Application Profile: Legionnaires' Disease Bacteria
Solution: Clean Cooling Tower Systems Including the Proper Microbicide
Cooling towers, evaporate condensers, air washers and their associated growth environments for all types of microorganisms. Some of this equipment, used in comfort air conditioning sites of outbreaks of Legionnaires’ Disease, was found to be largely untreated and therefore heavily fouled.
Since the first reported outbreak of Legionnaires’ Disease at a Philadelphia hotel in July 1976, there have been at least 11 epidemics of the disease resulting in 1011 cases and 156 deaths. Legionnaires’ Disease bacillus, isolated by the Center for Disease Control in Atlanta, was named appropriately
Legionella pneumophila. The bacterium grows most rapidly in a low oxygen, high nutrient, medium pH (6.9 – 7.0) environment. This environment is often present in a heavily fouled, untreated cooled system. The drift or mist from an infected cooling tower can be carried by the wind into building air intakes. At least one outbreak of the disease appears to have been caused in this manner. Control of Legionella PneumophilaProper cooling tower maintenance should include a complete water treatment program including corrosion inhibitors, dispersants, pH control agents (if necessary) and microbicides. This approach is necessary for the optimum functioning of the biocides included in a program. The Center for Disease Control supported this ‘clean system’ approach in its Morbidity and Morality Weekly Report of September 29, 1978, which reads in part: “It might be prudent to encourage adherence to recommendations that have been made for many years for satisfactory mechanical operation of small to medium size cooling towers and evaporative condensers which are used as part of the air conditioning systems for many public buildings and business establishments. Such units should be regularly treated with chemicals that have been tested and shown to be effective in preventing slime, corrosion, algae and high population of bacteria in cooling water or on water contact surfaces. Individuals or contractors skilled in maintenance of cooling towers should monitor treatment on a regular basis to assure that it has been carried out.”
In its Morbidity and Morality Weekly Report of June 22, 1979, the CDC cites their tests of didecyl dimethyl ammonium chloride (a quatemary amine) and 2, 2’-dibromo-3-nitrilopropionamide. According to this report, the quaternary ammonium based Microbicide was tested at normally recommended dosage rates and exposure times. Under these conditions, this Microbicide was effective in preventing recovery of Legionnaires’ Disease bacteria from the water. Testing was less complete on the nitrilopropionamide based Microbicide but initial results showed it also to be effective against the bacteria.
Mechanical Safeguards
The CDC also suggested that mechanical safeguards be built into cooling towers to prevent back syphonage. Back syphonage, the backflow of non-potable water into potable water systems, may occur whenever there is a water pressure below atmospheric pressure in a submerged potable water inlet to a tank of non-potable water. Submerged inlets connected to float valves are frequently found in cooling towers; and since many cooling towers are located on the top of buildings, the chance of low potable water pressure is greater than in other building locations.
An air gap with a vertical distance of twice the diameter of the water supply pipe above the overflow rim of the cooling tower reservoir is necessary to prevent any possibility of back syphonage.
With some simple mechanical safeguards and the use of a properly designed water treatment program, Legionella pneumophila doesn’t have the opportunity to be a problem in any open recirculating cooling system.