Measuring the results of a workshop on installing anaerobic digesters (AD) on smaller livestock farms

It is accepted generally that AD are efficacious technologies for reducing greenhouse gas (GHG) emissions from livestock operations (Pronto and Gooch, Reference Pronto and Gooch2009). In addition, AD technology has a number of other potential benefits including energy production for use on the farm and for sale, separation of manure solids for ease of use or export off farm, pathogen reduction leading to healthier labor and herd outcomes and odor control.

It is also clear that in the USA, research and extension efforts, including public financing of AD technology installations, have disproportionally been focused on larger farms- e.g., dairy farms with at least 500 milking cows. The US Department of Agriculture, land-grant universities, as well as private sector firms have tended to ignore smaller-scale livestock farms in regards to the development of appropriate AD technology interventions (Welsh et al., Reference Welsh2010; Woughter, Reference Woughter2014).

However, there seems to be a growing recognition that smaller farms maintain a large number of animals in total; and that GHG policy should encompass the methane and other gases produced by livestock on smaller farms- e.g., in the USA 34% of all milk cows reside on farms with less than 200 cows. In addition, social equity arguments concerning the need to spend public dollars on a broad constituency have proven effective- 90% of all dairy farms in the USA are smaller than 200 milking cows. For instance, Shelford and Gooch (Reference Shelford and Gooch2012: 2) of the Biological and Environmental Engineering Department at Cornell University found that:

And the agricultural engineering department at Ohio State University developed a 350-gallon digester appropriate for ‘95% of the nation's livestock farms’ because: ‘Most biodigesters are designed for dairy operations with 1000 or more cows (see http://www.ohiofarmer.com/story-small-scale-digester-fits-typical-farms-9-41588).’ In addition, private sector firms seem to have discovered the market for smaller livestock farms and developed designs and technologies more suited to smaller operations. For instance, the Wisconsin Dairy News published this article in 2012: Anaerobic Digesters Aren't Just for Large Farms (see http://www.americasdairyland.com/on-the-farm/environment/bioenergy/petersdigester#video). The article and embedded video argue that: ‘Wisconsin dairy farmers continue to adopt modern environmental agricultural practices and lead the nation in converting manure to renewable energy with large, on-farm anaerobic manure digesters. We head to a 200-cow dairy in Chaseburg that's testing out a smaller version.’

This discernible shift to be more inclusive of farms of smaller livestock farms in regards to research and development of AD technologies is the topic for this From the Field paper. In this paper, we detail the history of Clarkson University and Syracuse University researchers and the Cornell Cooperative Extension of St. Lawrence County, New York efforts to conduct research leading toward the development of viable AD systems for smaller livestock farms. Given the predominance of the dairy industry in New York, our work has focused primarily on smaller scale dairy operations. In addition, we conducted four workshops at the St. Lawrence County Extension facility for livestock farmers interested in smaller scale digester options. A power point presentation on the history of AD technology and its applications to smaller farms was presented to 39 attendees. The presentation included technical data produced by a pilot system developed by Clarkson University engineers. This paper presents elements of the educational materials developed for the workshop, as well as the results of pre and post-test surveys of farmers attending the workshops.

Background

This research evolved from a project to measure the willingness of northern New York dairy farmers to install AD technology on their farms and to develop a system for a local farmer who had received funding from the USDA to install a digester for his 650 cow farm (at that time in 2004). A mail survey was conducted as part of the effort to measure dairy farmer knowledge of AD technology and interest in using it to realize benefits to their farms such as power generation, odor control, pathogen reduction and enhanced nutrient management. We received over 400 useable responses from the survey. A surprising finding was that large numbers of smaller dairy farms stated an interest in AD technology. At the time this was unexpected as the extant literature emphasized the need for at least 500 cows to make a digester economically viable (see Welsh et al., Reference Welsh2010).

The survey findings set in motion an effort to develop a research program and pilot technology to extend to smaller scale livestock farmers in northern New York where Clarkson University is located. With funding from the US Environmental Protections Agency's People, Prosperity and Planet Program a pilot digester was developed to illustrate the capacity for producing sufficient amounts of biogas such that the rate of return for a smaller dairy farm installing the digester would be positive.

The estimated capital cost for that small-scale digester (for a 25 cow system) was US$2470 per cow, which is slightly higher than the estimated capital cost of a large-scale system. The current capital cost range for complete large-scale digester systems is estimated at US$1000–2000 per cow depending on herd size (Lazarus, Reference Lazarus2015). This capital cost is in line with estimates by Astill et al. (Reference Astill, Shumway and Frear2016) who estimated the capital cost of a large digester system (1600 cows) at US$1350 per cow. When using the generated biogas of the small system digester to displace propane instead of electricity a payback period of less than 7 years could be estimated which is well in line with payback periods of larger-scale digesters.

Specifically a 1:21 scale pilot digester was constructed at the Cornel Cooperative Extension Farm in Saint Lawrence County (Fig. 1) to collect long-term performance data and to be used during farm-digester workshops. The plug flow digester consists of a 1600-gallon gas-tight elliptical polyethylene tank (Plastic-Mart, Austin, TX, USA) insulated with 10.1 cm polystyrene sheathing (R-value = 22) and housing an internal tube heat exchanger (rated at approximately 100,000 BTU hr⁻¹). The heating loop uses a 50/50 water : ethylene glycol mixture and an on-demand hot water heater powered by produced biogas to maintain digester temperature at 98 F; excess produced biogas is flared. A 40-gallon hot water heater powered by propane serves as a back-up heater as needed; propane and biogas usage is metered. Thermocouples installed in the reactor vessel monitor tank temperatures. A 100-gallon horizontal polyethylene tank holds digester effluent for recirculation to the manure conditioner and leachate tanks. Excess effluent is pumped into a manure spreader for manure spreading (see Fig. 1).

Fig. 1.

Pilot-scale anaerobic digester; picture of physical system at workshop (top), physical layout schematic (center), plan view schematic (bottom).

To feed the digester, a chain-driven manure elevator drops raw manure into the manure conditioner, which is constructed from a 200-gallon conical-bottom polyethylene tank (US Plastic Corporation, Lima, OH, USA). Digester effluent is cycled to the manure conditioner, then air is injected into the bottom of the conical tank through a coarse bubble diffuser using a three horsepower blower (Ametek, Kent, Ohio, USA) to promote mixing and fiber separation. Additionally, two dry fermentation tanks were constructed from 250-gallon polyethylene totes with screened bottom grates placed in their interiors to keep fibers above the screen and out of the liquid leachate. The leachate tanks are designed not only for separated fibers, but also to accommodate solid co-digestate waste products from the farm such as waste silage, hay, or food scraps. When in use, dry fermentation tanks are seeded using 20 gallons of digester effluent which is recirculated through the leachate pile at a rate of 40-gallons per day to facilitate acidigenesis; after 1 week of operation an equal volume of leachate rich in VFAs was returned as feed to the anaerobic digester and replaced with fresh seed. A solids residence time of 60 days was maintained in each dry fermentation tank.

Feed mass and composition, temperature, biogas and propane flow rates and methane concentration were recorded on a continuous basis during the period of study. On average, the digester was fed daily with 84 kg of raw manure diluted with 154 kg of recycled effluent. Average digester biogas production was 2.4 m³ per day, or 2.0 m³ per cow per day at an average methane content slightly below 60%. The digester system generated approximately three times more energy than was need to heat the system. Excess energy could be used to offset the farm's heating fuel cost.

Workshops and assessment

This study used survey research methods to assess participants’ knowledge of anaerobic farm digestion as well as to assess the effectiveness of the workshop that the participants were invited to attend. Each producer participating in the study was asked to attend one of four AD technology training session at the Extension Learning Farm of St. Lawrence County. At the beginning of the training session, all participants were asked to complete a brief survey questionnaire. The survey focused on current knowledge and experience with AD technology as well as attitudes and opinions on whether the technology was appropriate for the participant's farm.

After completing the survey questionnaire, the training session began and the participants were asked to view and listen to a power point presentation on the viability of the technology for smaller livestock farms. The farmers were then given a brief tour and demonstration of the AD technology. Finally, after the presentation and tour were over, participants were asked to return to the classroom where the workshop took place and complete a brief post-training survey questionnaire identical to the pre-training survey.

All recruitment for this study was done by Cornell Cooperative Extension of St. Lawrence County. The target audience was producers who were currently operating a bovine livestock farm in St. Lawrence County with under 350 cows. A total of 39 individuals attended one of the four workshops that were held in the fall of 2015 and the spring of 2016; and, 34 individuals completed both the pre- and post-workshop survey questionnaires. However, six larger-scale farm operators ignored the recruitment criteria and attended the workshops and completed the surveys, as did four non-farmers. The larger operators and non-farming respondents were deleted from the final data set resulting in a sample size of 24 smaller farm operators.