The Department of Defense (DoD) is a significant consumer of energy and generator of solid waste. During fiscal year 2009 the DoD consumed 209 trillion British Thermal Units (BTUs) of energy (2.2 × 1017 J), excluding vehicle fuel. Further, during the same period the DoD generated 5.2 million tons of solid waste. In 2011, 164 million tons of municipal solid waste was discarded comprising 21.3% food waste. The energy content is about 130 trillion BTU or about 60% of the fiscal year (FY) 2009 DoD energy use. Much of this highly biodegradable waste is disposed in landfills where it is anaerobically digested into the greenhouse gases (GHG) methane and carbon dioxide. Anaerobic digestion of food waste in engineered reactors 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.
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 GHG emissions. Both quantitative and qualitative performance objectives were evaluated during the demonstration.
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 – a mixture of methane (CH4) and carbon dioxide (CO2). Two technologies were demonstrated for biogas purification biomethane. Hydrogen sulfide (H2S) and organosulfur compounds were removed using a mixed metal oxide media (SulfaTrap™). A triple-bed vacuum swing adsorption (VSA) unit was used for CO2 and moisture removal.
The demonstration was conducted at the United States Air Force Academy (USAFA) in Colorado Springs, Colorado. Four phases were conducted including I) equipment shakedown, II) startup, III) stable operation with diluted digester feed, and IV) modified process with concentrated digester feed. Biogas purification testing was conducted during Phase IV.
Energy conversion efficiency of food waste and canola oil (a surrogate for USAFA) grease trap waste to methane was 73±13% (Goal ³70%). When parasitic energy losses for the process (e.g., heating, pumping, and gas purification) were considered, the efficiency was 62% (goal ³50%). Volumetric methane production rate was not met (0.82±0.22 L/L/d [goal ³2]). This was a result of a dilute food waste/canola oil feed which was rectified later in the demonstration resulting in a rate of 2 L/L/d being observed at the end of the demonstration. Methane recovery during biogas purification by the VSA was 94±2.9% (goal ³80%). Hydrogen sulfide (H2S) in the treated biogas was 0.030±0.035 ppm (goal <4). CH4 in the treated biogas was 98±0.5% (goal ³95%) after correction for likely air contamination during sampling. Total solids reduction was 78±3.4% (goal ³60%). 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 acids 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% available during Phase III and 100% available during Phase IV (goal ³95%). 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 from nominally scaled food waste digester were –470 tons per year (i.e., GHG offset due to use of purified biomethane as vehicle fuel). By comparison, landfilling and composting would generate 530 and 180 tons/year, respectively.
Anaerobic digestion of food waste and a surrogate for grease trap waste (i.e., canola oil) was demonstrated to be capable of recovering potential energy content, reducing solid waste, and potentially producing a valuable, nutrient-rich end product. 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 capital and operations and maintenance (O&M) costs of a green field food waste digester and gas purification system was determined for three installation sizes (10,000; 20,000; and 40,000 personnel). Capital costs ranged from $0.93 (10,000 personnel) to $2.44 million (40,000 personnel). Net annual revenues (i.e., 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, O&M, 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 ton (40,000 personnel) to $50/wet ton (10,000 personnel). Compare these costs to average landfilling costs of $50/wet ton and composting costs ranging from $29 to $52/wet ton. This economic advantage combined with the minimized GHG emissions and dependence of petroleum-based fuels suggests that food waste digestion and biogas purification is advantageous.
The above project showed 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 and built. However, in recent years, a number of companies have emerged that specialize in manufacture of on-site anaerobic digestion systems. One important consideration for a military installation is whether they have the staff to operate and maintain what is essentially a wastewater treatment plant (WWTP). Clearly if the installation already had a WWTP on site such as USAFA then the implementation is much easier. Alternatives do exist as described in the Engineering Guidance Document. This document is intended to facilitate technology evaluation, selection, and implementation. The alternatives include transport to a local wastewater reclamation facility that has the capability of accepting food waste and fats, oils and grease.