Life-cycle environmental benefits and adoption of dairy anaerobic digesters in Vermont and New York


Bedding solids from post-digestion solid-liquid separation

Members of the Dartmouth IFES research team selected 30 farms in Vermont and 30 in New York to interview for their project on anaerobic digester adoption. Most farms had looked into anaerobic digester technology, and of those, a little over half had chosen to adopt it, while the others ultimately decided against its adoption. A couple of farms sampled had never considered adding an anaerobic digester.

Anaerobic digester (AD) technology is beneficial to dairy farms for a handful of reasons. The most well-known product of anaerobic digestion is the methane that is converted to CO2, which is then sold to the grid or net-metered for electricity. For most Vermont farmers with ADs, being able to generate revenue from this product was very beneficial to their farm operation, largely thanks to the Cow Power initiative, which pays upwards of $0.16/kWh. In New York State, however, farmers receive approximately $0.04/kWh, making the electricity sales unfeasible to rely on as a sole revenue generator. This is where the other co-products of anaerobic digestion come in.

Lesser-known benefits include the conversion of agricultural materials into marketable products, including solids into compost for farmers and gardeners, or bedding for cows. When the manure runs through a solid-liquid separator post-digestion, one of the two products is an odorless solid. This product saves farmers money (not having to purchase sawdust shavings for cow bedding can save up to millions of dollars each year), and selling compost to local gardeners can generate revenue for farmers.

Another co-product (or “intensification”) of ADs is the heat coming off the engine (GenSet), which can be used to heat the farm operation, allowing farmers to save large sums of money in annual fuel costs–sometimes their heating bill goes down to $0! Some farmers use the heat by-product to heat a vegetable greenhouse, which then provides the added benefit of generating revenue as a side buffer operation during poor milk years. One farm even heated their swimming pool with the heat coming off the GenSet!

VT farm with digester-heated greenhouse (source:

Nutrient management and odor control are additional benefits to adopting AD technology. Because manure is high in phosphorus content, running it through a digester and solid-liquid separator, and then selling the resulting solids as compost, moves the phosphorus off-site. In terms of odor control, most farmers with ADs would not necessarily say their farm smells better, but “different.” Nonetheless, many remarked on having received fewer odor-related complaints from neighbors once they added the digester.

While there clearly exist many benefits to AD technology, these co-products are often not enough for a farmer to justify investing in such a large undertaking—up to $3 million in capital costs. A farmer can be environmentally-minded and admire all the environmental benefits of a digester, but may have to ultimately decide against its adoption if the operation won’t pay for itself. And in addition to the cost aspect, adding a digester to the farm operation entails learning an entirely new skill set—all of a sudden a farmer adds mechanical engineering tasks to an already 12+hr day. With the unpredictability of the relatively new AD technology, the complexity of its moving parts, and the fact that many digester companies are based overseas, adding this mechanical operation to a farm operation comes with its headaches.

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Farmers are busy people! (and very organized)

One question the Dartmouth IFES group is interested in is whether farms with digesters will be better equipped for agricultural shifts affected by climate change, as affirmed by the FAO: farmers will “become more resilient and self-sufficient when it comes to energy and agricultural inputs, and through income diversification” (FAO, 2012). Along similar lines is the question of whether farms with ADs can more easily bounce back from the volatility of milk prices after poor milk years.