MYTH: Digesters prevent surface water and groundwater pollution caused by agricultural run-off. 

 

REALITY: Digesters alter, but do not reduce the nutrient content of manure. 
 

• Even in controlled conditions digestate delivers mixed results. For example, one study found that “application of liquid digestate did not ameliorate the nutrient losses from leachate and even caused more losses for nitrate,” in the context of land application on fields . 

• The nutrient content of digestate is even more unknown with every new load of co-digestates (food waste, food manufacturing waste, municipal wastewater, etc.) that enters the digester. If water is added to the digester, the volume of waste that needs to be handled by farmers can actually increase.

 

REALITY: Scientists emphasize the importance of responsible manure management for digestate which is severely lacking on Wisconsin farms. This is yet another issue with ‘digesters in theory’ versus ‘digesters in practice’. 

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• The Environmental Working Group and Midwest Environmental Advocates analyzed the nutrient concentration of soil in 9 Wisconsin Counties and found that:

  • More than 1,500 miles of streams and rivers, and 33 lakes, in the nine counties assessed have impaired waters due overwhelmingly to combined pollution from manure and commercial fertilizer.

  •  In eight of the nine counties, nitrogen from manure and fertilizer sold exceeded UW recommendations. 

  • In four of the nine counties, nitrogen from the two sources surpassed UW’s recommendations by more than 50 percent.”

• Reckless management of digestate can have serious consequences in terms of environmental tradeoffs: “...if improperly applied, digestate can harm plant growth and the soil (Rigby and Smith, 2013), and due to its chemical composition, it can lead to problems for its sustainable disposal. For example, early application of digestate and its longer retention time in the soil without usage by crops might cause the loss of nutrients and their translocation towards deeper soil layers or NO3 emissions into groundwater (Formowitz and Fritz, 2010).”

 

Context is important! Agricultural Runoff & Public Health

Digesters may reduce some methane emissions, but they do not address one of the top concerns rural communities have regarding CAFOs: pollution of drinking water sources. But what we do know is that pollutants from agricultural runoff has public health impacts on rural communities:

• Nitrates: A national study assessed the nitrate exposure and found that between 2300-12,594 cancer cases per year can be traced to nitrate contaminated groundwater. These cancers included colorectal, ovarian, thyroid, kidney, and bladder cancers. The same study also reported that, “2939 cases of very low birth weight, 1725 cases of very preterm birth, and 41 cases of neural tube defects could be related to nitrate exposure from drinking water.”

  • A Wisconsin-based study analyzed well water samples between 2010-2017 and estimated that nitrate contaminated well water caused at least 111-298 cases of cancer (colorectal, ovarian, thyroid, bladder, and kidney cancer) per year. They also estimated that consumption of nitrates also had impacts for pregnant women and children in utero such as low birth weight, preterm birth, and neural tube defects.