Comparative assessment on two full-scale food waste treatment plants with different anaerobic digestion processes

Single-phase anaerobic digestion (SPAD) and two-phase anaerobic digestion (TPAD) are compared based on two full-scale food waste treatment plants using a hybrid life cycle assessment (LCA) method combining process LCA and economic input output LCA. Plant S uses a SPAD process with thermal pretreatment and waste oil recovery, while Plant T adopts a TPAD process without pretreatment. Biogas yield, electricity consumption, and waste oil recovery are the three vital factors determining environmental benefits, and digested wastewater treatment is another key factor influencing economic performance. Plant S has 8.2% higher biogas yield than Plant T, but it also consumes steam and 1.7 times electricity on its pretreatment unit. Plant S can also recover waste oil for biodiesel production, reduce methane emission from landfill sites, and offset the negative effect of high energy consumption. As a consequence, Plant S has a carbon emission of -158.15 kg CO2-eq/t, a cumulative energy demand (CED) of -1868 MJ/t, and an energy efficiency of 64.6%; while the three values are -127.0 kg CO2-eq/t, 1713 MJ/t, and 59.2% for Plant T. Plant T achieves 66% higher reduction of acidic gases than Plant S due to the substitution effect of grid electricity, and it also gets 10% higher revenue with less cost. In the hypothetical optimal scenarios, Plant T should be upgraded by combining thermal pretreatment to enhance biogas yield and waste oil recovery, and the two plants should recycle digested effluent through land use other than traditional wastewater treatment. Thus, their carbon emissions and CEDs could decline to -326 CO2-eq/t and -2514 MJ/t for Plant and -243 kg CO2-eq/t and -1973 MJ/t for Plant S. (C) 2020 Elsevier Ltd. All rights reserved.