Engineered Systems - February 2008 - (Page 52) Eight Ways To Keep Legionella Out Check the cooling water regularly — preferably daily but at least twice weekly — for pH, total dissolved solids, disinfectant levels (if applicable), and other parameters required for evaluating the water treatment regimen. At least two times a week, test the total bacteria count (TBC) or Adenosine Triphosphate (ATP) level to evaluate the water treatment — a significant rise indicates a problem (e.g., a broken chemical pump). However, do not assume that a cooling tower is Legionella-safe just because the TBC is low. There is no reliable correlation between TBC and Legionella bacteria. The build-up of slime on this cooling tower indicates inadequate water treatment and a need for cleaning. In the vicinity of humidifier installations, check ductwork periodically for evidence of condensate pooling. In some cases, moisture from humidifiers pools downstream in the duct. t t For a load of less than 150 tons, consider alternatives to small cooling towers. Otherwise, be sure to monitor the water treatment program of small towers closely, and check Legionella levels regularly. If Legionella are not under control, even after changes in the treatment regimen, the small tower should be replaced with an air cooled unit, or its load shifted to another cooling tower. Small cooling towers have been implicated in numerous outbreaks of Legionnaires’ disease. This could be due to difficulty in treating water that is turned over frequently, but more likely is because small cooling towers do not get the maintenance attention that large ones do. Install and maintain high-efficiency drift eliminators. Consider replacing drift eliminators on older towers with new higher-efficiency models. Replace deteriorated eliminators. However, do not rely on high-efficiency drift eliminators to give total protection against transmission of Legionella bacteria. Some small droplets (smaller than 5 microns) will likely escape even the best eliminators. Legionella prevention is smart. Spending a modest amount of money on preventive measures yields a significant reduction in health and legal risk, and it may even save a life. ES Freije is president of HC Information Resources Inc. (Carlsbad, CA, www.hcinfo.com). He is an author, course instructor, and consultant specializing in Legionella and other waterborne pathogens. Freije earned a B.S. degree in mechanical engineering from Purdue University, a Water Treatment Plant Operations Specialist certificate from California State University, Sacramento, and is a Certified Water Specialist. He is the editor of Legionella Enews, an international newsletter that covers outbreaks and new technology. His book, Legionellae Control in Health Care Facilities: A Guide for Minimizing Risk, has sold in more than 30 countries. Portions of this article were excerpted from Freije’s recent publication Legionella Management Plan for Hotels. Freije can be reached by e-mail at mf@hcinfo.com. cti.org), OSHA, and the Association of Water Technologies (www. awt.org), particularly regarding the combination of oxidizing and nonoxidizing biocides. Follow ASHRAE Guideline 12-2000 regarding cooling tower shutdown and startup. If Legionella bacteria counts are above the target level, consider using side stream filtration to reduce suspended solids and the demand on the biocide. In most cases, filtering 3% to 5% of the circulated water to 10 to 15 microns is sufficient. A filter vendor can recommend a media based on a particle size analysis. Physically clean cooling towers at proper intervals. Studies indicate that cleaning alone has little effect on Legionella bacteria concentrations but is crucial for effective water treatment. Deposits become sites for bacteria and corrosion. A buildup of algae, fungi, protozoa, dirt, and biofilm puts a greater demand on a biocide and may prevent a biocide from making contact with microorganisms. The cleaning frequency will vary from tower to tower and depend on operating conditions and the effectiveness of the water treatment program. Clean basins monthly or more often if significant buildup of deposits occurs between cleanings. Ideally, the entire cooling tower system should be drained and flushed and all surfaces thoroughly cleaned two to four times a year. More frequent cleanings may be necessary if there is heavy scale deposition or excessive solids entering the system (e.g. if the tower is near a dusty construction site). A professional cooling tower cleaning service should determine the appropriate procedure for a given tower. Avoid conditions that promote Legionella growth in cooling towers. For example, remove dead legs (infrequently used piping with stagnant water) from the system. A significant volume of water may be contained in the piping that circulates water between the cooling tower and chiller. Biocides will not likely be effective in stagnant areas or dead legs. Keep pipe lengths as short as possible (e.g., to and from the heat exchanger) and the number of fittings to a minimum to minimize biofilm within piping. Biofilms are where Legionella bacteria flourish within cooling towers; the loosening of biofilm has been associated with outbreaks of Legionnaires’ disease. Shield the cold-water basin and other wet surfaces from sunlight to prevent algae growth. And keep kitchen exhaust, overhanging tree limbs, plants, and other sources of living organisms away from cooling towers. 52 En gi neer ed S y stem s February 2008 http://www.cti.org http://www.awt.org http://www.awt.org http://www.hcinfo.com
For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.