## 1 Introduction  
Water is a precious commodity with a rapidly dwindling supply. The scarcity of usable water has great geopolitical repercussions in the global circular economy. Consequently, wastewater stocks now represent a high-demand resource worldwide [1, 2]. The 17 United Nations Sustainable Development Goals (UN SDGs) exclusively devote SDG #6 to \"Water and sanitation\". Nevertheless, the circularity of water strongly interacts with several other crucial SDGs. To mention just a few, water accessibility impacts the pace of economic growth (SDG #8), which, in turn, is closely connected to the \"Sustainable consumption and production\" goal (SDG #12), and, eventually, it is water logistics that undisputably lay the foundations for the future of \"Sustainable cities\" (SDG #11) [3, 4, 5, 6]. Therefore, the water economy and its sparkplug--water innovation--are critical aspects of sustainable development. The circular economy's 10 R strategies--Refuse, Rethink, Reduce, Reuse, Repair, Refurbish, Remanufacture, Repurpose, Recycle, and Recover--certainly provide ample pathways for improvement [7; 8; 9; 10]. Besides being cost efficient, technologies are developing to identify and exploit opportunities for recycling, recovering, and reusing wastewater stocks. Every application is built to focus on very specific needs--domestic (potable/non-potable) use, agricultural or aquacultural use, industrial use, recreational use, landscape irrigation, etc. [11; 12; 13]. The purpose of water usage is important from an engineering perspective, because it also designates the preferred physicochemical process to be undertaken in order to regulate the nutrient content profile of the treated wastewater stocks. Of course, the origins of water feeder streams (blue, green, gray, black, or yellow) will lead to them requiring different treatment, because of characteristics related to their biochemical oxygen demand (BOD) and their chemical oxygen demand (COD), their nitrogen and phosphorus content, and the level of presence of solids, pathogens, and micropollutants. In recent water research, it has emerged that it is difficult to simultaneously exploit multiple types of resources solely relying on a single technology, especially when municipal and industrial wastewater stocks are so different [14]; only a narrow range of water products can be treated this way. The treatment of industrial wastewater streams is particularly demanding due to their potential to carry hazardous pollutants, thus making the removal of heavy metal ions a complex problem from an engineering and economic perspective [15; 16; 17]. There are numerous technologies that are currently available that depend on (1) classical removal techniques such as the membrane filtration, the adsorption unit operations, the Fenton processes, and advanced oxidation kinetics and (2) more contemporary combination-type applications such as membrane bioreactors and electrically assisted membrane adsorption bioreactors [18; 19; 20; 21]. To achieve the level of resource efficiency and technology effectiveness that the circular economy transition framework foresees, smart innovation in wastewater \"reuse, reduce, recycle and recover\" processes should be accelerated [22]. Regardless of the wastewater source and the type of technology employed, efficiency innovations should boost a separation unit's removal performance. To achieve optimal performance in wastewater treatment processes, it is essential to screen and model contaminant removal efficiencies [23; 24]. The high complexity of the chemical physics involved in the separation processes hampers the practical quantification of uncertainty in researching filtration phenomena at a plant scale. As a result, an extensive experimental search may be necessary in order to empirically profile and prioritize--with confidence--the main influences that control the quality/process characteristics.