Basics Industrial
Basics Industrial Composting: An Introduction Article contributed by Bruno De Wilde, Organic Waste Systems n.v., Gent, Belgium Biofilter (Photo: OWS) Industrial composting – Curing phase (Photo:VLACO vzw, Belgium) One of the major advantages of many bioplastics is the fact these are compostable. To support this claim one can use the European EN 13432 or American ASTM D.6400 standards. Yet these norms specifically refer to industrial composting which is just one, albeit the most important, option for biological (solid) waste treatment. Other options include home composting and biogasification. Industrial composting refers to centralised composting facilities where large amounts of biological waste, collected from many sources, are treated. The technological level can be rather different from one plant to another but they all have in common the fact that large volumes are treated and hence high operating temperatures can be maintained. Home or backyard composting refers to composting at (individual) household level. In contrast to composting in which oxygen plays an important role in the degradation of waste and which is therefore called “aerobic” biodegradation (aerobic = in the presence of oxygen), biogasification is a biological waste treatment system in the absence of oxygen, called “anaerobic” biodegradation. All these systems are “bio-inspired”, as in nature waste is also degraded either aerobically (e.g. in surface waters or on soil), or anaerobically (e.g. at the bottom of rivers and lakes where oxygen is depleted). Microorganisms consisting of bacteria and, in the case of aerobic conditions also fungi and actinomycetes (a kind of filamentous bacteria), will degrade waste (e.g. leaves of trees, dead animals, and other biomass) and convert it partially into new micro-organisms and humus but mainly into CO 2 and water, and in the case of anaerobic conditions also into methane (CH 4 ). Under aerobic conditions the biodegradation process will also release a certain amount of energy in the form of heat, which is mostly dispersed immediately and hence not measurable. However, when large quantities of biowaste are aerobically degrading (e.g. as in industrial composting) significant temperature increases can be measured. Under anaerobic conditions the energy is released in the form of methane and much less heat is generated. As methane can be used as a fuel for heating or for electricity production, biowaste in such cases is converted not only to compost but also to (useful) energy. 36 bioplastics MAGAZINE [03/07] Vol. 2
Basics Home composting (Photo: OWS) Biogasification plant, Brecht (Belgium) (Photo: OWS) Composting of municipal solid waste (MSW) is not really new. In the 1960s for example several composting plants were built for the treatment of mixed MSW. Yet this was never really successful as landfill was much cheaper and the compost produced was of inferior quality. Later, in the 1970s, several mass-burn incinerators were also built, offering another relatively cheap option for waste treatment. Only in the 1980s after some heavily publicised dioxin scandals which caused massburning to become very unpopular, was composting reconsidered. Nevertheless, it quickly became apparent that high-quality compost was an essential prerequisite and that this could only be obtained by source-separated waste collection - clean feedstock going in, clean product coming out. In areas of several countries (The Netherlands, Germany, etc.) biowaste was collected separately and composted to produce a high-quality compost. Biowaste was defined as kitchen and garden waste which comes directly from natural origin (“biogenic”). Anything “man-made” was forbidden in order not to compromise the quality of the compost. As mentioned already, the technology of industrial composting systems is quite variable. At the low-tech end windrow systems are being used in which the waste is aerated by placing it in long heaps to facilitate air diffusion into the waste. These windrows can be turned with different types of turning machines at different frequencies, again to promote aeration and accelerate degradation. Nowadays windrow systems are mainly used for garden waste. For biowaste, which includes also kitchen waste, more advanced systems are being used in order to avoid problems with odour and vermin. They mostly include an initial phase of some weeks of intensive forced aeration which is done either in “bay” or table systems, tunnels or containers. Afterwards this is followed by a maturing or curing phase in which the “semi-ripe“ compost is further gently aerated, either forced or by diffusion. In all systems a screening step is also included, which can be at the beginning, at an advanced stage or at the end, and which serves to retrieve non-compostable contaminants as well as objects too big to compost in a given time frame such as branches of wood. The goal is to obtain a nice, crumbly homogeneous compost. Because of the large quantities of waste, high temperatures are achieved (60-65°C) which are also needed to kill off pathogens in the waste. On the other hand, temperatures of 55-65°C as well as a relative humidity of almost 100% are needed for the population of micro-organisms to live and grow and do an efficient “bio-conversion job”. The consequences of adding bioplastics to biowaste and industrial composting include a potential threat but also some significant benefits. The major threat is obviously a decrease of the feedstock quality. It should be ascertained that only truly compostable materials are coming in, and not visually similar but non-compostable plastics or packaging. Hence, the importance of the communication aspect and the different compostability logo systems. Some composting systems might also need to be slightly technically modified (most often shifting the screening step in the process from the beginning or an intermediate stage to the end). The first benefit lies in a higher volume to be composted and hence higher income for the composting plant. In some cases the use of bioplastics will have a kind of snowball effect and convert larger volumes of waste from non-compostable into compostable (e.g. catering waste). On a more technical level the addition of bioplastics will increase and improve the C/N (carbon/ nitrogen) ratio of the biowaste leading to easier odour control (less ammonia production). Also the density of the biowaste will be decreased making aeration easier and more efficient and decreasing the need for the addition of structural material and energy input for aeration. Industrial composting plants are able to cope with large volumes of bioplastics as long as they are well mixed with other material such as kitchen and yard waste. As for all living organisms a balanced and varied diet is needed to stay healthy! bioplastics MAGAZINE [03/07] Vol. 2 37
