LAWR

nergy from waste (EfW) is a well-established technology that hasn't changed for some time. However, local authorities and the waste industry are beginning to look closely at advanced thermal processing technologies such as pyrolysis and gasification to complement conventional mass burn systems (also known as incinerators). Again, the choice of technology depends on the amount of waste produced in a given area and on the composition of the waste. In this article, I am concerned with the fraction of munici- pal solid waste that remains after the recyclable, compostable and digestible material has been removed. This residual municipal solid waste (or rMSW) forms the potential feed- stock for energy from waste (EfW) processes. Several of our European partners use EfW to treat most of their rMSW and we in the UK are increasing EfW to avoid the need for landfill. The amounts of rMSW generated and its composition can change for two reasons. Firstly, we may see a step change outside the waste industry that leads to a change in the waste feedstock itself. Secondly, adjustments in waste management policies and practice may result in a change in the amount of rMSW available for EfW or in its composition. I will investigate the potential impact of these effects by looking at previous changes in waste composition, consider- ing the current situation and the effect of the growth in recy- cling and then offering a few speculative suggestions about the future. Past changes Since the middle of the last century we have seen two step changes in rMSW, one caused by the virtual elimination of domestic fires [householders with fireplaces] and the other by the growth of plastics. The loss of domestic fires meant that much less paper was burned at home so found its way into the waste stream. At the same time the amount of ash (or 'screenings' or 'fines') fell with fewer people emptying their fire grates into the dustbins. This was demonstrated in work published by Professor Tony Bridgwater of Aston University in 1986 as shown in Figure 1. This clearly shows the rise in paper and organic material with the fall in screenings following the introduction of the 1956 and 1968 Clean Air Acts (which led to the reduction in domestic fires). The emergence of plastics in the 1970s and 1980s is also apparent. It is interesting to see what this would have meant if modern-day EfW was in place in the past. Using Professor Bridgwater's values, the calorific value (or heat con- tent) of the overall waste was 9.05 megajoules per kilogram (MJ kg)-1 in 1950 with an initial fall to 8.84 MJ kg-1 in 1960 and then a further reduction to 7.6 MJ kg-1 in 1980. This is probably due to the rise in low energy content organic mate- rial and the reduction in screenings (from the ash of domestic fires which still contained significant amounts of combustible material). From this analysis, we can conclude that for all four dec- ades, the waste would have made a perfectly acceptable feed- stock for EfW. Although there may have been some technical issues with the handling and burning of the fine screenings. Where are we today? Since the 1980s we've seen a continued increase in plastics in waste with the reduction in glass drink bottles and the growth in bottled water and rising popularity of disposable nappies. In Figure 2 we can compare Bridgwater's values for 1980 with some results from Wales taken in 2005. Care should always be taken when comparing two 'snapshots', but the growth in plastics is still dramatic. The reasons for the growth in organic waste and corresponding reduction in paper are not clear but could be due to the increased use of wheeled bins since 1980 – these bins make it easier for householders to get rid of their grass clippings, for example, in the domestic waste stream. Again it is possible to calculate the calorific value of the waste and in spite of the changes in composition remains unchanged at 7.6 MJ kg-1. E RWM WITH CIWM 2014 PREVIEW August 2014 Local Authority Waste & Recycling 21 Changing nature of waste: What it means for the future of EfW Consumer and business behaviours are changing. People are reading fewer newspapers and some firms are producing products with plastic instead of glass. But what does the changing nature of waste arisings mean for energy-from-waste? Dr Stephen Burnley finds out. Figure 1 - Past trends in waste composition

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