Purpose
Anaerobic digestion (AD) of dairy manure with and without food waste has mainly been implemented on a large scale in the US. The installed cost of these systems have economies of scale, and operations and maintenance costs need to be outweighed by adequate revenues from accepting food waste and/or producing energy, as well as reducing greenhouse gases (GHG). Scaling down AD systems is now technically feasible even on a micro-scale, but economic feasibility is still largely a challenge at small scales.
Local markets with competitive value structures for AD energy output and GHG reduction are needed to facilitate successful small and moderate scale AD systems. Analysis of energy values and consideration of food waste co-digestion with manure, as a way to expand revenues needed for economic feasibility at various scales, can help farmers and policymakers navigate opportunities.
What Did We Do?
We investigated the economic feasibility conditions of small, medium, and large AD systems processing dairy manure from 300 cows, 1000 cows, and 2,000 cows, respectively, in combination with varying amounts of food waste. The Cornell Manure-based Anaerobic Digester Simulation tool was further developed and then utilized to model the mesophilic, vessel-type AD of various food wastes and amounts in combination with varying dairy cow manure volumes to assess performance and associated economics. The breakeven capital cost of the full project was computed for each scenario (nine per dairy farm size) of manure to food waste ratio, tip fee revenue, energy output and revenue value. These were compared to estimated project costs based on multiple case studies to evaluate whether or not the breakeven cost was high enough to be considered an economically viable project.
What Have We Learned?
Key results from modeling these scenarios included that an AD to biomethane system can be economically feasible for a 300-cow dairy (300 lactating cow equivalents) only when food waste is co-digested in an equal volume with the manure and when tip fees reach $20 per ton and biomethane is valued at $25 per million BTU (MMBTU) or more. Biomethane sell price data collected from our collaborator, Energy Vision, was found to be as high as $70 per MMBTU if sold in the California transportation market (manure only AD), and between $12 and $28 in voluntary markets.
Additionally, a dairy farm with 1,000 lactating cow equivalents (e.g., 725 milk cows and 650 heifers), was found to achieve economic feasibility with 25% or more of food waste ratio to manure co-digested as long as both tip fees and energy revenue were high ($40 per ton tip and $35 per MMBTU biomethane). When food waste ratios increased to half the digester’s feedstock, economic feasibility was achievable at more moderate rates. The economic feasibility of manure-only AD continues to be challenged at small and moderate scale, while the addition of food waste with manure enables significantly higher revenues from substantially more energy production and tipping fees.
Future Plans
This project included detailed analysis of a small-scale co-digestion application at Cornell’s smaller Teaching Dairy operation to evaluate available equipment and biogas utilization options. A preliminary design is developed, capital funding secured, and initial operating period research and extension defined. The project is scheduled to be completed later this calendar year and will be utilized for various food waste and manure anaerobic digestion to energy system research and educational programming.
Authors
Presenting & corresponding author
Lauren Ray, Sr. Extension Associate, Cornell University – PRO-DAIRY, LER25@cornell.edu
Additional author
Peter Wright, Agricultural Engineer, Cornell University
Additional Information
https://cals.cornell.edu/pro-dairy/our-expertise/environmental-systems/manure-energy-systems
Acknowledgements
Funding was provided by the New York Farm Viability Initiative.
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