Chemical Treatment Fosters Zero Discharge by Making Cooling Water Reusable

Abstract

Over the past decade, the water requirements for cooling industrial manufacturing processes have changed dramatically. Once-through cooling has been largely replaced by open recirculating cooling water methods. This approach reduces water consumption by increasing the use of recycled water. Simplistically, the circulating cooling water flows through heat exchanger equipment and is cooled by passing through a cooling tower. The recycled water is cooled by evaporation of some of the circulating water as it passes through the tower. As a result of the evaporation process, the dissolved solids in the water become concentrated. The evaporated water is replaced by fresh makeup water. The dissolved solids content of the water is maintained by the rate of water discharge (blowdown). As the amount of dissolved solids increases, their solubility is exceeded and the solids tend to precipitate from the cooling water. The precipitated scale adheres to heat transfer surfaces and reduces heat transfer efficiency. In order to achieve zero discharge of water, it is paramount that the potential for scale formation and deposition be minimized. This can be accomplished through physical separation of scale-forming ions and particulate matter. Two widely used mechanical methods in this category are lime-soda side stream softening and vapor compression blowdown evaporation. Another approach is chemical treatment to promote scale inhibition and dispersion.