Biogas potential from cabbage and lettuce residuals:Effect of organic load and co-digestion

Current environmental and political pressures, the unstable perspective of the fuel prices, and the energy derived from fossil fuel reserves such as crude oil, coal, and natural gas have increased the industrial focus to bioenergy derived from renewable energy sources (RES), and encouraged technological progress in the biogas production sector [1–3]. In the European Union (EU), environmental and political pressures are boosted by the targets for climate and energy in 2030, stating a 27% improvement in the energy efficiency, a RES installed capacity of 27% for the whole EU energy supply, and 40% reduction of greenhouse gas (GHG) emissions [4–6]. Biogas is expected to play a key role for the EU to reach the energy targets due to the various biological sources that can be used for biogas production, its established application in a broad range of applications such as electricity production, heating and transportation, and the flexibility and storability of the energy carrier biogas [7,8]. The implementation of anaerobic digestion (AD) for biogas production is well-known as a useful renewable energy producer due to the arrangement of its main technological design and the significant advancements of the technology [9–14]. Co-digestion has been also widely examined and reported as an alternative technique to enhance biogas production by treating simultaneously different types of waste [15]. AD produces nutrient-rich digestate that is recycled to farmlands to act as organic fertilizer and diminish negative environmental impacts like odors, and eutrophication of freshwater systems which could occur if the disposal of such nutrients is not accurately controlled and managed [16,17]. Secondly, although evenly important, it converts the majority of organic materials into the monomers carbon dioxide and methane (i.e., biogas) convenient for energy production in multiple pathways [18]. In comparison with fossil fuels, biogas production by AD can diminish GHG emissions by using regionally available sources, and in comparison with other bioenergy production techniques with GHG emission reduction benefit, biogas production by AD is the least energy consuming process