The unique water-conservation strategy designed for Natomas Gateway Tower in Sacramento, Calif., is a marriage of simplicity and sustainability. Harbison-Mahoney-Higgins (HMH) Builders Inc. designed a system for the new high-rise commercial building — on track for Leadership in Energy and Environmental Design (LEED) Gold certification — that will use cooling-tower bleed water to irrigate all 14 acres of the property. This measure alone will save 750,000 gal. of water a year.
According to Stephen D. Miller, PE, the mechanical, electrical, and plumbing coordinator for the project, the facility will utilize Clearwater Systems' Dolphin Series pulsed-power water-treatment technology for condenser water. Two separate Dolphin System units were specified in lieu of chemical or other less environmentally friendly treatment measures because the owner desired a chemical-free system and HMH had used Dolphin System equipment successfully in the past. Most importantly, the chemical-free water treatment was “green,” allowing the owner to recycle “blowdown,” or discharged water, for irrigation.
While the Dolphin System saves water by limiting the volume of blowdown typically required for cooling-tower water management, a cooling tower still needs to be bled routinely to keep minerals from accumulating in the water. The engineering team decided that this blowdown, or bleed water, easily could be used to provide irrigation water for the Natomas property. The team then set out to design a system that would use an oversized tank specified to hold water for the building's fire-sprinkler systems to store bleed water from the cooling towers.
The design is remarkably simple. One of the Dolphin System units serves a 1,000-ton (primary source) cooling tower, while the other serves a 250-ton cooling tower used for constant chilled-water loads (electrical rooms, data rooms, etc.). The units are piped in series with the cooling towers and work continuously to eliminate bacteria and prevent the accumulation of minerals that lead to pipe scale.
From the cooling towers, blowdown water is piped into a holding tank required by code as a second fire-water source. Engineers simply increased the size of the tank from the required 12,000 gal. to 18,000 gal. As a safety measure, the water supply for irrigation simply connects to the tank above the 12,000-gal. mark. That way, the tank's fire-water capacity cannot be depleted as a result of water being drawn for irrigation.
“It works perfectly because the highest demand for irrigation water coincides with the highest cooling loads in northern California,” Miller said. “Summer is when Natomas will bleed their towers the most, so there will be more water in the tank for irrigation during the dry season.”
The application of the Dolphin System will qualify Natomas for a LEED credit in the “Innovation & Design Process” category and help earn the “Water Efficient Landscaping: No Potable Water Use or No Irrigation” credit in the “Water Efficiency” category.
The Dolphin System works by imparting high-frequency, pulsed electric signals into flowing cooling-tower water. These pulses encourage calcium carbonate (the precursor to pipe scale) in the water to precipitate as a powder, which then can be cleaned or filtered out of the system. These same electric pulses help control microorganism growth by interfering with bacteria cell-wall development.
Pulsed-power water treatment is becoming increasingly popular among facility owners desiring greener (chemical-free) operations and wanting to eliminate the expense associated with regular chemical treatment.
Grey-water-recycling systems frequently raise questions, primarily concerning backflow, about code compliance. To ensure backflow would not be a problem, HMH came up with another surprisingly simple solution.
Potable field lines feeding the fire-water/irrigation tank are equipped with backflow-prevention devices. Additionally, HMH designed the piping so that the feed pipes stop above the tank, so if the tank overflows, liquid cannot back up into the water supply. That was enough to satisfy concerns regarding the possibility of bacterial contamination.
“My advice for implementing a system like this is to give careful thought to the potential for backflow and then talk to local building officials early on to acclimate them to your plans,” Miller said.
Recycling cooling-tower water is one of many sustainable practices employed for the Natomas building. Others include the use of no-water urinals, a mechanical system exceeding the state energy code by 20 percent, and hospital-grade air filtration.
“The application of the Dolphin system at Natomas Gateway Tower is a magnificently simple system of engineering elegance,” Jerry Ackerman, communications director for Clearwater Systems, said. “It's one of those ideas where we all tend to say, ‘Why didn't we think of that one sooner?’”
Information and rendering courtesy of Clearwater Systems.