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Basics Anaerobic

Basics Anaerobic digestion plant (single-phase) at Würselen (Germany) Even though anaerobic digestion can be applied to very different types of waste streams, it is particularly suited to organic waste with a high moisture content such as kitchen waste and food waste. Anaerobic digestion plants have been built and have been operational for many years for the treatment of mixed, municipal solid waste, for biowaste (obtained after source separated waste collection), for residual waste and for many types of industrial waste. The major differences from aerobic composting include the production of energy, less odour production, less health risk (i.e. killing off of pathogens, typical for thermophilic digestion), less need for surface area (smaller footprint), and a higher level of technology. Consequently, anaerobic digestion is often the preferred biological waste treatment option in densely populated areas such as big cities or countries such as Japan or Korea. Recently, anaerobic digestion has also become an important player in the area of renewable energy production from energy crops (e.g. corn). The net energy yield per hectare is higher compared to the production of bio-diesel or bio-ethanol. Also, in bio-refineries, anaerobic digestion could play an important role with high-value plant parts being used for green chemistry and residual vegetable matter (after processing of low-value plant parts, such as stems and leaves or straw) being treated in anaerobic digestion for production of energy and compost. Current distribution and prospective of technology Figure 1 below gives an overview of the development of biogasification capacity in Europe in the last two decades. From just three plants in Europe with a total capacity of 87,000 tonnes per year in 1990, European anaerobic digestion facilities have now grown to a total of 171 plants with a digestion capacity of more than 5 million tonnes per year in 2010. Figure 2 gives an overview of the AD capacities in different European countries. Both the total capacity in a given country is quoted as well as the average capacity per plant. As can be seen, some countries tend to have smaller plants (e.g. Germany, Switzerland, Austria, …) while others have larger installations (e.g. Spain, France). These graphs also show that the anaerobic digestion capacity in Europe is increasing rapidly. Many digesters are being built in Mediterranean countries such as Spain and France. Most plants are dry and single-phase, and run at mesophilic temperatures. The evolution for the coming years can be deduced from the two graphs, the data for which are based on the bids for proposals published in the European Journal. Bioplastics and anaerobic digestion First of all, just as with aerobic composting, since anaerobic digestion is a biological waste treatment process, bioplastics Total Capacity Average Capacity 87.000 tpa 3 plants 281.000 tpa 18 plants 1.400.000 tpa 62 plants 3.470.000 tpa 116 plants 5.204.000 tpa 171 plants 1990 1995 2000 2005 2010 Installed Capacity (t/y) 1.600.000 1.400.000 1.200.000 1.000.000 800.000 600.000 400.000 200.000 0 Figure 1. Evolution of AD capacity in Europe (EU + EFTA countries) (with tpa = tons per annum) Figure 2. AD capacity in various European countries (2010) Germany Spain France Italy NL UK Switzerland Belgium Portugal Austria Sweden Malta Luxemburg Norway Denmark Poland Finland 80.000 70.000 60.000 50.000 40.000 30.000 20.000 10.000 0 bioplastics MAGAZINE [06/09] Vol. 4 33

Basics should be biodegradable in order to be compatible. Whether bioplastics are produced from renewable resources or not, doesn‘t matter. The key element is that they must be biodegradable under anaerobic conditions or at least be compatible with an anaerobic digestion process. Anaerobic digestion plant in (two-phase) at Vitoria (Spain) (all photos: OWS nv) Concerning technical preconditions of treating bioplastics in anaerobic digestion plants, a distinction must be made between wet and dry technologies. In general, wet technologies, especially in the pretreatment phase, cannot treat bioplastics easily: in the first pulping and hydrolysis phase they are removed either by flotation or by sedimentation and therefore are not really entering the digestion (except when bioplastics are quickly soluble or dispersible, which is rarely the case). A solution could be to add the bioplastics directly to the second step, the aerobic composting step (considering the retention time in this second step is much shorter than the residence time in a typical composting process). Another solution might be new developments in the pretreatment phase. In most dry systems, bioplastics can be added when some random conditions are fulfilled: they should be shredded (to reduce the particle size) before entering the digestion (just like biowaste itself) and sieving is better located at the end of the process in order to enable as much biodegradation and disintegration as possible in both the anaerobic digestion and the aerobic composting step. The major underlying reason why several bioplastics show a different biodegradation behavior in aerobic composting from their behavior in anaerobic digestion is the influence of fungi. Fungi are abundantly available and very active in aerobic composting while in anaerobic fermentation no fungi are active. Some polymers are mainly (or even only) degraded by fungi and not by bacteria and will therefore biodegrade in aerobic composting and not in anaerobic digestion - or only much slower. As a matter of fact, this is also the case for the natural polymer lignin which can be found in wood, straw, shells, etc. On the other hand, when bioplastics do also biodegrade in anaerobic fermentation there is a double benefit. First of all, energy is produced from the bioplastics in the form of biogas that can be converted to electricity. Secondly, as most bioplastics are very rich in carbon and do not contain nitrogen (or very little), the addition of bioplastics to biowaste will improve the C/N ratio of the mixture. Biowaste tends to be low in C/N, which is sometimes a problem in anaerobic digestion, by adding a carbon-rich substrate the C/N ratio is increased. So far, the knowledge of anaerobic biodegradation and treatability of bioplastics is limited and further research would be welcome. Ideally, bioplastics would biodegrade and also disintegrate during the anaerobic phase in an anaerobic digestion plant, just as the major part of ‘natural‘ biowaste does. However, if the bioplastic disintegrates during the anaerobic phase and then afterwards biodegrades completely during the aerobic stabilization phase or during the use of digestate or compost in soil, it can also considered to be compatible with anaerobic digestion. 34 bioplastics MAGAZINE [06/09] Vol. 4

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