Biocide Holding Time

Microbiological challenges in cooling water systems result in significant financial losses for businesses due to increased energy consumption, production downtime, unscheduled maintenance, and equipment replacement. Insufficient microbiological control may also lead to hazardous conditions, including the risk of Legionnaires’ disease contamination.

Although this article emphasizes biocide contact time, several additional factors must be evaluated when selecting a biocide, including:

• Types of microorganisms present

• Water characteristics

• Biocide properties and limitations

• Environmental restrictions

• Product label restrictions

• Feed methods

• Holding time

• Biocide contact time

Biocide contact time and the holding time index (HTI) are both critical considerations in the design of an effective biocide program. Contact time refers to the minimum duration a biocide must be maintained at an effective concentration to achieve microbial control. Fast-acting biocides may require contact times as brief as 30 minutes, whereas slow-acting biocides may necessitate periods exceeding six hours.

The holding time index (HTI), also referred to as half-life or retention time, quantifies the period required for a chemical in a cooling water system to be diluted to 50% of its initial concentration as a result of blowdown. For instance, if 100 ppm of biocide is introduced, the HTI specifies the time needed for the concentration to decrease to 50 ppm.

 HTI is determined by system volume and blowdown rate:

Cooling systems characterized by a small water volume relative to the blowdown rate exhibit a short HTI, whereas systems with larger volumes possess a longer HTI.

For example, a 500-gallon evaporative condenser with a 225 GPH blowdown rate yields an HTI of 1.6 hours. In contrast, the same blowdown rate in a 10,000-gallon system results in an HTI of 31 hours.

Systems with a short HTI, such as evaporative condensers or fluid coolers, should be treated with rapidly acting biocides. Otherwise, most of the biocide will be removed by blowdown before effective microbial control is achieved. Alternatively, a higher dosage of a slower-acting biocide may be used to maintain the required concentration for the necessary duration; however, this approach can be costly and must not exceed the maximum dosage specified on the product label. Rapid addition of non-oxidizing biocides is particularly important in low HTI systems.

Cooling towers with a long HTI, exceeding 48 hours, may accumulate elevated levels of contaminants, thereby complicating microbiological control and potentially necessitating increased biocide dosages.

HTI must also be considered when selecting scale and corrosion treatments, as essential inhibitors may degrade or be consumed in systems with extended HTI.

In systems subject to seasonal blowdown variations, such as most HVAC systems, HTI will also fluctuate seasonally.