News | March 16, 2000

Bacteria Counts and Cooling Tower Water: Debunking the Myths

Many people believe that bacteria counts performed on cooling tower waters can accurately gauge the levels of bacteria within that system. They rely on field test kits, such as the "Dip-Slide" test, to monitor bacteria levels. Then, they use those results to determine what amount of biocides they should introduce into their cooling system. The truth is, however, that this method is not only unreliable, but can actually have dire results on the whole system.

The "Dip-Slide" tests are ineffective because they show just the planktonic (free-floating) bacteria that happen to grow on the dip-slide during a particular testing period. Not only do testing periods vary widely from session to session, but it is the sessile (attached) bacteria, not planktonic bacteria, that cause the real corrosion and fouling damage. Sessile bacteria are not only more resilient to biocide treatment, they can also outnumber planktonic bacteria in the same system by thousands of times. It is entirely possible to have heavy slime deposits in your cooling tower even though a bulk water bacteria count has shown the presence of few bacteria.

Because the presence of sessile bacteria are not revealed in a bacteria count, low counts may lull personnel into decreasing biocide use, allowing the bacteria to grow unchecked and causing a multitude of problems, including:

  • Bacteria-driven corrosion from the formation of a sticky biofilm that not only causes corrosion, but that also attracts debris and solids, leading to under-deposit corrosion.
  • Decrease in cooling tower efficiency, leading to higher costs.
  • Biological health concerns.

There are other ways of attempting to monitor biological fouling, such as the use of removable coupons both with and without pressure gauges, but even these are unreliable and difficult to assess. "The best way to maintain biological control in a system is to obtain an accurate system volume, and then to dose the recommended biocides based on that volume. Two different but complementary biocides should be dosed regularly and alternately at recommended levels," said Bill Dignin, Biocide Product Manager at Garratt-Callahan Company.

By simply maintaining a strict adherence to a proper cooling tower treatment program, including regularly scheduled biocide treatments, the operator can ensure an efficiently performing cooling system, zero to minimal corrosion, and the general health and safety of the water. For more information, please contact a Garratt-Callahan Company representative at (650) 697-5811 or see us on the Web at