Assessment of flow cytometry for microbial water quality monitoring in cooling tower water and oxidizing biocide treatment efficiency.

The control of Legionella proliferation in cooling tower water circuits requires regular monitoring of water contamination and effective disinfection procedures. In this study, flow cytometry was assessed to monitor water contamination and disinfection treatment efficiency on bacterial cells regarding nucleic acid injury (SYBR® Green II), cell integrity (SYBR® Green II and propidium iodide) and metabolism activity (ChemChrome V6). A total of 27 cooling tower water samples were analyzed in order to assess water contamination levels regarding viable populations: standard culture, ATP measurement and flow cytometry methods were compared. Flow cytometry and plate counts methods showed a significant correlation for changes in concentrations despite a 1 to 2-log difference regarding absolute quantification. Concerning intracellular activity, the use of two different flow cytometers (FACSCanto™ II and Accuri™ C6) showed no statistical difference while a difference was observed between flow cytometry and usual methods (culture and ATP measurement). The standard culture and flow cytometry methods were also compared for in vitro bacteria inactivation measurements in the presence of 3 different types of oxidizing biocides commonly used for cooling tower disinfection. Reductions observed ranged between 1 and 2 log depending on (1) the detection method, (2) the bacterial population origin and/or (3) the active biocide molecule used. In conclusion, flow cytometry represents an efficient, accurate and fast approach to monitor water contamination and biocide treatment efficiency in cooling towers.

Removal of MS2, Qß and GA bacteriophages during drinking water treatment at pilot scale.

The removal of MS2, Qß and GA, F-specific RNA bacteriophages, potential surrogates for pathogenic waterborne viruses, was investigated during a conventional drinking water treatment at pilot scale by using river water, artificially and independently spiked with these bacteriophages. The objective of this work is to develop a standard system for assessing the effectiveness of drinking water plants with respect to the removal of MS2, Qß and GA bacteriophages by a conventional pre-treatment process (coagulation-flocculation-settling-sand filtration) followed or not by an ultrafiltration (UF) membrane (complete treatment process). The specific performances of three UF membranes alone were assessed by using (i) pre-treated water and (ii) 0.1 mM sterile phosphate buffer solution (PBS), spiked with bacteriophages. These UF membranes tested in this work were designed for drinking water treatment market and were also selected for research purpose. The hypothesis serving as base for this study was that the interfacial properties for these three bacteriophages, in terms of electrostatic charge and the degree of hydrophobicity, could induce variations in the removal performances achieved by drinking water treatments. The comparison of the results showed a similar behaviour for both MS2 and Qß surrogates whereas it was particularly atypical for the GA surrogate. The infectious character of MS2 and Qß bacteriophages was mostly removed after clarification followed by sand filtration processes (more than a 4.8-log reduction) while genomic copies were removed at more than a 4.0-log after the complete treatment process. On the contrary, GA bacteriophage was only slightly removed by clarification followed by sand filtration, with less than 1.7-log and 1.2-log reduction, respectively. After the complete treatment process achieved, GA bacteriophage was removed with less than 2.2-log and 1.6-log reduction, respectively. The effectiveness of the three UF membranes tested in terms of bacteriophages removal showed significant differences, especially for GA bacteriophage. These results could provide recommendations for drinking water suppliers in terms of selection criteria for membranes. MS2 bacteriophage is widely used as a surrogate for pathogenic waterborne viruses in Europe and the United States. In this study, the choice of MS2 bacteriophage as the best surrogate to be used for assessment of the effectiveness of drinking water treatment in removal of pathogenic waterborne viruses in worst conditions is clearly challenged. It was shown that GA bacteriophage is potentially a better surrogate as a worst case than MS2. Considering GA bacteriophage as the best surrogate in this study, a chlorine disinfection step could guaranteed a complete removal of this model and ensure the safety character of drinking water plants.