Converting Food Waste into Biomethane Fuel

Food waste is a source of greenhouse gas emissions and a potential energy source and the Department of Defense (DOD) is a significant consumer of energy and generator of solid waste.

During FY2009, the department consumed 209-trillion-BTU of energy, excluding vehicle fuel. Over the same period, DOD generated 5.2-million-T of solid waste. In 2011, of the 164-million-T of municipal solid waste discarded about 21.3 percent contained food waste.

The energy content of the food waste was about 130-trillion-BTU, or about 60 percent of the FY2009 energy use. Much of this highly biodegradable waste is disposed in landfills where it is naturally anaerobically digested into the greenhouse gases: methane and carbon dioxide.

Anaerobic digestion of food waste in engineered reactors in order to produce methane-rich biogas offers a sustainable alternative to current practices and a source of energy. Furthermore, this biogas can be purified to produce vehicle fuel and provide greenhouse gas offsets. A demonstration project sponsored by DOD’s Environmental Security Technology Certification Program (ESTCP) was conducted at the U.S. Air Force Academy in Colorado Springs, Colo., to pursue this technology and its potential benefits and adoption. ESTCP seeks to demonstrate and validate innovative technologies to enhance mission effectiveness and reduce the nation’s current and future environmental liabilities.

During the demonstration, four phases were conducted: equipment shakedown; startup; stable operation with diluted digester feed; and modified process with concentrated digester feed. Biogas purification testing was conducted during Phase IV.

APPROACH TO THE PROBLEM

Anaerobic digestion plus biogas purification was used to convert food waste to biomethane fuel (food-to-fuel). Anaerobic digestion is a process where a community of anaerobic microorganisms biodegrade organic matter and produce biogas, which is a mixture of methane (CH₄) and carbon dioxide (CO2).

One important consideration for an installation is whether there is staff available to operate and maintain what is essentially a wastewater treatment plant. If the installation already has a facility on site, such as at the Air Force Academy, then implementation is much easier.

There were two technologies demonstrated for biogas purification of biomethane. Hydrogen sulfide (H2S) and organosulfur compounds were removed using a mixed metal oxide media (SulfaTrap). A triple-bed vacuum swing adsorption unit was used for CO2 and moisture removal.

TECHNICAL & ECONOMIC RESULTS

Anaerobic digestion of food waste and canola oil showed the capability to recover potential energy content, reduce solid waste, and potentially produce a valuable, nutrient-rich end product. Canola oil served as a surrogate for grease trap waste, which is another energy-rich waste associated with food waste. Meanwhile, biogas purification was demonstrated to be capable of high methane recovery, and production of biomethane that was sufficiently pure to be compressed and used as vehicle fuel. When the processes are considered together, they provide a solid waste reduction technology that recovers energy, creates a greenhouse gas offset, and produces an end product. The process provides distinct advantages over landfilling and composting with respect to energy recovery and greenhouse gas offsets.

The performance objectives of this demonstration included various aspects of renewable energy conversion efficiency; digester capacity and stability; biogas purification; solids destruction and minimization of process residuals; operational reliability; and accounting of greenhouse gas emissions.

Both quantitative and qualitative performance objectives were evaluated during the demonstration. Energy conversion efficiency of food waste and canola oil to methane was 73±13 percent (goal ≥70 percent). When energy consumption by the process was considered (heating, pumping, and gas purification), the efficiency was 62 percent (goal ≥50 percent). Volumetric methane production rate was not met (0.82±0.22- L/L/d – goal ≥2-L/L/d]), likely as a result of feeding a diluted food waste/canola oil mixture. The target rate of 2-L/L/d was achieved at the end of the demonstration through the elimination of the food waste dilution step. Methane recovery during biogas purification by the triple-bed vacuum swing adsorption was 94±2.9 percent (goal ≥80 percent). H2S in the treated biogas was 0.030±0.035-ppm (goal <4-ppm). CH₄ in the treated biogas was 98±0.5 percent (goal ≥95 percent) after a correction for likely air contamination during sampling.

Total solids reduction was 78±3.4 percent (goal ≥60 percent). Digestate sulfide was 71-mg/L (goal <500-mg/L). The digestate was a liquid with low total suspended solids, high ammonia and volatile fatty acid concentrations, moderate concentrations of pathogens, and poor dewaterability. Compost amendment is possible, though odor could be a concern. The digestate may be useful as a liquid fertilizer considering the concentrations of ammonia and metal nutrients. The process was 93 percent available during Phase III and 100 percent available during Phase IV (goal ≥95 percent).

Mechanical malfunctions during Phase III were related to a digester mixer shaft seal that leaked. After start-up issues were resolved, the system was easily operated by a single operator working one shift per day, five days per week. The calculated greenhouse gas emissions generated from a nominally scaled food waste digester were -470-T/year. By comparison, landfilling and composting would generate +530-T/year and +180-T/year, respectively.

INVESTMENT CONSIDERATIONS

The capital and operations and maintenance costs of a green field food waste digester and gas purification system were determined for three installation sizes (10,000 personnel; 20,000 personnel; and 40,000 personnel). The capital costs ranged from $0.93 (10,000 personnel) to $2.44 million (40,000 personnel).

Net annual revenues (income from vehicle fuel minus operating and maintenance costs) ranged from -$20,000 (10,000 personnel) to $120,000 (40,000 personnel). When capital costs, operations and maintenance, and revenues were considered, the net present cost ranged from $1.28 million (10,000 personnel) to $280,000 (40,000 personnel). The costs for food waste digestion and vehicle fueling were as low as $4/wet-T (40,000 personnel) to $50/wet-T (10,000 personnel). Compare these costs to average landfilling costs of $50/wet-T and composting costs ranging from $29/wet-T to $52/wet-T.

This economic advantage, combined with the minimized greenhouse gas emissions and dependence of petroleum-based fuels, suggests that food waste digestion and biogas purification is advantageous.

BENEFITS & POTENTIAL IMPACT

The Air Force Academy ESTCP project demonstrated that anaerobic digestion of food waste at military bases is technologically feasible and can be cost competitive with alternative methods of food waste management, depending on the size of the installation. Often, anaerobic digestion systems are custom-designed. However, in recent years, a number of companies have emerged that specialize in the manufacture of on-site anaerobic digestion systems.

One important consideration for an installation is whether there is staff available to operate and maintain what is essentially a wastewater treatment plant. If the installation already has a facility on site, such as at the Air Force Academy, then implementation is much easier.

In the recent past, gasoline has exceeded $4/gal. At these prices, the value of the technology would be greater. Additionally, electricity prices vary greatly across the country and pricing in some areas and for larger customers can be more complicated. A treatment system that generates more power than it uses could sell excess power to the electric utility, but that may require a power purchase agreement as well as additional relays and switches to protect the grid. Another factor affecting cost effectiveness is local landfill tipping fees. Greater landfill tipping fees will result in the technology being more cost effective.

Patrick Evans, Ph.D. is Principal, CDM Smith; evanspj@ cdmsmith.com.

Andrea Leeson, Ph.D., is Deputy Director and Environmental Restoration Program Manager, Department of Defense Strategic Environmental Research and Development Program and Environmental Security Technology Certification Program; leeson.andrea@gmail.com.

Brig. Gen. Patrice Melançon, F.SAME, USAFR, is Mobilization Assistant to the Director of Logistics, Engineering & Force Protection, HQ Air Force Reserve Command, Robins AFB, Ga., and Watershed Engineering Department Manager, San Antonio River Authority; pmelancon@sara-tx.org.

This project was sponsored by ESTCP under Contract W912HQ10-C-0001 (ESTCP Project ER0933), and was supported by in-kind R&D funding from CDM Smith.

[Article first published in the January-February 2018 issue of The Military Engineer.]