The historical approach to urban water management (drinking water, rainwater, used water) has been “reinvented” many times over human history, most recently beginning in the industrialized cities of Europe and the United States (US) in the 19th and early 20th century.1,2 The spread of waterborne disease (e.g. cholera, typhoid) in urban areas caused by pollution of local water supplies lead to importation of uncontaminated water from remote sources. While this largely addressed drinking water related public health issues, it created the “problem” of sewage resulting from significantly increased volumes of contaminated (used water). The issue of sewage was subsequently addressed, along with drainage and flooding issues, by transporting the contaminated water out of the urban area for remote discharge. Pollution problems caused by these discharges compromised the quality of some drinking water sources, leading to development of drinking water treatment, and environmental degradation caused by pollution discharges lead to the development of used water (often called wastewater by others) treatment. Due to economies of scale for construction of these large- scale conveyance systems, and the limited treatment technologies available at the time, these systems were implemented as large-scale centralized systems, consisting of extensive piping networks and a small number of relatively large treatment facilities. While this general approach remained the norm throughout the 20th century, changes are occurring in the 21st century as described below.

The large-scale and centralized nature of the current urban water management system generally minimizes capital investment for the supporting infrastructure through economies of scale for facility construction, but often at the expense of efficient resource use. The large-scale, centralized systems are relatively energy-intensive (compared to alternatives), and minimize opportunities for resource recovery. Transport of water (e.g. drinking, used, reclaimed fit-for- purpose water) is energy-intensive, and these energy costs can be minimized if water supplies are produced locally and used water is treated for reuse locally. Combining various components of the used water stream for joint transport reduces resource recovery opportunities, as discussed below. While many factors were responsible for adoption of this approach during the 19th to early 20th century, two of the most important were the general availability of water and other resources, relative to demand, and the general lack of treatment technologies.

During the time that our current approach developed the global population was growing from 1 billion at the beginning of the 19th century to 2 billion in the first quarter of the 20th century, compared to the current global population of over 7 billion.3,4 Economic growth, which is the true determinant of water demand, has grown much faster. Moreover, the urban population has grown from around 20 to more than 50 percent of the total.5 Thus, while water and other resources were generally available in the 19th and early 20th century, this is no longer the case. Today, available sustainable water resources are generally fully allocated, and in many regions of the world are over-allocated.6 In fact, the growing water stress experienced throughout the world may be considered a result of the water management systems historically adopted.

Secondly, the general lack of technologies to reliably and cost-effectively treat contaminated water lead to the need to source relatively uncontaminated water supplies remotely, and to convey contaminated water for remote disposal. In contrast, treatment technologies are now available to treat relatively contaminated water to potable, and even higher, quality standards. Thus, the factors that principally resulted in development of the current urban water management system no longer exist.


Today we face increased resource scarcity (water and other resources), compared to the 19th and early 20th century when the current urban water management system evolved.7-9 Water resource scarcity is further exacerbated by climate change, which is decreasing available sustainable fresh water resources. Thus, it becomes necessary to implement systems that use available fresh water and other resources more efficiently. Fortunately, such systems exist and are being increasingly implemented.10-16 Table 1 contrasts some of the essential features of the historic approach to urban water management with the systems evolving to meet current and future needs. The evolving systems are integrated, multipurpose in nature, and rely much more heavily on local as compared to remote water supplies. These systems incorporate both centralized and distributed system components (often referred to as hybrid systems), and optimize operational features such as water use, energy, materials, and operational labor, rather than simply minimizing infrastructure cost. These systems are much more integrated into the urban systems that they are a major component of, thereby requiring significant institutional and financial changes.17 They are also increasing integrated into the evolving circular economy.18 While the “Future” scenario described in Table 1 certainly does not yet represent the norm, leading cities around the world are increasingly adopting these system components. As a result important examples existing internationally. Important components of the emerging paradigm are referred to as “One Water” and “Resource Recovery” and are deployed as components of integrated urban water management systems.

Table 1. Comparison of Historic and Future Approach to Urban Water Management

I.2.1  One Water

One important component of the evolving approach to urban water management can be referred to by many names, but one frequently used (and the favorite of the author) is “One Water”. One Water is based on the concept that all forms of water in the urban area (rainwater, groundwater, surface water, drinking water, used water) are linked and form a system that is best managed in an integrated fashion to provide effective urban water service. It is further recognized that the urban water cycle is connected to the broader environment, especially including the watershed where the urban area is located. To provide effective service the system must address the extreme conditions of drought and flooding (e.g. “too little” and “too much” water). The One Water approach addresses these conditions using a portfolio approach consisting of a combination of options, each one performing well over different conditions so that the combined system is resilient over a wide range of conditions. The portfolio components relative to water supply include surface and ground water, conservation, rainwater harvesting, water reclamation and reuse, and (as a last resort) brackish and seawater desalination.19,20 Likewise, the portfolio components relative to excessive water (storms, potentially leading to flooding) consist of conventional stormwater systems (including storage, piped conveyance, and physical flood protection, e.g. dikes), natural systems which capture and infiltrate water (green infrastructure), and designing the urban form to provide locations such as parks, etc. which can flood and be returned to service quickly and with minimal damage. In all cases the system components, and their relative sizes, are determined by local conditions.

I.2.2  Resource Recovery

The One Water approach is leading to urban water management systems using existing water supplies much more efficiently through conservation, rainwater harvesting, and reclamation and reuse. Other resources present in the urban water cycle can also be harvested, including energy, nutrients and other materials.13,15,21 Forms of energy include kinetic (the energy of flowing water), thermal, and chemical (such as the organic matter present in used water). We are all familiar with use of flowing water to generate electricity through hydropower systems. Thermal energy can be recovered from, or discharged to, water using existing heat exchange technology, including heat pumps. Organic matter can be captured from used water in the form of sludge produced through used water treatment and converted into biogas through anaerobic processes. Biogas can subsequently be used for a variety of purposes, such as in combined heat and power (CHP) systems, or upgraded to natural gas quality. Nutrients are recovered when biosolids products are produced for in agricultural use, and phosphorus is already being recovered as the slow release fertilizer product struvite (magnesium ammonium phosphate). Approaches to harvest other forms of carbon, nitrogen, and rare earth metals are also being investigated. Recovery and use of these resources can provide financial and strategic advantages to urban water utilities, along with broader life cycle advantages due to reduced need to extract these resources from the environment. Financial advantages result, both from the revenue generated by the recovered resources, but also because of the costs avoided in used water processing (such as reduced scaling in anaerobic digestion systems when struvite is recovered). Strategic advantages arise when desirable products are produced, rather than residuals (sludge) that are not perceived as useful to society. The result is increased public acceptance for the processing and management of these materials, rather than disposal.

I.2.3  Integrated Systems

The individual components of One Water and Resource Recovery systems are then combined into an integrated system that meets the needs of individual urban areas. As compared to the historic approach, forward-looking systems increasingly incorporate distributed components, along with traditional centralized systems.22 This arises because more recently developed treatment technologies (addressed below) allow source waters of various qualities (surface, ground, rain, and used) to be treated to meet the quality requirements for various uses – the concept of “fit for purpose” (as opposed to treating all water to potable quality) water production and use. While the “fit for purpose” concept is compatible with a fully centralized system, it becomes even more economical with a hybrid centralized and distributed system. Water production facilities can be located close to local water sources and areas of demand. For example, used water can be diverted out of the collection system and treated to a quality level appropriate for particular uses, such as irrigation, cooling, and domestic non-potable. Residuals from treatment can be returned to the collection system and conveyed to a larger, centralized treatment facility where recovery of energy and nutrients can be accomplished economically at the larger scale of such facilities. Source separation (separately collecting grey, black, and yellowater) is also an emerging trend which can provide inherent benefits from both resource efficiency and recovery perspectives.23

Figure 1 provides an illustration of such an integrated system incorporating centralized and distributed components. Both potable and non-potable water supplies are provided to municipal, commercial, and industrial customers. This example illustrates these water supplies being provided by local non-potable and potable water aquifers. Water supplies are supplemented, either directly or by supplementing the non-potable aquifer, by rainwater harvesting, stormwater infiltration, and wastewater reclamation (largely from greywater). Blackwater and yellowater are collected separately for resource recovery. Heat is recovered from the used water stream and the non-potable aquifer. Salts added through water use are concentrated into a saline water stream that is exported to a saline water aquifer. While not all components incorporated in this illustration will be included in all systems, the concept is illustrated.


New technologies and improved practices continue to develop and enable the integrated systems described above. While further technological advances are occurring and expected, Table 2 lists existing, well-developed technologies and practices that are currently enabling the systems described above. Technologies such as advanced oxidation, membranes, and ultraviolet (UV) treatment can be applied at various scales and with various water sources (ground, surface, rain, and used water) to produce product water meeting a wide range of fit-for-purpose quality requirements.24 The modular nature, performance resilience, and ability to remotely monitor performance allows these technologies to be applied at a wide range of scales, from small distributed to large centralized applications. Membranes can be coupled with biological treatment systems when treating waters containing biodegradable organics, forming the membrane bioreactor (MBR) and anaerobic membrane bioreactor (AnMBR) processes.25 Anaerobic systems can also be applied to a wide variety of water types and scales (distributed to centralized) to remove biodegradable organics with minimal energy input and recover the embedded chemical energy by conversion to biogas. Thermal hydrolysis (THP) is used in larger-scale centralized systems to pre- treat organic material prior to anaerobic treatment, thereby increasing biogas yield and reducing anaerobic treatment system size. Struvite precipitation can be applied at local (distributed) or centralized scales to recover phosphorus through conversion to fertilizer.

Source separation and fecal sludge management are alternatives to the traditional approach. Greywater is relatively uncontaminated (compared to blackwater and yellowater), and often represents the largest used water volume. Separate collection of greywater results in a water supply that requires less treatment than the combined used water stream, thereby allowing use of less energy- and chemical-intensive treatment systems to produce fit-for- purpose water supplies. Implementing this approach using many small-scale, distributed collection and treatment systems minimizes piping to collect the separated greywater and distribute the product water produced by appropriate treatment systems. Yellowater represents less than 1 percent of the combined used water volume but contains about

60 percent of the phosphorus and nearly 80 percent of the nitrogen. Diversion of this small volume, high nutrient concentration stream simplifies treatment of the remaining used water, and allows for increased capture of the nutrients it contains for reuse. Blackwater contains much of the organic matter but in a smaller volume, making anaerobic treatment for biogas production more efficient. Fecal sludge management represents application of these concepts in locations where traditional water supply and used water collection are not provided.26 Fecal matter, either with or without urine, is collected and periodically transported to a centralized location for processing to recover energy and nutrients in a manner which is protective of public health and the environment. Separate collection of fecal matter and urine further enables resource recovery.


The system components, technologies, and approaches described above are in various stages of development and application, but most have a significant number of full-scale applications in numerous settings. Advanced oxidation, membrane systems, and UV technologies are now widely applied in a variety of applications. Advanced oxidation is increasingly applied in advanced water treatment and water reuse applications, and it is receiving increased consideration for the control of micro-constituents (e.g. pharmaceuticals, hormones) in used water discharges. Membrane systems (micro- filtration, untra-filtration, nanofiltration, and reverse osmosis) have become standard technologies, applied in a wide range of treatment applications, and aerobic MBR’s have become a standard biological treatment technology, especially for water reuse applications. Anaerobic systems are widely used in industrial treatment applications and is a standard technology for the stabilization of the organic sludges produced in used water treatment. Interest in anaerobic systems for direct treatment of used water continues to grow. THP is increasingly used to pre-treat organic sludges produced in used water treatment prior to anaerobic treatment. A number of specific technologies to recovery phosphorus by struvite precipitation are available, and the number of installations is increasing rapidly.

Distributed system components are increasingly being added to existing centralized systems to increase capacity, improve level of service, increase resilience to the impacts of climate change, improve resource use efficiency, and improve resource recovery. Distributed rainwater capture and natural rainwater treatment systems which infiltrate captured water into local aquifers add to local water supplies and mitigate flooding and pollution caused by uncontrolled run- off. A significant number of applications already exist, and further applications are progressing on a global basis. These systems provide further value to their subject urban areas, for example by improved recreation and aesthetics along with reduced heat island effect. Water reclamation and reuse facilities provide a drought-resistant water supply while reducing pollution discharges. Locating such facilities adjacent to fit-for-purpose water demands that can be met with available quantities of used water reduces used and reclaimed water conveyance requirements. The concept of “sewer mining”, i.e. locating a water reclamation facility to meet local fit-for-purpose water supplies, is a well-established practice in several locations, including the arid Southwestern U.S. and Australia. Adding distributed system components in this fashion can supplement existing centralized systems and allow them to serve increasingly dense urban areas without the disruption associated with expanding the centralized system water distribution and used water collection system. Source separation can be incorporated into new construction and as existing buildings are renovated.

Separate greywater collection and treatment for reuse has been applied in such diverse locations as China (Qingdao) and California (San Francisco). Full- scale examples of urine diversion are just beginning to appear, but include examples in the U.S. and Europe (i.e. Paris).

Peri-urban areas can be served by distributed systems when a centralized system is either not present, or it is not cost-effective to extend the centralized system to the newly developing area. Fecal sludge management approaches can provide effective sanitation, resulting in the protection of public health and the environment. This approach is particularly applicable in locations such as informal settlements where conventional water supply may not be available, but is also certainly applicable when greywater is separately collected and managed as a local water supply. Examples are emerging rapidly, for example in sub- Saharan Africa. Combining distributed and centralized system components allows for phased upgrade and expansion of the urban water system as demand and the desired level of service increases. The success of these hybrid centralized and distributed systems is resulting in greatly expanded implementation. These systems are expected to become the norm over the next decade or two.


While new technologies will continue to develop, current technology is sufficient for the continued implementation of the One Water and Resource Recovery focused hybrid centralized and distributed approaches described above. A period of 15 to 20 years is generally required for new technologies to become material in the water sector, and significant changes in practices require even longer.27 Thus, it is unlikely that newly developing technologies will become material over the next 10 to 15 years, say by 2030. Technologies currently being translated into practice are generally consistent with the overall approach described above and, consequently, are unlikely to change the general direction of change and, most likely, will accelerate it. One trend that is expected to become material within this timeframe is the broader application of sensors, coupled with “big data” approaches to manage and optimize the use of both centralized and distributed infrastructure. Already a trend, these developments will serve to enable and accelerate implementation of these more complex and integrated, but higher performing, systems. Improved monitoring and analysis will also result in increased insights relative to superior approaches for integrating system components, leading to further improvements. These advances, coupled with the general learning resulting from the increasingly widespread application of these approaches, will further accelerate their evolutions and rate of adoption.


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Glen T. Daigger

Dr. Daigger is currently Professor of Engineering Practice at the University of Michigan and President and Founder of One Water Solutions, LLC, a water engineering and innovation firm. He previously served...

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