Further developing the bacterial growth potential method for ultra-pure drinking water produced by remineralization of reverse osmosis permeate

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 × 10 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 × 10 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 × 10 intact cells/mL compared with conventionally treated water (630 ± 70 × 10 intact cells/mL).

Measuring Bacterial Growth Potential of Ultra-Low Nutrient Drinking Water Produced by Reverse Osmosis: Effect of Sample Pre-treatment and Bacterial Inoculum

“…Expressing the BGP results in the most appropriate way is essential to accurately interpret the test outcome, allowing for achieving the objectives of this study. Currently, the BGP results are expressed either as BGP net for pre-treated (Vital et al, 2010;Park et al, 2016) and non-pre-treated (i.e., direct incubation) (Prest et al, 2016a;Nescerecka et al, 2018) samples, or as BGP max (Joret et al, 1991;Percherancier et al, 1996;Sousi et al, 2018). The results of the present study suggest that BGP max gives a better representation of the BGP of water, since the effect of initial cell count is ruled out (Figure 2).…”

“…Reverse osmosis (RO) filtration is capable of producing drinking water with ultra-low nutrient level, and thus, very low bacterial growth potential (BGP) (Park and Hu, 2010;Dixon et al, 2012). However, Sousi et al (2018) demonstrated that the existing BGP bioassays are not suitable for ultra-low nutrient water, such as remineralized RO permeate (BGP = 50-100 × 10 3 cells/mL), because of their high detection limit. Additionally, more care should be taken when measuring the BGP of ultra-low nutrient water samples due to the high susceptibility of this water type to sample preparation procedures that can introduce contamination (i.e., BGP increase).…”

“…Alternatively, some bioassays consider no sample pretreatment (i.e., direct incubation) (Prest et al, 2016a), arguing that it may affect the nutrient nature in water (Ross et al, 2013). However, Sousi et al (2018) demonstrated that pre-treatment is needed for ultra-low nutrient water samples in which initial cell count exceeded the available nutrients for bacterial growth (e.g., wash-out of bacterial loads from filtration units), leading to a decreasing number of cells over time directly after starting the bioassay. The main disadvantage of pasteurization is that it may affect the nature of organic compounds in water, whereas filtration may either remove or leach nutrients from/into water (Khan and Subramania-Pillai, 2006), resulting in unreliable BGP outcome.…”

“…Inoculating ultra-low nutrient water samples after pretreatment with natural bacteria is essential to initiate growth, where undesired nutrient addition can occur when using nutrient-rich water as an inoculum source (Sousi et al, 2018). Instead, ultra-low nutrient water (e.g., remineralized RO permeate) can be used as an inoculum source to reduce the undesired nutrient addition, especially that (remineralized) RO permeate bacteria are best adapted to grow in their own water without the need for an external inoculum as demonstrated by Sousi et al (2018), which was also observed for other water types (Farhat et al, 2018). However, investigating the ability of (remineralized) RO permeate bacteria to utilize organic carbon of various complexity levels is first needed, especially with the considerably low diversity of the RO permeate bacterial species (Belila et al, 2016).…”

“…The BGP bioassay based on cell count is considered for this study, where the results can be expressed as the maximum cell count obtained during the incubation period (Sousi et al, 2018), or as the net bacterial growth which is the difference between the initial and maximum cell counts as in the direct incubation methods (Prest et al, 2016a;Nescerecka et al, 2018).…”