While anaerobic digesters are growing in number on livestock operations, they tend to be built on larger farms. While anaerobic digesters can theoretically work for a farm of any size, there are economies of scale that come with these systems, and financial feasibility is often a challenge, especially for smaller operations digesting manure alone.
A Cornell University study looked at the practicality of co-digestion at various dairy herd sizes. Lauren Ray, a senior extension associate and engineer with Cornell University, shared these results during a CALS PRO-DAIRY webinar titled, “How small can co-digestion economically go?”
In New York, there are around 35 anaerobic digesters on dairy farms. Manure from 90,000 dairy cows — 15% of the state’s herd — is being processed through a digester. To evaluate how an anaerobic digester with co-digestion may work on more farms of various herd sizes, Ray shared the model farm and Cornell Manure-based Anaerobic Digester Simulation tool that was used in the study. The food products added to the anaerobic digester in the analysis included cheese whey, confectionery waste, and a small amount of fats, oils, and grease.
In their model, the waste was trucked from the food processor to the farm and loaded into a reception pit, which would then be mixed with dairy manure. This delivery and storage part is very important, especially on smaller farms, Ray noted. That’s because on-farm processes to handle the food waste, such as unpackaging or grinding, all add substantial cost and management requirements.
Co-digestion will also add to the amount of energy or biomethane produced during anaerobic digestion and will create more volume of digestate to be stored after the process. “That is a consideration and a cost,” Ray said.
The analysis looked at dairy farms with 150, 300, 1,000, and 2,000 lactating cow equivalents. Food waste was added at 10%, 25%, or 50% of the total digester feedstock that includes manure. The revenue estimates were based off energy production and possible payment for taking on food waste, but other revenue streams could be available depending on location.
Ray noted that in their model, co-digestion on a dairy with 150 lactating cow equivalents was not economically feasible. With half of the feedstock as food waste, the break-even cost would still be challenging to meet.
On a dairy with 300 lactating cow equivalents (about 225 milking cows plus 175 heifers), the project was feasible, but only when food waste was co-digested in an equal volume to the manure, tip fees reached $20 per ton, and biomethane was valued at $25 per million BTU (MMBTU) or more.
In comparison, a dairy with 1,000 lactating cow equivalents (725 milking cows plus 600 heifers), achieved economic feasibility with 25% or more of food waste added to manure, as long as both the tip fees and energy revenue were high (more than $40 per ton and more than $35 per MMBTU biomethane, respectively). If food waste was included at 50%, the project penciled out at more moderate tip and biomethane rates.
The economic feasibility improved dramatically when food waste was accepted at higher ratios, Ray noted. This comes from the ability to generate more energy and the extra revenue from taking on food waste. While co-digestion can work at a smaller scale, larger systems more easily achieve economic feasibility, she concluded. Ray encouraged farm owners to work with an experienced developer to explore opportunities for their business.
This article appeared in the May 2025 issue of Journal of Nutrient Management on page 19.
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