How to Perform a Toxicant Evaluation

Controlling microbiological growth in cooling systems is of the utmost importance. A biologically fouled system can reduce plant efficiency, damage equipment and in some cases pose a health risk (legionella).

The two most common methods of controlling microbiological activity are by utilizing oxidizing (bleach, chlorine gas, bromine based products) and non-oxidizing biocides. This technical tip will discuss optimizing the use of the latter.

One will often hear that the "bugs" in a cooling system become immune to the non-oxidizing biocide utilized. This is not entirely accurate. Rather, the microbiological organisms that are less susceptable to the biocide(s) utilized increase in number through natural selection. Therefore, if you one has been using the same combination of non-oxidizing biocides for a long period of time, or the dose required to control activity levels increases, it is suggested that a toxicant evaluation be performed. Simply put, a toxicant evaluation is measuring the effectiveness of several non-oxidizing biocides on the system water in question. In other words, which biocides provide the greatest "kill". The following is the procedure for determining the most effective non-oxiding biocides for your particle system.

1. Obtain samples a number of different biocides, along with the price per gallon. Your current, as well as other water treatment companies should be able to provide you with 2 ounce samples.
2. Obtain a sample of your system water prior to any biocide addition. For example, if you normally add biocides on Mondays and Thursdays, it would be best to obtain the sample immediately prior to the Monday addition since this would have result in the longest period of time passing since the last addition.
3. Separate your water sample into the number of biocides to be tested times 2, plus 1. For example, if you were testing 5 biocides you should separate your water into 5 times 2, plus 1, or 11 samples. This allows you to have two different dosages for each of the biocides plus a control sample that receives no biocide addition.
4. Calculate "equal cost" dosages of the biocides and add them to the sample. We suggest starting with 50ppm and 100ppm of the lowest cost biocide and factoring the dosages of the higher cost products down from there.
5. Measure the biological activity in each sample after 1 hour, 4 hours, and 24 hours.
6. Calculate the percent "kill" by comparing the innoculated sample activity with the control sample. Record your data.
7. Assuming you have dosed the samples on an "equal cost" basis, the product(s) that have the highest kill provide you with the greatest "bang for your buck", and should be considered for use in your system.

If you don’t feel like performing this work on your own you can go to an independent lab or ask Thermidaire to do so for you so that you are sure of obtaining the best results and cost performance.