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Atomic Force Microscopy (AFM) for In-Situ Biofilm Surface Characterization during Free Chlorine and Monochloramine Exposure
Citation:
Daniels, S., D. Wahman, AND J. Pressman. Atomic Force Microscopy (AFM) for In-Situ Biofilm Surface Characterization during Free Chlorine and Monochloramine Exposure. Presented at Water Quality Technology Conference, New Orleans, LA, November 16 - 20, 2014.
Impact/Purpose:
Information will be helpful for drinking water utilities to manage disinfection in their distribution systems
Description:
Drinking water distribution system biofilm are attached to pipe walls and found in sediments. These biofilms are complex and contain a variety of microorganisms embedded in a matrix with extracellular polymeric substances (EPS), providing protection from disinfection. Without proper monitoring, biofilm growth may lead to enhanced corrosion in pipes and drinking water deterioration. In an attempt to control biofilm growth, drinking water utilities use disinfectants (e.g., free chlorine and monochloramine) along with recommended operational practices (e.g., flushing). However, disinfectant effectiveness depends on the biofilm thickness, composition and the type of disinfectant used. Drinking water utilities typically monitor bulk water parameters to determine water quality because they cannot easily assess the direct impact to biofilm. For example, which disinfectant and at what concentrations does biofilm slough from the pipe surface compared to disinfection in-place? Thus, there is a need to provide insight on biofilm impacts on the microscale versus the macroscale for disinfection. This study used atomic force microscopy (AFM) as a novel technique for in situ characterization of biofilm structural changes at the nanoscale during free chlorine and monochloramine biofilm exposure. The primary advantage of AFM analysis is the ability to image the surface morphology in liquid without fixing the sample (i.e. dehydration) as required by other techniques. Nitrifying biofilm that are representative of those found in chloraminated drinking water were grown on polycarbonate slides in annular reactors (ARs) and fed dechlorinated tap water supplemented with monochloramine and free ammonia. Slides containing biofilm were removed from the ARs, placed in a specially designed AFM flow cell, and exposed to 4 mg Cl2/L free chlorine or monochloramine solutions with a continuous flow at a velocity of 7.5 cm/min, for various time periods (30 minutes to 24 hours). AFM images were acquired at defined times throughout the exposure by raster scanning a sharp tip in direct contact with the surface to generate a 3D topography of the biofilm. The AFM data revealed a densely packed and heterogeneous biofilm structure before free chlorine or monochloramine treatment. Biofilm images after free chlorine or monochloramine exposure showed substantial impact to biofilm structure with detachment upon free chlorine exposure. Confocal scanning laser microscopy was used as a complimentary surface technique to elucidate the average biofilm thickness and density, viable and non-viable bacteria distribution, and EPS abundance. FT-IR was also used to qualitatively identify biofilm chemical composition (e.g., proteins and polysaccharides). Combining these techniques provides insight into the effects of chlorine and chloramines on biofilm surface properties which may help utilities optimize their distribution system operations.
URLs/Downloads:
http://www.awwa.org/Portals/0/files/education/conferences/wqtc/wqtc14/pdfs/WQTC14FinalProgramWeb.pdf