(HALF)MYTH: “Digesters reduce methane, a powerful greenhouse gas that contributes to climate change.”

 

REALITY: Digesters are a “trade-off” because they increase other greenhouse gasses and hazardous gasses during the biogas production process. 
 

• Ammonia: According to the University of Wisconsin, “Digestion often shifts the form of the nitrogen to more ammonium. When digested manure is field applied, much of the ammonium will be released as a gas (ammonia) unless it is incorporated into the soil.” Since ammonia has a higher potential for volatilization (to turn into a gas) when spread on fields, this has the potential to dramatically increase the creation of fine particulate matter (PM2.5) which is already the leading cause of air pollution deaths from agricultural production . Holly et al. (2017) found in their study of digestate in Wisconsin that in storage, ammonia emissions increased 81%. In other words, digesters have found a way to reduce methane, but at the same time, they have begun producing higher levels of ammonia which is worse for human health. 

  • Health Impacts: “The most frequently reported health complaints from [ammonia] exposure include eye, nose, and throat irritation, headache, nausea, diarrhea, hoarseness, sore throat, cough, chest tightness, nasal congestion, palpitations, shortness of breath, stress, drowsiness, and alterations in mood (Schiffman and Williams, 2005; Wing and Wolf, 2000).”

  • “Increasing PM2.5 concentrations can cause serious implications to human health, such as worsening asthma symptoms (18% increase in occurrence with an in crease of 10 μg/m3) and an increased risk of cardiopulmonary mortality (a rise of 6–13% per 10 μg/m3 PM2.5 under long-term exposure) (World Health Organisation, 2013; Yu et al., 2000)...Reduced life expectancy is also associated with increasing PM2.5 levels, as is highlighted in a study conducted by Apte et al. (2018). These authors state that the probability of death above the age of 60 increases with exposure to PM2.5 depending on risk factors and disease rates.”

• Volatile Organic Compounds (VOCs): VOCs are gasses that are emitted into the air typically from industrial products or processes

  • When Zheng et al. (2019) studied the VOCs released from digestate during storage, they detected 49 different types of VOCs. Almost a third (32.77%) of the VOCs emitted from digestate were hazardous to human health: 8 of the compounds were carcinogenic and 14 were known to cause organ damage in humans.

• Nitrous Oxide (N₂O)- Nitrous oxide is a climate super pollutant that has 300x more global warming potential than carbon dioxide. Methane stays in the atmosphere for about 12 years while nitrous oxide stays in the atmosphere for at least 100 years. 

  • There is evidence that, depending on local weather conditions, spreading digestate on fields can increase nitrous oxide emissions (Holly et al. 2017). 

  • In real world conditions, researchers in Germany spread digestate on fields and monitored how it changed over two years. They found that, “Between 16–33% (1st year) and 17–38% (2nd year) of N2O emissions originated from digestate N, indicating that digestate application triggered N2O production and release mainly from soil N.”

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REALITY: Digesters are shown to reduce some methane emissions, but not enough.
 

• Digesters only reduce methane emissions from manure management (9% of US methane emissions), not enteric fermentation from cow burps and flatulence (27% of US methane emissions).

  • In 2021, digesters reduced agricultural emissions from enteric fermentation and manure management (265 million metric tons of CO2 equivalent) by 9.75 million metric tons of CO2 equivalent (both direct and indirect reductions) which equals only 3.7% of emissions from animal agriculture. 

• “Even if methane digesters were installed on every single US dairy farm and worked at optimal efficiency, this would still fall short of reducing the US dairy industry’s total GHG emissions by 25%,”. And that is just the dairy industry! Their calculations did not include methane emissions from the beef, pork, or poultry industries. 

• Using NASA satellite data, researchers just discovered that over a dozen factory farms with digesters emitted so much methane into the atmosphere that the plumes could be detected from space. They found that, “Just a single hour of pluming at these rates releases the carbon dioxide (C02) equivalent of driving a passenger car 1.1 million miles. That's the same as driving around the equator 45 times.” 

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