Farm-fabricated on-farm composting equipment project: Aerating equipment

Final Report for ONE05-036

Project Type: Partnership
Funds awarded in 2005: $9,980.00
Projected End Date: 12/31/2007
Matching Non-Federal Funds: $20,182.00
Region: Northeast
State: Vermont
Project Leader:
Tom Gilbert
The Highfields Institute
Expand All

Project Information


The Farm-fabricated On-Farm Composting Equipment Project is an effort to increase the availability of affordable composting technologies to farmers in order to further promote the more widespread integration of composting and compost use in agriculture. SARE funds were used to explore pile aerating equipment. Through this project we developed two significantly different aerating technologies with the intention of creating designs that cold be replicated by farmers or local fabrication shops for farmers.

The first implement is a tractor-drawn, PTO-powered windrow turner, which is pulled to the side of the tractor through the pile as it agitates the compost with a rotating drum. The second design is a set of four foot “Aerating Tines” that are mounted to a tractor bucket and used to sift up through the pile.
Fabrication manuals were developed for each implement. The manuals are now available at a low cost and will be available as a free download from the Highfields website.

Project Objectives:

Increase the adoption of on-farm composting by addressing equipment cost and management time issues that have been identified as limiting factors in the widespread adoption of composting as a manure management practice.

Develop two different pieces of compost aerating equipment that can be fabricated on the farm and are compatible with common farm equipment.

Make the two different pieces of equipment represent different levels of capital investment, fabrication difficulty, and aeration strategy to meet the diverse needs of different farms.

Make these equipment designs available to farmers through fabrication manuals to help address farmers’ concerns about the investment of time and/ or capital in considering on-farm composting.

Inform farmers about the availability of these designs and their associated fabrication manuals through demonstrations, events, agricultural media, and other outreach mechanisms.


Materials and methods:

Both the windrow turner and the aerating tines were designed with common goals, however the parameters within which each implement was evaluated were different, largely based on presumed factors of scale. Common design goals included: affordability, design accessibility for fabrication on the farm, compatibility with existing farm equipment and on-farm composting scenarios, and aeration and time effectiveness. Individually goals were established for each implement. In designing the turner we also sought to overcome common flaws in available designs, such as dragging the far side of the turner. Specific design parameters for the turner included a tunnel opening of 5’h x 10’w, the capacity to turn a pile with a bulk density of 1500 pounds per cubic yard, capacity to operate on 45-60 HP, and PTO-drive. Specific criteria for the tines included a tine length of at least 48 inches from the bucket blade, a width of at least four feet, and the capacity to lift the profile of the pile at a bulk density of 1500 pounds per cubic yard momentarily before the material falls between the tines (to achieve more effective mixing and aeration). Bulk densities of 1500 pounds per cubic yard, which is denser than preferred pile conditions, were used to ensure that the implements were capable of handling the common problem of overly dense compost mixes. Preferable pile bulk density is 1000 pounds per cubic yard.

A range of commercially available and farm-fabricated windrow turner designs were assessed for strengths and weaknesses. Students from a Design Communications class in the Vermont Technical College Engineering Department developed three different designs using Computer Animated Design (CAD). One design, developed by Eric Bornemann, was utilized for the project. During fabrication the design was altered to simplify fabrication and work with available materials by fabricator and mechanic Frank Sauer.

The Aerating Tines are an original design that was conceptually developed by Highfields staff and designed for fabrication by Bornemann and Greene Corp., a machinist and fabrication shop in Morrisville, VT. Tine dimensions were established for the project by a Materials Strength class at the Vermont Technical College Engineering Department. The class provided the project with a excel spreadsheet format for determining the stock size of the tines based on the materials being used. This stress calculator assumed that the tines must be able to hold roughly two cubic yards of compost with a bulk density of 1500 pounds per cubic yard, or 3,000 pounds.

Trials of these designs were not conducted to the extent that we had initially intended as a result of the extended delays incurred during the design and fabrication process. Three 30 foot piles of food scraps, wood chip, horse manure, shredded office paper, and hay were managed for two months with the turner, Aerating Tines, and a standard tractor bucket “rolling” method. Pile height and width varied according to the capacity of the implement. Piles aerated with the tractor bucket were built to 7 feet high and 12 feet wide, the turner pile was 4.5 feet high and 8 feet wide, and the Aerating Tine pile was 5.5 feet high by 8 feet wide. Both the turner and Aerating Tine piles required use of the bucket loader to reform piles to appropriate dimensions. Limited data was collected during these trials. Pile temperature (36” temperature probe), estimated pile moisture (squeeze test), odor (descriptive), mycelium development (observational), physical decomposition rates (observational), aerating rate (yards per hour), space use (cubic yards handled per square foot of pad space), work space requirements (feet from pile), and pile oxygen content (oxygen monitoring probe). The data gathered on pile oxygen is unreliable due to problems with the oxygen monitoring equipment.

During the trial period the equipment was demonstrated to two groups of farmers who came out to review the equipment and provide feedback on the designs. Over one dozen farmers and fabricators participated in these Equipment Review Days. Excellent insights in the design and operation of the turner were gathered and incorporated into the design.

Economic analysis was performed by taking basic information collected during the two month trial and extrapolating on it using an Excel calculator created for this project. Built into this calculator are several assumptions, the most critical of which is the number of times the windrow will be turned over the composting period. Having not assessed the implements over a full composting period it is challenging to determine how similar the aerating requirements will be for each strategy. This assumption has the potential to skew the projections. Other assumptions included the pile shape, and the operation and maintenance costs per hour for the tractor. Pile shape significantly impacts the amount of material the windrow holds per linear foot. Building piles to match the dimensions of the turner generally resulted in a more triangular profile, so a cross-sectional factor of 0.5 was assigned to the turner for the comparative economic analysis. The windrows managed with the “rolling” method and aerating tines generally maintained a parabolic shape and were therefore assigned a cross-sectional factor of 0.66. Operation and maintenance costs were set at $50/ hr, which includes labor. Other factors contributing to the overall cost of producing compost were not included, such as pile monitoring, because it was assumed that these costs would remain constant between all aeration strategies.

Research results and discussion:

See Appendix A – “Product Comparison” - write-up on summary of product performance. See Appendix B – “Aerating Strategy Processing Rates and Costs” – for processing rates and operations costs.

Design Process – The design process took significantly longer than we had anticipated, largely because we decided to expand the scope of this aspect of the project, recognizing how critical of a step it was within the larger project. Eventually we sent our fabricator to Quebec to visit a farmer there with a farm-fabricated turner design that seemed to offer some promise. We also developed a partnership with the Vermont Technical College Engineering Department to involve students in the design of both the turner and the Aerating Tines. While the time-frame for the design process impeded how rapidly we could implement the project, the quality that these other partners brought to the project improved the design outcomes significantly.

Implement Design and Performance – Both designs carry successes and challenges. We were successful in designing and fabricating the implements to fulfill their function, however flaws in each design persist; neither design has thus far demonstrated significant improvements in management efficiencies and aeration efficacy in comparison to the baseline pile aeration and mixing method, a tractor with a bucket. When performing well, under specific conditions, the turner is very effective at processing the windrow quickly – 12 linear feet of pile per minute – however, due to the small pile dimension required by the turner, the turner only aerates 20-27 cubic feet per linear foot compared to the front end loader that aerates 42-55 cubic feet per linear foot. The turner processes 12 feet of pile and 9-12 cubic yards per minute and the bucket processes two feet of pile and 3-4 cubic yards per minute.

The challenge in evaluating these implements in comparison to one another resides in the significant difference in each aeration strategy. For instance, when using the turner, the 16-21 cubic feet turned per linear foot was thoroughly and completely turned, whereas only the portion of the pile that is “rolled” with the bucket is directly aerated. “Rolling” piles with the tractor bucket is a more passive strategy of aeration and tends to minimize the amount of material directly handled (and thus, directly turned), however the effect of the action seems to resonate throughout the pile. Additionally, the throughput for the turner does not reflect the time associated with managing the piles with the turner when the performance is not optimal. As a result, without reliable oxygen readings, it is challenging to assess the efficacy of pile aeration per linear foot of the windrow against the processing rate per foot; however it seems that given the cost of fabricating the turner, the smaller pile size, the inconsistency of its performance, and the additional bucket work required to trim piles between turns, the turner is not an effective investment at this time.

The tine design has been successful in that the tines hold up to the stress of being lifted through the pile without bending, the tine attachment mounts to the tractor bucket easily and securely, and the tine spacing allows the windrow contents to fall between the tines as they are lifted. Additionally, the labor and materials cost to fabricate the tines met the initial criteria established for economic and technical accessibility. We would like to see improved mixing effects if possibly. Improved performance may be possible by loading the tractor’s tires. We have been operating the tines with a 250-300 pound concrete block mounted on the 3-point hitch of the tractor to counter balance the density of the pile, however it is thought that loading the tires would be more effective at improving the tractor’s leverage. Additionally, some of the slow processing rate can likely be attributed to the bucket hydraulics on tractor we operate at the West Hill Site, which are well-known as slow and relatively weak. A tractor with quicker, more aggressive front end hydraulics may be able to improve the aerating efficiency of the tines, which we found significantly slower per cubic yard than the other two aeration strategies. The tines process roughly two feet of linear pile and 2-3 cubic yards per minute.

Farmer participation in the design and review of both implements, but especially the turner, was extensive. (See Appendix E for list of farmer participants) Farmers around the US and in Canada operating farm-fabricated or commercial turners were contacted and in some cases visited. Two farmers, Eric Senecal and Gary Davis, were part of our early design development process, two farmers, Gary Davis and Ken Menard, were involved in the review of the turner fabrication manuals, a total of a dozen farmers participated in our equipment review days, and seven farmers attended our field day for the project.

Research conclusions:

The potential outcomes from this project have not been fully realized due to a delay in the project timeline, the higher than expected research and development costs, and challenges in refining the designs, however some impacts have resulted from the project and future impacts seem promising. We have shared our work on these implements with at least 120 attendees at Highfields Institute events and workshops. This audience has mostly included farmers, as well as professionals in agriculture, recycling and water quality. Through our design development work we have networked and collaborated with roughly fifteen farmers (two farmers are partners on the project), four Vermont Technical College (VTC) Engineering Department faculty, approximately 20 VTC engineering students, two fabricators, three composting industry professionals and researchers, and four farm and construction equipment dealers.

In November 2007 we hosted a field day to demonstrate the turner and Aerating Tines, as well as the pile “rolling” technique. Field day evaluations revealed that 75% of attendees would consider investing $500 to fabricate a set of aerating tines and that 17% of attendees would invest $5,000 to fabricate the turner. Of the self-identified seven farmers that attended the field day 100% expressed interest in making a set of tines, of which 80% stated that they or someone on their farm had both the tools and skills necessary to fabricate the tines, and 40% expressed an interest in building the windrow turner. It should be noted that this estimated cost presented to farmers was nearly $1,500 greater than the cost quoted to farmers on the survey, suggesting that with the lower cost the interest in adopting this technology might increase. Of the farmers attending the field day 60% said they or someone on their farm had both the skills and tools to fabricate the turner. We believe these responses suggest that there is significant interest among farmers in these technologies, as well as the capacity to produce these designs on the farm. Gary Davis, of Davis Farm in Jericho, who was not at the field day but was a design reviewer, stated that “I can’t wait to get started!” Additionally, we have orders from three producers for copies of the fabrication manuals when they are available.

We anticipate future impacts from this project to include secondary projects at the Highfields Institute to assess the impacts of these three aerating strategies on compost product quality. With expanded research in this area farmers will be able to make investments in management strategies based on a multi-pronged bottom line – economic, agronomic, logistical and infrastructure constraints, feedstocks, and end-use goals. For instance, vegetable growers are likely to have distinctly different needs than dairy farmers in terms of processing rates (generally handling fewer materials), product quality and end-use goals (may desire compost capable of suppressing specific plant pathogens), and their feedstocks may be different. For these reasons, a greater understanding of how equipment choices and aeration strategies impact various aspects of composting and compost utilization will enable producers to become significantly more effective composters, the implications of which are likely to extend well beyond the compost site and into other aspects of farm management, such as nutrient, disease and weed management.

Participation Summary

Education & Outreach Activities and Participation Summary

Participation Summary:

Education/outreach description:

Outreach over the life of the project has included a variety of incidental outreach efforts, such as sharing the project and design concepts with a class of engineering students and several professors at Vermont Technical College and communicating with a wide variety of farmers, Extension agents, agricultural educators, and technical service providers operating or designing turners. At the outset of the grant the project also received some publicity in the Times Argus, a large daily paper in central Vermont, in an article reviewing NE SARE awards to Vermont organizations. Focused outreach has largely occurred through two avenues and is lined up to occur through a third. To date most of our outreach efforts have been focused on turning farmers out to our Equipment Design Reviews. These were extremely informal afternoon gatherings at our composting research and education facility where we demonstrated the turner and the Aerating Tines for assembled farmers and project partners. Participants learned about the implements and increased their understanding of compost ecology, while we gathered extremely insightful and relevant feedback that was incorporated into design modifications. In November we also held a field day on the site to demonstrate the equipment. The field day was widely publicized in Vermont. Outreach advertising the field day was extensive with listings in multiple agricultural calendars, flyer and email announcements. The field day attracted 12 people.

We have also produced fabrication manuals (Appendix C) to share the designs with interested farmers and composters. These manuals will be an invaluable tool for disseminating the information assembled in this project in an effective manner. Outreach materials have been produced to advertise the manuals to the farming community. A flier advertising the manuals is shown in Appendix D. When the designs have been fully refined, we will disseminate these fliers to important agricultural outlets in the state, such as NRCS and NRCD offices, Extension offices, agricultural organizations, and feed stores and other relevant locations. Additionally, the manuals will be advertised on Highfields’ website, the website of other relevant organizations, through the newsletters of other agricultural organizations, and in various trade publications, such as Biocycle, Farm Show Magazine, Farming, Growing, and the Natural Farmer. An article for submission to some of these publications is largely complete and we are waiting to incorporate updated data and information before submitting it for printing.

Our outreach efforts were dampened by our delayed progress in designing and fabricating the turner. As a result, the designs are not as complete and refined as we had hoped to have them prior to releasing them to the farm community at larger. We have secured some funding that we will reserve for further testing and outreach. Through this grant we established the primary outreach tools and will apply them to a more focused outreach effort when we believe the designs merit dissemination.

Project Outcomes

Project outcomes:

Assessing the economics of pile turning can be challenging due to the variety of variables that can change in any given composting operation, which can significantly impact costs. When only accounting for the actual time and costs required to aerate a pile, the turner, at an estimated cost of $3,500 in materials (amortized over five years), becomes comparable to the operational costs of pile “rolling” when the total volume processed reached roughly 1000 cubic yards per year. This accounts for the time required to maneuver the turner around the composting site and line it up with piles, which can require as much as twice the time to process short 30’ piles or half of the processing time required to turn a 100’ pile. As a result, the fewer but longer the piles managed with the turner, the more cost effective. In other words, the turner is most cost effective in scenarios, like composting the bed pack from a deep bedded pack barn, where all of the material is cleaned from the barn once or twice a year and handled in a more consolidated fashion, rather than operations that take in small amounts of material on a regular basis and generally handle shorter piles. Around 1000 cubic yards of material the economy of scale begins to allow the turner to break even in cost compared to the “rolling” method and by 2500 cubic yards the cost to operate the turner annually drops to 80% of the tractor. As was to be expected, the turner becomes a tool for handling volume of material.

The tines become more cost effective than the turner at 300 yards or less, however the “rolling” method is more cost effective than the tines at nearly any scale – generally around 50-60% of the cost, depending on the scale. Assuming the existing functional challenges we are still working on with the turner are resolved, the turner would be an appropriate aeration tool for a farm with a herd of roughly 40 milking animals or greater, or anyone handling at least 1200 yards or more of raw feedstocks. At 1200 cubic yards per year or less, “rolling” piles with the tractor bucket appears to be the most cost effective aeration strategy. For more information regarding specific costs, refer to Appendix B “Aerating Strategy Processing Rates and Costs,” keeping in mind that these scenarios are based on specific assumptions about the site, piles and management strategy.

Farmer Adoption

No farmers have adopted these designs at this time. We have not felt that our designs are adequately refined to fully promote adoption of among farmers. A survey of field day participants revealed that 100% of farmers present at the field day were interested in adopting the Aerating Tines and 40% were interested in fabricating the turner. See “Impacts” section for additional information about potential farmer adoption.

Assessment of Project Approach and Areas of Further Study:

Areas needing additional study

Immediately, the two designs we have been working on will require additional study. We are presently seeking additional funding to continue with design development and research. Extended, replicated trials need to be conducted to further evaluate these designs. The Canaday Family Charitable Trust who has already provided matching funds for this project has expressed interest in receiving a proposal to support continued efforts on this project. Outside of work to complete these specific designs, inquiries into the impacts of different pile management strategies on product quality, economics, and other processing considerations will be extremely useful to producers. As our understanding of the potential impacts of compost to foster specific attributes in our soils grow, it will be increasingly important to understand how our management strategies impact agronomic qualities. For instance, does the intensive beating action of the windrow turner exacerbate the volatilization of carbon and nitrogen during composting? Some existing studies suggest it does, but additional research is necessary to confirm this and better understand potential variables. Additionally, if the turner does “burn” carbon in the compost, what are its impacts, or those of other aerating mechanisms, on the biological communities in the soil, and what are the potential ramifications of this on growing conditions? Additionally, greater understanding of pile oxygen dynamics are important and will inform the parameters within which we evaluate aerating systems. There is increasing discrepancy about what the minimum thresholds are for dissolved oxygen in the compost pile are, and how lower oxygen levels, between 2-5%, during active composting influence the process and product quality.

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.