Cooling towers and water quality

A Review of the potential effect of cool ing t wer blowd wn on the quality of the receiving water has been presented by Lee and Stratton1 and Stratton.2 They have discussed problems that may be en countered in meeting stringent water qual ity standards for cooling tower blowdown water that is discharged to natural waters.3 This paper presents the results of a sur vey of selected cooling tower blowdown waters for selected chemical parameters and discusses the potential aquatic environ mental effects of these blowdown waters. The evaporative cooling tower disposes of waste heat to the atmosphere by the evaporation of water. Heat is disposed of in this manner for the convenience of oper ating a closed-cycle cooling water recircula tion system or to avoid disposal of heated effluent that may result in ecological degra dation. Evaporative cooling towers are used extensively to remove heat from com mercial air conditioning plants, from ther mal power plants, and from many industrial processes or large pieces of machinery that generate waste heat. They range in size from units circulating a few hundred gal lons per minute to units circulating over 1,200 cfs (20,400 cu m/min). It is expected that the number of operat ing cooling towers will continue to increase rapidly to meet the needs of the continued expansion of air conditioning and the neces sity for industries to meet increasingly more stringent thermal criteria. The rate of expansion of the electric generating in dustry, coupled with concern for the effects of thermal pollution, promises a rapid in crease in the number of large cooling towers. Makeup water must be added continu ously to a wet cooling tower system to make up for the evaporative loss. As a result of continuous evaporation, the sys tern will concentrate natural salts added with the makeup water. A limit is soon reached at which large amounts of chemi cal deposits, principally calcium carbonate and calcium sulfate, form, thereby inhibit ing operation of the system. For this rea son, a portion of the recirculating water is discharged either continuously or intermit tently from the tower as blowdown. Blowdown of the brine from the cooling tower basin affords control of the concen tration of salts and, hence, scaling in the system. For a large cooling tower, blow down is usually continuous and amounts to 0.5 to 3 percent of the recirculating water flow rate. Smaller towers frequently are designed for intermittent blowdown in vol umes of about 5,000 gal (19,000 1). By blowdown, the concentration of salts in the system is generally maintained at a factor of two to eight times the concentration in the makeup water.