CHECK OUT: What Happens Inside

If water and sewer services are purchased from a municipal or public utility, reducing blowdown and make-up water requirements will trigger a series of resource and cost savings in those utilities. If the site operates its own water treatment and wastewater treatment facilities, reducing blowdown and make-up water requirements will allow the facility to realize benefits. The blowdown water typically contains organic material, and the local wastewater treatment facility will charge extra sewage fees for accepting the water. These costs can be significant in the overall costs of operating a cooling tower. Discharge of the blowdown water to the environment on site is coming under increasing EPA regulation due to the contaminants typically found in blowdown water.

A Typical Process

A cooling tower functions to cool a circulating volume of water. The tower acts as a heat exchanger by driving ambient air through falling water, causing some of the warmed water to evaporate (evaporation gives off heat, providing cooling), and then circulating cooler water back through whatever equipment needs cooling (such as a chiller condenser). Typically, chemicals such as chlorine and chelating agents are added to cooling tower water to control biological growth (called "biofilm") and inhibit mineral build-up (called "scale"). The control of biofilm and scale is essential in maintaining cooling tower heat transfer efficiency.

As the water volume in the tower is reduced through evaporation and drift, the concentration of these chemicals and their byproducts increases. Cooling towers also pick up contaminants from the ambient air. To maintain chemical and contaminant concentrations at a prudent level, water is periodically removed from the system through a process called "blowdown" or "bleed off". The blowdown water and the water lost through evaporation and drift are replaced with fresh "make up" water (which will also contain minerals and other impurities). Blowdown water must subsequently be discharged to a local wastewater treatment facility or discharged on site to the environment. Blowdown water from a cooling tower can be sent to a municipal drain, or it may require on site pretreatment prior to disposal to a drain. In some cases, blowdown may be stored on site and then retrieved by a disposal service.