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Aqua Technical Services president, Frank Rosa, explains why cooling towers should not be blamed for the recent outbreak of Legionnaires Disease in Sydney.

True to form, cooling towers are blamed for the recent LD outbreak because they stick out like sore thumbs and are found to be loaded with legionella. Instead, the focus should be on the real cause.

Let us “peel the onion” and get to the root of the problem, then, and only then, will we be in a position to address it.

We must consider the basics, legionella are natural aquatic organisms about 0.3 microns (mμ) by 1.0 mμ, bacteria, that inhabit ponds, lakes, rivers and, to an extent, estuaries. They are not to be found in the Sahara Desert (too dry) or at the Poles of the earth (too cold) under normal conditions.

Even in normal aquatic conditions they are not true pathogens to humans as they are to protozoa (amoeba).

One would be hard-pressed to locate one victim of legionellosis (the illness) among the passengers of the “Maid of the Mist” at Niagara Falls, NY or any cascading waterfall or from a river. So the question is why cooling towers, hot tubs and showers?

The answer seems to be evasive; it is heat that turns legionella from a Dr. Jekyll to a Mr. Hyde. Let us backtrack a tad, we have legionella in aquatic systems from whence we draw our water supply. That water supply is loaded with all kinds of life, even at the Poles, so prudence dictates we do “something” to protect those drinking the water from getting sick.

That something is chlorination, used as far back as the late 1890s, and used to stop an epidemic of typhoid in England.

Unfortunately, the chlorination of water, and to some extent, “filtration”, has been associated with the term “germ free”, or sterile, to the untrained in microbiology (and it continues today).

We must differentiate between “germ free” and sterile, as water suppliers are only obligated, at least in the USA and perhaps the UK, to provide potable water (ie it will not induce illness when imbibed, it is not sterile water that is bacteria free).

When chlorine is added to water it will adversely affect planktonic, free-swimming, organisms. It will kill some, hopefully all free-swimming human pathogens, and prevent others from multiplying or reproducing. However, nature always finds a way, and we soon learn that the little blighters have the means to evade being eradicated.

Just as a shark can detect blood from a great distance so can the microscopic population of the water detect deleterious changes in its environment. They will dive under the slippery bio-film deposited by some bacteria and thus escape in that fashion.

Others, legionella is one of them, may invade larger organisms (acanthamoeba) and find refuge inside that cell, multiplying within and surviving the chlorine attack within a cyst of the dead amoeba to be released when conditions no longer pose risk, i.e. no chlorine.

Where do we find ideal conditions? The potable hot water system is one ideal location when the water temperature inside the generator (hot water tank) is kept below 140ºF(60ºC), hot water tubs (spas) that cannot be cleaned and condensers in Chillers.

When the cold water supply enters the Potable H/W tank all the chlorine is destroyed, since the active ingredient is nothing more than chlorine gas dissolved in water. You know what happens when you open a can of warm soda, the gas comes out. For those with a penchant for chemistry, here’s the reaction for chlorine:
2Clº + HOH ↔ HOCl + HCl

In order to retain the chlorine one adds a bit of Caustic (Lye) [NaOH] to drive the reaction to the right by tying up the HCL, hydrogen chloride. The same thing happens in a hot tub, note the closeness of the nose to the bubbles. The condenser side of the centrifugal chiller is similarly hot, about 185ºF/85ºC skin temp in order to release BTU’s,.

An absorption machine has a two-pass condenser, with problems expected in the second pass. Heat drives off the chlorine and converts Dr. Jekyll to Mr. Hyde. When one adds additional contaminated water to the system one is assisting in increasing the bacterial population of those organisms that have succeeded in protecting themselves.

In the three cases illustrated, the equipment was the disseminator, but not the cause of the problem; it was the contamination in the water supply.
So, what’s the solution? The first line of defense should be a dual bank of properly sized 5 mµ bag filters in the main entering the building with a wrap around rechlorination system, with no dead ends, to assist in maintaining a 0.5 ppm residual of free chlorine in the distal ends of the system.

The make-up to the Potable H/W loop and cooling tower should have a dual bank of properly sized 1.0 mµ filter bags. Since there will be no chlorine in the H/W loop and cooling tower one may be tempted to install a Uv system. This may be okay in the H/W loop, but not the tower because Uv does not kill, it inactivates1.

The cooling tower should have a bag filtration system (not sand as sand is a wonderful amplifier of bacteria) and periodic doses of chlorine at 1.0 ppm at a pH as close to 7.0 as possible to ensure maximum HOCL presence.

Some have installed Copper/Silver Ionization systems in the potable H/W loop as a means to address the planktonic bacteria – but this article will not support that approach, too many unanswered questions and it cannot and it cannot be defended in a court of law.

About the Author - Frank Rosa is the president of Aqua Technical Services. He is an internationally renowned chemist and biologist with 50+ years experience in water treatment of potable and HVAC systems. Rosa designed and installed the first wrap-around filtration system with rechlorination in a hospital which has since become the defacto standard as the first line of defense in health care facilities. He is a consultant on waterside problems to industry as well as a technical consultant for barristers seeking answers for their clients. Rosa is an ASHRAE Life Member and former chapter president.

References:

Water Res. 2004 Jun;38(11):2757-63.
Photoreactivation of Legionella pneumophila after inactivation by low- or medium-pressure ultraviolet lamp.
See also http://trishul.sci.gu.edu.au/courses/ss12bmi/repair.html