Holding Time Index
As a general rule of thumb, Holding Time Index is a useful cooling tower calculation, but what does it present, and what decisions can be derived from it?
Every cooling tower presents a unique combination of metals, equipment, water chemistry, temperatures, and operational characteristics that make water treatment decisions prescription-based. As there are several issues that need to be addressed chemically, the focus of this article is biological control. Biofouling is a frequent and persistent problem in industrial water systems. The control ranges of temperature and pH, aeration, and abundance of nutrients in these systems provide an excellent environment for the growth of several biological species. When considering which biocides will perform the best, the holding time index is a useful calculation.
The Holding Time Index (HTI) is a calculated expression of the half-life of a chemical added to a given cooling water system. In other words, it is the length of time for a given chemical to be diluted to 50% of its original concentration. Half-life calculations are quite popular in the radioactivity industry and the same principle applies to water treatment. The HTI also presents the amount of time required to concentrate the makeup solids by a factor of 2. As several important decisions can be made based on the HTI, deciding which non-oxidizing biocide is certainly one of them. The HTI can be calculated as follows:
For example, if the system volume is 4,000 gallons and the blowdown rate is 10 gallons/min, then the HTI = ln(2) X [(4,000 gallons) / (10 gallons/min)] = 277 minutes or 4.6 hours. This becomes useful when deciding which non-oxidizing biocide will be most effective as each requires a certain amount of time for the chemical to be effective at its prescribed concentration, otherwise known as a biocide’s “contact time”. A biocide such as carbamates require a contact time of 6 to 8 hours, making it not an ideal choice for the above scenario.
Often non-oxidizers are intermittently slug-fed into a system based upon the biocide’s retention time and a typical aim is for 20% of its original concentration dose. The retention time of a biocide in a cooling tower (time elapsed until target re-dose point) can be expressed as:
The 1.6 is a rough estimate based on multiplying the ln(2) x 2 which would present a 25% decay. The 20% chemical residual is a fair estimate when biocides are being alternated. From these formulas, a biocide dilution curve can be computer generated to present a visual of a chemical’s depletion. Note that the HTI only accounts for the bleed-off rate and the actual dilution may occur more rapidly. A fluorescent tracer can be introduced/monitored to better reflect actual retention rates.
With a minimal amount of information, the HTI is a powerful calculation in the water treatment industry. Not only can a biocide be chosen, but a practical time for it to be re-dosed can also be calculated. This information is vital to ensure that biocide introductions are being maintained within their respective EPA regulatory approvals while maintaining effectiveness.
List of Resources:
 Lane, R. W. (1993). Control of Scale and Corrosion in Building Water Systems (1st Ed). McGraw-Hill.
 Golden, C. (n.d.). Basic Calculations Necessary to Survive in the Field—Part I: Cooling Water. Association of Water Technologies. Retrieved June 4, 2022, from https://www.awt.org/pub/?id=019E3A00-DF82-84B1-E8A9-3599FC74D720
 Industrial, T. (2018, March 22). Calculating HTI Allows You to Properly Protect and Operate Open Cooling Tower Systems. Taylor Industries. Retrieved June 4, 2022, from https://taylorindustrial.medium.com/calculating-hti-allows-you-to-properly-protect-and-operate-open-cooling-tower-systems-b131af9b3f7d
 Zaffalon, A. (2021, January 20). Cooling Tower Basic Calculations. Yamatho Supply LLC. Retrieved June 4, 2022, from https://www.yamathosupply.com/blogs/news/cooling-tower-basic-calculations-2