Abstract: A cooling tower system and associated method is disclosed for controlling an aqueous coolant circulated through a cooling tower without requiring discharge of the coolant during recirculation. The coolant pumped from the cooling tower is filtered and monitored for characteristics. Generally acceptable levels of pH, temperature and TDS are maintained in the coolant to reduce the corrosive and scale-forming tendencies thereof and with those levels being maintained and monitored, the calcium content of the coolant is then adjusted in order to balance the Langelier Saturation Index (Sl) at a zero level. A computer is programmed to resolve control algorithms for Sl=0 and, using stored tabular data corresponding to the respective values of the monitored characteristics of pH, temperature, alkalinity and TDS, the computer calculates the value of calcium necessary to maintain a zero index.