Direct Energy Balance of Anaerobic Digestion (AD) Toward Sustainability

Currently, the biogas production from organic waste (OW) refuses through AD has been and continues to be one of the most widely used processes for energy production. Usually, AD is carried out in thermophilic (TC) or mesophilic conditions (MC); some authors claim that it is far better to work in TC than MC, others explain that there are no major differences, while some others affirm that the MC has highest performance. Even if each in of single case, the statement is true, it is necessary nowadays to have a valid and objective criterion in order to compare different studies and mainly to assess the energetic sustainability of AD. To this purpose, in this paper, an Energy Sustainability Index (ESI) parameter is candidate. The ESI is the ratio between the energy obtained under form of H2 and/or CH4, and that spent as direct energy to heat the fermenting broth at the working temperature and when present, the energy spent as heat in the broth pretreatment. In Author’s opinion this fact is the first check toward sustainability of AD technology as mean to produce energy. Only in the case of ESI > 1, it is possible to consider others energy spent to produce the energy, in fact, for ESI < 1 the energy obtained is less than the spent one, i.e. the process is unsustainable. About 30 studies were initially taken into account, but only 15 of them provided the necessary information to carry out the study. Among 15, only 3 studies proved to be truly energetically sustainable, with an ESI > 1. The ESI here proposed represents the first step of a more detailed energy sustainability evaluation procedure, performed using a LCA (life cycle assessment) approach.