The world is currently witnessing a rapid increase in sewage sludge (SS) production, due to the increased demand for wastewater treatment. Therefore, SS management is crucial for the economic and environmental sustainability of wastewater treatment plants. The recovery of nutrients from SS has been identified as a fundamental step to enable the transition from a linear to a circular economy, turning SS into an economic and sustainable source of materials. SS is often treated via anaerobic digestion, to pursue energy recovery via biogas generation. Anaerobically digested sewage sludge (ADS) is a valuable source of organic matter and nutrients, and significant advances have been made in recent years in methods and technologies for nutrient recovery from ADS. The purpose of this study is to provide a comprehensive overview, describing the advantages and drawbacks of the available and emerging technologies for recovery of nitrogen (N), phosphorus (P), and potassium (K) from ADS. This work critically reviews the established and novel technologies, which are classified by their ability to recover a specific nutrient (ammonia stripping) or to allow the simultaneous recovery of multiple elements (struvite precipitation, ion exchange, membrane technologies, and thermal treatments). This study compares the described technologies in terms of nutrient recovery efficiency, capital, and operational costs, as well as their feasibility for full-scale application, revealing the current state of the art and future perspectives on this topic.

The increasing production of sewage sludge at global level has addressed the search for technical solutions to take advantage from it, reducing the environmental burden originating from its disposal. Anaerobic digestion is a suitable option to handle sewage sludge in accordance with circular economy principles, as it generates a methane-rich biogas and a digestate with potential fertilizing properties. Several techniques have been proposed to enhance anaerobic digestion performances and, among these, the application of static magnetic field (SMF) has recently gained attention. Nonetheless, the effects of high-intensity SMF on the sewage sludge destined to anaerobic digestion and its impact on the anaerobic digestion process have not been evaluated yet. This study aims to determine the effects of a 1.5 T SMF on the chemical composition of sewage sludge as well as on methane generation during anaerobic digestion. The main parameters influencing the SMF (i.e., flow rate, mixing ratio of magnetized to non-magnetized sludge, number of pumping cycles, and total solid content) were varied to evaluate the impact of different exposure conditions on the chemical characteristics and methane potential of sewage sludge. An extensive exposure to the high-intensity SMF applied resulted in a 24% decrease of methane production, reduced the concentration of the monitored ionic species (i.e., NH4+, NO3−, PO43−, SO42− and Mg2+) in the liquid phase of sewage sludge, and promoted the precipitation of compounds with valuable fertilizing properties, e.g., struvite. These outcomes suggest that high-intensity SMF, although negatively influencing methane generation, can promote the precipitation, and possibly the recovery/recycle of valuable compounds from sewage sludge, enhancing its proper management in a circular economy perspective. Graphical abstract

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