Kinetic Modeling of Thermophilic Anaerobic Digestion of Lemnaceae for Biogas Production

Abstract

Anaerobic digestion of sustainably sourced biomass to generate biogas is a vital form of renewable energy that provides significant benefits to the environment. Lemnaceae, commonly referred to as duckweed, has shown great potential as a next-generation biomass feedstock for anaerobic digestion due to its rapid growth rates, low lignin content, and ability to remove nutrients from wastewater. However, research in this area is largely focused on the mesophilic (35 °C) anaerobic digestion of duckweed. For the first time, batch thermophilic anaerobic digestion was performed using three different duckweed varieties grown on swine lagoon wastewater to ascertain the biochemical methane potential (BMP) of the biomasses and estimate parameters associated with the kinetics of the digestion process. The BMPs of the three duckweed varieties were 205 ± 5, 217 ± 5, and 262 ± 7 mL CH₄ g⁻¹ volatile solids (VS) for the local variety (OxNC), Lemna gibba (8678), and Lemna gibba (7741), respectively. Four kinetic models were fitted to the experimental data: first order, modified Gompertz, transference, and logistic function. Unique to this study, inoculum from continuous thermophilic anaerobic digesters processing identical feedstocks was used during the BMP, causing the absence of a lag phase. The first-order model predicted the hydrolysis constant (k) to be 0.205–0.285 day⁻¹, which is similar to the hydrolysis constants reported in the literature for effective anaerobic digestion systems, thereby demonstrating that duckweed biomass has viable degradation rates. In this work, BMP experimentation and kinetic modeling have demonstrated the viability of anaerobically digesting multiple varieties of duckweed biomass under thermophilic conditions.