Hydrothermal Carbonization of Aquatic Biomass: a Promising Solution for the Invasive Species Myriophyllum Aquaticum

Myriophyllum aquaticum (Vell.) Verdc., an invasive aquatic plant from South America, is to date worldwide distributed. In Tuscany M. aquaticum has colonized much of the surface water network of reclamation channels, forcing the reclamation authority to its removal, with the production of a huge amount of wet biomass to be disposed as a biological waste. In the present study the hydrothermal carbonization (HTC) process was proposed and investigated as a path for the valorisation of this waste biomass. Performed in aqueous conditions at moderate temperatures, HTC does not require energy-intensive pre-drying steps, making HTC applicable to biomass feedstocks with high moisture contents. HTC leads to a carbonaceous solid product, referred to as hydrochar, with different potential applications, such as solid biofuel, or soil-improving material. An experimental investigation was carried out performing HTC tests in a laboratory-scale reactor. The joint effect of operating parameters (temperature, residence time and solid load) on process yields and hydrochar properties was investigated by Design of Experiments – Response Surface Methodology (DoE-RSM), a statistical and mathematical approach for process analysis, prediction, and optimization. The results obtained demonstrated the feasibility of HTC for M. aquaticum, suggesting HTC as a promising treatment path. Beyond the use as biofuel, the suitability of the hydrochar produced from M. aquaticum as organic growth medium for vegetable seedlings in horticulture and gardening or soil amendment was evaluated. Potential phytotoxic effects of M. aquaticum hydrochar were evaluated through bioassays using cress seeds (Lepidium sativum L.) as a model species.