climate

REVIEW

Taking the “Waste” Out of “Wastewater” for Human Water Security and Ecosystem Sustainability Stanley B. Grant,1,2* Jean-Daniel Saphores,1,3 David L. Feldman,3 Andrew J. Hamilton,4

Tim D. Fletcher,5 Perran L. M. Cook,6 Michael Stewardson,2 Brett F. Sanders,1 Lisa A. Levin,7

Richard F. Ambrose,8 Ana Deletic,9 Rebekah Brown,10 Sunny C. Jiang,1 Diego Rosso,1

William J. Cooper,1 Ivan Marusic11

Humans create vast quantities of wastewater through inefficiencies and poor management of water systems. The wasting of water poses sustainability challenges, depletes energy reserves, and undermines human water security and ecosystem health. Here we review emerging approaches for reusing wastewater and minimizing its generation. These complementary options make the most of scarce freshwater resources, serve the varying water needs of both developed and developing countries, and confer a variety of environmental benefits. Their widespread adoption will require changing how freshwater is sourced, used, managed, and priced.

M ore than 4 billion people live in parts of the world where freshwater scarcity directly threatens human water secu-

rity or river biodiversity (1). Threats to human water security can be overcome by building cen- tralized infrastructure that harvests, stores, treats, and transports water for agricultural, industrial, and municipal uses. For countries that can af- ford it, this approach has greatly benefited hu- man health and economic development, but it is often energy-intensive and comes at a steep ec- ological price. In the developing world, on the other hand, an estimated 1 billion people lack access to safe affordable drinking water, 2.7 bil- lion lack access to sanitation, and many millions die each year from preventable waterborne dis-

eases (2). Thus, developed and developing coun- tries face separate but overlapping challenges. In developed countries, existing water infrastruc- ture needs reengineering to sustain a high stan- dard of living while reducing its environmental footprint and sustaining or restoring biodiver- sity. In developing countries, affordable infra- structure is needed to satisfy the water needs of humans and to preserve aquatic ecosystems (1). Meeting these twin challenges will require strik- ing a balance between delivering new sources of water and using water more productively through pricing, conservation, and wastewater reuse.

How Is Water Used and Wasted? Water use can be classified as consumptive or nonconsumptive, depending on how readily the used water can be reused. Consumptive use con- verts water into a form that cannot be reused. A portion of the water used for irrigation, for ex- ample, is evaporated, transpired, and incorpo- rated into plant biomass. This consumed water is unavailable for reuse in the watershed over time scales of practical interest. In contrast, after non- consumptive use, water can be captured, treated, and reused. If a nonconsumptive use degrades the quality of the water (for example, by adding contaminants), it is said to generate wastewater. An example of nonconsumptive use is the flush- ing of a toilet, which converts drinking water into domestic wastewater. In principle, domestic wastewater can be collected, treated to remove human pathogens and other contaminants, and then reused for potable or nonpotable purposes. Globally, the largest consumptive use of water is for agriculture, whereas the largest nonconsump- tive use of water is for industrial and municipal supplies (3).

What Is Water Productivity and How Can It Be Improved? Addressing threats to human water security and biodiversity will require getting the most out of locally available water resources. But what does that mean in practice? One way to evaluate wa- ter use is to consider its “productivity,” defined as the value of goods and services produced per unit of water used. By improving water produc- tivity, communities can enjoy the same goods and services, generate less wastewater, and leave more freshwater in streams, rivers, lakes, and coastal estuaries to support biodiversity. Because less water is harvested, treated, and transported, fossil fuel consumption and greenhouse gas emis- sions are reduced. Although water productivity has steadily improved in the United States since the mid-1970s, additional gains are possible both here and around the world (4). In this Review, we focus on three general strategies for improving water productivity (Fig. 1): substituting higher- quality water with lower-quality water where ap- propriate, regenerating higher-quality water from lower-quality water by treatment, and reducing the volume of higher-quality water used to gen- erate goods and services.

What Are the Opportunities for Substituting? Many municipal, industrial, and agricultural uses can be satisfied by lower-quality water. For ex- ample, treated domestic wastewater that would not be suitable for municipal water supplies may be perfectly suitable for industrial cooling and landscape irrigation, to name a few (5). Although the use of treated wastewater in the United States is currently limited (<5% of municipal supply), it could be expanded to 17 teraliters per year (Tl year–1) (~27% of municipal supply), providing a new drought-resistant source of water in coastal areas where treated wastewater is currently dis- charged to the sea (6). Large-scale (centralized) wastewater treatment and potable substitution schemes can reduce overall energy consumption and reduce greenhouse gas emissions. In southern California, substituting potable water with treated wastewater consumes less energy and generates fewer greenhouse gases as compared to interbasin transfers of water or desalination of seawater or brackish groundwater (7).

Treated domestic wastewater is not the only lower-quality water that can be exploited in po- table substitution schemes. Hong Kong’s dual wa- ter system, which has been in operation for over 50 years, supplies seawater for toilet flushing to 80% of its 7 million residents, cutting municipal water use in the city by 20% (8). A triple-water distribution system at Hong Kong’s International Airport, consisting of freshwater, seawater, and treated graywater from sinks and aircraft wash- down, cuts municipal water use by over 50% (8).

Potable substitution can also be implemented at neighborhood and single-home scales (Fig. 2). Rainwater (from roofs) and graywater (from

SPECIALSECTION

1Department of Civil and Environmental Engineering, E4130 Engineering Gateway, University of California, Irvine, CA 92697-2175, USA. 2Department of Infrastructure Engineer- ing, Melbourne School of Engineering, Engineering Block D, University of Melbourne, Parkville 3010, Victoria, Australia. 3Department of Planning, Policy, and Design, 202 Social Ecol- ogy I, University of California, Irvine, CA 92697-7075, USA. 4Department of Agriculture and Food Systems, University of Melbourne, 940 Dookie-Nalinga Road, Dookie College, Victoria 3647, Australia. 5Melbourne School of Land and Environment, University of Melbourne, Burnley Campus, 500 Yarra Boule- vard, Richmond, Victoria 3121, Australia. 6Water Studies Centre, School of Chemistry, Monash University, Victoria 3800, Australia. 7Center for Marine Biodiversity and Conservation, Scripps Insti- tution of Oceanography, La Jolla, CA 92093-0218, USA. 8En- vironmental Science and Engineering Program, University of California, Los Angeles, CA 90095-1772, USA. 9Monash Water for Liveability, Department of Civil Engineering, Building 60, Monash University, Victoria 3800, Australia. 10Monash Water for Liveability, School of Geography and Environmental Sci- ence, Monash University, Clayton, Victoria 3800, Australia. 11Department of Mechanical Engineering, Melbourne School of Engineering, Engineering Block E, University of Melbourne, Parkville 3010, Victoria, Australia.

*To whom correspondence should be addressed. E-mail: [email protected]

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laundry, dishwashing, and bathing) can be used in place of drinking water for a variety of activ- ities. The reuse of graywater for toilet flushing and yard irrigation can cut household municipal water use by 50% or more (9). The energy cost, water savings, and reliability associated with rain- water harvesting depend on engineering consid- erations (e.g., contributing roof area and storage tank volume), local climate, connected end uses (e.g., toilet, laundry, and hot water), and temporal patterns (10). In a case study of a model home in Melbourne, Australia, the use of rainwater tanks to supply water for laundry, dishwashing, toilets, and an outside garden reduced household munic- ipal water use by 40% (9). How- ever, even in Melbourne, where rainwater-harvesting schemes are commonplace, they contribute a modest 5 gigaliters (Gl) year−1

to the city’s overall water bud- get, which represents 1.2% of the city’s total water use and 1.4% of its municipal supply (11).

Stormwater runoff from roads and other impermeable surfaces is another locally available source of water, but here the challenge is harvesting and storing the run- off (which can be generated over very short periods of time) and adequately removing contaminants (pathogens, metals, and organic pollutants). These challenges can be overcome through the integra- tion of natural treatment systems into the urban landscape, includ- ing green roofs, rain gardens, bio- filters, and constructed wetlands (12). Processes responsible for pollutant removal in natural treat- ment systems include (12–15) gravitational sedimentation of large particles, pathogen remov- al by solar ultraviolet (UV) inac- tivation and predation, filtration of colloidal contaminants, oxi- dation of labile organics by hy- drolysis and sunlight-generated reactive oxygen species, precip- itation of metals, and nitrogen removal by bacte- rially mediated nitrification and denitrification in sediments. Plants play a key role, taking up excess nutrients and serving as both a source of organic carbon to fuel denitrification, and a source of oxygen through their root systems to fuel nitri- fication. As runoff moves through natural treat- ment systems, a portion of the water returns to the atmosphere (evapotranspiration); a portion infiltrates into the subsurface (groundwater re- charge); and the rest can be harvested, stored, and ultimately used for nonpotable purposes. In Melbourne, stormwater harvesting is a rela- tively minor component (5 Gl year−1 or 1.4% of

municipal water use) of the city’s water budget (11), but including stormwater reuse schemes in new greenfield and brownfield developments until 2050 could result in a sevenfold increase in nonpotable water availability for the city (35 Gl year−1 or 9.8% of municipal water use) (16).

Integrating natural treatment systems into urban landscapes confers many benefits beyond improving human water security. In warmer cli- mates, the evapotranspiration of runoff moder- ates the urban heat island effect (17), whereas infiltration recharges the groundwater and pro- vides environmental water for local wetlands and riparian zones (12). The construction of new

wetlands or reinvigoration of existing wetlands creates habitats for resident and migratory spe- cies and sustains biodiversity by enhancing habitat heterogeneity, connectivity, and food web sup- port (18). When storm water is locally detained and retained throughout the catchment, less run- off enters rivers and streams, pollutant loads are reduced, and flow regimes more closely resem- ble predevelopment conditions (19). As a result, streams are less likely to overtop their banks and cause flooding (20), and the negative effects of urbanization on stream health and function, col- lectively known as the “urban stream syndrome” (21), can be mitigated (22).

What Are the Opportunities for Regeneration? With adequate treatment, higher-quality water can be regenerated from wastewater. Because additional goods and services are produced every time a parcel of water is recycled, regeneration has the potential to significantly increase water productivity. A prime example of regeneration is potable reuse, in which wastewater is treated with conventional and advanced methods and then added back to the water supply either directly (direct potable reuse) or indirectly, by holding the water for a time in groundwater or surface-water reservoirs (indirect potable reuse) (5, 6).

Apart from a few small-scale facilities, direct potable reuse is not practiced in the United States. However, several indirect potable reuse facilities are operational. The world’s largest is the Ground- water Replenishment System (GWRS) in Foun- tain Valley, California, which treats up to 97 Gl year−1 of domestic wastewater using conventional (primary and secondary sewage treatment) and ad- vanced (microfiltration, reverse osmosis, and UV disinfection) techniques (23). Water produced by the GWRS provides approximately 20% of the water needed to maintain the local groundwater aquifer in Orange County, a primary source of mu- nicipal supply for more than 2 million residents.

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a Singapore wastewater reuse

b Eliminate non-revenue water (poorly run utility) b Dual-flush toilets in Florianopolis (best case)

b Dual-flush toilets in Florianopolis (worst case) e High-efficiency toilet

e High-efficiency shower head b Elimination of non-revenue water (well-run utility)

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Scale of interest

Fig. 1. (Left) Three complementary approaches for improving the productivity of higher-quality water. The water level in each glass shows how much water is used in producing a fixed value of goods and services. Substitution uses lower- quality water in place of higher-quality water for some activities. Regeneration transforms lower-quality water into higher-quality water by treatment. Reduction achieves the same value of goods and services using less higher-quality water. In these hypothetical examples, each option cuts by half the use of higher-quality water and therefore doubles its productivity. (Right) Percent increase in water productivity associated with the 21 case studies described in the text (51). These productivity improvements are illustrative only and will vary substantially in practice. The scale at which a particular water-saving intervention was implemented is indicated. The bars are color-coded to match the three general approaches for improving water productivity.

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Internationally, the longest-running example of direct potable reuse is in Windhoek, Namibia, where recycled wastewater (mostly domestic sew- age) has been added to the potable water distribu- tion system more or less continuously since the late 1960s with no obvious adverse health effects among the population of several hundred thou- sand (24). The current facility produces enough water (7.7 Gl year−1) to meet approximately 35% of the city’s municipal water needs.

Among the centralized options for augment- ing potable water supplies, potable reuse is pref- erable to interbasin water transfers for several reasons (25): (i) Interbasin water transfers re- duce the water available at the source for critical ecosystems and agricultural production; (ii) trans- porting water over long distances can be energy- and carbon-footprint–intensive; and (iii) the water transmission systems are vulnerable to disruption by natural and human-made disasters, such as earthquakes and acts of terrorism. All three prob- lems are evident in California, where the southern part of the state has long relied on water imported from sources located hundreds of kilometers to the east and north. In 2001, an estimated 4% of

the electric power consumption in California was used for water supply and treatment (largely trans- portation) for urban and agricultural users; this estimate increases to 7% if end uses in agricul- ture (which are mainly related to pumping) are included (26). The depletion of source waters in the state has led to habitat deterioration, the de- cline and extinction of native fish species, the near-collapse of the Sacramento–San Joaquin River Delta ecosystem (27), and the desiccation of Owens Lake, whose dry lake bed is arguably the single largest source of asthma- and cancer- inducing respirable suspended particles in the United States (28). Potable reuse also has advan- tages relative to the desalination of seawater. By one estimate, potable reuse consumes less than one-half the energy [~1000 to 1500 kilowatt-hours per megaliter (kWh Ml−1)] beyond conventional treatment) required for the desalination of sea- water (~3400 to 4000 kWh Ml−1) (25).

Relative to the classification scheme presented in Fig. 1, some nonpotable wastewater reuse is best described as regeneration, provided that the treated effluent replaces water of equal or lower quality, such as river diversions (Fig. 2). For exam-

ple, 73% of Israel’s municipal sewage is treated and reused for agricultural irrigation, which is equal to roughly 5% of the country’s total water use (29) and 13% of its municipal supply. In Singa- pore, 27 Gl year−1 of highly treated domestic waste- water is used primarily for industrial applications, which is equal to 5% of its total water use and 9% of its municipal supply (30).

Relatively low-energy centralized approaches for nonpotable wastewater reuse are also availa- ble, such as waste stabilization ponds (WSPs), in which sewage is directed through a series of open- air shallow ponds where physical processes (floc- culation and gravitational sedimentation), microbial processes (algal growth, aerobic and anaerobic heterotrophic metabolism, nitrification, and deni- trification), and exposure to sunlight jointly remove pathogens, organic contaminants, and nitrogen (31). Effluent from WSPs can irrigate crops (Fig. 2) or recharge groundwater aquifers, and the ponds themselves may provide a much needed quasi- wetland habitat for waterbird conservation (18). The world’s largest WSP system, the Western Treatment Plant in Melbourne, produces 40 Gl year−1 of treated wastewater, equivalent to 11%

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Fig. 2. Practical examples of substitution (A), regeneration (B), and reduction (C) at the household scale. Substitution includes watering a garden with rain- water from a rainwater tank and flushing toilets and washing laundry with treated stormwater effluent from a biofilter. For regeneration, a waste stabilization pond (WSP) transforms sewage from the house into high-quality water used for irri-

gating an orchard. Reduction includes repairing leaks in the water distribution system, drip irrigation, a dual-flush toilet, a low-flow shower rose, and a front- loading clothes washer. Other water infrastructure elements shown include a conventional drinking water plant (DWTP); a conventional wastewater treat- ment plant (WWTP); and a river diversion (supplying the orchard).

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of Melbourne’s municipal supply, and uses ap- proximately 500 kWh Ml−1 less energy than con- ventional wastewater treatment (32). Recycled water from the Western Treatment Plant is used for a variety of nonpotable applications, includ- ing in-plant uses and dual pipe schemes for the irrigation of agricultural crops, gardens, golf courses, and conservation areas.

Primary concerns associated with wastewater reuse include the buildup of contaminants and salts in soils (in the case of wastewater irriga- tion) and the possibility that incomplete removal of chemical or microbiological hazards during treatment may cause disease in an exposed pop- ulation (6). Disease risk can be evaluated on a case-by-case basis using a statistical framework, such as quantitative microbial risk assessment, that predicts a population’s disease burden, given the types and concentrations of pathogens that are likely to be present in the water, as well as particular exposure scenarios (33).

What Are the Opportunities for Reduction? Water productivity can also be improved by re- ducing the volume of water used to produce a fixed value of goods and services. A modeling study of the water supply system in Florianopolis, Brazil, concluded that replacing single-flush toilets with dual-flush toilets would reduce mu- nicipal water use in the city by 14 to 28% and reduce energy use at upstream (drinking water) and downstream (wastewater) treatment plants by 4 GWh year–1—enough energy to supply 1000 additional households (34). An analysis of 96 owner-occupied single-family homes in Califor- nia, Washington, and Florida concluded that the installation of high-efficiency showerheads, toi- lets, and clothes washers reduced household use of municipal water by 10.9, 13.3, and 14.5%, re-

spectively (35). Because water is not technically required for bathroom waste disposal, the instal- lation of composting toilets and waterless urinals can reduce municipal water use even further (36).

Agriculture accounts for the majority of glob- al freshwater withdrawals (37), and thus even small improvements in water productivity in this sector can result in substantial water savings. Water savings can be achieved by switching to less–water-consuming crops, laser-leveling of fields, reducing nonproductive evaporation of water from soil or supply canals, changing irri- gation scheduling, and adopting more efficient sprinkler systems, including microirrigation tech- niques (drip irrigation and microsprinklers) that precisely deliver water to plant roots (37). These approaches could help mitigate escalating water demand associated with growing energy crops, such as corn, particularly if projected increases in U.S. biofuel production are realized (38).

Drinking water is lost after it leaves treat- ment plants because of physical leaks in urban water distribution systems and poor accounting. Worldwide, the total volume of this “nonrevenue water” is estimated to be 49 Tl per year (39). Pipeline losses range from over 50% in much of the developing world to less than 10% in well- run utilities (39). The World Bank estimates that if just half of the losses in developing countries were eliminated, $1.6 billion would be saved an- nually in production and pumping costs, and drinking water could be extended to an addi- tional 90 million people without the need for new treatment facilities (39).

What Is the Role of Water Quality? Protection of water quality is also a priority. The Catskill Mountains supply drinking water for New York City. When agriculture and residen-

tial developments threatened surface-water qual- ity, the city considered building an $8 billion water treatment plant, but instead opted to spend $1 billion buying land and restoring habitat in the water supply catchment. This approach obviated the need for a treatment plant, saved the city bil- lions of dollars in capital and ongoing operations and maintenance costs, and preserved a critical ecosystem (40).

What Is the Right Mix of Wastewater Reuse and Water-Saving Schemes? The wastewater reuse and water-saving schemes described above are each tailored to a particular scale of implementation (from single homes to entire countries), population density (from urban to rural), and level of technological sophistication (from high-tech to low-tech) (Fig. 3A). Potable substitution schemes using advanced wastewater treatment may be feasible in an urban context, but not in a rural context. Furthermore, no single scheme simultaneously maximizes wastewater re- use, minimizes wastewater generation, and min- imizes stormwater runoff (Fig. 3B). How does a community identify the right mix of schemes that will optimize their water systems? One study eval- uated infrastructure options for a hypothetical res- idential development in the southeast of England, and concluded that every community has a tech- nological state-of-the-art equilibrium beyond which tradeoffs are required (41). Wastewater reuse and water-saving schemes can improve water use, en- ergy use, and land use up to the equilibrium point. Beyond the equilibrium point, further reductions in water use require increasing either energy use (if high-tech options are used) or land use (if low-tech options are used) (41). Human behav- ior should also be considered in the assessments of optimal water management strategies, as was

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rainwater harvesting, graywater reuse,

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Fig. 3. Wastewater reuse and water-saving schemes discussed in the text, arrayed relative to a few key attributes (A) and performance indicators (B). The color scheme matches that in Fig. 1; substituting, red; regenerating, blue; and reducing, green. Certain schemes can be classified in more than one way;

for example, wastewater recycling with wastewater stabilization ponds might be considered blue or red, depending on the end use and what water is being substituted or regenerated. The adoption of water-saving schemes is influ- enced by policy, law, regulations, markets, and incentives.

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done in an elegant systems modeling study of water supply options in Chennai, India (42).

Because end-user behavior affects all aspects of water and wastewater management (Fig. 3B), changing attitudes and expectations may be more effective than finding infrastructure solutions to water scarcity. Turf grass consumes upward of 75% of residential drinking water in arid and semi-arid areas of the United States (43). If wa- ter resources in this region continue to dwindle, reducing the volume of water used for yard ir- rigation (for example, by implementing advanced irrigation technologies) may not be sufficient. Homeowners may have no choice but to replace turf grass with xeric landscaping (44). Such whole- sale rethinking of our relationship with water is an example of a “soft-path” approach to water management. As a general principle, the soft- path approach is characterized by (4, 45) (i) viewing water as a service rather than an end in itself, (ii) adopting ecological sustainability as a fundamental criterion, (iii) matching the qual- ity of water delivered to that needed by its use, (iv) planning from the future back to the present, and (v) ensuring community and citizen involve- ment in water management planning.

What Are the Main Roadblocks? Efforts to improve water productivity will require overcoming economic, planning, regulatory, in- stitutional, and public acceptance challenges. Key obstacles include uncertainty regarding the lon- gevity and maintenance costs of infrastructure; upfront costs for piping, storage, and land; quan- tifying unpriced benefits; and overcoming water underpricing.

The first obstacle will probably resolve as experience is gained by vendors who develop wastewater-reuse and water-saving infrastructure, by community planning agencies that introduce them, and by municipal agencies or households that maintain them.

Underpricing of water is more serious because it leads to excess water demand and revenue short- falls for water utilities. Underfunded utilities tend not to maintain infrastructure or repair leaks, ade- quately treat wastewater (spoiling scarce water re- sources), or extend service to the urban poor, who are then forced to buy expensive water from street vendors (46). Policies that inhibit full-cost pricing for water to ensure social equity or for other reasons may exacerbate this situation (47). When imple- mented in ways that ensure utility accountability to users and fair rate structures, full-cost pricing may help manage water resources sustainably and equitably (48), sending appropriate and realistic price signals that reflect the true cost (including externalities) of water use.

Many of the approaches discussed in this review require fundamental changes to the built environ- ment, which can be promoted through a multi- objective approach that fosters collaboration with developers, a culture of innovation, and community

implementation capacity (49). Distributed water infrastructure can be introduced as part of com- prehensive planning strategies that promote com- pact urban forms with mixed land uses and a focus on urban amenities, encourage alternative forms of transportation to permit narrower streets and reduce demand for parking, foster energy saving and waste recycling, promote water savings, and reduce liability for innovative developers.

Regulatory changes are needed to promote water conservation by mandating water-efficient plumbing, fixtures, and appliances. Regulatory changes are also needed to provide a consistent risk-management framework for water recycling schemes. Australian water reuse regulations, for example, emphasize protecting human health, which may foster a more favorable regulatory en- vironment than in the United States, where wa- ter laws emphasize environmental health (6).

Finally, lessons from wastewater recycling systems indicate that the adoption of new water infrastructure depends on public acceptance. Pub- lic support for wastewater reuse, for example, is higher for uses such as landscape irrigation or car washing that minimize human contact (6). Public acceptance is also a necessary condition for utilities to embrace wastewater reuse and for firms to incorporate it in their production pro- cesses, provided that recycling wastewater nei- ther substantially increases their costs nor triggers additional regulatory scrutiny.

To increase the likelihood of public accept- ance, decision-makers should first demonstrate why changes are required to avert water shortages and that these water-saving schemes are safe. In addition, concerted and sustained efforts must fo- cus on properly maintaining water infrastructure, especially when it pertains to wastewater reuse; on allowing stakeholders to monitor the uses and op- erations of wastewater recycling; and on vigilantly ensuring the protection of public and environ- mental health (50). Overcoming obstacles to the widespread adoption of wastewater recycling and water-saving measures is a sine quo non for meet- ing the water challenges of the future.

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Focus on Sanitation (World Health Organization, 2008); www.who.int/water_sanitation_health/monitoring/ jmp2008/en/index.html.

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4. P. H. Gleick, Science 302, 1524 (2003). 5. U.S. Environmental Protection Agency (EPA), Guidelines

for Water Reuse (EPA/624/R-04/108, U.S. EPA, Washington, DC, 2004).

6. National Research Council, Water Reuse: Potential for Expanding the Nation's Water Supply Through Reuse of Municipal Wastewater (National Academies Press, Washingtion, DC, 2012).

7. J. R. Stokes, A. Horvath, Environ. Sci. Technol. 43, 2680 (2009).

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11. Living Melbourne, Living Victoria Roadmap (Victorian Government Department of Sustainability and Environment, Melbourne, Australia, 2011).

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13. B. S. Razavi, S. Ben Abdelmelek, W. Song, K. E. O’Shea, W. J. Cooper, Water Res. 45, 625 (2011).

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44, v (2009). 21. C. J. Walsh et al., J. N. Am. Benthol. Soc. 24, 706 (2005). 22. C. J. Walsh, T. D. Fletcher, A. R. Ladson, J. N. Am.

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(IRC International Water and Sanitation Centre, The Hague, Netherlands, 2004).

32. C. Saliba, K. Gan, Energy Density Maps in Water Demand Management, Report to the Yarra Valley Water District (report E6109, Yarra Valley Water, Melbourne, Australia, 2012).

33. World Health Organization, Guidelines for the Safe Use of Wastewater, Excreta and Greywater (WHO, Geneva, Switzerland, 2006)

34. L. C. Proença, E. Ghisi, D. D. F. Tavares, G. M. Coelho, Resour. Conserv. Recycling 55, 960 (2011).

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Sanitation and Water (Copenhagen Consensus 2008

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SPECIALSECTION

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Challenge Paper, Copenhagen, Denmark, Copenhagen Consensus Center, 29 April 2008).

47. I. Aguilar-Benitez, J.-D. Saphores, Water Resour. Res. 44, W08414 (2008).

48. K. Bithas, Ecol. Econ. 68, 221 (2008). 49. M. A. Farrelly, R. R. Brown, Glob. Environ. Change 21,

721 (2011). 50. S. Dolnicar, A. Hurlimann, Water Sci. Technol. 60, 1433

(2009). 51. Water productivity P is defined as the value of goods and

services V generated per unit of water used Q: P = V/Q.

The percent change in potable water productivity associated with the implementation of a water-saving scheme is DP ≡ 100(P2 − P1)/P1, where P1 and P2 represent the productivity of potable water before and after the scheme is implemented (not all water-saving schemes discussed in the text directly reduced potable water use, but for consistency across the 21 case studies, water savings were benchmarked relative to potable water use at the appropriate scale). Assuming that the water-saving scheme does not change the value of goods and services produced, DP can be calculated

directly from potable (municipal) water use before (Q1) and after (Q2) scheme implementation: DP ¼ 100(Q1/Q2 − 1).

Acknowledgments: The authors thank H.-F. Mok and C. Jayasuriya for preparation of Fig. 2; J. Famiglietti, K. Low, and three anonymous reviewers for providing comments on the paper; and the University of California, Irvine, HSSoE Research Ignition Fund for funding to S.B.G.

10.1126/science.1216852

REVIEW

Conversion of Wastes into Bioelectricity and Chemicals by Using Microbial Electrochemical Technologies Bruce E. Logan1* and Korneel Rabaey2

Waste biomass is a cheap and relatively abundant source of electrons for microbes capable of producing electrical current outside the cell. Rapidly developing microbial electrochemical technologies, such as microbial fuel cells, are part of a diverse platform of future sustainable energy and chemical production technologies. We review the key advances that will enable the use of exoelectrogenic microorganisms to generate biofuels, hydrogen gas, methane, and other valuable inorganic and organic chemicals. Moreover, we examine the key challenges for implementing these systems and compare them to similar renewable energy technologies. Although commercial development is already underway in several different applications, ranging from wastewater treatment to industrial chemical production, further research is needed regarding efficiency, scalability, system lifetimes, and reliability.

T here is substantial energy in organic mat- ter that is currently wasted or lost in treat- ment processes. Treatment of organic-rich

wastewater currently consumes about 15 GW, or about 3% of all electrical power produced in the United States (1), but domestic, industrial, and animal wastewater together contain ~1.5 × 1011

kilowatt-hour (kWh) of potential energy (~17 GW of power) (2). Capturing part of this energy would provide a new source of electrical power that would also avoid the consumption of energy for waste- water treatment. Furthermore, agricultural practices could be modified to annually produce an addi- tional 1.34 billion tons of biomass for energy pro- duction, without affecting food production (3), which is equivalent to more than 600 GW of con- tinuous power. These different sources of waste organic matter can be a rich resource for energy pro- duction if we can develop cost-effective methods for harnessing this energy. Alternatively, we could capture this waste biomass energy in industrial pro-

cesses to make other useful chemicals, such as bio- fuels or industrial chemicals, that currently require electricity or organic substrates for this purpose.

Recently developed microbial electrochemical technologies (METs) that use microorganisms to catalyze different electrochemical reactions, such as microbial fuel cells (MFCs) that generated elec- trical power, are promising approaches for captur- ing the energy in waste biomass for diverse purposes. Energy production by electrochemical processes or conventional combustion requires a fuel to pro- vide electrons and an electron acceptor (oxidizer). In METs, organic matter is the fuel, and oxygen is the primary oxidizer for aerobic respiration by bacteria. However, many other soluble chemical species can serve as oxidizers for anaerobic bacteria, including nitrate, sulfate, and carbon dioxide. Bacteria known as exoelectrogens have the ability to transfer elec- trons outside the cell to insoluble electron acceptors, such as iron and other metal oxides, or to electrodes in bioelectrochemical systems. The most common- ly studied microorganisms are various Geobacter and Shewanella spp., but many other bacteria have been found to possess exoelectrogenic abilities (4). Electrons are transferred by these bacteria out- side the cell indirectly, by using electron shuttles such as flavins and phenazines (5–7), or directly by using outer membrane proteins (8). These mecha- nisms can occur in combination with self-produced conductive pili called nanowires (9, 10). In con-

trast, electrotrophic microorganisms can directly or indirectly accept electrons into the cell (11).

How Do Microorganisms Generate Electricity from Organic Matter? The use of exoelectrogenic microorganisms in MFCs allows electrical power generation from nearly any source of biodegradable organic or in- organic matter in water that does not directly re- quire oxygen as a part of the degradation process. These organic sources include simple molecules such as acetate, ethanol, glucose, and hydrogen gas; polymers such as polysaccharides, proteins, and cel- lulose; and many types of wastewaters from domes- tic, food processing, and animal sources (12, 13) (Fig. 1). In an MFC, bacteria release electrons to the anode and protons into solution, resulting in a negative anode potential of about –0.2 V (versus a standard hydrogen electrode) that is generally only slightly more positive than that of the half- cell reaction for the substrate (e.g., a midpoint po- tential at pH = 7 of –0.28 V for acetate) (14).In most cases, oxygen in air is used as a sustainable oxidizer at the cathode, with a typical maximum potential of +0.3 V, producing an overall maximum cell po- tential of +0.5 V. Cathode potentials obtained in MFCs are considerably lower than theoretical val- ues (~+0.8 V, with oxygen) even with Pt-catalyzed cathodes (15, 16) (Fig. 1). One of the most promis- ing nonprecious metal materials used for oxygen reduction in MFCs is activated carbon, because it is both inexpensive and renewably produced from waste biomass (17). Nitrate is an alternate electron acceptor that produces comparable cell voltages be- cause of its high solubility relative to oxygen (18). Voltages cannot be increased by linking MFCs in series as is done with batteries (19, 20). However, higher voltages can be captured from arrays of MFCs by wiring them to charge capacitors in par- allel and then discharging the capacitors in series, resulting in nearly additive voltages from the in- dividual MFCs (21).

The power densities produced by MFCs are lower than those possible by using hydrogen fuel cells because of high internal resistances, the limited temperature and solution conditions toler- ated by microorganisms, substrate degradability, and biofilm kinetics. Hydrogen fuel cells use an ion-exchange membrane as a solid electrolyte for charge transfer. Membranes are not required in MFCs, and using a membrane between the anode and cathode can add internal resistance, which will

1Department of Civil and Environmental Engineering, 212 Sackett Building, The Pennsylvania State University, University Park, PA 16802, USA. 2Laboratory of Microbial Ecology and Technology (LabMET), Ghent University, Coupure Links 653, 9000 Ghent, Belgium, and Advanced Water Management Centre, The University of Queensland, Brisbane, QLD 4072, Australia.

*To whom correspondence should be addressed. E-mail: [email protected]

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Sustainability Taking the ''Waste'' Out of ''Wastewater'' for Human Water Security and Ecosystem

William J. Cooper and Ivan Marusic Rosso,Stewardson, Brett F. Sanders, Lisa A. Levin, Richard F. Ambrose, Ana Deletic, Rebekah Brown, Sunny C. Jiang, Diego

Stanley B. Grant, Jean-Daniel Saphores, David L. Feldman, Andrew J. Hamilton, Tim D. Fletcher, Perran L. M. Cook, Michael

DOI: 10.1126/science.1216852 (6095), 681-686.337Science

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(print ISSN 0036-8075; online ISSN 1095-9203) is published by the American Association for the Advancement ofScience

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