Are closed-circuit cooling towers sources of pollution?

As closed-circuit cooling towers see increasing use in industrial cooling, questions have naturally arisen regarding potential pollution—such as whether the towers themselves pollute or if their exhaust emissions are harmful. Let us examine whether closed-circuit cooling towers truly generate pollution.
Closed-circuit cooling towers produce two types of emissions: evaporation and drift. Evaporative emissions consist of pure water vapor, though they may carry volatile contaminants. Drift emissions consist of water droplets containing dissolved and suspended solids; the chemical composition of these droplets mirrors that of the water circulating through the tower. Water treatment additives—used for corrosion, scale, and fouling control, as well as biocidal action—can generate VOCs, particulate matter, and toxic compounds. If industrial processes leak into, contaminate, or directly contact the cooling water, these contaminants must be accounted for when estimating the sources and quantities of air pollutants in Washington State.

Drift droplets remain suspended in the air, whether they are released as volatile gases or remain dissolved in the water. Closed-circuit cooling towers draw in ambient air, which can contaminate the cooling water. Calculating pollution levels requires applying best engineering judgment based on specific process chemistry, potential chemical exposure, tower water analysis, and material balances. While EPA emission factors serve as a useful guide, site-specific analysis is preferable. Particulate matter is a standard metric for cooling tower emissions; "drift" releases result in the emission of water treatment chemicals (such as salts) and inhalable particulates into the area surrounding the tower. AP-42 Emission Factor 13.4 can be used to estimate particulate matter and drift emissions. NESHAP regulations regarding industrial process cooling towers (Rule 94-21957) have effectively eliminated the use of chromium-based water treatment in these systems, removing a major source of chromium emissions from evaporative condensers and large-scale chemical treatment processes.
The chromium previously used in these systems was in the hexavalent (Cr+6) form, a toxic air pollutant. Phosphates and polymer dispersants are common choices for water treatment, while molybdates and zinc are also utilized. The use of chromium-based water treatment in industrial process cooling towers is discouraged, as it would trigger NESHAP compliance requirements. Biocides are added to cooling towers to inhibit microbial growth; Washington State has reported the use of chlorine, bleach, and bromine in these systems. Other biocidal systems may also be used. Water treatment systems are constantly evolving, and suppliers often keep the chemical composition of their systems confidential. While they may disclose the chemicals used for regulatory compliance, they typically keep the specific formulations proprietary.