One water – evolving roles of our precious resource and critical challenges


 This article presents the evolving challenges and roles of our water resources in this contemporary world. First, water quality issues surrounding water supplies are discussed. Potential pathways to address the water quality challenges are presented, which include technological approaches for minimizing waste and enhancing resource recovery. Focused discussions on emerging global pollutants such as microplastics and PFAS (per- and poly-fluoro alkyl substances) and treatment alternatives are included. Next, the roles of used water (wastewater) in the wake of circular economy and recent outbreaks are discussed. The potential for energy and resource recovery possibilities and the critical role of wastewater treatment plants in controlling the spread of outbreaks are discussed in detail. Finally, perspectives on some of the key developments essential for transforming our water infrastructure, addressing water-centered socio-economic issues and the critical needs of digitalization in water sector operations are presented.


GRAPHICAL ABSTRACT INTRODUCTION
The basic needs of sanitation and hygiene can never be overstressed and these cannot be met without the use of freshwater sources (Gude a). Worldwide population growth and economic development coupled with higher living standards have escalated the global freshwater demands over the past few decades (Gude ). While the number of inhabitants increases, freshwater reserves at the global level remain finite, which need to be protected and preserved for longevity, so that the critical needs of our society can be met in a sustainable manner. It is estimated that by 2050, more than 5 billion will face water scarcity (WWAP ). At the global level, we have only one 'water' source whether it is called brackish water, freshwater, ground water, hypersaline water, saline water, seawater, surface water and 'used' or wastewater, which is impacted by climate change, irregular hydrologic cycles, excess withdrawals, and anthropogenic pollution (Figure 1).
There is a pressing necessity to identify the trends of water supply demands, sources of pollution, and inadequacies in infrastructure and operations so that timely and effective remedies can be developed.
Global freshwater consumption has increased by eightfold over the past century, and the freshwater withdrawals have increased by threefold with respect to the population growth (Wada & Bierkens ; Gude a). Several river basins and aquifers are under exploitation worldwide, which could impact the ability to provide freshwater sources to 25% of the world population in near future (Soligno et al. ). While groundwater is a critical resource for sustainable development in many regions of the world, overexploitation of the resource could lead to global water scarcity and other compounding issues related to people, economy and the environment (Gude a; Huang et al.

; Gleeson et al. ).
The availability of water sources (quantity) will also affect the quality of the resource often impacted by human extraction and exploitation causing impairment of the resource. Ensuring water sources of adequate quality for beneficial uses is paramount to sustainable development.
Enabling technologies should be developed to ensure highquality and reliable water supplies. In recent decades, the water industry has been faced with several unprecedented challenges in meeting both the quantity and quality of water resources required for various beneficial uses and ecosystem protection. The purpose of this article is threefold: (1) to discuss and present management and technological approaches required for demand mitigation and supply enhancement in order to address both water quality and quantity issues, especially considering the ubiquitous nature of microplastics and PFAS; (2) to discuss the potential roles of wastewater treatment facilities in circular economy and outbreak control and management; and (3) to present some of the key developments essential to transform our water infrastructure, to address water-centered socio-economic issues and the critical needs of ensuring resiliency in water sector operations.
The purpose of this article is to discuss how we may progress from conceptualizing our wastewater treatment systems as waste management, to resource recovery and human health management systems. To do this, an overview of some emerging water quality issues and the state of the art of the treatment technologies by which they can be addressed is presented. The emerging role of wastewater treatment plants for informing public health policy regarding the SARS-CoV2 outbreak is discussed. Finally, some suggestions for focus areas that can lead to transformation of our water infrastructure are presented.

WATER QUALITY CHALLENGES
The major scientific and technological advancements in recent decades have led to the creation and production of numerous synthetic chemical and biological compounds that now pose a threat to our water sources in a number of ways. These include chemical and biological contaminants, pharmaceuticals and personal care products, metals, pesticides, and more recently, microplastics and PFAS (per-and poly-fluoroalkyl substances) as shown in Figure 2.
Considering current challenges faced by the water industry, the order of water pollutants in terms of their priority is listed as follows: PFAS, point and non-point source pollution, chemical spills and cyanotoxins, CSOs (combined sewer overflows), lead and copper from aging infrastructure, nutrient removals, pathogens, perchlorates, arsenic and radionuclides (AWWA ). While there has been a great deal of effort put into developing solutions for removing metals, pesticides, biological contaminants, and pharmaceuticals and personal care products over the past few decades, solutions are yet to be developed for emerging constituents such as PFAS and microplastics. PFAS rose to the top 2020 regulatory concern after placing second in 2019 and ninth in 2018. The USEPA (The United States Environmental Protection Agency) has proposed setting national drinking water standards for two of the most common and studied types of PFAS chemicals and is seeking comment on potential monitoring requirements and regulatory approaches for the chemicals. In the meantime, numerous It has been estimated that some 1,000-1,000,000 fibers are released from a single garment. Around 35% of microplastics occurring in oceans are from synthetic fibers and textiles (Prata ). In general, the plastic pollutants can be identified by their size as mesoplastics, microplastics, and nanoplastics (Andrady ). Spectroscopy and imaging can be used to identify the mesoplastic pollutants, while fluorescent techniques and microscopy can be used to analyze the presence of microplastics. Electron microscopy can be used to identify the presence of nanoplastics. Polymer identification methods including Fourier-transform infrared spectroscopy (FTIR), inductively coupled plasma mass spectrometry (ICP-MS), and Raman spectroscopy can be used to quantify microplastic particle (MP) concentrations in food sources and drinks (Cox et al. ).
Some of the pathways that may contribute to the degradation of microplastics in the environment are biodegradation, photodegradation, thermooxidative degradation, thermal degradation and hydrolysis. Solar-assisted UV degradation is a very efficient mechanism; however, it is significantly impeded by moisture or under water environment.
Possible consumption of microplastics can lead to numerous health impacts (Cox et al. ).
Human consumption of microplastics is another evolving concern. Microplastics are present in various food sources and drinks including bottled water. The concentrations of microplastics vary across the sources. For example, seafood (fish, bivalves and crustacean) may con-  Wastewater treatment plants receive effluents with high concentrations of microplastics. Primary, secondary and tertiary treatment schemes enable the removal of microplastics to a great extent, but a significant portion of these particles is discharged through effluents into the receiving environment.
In regions where strict regulations are not maintained for secondary wastewater treatment, microplastics escape through the system (Prata ).
There are several methods to remove microplastics from the water sources. These include chemical (advanced oxidation, coagulation-flocculation, chlorination and electrocoagulation), physical (adsorption, granular sand filtration, sedimentation, ultrafiltration and reverse osmosis) and biological processes (anaerobic digesters, membrane bioreactors, activated sludge and other biological wastewater treatment units). The removal efficiencies of these processes depend on the chemical and physical characteristics of microplastics. There is a wide range of removal efficiencies reported for microplastics removal. These vary between 8 and 99.5% (Zhang & Chen ).  better performance in removing these substances, but these processes usually generate a concentrated stream (Woodard et al. ). Sorption using highly active carbon surfaces seems to be a promising alternative, but this technique is not able to capture short-chain PFAS which is another concern. Advanced oxidation and reduction processes were also explored equally for efficacy in breaking down of these substances. High thermal, high pressure and high radiation and sonic frequency treatment methods also All of these have some degree of removal capacity but each with their own drawbacks such as high capital costs, low treatment volumes and high-specific energy consumption and residual products.
There are many technologies that can be employed for PFAS removal from water supplies and wastewater sources.
These are classified as 'demonstrated', 'partially demonstrated' and 'developing' depending on their level of readiness for wide application. Demonstrated technologies include adsorption (activated carbon, granular-activated carbon and -reactivated carbon).
In granular-activated carbon system, the performance of

Disinfectants for biological contaminants
Yet another emerging area of concern is the microbial contamination of drinking water supplies. Drinking water distribution systems are susceptible to microbial fouling due to water stagnation or suspension in water lines (Ling et al. ). It is reported that the number of microbial cells could range from 10 6 to 10 9 per liter (Hull et al.

EVOLVING ROLES OF WATER SOURCES
Used water (also known as wastewater) The amount of wastewater (used water) generated at community levels is in proportion to the water usage by the community. The used water should be treated to remove the pollutants and recover the resource. Physical, chemical and biological processes employed in primary and secondary treatment schemes enable this goal to be achieved, while the secondary (biological) treatment is mandatory and enforced as a standard to protect the environment and public health. With the realization that the discharged used water is going to interact with potential water supply sources in space and time across the regions, it is critical to ensure proper treatment of the used water to protect the water supplies. Energy-efficient and cost-effective treatment processes for the removal of major nutrients (carbon, nitrogen and phosphorous) and other deleterious compounds are being developed on a continuous basis to meet the evergrowing challenges of protecting our environment and receiving water bodies.

Used water valued as a resource
The roles of wastewater treatment plants are evolving recently in different directions that require dynamic shifts to accomplish the new goals and objectives surrounding the performance of these facilities. For example, once considered as facilities to prevent pollution of receiving water bodies, these facilities are now valued as resource recovery facilities.
Specific energy consumption of used water depends on its characteristics, quantity and treatment scheme, and the energy demand for used water treatment can be burdensome at times (Gude ). However, used water is now increasingly recognized as a valuable resource for water, energy and nutrients and other valuable bio-or energy-products (Gude b).
The energy embedded in the organic matter of used water is a motivating factor for potential energy recovery. Nutrients such as nitrogen and phosphorus are other valuable commodities required in agricultural and irrigation applications. Above all, water itself is a major and invaluable resource that could be recovered from these operations.
Here, W R is the amount of water that is recovered or recycled, and W T is the total wastewater that was treated in the wastewater treatment plant.

Circular economy index of energy source (CEI E ) in
wastewater can be written as: Here, E R is the amount of water that is recovered and E W is the total energy content in wastewater that was treated in the wastewater treatment plant.

Circular economy index of nutrients (CEI N ) in waste-
water can be written as: Here, N R is the amount of nutrients that is recovered or recycled and N W is the total amount of nutrients in wastewater that was treated in wastewater treatment plant.
Total circular economy index:

Future scenarios for water technologies
New technologies, in general, are hyped in their potential for their performance, usually in an effort to increase their visibility (see Figure 5(a)). However, their actual performance may be significantly lower than the expected or project potential. This is called actual potential or performance. This stage clearly provides an opportunity for realization of actual and practically feasible technological potential and the scope for improvements can then be determined. Further efforts of redesigning and improving the process performance will enable the practical and demon-

WHAT ELSE DO WE NEED MOVING FORWARD?
Under 'one water' concept, water and used water interact in hydrological cycles. Therefore, there should be a proper coordination between the water supply and used water management sectors to enable efficient resource recovery, human health protection and waste management. Within the water sector, water and wastewater (used water) utilities operate independently and coordination between the two utilities is lacking in many municipalities. There is a growing importance and need for improving communication between different utilities to protect water sources. In addition to effective communication and coordination, other critical areas where significant efforts are warranted to make a holistic difference in this sector are aging infrastructure, dwindling workforce, inefficient process control, and socio-economic differences. These challenges will be discussed next.

Investing in water infrastructure and workforce
Aging infrastructure has become a major issue in many developed countries (Sakai et al. ). Utilities recognize the critical need to invest in infrastructure improvement projects. Still, their limited resources require striking the right balance between addressing emerging needs and executing

Socioeconomics and water resource education
The effects of changes in social and economic factors, such as population growth and water consumption, might be as important or even more important than climate change in affecting the hydrological cycle and increasing water scarcity risk (Koutroulis et al. ). There is a significant imbalance in the origin of the blue water (fresh surface and ground water sources) increasingly consumed by developed and developing countries. On average, developed (economically advanced) countries tend to increase their affluence by intensifying mainly the use of foreign water resources; conversely, the affluence growth in developing regions mostly relies on the use of local water resources.
As a matter of fact, despite the water resource status, devel- Finally, education and outreach efforts should address the issues at the nexus of the consumers, manufacturers, regulators, stakeholders, investors, and water utilities.
Individual choices that lead to water pollution, utility owner preferences that impact water system designs and regulatory enforcement of effluent and water supply standards and ultimately, the financial derivatives and investment interests should be revisited to understand the evolving roles and then overcome the challenges of our precious resource.