Bioenergy recovery from waste: comparison of different treatment scenarios by LCA

Anaerobic digestion of sewage sludge generally produces an amount of biogas that is not enough to cover the energy requirements of the digester. One possibility to increase the biogas production is to co-digest, together with sewage sludge, other substrates, for istance the organic fraction of municipal solid waste. Alternatively, a preliminary step of dark co-fermentation of those mixed substrates can be applied. In this work, such possible cases are compared by Life Cycle Assessment approach. The study was carried out with reference to the Viareggio wastewater treatment plant, Italy. Anaerobic co-digestion of sewage sludge and the organic fraction of municipal solid waste emerges as the best treatment option in terms of environmental impacts. However, also dark co-fermentation presents, albeit less, benefits and a reduction in environmental burdens. The robustness of the results is explored by the sensitivity analysis with respect to the effective thermal energy use.