Abstract
Although water produced by reverse osmosis (RO) filtration has low bacterial growth potential (BGP), post-treatment of RO permeate, which is necessary prior to distribution and human consumption, needs to be examined because of the potential re-introduction of nutrients/contaminants. In this study, drinking water produced from anaerobic groundwater by RO and post-treatment (ion exchange, calcite contactors, and aeration) was compared with that produced by conventional treatment comprising (dry) sand filtration, pellet softening, rapid sand filtration, activated carbon filtration, and UV disinfection. The multi-parametric assessment of biological stability included bacterial quantification, nutrient concentration and composition as well as bacterial community composition and diversity. Results showed that RO permeate remineralised in the laboratory has an extremely low BGP (50 ± 12 × 103 ICC/mL), which increased to 130 ± 10 × 103 ICC/mL after site post-treatment. Despite the negative impact of post-treatment, the BGP of the finished RO-treated water was >75% lower than that of conventionally treated water. Organic carbon limited bacterial growth in both RO-treated and conventionally treated waters. The increased BGP in RO-treated water was caused by the re-introduction of nutrients during post-treatment. Similarly, OTUs introduced during post-treatment, assigned to the phyla of Proteobacteria and Bacteroidetes (75–85%), were not present in the source groundwater. Conversely, conventionally treated water shared some OTUs with the source groundwater. It is clear that RO-based treatment achieved an extremely low BGP, which can be further improved by optimising post-treatment, such as using high purity calcite. The multi-parametric approach adopted in this study can offer insights into growth characteristics including limiting nutrients (why) and dominating genera growing (who), which is essential to manage microbiological water quality in water treatment and distribution systems.
Highlights
IntroductionAs the type and composition of nutrients are important for both bacterial growth and shaping the bacterial community (Elhadidy et al, 2016; Nescerecka et al, 2018), including a detailed analysis of carbon, phosphate, and nitrogen fractions in the multi-parametric approach is necessary to understand the driving force for bacterial growth
The trends in intact cell count (ICC) and adenosine triphosphate (ATP) concentration across the conventional and reverse osmosis (RO)-based treatment lines from two sampling campaigns are shown in Fig. 2 and Fig. S1, where similar trends were observed except for concentrations in the source anaerobic groundwater (AGW)
Intact cell count (ICC) and ATP concentration in the source anaerobic groundwater (AGW) were 215 ± 40 × 103 ICC/mL and ~0.5 ng ATP/L (Fig. 2), which increased by a factor 3 and 28, respectively, after dry sand filtration (DSF) and softening (SOF) in the conventional treatment line where oxygen was introduced in the water
Summary
As the type and composition of nutrients are important for both bacterial growth and shaping the bacterial community (Elhadidy et al, 2016; Nescerecka et al, 2018), including a detailed analysis of carbon, phosphate, and nitrogen fractions in the multi-parametric approach is necessary to understand the driving force for bacterial growth. Such an integral multi-parametric approach will be especially powerful for comparing treatment plant performance, and/or diagnosing problems related to bacterial growth in drinking water
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