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.

This research focuses on the management of food waste in small communities such as the one at the Reception and Identification Center (RIC) of Kara Tepe in Lesvos (Greece), using the method of home composting. To understand production patterns and possible variations of the characteristics of food residues, a sampling methodology was designed for obtaining representative samples of the whole population of the center. Two composting trials were carried out using household composters with a tailored active aeration system. The first trial contained food residues and olive leaves as a bulking agent (Κ1), and the second one contained food residues, olive leaves and two-phase olive pomace (K2) to investigate co-composting with local agricultural byproducts. Results indicate that the compost produced in the first trial (K1) was stable but phytotoxic and unsuitable for use as a soil amendment. The compost produced in the second trial (K2) was also stable and non-phytotoxic but potentially harmful for very sensitive plants if used as a soil amendment because of high pH values. In addition, an analysis of the operational cost of the proposed treatment process was performed, involving the estimation of electricity and water consumption. The findings indicate that 304 home composters can cover the requirements of a population of this size (approximately 1300 people), at a cost of a mere 3.36 €/day. Given the unique challenges faced by island communities in contrast to urban areas, including differences in strategy development, waste management practices, infrastructure, cultural considerations and innovation potential, the findings serve as a foundational guide for similar island communities in Mediterranean areas (for policy and decision makers, stakeholders, academia, etc.) to develop tailored strategies within the framework of the circular economy that particularly focus on composting for reducing food waste and utilizing byproducts. Therefore, it is feasible to exploit the proposed methodology in similar-sized communities—villages or even a block of flats—for the on-site treatment and utilization of food residues since it can provide a good-quality compost at a reasonable cost.

Biowaste management is in the center of attention in recent years, due to the increased focus on Circular Economy practices. Recent management solutions promote exploration for small and medium scale processing and monitoring practices. In this context, two pathways of biowaste valorization are analyzed: composting and anaerobic digestion. This study focuses on energy production with the hydrothermal carbonization (HTC) of anaerobic residues from biogas production. Lesvos has numerous processing facilities and olive mills, and therefore Olive Mill Wastewater (OMWW) are the main biowaste streams in the island, along with olive tree pruning, the organic fraction of municipal solid waste and sludge from wastewater treatment facilities. Most studies present applications on specific types of biowaste, while this study aims to co-treat several biowaste streams. A holistic experimental set-up that combines aerobic and anaerobic treatment strategies was developed taking into consideration the hydrothermal treatment of AD digestate along with locally available biowaste, like OMWW, sludge etc. A main target objective is to investigate if solid fraction could have a high heating potential In addition, HTC was able significantly to reduce the COD of OMWW while resulting in hydrochars with increased heating values.