Effect of hydrodynamic cavitation on flocs structure in sewage sludge to increase stabilization for efficient and safe reuse in agriculture

Sewage sludge is the by-product of wastewater treatment processes. Its reuse is central to a circular economy approach and offers a sustainable alternative to its disposal. Treated sludge contains a wide range of nutrients (mainly nitrogen, phosphorus, and potassium), which favor its sustainable employment for agricultural purposes (land-spreading, compost production) and environmental requalification interventions (forestry, silviculture, land reclamation and revegetation). However, if not properly treated, sewage sludge can contain various contaminants such as heavy metals, organic pollutants, pathogens, and other emerging contaminants, which pose a threat for crops production and human health. Hydrodynamic cavitation (HC) is an eco-friendly and cost-efficient pretreatment that can enhance sewage sludge stabilization in both anaerobic and aerobic digestion units, thereby making safe its management and disposal. In this study, HC was used for the gradual disintegration of activated sludge (reaching a maximum disintegration degree (DDPCOD) of 19.2% after 8 h of treatment), and the solubilization of the dissolved organic matter (increasing the Soluble Chemical Oxygen Demand (SCOD) from 244 to 4,578 mg L−1 after 8 h of treatment). Then, both dynamic light scattering analysis and stereoscopic microscope observations proved that HC can also lead to a size reduction of sludge suspended particles. In addition to evaluate the HC treatment efficiency, in this work was also provided a brief discussion on the possible procedures to be followed for the safe and efficient sewage sludge disposal on land after it has been HC-treated.