Palo Verde Nuclear Generating Station Wastewater Cooling System

Palo Verde Nuclear Generating Stations sits 45 miles west of Phoenix.  It is the largest power plant in the country, and the desert facility uses something no other nuclear power plant in the world does: wastewater.  Wastewater from Phoenix’s 91st Avenue wastewater treatment facility and Tolleson’s facility is piped to the nuclear facility underground -- essentially in one giant 36.5-mile pipe -- for use in Palo Verde's cooling towers.

The wastewater moves through the first 28.5 miles of the pipe by gravity alone; it's pumped the remaining 8 miles to Palo Verde. Once on-site at the APS-operated facility, the water is purified again.

Next, the water is channeled into reservoirs at the facility for temporary storage before it is used in the cooling towers near Palo Verde's three reactors. The plant needs 60,000 gallons of treated wastewater per minute.  The plant’s three nuclear reactors each require 20,000 gallons of water per minute.

Palo Verde Facility View From Above.png

Plant managers have to maintain a constant flow of wastewater to the on-site reservoirs to make up for water lost to evaporation.

Seven Southwestern companies own the plant.  Its largest stakeholder is Arizona Public Services (29.1%), but it is also owned by Salt River Project, El Paso Electric, Southern California Edison, Public Service Co. of New Mexico, Southern California Public Power Authority, and L. A. Dept. of Water & Power.

The plant, in commercial operation since 1986, produces power for about 4.1 million people across regions in New Mexico, Texas, California and Arizona.  Each of the turbines within the reactors generates 1,400 megawatts of power.

Palo Verde supplies 80 percent of Arizona's carbon-free electricity.  Only 1.3 percent of the APS energy portfolio came from renewable energy in 2014, while 27.2 percent came from nuclear. Coal leads all sources with 33.5 percent, according to the plant's owner.

If the wastewater supply were to be suddenly cut off, Palo Verde keeps more than a 13-day supply of water in its reservoirs, designed to give facility staff enough time to fix any issue.

According to Nuclear Engineering International: 

Fresh water in the Phoenix, Arizona area comes from the Verde, Salt and Colorado rivers. After it is used by the two million people living in the area, it reaches the 91st Street water treatment plant in Phoenix. This plant, built before Palo Verde but expanded several times since then, actually serves four areas in the metropolitan Phoenix area: Phoenix, Scottsdale, Mesa, Tempe and Glendale, although the city of Phoenix owns more than 50%. Its rated capacity is 204 million gallons/day, but its operational amount is 130-150 million gallons/day. Another treatment plant nearby, capacity 17 million gallons/day, serves the city of Tolleson. Both plants carry out primary and secondary water treatment: primary sludge settling and separation, and degradation of the biological constituents of sewage.

Normally, wastewater treatment plants pump the treated effluent back into a river. At this plant, a 114-in (3m)-diameter pipe diverts up to 90,000 million gallons/day toward Palo Verde; another pipe also takes up to 6 million gallons/day from Tolleson. (Other effluent streams go to irrigation in Buckeye, Arizona, and an artificial wetlands area next door, and the Salt and Gila Rivers). The utility consortium that owns the three-unit Palo Verde Generating Station buys the water from the city; it is authorized to take up to 24 billion gallons/year, or an average of 65 million gallons/day, but uses more like 21-22 billion gallons/year (and up to 2.2 billion gallons/year from Tolleson). It pays based on a unit of an acre-foot, 326,000 gallons/year or 1233.5 m3, in a price that is escalated per year.

Water flows 100 ft downhill inside the buried pipeline for about 28 miles (6 miles at 114 in diameter pipe, 22.5 miles at 96 in diameter pipe) until it reaches the Hassayampa Pumping Station, which pushes it 125 ft uphill the remaining eight miles to the site treatment plant in a 66 in-diameter pipe.

(AZFamily, Tucson.com, 1/1/2016, Nuclear Engineering International, 6/25/2013)