Non-Chemical Water Treatment for Cooling Towers
Scale forms when dissolved minerals (generally calcium carbonate) and other solids in the water crystallize on surfaces. This further reduces the system’s efficiency by clogging water paths and insulating heat-transfer areas: a 1/32" (0.8 mm) layer of scale can increase energy use by close to 10%. Evaporation exacerbates scale formation, and adding make-up water introduces more minerals to the process. This is partially controlled by “blow-down,” a process of replacing some of the solids-laden recirculating water with make-up water.
Treating cooling-tower water to prevent biological fouling, scale, and corrosion is a complex, highly monitored process. Most of the dirty work in the half-million cooling towers in the U.S. is accomplished with biocidal, conditioning, dispersant, and scale-inhibiting chemicals, including chlorine, various brominated compounds, phosphates, molybdenates, acids (including sulfuric acid), and zinc compounds (which are now banned for cooling-tower use in about half of U.S. states). While chemicals do get the job done, there are considerations beyond the tower. Regulations are increasingly stringent for chemical storage, handling, and disposal; in many jurisdictions, chemically treated cooling-tower blow-down water itself is regulated. There are also consequences to worker, public, and environmental health due to accidental spills, chronic chemical exposure (even at low levels), and bioaccumulation of persistent chemicals in the food chain. Reducing or eliminating chemical usage in the treatment of cooling-tower water is an appealing thought.
Published April 1, 2005