Utilization of Rainwater as a Supplementary Water Source for Cooling Tower Makeup: A Sustainability Strategy for Potable Water Use Reduction

The use of rainwater as a supplementary water source for cooling water makeup was explored in an effort to reduce the potable water demand of theUniversityofTennessee’sKnoxvillecampus. A water quality analysis involving the measurement of parameters relevant to cooling tower operation was conducted on tap water currently used for makeup supply and rainwater collected from the roof of a campus building. In anticipation of limited rainwater supplies due to issues of catchment surface area, collection efficiency, storage capacity, and climatic conditions, blends of rainwater and tap water were also analyzed. The dissolved solids concentration of the rainwater was significantly lower than in tap water, which indicated that a higher number of cycles of concentration (COC) could be achieved should rainwater be used in the makeup water source. Predicted COC values for the rainwater/tap water blends were calculated based on silica and conductivity measurements and were higher than the 4 ± 0.6 COC at which the campus towers were operating at during the investigation. A back-calculation using the blended COC values was used to determine potential makeup demand reductions. The replacement of tap water with rainwater in the makeup supply was found to contribute to the total tap water savings to a greater extent however, than increasing COC. Based on a rainwater supply estimate of 116 million gallons, it is possible that anywhere from a 20-60% blend may be utilized in each of the campus cooling towers if the average annual heat load is between 50-100%. At the current price of water and wastewater, the maximum cost savings associated with these results is about $343,000 for the case in which a 40% blend is used for cooling towers operating at a 75% heat load.