## 1 Introduction  
Water resources are under pressure worldwide, resulting in scarcity and deterioration of freshwater quality. Water scarcity affects already a large part of the world's population, and the United Nations indicates that two-thirds may face water shortages by 2030 [1]. The continuous decrease in clean freshwater availability is the result of the unique combination of the sweeping growth of population, urbanization and pollutant release as well as climate change and the drought crisis. To address the imbalance between limited available water supply and increasing water demand, it will be unavoidable to reduce freshwater source consumption and wastewater discharge by increasing its management according to the circular economy rules: reduction, reuse, recycling and recovery. Reclaimed water can be utilized via indirect and direct potable recycling, as well as non-potable reuse in irrigation (both agricultural and landscape), recharge of aquifers, seawater barriers, industrial applications, dual-distribution systems for toilet flushing, other urban uses and aquaculture industries. Water treatment and reuse is not a recently introduced practice, and it is not restricted to arid and semi-arid regions. It involves, and will involve more and more in the future, all continents, including those with sufficient water resources. The European Union, Canada, the USA, China, Japan, Israel, Australia and Brazil have already implemented water reuse policies [2]. Wastewater (WW) remediation has passed through different development stages with time, exploiting more and more effective regeneration technologies and hybrid procedures to transform the escalating quantity of wastewater from municipal, industrial and agricultural activities, often containing several toxic pollutants, in a product safe for the environment as a whole. The present global water crisis is further pushing research efforts aimed to find more suitable procedures for effective, economically feasible and sustainable remediation and reuse of wastewater, preserving freshwater sources for drinking water use and, ideally, to produce safe drinking water by reclaiming WW. A rapid survey of the literature published during the last decade illustrates the continuous, massive research effort and the important advances in physical, chemical and biological-based solutions proposed to cope with wastewater characteristics and the expected end-use of reclaimed water. The thousands of papers published during the last few years witness the importance of developing effective protocols for wastewater management in the viewpoint of circular use. Hundreds of review papers have attempted to organize little portions of the huge amount of the newly proposed materials and methodologies. To mention few examples, reviews were focused on heavy metal and metalloid removal from industrial and urban wastewater [3; 4; 5; 6]. Others addressed methods for the complete degradation of persistent pollutants such as dyes, detergents, contaminants of emerging concern like pharmaceuticals, endocrine-disrupting compounds, PFAS, organochlorines, etc. [7; 8; 9; 10; 11; 12; 13; 14], or the removal of nowadays ubiquitous microplastics [15; 16]. All aspects of municipal wastewater, harvested rain and stormwater treatment concerning both chemical pollutant degradation and reclaimed water microbiological quality are continuously evolving and reviewed [17; 18; 19]. In a similar way, the remediation and reuse of industrial WW is an important topic, especially because the water quality requested for use at the same plant is not so pressing [20; 21; 22]. Finally, several review papers are focused on specific cleaning techniques or propose innovative combinations of materials and treatments. Examples include those about several, catalyzed and/or combined advanced oxidation (and reduction) processes [23; 24; 25; 26; 27], the development of new nanomaterials and nanocomposites [28; 29; 30; 31] and new sustainable absorbent materials [32; 33; 34] and the highest-performing filtration membranes [35; 36; 37].