Biomass conversion strategies and wastewater reuse: a deep focus on hydrothermal liquefaction as a circular economy approach

A wide variety of eco-friendly and at zero waste techniques are developed for biomass conversion and valorization of its residues and by-products such as water fraction and organic residues which could be further utilized. The wastewater reuse is one of the best strategies for water security, sustainability, and resilience. To date, the municipal wastewater was the most widely used, nowadays the innovative technologies for biomass conversion and energy production allow the recovery of wastewater with better and safer features than the municipal effluents. Depending on the moisture content of the starting feedstock, the hydrothermal liquefaction process (HTL) generates also up to 95% of wastewater (HTL–WW) generally rich in nitrogen, phosphorus, and sulfate as well as micronutrients and minerals. Although it is currently recycled through various biological systems such as microalgae cultivation and anaerobic digestion, the possibility of using the wastewater from HTL process as irrigation water for agricultural purpose is discussed representing a source of crop nutrients for the high amount of organic and inorganic compounds and a new approach in contributing to reduce the increasing pressure on freshwater resources.