Technology▼Solutions Microbes manufacture plastic from food waste
JIAN YU; INSET: GUOCHENG DU
be more cost-effective overall than available technologies. Researchers at the University of “It has long been known that you Infrastructure, degradation prodHawaii have developed a novel can convert practically any type of ucts, and energy consumption issues process that employs microbes to conbiomass to sugars with appropriate may outweigh operating costs. Selke vert food waste into biodegradable chemical or biological treatments, and says, “Whenever you talk about biothermoplastics. “We believe that if we that certain organisms can make these degradable plastics, you have to think can produce plastics from a renewable PHAs from sugars. So the basic conabout where they are really going to source [such as food waste], we can cepts were known,” says Susan Selke, a go [and] what markets are there for achieve some sustainable technology professor in plastics and packaging at these materials, where biodegradabiliwhich can produce the plastics from Michigan State University. What sets ty is really an asset rather than just natural resources instead of petroleum Yu’s work apart, Selke says, is his apsomething that sounds good, [and and hydrocarbon raw materials,” says proach. “I haven’t seen them put tothat] depends on the waste managelead researcher Jian Yu. gether in this way ment infrastructure.” Although some Over the years, others have before,” she says. locations in California and Europe attempted to create plastics sort out compostable materials, from various renewable remost trash goes directly into landsources and waste streams. fills designed to prevent decompoLandfill limitations in Europe sition and greenhouse gas release. and Japan and global issues reFor example, Selke has seen carrots lated to incinerating solid still orange inside and grass clipwaste drive these projects, but pings that are still green after years cost and logistics associated with in a landfill. Selke says that prodthe starting materials have historiucts that end up in sewage systems cally limited commercialization. and marine environments, such as Now, Yu and Guocheng Du use a plastic tampon applicators and bioreactor technology and natural bait containers, may be examples microbes to manufacture polyhyof good markets for biodegradable droxylalkanoate (PHA) from food plastics, and Barber says “flushwaste (Environ. Sci. Technol. 2002, able” wet wipes containing poly36, 5511–5516). mer fibers are in the works. PHAs are polyesters with many However, these potential markets of the same physical properties of represent only a fraction of total polyethylene, polypropylene, synplastics consumption in the United thetic polyesters, and acrylics, but States alone. The food waste ferments to organic acids in another they can come from renewable Whether biodegradable plastics reactor, and these are then transferred to this bubble sources. Current uses for PHAs really solve pollution problems is column reactor. Here, Ralstonia eutropha bacteria include packaging, disposable syanother issue, according to Tillman synthesize and store polyhydroxyalkanoate polymer ringes and bottles, fast food conGerngross, a professor at Dartwithin their cells. At this early stage, the turbidity tainers, and coatings. mouth College, who nonetheless indicates that the polymer composes ~20−30% of the Capitalizing on microbes to praised the “high quality” of Yu’s cell mass. Inset: Observed through a microscope, R. eutropha cells swell with PHA. generate PHAs is not new, and work. Gerngross says, “In essence, other researchers have seen what you have done [with bio“trash-to-cash” schemes before. The Using food waste brings the cost degradable plastics] is converted the British company ICI started feeding of the raw materials essentially to solid waste disposal problem into an pure sugars to microbes that naturalzero, so only processing costs remain, air pollution problem. Some people ly create PHAs over a decade ago. Yu says. Barber agrees there is interest may argue that is a good idea, [but] if Metabolix, a small company in in making something of value from you look at the international efforts to Cambridge, Mass., now holds patents organic waste and commends Yu’s reduce greenhouse gas emissions, I for processes that generate the polygroup for using membrane filtration think it flies in the face of those efmer for “well under a dollar a pound,” to “sidestep the issues of contaminaforts.” It’s also important to analyze in which cells swell to at least 80% tion that one encounters using a lot how much fossil fuel-based energy is dry weight PHA in under 40 hours, of the scrap materials directly”, but he used to generate any biodegradable according to the CEO James Barber. is still skeptical that this process will plastic, no matter what the source, © 2003 American Chemical Society
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because it is often greater than conventional polymers, he adds. Although Yu has not completed a full energy assessment for his process, he says using microbes reduces energy needs for purifying acids. In addition, he believes his process completes the carbon cycle, instead of adding to it. Barber called Gerngross’s energy arguments “outdated” and says the state of the art has improved significantly since the mid-1990s and continues to improve. “Now the amount of carbon fossil energy [which includes running farm equipment to grow corn, processing sugars, and producing PHAs through fermentation and extraction] is not greater. In fact, it is less than what would be found with [petroleum] materials.” In addition, he says, “We don’t talk a lot about biodegradability except where that is a benefit [because] what is really important here is a more sustainable family of products.” Yu’s approach is built on work that he and Yingtao Si published at Hong Kong University of Science & Technology. They first made PHAs from wastewater (Environ. Sci. Technol. 2001, 35, 3584–3588). However, the volume of water required made it un-
feasible for large-scale work, says Yu, so we moved to another waste stream. For the food waste work, Yu and his colleagues visited the University of Hawaii’s campus cafeteria to collect scraps from food preparation. Yu says they did not target any particular composition, but they estimate the collection mimics a typical American diet that is largely starch, with some meat and vegetables. Converting food waste to PHA on a lab scale required two bioreactors coupled by a membrane. In the first reactor, the raw food waste was anaerobically fermented to more bioavailable organic acids because microbes rarely produce a consistent product from heterogeneous food scraps. Lactic, butyric, acetic, and propionic acid concentrations built up over a 20-day period and then continuously moved through the membrane into the second reactor by molecular diffusion. There, a culture of the bacteria Ralstonia eutropha used the acids as a carbon source and stored the resultant PHA in their cells, in the same way that green plants store starch. The bacteria contained 72.6% dry weight PHA after 80 hours, which was extracted with dichloro-
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methane in the lab, but could be done with enzymes. Although they investigated only R. eutropha, Yu says “more than 60” species of microorganisms can do PHA conversion, including genetically engineered E. coli and Pseudomonas spp. They also report that R. eutropha creates different monomers from the different acids, and controlling the acid profile during waste fermentation ultimately helps produce polymers with the desired properties. Yu says one company is interested in exploring the technology because no waste material is produced from this process—even the remaining solid mass from fermentation can be sold as soil conditioner. The University of Hawaii has patented Yu’s process and is currently in licensing negotiations with two companies who believe they can combine it with other technologies to overcome cost and environmental issues. Yu says one pilot design that incorporates a commodity chemical to eliminate the need for extraction with organic solvents should be complete this summer and producing polymer by the end of the year. —RACHEL PETKEWICH
