Assessing the effectiveness of near real-time flow cytometry in monitoring ozone disinfection in a full-scale drinking water treatment plant

While real-time monitoring of physicochemical parameters has widely been incorporated into drinking water treatment systems, real-time microbial monitoring has lagged behind, resulting in the use of surrogate parameters (disinfectant residual, applied dose, concentration x time [CT]) to assess disinfection system performance. Near real-time flow cytometry (NRT-FCM) allows for automated quantification of total and intact microbial cells but has not been widely implemented in full-scale systems. This study sought to investigate the feasibility of NRT-FCM for full-scale drinking water ozone disinfection system performance monitoring. A water treatment plant with high lime solids turbidity in the ozone contactor influent was selected to evaluate the NRT-FCM in challenging conditions. Total and intact cell counts were monitored for 40 days and compared to surrogate parameters (ozone residual, ozone dose, and CT) and grab sample assay results for cellular adenosine triphosphate (cATP), heterotrophic plate counts (HPC), impedance flow cytometry, and 16S rRNA gene sequencing. NRT-FCM provided insight into the dynamics of the full-scale ozone system, including offering early warning of increased contactor effluent cell concentrations, which was not observed using surrogate measures. A strong correlation between log intact cell removal and CT was also not observed (Kendalls tau= -0.09, p=0.04). Positive correlations were observed between intact cell counts and cATP levels (Kendalls tau=0.40, p<0.01), HPC (Kendalls tau=0.20, p<0.01), and impedance flow cytometry results (Kendalls tau=0.30, p<0.01). However, 16S rRNA gene sequencing results showed that passage through the ozone contactor significantly changed the microbial community (p<0.05), supporting the hypothesis that regrowth was occurring in the later chambers of the contactor. This study demonstrates the utility of direct, near real-time microbial analysis for monitoring full-scale disinfection systems. Graphical Abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=76 SRC="FIGDIR/small/23300640v1_ufig1.gif" ALT="Figure 1"> View larger version (19K): org.highwire.dtl.DTLVardef@13f5e68org.highwire.dtl.DTLVardef@14f33d3org.highwire.dtl.DTLVardef@d3821dorg.highwire.dtl.DTLVardef@373af_HPS_FORMAT_FIGEXP M_FIG C_FIG HighlightsO_LINear real-time flow cytometry (NRT-FCM) was effective for ozone system monitoring. C_LIO_LIIntact microbial cell counts were consistent with cellular ATP and HPC results. C_LIO_LINRT-FCM provided early detection of increased effluent cell concentrations. C_LIO_LIThe ozone contactor influent and effluent microbial communities were distinct. C_LI