Basics Basics of
Basics Basics of Anaerobic Digestion Article contributed by Bruno de Wilde Organic Waste Systems nv, Ghent, Belgium Anaerobic digestion: another way of biological solid waste treatment Within biological solid waste treatment a distinction can be made between two major categories, one being aerobic composting and the other being anaerobic digestion (AD) or biogasification . In composting, organic matter is degraded by a microbial population consisting of bacteria and fungi, that consume the organic matter together with oxygen and produce CO 2 , water, biomass (compost or humus) and a lot of heat. Due to this exothermic process, the temperature in a composting pile increases significantly. In anaerobic digestion, organic matter is degraded by a microbial population consisting of bacteria in the absence of oxygen and producing CH 4 (methane) and CO 2 (this mixture often being referred to as ‘biogas‘) and compost with practically no exothermic heat. When collected properly this biogas can be exploited in a CHP (Combined Heat and Power) system, producing electricity and heat, or can be upgraded to biomethane. To put it simply, the energy present in wet, organic waste is released as biogas instead of heat as in composting. Typically from 1 tonne of biowaste 120 m 3 of biogas can be produced, with a total electricity yield of 250 kWh and a net electricity yield of 200 kWh. In industrial composting the different technologies are rather similar and the differences lie in relatively minor aspects (e.g. aeration by over- pressure or under-pressure, Aerobic composting (open windrow) at Tenneville (Belgium) the form of the waste heaps, etc) with little or no consequence for the treatment of bioplastics. In contrast, rather different technologies can be distinguished in anaerobic digestion. One distinction between different technologies is the temperature at which the anaerobic digestion is operated. Temperature is externally controlled and digesters are run either at mesophilic temperature (35-40°C), or at thermophilic temperature (50-55°C). These are two distinct temperature zones at which different types of anaerobic bacteria show maximum activity (namely mesophilic and thermophilic bacteria). The rate of activity is higher at thermophilic temperature. Further, anaerobic digestion can be a singlephase or a two-phase process. In a single-phase process the complete digestion takes place in one unit or digester. In two-phase fermentation the first phase (hydrolysis and acidification) and the subsequent methanogenic phase are run in separate tanks. The distinction between single-phase and two-phase is referred to as a distinction between dry and wet fermentation systems. In dry anaerobic digestion the process is run at a moisture content of < 85%, while in wet systems the process is run at a moisture level of >85%. These technical differences have rather far-reaching consequences with regard to the treatment of bioplastics. For example, certain bioplastics (e.g. PLA) need an elevated temperature (50-60°C) to start biodegrading. In thermophilic anaerobic digestion this temperature is met and these bioplastics will degrade. However, in mesophilic anaerobic digestion where the temperature is lower, these bioplastics will not readily biodegrade. Practically all commercial anaerobic digestion systems feature a combination of an anaerobic fermentation first step, and a subsequent, aerobic composting, stabilisation second step. Since fermentation is something of a mixed process the output is not fully stabilised or fermented (note: mixing can be done in the reactor or outside the reactor by blending residue output with new feedstock input). In order to reduce the residual biological activity and to obtain complete maturity of the compost end product, the residue from the anaerobic digestion phase is therefore aerobically composted for a short time (typically for 2-4 weeks). 42 bioplastics MAGAZINE [06/09] Vol. 4
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 43
