Stop Flushing Water Down the Drain

A mechanical engineer explains how reusing wastewater at the town and even household levels can save both water and energy

From  Thirst for Power: Energy, Water and Human Survival,  by Michael E. Webber. Copyright © 2016, by Michael E. Webber. Excerpted by permission of Yale University Press. All rights reserved.

One of the approaches to saving water and energy is to put the waste streams from water and energy systems to good use. There are a variety of ways to accomplish this goal. In particular, there are 30 to 40 billion gallons of treated effluent generated each day in the U.S. Because the wastewater is generated in cities, the wastewater plants are nearby. That means the effluent is an abundant source of water that is generally co-located with population centers. And it is typically overlooked as a source. If that treated effluent is “reclaimed”—or used again—it can be a reliable supply of water. Reclaimed water is distributed in purple piping systems so that it can be distinguished from the treated drinking water and sewage.

Effluent is usually returned to lakes or rivers, or ejected into aquifers or oceans. While our ecosystems depend on many of those returns, it is also possible to use the effluent again before returning it to the watershed—for example, running the effluent through a power plant as a source of coolant. If the water does not need to be returned to a nearby river then it could be used for consumptive purposes as an alternative to freshwater—for example, for irrigating crops, hydraulic fracturing or industrial purposes. The “toilet to tap” approach saves energy compared with desalination to make brackish water or seawater potable. Because it is already used around the world in places like Singapore, Israel and Southern California, and out of this world aboard the International Space Station, it is a proven technology.

For many municipalities, closing the loop with their waste streams to turn the wastewater into drinking water or for other water purposes might make a lot more sense from an economic or energetic perspective. Austin, Texas, has set up an extensive purple-piping network that brings the treated effluent from the wastewater treatment plant to downtown Austin, some high-density neighborhoods near the urban core and the University of Texas, where it is available for irrigation and cooling. This approach is sensible since drinking water is not needed for these applications and because treated effluent in Austin is much cheaper to buy than drinking water.

The original customers for the effluent were local golf courses that used the water for their extensive irrigation requirements. Since then, more customers have emerged. The effluent can be used within buildings for nonpotable purposes such as toilet flushing. In this case rather than “toilet-to-tap” it would be “toilet-to-toilet.” For organizations that have the up-front capital to invest in the purple-piping infrastructure such as the city of Austin or the University of Texas, reusing effluent is a cost-effective option. Generally speaking, reuse projects are more expensive than water conservation programs, yet more affordable than seawater desalination. It is also a resilient, drought-resistant source of water supply. However, despite those benefits, it does pose some financial, health and performance risks. Just one risk is the possibility that the purple-piping system will accidentally get cross-connected with the drinking water system, meaning treated effluent that isn’t intended for drinking will be used for potable applications. And the billing rates for reclaimed water are typically set at a level below what is needed to recover the full costs of building the system, which means the customers are subsidized. While those customers surely benefit from the lower costs, the discounted rates put the long-term financial health of the reclaimed water system at risk.

One place where effluent can displace the use of freshwater is energy production. In particular, hydraulic fracturing is a rapidly growing technique that uses water to improve productivity from wells in many more locations than before. In some areas, those water needs are competing with other users. At the same time, hydraulic fracturing does not require drinking water or even freshwater, for that matter. Effluent might be a useful alternative, and its use has already been demonstrated by some shale producers in Texas, where water scarcity encourages experimentation with water alternatives.

Effluent can also be used for power plant cooling. In 2010 there were already 46 power plants out of more than 1,000 in the U.S. that used reclaimed water for cooling tower makeup. Another handful of facilities use reclaimed water for cooling ponds, air scrubbers, as injected pressure at geothermal fields and as boiler feedwater. Those include the very large nuclear power plants at Palo Verde in Arizona, which use the municipal wastewater from Phoenix and other cities nearby. While reclaimed water was first used for power plant cooling in 1967 in Burbank, California, in total only about a dozen of these cooling systems were built before 1990. After that, the pace of using effluent for cooling water accelerated, reflecting the increasing pressures on water resources in general.

Generally, effluent is safe to use for power plant cooling, although some minimum secondary treatments are recommended for disinfection and to prevent problems with equipment. For example, additional treatment can be useful as a way to prevent scaling, corrosion, fouling, foaming or biological growth, all of which can degrade performance. In addition, there are concerns that windblown spray from the use of reclaimed water for cooling might reach workers or the general public, a risk that can be mitigated by setting the cooling tower at least 90 meters from the public or using higher levels of disinfection.

A general rule of thumb is that it makes sense to ship reclaimed water up to 25 miles for power plant cooling, beyond which the pumping requirements outweigh the benefits. In places where water is scarcer, the 25-mile rule is not as relevant. For example, the Palo Verde plants, located in a desert, must pipe the reclaimed water dozens of miles from the treatment facility to the power plant. The cities of Phoenix, Glendale, Scottsdale, Tempe, Mesa and Tolleson all provide the wastewater for the power plant. The wastewater from Phoenix moves nearly 30 miles downhill using the force of gravity, then is pumped uphill for eight miles to the nuclear facility. The other wastewater streams move even farther, up to 60 miles in some cases. Although that seems like a long distance, it is much shorter than the hundreds of miles that freshwater is piped from the Colorado River.

Palo Verde is the only nuclear facility that uses 100 percent reclaimed water for its cooling. It is also the only nuclear facility whose cooling systems do not return water to the environment. The water reclamation facility can handle a total flow of 90 million gallons per day, and uses 25 billion gallons annually. And while the power plant operators are willing to pay a high price and would like more wastewater, they now face competition from cities and other industrial users, who also want that water.

Scaling up the idea at other plants, the tens of billions of gallons of wastewater generated each day across the U.S. are more than enough to satisfy the billions of gallons of water consumed each day at power plants. The question of practicality becomes one of distance, cost and other tradeoffs. For example, large wastewater treatment plants and large power plants might not be close to each other. Or the times of day or year when wastewater flows are highest may not correspond with the times of day or year when water is needed the most by power plants. An, many regions rely on wastewater discharges to streams, lakes or aquifers. Diverting those discharges for power plant use instead might deprive those ecosystems of badly needed water. Despite these obstacles, it is clear that using reclaimed water for power plant cooling remains mostly an untapped opportunity. While only a few dozen plants use effluent today, there are hundreds that could.

Effluent can also be used for nonpotable industrial processes, such as firefighting, or as a heat transfer fluid in the form of steam for distributing heat or chilled water for cooling. And water is a necessary ingredient for snowmaking for ski resorts. Larger ski resorts often use snowmaking to increase the skiable terrain or to extend their operable season. A posh ski resort in Killington, Vermont, that caters to Bostonians and New Yorkers looking for a large ski hill within a half-day’s drive of their city, uses its snowmaking as a key selling point. The snowmaking system uses more than 720,000 gallons of water per hour at full force, operates 1,500 snow guns with 88 miles of pipe, and can add one foot of snow to 80 acres in an hour. Killington uses effluent as its water source for snowmaking, noting in its promotional materials that it has “a virtually endless supply of water.” Given that wastewater is continuously generated, this claim is true.

Although effluent has its advantages in supply and cost, its use can generate criticism. When Killington first announced this plan in 1987, detractors poking fun at Killington created a fake organization named “Vermont Association for Sanitary Skiing” to generate opposition; they also created the bumper sticker “Killington: where the affluent meet the effluent.” This clever slogan is reminiscent of the “toilet to tap” phrasing in terms of its intent to generate a negative impression. And it worked: several Vermonters whom I befriended while writing this book over the river at Dartmouth College said they would never ski at Killington for precisely this reason. However, it turns out that Killington was simply decades ahead of its time in recognizing that properly managing a scarce resource is a sensible thing to do. Using effluent also offered cost savings compared with building reservoirs or using municipal drinking water. The snowmaking was good for business because it helped improve skiing conditions and allowed the resort to extend its ski season, both of which increased revenues. This case is yet another example of how thoughtful management of resources can carry economic and environmental benefits simultaneously.

Interestingly enough, one of the initial critics—Bill Mares, who was one half of a duo of legislators who were poking fun with their bogus organization and clever slogan—issued a public apology in 2012 for the gimmick. In an interview, he noted that he and his legislator compatriot found the whole episode amusing, but 25 years later in the context of heightened awareness of water scarcity, he admitted, “Now the joke is on me. Killington was way ahead of its time.”

* * *

Wastewater from homes can also be reused, with potentially great savings. If homes are built with distinct plumbing systems that separate water streams, then the domestic graywater from showers, sinks and clothes washers can be used again within the home. Graywater can flush toilets inside or water flowerbeds outside. Since it is not necessary to use drinking water—the most energy-intensive form of water—for these applications, graywater usually represents an energy savings for nonpotable applications. In many ways it is ridiculous that we use the world’s cleanest water for toilet flushing in the first place, so this approach seems sensible by comparison. With minimal treatment to remove the organic components, it can also be used again for washing. While graywater reuse can help avoid using the most energy-intensive water, if everyone does it at a large scale, then the sewers might not have enough water to operate effectively. These kinds of unintended consequences should be planned for.

In addition, rainwater can be collected from storm runoff through roof systems of gutters into rain barrels. That water can then be used for irrigation. Notably, while rainwater might be very clean when it lands on the roof, it subsequently might pick up chemicals that make it unsafe to drink.

It is also important to note that if you have a graywater system at your home, then it would be good to use biodegradable soaps and avoid putting toxics down your drains, lest they end up on your plants a few hours later. The International Plumbing Code, which is only used in certain jurisdictions in the U.S., allows for graywater from showers and bathtubs to be used for flushing toilets. However, the Uniform Plumbing Code, which is used more widely in the U.S., prohibits graywater use indoors.

Harvesting graywater is not only a way to supply more water, it is also an option for harvesting heat. The water from sinks, showers and laundry machines is often still hot when it goes down the drain. That heat is typically lost through the pipes to the ground as the wastewater moves along. However, with a heat recovery device, it can be used to preheat incoming freshwater to the water heater tank, saving energy.

Overall, graywater use can spare some freshwater withdrawals and might save energy for heating, pumping and treatment. For some rural settings where water is really scarce, graywater might also be contemplated as a source of potable water. In these cases, graywater reuse for potable needs might actually be more energy intensive than getting freshwater straight from a well. Many rural areas get water from wells, especially for remote domestic purposes or small farms or ranches. For these applications, the groundwater is often very clean, and so the energy requirements are simply for pumping. By contrast, graywater reuse for potable purposes would require additional treatment to remove the organic components and pathogens, driving its energy intensity higher than simple pumping. In those cases it makes more sense to use the graywater only for irrigation. However, in municipal areas that already have advanced water treatment systems in place, graywater reuse could make a lot of sense.

ABOUT THE AUTHOR

Michael E. Webber is a professor of mechanical engineering, deputy director of the Energy Institute and the faculty affiliate of the Center for Research in Water Resources at the University of Texas at Austin.

Attached link

Media

Taxonomy

• Water Reuse & Recycling
• Industrial Water Reuse
• Water Recycling
• Reuse
• Grey Water
• Nutrient Recovery & Reuse