Slashing salinity, saving water

The city of Scottsdale, Ariz., wanted to support its vibrant golf economy without using its precious drinking water supplies for irrigation. The solution? Twenty-three golf courses, a partnership and technology.


Slashing salinity, saving water
A peek inside the Advanced Water Treatment facility in Scottsdale, Ariz., the result of a partnership between the city and 23 private golf courses. Here, roughly half of Scottsdale’s wastewater is transformed into water that can not only be reused, but that has a low enough salt content that it reduces the amount of water the golf courses require. Photo courtesy of the city of Scottsdale

Since 1998, the city of Scottsdale, Ariz., has been treating about half of its wastewater to near drinking water standards. It’s an expensive process that uses reverse osmosis technology, but the city isn’t footing the bill alone — the cost to build and the ongoing costs to operate the plant are shared between Scottsdale and 23 private golf courses in north Scottsdale that use the resulting water to keep their greens pristine.

Keeping golf courses green is more difficult when irrigation water has a high salt content. Saltier water means watering the greens more often, along with applying more fertilizer. Before the construction of Scottsdale’s Advanced Water Treatment facility (AWT), the golf courses were using untreated, or “raw,” Colorado River water delivered to the city by the Central Arizona Project aqueduct. The raw water had a salt content of about 600 to 650 milligrams per liter. Wastewater recycled at the AWT is essentially salt-free, and when blended with raw Central Arizona Project water, the resulting water has a sodium concentration below 125 milligrams per liter, the level established in the agreement between the city and the golf courses. Using this low-salinity recycled water means the courses require less water and fertilizer to maintain higher-quality greens. It also means the city can save more raw surface water to treat to drinking water standards.

The water that flows out of homes from sinks, showers, toilets and laundry and into the sewage system is traditionally called “wastewater,” but that term no longer really applies for the 10 largest municipalities in the Phoenix area, which are represented by the Arizona Municipal Water Users Association (AMWUA). Nearly 100 percent of what was once wasted effluent is treated by the cities via various means and then put back to use. Among those reuses, treated wastewater from five AMWUA cities — including the wastewater from the other half of Scottsdale — is sent through a 36-mile pipe to cool the Palo Verde Nuclear Generating Station. Cities also use the reclaimed water to irrigate parks, create fishing lakes and wetlands, or save underground for future use, which is called “recharging” the aquifers.

All reclaimed water used in the cities’ parks, golf courses, homeowners association common areas, school playgrounds or other locations is treated to what the state deems “A+” quality. Water that is “A” quality has been treated and disinfected until there are no routine, detectable disease-causing bacteria present. It moves up to A+ when the treatment process also removes nitrogen compounds, which can contaminate groundwater. Scottsdale’s AWT goes one giant step further, employing reverse osmosis to remove mainly salts and inorganic materials from the recycled water produced at the plant.

Wastewater always has a high salt content because it passes through homes and businesses, picking up salt from cooling towers and food and other waste. Chemicals used to treat wastewater also add salt to the end product. The salinity of wastewater from homes in northern Scottsdale is particularly high — around 1,100 milligrams per liter. (The city estimates that water softeners account for more than 30 percent of the total salt concentration in the wastewater system. These products work by exchanging salt for hard minerals such as calcium and magnesium, providing soft water for homeowners but introducing the salty brine discharge into the sewer system.)

Scottsdale’s AWT is capable of producing 20 million gallons of low-salinity recycled water per day, although current rates of wastewater flowing into the plant are closer to 10 million gallons. In summer, the golf courses use all of that water. During summer, when temperatures climb into the 110-degree range or higher, recycled water is supplemented with raw Central Arizona Project surface water to meet irrigation demands. During winter months, the golf courses typically need an average of only 3 to 5 million gallons of water per day, and given a few rainy days, that need may drop to zero. Any additional low-salinity water supplied by the plant is recharged underground for future use. Scottsdale also uses ozone and UV treatment on the water it recharges to preserve water quality in the underlying aquifer. These processes eliminate “emerging contaminants,” which are contaminants the EPA is studying but has not yet regulated.

Two decades ago, the city of Scottsdale wanted to support its vibrant golf economy, but didn’t want to use its precious drinking water supplies for irrigation. The dilemma was solved thanks to 23 golf courses, a partnership and technology. Today they call that innovation. Twenty years ago, it was just smart.

Editor’s note: This article originally appeared on the Arizona Municipal Water Users Association blog on March 27, 2017.

Warren Tenney is the executive director of the Arizona Municipal Water Users Association, which represents the 10 largest municipalities in the Phoenix area.