Not only do cooling towers consume a great deal of water; they also frequently use water inefficiently.
These tips from A Water Conservation Guide for Commercial, Institutional and Industrial Users, courtesy of the New Mexico Office of the State Engineer, highlight ways to improve the water-use efficiency of a cooling tower by reducing the water lost to bleed-off.
Operate bleed-off on a continuous basis, rather than by the "batch" method
Most cooling towers are bled-off automatically when the mineral concentration, as measured by conductivity, reaches a specified level. This is usually done by the batch method, releasing large quantities for a preset period of time or until the conductivity reaches a preset low level.
Unfortunately, this method can lead to wide fluctuations in the conductivity, which wastes water. Instead, try to operate the bleed-off on a more continuous basis, keeping the conductivity closer to the limits. Set the bleed-off timer for a shorter time, or set the low-end conductivity higher, not much less than the bleed-off start level.
Install conductivity and flow meters on make-up and bleed-off lines
This will allow the operator to closely monitor the volumes of water being used and verify that the system is operating at optimum parameters. Meters that display total water flow as well as current rate of flow are most useful.
Read meters regularly
Keep a log of make-up and bleed-off consumption and dissolved solid concentration, evaporation, cooling load and concentration ratio.
Add an automatic control to shut off the unit when it is not being used (i.e. at night or on weekends).
Select your treatment vendor with care
Tell vendors that water conservation is a high priority, and ask them to estimate the quantities and costs of treatment chemicals and volumes of bleed-off water, as well as the expected concentration ratio. Keep in mind that some vendors may be reluctant to improve water efficiency because it means the facility will purchase less treatment chemicals.
Adjust pH by adding sulfuric acid
Carefully adding a controlled amount of sulfuric acid to the cooling tower water lowers the pH and prevents scale by converting a portion of the scale-forming minerals into more soluble forms. This option may reduce water consumption by up to 25 percent. Make sure that workers are fully trained in the proper handling of acids. Also note that acid overdoses can severely damage a cooling system, so use a timer and add acid at points where the flow of water is well-mixed and reasonably rapid. Also be aware that by lowering the pH, you may have to add a corrosion inhibitor.
Install sidestream filtration
Routing cooling tower water through a rapid sand filter or high-efficiency cartridge filter is a particularly good method for improving water quality in places where airborne contaminants and water cloudiness are common and for systems with narrow passages susceptible to clogging. Filtration improves cooling tower efficiency and cuts down on the need for maintenance. Sidestream filtration is particularly helpful if your cooling tower is subject to dusty atmospheric conditions.
Treat water with ozone
Ozone not only kills viruses and bacteria, but it may also control corrosion by oxidizing inorganics and soluble ions. Ozonation can improve water quality without the need for additional chemicals. (Note: While ozone is a powerful oxidizing agent, its effective life is less than one hour. This means it must be produced at the site. Ozone is also highly corrosive. Materials compatibility must be considered in any system that uses ozone.)
Recycle and reuse
Even if you have done everything possible to improve water quality, you can still save water by finding other uses for bleed-off water. You may also discover additional sources of make-up water such as single-pass cooling systems used elsewhere in the facility. Some high-quality municipal treated wastewater may also be acceptable provided the tower is operated at somewhat conservative concentration ratios.
Explore other options
Some vendors claim that magnets and electrostatic field generators dislodge mineral deposits and scale without the use of chemicals. Be aware that these claims are unsubstantiated. Investigate these systems thoroughly before a possible purchase.
The complete guide can be accessed at www.seo.state.nm.us.
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