Bioplastics from Waste
Bioplastics from Waste Streams Microbial Community Engineering By Leonie Marang Yang Jiang Jelmer Tamis Helena Moralejo-Gárate Mark C.M. van Loosdrecht and Robbert Kleerebezem all: Delft University of Technology Delft, The Netherlands Producing Bioplastic from Waste Production of waste is a sign of inefficiency. The amounts of waste generated in agro-industrial production chains are nevertheless enormous. Effective reclamation and valorisation of these heterogeneous organic residues is one of the main challenges towards the establishment of a sustainable society. In recent years the Environmental Biotechnology group at Delft University of Technology developed a biotechnological process in which organic waste streams are used to produce bioplastic - thus converting the waste into a resource. Polyhydroxyalkanoates The polymer that is produced is a polyhydroxyalkanoate, or in short PHA. PHAs are storage polymers accumulated by many different groups of bacteria in nature as an energy reserve similar to fat storage by animals. PHA is therefore a bioplastic that, besides being produced from renewable resources, is fully biodegradable and the only bioplastic completely synthesized by microorganisms. Chemically, PHA is a polyester of hydroxy fatty acids. Many different hydroxy fatty acids have the potential to be incorporated into the polymer – over 90 different monomer units have been identified. Interestingly, the type of monomer that is produced and incorporated in the polymer depends mainly on the available substrate, i.e., on what you feed to the bacteria. The properties of the polymer can thus be tuned by adjusting the composition of the substrate. In general, the properties of the most common PHAs are similar to those of polypropylene (PP). Engineering the Environment instead of Bacteria In traditional biotechnological processes pure cultures of a specific bacterium are used. These bacteria have often been genetically modified to improve the productivity (Figure 1). At this moment, PHAs are being commercially produced according to this approach. However, the cultivation of these bacteria requires sterile equipment and well-defined substrates such as glucose. This is not desirable for two reasons. Firstly this results in a high cost price – PHA is currently still 2-5 times more expensive than comparable petroleum-based plastics. Secondly, the use of pure glucose for bioplastic production competes with food production. To make PHA a truly sustainable bioplastic the researchers at Delft University of Technology use an alternative approach: microbial community engineering. The conceptual idea of microbial community engineering is that genetic engineering is often not needed when recognizing that microorganisms in nature already provide us with a wealth of catalytic potential. The Dutch microbiologist L. Baas Becking once stated that “Everything is everywhere, but the environment selects”. Inspired by this statement, the team at Delft works on the engineering of the environment instead of the bacterium to select a natural bacterium that thrives under the conditions that they chose. The Environment Selects In order to create an environment in which PHA-storing bacteria can be selected, a natural bacterial community is subjected to alternating periods of substrate presence (feast) and absence (famine). During the feast phase, when substrate is present, bacteria can use the substrate either for growth or storage. During the subsequent famine phase only those bacteria that stored can continue to grow and thus increase in number. Bacteria that did not store substrate cannot grow. Therefore, bacteria that quickly store a lot of substrate as soon as it becomes available have a competitive advantage over bacteria that use substrate only for growth during the feast phase. Before feeding new substrate to the enrichment reactor, part of the bacteria is removed. In this way, the number of bacteria in the reactor is being controlled and it is assured that only bacteria that are able to store enough PHA can survive in the system. After repeating this selection cycle numerous times, the microbial community is enriched with PHA-producing bacteria, whereas non-PHA producers are washed out. Eventually, the 22 bioplastics MAGAZINE [04/12] Vol. 7
Bioplastics from Waste Streams WORK HORSE GENOME ANALYSIS GENETIC ENGINEERING INDUSTRIAL BIOTECHNOLOGY MICROBIAL COMMUNITY ENGINEERING MICROBIAL COMMUNITY SELECTIVE PRESSURE DOMINANT WORK HORSE Photo: iStockphoto.com/ MiguelMalo Figure 1: Industrial biotechnology versus microbial community engineering. bacterium that can produce the largest amount of PHA at the highest rate will dominate the microbial community. Producing the Polymer Although the enrichment of a microbial community with a high storage capacity is the key to the production of PHA from waste, the overall process consists of four steps (Figure 2). In the first step, the organic waste, mainly consisting of carbohydrates, is converted to a mixture of volatile fatty acids by anaerobic fermentation. These fatty acids are more suitable for PHA production than the original carbohydrates, and will be used as a substrate in the following two steps. The second step is the enrichment of PHA-producing bacteria, as described above. Once a stable enrichment is obtained, this reactor will be operated as a microbial community production step. The microbial community that is harvested from the enrichment reactor at the end of each cycle is used for the actual production of PHA. In this third step, in order to produce large amounts of PHA, the microbial community is continuously fed with substrate and the bacteria will store as much PHA as they can. Under these conditions, the bacteria produce up to nine times their own dry weight of PHA (Figure 3). Comparing these natural bacteria with their genetically modified competitors from industry, they can accumulate similar amounts of PHA and are able to achieve these high PHA contents in a shorter period of time. In the fourth and final step, the PHA is recovered from the cells and purified for its use as bioplastic. A Bright and Natural Future Using enrichments of natural bacteria for the production of PHA has several advantages. First, instead of glucose, organic waste streams can be used as the substrate. This will reduce the substrate costs, especially since waste (water) currently has a negative value. Second, through Figure 2: Schematic overview of the PHA production process: converting organic waste streams into a versatile biopolymer that, for one, can be used as a bioplastic. AGRICULTURAL RESIDUES BIOPLASTICS ANAEROBIC FERMENTATION ORGANIC WASTE fatty acids MICROBIAL ENRICHMENT BIOCHEMICALS BIOPOLYMER PRODUCTION biomass INDUSTRIAL WASTE biomass & biopolymer PRODUCT RECOVERY BIOFUELS bioplastics MAGAZINE [04/12] Vol. 7 23
