Energy sustainability of pretreated Organic waste market (owm) in two-stage anaerobic digestion (AD)

AD producing biogas and fertilizers is a well established biotechnological process used in practice for over a century for the disposal of waste-water and solid organic wastes. A two-steps AD process combining H2 and CH4 production was firstly proposed in 1971 by Pohland and Ghosh, (1971) and now has taken quite attention thanks to the possible increase of the overall efficiency of the process. On the other hand, pretreatment methods for improving the AD performances have been the focus of a large number of scientific studies over the past two decades. In the light to improve the energy production, however, the energetic sustainability of different pretreatments is an issue that has not yet been addressed; the majority of studies conducted up to date focus on the evaluation of the increase of energy production. In this study, two-stage AD was experimentally tested to evaluate the maximum biogas production from the Organic Waste Market (OWM), with the purpose of comparing two pretreatments, alkaline (NaOH) and thermo-alkaline (NaOH + heat) to enlighten their energetic sustainability. The sustainability was assessed through an Energy Sustainability Index (ESI), as ratio between the obtained energy and the energy spent to carry out the pretreatments. The total energy obtained was 503.28 kJ/L and 372.93 kJ/L for alkaline and thermo-alkaline, respectively. The total energy spent for the pretreatments was 48.00 kJ/L (alkaline) and 1004.42 kJ/L (thermo-alkaline) with an ESI of 10.49 and 0.37, respectively. A comparison results with OWM pretreated with NaOH with literature data show that the energy obtained is 64% and 57% higher than that obtained with two-steps AD and one-step AD, respectively.