|Content Type||Conference Paper|
|Title||Non-Oxidizing Biocides For Legionella Control|
|Authors||Steve Najmy, Sr. Technical Specialist, The Dow Chemical Company|
|Source||CORROSION 2007, March 11 - 15, 2007 , Nashville, Tennessee|
|Copyright||2007. NACE International|
|Keywords||Legionella, non-oxidizing biocide, biofilm, protozoa, amoebae, glutaraldehyde, DBNPA,Isothiazolone, polyquat, carbamate|
Control of Legionella bacteria came into focus after 1976. Oxidizing and non-oxidizing biocides already approved for use in cooling towers were tested in the laboratory to determine their efficacy against Legionella bacteria. This paper summarizes some of these laboratory studies.
There have been many studies over the years to explore the use of non-oxidizing biocides and their effectiveness in preventing the growth and proliferation of Legionella bacteria. A review of the literature suggests an evolution for the design and scope of lab-scale investigations to better approximate the real-world conditions that help to protect and populate Legionella. In fact, more recent studies include the efficacy of various biocides not only against the Legionella bacteria but also Legionella harboring protozoa, while another research project uses a model water test system to better evaluate biocide efficacy against sessile forms. This paper provides a summary of some of the previous published studies since it provides new insight into better understanding the research needs and challenges to prevent another onset of a serious public health issue.
NON-OXIDIZING BIOCIDES VS. OXIDIZERS
Depending on cost and performance, non-oxidizing biocides are becoming more popular either alone, or in combination with oxidizing biocides for use in recirculating cooling water systems. Cost includes both the operational cost to treat but also the cost to repair/replace equipment and any associated piping & instrumentation. Compared to halogenated oxidizers, non-oxidizing biocides are more compatible with metal-based materials of construction commonly used in cooling water and other recirculating water systems. Therefore, over time, non-oxidizing biocides can be less expensive. As for performance, a combination treatment program can be an effective strategy for side-stepping nature's way of tolerance and selection.
NON-OXIDIZING BIOCIDES FOR LEGIONELLA CONTROL
Table 1 provides a list of the non-oxidizing biocides presented in this paper. Rather than trying to capture all of the research results from the published literature on the use of non-oxidizing biocides for Legionella control, the information presented here is intended to instruct the reader that various methodology has been employed to study Legionella control in a laboratory environment.
(available in full paper)
Table 2 summarizes the results of various studies for DBNPA efficacy. A couple of early studies showed very high population kills on the order of 5 to 6 log reductions of planktonic Legionella at active concentrations between 2 to 5-ppm and exposure times of only 2 to 3 hours. However, a study back in 1993 reported higher concentration requirements and longer exposure times for Legionella colonies associated with a biofilm.
(available in full paper)
Another research project utilized a model water system to better approximate real world conditions - enabling biofilm formation which then can help to protect the Legionella organisms. The overall results of this study show the performance of the non-oxidizing biocides to be significantly reduced when compared with bench testing under static closed conditions.
Monitoring for recovery and re-growth was a key element of the model water test system research. In summary, DBNPA did not provide lasting control at typical use levels.
|File Size||183 KB||7|