HTI’s Effect on Biocide Performance

In the last post we talked about the Holding Time Index (HTI) of cooling water systems. We discussed how the HTI is the half-life of water and treatment chemical in the system, which can be calculated by dividing the system’s water volume (V) by the blowdown rate (BD) and multiplying this value by the natural log of 2 [ln(2) = 0.693]. Knowing a system’s HTI can give you approximate concentrations of treatment chemicals over time, a very important factor to know when dosing non-oxidizing biocides. So, why is this essential?

All non-oxidizing biocides have effective minimum dosage rates and minimum contact times to assure they produce the intended results. Biocide concentrations not meeting those minimums are non-effective and lead to wasted chemical.

Let’s say you need 100 ppm of a biocide for 24 hours to sterilize a cooling tower system. If you add 100 ppm to the system and the system operates normally and bleeds water based on conductivity, then the biocide concentration will be reduced each time water is bled and fresh water comes into the system. If the system’s HTI is 24 hours, then, by definition, only 50 ppm will be present after 24 hours. The biocide will not have been able to properly do its job. Therefore, in order to maintain 100 ppm throughout the entire 24 hours, you will need to add 200 ppm of chemical into the system.

After consulting with my esteemed colleague James McDonald, we have determined the equation for calculating the initial minimal dosage required given the desired treatment conditions (Ci):

Ci = 10^[(Log Cf) + (0.30 x T/HTI)]

Or, the initial dosage (Ci) is equal to 10 to the power of (^) the log10 of the final desired dosage concentration (Cf) plus the factor of 0.30 times the contact time for the chemical (T) divided by the system’s Holding Time Index (HTI). Whew! (Thanks, James!)

Using the formula above, we can calculate that if we want to dose 100 ppm (Cf) of biocide for a minimum of 24 hours (T) in a system that has an HTI of 24 hours, we will need at least 200 ppm (Ci) of biocide for our initial dosage. (I say “at least” because there will be some biocide consumed by the demand in the water that needs to be accounted for when determining the amount of chemical to add.) If the system has an HTI of 48 hours, the calculation shows I will only need 141 ppm (Ci) of product. The lower dosage amount is due to the reduced blowdown rate (BD) compared to the system volume (V) from the higher HTI (48 hours vs 24 hours in the previous example). At higher HTIs, the chemical has a chance to be in the water at greater levels for a longer period of time.

Based on this discussion you can see that HTI plays a significant role in making sure you add the correct amount of non-oxidizing biocide to assure desired results.

Finally, to be even more certain you are getting the best results from your efforts, you can always test for the biocide residual at the end of the prescribed contact time.

Please let me know if you have any questions or comments.