By Warren Tenney
Since 1998, the City of Scottsdale 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 paying for all of it. The cost to build and operate the plant is shared between Scottsdale and 23 private north Scottsdale golf courses that use the water to keep their greens pristine.
It’s more difficult to keep golf courses green when irrigation water has a high salt content. Saltier water means watering the greens more often and using more fertilizer. Before building the Advanced Water Treatment (AWT) Facility, the golf courses used untreated or “raw” Colorado River water delivered to the city by Central Arizona Project. The raw water has a salt content of about 600 to 650 milligrams per liter (mg/l). Wastewater recycled at the Advanced Water Treatment Facility is blended with raw CAP water and yields water with a sodium concentration below 125 mg/l, a level required to fulfill the city’s agreement with the golf courses. Using this low-salinity recycled water means the courses can use less water and fertilizer to maintain higher-quality greens. It also means the city can save more raw water to treat for drinking.
The water that flows out of your home from your sink, shower, toilets and laundry and into the sewage system is traditionally called wastewater. The name no longer really applies for AMWUA cities. Nearly 100 percent of what was once wasted effluent is treated by the cities and put back to use. In other words, water is reused. Treated wastewater from five AMWUA cities – including the other half of Scottsdale’s wastewater – is sent through a 36-mile pipe to cool the Palo Verde Nuclear Generating Station. Cities also use treated wastewater to irrigate parks, create fishing lakes and wetlands, or save underground for future use, which is called “recharging” the aquifers. Wastewater is now a renewable water source, most often called recycled water or reclaimed water.
All reclaimed water used in parks, golf courses, HOA common areas, school playgrounds or church campuses is treated to what the state deems as A+ quality. Grade A quality means the water is treated and disinfected until there are no routine, detectable disease-causing bacteria. It moves up to A+ when the treatment process also removes nitrogen compounds, which can contaminate groundwater. Scottsdale’s advanced treatment plant goes one giant step further. It uses 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, food and other waste. Chemicals used to treat wastewater also add salt to the end product. The salinity of Scottsdale’s wastewater from homes in the northern part of the city is particularly high, around 1,100 milligrams per liter. Scottsdale estimates that water softeners account for more than 30 percent of the total salt concentration in the wastewater system. Water softeners work by exchanging salt for hard minerals, such as calcium and magnesium, creating soft water for homeowners but dumping the salty brine discharge into the sewer system.
The advanced treatment plant is capable of producing 20 million gallons a day of low-salinity recycled water, although current rates of wastewater flowing into the plant are closer to 10 million gallons. In the summer, the golf courses use all of that water. When temperatures climb into the 110-degree range and above, recycled water is supplemented with raw water to meet summer irrigation demands. During winter months, the golf courses typically need an average of only 3 to 5 million gallons per day and, after a few rainy days, that need may drop to zero. Any additional low-salinity water produced at the plant is saved underground at the Scottsdale Water Campus for future use. Scottsdale also uses ozone and UV treatments on the water it saves underground to recharge the aquifer. These processes remove “emerging contaminants,” which are contaminants the U.S. Environmental Protection Agency is studying, but has not yet regulated.
Scottsdale wanted to support its golf economy, but not use its precious drinking water supplies for irrigation. The dilemma was solved nearly two decades ago by a public-private partnership and technology. Today they call that innovation. Twenty years ago, it was just smart.
For 48 years, Arizona Municipal Water Users Association has worked to protect our member cities’ ability to provide assured, safe and sustainable water supplies to their communities. For more water information visit www.amwua.org.