A group of researchers from South Dakota School of Mines and Technology (SDSM&T; Rapid City; www.sdsmt.edu) has developed a modular technology known as the Gas Cube, which can convert a wide variety of liquid and solid waste into methane. Having a viable, flexible solution for waste management in remote locations is paramount for saving costs and ensuring sustainability. The portable technology’s first application will be treating kitchen waste from a U.S. Air Force base in a remote location. Along with food waste, SDSM&T says that the Gas Cube is also suitable for handling cardboard, agricultural residues or even wastewater.
In the process (diagram) the solid waste is first fed to a shredder. Then, in a continuously stirred tank reactor (CSTR), hydrolytic microorganisms convert the waste materials into sugars, and fermenting microbes further break down the sugars into fatty acids. Remaining solids are filtered out and given more time to react. The fatty acids are then fed to a vertical chamber for a process dubbed upflow anaerobic sludge blanket (UASB). Here, methanogenic microbes transform them into CH4, which can be used at the site to run generators or other equipment. The ability to handle a mixture of food and cardboard waste, along with the adoption of a two-stage anaerobic digestion process to a portable, skid-mounted unit, make the Gas Cube especially unique, says Patrick Gilcrease, professor of chemical and biological engineering at SDSM&T.
The Gas Cube has been demonstrated using 2-L reactors, and a field prototype unit is currently undergoing tests. “Rather than further scaleup, the plan is to send multiple truck-mounted units to handle more waste at a given site,” explains Gilcrease. “If prototype tests are successful, the next steps will be testing alternative wastes that could benefit from small-scale onsite treatment. Microbreweries are one possibility,” he adds. Another task will be to reduce the unit’s weight for ease of transportation. Currently, the prototype uses carbon-steel tanks, but the use of plastic tanks would greatly decrease weight.
