Abstract
Ensuring the biological stability of drinking water is essential for modern drinking water supply. To understand and manage the biological stability, it is critical that the bacterial growth in drinking water can be measured. Nowadays, advance treatment technologies, such as reverse osmosis (RO), are increasingly applied in drinking water purification where the produced water is characterized by low levels of nutrients and cell counts. The challenge is, therefore, how to measure the low bacterial growth potential (BGP) of such ultra-pure water using the available methods which were originally developed for conventionally treated drinking water. In this study, we proposed a protocol to assess BGP of ultra-pure drinking water produced by RO and post-treatment (including remineralization). Natural bacterial consortium from conventional drinking water was added to all water samples during this study to ensure the presence of a wide range of bacterial strains. The method development included developing an ultra-pure blank with high reproducibility to lower the detection limit of the BGP method (50 ± 20 × 103 intact cells/mL) compared with conventional blanks such as bottled spring water, deep groundwater treated by aeration and slow sand filtrate of surface water supply. The ultra-low blank consists of RO permeate after adjusting its pH and essential mineral content under controlled laboratory conditions to ensure carbon limitation. Regarding the test protocol, inoculum concentrations of >10 × 103 intact cells/mL may have a significant contribution to the measured low levels of BGP. Pasteurization of water samples before measuring BGP is necessary to ensure reliable bacterial growth curves. The optimized method was used to assess BGP of ultra-pure drinking water produced by RO membranes and post-treatment (including remineralization), where the BGP has decreased more than 6-fold to a level of 90 ± 20 × 103 intact cells/mL compared with conventionally treated water (630 ± 70 × 103 intact cells/mL).
Highlights
Bacterialgrowth in drinking water supply systems may leadM
Conventionally treated groundwater (CTW) has an average bacterial growth potential (BGP) of 630 ± 70 Â 103 intact cells/mL, whereas the BGP of reverse osmosis (RO)-treated groundwater after post-treatment was reduced considerably to 90 ± 20 Â 103 intact cells/mL, which can no longer be affirmed by the BGP detection limit using conventional blanks
This observation can be confirmed by the constant initial cell count of the lab-Remin blank (~2 Â 103 intact cells/mL, the detection limit of flow cytometry (FCM)) measured on different sampling dates, whereas up to 40% variation was observed in the initial cell count of conventional blanks
Summary
Bacterial (re)growth in drinking water supply systems may leadM. Sousi et al / Water Research 145 (2018) 687e696 water that contains low concentrations of readily available nutrients for bacterial growth (van der Kooij, 2000; Hammes et al, 2010; Liu et al, 2013b), e.g., assimilable organic carbon (AOC) below 10 mg ac-C eq/L (van der Kooij, 1992). Several batch tests have been developed to assess the potential of drinking water to support bacterial growth using different techniques to measure the increase in cell abundance Those techniques include: turbidity (Withers and Drikas, 1998; Page et al, 2002), colony count (van der Kooij et al, 1982; Joret et al, 1991; van der Kooij, 1992; Sack et al, 2010, 2011), biomass volume (Servais et al, 1987), cell number (Hammes and Egli, 2005; Dixon et al, 2012; Park et al, 2016; Prest et al, 2016) and cell activity based on adenosine tri-phosphate (ATP) (van der Kooij and Veenendaal, 2001, 2014; van der Kooij et al, 2017). Employing flow cytometry (FCM) for cell count was evaluated and applied in BGP tests (Dixon et al, 2012; Prest et al, 2016) because of its advantages of being simple, rapid, reproducible and informative (Prest et al, 2013; van Nevel et al, 2017)
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