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Author: Glen T. Daigger
The challenge of effective water management can also be an opportunity for enhancing the urban environment.
The National Academy of Engineering included urban water supply in the top five engineering achievements of the 20th century (Constable and Somerville, 2003), and in a survey by the British Medical Journal (2007), sanitation was the single most important contributor to improving public health in the past 150 years. Clearly, efficient water management is crucial to public health, a viable economy, and a livable urban environment.
Effective, efficient management of water resources is essential to a sustainable urban area. Water must be supplied for domestic, commercial, and industrial use, as well as irrigation and maintaining and enhancing local environments (e.g., urban streams). In addition, storm water must be managed to prevent flooding and environmental damage, and used water, which contains heat, organic matter, nutrients, and other constituents that can be extracted and reused, must be collected and managed.
Historically, with the exception of certain locations, such as the desert Southwest of the United States, water has been available in sufficient quantities, and providing supporting infrastructure has been relatively straightforward (Novotny and Brown, 2007; Solomon, 2010). Governments plan, implement, operate, and manage the physical assets (infrastructure) and institutions (urban water management utilities) to ensure adequate water management services, often the single most costly infrastructure investment by a municipal government. As populations increase, however, water is becoming increasingly scarce, leading to competition among users, one of which is urban areas (Daigger, 2007).
Enlightened professionals are also becoming increasingly aware that water is not only an essential public service, but can also be a vehicle for enhancing the urban environment (Novotny and Brown, 2007). For example, managing storm water by taking advantage of natural systems not only relieves the burden on infrastructure, but also enhances natural areas, reduces heat-island effects, and contributes to a pleasing, livable urban environment. In short, the challenges of providing urban water management services, which some consider a problem, can also be considered an opportunity for enhancing the urban environment. For example, the International Water Association Cities of the Future Program (IWA, 2011) promotes the idea of water-centric urban design (Hao et al., 2010; Novotny and Brown, 2007).
In this article, I describe a new approach to supplying and managing water and resource infrastructure to achieve urban sustainability. Examples of system components are also identified, as are challenges to implementing higher performing systems.
Sustainable Urban Water and Resource Management
Urban water and resource management involves the following steps: collecting water in sufficient quantities to meet needs throughout the urban area; treating collected water to achieve the quality required for specific purposes; distributing water to end users; collecting used water; treating used water for reuse, including for environmental enhancement; managing residuals from treatment processes; and extracting useful materials, such as heat, energy, organic matter, and nutrients, from the used water stream.
This approach differs from the historical approach in several respects (Daigger, 2009). First, water-supply options today include not only imported surface and groundwater, but also locally collected rainwater (“rainwater harvesting”) and used water for reclamation and reuse. Second, all used water is reused, either to meet water-supply needs or to enhance and restore the environment.
Finally, the waste stream (used water) is no longer viewed as a necessary “evil” that must be managed to minimize harm. Instead, it is considered a resource from which useful products can be extracted. Heat can be extracted directly. Organic matter can be removed and used for energy production and the production of soil-conditioning products. Nutrients can also be extracted and re-used.
Single purpose systems for drinking water, storm water, and used water
Infrastructure for implementing this newly defined system requires a significantly different approach to urban water and resource management (Table 1). For several reasons, water supply has historically depended on the importation of sufficient quantities of relatively pristine water from remote sources. First, because of the lack of pollution-control systems and technologies, local water supplies inevitably became polluted, making it impossible to produce safe drinking water in sufficient quantities. Thus, remote sources of water had to be imported.
This situation has changed, however, most importantly because of the development of effective treatment technologies that can produce clean water from a wide variety of sources (Daigger, 2003, 2008). In addition, the availability of remote, pristine source waters has diminished greatly in comparison to the human population. These trends have combined to make the use of local water supplies necessary.
Second, the historic system evolved when water was abundant and energy was inexpensive. Thus, the least expensive systems were those that optimized infrastructure costs. Moreover, because water was inexpensive, economies of scale led to the selection of systems focused on meeting demand rather than managing consumption, which often also increased water use. Today, with limited water supplies and expensive energy, evolving systems focus much more on increasing water efficiency and minimizing energy use. Water demand is also managed to ensure a sufficient supply.
Third, water managements systems have evolved from single-purpose to multipurpose systems. Urban water and resource management systems were historically implemented sequentially as specific needs were identified and funding was obtained. As a result, systems for handling drinking water, storm water, and used water were often separate (except for sewers, which collected and conveyed both storm water and used water). These separate systems also provided services independently. Today, we know that many benefits are provided by integrating these functions into a single system (Daigger, 2008, 2009; Hao et al., 2010; Novotny and Brown, 2007).
Finally, systems have evolved from a centralized to a hybrid configuration that includes both centralized and decentralized components (Daigger, 2009). Because water was historically imported from outside the urban area, the most cost-effective infrastructure was a single or small number of systems, referred to as centralized systems, that treated and distributed water throughout the urban area. Similarly, because storm water had to be collected and removed from the urban area (because it was polluted), the most cost-effective approach was a single or small number of collection and conveyance systems. The same held true for the used water system.
The following sections address two key components of the evolving urban water and resource management infrastructure paradigm: (1) hybrid systems; and (2) water-supply and used-water source separation.
Hybrid Systems
Enabled by improved treatment technologies, local water resources are becoming increasingly usable, and treatment systems are being distributed throughout service areas. The resulting hybrid systems include both centralized and decentralized components. Three illustrative examples of hybrid systems are described below.
Green Infrastructure
Traditional, centralized storm water-management systems generally consist of drains and collection points that direct rainwater into pipes that convey it to existing streams and waterways. The objective is to collect and remove storm water rapidly to prevent local flooding.
Although these systems achieve their objective, they also have side effects. First, storm water picks up pollutants from urban surfaces, and those pollutants are thus conveyed into local waterways. Second, by inhibiting the infiltration of rainwater into the local groundwater, the system ultimately depletes local water resources. Third, fast-moving water directed through local waterways causes significant erosion.
Analyses of the hydrology of urban areas have shown that rainfall often has two principal components: (1) long-duration, low-intensity storms that produce significant volumes of storm water, which tends to build up pollution because of their frequency; and (2) short-duration, high-intensity storms, which are less frequent and therefore carry less pollution overall (Daigger, 2009).
Distributed storm water management and rainwater-harvesting systems collect rainwater and direct it either to storage areas for later use or to natural systems (e.g., swales and bioretention structures) that reduce the velocity of the water, infiltrate rainwater into the ground, and thereby remove pollutants. Such systems are often referred to as “green infrastructure,” because (1) they typically rely on plants to control pollution and (2) they maintain or increase local water resources by recharging groundwater.
Although peak storm water flow rates are reduced by the processing capacity of distributed systems, rapid conveyance is usually necessary to deal with high-intensity storms. These supplementary systems, coupled with healthy local streams and waterways, prevent erosion.
Green infrastructure systems also have added benefits, such as the restoration of local ecosystems, reductions in urban heat-island effects as a result of replacing impermeable surfaces with natural surfaces that reflect less heat, and a more aesthetic, livable urban environment. Progressive urban areas (e.g., Portland, Oregon; Seattle, Washington; Philadelphia, Pennsylvania) are adopting this hybrid approach.
Used-Water Reclamation and Reuse Systems
Another hybrid system is the distributed used-water reclamation and reuse system (Jimenez and Asano, 2008). Driven by the increasing scarcity of water and enabled by modern treatment technology, used water is increasingly being reclaimed and reused in a variety of ways. Non-potable water produced from used water can be reused for irrigation and to supply industry. Potable water is achieved by treating used water to levels beyond those required for typical drinking water and introducing it into the local ground or surface water, where it mixes with the existing water supply. Water can then be withdrawn for further treatment and distribution.
Used-water reclamation and reuse require extensive collection and distribution systems, especially for centralized, non-potable systems that necessitate separate distribution for potable and non-potable water. The need for dual water-distribution systems presents a significant cost barrier, especially in existing urban areas, as well as a substantial increase in energy to convey both water supplies.
In distributed used-water reclamation and reuse systems, treatment facilities are located adjacent to used-water pipelines. When sufficient capacity has been reached, enough used water can be removed and reclaimed to meet non-potable water demands in a modest service area. This approach not only reduces the size of the non-potable water-distribution system, but also reduces the required size of the used-water conveyance system downstream of the diversion point. Thus, significant system savings in cost and energy can be realized.
In-Home Treatment Devices
In-home devices can also be used to provide potable water. Generally purchased and installed on an elective basis by individual homeowners, these devices provide good quality water for most purposes, thereby reducing the amount of water that must be treated to a higher standard for truly potable purposes. In fact, the widespread consumption of bottled water represents another approach to achieving the same end.
Separation of Potable and Non-potable Water Supplies
We now turn to systems that provide separate non-potable and potable water supplies (Jimenez and Asano, 2008). A relatively small volume of water, on the order of less than 40 liters per person per day (L/capita-day), is needed for truly potable purposes (e.g., direct consumption and food preparation). A much larger volume of water, ranging from 100 to 400 L/capita-day, is used for other purposes (e.g., laundry, toilet flushing, bathing, and outdoor water use).
When water supplies were pristine and required little or no treatment to meet potable water standards, the provision of separate potable and non-potable supplies made little sense. Today, however, pristine water supplies are limited, and alternate water supplies must be used. However, because of the small amount of water used for drinking and cooking, treating all water to potable standards makes little sense. Moreover, water quality deteriorates in the distribution system. Thus water exiting a drinking-water treatment plant may exceed potable water-quality standards, but water that reaches the consumer may not.
One approach to addressing this problem, referred to as distributed water treatment (Weber, 2004), has been contemplated but not implemented at full scale. This approach consists of treating water on a centralized basis to non-potable standards, distributing it through a centralized system, and using some of it to supply distributed treatment systems that can treat small, necessary quantities of water to potable standards. A second approach is dual distribution of non-potable and potable water.
Numerous examples can be found of both approaches, but in general, treatment for potable water continues to be centralized, while treatment of non-potable water is either centralized or decentralized. Decentralized systems are less expensive, however, and have the advantage of treating locally harvested rainwater and reclaimed used water to non-potable standards, as required, and distributing it to meet local needs.
Another idea being evaluated and selectively implemented is the separation of used water into various components (Daigger, 2009; Henze and Ledin, 2001). Collecting used-water components made little sense when an abundant water supply was available to convey it, minimal treatment was required to dispose of it, and technologies for recovering resources from it were limited. Today, however, treatment requirements are significant, and many options are available for extracting energy and nutrients from used water.
The logic for separating components is based on an analysis of the domestic wastewater stream (Figure 1). The principal constituents in this used water stream are biodegradable organic matter, expressed by five-day biochemical oxygen demand (BOD5), and nutrients (nitrogen, phosphorus, and potassium). Organic matter can either be treated by conventional technology, which requires significant amounts of energy, or it can be used as a source of energy in and of itself. The nutrients have obvious value for agriculture if they can be recovered in a useful form. Figure 1
Figure 1 also shows that the three principal contributors to the used water stream are grey water, black water, and yellow water. Grey water, which is used for laundry, bathing, and similar purposes, is the largest volume of domestic water. It also contains the most heat, as is apparent from the way it is used. When separated out, grey water, which is only modestly polluted, can be readily treated to non-potable standards.
Black water (feces) and kitchen waste, a relatively small volume of water, contain most of the organic matter in used domestic water. A variety of technologies are available for converting this organic matter into useful energy.
Most of the nutrients in used water are contained in yellow water (urine), less than 1 percent of the total volume of used water (generally 1 to 2 L/capita-day). Because the body excretes most unused pharmaceuticals and hormones through the kidneys, yellow water also contains a disproportionate amount of these materials. Thus separating out yellow water reduces the treatment required for grey and black water.
Challenges and Opportunities
Even though much remains to be learned about the potential of highly integrated urban water and resource management systems, we already know they have significant advantages. The direction of change is clear—from centralized, single-purpose components to hybrid, integrated systems.
The transition from past, centralized systems to hybrid, integrated systems presents many challenges (Table 2). In the past, the various components of urban water and resource management systems were managed separately, often by different utilities or different departments in a utility. Integrated systems will require a new management structure, hence institutional reform.
Professional education and practice have also historically been organized according to system components (e.g., drinking water, storm water, and used water). To accelerate the transition to integrated, higher performing systems, education, professional practice, and utilities’ institutional structures must also become more integrated.
Management of a hybrid, integrated system is necessarily more complex than management of a traditional system. Distributed water treatment and integrated potable and non-potable water supplies, storm water, and used water significantly increase the complexity of management and will require the development of new managerial systems.
In the future, the responsibility for system management may also be in private rather than public hands, which has raised concerns that public health and environmental protection might be compromised. Fortunately, we already have technology to manage increasingly distributed systems, and this technology will no doubt improve with “learning by doing.”
Urban water and resource management utilities have traditionally been financed on the basis of “volume”—the volume of water sold and the volume of used water collected and treated. As happened with electrical utilities, when the volume sold decreases, financial resources for the utility also decrease, which may compromise the availability of financing for new systems. Thus we will need a funding approach based on service rather than volume.
The most difficult challenge, however, may be associated with planning and implementing urban infrastructure as a whole. Historically, the planning and expansion of urban areas occurred with minimal consideration of water and resource management, often with the assumption that traditional, centralized systems would be used.
However, evidence is accumulating that water can be a central feature of sustainable urban areas, and the concept of water-centric urban areas is becoming more common (Hao et al., 2010; Novotny and Brown, 2007). Achieving this vision will require that water professionals become strategic partners with urban planners. The International Water Association Cities of the Future Program promotes such partnerships (IWA, 2011).
Conclusions
In conclusion, urban water and resource management systems are evolving in a clear and unified direction: (1) from the use of remote water supplies to the use of local water supplies, such as rainwater and reclaimed used water; (2) from optimizing the cost of infrastructure to optimizing water use, energy production, and nutrient extraction; (3) from independent, single-purpose components to integrated, multi-purpose systems; and (4) from centralized systems to hybrid systems that incorporate centralized and decentralized components. These changes are necessitating changes in institutions, system management, financing, and urban planning.
References
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