Methane is a more potent heat-trapping gas than carbon dioxide, around 80 times more powerful. Though it does not stay as long in the atmosphere as carbon dioxide, emissions are more than 80 times more potent than carbon dioxide to cause damage to our climate.
Between the two GHG, carbon dioxide stays longer (between 20-200 years) in the atmosphere than methane (around 12 years); however, comparing one tonne of each gas in twenty years, methane’s heat-trapping effects is 86 times more potent (Economist, 2021).
Experts agree that if methane emissions are halved in the next 30 years, it can cut back 0.18°C off global temperature by 2050. Scientists estimate that between 550m and 880m tonnes of methane were released annually between 2008 to 2017, and 60% is due to three types of human activities: farming, burning of fossil fuel, and landfills and sewage-treatment plants—each accounting for a third of total emissions.
This article focuses on landfill and sewage-treatment plants methane emissions, how plants manage their methane emissions, what approaches or technology they use to reduce methane emissions, and the implications these changes will have in sewer treatments worldwide.
A study by Liu et al., Methane emissions from sewers, find that sewer systems emit significant amounts of methane; however, according to researchers, there are no large methane sinks or methanotrophic activities – a biochemical process that breaks down methane, exists in sewers plants to date.
Methane emissions would occur in gravity sewers, pumping stations, and inlets of wastewater treatment plants. However, the amounts of methane produced are affected by several factors like wastewater retention time, volume to area ratio, temperature, and organic matter concentration in sewers.
Due to the sensitivity of methanogens to environmental conditions, most of the chemicals applied to control sulphide in sewers also suppresses methane production, a finding that the study investigates for possible methane mitigation. The study also looked at methane production, quantification, and modelling of methane in wastewater treatment plants.
A study by Nisbet et al. says that technical developments make it easier to quantify methane emissions from landfills and sewage facilities. Fast-growing cities in developing and tropical countries, their poorly regulated landfill areas sewage plants are increasingly becoming hotspots for methane emissions. However, cost-effective methods are available to mitigate methane emissions.
According to the study, the initial step to mitigate methane emissions is to locate the leaks or emissions. The easiest way to find leaks is from “tractable emissions”, referring to apparent locations or methane emissions sources of methane emissions such as gas industry, urban wastewater and sewage, landfills, underground coal mines, etc. Improvements in using drone systems (AUV), which are cost-effective, are increasingly used to detect leaks or identify “super emitters”.
According to the study, when leaks are located, it is easily quantifiable and can be ended quickly, cheaply, and even profitably.
Reducing methane emissions from urban wastewater and sewage poses some challenges as well. Biogas extraction can still produce leaks and drains which are susceptible to a methane explosion. Methane production in the sewage can deplete biodegradable chemical oxygen demands, which is detrimental to wastewater treatment processes.
Methane removal from pipes running from buildings to wastewater treatment plants can be done by installing small bioreactors along the sewer’s length by harvesting methane-rich air above the sewage for soil oxidation.
Other techniques used to reduce methane emissions in wastewater treatment plants are submerged underwater bioreactors, halving methane emission. Floating bioreactors on wastewater settling pools in agricultural settings can oxidate 67% of methane, and one can apply the technology in urban wastewater treatments.
Other methane mitigation methods used sponge reactor for anaerobic sludge effluents and anaerobic sludge blanket reactors that can remove up to 50% methane. Generally, methane removal in wastewater treatments plants is inexpensive and self-financing because of methane’s potential to be converted to energy.
Landfills in large urban areas are enormous sources of methane emissions. Still, they can be easily mitigated through capture and piping or simply by adding a thick soil cover to host methanotrophic bacteria. Bacterial methane oxidation is a low-cost methane mitigation option for developing countries. Researchers also suggest the viability of methane oxidation in oil walls and sanitary landfills in Alberta, Canada and can be enhanced by adding biochar.
Soil oxidation not only works as passive bio-cover systems, but it has a high mitigation efficiency of up to 80%. One can enhance mitigation efficiency by adding local compost materials like garden and kitchen waste. Even in freezing conditions, methane consumption in landfill continues to occur due to its endemic exothermic state.
In some developed nations, particularly in Europe, landfill emissions are now heavily regulated and not a problem than in developing countries, except for California. Their landfills are a source of significant methane emissions in the state, accounting for 41% of their total methane emissions.
Landfills in developing countries are typically uncontrolled, poorly managed, and susceptible to burning. Widespread landfill burning can emit enormous amounts of methane and harmful air pollution.
A solution to capture methane emissions from landfills is to cover it with a geomembrane and fitted with a gas capture piping that can deliver gas to power electricity generators. However, as a warning, anaerobic biodigesters can become a source of methane emissions if not managed carefully.
If methane cannot be harvested and converted to power, biofiltration can be a removal solution. But these methods require ongoing investment, commitment, and management to maintain pipe integrity to avoid fractures and leakages.
The study also presents mitigation solutions to other methane sources like deep coal, open-cast coal, agriculture, biomass burning in the tropics. And “intractable emissions” or tricky ones from moving animals and food productions -emissions that are not easily stopped. Researchers describe existing methods to quantify and locate these methane emissions.
As the studies have shown, there are several methane mitigation solutions and technology available and are being implemented worldwide, ranging from low-costs and nature-based solutions to oxidate methane and those designed to capture methane for fuel.
Thankfully technologies are available, continuously improved, and scaled up to detect methane leaks. Detecting and quantifying methane leaks is the first and critical step in methane emissions mitigation.
To read the studies mentioned in this article, click the links below:
Those who worry about CO2 should worry about methane, too. (2021 April 3). The Economist. Retrieved from https://www.economist.com/science-and-technology/2021/04/03/those-who-worry-about-co2-should-worry-about-methane-too
Yiwen Liu, Bing-Jie Ni, Keshab R. Sharma, Zhiguo Yuan, Methane emission from sewers, Science of The Total Environment, Volumes 524–525, 2015, Pages 40-51, ISSN 0048-9697, https://doi.org/10.1016/j.scitotenv.2015.04.029.
Nisbet, E. G., Fisher, R. E., Lowry, D., France, J. L., Allen, G., Bakkaloglu, S., et al. (2020). Methane mitigation: methods to reduce emissions, on the path to the Paris agreement. Reviews of Geophysics, 58, e2019RG000675. https://doi.org/10.1029/2019RG000675