Energy Production from Cattle Manure within a Life Cycle Assessment Framework: Statistical Optimization of Co-Digestion, Pretreatment, and Thermal Conditions

Waste to energy processes from anaerobic digestion (WtE-AD) from cattle manure (CM) have low CH4 yields due to CM's structural composition. The search for alternatives to increase the energy yields of these processes must consider the optimization of operating parameters within a framework of mitigating the environmental footprint. The goal of this paper is to provide a statistical optimization strategy based on experimental designs to improve CH4 yields and reduce the environmental profile of CM valorization through a WtE-AD process. Biochemical methane potential tests were conducted to determine the energetic and environmental effects that alkaline pretreatments, different AD temperatures, and co-digestion formulations with fruit and vegetable waste (FVW) have on the WtE-AD process from CM. The evaluation was conducted following a life cycle assessment approach through energy balances. The results indicate that the highest CH4 yield (384.3 mL CH4 g VS-1) and the lowest environmental impact (-0.06 kg CO2 eq kWh(-1) of electricity production) were achieved with the co-digestion of CM with FVW 1:1, pretreatment with 10 g NaOH 100 g(-1) of VS of CM, and a temperature of 45 degrees C. It was found that the CM pretreatment with NaOH substantially increases the energy profile of the WtE-AD process without compromising the environmental impact since greenhouse gas emissions in chemical production are negligible.