# A Cyclic Graywater Treatment Model for Sustainable Wastewater Management Applied in a Small Scale
## 1 Introduction  
Water is a vital resource, indispensable for human survival, ecosystem function, and global food security. Its sustainable management is, therefore, a critical challenge for ensuring the resilience of both human societies and the natural environment. The global population is projected to reach nearly 9.7 billion by 2050 [1], with direct implication in the increasing of domestic and agricultural water needs. The global demand for water is increasing by approximately 1% annually, with projections indicating a 55% rise from 2000 to 2050 [2]. Clean water scarcity has become a major problem for the entire world's population. Water quantity and quality problems must be intertwined, since areas with acute water scarcity frequently struggle with serious pollution [3,4]. Climate changeexacerbates water scarcity and quality risks in urban areas, with 88.8-99.7% of large cities expected to face increased water challenges by 2050, primarily due to rising demand, population growth, and agricultural runoff [5]. Graywater, which can be defined as domestic wastewater, excluding toilet waste, presents a viable solution to global water scarcity, constitutes 60-75% of household wastewater, and can reach 90% with the use of vacuum toilets [6]. The implementation of graywater reuse systems can lead to substantial water savings across different building typologies, with reductions of 46% for hotels, 44% for residential buildings, and 29% for industrial facilities [7]. Graywater is generally considered safer to handle than black water due to its lower contamination levels, making it suitable for non-potable applications, such as toilet flushing and landscing [8]. Agriculture, which accounts for over 76% of total water consumption in Tunisia, relies heavily on water resources. In response to increasing irrigation demands and water scarcity, treated wastewater--more specifically, graywater--has emerged as a crucial alternative resource [9; 10]. Studies indicate that irrigation with graywater can improve soil properties by increasing organic carbon content and enhancing nutrient availability, thereby supporting crop growth [11]. Indeed, treated graywater has been linked to significant increases in crop yield, such as olive trees, which thrived under irrigation with treated graywater [12]. Depending on domestic activities, graywater's composition varies. For example, kitchen wastewater has a high biochemical oxygen demand (BOD), indicating a significant amount of organic matter, whereas laundry effluent usually has a higher pH and chemical oxygen demand (COD) due to the use of certain soaps and detergents [6]. Given the varying composition of graywater, effective treatment methods are essential to ensure its safe and efficient reuse. These methods aim to address specific contaminants depending on the source of the graywater, ultimately enabling its application in recycling applications while maintaining environmental and health standards. Treatment methods, including the use of polymers such as Poly Aluminum Chloride (PAC), have been demonstrated to effectively reduce contaminants, enhance water quality, and minimize sludge production [13]. In particular, PAC excels in coagulating and removing turbidity, suspended solids, and organic contaminants across a broad pH range, requiring lower dosages and producing less waste, making it a cost-efficient and operationally advantageous option for graywater treatment. Moreover, Vertical Treatment Systems have shown high efficiency in contaminant removal, with studies reporting a 90% reduction in BOD\({}_{5}\) and an 83% reduction in COD, making treated graywater suitable for irrigation [12]. Similarly, constructed wetlands (CWs) employing a mixed filter bed of seashells, ceramic brick fragments, and sand have demonstrated remarkable efficacy in removing organic matter, anionic surfactants, and total phosphorus, highlighting their potential as an effective and sustainable graywater treatment method [14]. Ref.