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3D Chlorine and Monochloramine Penetration and Nitrifying Biofilm Activity and Viability: Periodic Chlorine Switch Implications
Citation:
Lee, W., D. Wahman, AND J. Pressman. 3D Chlorine and Monochloramine Penetration and Nitrifying Biofilm Activity and Viability: Periodic Chlorine Switch Implications. In Proceedings, AWWA Water Quality Technology Conference, Toronto, ON, CANADA, November 04 - 08, 2012. American Water Works Association, Denver, CO, 999, (2012).
Impact/Purpose:
To disseminate information to the drinking water community on conducting free chlorine burns in chloraminated drinking water distribution systems
Description:
Biofilm formation in drinking water distribution systems has been associated with water quality degradation and may result in non-compliance with existing regulations. United States water utilities report biofilm survival and regrowth despite disinfectant presence, and systems that use chloramines to comply with disinfection by-product regulations will release ammonia which serves as a growth substrate for ammonia-oxidizing bacteria (AOB). If unchecked, these AOB can accelerate chloramine loss, leading to disinfectant depletion. Once this occurs, utilities have very few options available to correct the situation and often switch to free chlorine for a period of time to remove the growth substrate (ammonia) and inactivate biofilm. Currently, the only method to assess effectiveness of the switch to free chlorine is to measure free chlorine concentrations in the bulk fluid, but this may be unrepresentative of the free chlorine concentration in the biofilm. The current research seeks to provide a better understanding of free chlorine application to nitrifying biofilm by monitoring free chlorine penetration over an extended period of time and assessing activity and viability through the entire biofilm depth to the substratum. Three biofilm application scenarios were monitored under the same conditions and equivalent chlorine concentrations: (1) free chlorine application until full biofilm penetration, (2) monochloramine application until full biofilm penetration, and (3) monochloramine application until full biofilm penetration followed by free chlorine application until full biofilm penetration. During application, three-dimensional mapping of free chlorine and monochloramine biofilm penetration was correlated spatially and temporally with aerobic microbial activity and viability using a combination of (1) microelectrode measurements for disinfectant penetration and activity measurements and (2) Live/Dead BacLight staining, cryoembedding, cross-sectioning, and image analysis using confocal laser scanning microscopy (CLSM) for viability measurements throughout the entire biofilm depth to the substratum. These studies extended previous research where viability analysis was limited to the upper biofilm (50 μM depth) and free chlorine penetration was not allowed to proceed to completion, allowing direct comparisons of disinfectant penetration and activity and viability throughout the entire biofilm thickness. In general, free chlorine biofilm penetration compared to monochloramine (1) was slower, (2) led to a decrease in biofilm thickness from sloughing, (3) corresponded directly with a decrease in viability; compared to complete penetration with monochloramine where viable bacteria remained. Based on these results, implications for drinking water distribution system operation will be highlighted, including estimations of free chlorine exposure required to completely inactive “model” drinking water distribution system biofilm.
URLs/Downloads:
http://wcwwa.ca/events/event/2012-water-quality-technology-conference-and-exposition-wqtc/
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