A Case Study: Anaerobic Digestion of Dairy Manure and Food Processing Waste with Renewable Energy, Composting and Manure Injection (FS-2023-0694)

University of Maryland researchers monitored the transformations in nutrients, solids, and energy production over 13 months at a farm-scale anaerobic digestion system in Cecil County, MD that included composting and manure injection. Anaerobic digestion is a biological process that creates renewable energy in the form of biogas from substrates, such as manure, food waste, and wastewater sludge. The sustainability of anaerobic digestion, composting, and manure injection was quantified based on energy production and using a life cycle assessment (LCA) approach that calculated material and energy inputs and outputs. The biogas produced in the anaerobic digester was used directly to generate electricity in a combined heat and power generator, with excess heat captured from generator used to heat the anaerobic digester and maintain a constant temperature inside the anaerobic digester of 97°F for optimal microbial activity throughout the year. The solid separator removed solids from the dairy manure, with the separated solids composted to generate a value-added product. The liquid from the anaerobic digester was injected into the fields to provide nutrients closer to the plant roots and reduce both runoff potential and ammonia volatilization to protect waterways. Additionally, odor from the manure and food processing residuals was significantly reduced using anaerobic digestion and composting of manure solids, resulting in little to no on-farm odor concerns or drifting of odor off-farm, which drastically reduces both the on-farm and local community impact of manure management.

Dairy manure can be anaerobically digested with or without solid separation. On this farm (home to 750 dairy cows), the liquid portion of the manure was separated (72% of the dairy manure volume) and added to the anaerobic digester, while the separated solids (28%) were composted. The liquid fraction of dairy manure has higher concentrations of the dissolved organics used for biogas conversion. The more fibrous solids take longer to decompose and were separated from the waste stream prior to anaerobic digestion in this system. Some anaerobic digestion systems perform solid separation after digestion, with the digested solids used for cattle bedding. In this system, sand was used as the bedding substrate for the dairy cows in the barn, with the sand separated from the manure waste stream in a gravity sand lane prior to anaerobic digestion. The separated solids in the pre-digested manure were used for composting, and the liquid effluent from the anaerobic digester was recirculated and used for flushing the dairy barns. Food processing waste (36,000 gallons/week) was added directly to the anaerobic digester and co-digested with the liquid dairy manure (120,000 gal/week). The food processing waste consisted of cranberry processing waste, which has a high sugar content, and poultry processing waste. The poultry processing waste is treated with a process known as dissolved air flotation (DAF), where a coagulant and air are added to the liquid waste stream. This brings the dissolved oil, proteins, and carbohydrates to the top of the liquid for collection as a concentrated DAF substrate that is collected and added to the anaerobic digester, leaving behind a cleaner liquid stream for final treatment and disposal at the processing plant.

The dairy manure and food waste co-digestion and compost system process operated as follows:

1. Dairy manure and sand bedding material was flushed from the barn using the anaerobic digester effluent into a gravity-based sand separation lane. The sand bedding material was gravity settled, separated, and dried to be reutilized in the barn.
2. Dairy manure was processed through a screw-press, followed by a vibrating solid separation unit to separate the solid and liquid fractions.
3. The dairy manure solids were transported to the composting facility via a conveyor belt.
4. The liquid manure was pumped to the anaerobic digester, which operated at 97°F.
5. Food processing waste, consisting of poultry processing DAF and cranberry processing waste, was stored in two separate open containers and mixed using one part cranberry waste to three parts DAF before being pumped into the digester and mixed with the liquid dairy manure.
6. The digester waste contents are mixed by a motor that pulls the waste from inside the digester through an external radiant heater.
7. The excess energy is captured as radiant heat by the tubes surrounding the combined heat and power (CHP) electricity generator.
8. After absorbing the radiant heating, the waste content is pushed back into the digester, creating heat vortexes inside the anaerobic digester that internally mixes the wastes.
9. The generated biogas passes through an iron-based system to remove hydrogen sulfide (H₂S) prior to proceeding to the CHP to make electricity. Removing H₂S protects the generator from corrosion.
10. The anaerobic digester effluent is stored in an open lagoon that is used both for flushing the lanes of the dairy barns and for crop fertilization via surface application or manure injection.

The farm-scale anaerobic digestion system processed 6,210,000 gallons of dairy manure per year, with an average energy production of 2,000 megawatt hours (MWh)/year from 64 million cubic feet of biogas produced per month. The energy is derived from the methane (CH₄) content of the biogas. The CH₄ is produced from the organic matter, known as the volatile solids (VS) in the manure and food processing waste. The efficiency of producing energy was 631 L of CH₄ produced from each kg of VS processed (631 L of CH₄/kg VS). The produced biogas contained 56.2% CH₄, while the H₂S concentration in the biogas averaged 212 ppm before the scrubber (0.02% of the biogas content), and 4.4 ppm after the scrubber (0.0004% of the biogas content). During combustion in the CHP generator (or flare), this CH₄ and H₂S are combusted, eliminating the greenhouse gas emissions from CH₄ and odor concerns from H₂S release with open lagoon storage without a digestion system employed.

Throughout the monitoring period, 72.2% of the biogas was used for electricity generation, and 27.8% of the biogas was flared and not converted to renewable energy, as the 240-kW generator was run near capacity and more biogas was produced than the CHP could process. The generator processed 1,320,000 m³ of biogas/year, producing an average of 224 kW/hour of electricity. A higher capacity generator would be able to process more of the produced biogas into electricity.

The solids separator produced 4,700 m³ of separated solids annually, generating 4,200 tons of compost that contained 1.2% N, 0.24% P, and 0.43% K. Dragline manure injection and surface application were analyzed in the University of Maryland Bioenergy and Bioprocessing lab for ammonia (NH₃) volatilization. Manure injection analysis showed that the amount of ammonia (NH₃) lost to the atmosphere was negligible (below detection limit), while surface application without using injection reached a maximum of 16.4 mg/L NH₃ volatilized 24 hours after application.

University of Maryland researchers conducted a bioenergy potential test in a laboratory environment with individual substrates anaerobically digested individually and mixtures of poultry processing DAF and cranberry processing waste co-digested to compare the farm-scale energy production results to a controlled lab environment. The results showed that co-digestion of dairy manure without solid separation, DAF, and cranberry had the highest CH₄ efficiency (987 L CH₄ /kg VS), which was similar (958 L CH₄/kg VS) to the substrates used on the farm (dairy manure with solids separated, DAF, and cranberry). The food processing waste and dairy manure mixture was significantly higher than digestion of only liquid dairy manure (224 L CH₄/kg VS). The lab-based energy production efficiency was higher than on-farm results (631 L CH₄/kg VS), which is expected due to the ideal conditions of the lab-based conditions. The food processing waste (DAF and cranberry) digested in separate reactors had high energy potential (922 mL CH₄ and 909 mL CH₄, respectively) but not as high as co-digesting the food residuals with dairy manure due to the buffering capacity of the manure substrate.

A life cycle assessment (LCA) is used to calculate the environmental impact of technologies or processing systems. In this case, the analysis includes dairy manure storage, food processing waste transportation, energy production, energy use during operation, the anaerobic digestion and composting system components, and the fertilizer produced. A LCA estimates the environmental impact, including emissions, toxicity, and eutrophication (i.e., nutrient runoff) potential, of the entire process from manufacturing the system components to the energy and fertilizer produced and utilized at each stage (known as a “cradle to grave” analysis). The results can help in understanding the impact of each process and how to alter processes with high impacts to be more sustainable.

The case study’s LCA showed that the co-digestion and composting system at this farm lowered greenhouse gas emission by 81% (4,495 T CO₂eq/year) and lowered eutrophication by 447% compared to the baseline scenario of no anaerobic digestion and no composting with open lagoon dairy manure storage (23,751 T CO₂eq/year). Anaerobically digesting the food processing waste increased the renewable energy produced by 77%, thereby reducing greenhouse gas emissions, but resulted in small increases in fossil fuel depletion (3%) and ozone depletion (6%) due to the emissions associated with the transportation of food waste to the anaerobic digester (DAF was transported from 30 miles away and cranberry waste from 100 miles away). Sourcing local food processing residuals would increase overall sustainability. Additionally, using all the produced biogas in a higher capacity CHP generator would be more sustainable than flaring nearly 1/3 of the produced biogas due to the generator being operated at capacity. The LCA showed the positive impact of having anaerobic digestion and composting on-farm, with large reductions in environmental impacts, eutrophication, and greenhouse gas emissions by adding an anaerobic digester and producing renewable energy and fertilizer from dairy manure and food processing waste.

• The renewable energy produced from the digester used in a CHP generated a net positive energy production, producing 2,000 MWh/year, which is enough electricity to power 190 homes annually.
• The hydrogen sulfide (H₂S) scrubbing system was near 100% efficiency for the entirety of the monitoring period, eliminating potential corrosion in the CHP and odor associated with H₂S when open lagoon storage without an anaerobic digestion system employed.
• The lab-based energy analysis showed that co-digesting poultry processing DAF, cranberry processing waste, and liquid manure had high bioenergy conversion efficiency, with co-digestion producing more energy than digesting the food processing waste or dairy manure separately.
• The LCA showed that the anaerobic digestion system reduced GHG emissions by 81%, and if the food waste residuals were located closer to the digester and 100% of the biogas was used in the CHP generator for renewable electricity generation, the reductions would be even greater.

This study evaluated a farm-scale anaerobic digestion system for biogas and compost production. Over 13 months, monthly samples were collected from nine sampling points to verify the system performance. The results showed that using anaerobic digestion to convert the dairy manure and food processing waste to biogas and renewable electricity and compost is possible in Maryland. The total biogas production was 64,000,000 ft³/yr from the system, which generated 2,000 MWh/yr, with 4,200 metric tons of compost produced to be used as fertilizer. The life cycle assessment (LCA) showed that current operation decreased environmental impacts of manure management, especially greenhouse gas emissions and eutrophication. Lowering transportation distances of the food processing waste would further increase sustainability.

Dr. Stephanie Lansing, University of Maryland, Department of Environmental Science and Technology, College Park, MD. 20742 Tel: 301-405-1197 | Email: slansing@umd.edu

For more information on the Animal Waste Technology factsheet series and the Maryland Animal Waste Technology assessment submitted to the Maryland Department of Agriculture go to https://go.umd.edu/AWTF

This work was supported by the Maryland Department of Agriculture.