## 1 Introduction  
The planetary priorities for clean water and sanitation have been established by the United Nations in the Sustainable Development Goal (SDG) #6: \"80% of wastewater goes into waterways without adequate treatment\" [1]. \"Since water stress affects more than 2 billion people (with this figure projected to increase)\", consequently, \"80% of the countries have laid the foundations for integrated water resources management\". Already, more than four billion people experience the effects of water scarcity as a two-end problem: (1) water shortage due to water unavailability, and (2) water stress due to growing consumption [2; 3; 4]. A water footing framework facilitates the tracking down and mapping of water flows, thus permitting a resource assessment from an environmental management perspective [5; 6; 7]. There is a strong interplay among the different framework aspects, which implicate the dimensions of the water quantity and quality on the 'three-colored' water-footing avenues: (1) green (rainwater), (2) blue (ground and surface water) and (3) grey (polluted water) [8; 9; 10; 11; 12; 13; 14; 15; 16]. As agriculture may consume up to 92% of the green, blue and grey water-footing flows, this area becomes the spotlight for finding opportunities to manage water availability and consumption [17; 18; 19; 20; 21]. Grey water attracts a fair share of the evolving technological ingenuity due to the innate complexities of coping with wastewater physicochemical phenomena. Wastewater reuse is indispensable, particularly when there are great supply shortages in arid and semi-arid regions [22; 23; 24; 25; 26; 27]. Improve wastewater recycling efficiency is imperative to continuously developing and optimizing a broad range of older and current separation treatments [28; 29; 30; 31; 32]. Water separation techniques that exploit the modern advances of nanofiltration and ultrafiltration processes are becoming popular in containing pollution by regulating contaminant removal from wastewater systems [33; 34; 35; 36; 37; 38; 39; 40; 41; 42; 43; 44; 45]. To achieve sustainable development goals, in an endeavor to curb pollution, innovative solutions (SDG #9) should encompass lean, green and artificial intelligence technologies, which are proven in boosting operational performance in production [46; 47; 48; 49; 50; 51; 52]. Green, lean and six sigma (modern quality management) practices can be the catalysts for orchestrating the manufacturing of prosperous products by optimizing upstream processes. By combining a rapid-response mentality with outcome effectuation through proper project selection rules, they are bound to assist operations in remaining profitable in a circular economy [53; 54]. Generally speaking, waste reduction projects require an integrated lean-green approach to be effective in deploying and monitoring the right environmental measures and, hence, in accomplishing the aim of process performance enhancement [55; 56]. Lean six sigma projects, which are supported by artificial intelligence know-how, lead to superior business results because they accelerate the innovation cycle time [57; 58]. The sustainability factor is vital in improving water purification processes [59]; it stands as a broad indicator which tracks down checks and balances in production while ascertaining which operations are conducted in an environmentally sound manner. Ultrafiltration- and nanofiltration-membrane processes cover a wide range of water purity demands to complement reverse osmosis treatment capabilities. The tailored design of nanofiltration membranes can push the limits of separation by optimizing their property performance relationships [60]. At any rate, it is noted that optimizing water quality is a challenging task because of the ever-present influence and extent of uncertainty in the influx water volume [61].