This project studied the application of biodigestion on small farms to convert organic waste into bioenergy and biofertilizers. The first goal of this project was to measure and analyses potential feedstocks for biodigestion on small farms. These included organic wastes from a local small farm, biodiesel production byproducts, and food waste from local schools and restaurants. The second goal of this project was to demonstrate a functional biodigester to the small farming community. This included both demonstration of biodigestion, as well as biogas storage, cleanup, and usage. To complete this goal, a small-farm-scale biodigester was constructed for demonstration and usage.
The purpose of this project was to study the potential applications of bioenergy and biofertilizer production on small farms. The US EPA AgStar program outlines the bioenergy opportunities in the US for larger livestock operations (US EPA). Bioenergy options, however, are not just for large farms and cities. On the contrary, these emerging renewable energies must ensure economic benefits for family farms and rural communities (Wilkie, 2007). There are many small farms in the US that could produce bioenergy and biofertilizer from the variety of different organic wastes they produce. This project will study these different feedstocks and demonstrate the feasibility of biodigestion on small farms.
The process of anaerobic digestion, or biodigestion, has occurred naturally for millions of years. It is essentially the microbial decomposition of organic material in the absence of oxygen. As the material breaks down, methane is released. Under controlled conditions, this methane can be captured and used as an energy source (Wilkie, 2008). Biogas, as the gaseous product is termed, is composed largely of methane, and is a readily usable fuel that can be burned directly for cooking or heating water. Biogas can contain hydrogen sulfide that can cause problems for equipment using biogas. It may be necessary to remove this hydrogen sulfide prior to use. A simple technique is to use steel wool or iron-impregnated woodchips to scrub the hydrogen sulfide from the biogas. Biodigestion also produces a liquid and solid effluent. Because the bacterial metabolism consumes only the carbonaceous component of the material, the nutrients from the organic material remain within the effluent as a high-quality organic fertilizer (Arnott, 1985). Biodigestion captures both energy and nutrients from organic materials using a natural process that requires little to no input of energy and chemicals.
Nearly any type of organic material can be used as a feedstock for biodigestion. Small farms produce significant amounts of organic wastes, such as culled vegetables, crop residues, processing waste, and animal manure. Currently most of these wastes on small farms are composted, land applied, or hauled off-site. Through biodigestion of these wastes, both the energy and nutrients from the waste are captured and recycled for use on the farm. The variety of organic material available for biodigestion means that biodigestion can benefit every small farm. Along with plant and animal wastes, byproducts of biodiesel production, including glycerol and washwater, are potential feedstocks for biodigestion for small farms that produce their own biodiesel. In addition, organic wastes from the community can be brought onto the farm for additional production of biogas and biofertilizer, including food waste from schools or restaurants or biodiesel byproducts from community producers. The first goal of this project (Objective 1) is to study and analyze potential feedstocks for small-farm biodigestion. In order to accomplish this goal, waste audits were conducted at a variety of locations to determine the amount and type of organic waste that is generated. Samples of these materials were analyzed to estimate methane production potential.
Demonstrating the feasibility of on-farm biodigestion will make great strides towards the implementation of this sustainable technology on small farms in the region. Therefore, the second goal (Objectives 2 and 3) was to construct and demonstrate a functioning small-farm-scale biodigester with integrated biogas storage and clean-up to remove hydrogen sulfide. By giving small farmers the opportunity to see biodigestion with a reactor that can be replicated on their own farms, these farmers will be more likely to adapt this technology.
- Determine biogas potential of various organic wastes produced by small farms.
Demonstrate effective methods of biogas clean-up and storage.
Demonstrate a functioning biogas reactor and storage system to the small-farm community.
Small Farm Waste audit
An on-farm waste audit was conducted at a local small farm (Crones’ Cradle Conserve) located in Citra, FL. Crone’s Cradle Conserve is a small organic farm with approximately 2/3 acre under cultivation that produces a variety of produce, farm-kitchen processed foods, and greenhouse plants. They also keep approximately two dozen rabbits and a pig. The audit occurred over a 3-week period in the summer season (June and July) during which all organic waste produced each day from farm activities was collected, categorized, and weighed, and samples were taken for laboratory analysis. There were eight categories of organic waste: row clearings, weeds, greenhouse waste, vegetable culls, processing waste, harvest waste, rabbit manure, and pig manure. Row clearings were unproductive or diseased crop plants that were removed from the rows. Greenhouse waste included any culled plants, weeds, and organic potting medium from the greenhouses. Vegetable culls were any rotten or unmarketable produce. Processing waste was material from the on-farm kitchen that produced jams, breads, canned items, etc. Harvest waste was any material harvested with the vegetables but is discarded prior to market (e.g. corn husks, onion tops). All samples were measured for total solids (TS) and volatile solids (VS). Literature values were used to estimate methane production potential.
Biodiesel Production Byproduct audit
Glycerin and washwater are byproducts of biodiesel production and are waste products that a small farmer would have to address should the farmer decide to produce their own biodiesel from used or virgin vegetable oils. In order to characterize the byproducts produced from biodiesel production and determine their potential as a feedstock for biodigestion, we worked with the Alachua County Hazardous Waste Collection Center, which produces biodiesel from used vegetable oil collected from the community. The biodiesel is produced using an off-the-shelf, self-contained system (Biopro 190) that produces approximately 190 L of biodiesel from approximately 190 L of used vegetable oil per batch. Over 12 weeks, the waste products produced from each weekly batch was measured and samples were taken for laboratory analysis. Chemical oxygen demand (COD) was measured on each sample to estimate the methane production potential.
Food Waste audits
Additional audits were conducted at local schools and restaurants to determine the methane and biofertilizer production potential if small farmers were to partner with these establishments to collect the food waste for biodigestion. Audits were conducted at three local schools: J.J. Finley Elementary (a public elementary school), Lofton High School (a public alternative high school), and Oak Hall School (a private middle and high school). Each audit was conducted for 2 weeks during which all post-consumer waste was collected from the cafeteria and sorted to determine the amount of food waste generated. Lofton and J.J. Finley both served breakfast and waste from breakfast was included in the audit. Additionally at Lofton, waste from the cafeteria’s kitchen was included in the audit. At Oak Hall, lunch was not served to students from a kitchen, rather each student brought their own lunch or food was catered via box lunches from outside vendors. Waste audits were also conducted at three local restaurants: Rolls ‘n Bowls (a quick-serve Asian restaurants), Satchel’s Pizza (a full-service pizza restaurant), and The Top (a full-serve restaurant). Each audit was conducted for 2 weeks during which all pre-consumer waste from the kitchen and post-consumer waste from the dining room (excluding Roll ‘n Bowls, which did not have a formal dining room) was collected and sorted separately to determine the amount of food waste generated. Once all food waste was separated and weighed for each day at each location, samples were taken for laboratory analysis. Total solids, VS, and COD were measured to determine biogas production potential. Total nitrogen (TN) and total phosphorous (TP) were measured on the samples to determine nutrients available for use as a biofertilizer after biodigestion.
Total Solids and Volatile Solids
Total solids and VS were measured according to standard methods (APHA, 2005). Triplicate 100 g representative samples were weighed into pre-ashed, pre-weighed 200 mL disposable aluminum dishes. Samples were dried at 103°C in a drying oven (Precision Model STG 80) for 24 hours. Dried samples were placed in a desiccator to cool to room temperature, then weighed to record TS. To measure VS, dried samples were ashed for 2 hours in an ashing furnace (Thermolyne 30400) at 550°C. Ashed samples were placed in a desiccator to cool to room temperature and then weighed. Ash weight was subtracted from TS to calculate VS.
Chemical Oxygen Demand
Chemical oxygen demand is the oxidative potential of organic matter within a solid or aqueous solution. It is an important measure when studying anaerobic digestion because it determines the maximum theoretical methane yield of a substrate. Stoichiometrically, 1 g of COD can generate 0.35 L of methane at standard temperature and pressure (STP: 0°C, 1 bar). Chemical oxygen demand was measured according to standard methods (APHA, 2005). Samples were blended to homogenize, diluted to the required concentration as needed, and mixed continuously when sampling in order to obtain a representative sample. A large-bored, disposable pipette was used to transfer 2 mL of the diluted sample to a HACH High-range COD tube, which measures COD through a colorimetric change. Tubes were digested for 2 hours at 150°C in a HACH Model 45600 COD reactor. Digested tubes were read on a HACH 890 colorimeter.
Total Nitrogen and Total Phosphorus
Total nitrogen and TP were measured by blending a 25 g representative sample of food waste in a 360 mL stainless steel blender for 1 minute. Triplicate 0.25 g representative subsamples of the blended food waste were weighed onto 1 KimWipe tissue (1 ply, 11 x 21 cm). Samples, including KimWipe, were digested using a modification of the aluminum block digestion procedure of Gallaher et al. (1975). Catalyst used was 1.5 g of 9:1 K2SO4:CuSO4, and digestion was conducted for at least 4 h at 375°C using 6 ml of H2SO4 and 2 ml H2O2. Nitrogen and phosphorus in the digestate were measured by semiautomated colorimetry (Hambleton, 1977). A blank KimWipe was also measured to correct for the TN and TP in the tissue.
A biogas storage unit was integrated into the small-farm biodigester (see Objective 3). The biogas collector consisted of a 3’ length of 2.5’ diameter corrugated HDPE double-wall tubing with a volume of 400 L. One end of the tube was sealed by welding a circle of plastic inside the tube approximately 1” from the end. A ½” PVC ball valve was placed through the plastic to allow the biogas to be used during operation. Concrete was then poured into this 1” space on top of the plastic creating a disk than provides structural integrity as well as weight to create pressure for the biogas to maintain a stable flame when combusted. The concrete was coated with rubber sealant to prevent any potential gas leaks. Flexible rubber tubing connected the valve on top of the gas collector to PVC tubing on the outside of the biodigester that connected to a valve approximately 2’ off the ground. This valve was connected to the biogas scrubbing unit. This unit consisted of a gastight plastic canister with barb valves at the top and bottom. Steel wool was placed into the canister to scrub hydrogen sulfide from the biogas. Sampling valves were placed before and after the biogas scrubbing unit to measure the reduction in hydrogen sulfide with the unit. Rubber tubing connected the scrubbing unit to a biogas cook top for demonstration and utilization.
In order to demonstrate anaerobic digestion to the small farms community, two biodigesters were utilized in this project. The first was a portable biodigester that was constructed using funds from another project. The portability of this biodigester allowed us to demonstrate biodigestion at events away from our lab. These events are listed under the Outreach section of this report. The second biodigester, which was built exclusively for this project, was constructed as a demonstration unit to showcase a low-cost, easy-to-assemble biodigester that would be ideal for the small farming community. This biodigester consisted of two vertical lengths of corrugate HDPE double-wall tubing. The larger tube (3’ diameter) is 7’ and served as the biodigester. The smaller tube (2.5’ diameter) was 3’ tall and served as the biogas collector (described above) and was placed inside of the larger tube. One end of the biodigester tube was sealed with a circle of plastic and concrete to seal the bottom and act as a base for the unit. The tubes’ diameters were selected to minimize space around the gas collector and reduce the exposure of the biodigester contents to the atmosphere. The working volume of the biodigester was approximately 900 L. Fixed microbial medium was contained within the biodigester to allow attached growth of the anaerobic microbial consortia, which increased the biological stability of the biodigester and allowed for greater biodigestion efficiency.
PVC plumbing outside of the biodigester allowed feedstock to be fed to the biodigester and effluent to exit the biodigester. A manual bilge pump pumped the feedstock into the biodigester on top of the fixed media. A vertical length of PVC (5’ long) acted as a displacement/effluent line to maintain the volume in the biodigester. An inverted u-trap on top of the effluent line allowed the effluent, displaced when feeding the biodigester, to be recovered as biofertilizer. Valves on the influent and effluent line can be closed to mix the biodigester using the manual pump. An additional valve is provided in the PVC plumbing for sampling the biodigester content. To inoculate the biodigester with the anaerobic microbial consortia needed for anaerobic digestion, we used 900L of flushed dairy manure obtained from the University of Florida Dairy Unit. Once the biodigester began producing methane, we began feeding the biodigester various organic wastes obtained through the waste audits to produce sufficient gas for demonstration activities.
Small Farm Waste audit
Table 1 shows TS, VS, and estimated methane production for each category of waste collected at Crones’ Cradle Conserve. On a weekly basis, the total amount of organic waste generated from Crones’ Cradle Conserve was 379.0, 545.2, and 524.5 kg for Weeks 1, 2, and 3, respectively (Table 2). On an ash-free, dry weight basis (i.e. VS), 59.7±2.4, 67.8±5.0, and 51.8±2.6 kg VS were generated for Weeks 1, 2, and 3, respectively. For all three weeks, the largest category was row clearings followed by weeds for Weeks 1 and 2 and culls for Week 3. Due to the heterogeneity of each category of waste collected as well as the specificity of the individual components collected in the audit, it is difficult to make specific estimates of the methane production potential of these wastes that would be applicable to different small farms. A range of methane production data have been reported in the literature for various organic wastes, which can provide a general range for the estimated methane production potential of the on-farm organic wastes. Gunaseelan (2004) reported methane potential values of 0.19 to 0.41 m3/kg VS for various vegetable wastes. These values included leaves, stems, whole plants, culled vegetables, peels, and processing waste from different crops. These types of materials were represented in the row clearing, culls, harvest waste, greenhouse waste, and processing waste categories. Chynoweth et al. (1993) reported a range of 0.16-0.39 m3/kg VS for methane potential from grasses. The weeds category of our audit was predominantly represented by grassy weeds. Masse et al. (2011) reported methane potential values from swine manure of 0.2 to 0.4 m3/kg VS, and Aubart and Bully (1984) found methane production values of 0.23 m3/kg VS for rabbit manure. Applying these ranges in the literature to the waste generated from Crones’ Cradle Conserve, an estimated 11.1 to 23.0 m3, 11.9 to 27.1 m3, and 9.75 to 21.1 m3 of methane could be generated for Weeks 1, 2, and 3, respectively.
It is important to note, that these results are only representative of the organic waste this farm produced during the summer season. It is expected that there would be significant variability throughout the year. For example, row clearings were high during the audit because many of the rows of spring crops were being removed for replanting. Weeds were also high due to increased weed growth during the summer. During harvesting periods it would be expected that culls and harvesting waste would be greater. Organic waste generation would also be significantly reduced during the non-growing season. Different farms growing different crops or livestock would have much different organic waste profiles. Crones’ Cradle Conserve practices organic production so most of the weeds were hand pulled allowing them to be collected and potentially biodigested. A farmer that sprays weeds would not have this option. Nevertheless the results of the waste audit at Crones’ Cradle Conserve indicated that even a very small farm, at only 2/3 acre under cultivation, can generate a significant amount of methane through biodigestion to offset their energy needs.
Biodiesel Production Byproduct audit
Table 3 shows the COD and methane potential of the glycerol and washwater collected during the audit. The mean weekly generation of glycerol and washwater at a 1 batch/week rate was 35.5±1.4 and 198.9±7.9 L/week, respectively, from the 190 L biodiesel production unit evaluated in this audit (Table 4). The mean COD concentration of glycerol and washwater samples was 1677±59 and 55.5±11.7 g COD/L, respectively (Table 3). Using an assumption of 90% COD conversion to methane, the estimated methane potential of these byproducts was 529±19 and 17.5±3.7 L methane/L for glycerol and washwater, respectively. Therefore, the methane production potential of the byproducts from the biodiesel unit at the Alachua County Hazardous Waste Collection Center was 18.7±0.9 and 2.3±0.1 m3/week.
If a small farmer either chooses to produce their own biodiesel or partner with a biodiesel producer to collect their waste products, the farmer could benefit through by biodigestion of the byproducts and generate approximately 21 m3 of methane per 50 gallons of biodiesel produce. Glycerol and/or washwater could be added into a biodigester with other feedstocks (manure, food waste, farm waste) to reap the benefit of this additional load of organic material.
Food Waste from Schools and Restaurants
Table 5 shows the physiochemical parameters of food waste collected from the schools during the waste audit. The mean daily generation of food waste from each school was 8.2±0.8 kg/day (3.1±0.5 kg VS/day, 4.3±0.6 kg COD/day) at Oak Hall School, 36.8±5.7 kg/day (9.5±0.9 kg VS/day, 13.7±1.3 kg COD/day) at J.J. Finley, and 13.2±2.3 kg/day (3.8±0.8 kg VS/day, 5.5±0.9 kg COD/day) at Lofton (Table 6). The daily food waste generated per student was 24.7±2.3 g/day/student from Oak Hall (355 students), 90.4±13.3 g/day/student from J.J. Finley (436 students), and 60.1±9.3 g/day/student from Lofton (247 students) (Table 7). The low generation from Oak Hall was likely due to the fact that Oak Hall does not have a full cafeteria. The weekly methane production potential through biodigestion of the schools’ food waste, assuming 90% COD conversion, was 6.8±0.9 m3/week for Oak Hall, 21.5±2.0 m3/week for J.J. Finley, and 8.7±1.5 m3/week for Lofton.
Table 8 shows the physiochemical parameters of food waste collected from the restaurants during the audit. The mean daily generation of food waste from each location was 20.3±5.9 kg/day (4.5±2.2 kg VS/day, 5.7±2.8 kg COD/day) from Rolls ‘n Bowls, 36.4±11.4 kg/day (9.3±2.8 kg VS/day, 13.2±4.1 kg COD/day) from Satchel’s, and 56.0±15.2 kg/day (11.2±3.8 kg VS/day, 18.6±7.1 kg COD/day) from The Top (Table 9). The daily food waste generated per customer was 75.0±21.6 g/customer/day from Rolls ‘n Bowls (mean 273 customers/day), 82.2±19.9 g/ day from Satchel’s (mean 442 customers/day), and 186.9±47.1 g/customer/day from The Top (mean 303 customers/day) (Table 10). The high generation of food waste from The Top is likely due to the fact that it was a full-service restaurant with a full kitchen preparing fresh foods. The weekly methane production potential through biodigestion of the restaurants’ food waste, assuming 90% COD conversion, was 12.6±6.3 m3/week for Rolls ‘n Bowls (open 7 days/week), 20.8±6.4 m3/week for Satchel’s (open 5 days/week), and 35.2±13.4 m3/week for The Top (open 6 days/week). At Satchel’s 84% of the methane potential was from dining room food waste, while at The Top 58% of methane potential was from dining room food waste.
The mean total nitrogen (TN) and total phosphorus (TP) of all school food waste samples were 2.97±0.67% dry weight and 0.53±0.22% dry weight, respectively. The nitrogen and phosphorus recoverable in biofertilizer through biodigestion of the food waste from each school was 0.51±0.07 kg TN/week and 0.07±0.02 kg TP/week from Oak Hall, 1.58±0.23 kg TN/week and 0.26±0.05 kg TP/week from JJ Finley, and 0.58±.014 kg TN/week and 0.11±0.03 kg TP/week from Lofton. The TN and TP of all restaurant food waste samples were 3.06±0.89% dry weight and 0.34±0.10% dry weight, respectively. The nitrogen and phosphorus recoverable in biofertilizer through biodigestion of the food waste from each restaurant was 0.97±0.55 kg TN/week and 0.10±0.04 kg TP/week from Rolls ‘n Bowls, 1.45±0.48 kg TN/week and 0.15±0.05 kg TP/week from Satchel’s, and 2.65±1.44 kg TN/week and 0.28±0.13 kg TP/week from The Top.
The data from these audits helps to inform small farmers of the potential bioenergy and biofertilizer available by partnering with local schools and restaurants to collect their food waste for on-farm biogas and biofertilizer production. When partnering with a school or restaurant, it is important to decide the type and size of the establishment because food waste generation varies widely between locations. It is also important to consider seasonal variation; particularly with schools where little or no food waste would be generated during the summer, and biogas and biofertilizer production would be reduced during this time. Connecting small farmers to local schools and restaurants will help to strengthen communities by creating a sustainable cycle in which the community’s food waste is sustainably handled while generating bioenergy and biofertilizer for the small farmer. There could also be additional kickbacks for the small farmer by having a close partner to which to sell their produce and food products.
Utilizing the portable biodigester, we were able to demonstrate small-scale biodigestion at numerous events in the small-farming community. These events are listed below under the Outreach section. The small-farm demonstration biodigester has been used as an example of what a small-farmer can construct to digest a variety of on-farm or community-sourced organic wastes. The biodigester can be completely assembled using readily-available parts found at local hardware stores or agriculture supply catalogs. No electricity or external input is required by the biodigester so it can be constructed anywhere on the property wherever is most convenient for the famer. The amount of feedstock fed to the biodigester depends upon the type of feedstock the farmer selects and the optimal operating conditions for the feedstock. This is subject for future research. The biogas collector holds 400 L of biogas, which assuming a methane concentration of 75%, is approximately 10,600 BTU of energy. The rate of biogas production and the concentration of methane in the biogas depend on the operating conditions of the biodigester; however it could be feasible to fill the 400L biogas collector multiple times daily. For the demonstration unit we had the biogas connected to a small biogas stove, which may be ideal for a small farmer for cooking/processing food. The farmer could also power a modified propane or natural gas refrigerator or heat water or a greenhouse, if needed. The biogas scrubbing unit helps to remove hydrogen sulfide to prevent corrosion of equipment. Because the biodigester is scalable, a larger biodigester could be constructed using a similar design or several biodigesters could be constructed to process a larger amount of waste and produce a greater volume of gas. With larger volumes of biogas, additional uses of the biogas, including electricity production are possible.
Educational & Outreach Activities
We have developed a website to share our activities for this project. The website is available at: http://biogas.ifas.ufl.edu/SARE
- “Anaerobic Digestion: Sustainable Energy and Nutrients from Food Waste”. Florida Campus and Community Sustainability Conference, Tampa, FL. October 8, 2009.
“Biofertilizer Potential of Food Waste Anaerobic Digestion on Small Farms”. Joint Meeting of the Florida State Horticultural Society and the Soil and Crop Science Society of Florida, Crystal River, FL. June 6-8, 2010.
“Anaerobic Digestion and Algae Farming: Energy and Nutrients for Small Farms”. IFAS Florida Small Farms and Alternative Enterprises Conference, Kissimmee, FL. July 31-August 1, 2010.
Poster: “Energy and Nutrient Resources from Organic Wastes for Small Farms”. 11th Annual Soil and Water Science Department Research Forum, Gainesville, FL. September 10, 2010.
“Diverting Food Waste for Bioenergy”. Southeast Bioenergy Conference, Tifton, GA. August 11, 2011.
Poster: “Anaerobic Digestion Potential of Organic Wastes from Small Farms”. 12th Annual Soil and Water Science Department Research Forum, Gainesville, FL. September 9, 2011.
- IFAS Agricultural Enterprises Workshop for North Florida, Live Oak, FL. November 5, 2009.
1st Annual North Central Florida Farm-to-Restaurant Workshop, Gainesville, FL. July 19, 2010.
Sunbelt Agricultural Exposition, Moultrie, GA. October 19-21, 2010.
UF/IFAS Tri-county Biofuel Symposium, Bunnell, FL. June 9, 2011.
2nd Annual North Central Florida Farm-to-Restaurant Workshop, Gainesville, FL. August 1, 2011.
Southeast Bioenergy Conference, Tifton, GA. August 10, 2011.
Fall Natural Foods Gala at Crones’ Cradle Conserve, Citra, FL. October 15, 2011.
There are two primary outcomes of this project. The first outcome is that the waste audits and feedstock analyses have allowed us to create a database of organic wastes available to small farms as potential feedstocks for biodigestion. This database will be made available on our website in the future. The second outcome is the small-farm-scale biodigester we constructed for this project that is now a permanent fixture at our laboratory. Visitors to our labs, which include small farmers, extension agents, educators, students, and business people, will be able to see and learn about a functional biodigester that can be assembled using readily available materials and without any energy or technological inputs. The biodigester will also be the centerpiece for future field days to be held with the UF/IFAS Small Farm and Alternative Enterprises.
Areas needing additional study
Further research is needed to develop a full-scale feasibility study and pilot project for biodigestion on small farms. These studies are needed to investigate to what extent the methane production estimates developed by this study are replicable using a small-farm-scale biodigester. More research is needed on the best use of the biogas on small farms and developing off-the-shelf biogas units (e.g. stoves or refrigerators) for use on the farm. There is also need of crop production studies to investigate the use of biofertilizer as an amendment or replacement for synthetic fertilizer. Economic analysis is needed to determine the economic feasible of this technology on small farms.
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