With the current rate of consumption, it is estimated that the world’s water supply will fall short of demand by 40% by 2030. Numerous global regions are already witnessing demand outstripping supply, and in other areas, water scarcity is impeding economic development [
1]. As economic growth and unpredictable weather patterns intensify competition for water resources, the threat of a worldwide food crisis looms, potentially affecting businesses, governments, citizens, and farmers. The World Resources Institute (WRI) predicts that 33 countries will face severe water stress by 2040 when assessing future water stress situations. Many of these countries, 14 to be exact, are in the Middle East, with 9 scoring the highest possible score of 5, including the United Arab Emirates (UAE), highlighting a particularly severe water stress situation [
2]. While water makes up 70% of our Earth’s surface, only 3% of it is not saltwater. Saline water, which accounts for 97% of the total, is unsuitable for human consumption, agriculture, or industrial cleaning without substantial energy expenditure and desalination processes. However, this saline water has limited use, such as in certain types of industrial cooling. Over the past century, our water sources have remained static, but the global population has skyrocketed [
3]. A staggering 80% of all freshwater drawn globally is used for irrigating food crops, which establishes agriculture as the primary consumer of water. Additionally, pollution is swiftly contaminating both surface and groundwater, restricting their use for irrigation. For example, over 80% of wastewater from human activities is discharged into oceans and rivers without any prior treatment. This practice results in considerable water pollution, rendering these bodies of water unfit for irrigation [
4,
5,
6,
7]. Wastewater treatment is an increasingly recognized crucial approach to water resource management. The goal is to remove pollutants from the wastewater, rendering it safe enough to be discharged back into the environment. The different methods for treatment vary depending on the source of wastewater such as irrigation wastewater, industrial wastewater, etc. For instance, utilizing treated wastewater can be seen as a resource recovery strategy in small to medium-sized agricultural areas. This is because it not only fulfills irrigation needs, but it could also potentially serve as a source of nutrients for crops [
3,
4,
8]. Wastewater can be divided into five categories based on what it contains: blackwater, which consists of fecal sludge, urine, and feces; greywater, comprised of water from washing and bathing; blue water, which is stormwater and urban runoff; and green water and red water, which refer to agricultural and industrial waste, respectively. Among the different types of wastewater, greywater treatment is the main focus of this article. Greywater treatment contributes to water demand management by encouraging the preservation of premium freshwater and lowering both environmental pollution and total supply costs [
9]. Greywater is used–untreated water from appliances such as bathtubs, showers, washbasins in bathrooms, clothes washers, and laundry tubs [
10]. Greywater generated in malls, restaurants, and university buildings is generally dilute and it will later become concentrated when it is merged into the main sewage collection line. It would be more economical and environmentally friendly if the greywater is treated locally using small modular wastewater treating units that produce treated water amenable for irrigation or horticulture [
11]. Recycling domestic wastewater like greywater using centralized and decentralized treatment methods is a highly effective strategy to reduce worldwide water demand. The choice between these systems is guided by location accessibility, economic circumstances, and the availability of treatment facilities [
12].
Figure 1a,b show a schematic of centralized and decentralized treatment facilities used for wastewater treatment, respectively.
In many developed and developing countries, conventional centralized wastewater treatment plants are regarded as primary wastewater management solutions in which various types of wastewater, such as domestic, commercial, industrial, and hospital wastewater, together with storm and runoff water, are processed at a central treatment facility that is planned, developed, and run by a government or private organizations. On the other hand, a decentralized wastewater treatment facility treats wastewater very near the generation point on a small scale utilizing cheaper and simpler technologies [
13,
14,
15]. In developed nations, chemical processes often have their own wastewater treatment facilities to manage the industrial waste generated by their activities. However, in developing countries such as Yemen, all wastewater from different industries is typically collected by a single sewage treatment plant. This can lead to an overload of the plant’s capacity, potentially compromising its effectiveness and resulting in lower-quality treated effluents [
16,
17,
18,
19]. Due to centralized systems’ high capital and operational costs, decentralized systems appear vital, cost-effective, reliable, and environmentally sustainable, especially for developed countries that cannot afford these massive expenses [
20,
21]. Owing to its benefits, the number of decentralized wastewater systems has grown significantly; more than 1000 systems have been constructed in China [
22]. Brazil, for instance, has acknowledged decentralized wastewater treatment plants by law and is a component of the plan of the National Sanitation Strategy to treat 86.5% of all produced wastewater by 2023, increasing the current treatment index by 49% [
18]. For small and medium-sized agricultural regions, using reclaimed wastewater can be a resource recovery alternative since it not only meets irrigation demands but also has the potential to be a source of nutrients for plants [
23,
24]. The substantial distance between treatment facilities and agricultural zones can pose a significant challenge in reusing water treated at centralized wastewater plants. This is because it necessitates the establishment of a distribution network for the treated water, which could result in duplicating the existing infrastructure for treatment, reclamation, and discharge. Decentralized wastewater treatment plants (DWWTPs) not only ease the load on centralized wastewater treatment plants (CWWTPs) and cut down on pumping costs, but they can also tackle the hurdles associated with reusing treated wastewater. This is possible by utilizing the treated water in nearby green spaces and agricultural lands [
25,
26,
27,
28]. Treated greywater has a variety of uses, including toilet flushing, car washing, fire prevention, and irrigating green spaces like parks and schoolyards. Its use for irrigation is becoming increasingly common, especially in desert regions, where it could reduce drinking water consumption by as much as 50% [
29,
30].
In this research article, the potential for treating greywater generated at Abu Dhabi University (ADU) located in the United Arab Emirates, is evaluated by conducting experiments at a lab scale and scaling it up to the design requirements. ADU consumes almost 100,000 m3 of fresh water annually, costing around USD 225,000. Although this water is used for multiple purposes, a significant amount becomes greywater. This greywater is merged into a sewer, connected to the city’s main pipeline leading to the CWWTP located in the Al Wathba region of the Abu Dhabi Emirate. The greywater produced at ADU is relatively low in concentration and can be treated in the proposed decentralized wastewater treatment plant (DWWTP) on campus. The treated water from this process can then be employed for campus horticultural needs or marketed to external vendors. Using the results obtained from the lab tests, a scaled-up version of the DWWTP is designed, and an economic analysis is run to confirm the profitability and the environmental benefits associated with the proposed treatment facility.