There is substantial combustion of biowastes for the generation of electric power, which in principle may be used for traction. There is also substantial anaerobic con­version of biomass wastes into methane, which in turn can be used for automotive purposes. Biodiesel made from waste fats and oils (yellow grease) is currently the main biowaste-derived road transport biofuel. As pointed out in Chap. 1, there are a wide variety of processes that have been proposed to convert wastes into liquid biofuels, such as ethanol, methanol and Fischer-Tropsch liquid transport fuels, and there is also the possibility for conversion into H2.

How does one evaluate the environmental impact of using biowastes to power transport? This is a tricky question. We will illustrate this with two examples, drawn from major applications of such wastes: burning to generate electricity and conver­sion into methane. Firstly, the allocation of emissions has a large impact on those evaluations. In this context, one may consider the example of electricity production from chicken manure in the European Union (Reijnders and Huijbregts 2005). At the time of this study, such manure had a negative price. Reijnders and Huijbregts (2005) calculated the emissions of greenhouse gases associated with electricity pro­duction using allocation on the basis of actual prices, allocation on the basis of prices but with an assumed zero price for manure and allocation on the basis of energetic outputs of chicken production. The results thereof are shown in Table 4.2.

As also pointed out in Chap. 2, so far it has been usual in life cycle assessments of biofuels from wastes to allocate on the basis of a zero price, implying that the life cycle leading to the generation of wastes has no impact on the environmental eval­uation of such biofuels. A first problem with this approach is that when the current

waste indeed turns out to be viable as a source of biofuel production, it will turn into a ‘secondary resource’ which may well have a positive price. A second complication centres around the reference to be used and the ‘normal fate’ of the waste used. This again may have a very large impact. For instance, a study about Dutch projects to convert manure into methane (Zwart et al. 2006) concluded that the energetic out­put (methane) was roughly the same as the energetic input (fossil fuels). However, Zwart et al. (2007) calculated a very low net greenhouse gas emissions for fermen­tation of manure, because they did not compare greenhouse gas emissions linked to methane from manure with the life cycle emissions of natural gas, but rather they compared fermentation with other ways of handling manure. Also, for the energetic application of manure, they assumed a major reduction in the emission of methane and N2O due to a much-reduced storage time for manure. So estimates about the environmental impacts of biowastes used for fuelling transport are dependent on subjective assumptions.

Zah et al. (2007) have studied emissions associated with methane production from a variety of wastes, using allocation on the basis of prices and a zero value of the waste itself. Thus, the calculation of emissions linked to methane production from wastes was restricted to the waste-to-wheel stages of the life cycle. Compar­ison was with natural gas. The wastes considered were: sewage sludge, biowaste, manure and manure plus co-substrates. The emission of greenhouse gases for the production of methane from these wastes was in the order of 50-80% of the fossil reference. When allocation would have been seed-to-wheel on the basis of energy or mass, the emission of greenhouse gases linked to waste-based methane production would have been much higher (cf. Table 4.2).