Alternative Water Sources: Supply-Side Solutions for Green Buildings
Water efficiency should be a top priority for our buildings. At the same time, finding alternative sources of water is also important for sustainability and passive survivability. Several sources of water can be harvested at the building level as well as through municipal-scale wastewater treatment and desalination.
The severe 2007 drought throughout the southeast U.S. was a wake-up call. This drought taught us that even areas of the country we have long assumed to have plenty of water are not immune from water shortages, and it showed us how woefully inadequately prepared we were—and still are— to respond to severe drought.
Changing precipitation patterns, including increased drought in some parts of the country, are predicted with global climate change. Yet even without the wild card of climate change, unsustainable groundwater extractions for burgeoning populations in the western U.S. and throughout the world have become front-page news.
While using water more efficiently should remain the top priority, we also need to take a look at alternative sources of water, including those we can harvest at the building level. Providing alternative water supplies in buildings can also help achieve resilience—maintaining livable conditions when electricity, heating fuel, or municipal water are lost for extended periods of time. This article examines the spectrum of unconventional water sources that can be used in and around buildings.
Potable versus Nonpotable Uses
In identifying alternative sources of water, the first consideration is what those sources will be used for. Potable water, which we can use for drinking, cooking, and bathing, among other uses, must meet a high level of purity and safety. Nonpotable water is less pure but, when handled properly, it can be fine for landscape irrigation, makeup water for cooling towers, and toilet flushing. Many alternative water sources are best suited to nonpotable uses, though some can be made potable with additional treatment.
If we can provide separate plumbing in and around buildings for potable and nonpotable water, it opens up significant new options for water supply. Installing separate supply piping for landscape irrigation and cooling-tower makeup water is fairly easy, while installing separate nonpotable supply plumbing for toilet flushing, which requires dual piping throughout a building, is more difficult.
Described in this article and summarized in the table are the most prevalent alternative water sources for use in and around buildings. Scale of delivery can dictate what solutions to focus on. Graywater, for example, is typically dealt with only on an individual-building scale. With rainwater collection or air-conditioner condensate, it may be possible to aggregate water from several roofs in a university or office campus and use common storage, improving the economic viability of such systems. Using reclaimed water (treated wastewater) and desalination water is usually viable only when a municipality supplies that water.
Graywater Collection and Reuse
As the term is most commonly used, graywater refers to wastewater from clothes washers, showers, bathtubs, and lavatory faucets—and not water from toilets, kitchen sinks, and dishwashers. Graywater is collected with separate drain lines (requiring a building to be dual-plumbed for graywater and other wastewater drainage), filtered to remove large particles, and stored until use for landscape irrigation—usually below ground.
Because graywater generally contains organic matter (such as hair, textile fibers, and skin cells), it should not be stored for long. Bacteria in the water quickly break down this organic matter, using up dissolved oxygen in the process (chemists refer to organic matter in water as biochemical oxygen demand or BOD). Once the oxygen is depleted, different bacteria take over and the decomposition becomes anaerobic, which can produce smelly methane and hydrogen sulfide gas. For this reason, most graywater systems quickly deliver water to irrigation piping. If a system stores graywater, it is usually to allow just enough to build up that it can be discharged in larger doses that fill the irrigation pipes and provide even distribution.
Water conservation expert John Koeller, P.E., predicts that graywater will be “the next major breakthrough” in the field of water savings. He expects to see significant developments of both whole-building systems and units that are tied directly to lavatory sinks or clothes washers.
Several whole-building graywater systems are available. These systems necessitate reconfiguring drainage piping, so are less expensive when installed in new construction than when retrofit for existing buildings.
In addition to these whole-house or whole-building graywater systems, there are products that capture wastewater from an individual lavatory sink or clothes washer. Some can store it in an under-counter tank to flush an adjacent toilet.
The biggest challenges facing graywater today are regulatory. Most states do not permit separate collection and use of graywater, though severe droughts have helped to ease those restrictions in some regions. California, Arizona, New Mexico, Nevada, Utah, Texas, and Montana all have statewide regulations permitting graywater use, though in some cases local regulations can supersede statewide rules. These regulations primarily address use of graywater for landscape irrigation, and they have important differences. California permits only subsurface irrigation, for example, and vegetable crops are specifically excluded in most states that allow the use of graywater. Arizona’s law is tiered, with different standards and levels of review required depending on the daily flow. Some localities have adopted regulation that allows graywater to be used, but only for toilet flushing—and only after it has been filtered, disinfected, and dyed blue or green to distinguish it from potable water. Key aspects of a sampling of the country’s graywater regulations are described in the table above.
A few places in North America rely almost totally on harvested rainwater. In the town of Volcano, Hawaii, near Hilo, the rock is so porous that wells would have to be thousands of feet deep to reach freshwater. It’s far less expensive to simply capture and store the abundant rainfall. The vast majority of homes and businesses there rely on harvested rainwater for all of their water needs—potable and nonpotable—storing the collected water in large, covered, above-ground cisterns (usually swimming pools adapted for this purpose).
Rainwater harvesting is also common as a sole residential water source on the island of St. John in the Caribbean, and it is fairly widely used in parts of Texas, Kentucky, Ohio, and the Pacific Northwest—especially in locations where the groundwater is brackish, very hard, or in limited supply. “There are pockets around the country,” says Hari Krishna, Ph.D., of the Texas Water Development Board in Austin and founder of the American Rainwater Catchment Systems Association. Less sophisticated rain-barrel systems for irrigating gardens and lawns are common in the Pacific Northwest and seen occasionally in the rest of the country. Krishna estimates that there are about 10,000 rainwater-harvesting systems in Texas, including about 1,000 that serve as the sole water source for a home.
Rain barrels are very simple—most often a covered container connected to the downspout beneath the eaves of a house, with an overflow and a spigot attached to a hose. Whole-house and commercial rainwater-harvesting systems are significantly more involved (see EBN ).
- a smooth roof surface that neither leaches chemicals into the water nor traps organic matter that could contaminate harvested water;
- a first-flush system to divert the first rain that falls during a storm, carrying off accumulated particulate matter;
- a coarse filter to keep out leaves and other detritus;
- a cistern large enough to serve expected water needs (almost always the most expensive component); and
- for potable water systems, a treatment system to purify at least the water used for drinking and cooking.
According to Krishna, ultraviolet (UV) light is the most common treatment for potable rainwater systems. With proper first-flush systems, filtration, and UV treatment, Krishna says chemical treatment should be unnecessary, especially since rainwater starts out so clean (assuming there are no severe local sources of air pollution). “Rainwater is one of the purest sources of water that exists in the world,” he says, noting that rainwater’s low mineral content makes water softening unnecessary (see EBN , for more on water treatment).
Most rainwater harvesting systems are made up of components that were intended for other purposes, but that may be changing. Several Australian companies, including BlueScope Water, are entering the North American market. BlueScope and its companion company, Pioneer Tanks, offer rectangular storage tanks that fit into walls or under the floor. With harvested rainwater proving increasingly attractive for cooling-tower makeup water in commercial buildings and for landscape irrigation, we can expect to see specialized components designed for commercial buildings as well. “The market is growing,” Krishna told EBN. While some experts suggest that a minimum of 20"–24" (500–600 mm) of rainfall is required annually for harvested rainwater to serve whole-house needs, Krishna says that in areas in Texas receiving as little as 15" (380 mm) of rainfall per year, there are homes that have relied on 100% rainwater for years.
Landscape-Scale Stormwater Harvesting
Stormwater is nearly always managed in the landscape surrounding a building, and it is commonly channeled into retention ponds. Occasionally, such retention ponds are designed so that water from them can be pumped out for nonpotable uses in and around a building. At the Heifer International headquarters building in Little Rock, Arkansas, completed in 2006, stormwater from parking areas is captured in a large retention pond, from which water is drawn off for landscape irrigation and an innovative cooling system for parts of the 94,000-ft2 (8,700-m2), five-story building.
Cooling systems rely on evaporator coils through which refrigerant fluid changes from liquid to vapor, cooling the coils in the process. Air blowing past the coils cools off as it goes by, and moisture from the air condenses on the coils. Condensate drains carry away the water, usually into the sewer. Instead of wasting it, more and more buildings, especially in parts of the country with hot, humid summers, are capturing that condensate for reuse.
In large commercial buildings, condensate recovery often produces enough water to supply all of the landscape irrigation needs or a significant portion of makeup water for cooling towers. In San Antonio, Texas, with its high temperatures and high humidity, condensate recovery is an easy choice. “When you take the humidity out of the air, that condensate water is a huge volume in a large building,” says Karen Guz, director of the Conservation Department for the San Antonio Water System. In the ASHRAE Journal, Guz reported that the San Antonio Public Library is producing a gallon of condensate per minute, or over 1,400 gallons (5,300 l) per day, which is used for irrigation. The downtown Rivercenter Mall produces 250 gallons (950 l) of condensate per day, which is used to replenish cooling-tower losses—this condensate recovery system paid for itself in less than six months, according to Guz.
If condensate is being used only for cooling-tower makeup, the condensate can often be fed directly into the cooling tower without storage—because condensate produced in a building will never exceed the evaporative losses from the cooling tower. This can reduce costs significantly, according to Guz. Using condensate as a source of irrigation water is more expensive, as it requires storage and a system to pressurize the water.
In San Antonio, it is becoming more common to combine rainwater harvesting and condensate recovery for use in irrigation—the city refers to this as rainwater plus. “The combination is great,” Guz told BuildingGreen. “Our rainfall patterns are so erratic that a rainwater system by itself must have an enormous, expensive tank in order to go through the long periods we can go without rain.” Because the production of condensate is fairly steady, and increases as the weather gets hotter, smaller storage tanks are sufficient.
Air-conditioning condensate recovery is most practical in climates with high cooling-season humidity. Along with the obvious places like Houston, San Antonio, Atlanta, and Miami, it also makes sense in cities like Philadelphia, Chicago, and New York, which experience high humidity that coincides with the greatest cooling loads. Condensate recovery is especially attractive in facilities like shopping centers, where there is a high degree of air exchange.
The quantity of condensate water produced depends on the temperature and humidity conditions (both outdoors and indoors) and the amount of cooling being provided. Guz has developed a rule of thumb for large buildings in the summer months of 0.1 to 0.3 gallons (0.4–1.1 l) of condensate per ton of air conditioning for every hour that the cooling system operates. In the San Antonio climate during peak summertime months, this translates into roughly 0.5–0.6 gallons per hour for every 1,000 ft2 of cooled area (20–24 l/hr per 1,000 m2).
While air-conditioner condensate is inherently pure—it is essentially distilled water—there is potential for contamination, especially if it sits in a warm environment. For this reason, chlorine is usually used to treat condensate. San Antonio hasn’t experienced problems with the moderate chlorine concentrations in its irrigation water, according to Guz, but chlorine could harm some plants.
A lot of water is lost from cooling towers through evaporation and drift losses. Water is also intentionally drawn off—a process referred to as blow- down—because minerals and other contaminants become more concentrated as a result of evaporation. Typically, the blowdown water is drained into sewer lines, but it can be collected and reused for applications where the salinity or mineral content is acceptable.
Water conservation expert Bill Hoffman, P.E., of Austin, Texas, suggests that if blowdown water is being reused, the “cycles of concentration” (a measure of how concentrated the minerals become due to evaporation) shouldn’t exceed two or three. The building housing the San Antonio Water System, however, has been using blowdown water for eight years, according to Guz, and “during the past two years the cooling-tower operations have been documented at no less than four cycles of concentration with no ill effects on the plants (though there may be additional dilution from groundwater that is also captured in French drains).”
It is also possible to treat the water in cooling towers to remove minerals—for example by chemical precipitation or by using reverse osmosis (see discussion of desalination below)—but this is costly and rarely practiced.
While air-conditioner condensate is inherently pure (at least when first produced), that is not the case with blowdown water. Along with concentrating minerals, cooling towers also concentrate bacteria and other contaminants, including Legionnella (a bacteria that causes potentially fatal Legionnaires’ disease). If blowdown water is used for irrigation, treatment is essential.
Building-Scale Treated Wastewater
While it is relativley uncommon, an increasing number of large buildings are treating their wastewater onsite and producing nonpotable water for landscape irrigation and toilet flushing. The Solaire in New York City was one of the first examples of this approach. An advanced, multistep, biological treatment and micro-filtration process treats 100% of the wastewater produced in the 28-story, 293-unit, LEED Gold apartment building. The treated wastewater is used for all the toilets in the building, the building’s cooling tower, and all landscape irrigation requirements, including 5,000 gallons per day (19,000 l/d) for an adjacent part. Potable water use in the Solaire was reduced by 50%.
Similarly, the Audubon Center at Debs Park in Los Angeles treats 100% of its wastewater onsite—and, in fact, has no connection to the municipal sewer. This treatment system consists of a hybrid anaerobic/aerobic treatment and filtration process and a peracetic acid and UV advanced oxidation disinfection process. This treated wastewater will be used for toilet flushing. Overall, this building has achieved a 70% reduction in potable water use.
Ecological wastewater treatment provides another option for more comprehensive onsite treatment of both graywater and blackwater.
Municipal-Scale Treated Wastewater
The first U.S. municipality to distribute treated wastewater through separate piping was Grand Canyon Village, Arizona, beginning in 1926. Freshwater had to be trucked in, so wastewater was too valuable to discard. The town’s small system reclaimed and treated wastewater for use in landscape irrigation and toilet flushing. Similarly, in 1942, Sparrows Point, Maryland (near Baltimore), built a 4.5-mile (7.2-km) pipeline to supply treated wastewater to the Bethlehem Steel factory for process use and cooling.
The first large-scale, municipal reclaimed water system began operation in 1977 in St. Petersburg, Florida. By 2008, the city was delivering 21 million gallons (80 million l) per day to 4 cemeteries, 7 golf courses, 64 schools, 92 parks and recreation facilities, 339 businesses, 135 multifamily housing projects, and 10,200 single-family homes. This reclaimed water, supplied by 299 miles (481 km) of pipe, is used primarily for landscape irrigation, though some commercial customers use the water for industrial processes and cooling towers. The city uses about two-thirds of all available treated effluent each year, according to Patricia Anderson, the water resources director for the City of St. Petersburg.
Irrigation remains the largest use of reclaimed water in the U.S., according to James Crook, Ph.D., P.E., a water reuse expert in Boston. The earliest systems provided mostly agricultural water, but urban irrigation for parks, playgrounds, and lawns has been growing rapidly. Cooling is the next major use—for power plants, industrial processes, and makeup water for cooling towers. Use for toilet and urinal flushing in commercial buildings is less common but quickly gaining popularity, especially in the green building community. In many places where reclaimed water is used, its piping is painted purple to distinguish it as nonpotable.
Some regions of California now mandate that commercial buildings be dual-plumbed so that they can use reclaimed water (referred to as “recycled water” in California) for toilet and urinal flushing. In the Irvine Ranch Water District south of Los Angeles, for example, all buildings seven stories or taller are required to include dual plumbing for reclaimed water use.
With reclaimed water use, the question often comes up as to whether this water can be deemed suitable for potable uses—so-called “direct potable water reuse” or pipe-to-pipe reuse. Some experts think that cities in the U.S. will eventually accept direct potable water reuse, though public resistance is strong. Denver and San Diego have studied the idea, according to Crook, but an effort to permit potable reuse in San Diego was defeated several years ago. Crook believes that we won’t get direct potable water reuse until water-quality monitoring improves to the point that safety problems can be identified almost immediately.
But some suggest that direct potable water reuse isn’t that different from what we’re doing today. “Proponents say, ‘Hey, we’re doing it now,’” says Crook. Many of our rivers are dotted with cities and towns, each with drinking water intakes and sewage treatment plant outlets along the river. The Colorado River, which provides drinking water for San Diego, Los Angeles, Phoenix, Las Vegas, and many other municipalities, has more than 450 wastewater discharge permits along its course. The U.S. Environmental Protection Agency (EPA) refers to this as “surface water augmentation for indirect potable reuse.”
Indirect potable water reuse can also occur through aquifer storage. A large indirect potable water reuse system is used in Orange County, California. Each day 70 million gallons (260 million l) of highly treated wastewater are injected into an underground aquifer, from which drinking water is withdrawn for 2.3 million Californians in 20 cities. In EPA parlance, this is “groundwater recharge for indirect potable reuse,” and, because there is dilution as well as physical separation of wastewater delivery and drinking-water withdraw, it is more acceptable to the public than direct potable water reuse.
Of concern with either direct or indirect potable water reuse is the prevalence of pharmaceutical and illicit drugs in the wastewater. Studies have shown that wastewater treatment plants do not effectively remove these pollutants, so they would be found in reused water. This is indeed a concern with potable water reuse, but it is also a concern with conventional public water supplies—from both surface water and groundwater sources. Pharmaceuticals are entering our aquifers as well as our rivers, creating a problem with no simple solutions.
Desalination is the process of removing salts (and other impurities) from seawater or brackish water. Ninety-seven percent of the world’s water is saline, so tapping this resource as a freshwater source has long been attractive. While there are a few systems in the world that use seawater directly for toilet flushing and certain other nonpotable uses (see sidebar), the highly corrosive nature of seawater makes this impractical; desalination is required for widespread use of seawater.
Interest in desalination emerged at least as far back as the 1700s. As secretary of state, Thomas Jefferson considered a plan in 1790 to install desalination systems on ships, and a British patent was issued in 1852 for a desalination device. The first desalination plant on land was built on the island of Curaçao in the Netherlands Antilles in 1928. Saudi Arabia built its first plant in 1938.
Worldwide desalination capacity gradually grew from almost nothing in 1960 to about 9.5 billion gallons (36 million m3) per day in 2005, according to the Pacific Institute, a California-based think-tank focused on water issues. Half of this capacity was in the Middle East, where inexpensive energy makes the process more feasible; the U.S. is also one of the largest users of desalination.
While early desalination plants relied on evaporation and condensation, most plants rely on reverse osmosis (RO), a less energy-intensive technology using selective membranes. One of the largest desalination plants in the U.S.—run by Tampa Bay Water, Florida’s largest wholesale water supplier—produces about 25 million gallons (95 million l) per day using an RO system.
From an energy and environmental standpoint, desalination has a number of drawbacks. First, it is energy intensive, even where RO technology is used instead of evaporation. Current best practices with RO require about 12 kWh per 1,000 gallons (3.2 kWh/1,000 liters); with this energy intensity, between a third and a half of the total cost of desalination goes to energy. The figure below shows the relative energy intensities of various freshwater sources in San Diego, California. The high energy intensity of desalination in San Diego is comparable to that of Tampa Bay’s plant, which uses approximately 4,476 kWh/acre-foot (3.629 kWh/m3).
Second, along with producing freshwater, desalination also produces brine with about twice the salinity of the source water. The concentrated brine also often contains elevated levels of constituents found in seawater, such as manganese, lead, and iodine, as well as chemicals from urban and agricultural runoff. When brackish groundwater is desalinated, the resultant brine is usually deposited into evaporation ponds, reinjected into the ground through deep wells, or piped to the ocean.
When seawater is desalinated, the brine is usually piped some distance out to sea, though it may be mixed with treated wastewater or power-plant cooling water first. If it is not diluted first, the desalination brine is more dense than seawater, so it sinks, creating plumes of higher-salinity seawater on the ocean bottom—where sea life is concentrated. Few studies have been conducted to determine the risk of brine discharge into the ocean, and potential risks are significant.
Desalination is also well suited to solar-thermal power plants, according to a study by the German Aerospace Center, “Aqua-CSP: Concentrating Solar Power for Seawater Desalination.” In response to the heavy environmental impacts of conventional desalination systems, the researchers analyzed the use of concentrating solar power (CSP) to support desalination measures in urban centers throughout the Middle East and North Africa. They determined that implementing large-scale CSP desalination systems in those areas would not only avoid some environmental impacts from conventional desalination methods but would also become cost-competitive with desalination plants fueled by nonrenewable energy sources.
Future water shortages in the U.S. and internationally will necessitate reducing demand and increasing supply. The starting point, almost always, should be water conservation, but using unconventional supplies should not be overlooked. This article has introduced some of the options.
Relative to green building, the most exciting alternative sources are those that can be harvested onsite: especially rooftop rainwater, graywater, and air-conditioner condensate. Beyond these site-specific options are municipal water sources, including treated wastewater and desalinated water. Using these sources appropriately and continuing to maximize water conservation opportunities will help us stave off the water shortages looming on the horizon.
Wilson, A., & Navaro, R. (2008, April 29). Alternative Water Sources: Supply-Side Solutions for Green Buildings. Retrieved from https://www.buildinggreen.com/feature/alternative-water-sources-supply-side-solutions-green-buildings