Specifier’s Notebook: Treating Legionella
For potable cold water , one can start by filtering all the water entering the facility at 5um. If the water supplier has difficulty maintaining the correct chlorine reserve in the facility, then install a wrap around chlorine feed system. The 5um filter will trap large particulates, hopefully slime sloughed off from within the distribution piping. Is the cold water a Legionella problem? Well, some microbiologists are of the opinion that bacteria exhibit exogenous dormancy, cold water keeps them in a “safe” state. I hold the opinion of heat induced pathogenicity.
Potable hot water systems are more of a problem area, primarily due to fear, well-intentioned code and ignorance. The use of “swabbing,” a qualitative test for LB within pipe surfaces, coupled with new diagnostic and detection techniques has shed light on this avenue of LB amplification and Legionnaires’ disease (LD) proliferation.
While it is an established fact that LB cannot survive at temperatures above 140°F, codes prevents health-care facilities from taking that route. Vendors of instantaneous hot water heaters tout the benefits for LB control, forgetting the necessary tempering that occurs downstream of the device. One must also consider that the high temperatures will cook whatever enters the device, only to serve as food for the citizens of any slime city waiting after tempering occurs. For energy conservation there may be merit to the heater. However, for Legionella control it is futile.
Increasing the temperature of the entire system to >140°F on a monthly basis has been used at two facilities known to me with satisfactory control results (i.e., no illness, no deaths attributable to LD with a raw water supply known to be prone to LB). Is this a good method? Hard to say, with many reported failures, however the correct course of action would be to alter code to allow >140°F water and mandate the use of thermo pinch valves on faucets and shower heads. With these valves, if the water temperature goes above a certain setpoint, they shut off and must be manually reset. These devices screw into the faucet like an aerator and tempering is done at point of use. As a side benefit one can unscrew them and flush the immediate 4 to 5 inches of plumbing with >140°F water periodically if the room went unused for some time.
What about hyperchlorination? This phrase has many definitions. I prefer to set limits and define in that manner. Since hypochlorous acid (HOCl) is the active killing agent obtained from bleach, it makes sense to generate that ingredient rather than use the hypochlorite, which is substantially less effective. Our definition is to maintain greater than 10 ppm free chlorine while keeping the pH within 5-6 via the addition of any acid for 24 hours. This would be a bit “touchy” for the average maintenance worker, but should be within the capabilities of any competent water-treatment professional with HVAC experience. Either way, be prepared for the foam, lots of foam, that will be generated as the organic slime is oxidized and lots of carbon dioxide is generated. It is also important to remember that continuous chlorination of the water will reduce pipe longevity considerably.
And how about using a copper/silver treatment? Certain papers on proprietary systems would make one believe this is the route to take. However, the failures of these systems are never shown, or they are brushed aside like so much meaningless disinformation. The ability of biofilms to protect their citizens from heavy metal attack and the protective nature of MT is the reality. (For a lengthy discussion of this issue, visit the Aquatech Web site and click on “Discussion on Copper/Silver”).
For those seriously contemplating copper/silver technology (which can cost up to $30,000), there is a far less expensive approach ($2,000-$6,000) that uses copper and silver salts fed by chemical feed pumps. One can guarantee greater control of the Cu and Ag ions with this approach than with an electronic Cu/Ag device.
Cooling towers and open circulating water systems offer a dual threat, contamination from the potable water as well as outside sources due to the air-washing nature of water-cooled heat rejectors. Contrary to the opinion of some, these systems are involved in many significant outbreaks of LD. A properly designed sidestream filtration system (not a centrifugal separator) can be part of the solution. Another action is to apply a biocide to a continuously operating system, leaving no stagnant water. A generic water-meter-actuated chemical feed/bleed system with separate timers for feeding at the time of least load will work. When choosing sand filters, consider surface area, biofilm production and system upset before deciding.
Hot tubs, whirlpools and hydrotherapy tubs are another problem area. These are impossible to clean as they have internal piping that are prone to biofilm production. There is only one make on the market, at present, that has a unit that is capable of being manually cleaned 100%. All others need to be cleaned with sodium hypochlorite (bleach) at a pH of 5-6 so as to maximize the oxidation process. This cleaning process is generally beyond the capability of a homeowner, janitor, plumber or whirlpool vendor, and poisonous chlorine fumes will be generated.
The bottom line is that the microbial world is full of surprises, and the biggest problems lie with biofilm and MT issues. Both, unfortunately, have Legionella news being printed faster than journals can keep pace.
Frank Rosa is a graduate chemist with a minor in Biology who has 39 years of experience in water treatment. He is the author of two books on this subject, ‘Water Treatment Specification Manual’ (McGraw-Hill 1983) and ‘Legionnaires’ Disease Prevention & Control’ (BNP 1993). He also currently serves as president of Aqua Technical Services, Inc. in Syracuse, N.Y., which he started in 1986.