At present, 80.7% of the US population resides in urban areas (US Census Bureau 2012). Increased urbanization (paved areas and buildings) and urban population growth has exerted significant pressure on urban water demand and expansion of urban water infrastructure—i.e., potable water supplies, wastewater treatment and discharge, and urban stormwater runoff control. These infrastructure characteristics include: (1) centralized and large-scale systems serving large areas and populations; (2) dependence upon water and energy coming mostly from sources outside urban boundaries; (3) generation of significant volumes of wastewater; (4) generation of high volumes of stormwater runoff; (5) dependence upon extensive pipe networks that deliver potable water to consumers and drainage network to transport wastewater and stormwater runoff away from population centers; and (6) urban water-infrastructure traditionally planned and designed as separate systems.
Recognized problems associated with conventional urban water infrastructure include: (1) surface water pollution caused by urban stormwater runoff and wastewater discharge; (2) declining groundwater table caused by reduced infiltration and high water use; (3) saltwater intrusion in coastal aquifers caused by declining fresh groundwater table; (4) contaminant intrusion into tap water via water pipelines; (5) losses of energy intensive treated potable water via water pipeline leakage (20 to 30%); (6) increased competition for other urban uses of water such as aesthetics and recreation; and (7) high energy consumption and potential impact on climate change.
Urban water infrastructure consumes a significant portion of total national energy use. For example, in the US, about 56 billion kilowatt-hours (kWh) or 4% of total energy consumption is attributed to water and wastewater services (CRS 2013). The energy used for water treatment and delivery in the US is in the range of 0.07 to 0.92 kWh/m3 with an estimated average of 0.38 kWh/m3 (AWWARF 2007). Furthermore, fossil-fuel-based electricity used by water utilities can be a significant source of CO2 emission. As an example, Table 1 shows the annual carbon footprint of water consumption attributed to fossil-fuel-based electricity for single buildings (Younos et al. 2016).
Challenges facing global urban water management demand a significant need for a paradigm shift and new thinking to cope with the existing and emerging problems. The new paradigm in urban water management should accomplish the following objectives:
- Prevent source water pollution,
- Preserve groundwater quantity and quality,
- Prevent water supply leakage and potable water contamination via pipelines,
- Develop and use alternative water sources such as rainwater and wastewater,
- Energy use efficiency and conservation, and
- Develop and use renewable energy sources for water treatment and delivery.
These objectives can be achieved by developing a holistic approach integrating the interconnectedness of natural and engineered systems into urban water infrastructure system planning and design. The major components of the envisioned system include:
- Implementing decentralized water infrastructure using locally available water (captured rainwater, stormwater runoff, wastewater, salt water, and brackish water),
- Integrating innovative decentralized small-scale advanced water treatment technologies,
- Integrating renewable energy (solar, wind, micro-hydro power, geothermal, biomass, other), and
- Using locally available water and renewable energy sources in urban agriculture.
Figure 1. Rooftop rainwater capture and use in southwest Virginia mountaintop residential area
Captured rainwater has been used for household drinking with minimal treatment in the rural US and other countries for a long time. For example, Figure 1 shows rooftop rainwater capture and use in a mountaintop community of southwest Virginia (Younos et al. 1998). Recent small-scale water treatment technologies have made captured rainwater a suitable substitute for household potable water use (Hammerstrom and Younos 2014).
Advances in rooftop rainwater harvesting system technology have facilitated its implementation as a decentralized green technology in commercial buildings, schools, and other sites (Sojka et al. 2016). Captured rainwater is utilized for indoor uses such as flushing toilets, cooling, and laundry, with only a few cases of rainwater use for potable purposes. An excellent example is the Bullitt Center Living Building in Seattle (Figure 2). The building is designed as a net-zero water site—i.e., a site must not import water from outside the site or discharge water off the site. Potable-rainwater harvesting system is a key feature of the Bullitt Center (Morton 2013; One year in 2014).
Figure 3. Rainwater harvesting system at Hurt Park Community Garden, Roanoke, VA
Outdoor uses of captured rooftop rainwater include irrigation water for community gardens in food production. Figure 3 shows a typical community garden, which uses captured rooftop rainwater.
Recent innovative decentralized uses of captured surface stormwater runoff include using stormwater runoff for landscape maintenance and city aesthetics. Figure 4 shows installation of a 250,000-gallon (950,000-liter) cistern beneath the National Mall in Washington, D.C. for turf irrigation (NAP 2016). Figure 5 shows an ornamental water feature at the Cincinnati Zoo replenished by a stormwater capture system.
Figure 3. Rainwater harvesting system at Hurt Park Community Garden, Roanoke, VA
Advanced small-scale decentralized water treatment technologies allow decentralized systems to reuse wastewater and function as standalone infrastructure in locations such as shopping centers, high rise buildings and hotels, and dormitories and university campus buildings. A recent National Academies Press publication provides significant details on beneficial uses of greywater and stormwater including risk assessment, costs, and benefits (NAP 2016). Decentralized wastewater treatment and recycling is an attractive option in high-density population urban areas where water scarcity is prevalent. For example, Chen at al. (2016) discussed case studies and potential for wastewater recycling and reuse in Beijing’s highrise hotels. Figure 6 shows a typical example of a wastewater treatment unit in a Beijing hotel where 30% of generated wastewater is recycled within the building.
Figure 4. Captured Stormwater for Turf Irrigation, National Mall, Washington, DC
CONCLUSION
Centralized and conventional urban water-infrastructures are not only inadequate from water pollution control and water and energy conservation perspectives, but also from aging. These infrastructures are experiencing significant degradation, and upgrading of these systems has become cost prohibitive. These conditions provide a significant opportunity for implementing innovative solutions and transition to sustainable water management in urban environments.
The drought and water scarcity in California has pushed local governments and utilities to consider wastewater reuse as an alternative water source, and rainwater harvesting is encouraged as a water conservation measure. Las Vegas has established a wastewater reuse system. Portland, OR, is one of few cities that promote large-scale rainwater harvesting in commercial buildings.
Figure 4. Captured Stormwater for Turf Irrigation, National Mall, Washington, DC
Energy and water conservation resulting from decentralized systems reduces carbon dioxide emissions attributed to water consumption in urban environments and will mitigate climate change impact. Incorporating decentralized systems in urban areas also strengthens the nexus between water, food, and energy by promoting small agricultural plots and green roof projects in inner cities (FAO 2010). The implications are water and energy conservation, low-cost food production, and job creation in inner urban areas.
A few cases of integrating renewable energy in centralized water and wastewater treatment plants do exist (USEPA 2009). However, these are exceptions, and mostly not a component of a holistic approach that integrates green decentralized systems within the overall urban water management and sustainability. Futuristic decentralized systems will incorporate locally available, renewable energy resources (solar, wind, biomass, other) for small-scale water treatment and delivery in urban buildings and landscape irrigation.
Figure 5. Captured stormwater use in Cincinnati Zoo.
Across the US, perception, regulatory, and technical hurdles must be overcome. In many localities, captured rainwater is perceived as greywater, and citizens are reluctant to accept wastewater reuse as an alternative potable water source. Updating policies in zoning ordinances, and economic incentives to promote and, perhaps, mandate implementing green decentralized water infrastructure are needed. These challenges can be overcome by offering workshops and information seminars to municipal and local government water planners and engineers, regulators, and policy makers. A significant need also exists to upgrade college level curricula in educating and training future planners, scientists, and engineers, not only on technical and regulatory aspects of the urban water infrastructure, but also on understanding the interconnectedness of natural and engineered water systems that can be achieved by implementing decentralized green water infrastructure.
REFERENCES
AWWARF 2007. Energy Index Development for Benchmarking Water and Wastewater Utilities. American Water Works Association Research Foundation, Denver, CO.
Chen, Y., L. Zhu, J, Che et al. 2016. Reclaimed Water Use and Energy Consumption: Case Study in Hotel Industry, Beijing. Sustainable Water Management in Urban Environments (T. Younos and T.E. Parece, Eds.), Handbook of Environmental Chemistry (Chapter 3), Springer, Heidelberg, Germany, 47:57–82.
FAO 2010. Water Management for Urban and Peri-Urban Horticulture. Food
and Agriculture Organization of the United Nations, Rome, Italy. www.fao.org/nr/water/docs/fao_urban_agri_factsheet.pdf
Hammerstrom J., T. Younos 2014. “Single-Family Home Rainwater-Harvesting System Demonstration Project for Stormwater-Runoff Control and Utility-Water Saving.” The Cabell Brand Center Special Report No. 100–2014. www.cabellbrandcenter.org.
Morton, J. 2013. “A Path to Net-Zero Water.” Buildings 107(8):28.
NAP 2016. “Using Graywater and Stormwater to Enhance Local Water Supplies: An Assessment of Risks, Costs, and Benefits.” Committee on the Beneficial Use of Graywater and Stormwater: Water Science and Technology Board; Division on Earth and Life Studies; National Academies of Sciences, Engineering, and Medicine. www.nap.edu/21866.
One Year In 2014. Bullitt Center Exceeds Energy Goals, Architectural Record, 202(6):32.
Figure 6. Wastewater recovery, recycling, and reuse in a Beijing hotel
Parece, T. E., M. Lumpkin, J. B. Campbell 2016. “Irrigating Urban Agriculture with Harvested Rainwater: Case Study in Roanoke, Virginia, USA.” The Handbook of Environmental Chemistry Vol. 47. 351pp. Springer Publishers. pp. 235–264.
Sojka, S., T. Younos, D. Crawford 2016. Modern Urban Rainwater Harvesting Systems: Design, Case Studies, and Impacts. The Handbook of Environmental Chemistry Vol. 47. 351pp. Springer Publishers. pp. 209–234.
USEPA 2009. Massachusetts Energy Management Pilot Program for Drinking Water and Wastewater Case Study. US Environmental Protection Agency, EPA-832-F-09-014.
Younos T., R. Bohdan, E. Anderson, K. Ramsey, N. Cook, B. Ross, T. Dillaha, J. Poff 1998. “Evaluation of Rooftop Rainfall Collection-Cistern Storage Systems in Southwest Virginia.” VWRRC Special Report SR3-1998., Virginia Tech, Blacksburg, VA. 41 pp.
Younos, T., K. O’Neill, A. McAvoy 2016. Carbon Footprint of Water Consumption in Urban Environments: Mitigation Strategies. The Handbook of Environmental Chemistry Vol. 47. Springer Publishers. pp. 33–56.
