Pilot project will demonstrate feasibility of converting food waste from the Academy’s dining hall into biomethane fuel. Part IV
David L. Parry
BioCycle July 2013, Vol. 54, No. 7, p. 58
A pilot-scale facility to convert food waste into biomethane fuel will start up soon at the United States Air Force Academy (USAFA) in Colorado Springs, Colorado. The objective is to demonstrate use of anaerobic digestion (AD) as a viable means to manage food waste while also producing renewable energy and reducing the amount of high-moisture waste that is otherwise landfilled. The project is funded by the Department of Defense (DoD) Environmental Security Technology Certification Program (ESTCP). When completed, wider implementation at both forward operating bases and conventional bases for the United States DoD is anticipated.
This demonstration project is the continuation of research that began in the CDM Smith Laboratory in Bellevue, Washington, which was discussed in Part II of this article series (see “Codigestion Research Builds Facility Operator Confidence,” May 2013). Food waste samples were characterized and then digested in bench-scale anaerobic digesters under a variety of feeding and operating conditions. The lab work was able to show that AD was a viable process for managing food waste. It generated useful information on the condition under which food waste can be digested to produce the maximum amount of biogas in a given digester volume with stable operation, and provided valuable insights for digester organic loading rates and biogas production.
Another important finding was that the food waste was deficient in nickel, cobalt, and possibly molybdenum. The addition of these trace nutrients was necessary for continued stable operation when only food waste was fed to the digestion process. It was also determined that digesting food waste requires high total solids feed. Feeding the bench-scale digesters at rates typical for municipal sludge digestion proved too low to maintain stable operation.
Pilot Testing At USAFA
Food waste for the pilot testing will be collected from Mitchell Hall, the cadet cafeteria at the Air Force Academy. Mitchell Hall serves 4,500 cadets three meals a day, seven days a week. Food waste from the kitchen is sent through a commercial scale food disposer and then screened. In addition, the site also has a large grease trap, which can serve as a source of fats, oils and grease (FOG) for the study. The pilot plant will be located at the USAFA wastewater treatment plant (WWTP). The site was selected for the easy access to utilities, as well as the ability for the pilot plant to discharge excess sludge into the WWTP, as well as vent waste biogas to the plant flare. Otherwise, it will operate independently. A site plan of the proposed demonstration at the Air Force Academy is shown in Figure 1.
The pilot plant consists of two 500 gallon continuous stirred tank reactors (CSTR) anaerobic digesters, along with all of the necessary grinders, pumps and auxiliary equipment to facilitate loading of food waste into the system. The pilot plant will also have its own gas treatment, storage and analysis equipment. The purpose of having two digesters is that one will be used as a test while the other can be maintained as a control. Online gas-analyzers will be used to continuously collect gas production and composition data. Digester feed and sludge will periodically be sampled and either analyzed in house or sent to a contract laboratory. A pilot digester is shown in Figure 2.
The most innovative step of the gas clean-up will be removal of CO2. A vacuum swing adsorption (VSA) system produced by TDA Research Inc. will be tested for the removal of CO2 and moisture. The system will process biogas produced from the anaerobic digesters and separate the CO2 and moisture from the methane. The CO2 and moisture removal system is shown in Figure 3. In this system, the biogas will be pumped through a bed of proprietary media, which is able to adsorb carbon dioxide and moisture at ambient pressures and temperatures. The purified biogas (i.e., biomethane) is not adsorbed by the media and flows out of the media bed. The media is regenerated under vacuum where carbon dioxide and moisture desorb and are vented. The quality of the biogas will be monitored at each step in the treatment process while various digester conditions are tested to determine the impact on both the raw biogas, and the gas treatment process.
The pilot plant will be operated in four phases: Start-up, optimization, steady state and challenge. Throughout these experiments, the control will be fed 0.25 lbs VS/ft3/day (7g COD/L/day) with 10 percent of the COD coming from grease trap waste. This feeding rate proved stable during the laboratory work. During Phase I, the test digester will receive the same treatment as the control digester, in order to establish steady operation.
In Phase II, the test digester feed rate will be steadily increased from 0.25 lbs VS/ft3/day up to the maximum feeding rate that allows for steady operation. It is expected that the maximum feed rate will exceed 0.65 lbs VS/ft3/day. The specific energy loading rate (SELR, gCOD/day/gVS) will be monitored to account for the strength of the food waste and concentration in the digester. The digesters will be fed at 10 percent solids, with 10 percent of the COD coming from grease trap waste throughout Phases I-III.
In Phase IV, the test digester will undergo a series of challenges, including testing to determine the maximum feeding rate, and the maximum percentage of COD that can come from grease trap waste. The feed rate will be increased until the volatile fatty acids (VFA)/ alkalinity ratio exceeds 0.2, which will indicate instability. The reactor will then be brought back to steady conditions, and the proportion of grease trap waste will be increased until the VFA/alkalinity ratio exceeds 0.2. One of the success criteria for this demonstration will be determining the maximum stable loading rate that can be achieved with these digesters. Achieving a maximum loading rate of 0.2 VS/ft3/day with a corresponding SELR will be considered a success for this demonstration.
The economic viability of anaerobic digestion of food waste also will be evaluated during the operation of the pilot. Preliminary economic models suggest that this system would meet DoD and federal requirements for financing, however several key assumptions, such as solids retention time, volatile solids destruction and biogas production will be determined during the demonstration. Another success factor will include operability. This demonstration will be considered to have acceptable operability if it can be reliably operated by a single operator, and if shutdowns are infrequent. In addition, the digestate must be of a quality that it can be used for a soil amendment.
David L. Parry, PhD, PE, BCEE is a Senior Vice President with CDM Smith and is the Principal Investigator for the project. Pat Evans, PhD, also of CDM Smith, is Co-Principal Investigator.