Life-cycle assessment of biohythane production via two-stage anaerobic fermentation from microalgae and food waste

A life-cycle assessment was performed to evaluate the energy conversion characteristics and environmental impacts of industrial-scaled biohythane (i.e., biohydrogen and biomethane) production via two-stage fermentation from microalgae and food waste. The results showed that for the entire system, the ratio of net energy input to net energy output was 0.24. Biomass pretreatment and microalgae cultivation were the dominant energy consumption processes, corresponding to a consumption of 53.8% and 16.6% of total energy input, respectively. The net greenhouse gas emissions per upgraded biohythane for the entire system were 124 g CO2-eq MJ−1, whereas the absorption of carbon sources by microalgae photosynthesis corresponded to 49 g CO2-eq MJ−1. The dominant emissions were generated by electricity production (41.6%), CO2 release in pressurized water (27.8%), and energy recovery (19.8%). According to the nutrient balance analysis, 55.3% of nutrients in the liquid fermentation effluents, including carbon, nitrogen, and phosphorus sources, were recycled to maintain the microalgae growth conditions. Additionally, the uncertainties caused by changes in the key parameters and the lack of critical processes were quantitatively analysed.