, which collectively treat approximately 9 % of centralized wastewater in the US.

The results show that measured emissions were significantly higher than reported by current US inventoriesSpecifically, N₂O emissions were 1.9 times higher and CH₄ emissions were 2.4 times higher.

It is important to note that methane (CH₄) has a 100-year global warming potential (GWP₁₀₀) of 28, while nitrous oxide (N₂O) has a GWP₁₀₀ of 265Furthermore, N₂O remains in the atmosphere for an average of 116 years, thus having a long-term impact on the climate and damaging stratospheric ozone.

As a result of these findings, the updated CH₄ emission inventory for the US domestic wastewater sector is estimated to be 17.9 MtCO₂e per year, which is 2.4 times more than the US EPA inventory (7.6 MtCO₂e per year). N₂O emissions are estimated at 31.5 MtCO₂e per year, which is 1.9 times more than the previous inventory. The combined climate effect of CH₄ and N₂O leads to direct greenhouse gas emissions of 49.8 MtCO₂e per year.

An overlooked source of ammonia

In addition to greenhouse gases, WRRFs have also been shown to be an overlooked source of urban ammonia (NH₃). The detected NH₃ emissions from the domestic wastewater sector in the USA represent 86 kt NH₃ per year. Although ammonia was detected in only a small fraction of the facilities, this estimate is more than two orders of magnitude higher than current EPA estimates (<1 kt NH₃ per year) and is comparable to emissions from light-duty vehicles (excluding diesel). Ammonia reacts rapidly with acidic compounds, leading to the formation of fine particles that degrade air quality and negatively impact human health and ecosystems.

The phenomenon of "super-emitters" and mitigation options

The range of emissions is extremely variable, often varying by several orders of magnitude. A key finding concerns the concentration of emissions: just the top 10 % devices contribute to 74 % of the total carbon footprintThe top half of emitters are responsible for more than 98% of the measured carbon footprint.

These findings suggest that inefficiency in individual devices is not isolated to just one processFor example, devices that use anaerobic digestion (AD) for sludge treatment, emit on average 2.5 times more CH₄ in proportion to the waste processed than facilities without AD.

Therefore, it would Emission mitigation efforts should focus primarily on larger facilities and "super-emitters", which also usually have more resources to implement such measures. If the emissions of the nine largest emitters were reduced to the median values, emissions equivalent to 3.1 MtCO₂e per year.

Future challenges

As globalization and urbanization increase, the demand for centralized wastewater treatment will also increase. If the findings from the US are representative of global facilities, projections suggest that due to the growth of wastewater treatment its relative contribution to global greenhouse gas emissions will increase 2- to 17-fold by 2100.

To achieve sustainability goals and net-zero plans, It is necessary to pay more attention to emissions from wastewater treatment plants and implement more frequent and widespread monitoring. It is critically important to examine the relationship between nutrient removal from wastewater and air emissions to identify environmentally sustainable processes that benefit both air and water. JRi