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

The effect of different carbon sources on biofouling in membrane fouling simulators: microbial community and implications

Biofouling is a problem affecting the operation of nanofiltration systems due to the complexity of the carbon matrix affecting bacteria and biofilm growth. This study used membrane fouling simulators to investigate the effects of five different carbon sources on the biofouling of nanofiltration membranes. For all the carbon sources analyzed, the increase in pressure drop was most accelerated for acetate. The use of acetate as the single carbon source produced less adenosine triphosphate but more extracellular polymers than glucose. The microbial community was analyzed using 16 s rRNA. The use of more than a single carbon source produced an increase in bacteria diversity even at similar concentrations. The relative abundance of proteobacteria was the highest at the phylum level (95%) when a single carbon source was added. Additionally, it was found that the use of different carbon sources produced a shift in the microbial community, affecting the biofouling and pressure drop on membranes.

Evaluation of DNA extraction yield from a chlorinated drinking water distribution system

Desalination technology based on Reverse Osmosis (RO) membrane filtration has been resorted to provide high-quality drinking water. RO produced drinking water is characterized by a low bacterial cell concentration. Monitoring microbial quality and ensuring membrane-treated water safety has taken advantage of the rapid development of DNA-based techniques. However, the DNA extraction process from RO-based drinking water samples needs to be evaluated regarding the biomass amount (filtration volume) and residual disinfectant such as chlorine, as it can affect the DNA yield. We assessed the DNA recovery applied in drinking water microbiome studies as a function of (i) different filtration volumes, (ii) presence and absence of residual chlorine, and (iii) the addition of a known Escherichia coli concentration into the (sterile and non-sterile, chlorinated and dechlorinated) tap water prior filtration, and directly onto the (0.2 μm pore size, 47 mm diameter) mixed ester cellulose membrane filters without and after tap water filtration. Our findings demonstrated that the co-occurrence of residual chlorine and low biomass/cell density water samples (RO-treated water with a total cell concentration ranging between 2.47 × 102-1.5 × 103 cells/mL) failed to provide sufficient DNA quantity (below the threshold concentration required for sequencing-based procedures) irrespective of filtration volumes used (4, 20, 40, 60 L) and even after performing dechlorination. After exposure to tap water containing residual chlorine (0.2 mg/L), we observed a significant reduction of E. coli cell concentration and the degradation of its DNA (DNA yield was below detection limit) at a lower disinfectant level compared to what was previously reported, indicating that free-living bacteria and their DNA present in the drinking water are subject to the same conditions. The membrane spiking experiment confirmed no significant impact from any potential inhibitors (e.g. organic/inorganic components) present in the drinking water matrix on DNA extraction yield. We found that very low DNA content is likely to be the norm in chlorinated drinking water that gives hindsight to its limitation in providing robust results for any downstream molecular analyses for microbiome surveys. We advise that measurement of DNA yield is a necessary first step in chlorinated drinking water distribution systems (DWDSs) before conducting any downstream omics analyses such as amplicon sequencing to avoid inaccurate interpretations of results based on very low DNA content. This study expands a substantial source of bias in using DNA-based methods for low biomass samples typical in chlorinated DWDSs. Suggestions are provided for DNA-based research in drinking water with residual disinfectant.

Multi-parametric assessment of biological stability of drinking water produced from groundwater: Reverse osmosis vs. conventional treatment

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.