Untreated Wastewater May Be a Contributing Factor to Methane Emissions From Urban Areas

The study was led by Benjamin de Foy, Ph.D., professor of Earth and Atmospheric Sciences at Saint Louis University, and was titled "Investigating high methane emissions from urban areas detected by TROPOMI and their association with untreated wastewater."

The researchers discovered that methane emissions from untreated wastewater discharge are a significant contributor to global methane emissions and that improving wastewater treatment in urban areas could result in significant reductions in greenhouse gas emissions, assisting cities in their quest for carbon neutrality.

Improved Wastewater Treatment Could Lead To a Significant Reduction In Greenhouse Gas

(Photo : BEN STANSALL/AFP via Getty Images)

According to de Foy, reducing untreated wastewater discharges could reduce global methane emissions by up to 10%, with significant ecological and human benefits, as per Phys.org.

Carbon dioxide and methane are the two most significant contributors to climate change. Global methane concentrations increased at the fastest rates on record in 2021, and current estimates of methane emissions inventory are insufficient to explain recent trends.

Satellite remote sensing, such as the TROPOspheric Monitoring Instrument (TROPOMI) on board the Sentinel 5 Precursor satellite, is one method of assessing methane emissions. Since November 2017, this has been measuring methane and other air pollutants all over the world.

According to the findings, methane emissions from urban areas may be underestimated by a factor of three to four in the Emissions Database for Global Atmospheric Research (EDGAR) greenhouse gas emission inventory.

The study extrapolated the findings to 385 urban areas around the world with populations greater than 2 million people, implying that they could account for up to 22% of global methane emissions.

The 61 urban area emission estimates do not correspond to the total or sectoral EDGAR emission inventory.

They do, however, correlate with estimated untreated wastewater rates, which range from 33 kg of methane per person per year in cities with no untreated wastewater to 138 kg of methane per person per year in cities with the most untreated wastewater.

The study examined various scenarios for reducing emissions in 61 urban areas as well as all areas with a population of more than 2 million people.

By reducing the emissions from the 33 cities with medium to high levels of untreated wastewater to the mean emissions from cities with zero to low levels of untreated wastewater, 2% of the total global emissions could be reduced.

If all 61 cities reduced their emissions to the lowest possible level, total global methane emissions would be reduced by 6%.

According to the researchers' model, untreated wastewater accounts for a large portion of total methane emissions, rather than other options such as natural gas leaks or older infrastructure.

According to de Foy, the estimates of methane emissions, there is much more methane formation in the environment as a result of the release of untreated wastewater than is estimated in current inventories.

Some cities could cut their emissions by half or more by fully treating all of their wastewater.

More work is needed, according to the researchers, to bridge the gap between inventories and measurements in order to create a more refined global emission inventory and identify the varying emissions from city to city more precisely.

De Foy noted that there can be significant differences between countries, pointing out that Milwaukee has a significant methane enhancement but neighboring Minneapolis does not, which could be due to differences in how stormwater and sewage are handled.

Also Read: Greenhouse Gases Carbon Dioxide and Methane Dramatically Increase by 0.6% and 0.5% in 2022 [Study]

Decade To Deliver Greenhouse Gas Emissions Mitigation

When sewage is treated before being returned to the environment, several by-products are produced, including the potent greenhouse gases nitrous oxide, biomethane, and carbon dioxide - this happens all over the world, it's just science, as per Water UK.

The water sector is focusing on two areas to reduce process emissions. The first is to estimate process emissions more accurately, and the second is to use this information to develop solutions to reduce them.

Water companies around the world currently use generic emissions factors to estimate nitrous oxide and methane levels from process emissions.

The Intergovernmental Panel on Climate Change (IPCC) updated its wastewater treatment emissions factors in 2019, but different countries face different wastewater challenges.

Because of differences in climate, geography, and wastewater composition around the world, these emission factors are not representative of actual emissions at all sites.

To address this, the water sector is constantly improving its monitoring and measuring of emissions at various types of treatment sites, using new equipment to build a clearer picture of the process emissions unique to their technology and location.

Companies can find new ways to reduce process emissions with more accurate information, such as using cutting-edge technology to capture biomethane and use it to generate energy.

Nitrous oxide emissions, on the other hand, are less well understood.

Water companies are working on projects to figure out where and when nitrous oxide appears in the biochemical process, which will reveal where to tweak the treatment and deploy new technology to reduce this greenhouse gas.

Related article: Agricultural Greenhouse Gas Emissions Must Be Reduced According to Scientists