Cooling Tower Blowdown Reuse

Along with losing water through evaporation and drift loss (droplets of water that become airborne), water is drawn off cooling towers through the process of blowdown, the removal of water as minerals become more concentrated due to evaporation. In some places, this blowdown water is used for landscape irrigation.

Two 8,000-gallon cisterns at the Sally Cheever Girl Scouts Leadership Center in San Antonio store rainwater and air-conditioner condensate, which are then used for landscape irrigation. Photo courtesy Brian Wilkie, San Antonio Water System

Water conservation expert Bill Hoffman of Austin, Texas, recommends that there should be no more than two or three cycles of concentration-a measure of how concentrated the water becomes due to evaporation-when blowdown water is used for irrigation. The San Antonio Water System, however, has not had a problem irrigating with blowdown water exceeding four cycles of concentration, according to Guz. Plants relying on blowdown water for irrigation should be selected for their resistance to salinity.

Schematic of the seawater distribution system in Hong Kong. Seawater has been used for toilet flushing in Hong Kong since the 1950s. Photo courtesy Brian Wilkie, San Antonio Water System

Although air-conditioning condensate is inherently pure when first produced, blowdown water is not. Cooling towers concentrate not only minerals, but also bacteria and other contaminants, including Legionella (a bacteria that causes potentially fatal Legionnaires' disease). If blowdown water is reused for irrigation or any other purpose, it must be treated.

Seawater Used in Buildings

While the practice is rare, it is possible to use seawater as a nonpotable water source for toilet and urinal flushing and a few other uses. The small city of Avalon, California, on the island of Catalina, off the coast near Los Angeles, mandates that dual plumbing be provided and seawater used for all toilet flushing, both commercial and residential, as well as for fire suppression.

While Avalon's saltwater piping system is not without its problems, including corrosion of pipes and difficulty treating the saltwater at the sewage treatment plant, other, much larger cities around the world are apparently having greater success. Hong Kong has used seawater for toilet flushing since the 1950s, as have a number of Pacific atoll islands, including the Marshall Islands. In Hong Kong, roughly 80 percent of the city's nearly nine million residents rely on seawater for toilet flushing-using nearly 200 million gallons of seawater per day in 2007. The seawater is screened to remove larger particles, then disinfected with sodium hypochlorite before use in this nonpotable application.

Final Thoughts

The eight sources of nonpotable water covered here provide viable options for reducing consumption of highly purified potable water-a priority in green buildings today but likely soon to become a priority in all buildings. Fresh water is an increasingly limited resource in many regions, particularly with growing populations, the rising incidence of drought, and the looming influence of climate change. As water supply dwindles, potable water prices rise, making these nonpotable water options economically attractive.

Relative to water, the number-one priority in designing any building today, commercial or residential, should be water efficiency. Dramatic reductions can be made in water consumption, both indoor and out, in all parts of the country. After establishing an aggressive water conservation agenda, however, the next priority should be to dual-plumb buildings and provide one or more alternative sources of nonpotable water. The nonpotable water can be produced either on-site or piped from a utility.

ALLOWED USES OF RECLAIMED WATER IN CALIFORNIA

Source: California Code of Regulations, Title 22.