Energy balance and carbon dioxide emissions comparison through modified anaerobic digestion model No 1 for single-stage and two-stage anaerobic digestion of food waste

Abstract

Mathematical models are tools that allow the prediction and control of the anaerobic digestion (AD) process, ensuring process stability and maximizing methane production. In this study, the AD of food waste (FW) was evaluated in single-stage and two-stage semicontinuous configurations, with the following organic loading rates (OLR: kgVS⋅m−3⋅d−1): i. R1 (single-stage reactor): 0.7, 1.5, 3.0, 6.0; ii. R2 and R3 (two-stage acidogenic and methanogenic reactors): 3.0, 4.0, 9.0, 15 and 1.0, 2.0, 4.0, 7.0, respectively. Each configuration was analyzed in terms of the kinetics and energy balance, based on the ADM1 model (modified to consider lactic acid production, an acid not traditionally examined in this model). The proposed ADM1 accurately simulates (>80%) the behavior of pH, methane production and lactic acid formation for both configurations, with the single-stage presenting the best fit (R2 > 0.90 NRMSE <0.2); however, two-stage configuration showed better behavior in terms of the kinetic parameters associated with hydrolysis (kdis, kch, kpr, kli), with an increase of more than 50% in the hydrolysis constant, compared to single-stage. Additionally, the two-stage configuration achieved a significant impact on both energy efficiency (Re), up to 57.5% increase, and CO2 emissions, achieving a 47%. Both reactor configurations, resulted in lower CO2 emissions than landfill, which produced up to 72.4%. Furthermore, two-stage configuration has the potential to consider the modeling of other types of biotechnological and energy by-products.