Hydrothermal Carbonization of Dry Anaerobic Digestion Residues Derived from Food and Agro Wastes in Lesvos Island

Biowaste management is at the center of attention in recent years due to the increased focus on Circular Economy practices. Lesvos has numerous food processing facilities and olive mills, and therefore Olive Mill Wastewater (OMWW) is a wastewater stream that requires attention. In this study, a holistic experimental set-up that combines aerobic and anaerobic treatment strategies was developed taking into consideration the hydrothermal carbonization of AD digestate along with locally available OMWW. The study focuses on the hydrothermal carbonization (HTC) of anaerobic residues from biogas production, and food waste was co-utilized with spent coffee grounds (SCG). The reduced volatile solids of SCG have some effects on the final products. AD produced methane yields of 54.7% for the food waste and 52.4%. for the feedstock with added SCG. At the same time, the feedstock that contained SCG produced more hydrochar that reached up to 50% of the yield. Hydrothermal carbonization in a water medium produced liquids with basic pH values around 8 and conductivities of 4-5 mS/cm, while the samples that were treated in OMWW medium had pH values close to 5.5 and conductivities of around 12 mS/cm. The produced hydrochars have significant calorific values that exceeded 20 MJ/kg for almost all the samples. Overall, HTC with OMWW as a medium was able significantly reduce the COD of OMWW while resulting in hydrochars with increased heating values.

Automated summary

Biowaste management has gained increased attention due to the focus on Circular Economy practices. A study was conducted on Lesvos to address Olive Mill Wastewater (OMWW), which is a significant wastewater stream on the island due to the presence of food processing facilities and olive mills. The study developed a holistic experimental set-up that combines aerobic and anaerobic treatment strategies, focusing on the hydrothermal carbonization (HTC) of anaerobic residues from biogas production. The results showed that utilizing food waste and spent coffee grounds (SCG) together had an impact on the final products, with enhanced methane yields and hydrochar production. The study also highlighted the potential of using OMWW as a medium for HTC, resulting in reduced COD and hydrochars with increased heating values.