If one were to suggest that wastewater could be used as an efficient source of fresh water, a likely rejoinder would be serious doubt and, at the very least, a pinched face and a declaration along the lines of "Ew, that's gross."
Call it "the yuck factor."
Yet capturing and reusing wastewater for municipal and household use, agricultural and industrial production, and recharging depleted aquifers is precisely what researchers writing in the latest issue of Science suggest needs to happen in order to address the world's growing water crisis.
"[D]rought conditions are a serious problem both in the U.S. and elsewhere in the world, and with climate change and population growth that problem is likely to get worse," said Stanley Grant, lead author of the article and a professor of engineering at the University of California, Irvine, and the University of Melbourne. "And the options that we have for addressing the water needs of populations really are ... dwindling," he added.
Grant and his co-authors reviewed dozens of scientific papers examining ways wastewater might be reused or water used more efficiently. Their article appears in a special issue of Science called "Working With Waste," which focuses on the myriad problems of waste and how to reduce its burden in the future.
Historically, Grant said, nations have augmented water supplies by building dams or constructing pipelines or aqueducts for transferring water from a more plentiful source to one with a water deficit. More recently, he added, nations have turned to desalination plants that convert seawater to potable water.
"In many cases the rivers that we've been tapping are simply dry," Grant said. "Many of the world's largest rivers and most important rivers, like the Nile, for example, or even the Colorado River in the U.S., are many years not reaching their deltas, which obviously has very significant implications, not only for human water security but also for ecosystem sustainability."
Facing a drier future
Yet the temptation to tap rivers, oceans or aquifers, no matter how shortsighted those efforts may seem, is often difficult for governments to tamp down.
Consider the scope of the world's water crisis. According to the World Water Council, 1.1 billion people live without clean drinking water and 2.6 billion lack adequate sanitation. Millions more die each year from waterborne diseases.
The projected impacts of a warming atmosphere and oceans on the Earth's hydrological cycle -- dry regions likely becoming drier, while wet ones become more wet -- will likely exacerbate this already dire situation.
Given these acute demands for water and constraints on current -- and likely future -- availability, Grant said, "the real alternative, the really only alternative, is to improve what's called water productivity, which is essentially the amount of value services that are achieved with a given unit of water."
The methods for achieving greater water productivity, Grant and his colleagues suggest, are threefold: substituting higher-quality water with lower-quality water, regenerating higher-quality water from lower-quality water through treatment, and reducing the amount of high-quality water that is used to generate goods and services.
Municipalities in the United States currently use very little treated wastewater. But the authors suggest that 27 percent of municipal supplies could be treated and used, significantly improving drought resiliency in areas like Southern California. Treated wastewater applications, the authors say, include industrial cooling and landscape irrigation.
Another method for substituting for the use of high-quality fresh water, they say, occurs in Hong Kong, which for the past 50 years has operated a dual water system that delivers seawater for toilet flushing to 80 percent of its 7 million residents. A triple water system at Hong Kong International Airport includes fresh water, seawater, and treated water from sinks and aircraft washing, which reduces municipal water use by 50 percent.
Rainwater or reused water from bathing or kitchen activities -- often called gray water -- provides a method for replacing fresh water in toilet flushing and yard irrigation that could, the authors say, cut household water use by 50 percent or more.
Water substitution methods, whether using treated wastewater, seawater or gray water, are generally viewed as less controversial than the use of regenerated water, a process in which wastewater is treated and added back to the water supply.
Yet regeneration is already being used in the United States and abroad, reducing groundwater and surface water depletion as well as energy consumption.
The world's largest facility for the regeneration of wastewater is in Fountain Valley, Calif., where the Groundwater Replenishment System treats sewage water using conventional and advanced methods, such as microfiltration, reverse osmosis and ultraviolet disinfection.
The treated water is injected into Orange County's local groundwater aquifer, which 2 million residents rely on for their water needs. This technique -- called indirect reuse -- provides approximately 20 percent of the water needed to maintain the aquifer.
Direct reuse of wastewater is not practiced in the United States. But a facility in Windhoek, Namibia, has processed domestic sewage into potable water for human consumption since the late 1960s "with no obvious adverse health effects among the population of several hundred thousand," the authors state. The facility provides roughly 35 percent of the city's municipal water needs.
In Israel, they add, 73 percent of municipal sewage is treated and reused for irrigation, which is about 5 percent of the country's total water use. Singapore treats and reuses sewage water on a large scale.
The most straightforward -- and likely the least controversial -- method for improving water productivity that Grant and his co-authors suggest is expanding efforts at reducing freshwater consumption, such as replacing single-flush toilets with dual-flush toilets or installing high-efficiency shower heads and washing machines.
Need for strong public outreach
Agricultural production accounts for the greatest amount of global water consumption, and in this sector the authors suggest a variety of efficiency proposals such as improved irrigation systems and switching to crop varieties that consume less water.
The authors say the combined impact of substitution, reuse and water conservation would help to preserve freshwater supplies and coastal estuaries, as well as biodiversity. Tapping freshwater sources less, they add, means lower fossil fuel use and lower greenhouse gas emissions.
Yet improving water productivity requires an array of economic, planning and regulatory approaches, they say. Fundamental improvements to the built environment are required. Land must be bought, storage facilities and pipelines constructed. On a more discreet level, household efficiency efforts require promotional campaigns in order to win over a skeptical public.
Grant said the public can be moved beyond "the yuck factor," though, and embrace water substitution and wastewater reuse. There must be a very strong public outreach program so that a community can come to understand why these strategies are necessary or why there may not be any viable alternatives, he said, adding that transparency about how the system works and assurances of its safety are also vital.
"Once the public understands that no one is getting sick from these methods," he said, "then there's likely to be great support for them."