Subcritical water extraction as a circular economy approach to recover energy and agrochemicals from sewage sludge

Global population growth is creating severe pressure on wastewater treatment plants, and specifically on sludge management. Meanwhile, the global challenge of achieving food-security requires the development of green pest control practices that maximize crop productivity. The hydrothermal technology, using subcritical water as the conversion media has been intensively studied, mostly for energy recovery purposes. Here, we focused on the aqueous phase by-product of this process and studied the subcritical water extraction of sewage sludge to recover valuable agrochemicals, with high potential of pre-emergent herbicidal activity. Full characterization of hydrothermal extracts from different reaction temperatures (200–300 °C) and times (30–120 min) highlighted the formation of pyrazine derivatives. Seed germination bioassays with three different species suggested a positive correlation between reaction temperature and extract herbicidal activity. Moreover, differences in seed viability and final root elongation between the tested crop (Trriticum aestivum) and weeds (Lapidium sativum and Amaranthus palmeri) may indicate the competitive abilities that can play a key role in weed management. Our results suggest that subcritical water can be applied as a green solvent for extracting a valuable agrochemical from sewage sludge and improving the circular economy for wastewater treatment.