Abundance and composition of indigenous bacterial communities in a multi-step biofiltration-based drinking water treatment plant

Effective enzyme activity: A proposed monitoring methodology for biofiltration systems with or without ozone

"Effective Enzyme Activity", or simply "Effective Activity", is proposed as a biofiltration monitoring tool which combines enzyme activity with empty bed contact time (EBCT) to quantify biodegradation potential. The primary objective of this study was to evaluate the applicability of the Effective Activity concept for predicting water quality in biofiltration systems. This pilot-scale study evaluated eight different biofilter configurations in order to quantify impacts associated with filter media (anthracite/sand or granular activated carbon), pre-treatment (settled water with or without ozonation) and operating conditions (15- and 30-min EBCT, and backwash with or without chlorine). Microbial characterization included biomass concentration, as measured by adenosine triphosphate (ATP), in addition to esterase and phosphatase activity. Water quality parameters included dissolved organic carbon (DOC), trihalomethane (THM) formation potential (FP), haloacetic acid (HAA) FP, haloacetonitrile (HAN) FP, iodinated DBP FP (THMs and HAAs) and inorganic nutrients (phosphorus and nitrogen). Results confirmed the benefits to treated water quality associated with the application of an ozone residual of 0.5 mg/L, utilization of GAC filter media, eliminating chlorinated backwash, and extending EBCT. This study demonstrated a good relationship between effective esterase activity and reductions in DOC and THM FP, including those systems which incorporate pre-ozonation. As such, this study showed that Effective Activity may be appropriate for relating biomass characterization to treated water quality and highlights the importance of quantifying biomass activity in addition to quantity.

Unravelling the composition of tap and mineral water microbiota: Divergences between next-generation sequencing techniques and culture-based methods

The complex and highly diverse microbial environment of drinking water, consisting mainly of bacteria at different metabolic states, is still underexplored. The aim of this work was to characterize the bacterial communities in tap water and bottled mineral water, the two predominant sources of drinking water in modern societies. A total of 11 tap water samples from a range of locations and distribution networks and 10 brands of bottled natural mineral water were analysed using two approaches: a) heterotrophic plate counts by matrix-assisted laser desorption/ionization time of flight mass-spectrometry (MALDI-TOF MS) for the culturable heterotrophic communities, and b) Illumina amplicon sequencing for total bacteria including non-culturable bacteria. Culturable heterotrophic bacteria were isolated in WPCA (ISO) agar at 22 ± 2 °C for 72 h and 2046 isolates were identified using MALDI-TOF MS. The Bruker Daltonics Library and a previously customized library (Drinking Water Library) were used as reference databases. For the total bacteria fraction, DNA was extracted from 6 L of water and submitted to Illumina 16S rRNA sequencing of the v4 region. Significant differences were observed between mineral and tap water, with a general dominance of Alphaproteobacteria (mainly the genus Blastomonas) in tap water and Gammaproteobacteria in mineral water with Acidovorax being the dominant genus in 3 out of 7 mineral water brands. The bacterial communities in the different brands of mineral water were highly diverse and characteristic of each one. Moreover, the season in which the water was bottled also affected the species distribution, with some of them identified in only one season. Among the culturable bacteria, the most abundant phylum was Proteobacteria (around 85% of the isolates), followed by Actinobacteria, Firmicutes and Bacteroidetes. Proteobacteria was also the most abundant phylum detected with Illumina sequencing (>99% of the reads). The two methods gave distinct results at the different taxonomic levels and could therefore have a complimentary application in the study of microbiota in mineral water environments. MALDI-TOF MS is a promising method for the rapid identification of heterotrophic bacteria in routine water analysis in the bottling industry. SIGNIFICANCE AND IMPACT OF THE STUDY: The complementarity of MALDI-TOF MS and NGS in the assessment of bacterial community diversity has been demonstrated in water intended for human consumption. The two methods are suitable for routine use in the water industry for water quality management.

Effect of granular activated carbon pore-size distribution on biological activated carbon filter performance

Proper granular activated carbon (GAC) selection could improve the performance of biological activated carbon (BAC) filters through a combination of adsorption and biodegradation, while the GACs used in BAC filters are now mainly selected according to adsorption function, ignoring biodegradation. In this study, sand filter effluent obtained from a drinking water treatment plant was fed into continuous-flow bench-scale BAC columns operated in parallel over 245 days to examine the effects of GAC pore-size distribution on BAC filter performance, in terms of the dissolved organic carbon (DOC) and disinfection byproduct (DBP) precursors. A metagenomic analysis indicated that bacterial community structure played an important role in BAC filter performance. A significant correlation was found between metabolism-related proteins and the volume of micro-level macropores based on metaproteomic analysis. It is suggested that the adsorption saturation was dynamic and that adsorption played a role in the performance of the BAC filters throughout the 245-day operating period. Renewed adsorption capacity, or bioregeneration, was driven by bacterial metabolic activity. Such activity largely depended on the organic matter adsorbed by the GAC, in which micro-level macropores, especially those with diameters of 0.2-10 μm, played an important but previously unrecognized role. The results suggest that more attention should be paid to well-developed pores and pore-size distribution in the production and selection of GAC used for full-scale drinking water biofilters.

Reuse of ultrafiltered effluents for crop irrigation: On-site flow cytometry unveiled microbial removal patterns across a full-scale tertiary treatment

Reuse of treated wastewater for crop irrigation has been widely adopted to mitigate the effects of water scarcity on agricultural yields and to help preserving the integrity of aquatic ecosystems. This paper presents the outcomes of one-year monitoring of a full-scale agro-industrial wastewater treatment plant designed for water reuse, with a multistage tertiary treatment based on sand filtration, membrane ultrafiltration, storage and on-demand UV disinfection. We aimed to test flow cytometry as a monitoring tool to provide on-site indications on tertiary treatment performances and on the quality of treated wastewater along the treatment scheme. Membrane ultrafiltration retained prokaryotic cells and E. coli (>3 log). During storage of treated effluents, a significant decay of E. coli was observed together with the growth of prokaryotic and eukaryotic cells, and the UV disinfection was effective only against fecal indicators. The microbial quality of the treated effluent was comparable to the control groundwater locally used for irrigation. On-site rapid assessments by flow cytometry allowed unveiling crucial aspects affecting the microbiological quality of ultrafiltration permeate and treated effluent immediately after sampling, including plant operating performances and microbial removal patterns across the treatment train.

Metagenomics Unravels Differential Microbiome Composition and Metabolic Potential in Rapid Sand Filters Purifying Surface Water Versus Groundwater

Designed for retaining suspended particles, rapid sand filters (RSFs) are widely used in drinking water treatment. There is increasing evidence that microbial processes within RSFs contribute to the transformation and removal of organic carbon, nitrogen, and metal pollutants. Here, we linked microbial composition and functional profiles with the treatment performance of 12 different RSFs that significantly removed influent ammonium and manganese (Mn). Metagenomic analyses showed that chemoautotrophic or methanotrophic bacteria were prevalent in the groundwater filters, and chemoheterotrophic bacteria encoding more carbohydrate- and xenobiotic-metabolizing genes were more abundant in the surface water filters. Approximately 92% of ammonium was transformed into nitrate, with a critical contribution from comammox Nitrospira. The composition of comammox amoA differed between groundwater and surface water filters, with clade A dominating groundwater filters (78.0 ± 12.0%) and clade B dominating surface water filters (91.9 ± 8.9%). Further, we identified six bacterial genera encoding known Mn(II)-oxidizing genes in the RSFs, with Pseudomonas accounting for 71.1%. These Mn(II)-oxidizing bacteria might promote Mn(II) oxidation and thus increase the removal of influent Mn. Overall, our study gave a comprehensive investigation of microbiome in RSFs and highlighted the roles of comammox and Mn(II)-oxidizing bacteria in water purification.

Disinfection exhibits systematic impacts on the drinking water microbiome

Limiting microbial growth during drinking water distribution is achieved either by maintaining a disinfectant residual or through nutrient limitation without using a disinfectant. The impact of these contrasting approaches on the drinking water microbiome is not systematically understood. We use genome-resolved metagenomics to compare the structure, metabolic traits, and population genomes of drinking water microbiome samples from bulk drinking water across multiple full-scale disinfected and non-disinfected drinking water systems. Microbial communities cluster at the structural- and functional potential-level based on the presence/absence of a disinfectant residual. Disinfectant residual alone explained 17 and 6.5% of the variance in structure and functional potential of the drinking water microbiome, respectively, despite including multiple drinking water systems with variable source waters and source water communities and treatment strategies. The drinking water microbiome is structurally and functionally less diverse and variable across disinfected compared to non-disinfected systems. While bacteria were the most abundant domain, archaea and eukaryota were more abundant in non-disinfected and disinfected systems, respectively. Community-level differences in functional potential were driven by enrichment of genes associated with carbon and nitrogen fixation in non-disinfected systems and γ-aminobutyrate metabolism in disinfected systems likely associated with the recycling of amino acids. Genome-level analyses for a subset of phylogenetically-related microorganisms suggests that disinfection selects for microorganisms capable of using fatty acids, presumably from microbial decay products, via the glyoxylate cycle. Overall, we find that disinfection exhibits systematic selective pressures on the drinking water microbiome and may select for microorganisms able to utilize microbial decay products originating from disinfection-inactivated microorganisms. Video abstract.

Single molecule sequencing reveals response of manganese-oxidizing microbiome to different biofilter media in drinking water systems

Rapid sand biofiltration (RSBF) is widely used for the removal of contaminants from drinking water treatment systems. Biofilm microbiomes in the biofilter media play essential roles in biotransformation of contaminants, but is not comprehensively understood. This study reports on Mn(II) oxidation and the core microbiomes in magnetite sand RSBF (MagS-RSBF) and manganese sand RSBF (MnS-RSBF). MnS-RSBF showed a relatively higher Mn(II) removal rate (40-91.2%) than MagS-RSBF during the start-up. MagS-RSBF and MnS-RSBF had similar Mn(II) removal rates (94.13% and 99.16%) over stable operation for 80 days. Mn(II) removal rates at different depths in the MnS-RSBF reactor significantly changed with operation time, and the filter in the upper layer of MnS-RSBF made the largest contribution to Mn(II) oxidation once operation had stabilized. PacBio single molecule sequencing of full-length 16S rRNA gene indicated that biofilter medium had a significant impact on the core microbiomes of the biofilms from the two biofilters. The magnetite sand biofilter facilitated the enrichment of Mn(II)-oxidizing biofilms. The dominant populations consisted of Pedomicrobium, Pseudomonas, and Hyphomicrobium in the RSBF, which have been affiliated with putative manganese-oxidizing bacteria (MnOB). The relative abundance of Pedomicrobium manganicum increased with operation time in both RSBF reactors. In addition, Nordella oligomobilis and Derxia gummosa were statistically correlated with Mn(II) oxidation. Species-species co-occurrence networks indicated that the microbiome of MnS-RSBF had more complex correlations than that of MagS-RSBF, implying that biofilter medium substantially shaped the microbial community in the RSBF. Hyphomicrobium and nitrite-oxidizing Nitrospira moscoviensis were positively correlated. The core microbiomes' composition of both RSBF reactors converged over operation time. A hybrid biofilter medium with magnetite sand and manganese sand may therefore be best in rapid sand filtration for Mn(II) oxidation.

Co-selection and stability of bacterial antibiotic resistance by arsenic pollution accidents in source water

Frequent heavy-metal pollution accidents severely deteriorated the source water quality of drinking water treatment plants (DWTP). Limited data have explicitly addressed the impact of these incidents on bacterial antibiotic resistance (BAR). In present study, we investigated the shift of antibiotic resistome caused by heavy metal pollution incidents via simulating an arsenic shock loading [As (III)], along with the associated risks imposed on drinking water systems. The results indicated that a quick co-selection of antibiotic resistant bacteria (ARB) was achieved after exposure to 0.2-1 mg/L As (III) for only 6 h, meanwhile, there was an increase of relative abundance of antibiotic resistance genes (ARGs) and mobile genetic elements. Most of the co-selected BAR could be maintained for at least 4 days in the absence of As (III) and antibiotics, implying that the pollution in source water possibly contributed to the preservation and proliferation of antibiotic resistance determinants in the subsequent DWTP. Bacterial community structure analysis showed a strong correlation between bacterial community shift and BAR promotion, and enrichment of opportunistic bacteria (e.g. Escherichia-Shigella, Empedobacter sp. and Elizabethkingia sp.). The results indicated a potential epidemiological threat to the public due to accident-level arsenic contamination in the source water. This study gave insight into understanding the source water pollution accidents from the perspective of bio-hazard and biological risks, and highlighted a neglected important source of BAR in drinking water systems.

flowEMMi: an automated model-based clustering tool for microbial cytometric data

Background

Flow cytometry (FCM) is a powerful single-cell based measurement method to ascertain multidimensional optical properties of millions of cells. FCM is widely used in medical diagnostics and health research. There is also a broad range of applications in the analysis of complex microbial communities. The main concern in microbial community analyses is to track the dynamics of microbial subcommunities. So far, this can be achieved with the help of time-consuming manual clustering procedures that require extensive user-dependent input. In addition, several tools have recently been developed by using different approaches which, however, focus mainly on the clustering of medical FCM data or of microbial samples with a well-known background, while much less work has been done on high-throughput, online algorithms for two-channel FCM.

Results

We bridge this gap with flowEMMi, a model-based clustering tool based on multivariate Gaussian mixture models with subsampling and foreground/background separation. These extensions provide a fast and accurate identification of cell clusters in FCM data, in particular for microbial community FCM data that are often affected by irrelevant information like technical noise, beads or cell debris. flowEMMi outperforms other available tools with regard to running time and information content of the clustering results and provides near-online results and optional heuristics to reduce the running-time further.

Conclusions

flowEMMi is a useful tool for the automated cluster analysis of microbial FCM data. It overcomes the user-dependent and time-consuming manual clustering procedure and provides consistent results with ancillary information and statistical proof.

Quantifying the contribution of microbial immigration in engineered water systems

Immigration is a process that can influence the assembly of microbial communities in natural and engineered environments. However, it remains challenging to quantitatively evaluate the contribution of this process to the microbial diversity and function in the receiving ecosystems. Currently used methods, i.e., counting shared microbial species, microbial source tracking, and neutral community model, rely on abundance profile to reveal the extent of overlapping between the upstream and downstream communities. Thus, they cannot suggest the quantitative contribution of immigrants to the downstream community function because activities of individual immigrants are not considered after entering the receiving environment. This limitation can be overcome by using an approach that couples a mass balance model with high-throughput DNA sequencing, i.e., ecogenomics-based mass balance. It calculates the net growth rate of individual microbial immigrants and partitions the entire community into active populations that contribute to the community function and inactive ones that carry minimal function. Linking activities of immigrants to their abundance further provides quantification of the contribution from an upstream environment to the downstream community. Considering only active populations can improve the accuracy of identifying key environmental parameters dictating process performance using methods such as machine learning.

Betaproteobacteria are predominant in drinking water: are there reasons for concern?

Betaproteobacteria include some of the most abundant and ubiquitous bacterial genera that can be found in drinking water, including mineral water. The combination of physiology and ecology traits place some Betaproteobacteria in the list of potential, yet sometimes neglected, opportunistic pathogens that can be transmitted by water or aqueous solutions. Indeed, some drinking water Betaproteobacteria with intrinsic and sometimes acquired antibiotic resistance, harbouring virulence factors and often found in biofilm structures, can persist after water disinfection and reach the consumer. This literature review summarises and discusses the current knowledge about the occurrence and implications of Betaproteobacteria in drinking water. Although the sparse knowledge on the ecology and physiology of Betaproteobacteria thriving in tap or bottled natural mineral/spring drinking water (DW) is an evidence of this review, it is demonstrated that DW holds a high diversity of Betaproteobacteria, whose presence may not be innocuous. Frequently belonging to genera also found in humans, DW Betaproteobacteria are ubiquitous in different habitats, have the potential to resist antibiotics either due to intrinsic or acquired mechanisms, and hold different virulence factors. The combination of these factors places DW Betaproteobacteria in the list of candidates of emerging opportunistic pathogens. Improved bacterial identification of clinical isolates associated with opportunistic infections and additional genomic and physiological studies may contribute to elucidate the potential impact of these bacteria.

Small-Scale Heterogeneity in Drinking Water Biofilms

Biofilm heterogeneity has been characterized on various scales for both natural and engineered ecosystems. This heterogeneity has been attributed to spatial differences in environmental factors. Understanding their impact on localized biofilm heterogeneity in building plumbing systems is important for both management and representative sampling strategies. We assessed heterogeneity within the confined engineered ecosystem of a shower hose by high-resolution sampling (200 individual biofilm sections per hose) on varying scales (μm to m). We postulated that a biofilm grown on a single material under uniform conditions should be homogeneous in its structure, bacterial numbers, and community composition. A biofilm grown for 12 months under controlled laboratory conditions, showed homogeneity on large-scale. However, some small-scale heterogeneity was clearly observed. For example, biofilm thickness of cm-sections varied up to 4-fold, total cell concentrations (TCC) 3-fold, and relative abundance of dominant taxa up to 5-fold. A biofilm grown under real (i.e., uncontrolled) use conditions developed considerably more heterogeneity in all variables which was attributed to more discontinuity in environmental conditions. Interestingly, biofilm communities from both hoses showed comparably low diversity, with <400 taxa each, and only three taxa accounting for 57%, respectively, 73% of the community. This low diversity was attributed to a strong selective pressure, originating in migrating carbon from the flexible hoses as major carbon source. High-resolution sampling strategy enabled detailed analysis of spatial heterogeneity within an individual drinking water biofilm. This study gives insight into biofilm structure and community composition on cm-to m-scale and is useful for decision-making on sampling strategies in biofilm research and monitoring.

Microbial Dynamics of Biosand Filters and Contributions of the Microbial Food Web to Effective Treatment of Wastewater-Impacted Water Sources

Biosand filtration systems are widely used for drinking water treatment, from household-level, intermittently operated filters to large-scale continuous municipal systems. While it is well-established that microbial activity within the filter is essential for the removal of potential pathogens and other contaminants, the microbial ecology of these systems and how microbial succession relates to their performance remain poorly resolved. We determined how different source waters influence the composition, temporal dynamics, and performance of microbial communities in intermittently operated biosand filters. We operated lab-scale biosand filters, adding daily inputs from two contrasting water sources with differing nutrient concentrations and found that total coliform removal increased and became less variable after 4 weeks, regardless of water source. Total effluent biomass was also lower than total influent biomass for both water sources. Bacterial community composition, assessed via cultivation-independent DNA sequencing, varied by water source, sample type (influent, effluent, or sand), and time. Despite these differences, we identified specific taxa that were consistently removed, including common aquatic and wastewater bacteria. In contrast, taxa consistently more abundant in the sand and effluent included predatory, intracellular, and symbiotic bacteria.IMPORTANCE Although microbial activities are known to contribute to the effectiveness of biosand filtration for drinking water treatment, we have a limited understanding of what microbial groups are most effectively removed, colonize the sand, or make it through the filter. This study tracked the microbial communities in the influent, sand, and effluent of lab-scale, intermittently operated biosand filters over 8 weeks. These results represent the most detailed and time-resolved investigation of the microbial communities in biosand filters typical of those implemented at the household level in many developing countries. We show the importance of the microbial food web in biosand filtration, and we identified taxa that are preferentially removed from wastewater-impacted water sources. We found consistent patterns in filter effectiveness from source waters with differing nutrient loads and, likewise, identified specific bacterial taxa that were consistently more abundant in effluent waters, taxa that are important targets for further study and posttreatment.

Impact of carbon-based nutrient enhancement on biofiltration performance for drinking water treatment

Incorporation of a carbon-based nutrient enhancement strategy for drinking water biofiltration is an attractive option, especially for source waters which contain recalcitrant organics. This study compared biofilters that were operated in parallel and individually enhanced with amino acids (including alanine, phenylalanine, and tryptophan), inulin, and sucrose to increase biomass concentration and promote biodegradation of dissolved organic carbon (DOC) in the source water, including disinfection by-product (DBP) precursors. Biomass activity was characterized by measuring adenosine tri-phosphate (ATP), dissolved oxygen (DO) consumption, and through the use of laccase and esterase enzyme assays. Performance was evaluated in terms of headloss, turbidity, pH, DOC, UV₂₅₄, and DBP formation potential (DBP FP). The introduction of carbon-based nutrients significantly increased biomass activity, where ATP values peaked at 976 ng/g of filter media, 853 ng/g, and 513 ng/g for amino acids, inulin, and sucrose-spiked biofilters, respectively, while a non-spiked control only reached 104 ng/g. DO utilization by the enhanced biofilters was significantly higher than the control, with a strong correlation between ATP and DO uptake observed for all filters (R² > 0.74). Laccase and esterase enzyme activities of enhanced biofilters were also elevated (p > 0.05), suggesting greater biodegradation potential. Operational parameters such as headloss development and turbidity control were not impaired by carbon supplementation strategies or an increase in biomass concentration and activity. However, the enhancement strategy did not provide improvement in terms of source water carbon removal (DOC and UV₂₅₄) or DBP FP when treated filters were compared to a control.

Purification of harvested rainwater using slow sand filters with low-cost materials: Bacterial community structure and purifying effect

Slow sand filters (SSFs) have been shown to effectively improve water quality. The aim of the present study was to obtain low-cost materials (LCMs) as filter mediums (FMs) to efficiently purify harvested rainwater and to document the relationship between bacterial community structure and water purification. The red clay was mixed with crushed limestone and crushed brick, respectively. The mixtures or brick powder were used as the filter media for SSFs. Laboratory column tests were conducted in conjunction with the monitoring of representative water quality parameters (COD, NH₄⁺, CFU and total coliforms) to estimate the performance of low-cost material slow sand filters (LCM-SSFs), including the time needed for biofilm maturation. The relationship between bacterial community structure and SSF performance was determined using a combination of 16S rRNA gene sequencing and an array of statistical techniques. The results demonstrated that LCM-SSFs perform well in purifying harvested rainwater, and are of superior economic benefit. LCMs had a stronger adsorptivity than quartz sand, which enhanced the purification of harvested rainwater before the biofilms matured, and shorten the time required for biofilm maturation. During the 90-day laboratory experiment, a mixture of crushed limestone and red clay exhibited the best performance. The abundance of Opitutae could be used as a potential indicator of NH₄⁺ removal efficiency by SSFs. Schmutzdecke was characterized by abundant, diverse and evenly distributed bacterial communities that produced rich, stable and robust environmental functions, and that possessed an excellent purifying capacity. Environmental conditions associated with low ecological stress, such as neutral pH filter mediums and lucifugal experimental conditions, were conducive to the diversity and evenness of effluent bacterial communities and improved the performance of LCM-SSFs in purifying harvested rainwater.

Slow sand filtration for biofouling reduction in seawater desalination by reverse osmosis

Control of the organic substrate pool that determines the microbial growth potential (MGP) of feedwater in seawater reverse osmosis (SWRO) is a challenge unresolved in conventional or advanced membrane pretreatment. Slow sand filtration (SSF) combines filtration with biodegradation, but its capability of reducing MGP, proteins and carbohydrates on seawater feeds is not known. Two SSF, one constructed with new media (newSSF) and one from a previous filtration run (oldSSF), reduced MGP as measured in a growth assay with the marine organism Pseudoalteromonas songiae by one order of magnitude after maturation periods of 76 and 61 days, respectively. The reduction of the amount of biopolymers deposited on the surfaces of SWRO membranes in laminar fluid flow cells was significant with filtrates from biologically non-acclimated SSF (proteins: 60% (oldSSF) and -66% (new SSF), carbohydrates: 75% (oldSSF) and -70% (newSSF)) and an even greater reduction was observed after filter maturation (proteins: 81% (oldSSF) and -76% (new SSF), carbohydrates: 88% (oldSSF) and -88% (newSSF). Turbidity was less than 0.3 nephelometric turbidity units (NTU) and silt density index (SDI) < 4 immediately after startup and during the 181 days operating period regardless of the oscillations of the raw sea water quality. Filtration and biological activity were restricted to the top 30 cm of the media column, with no significant further contribution of the deeper media layers to filtrate quality.

The contribution of biofilm to nitrogenous disinfection by-product formation in full-scale cyclically-operated drinking water biofilters

Biofiltration has been shown to be effective for disinfection by-product (DBP) precursor control, however few studies have considered its role in the potential formation of DBPs. Biofilm is composed of heterogeneous bacteria as well as extracellular polymeric substances (EPS). The objective of this study was to determine the contribution of biofilm-related materials such as EPS to form nitrogen-containing DBPs upon chloramination, and to determine the influence of cyclical (scheduled on-off) biofilter operation on DBP precursor removal. Biologically active media was sampled from a full-scale biofilter operating under cold-water conditions (3.6 ± 0.5 °C) and extracted using a cation exchange resin into a phosphate buffer solution. Biomass concentrations, as determined using adenosine triphosphate (ATP) measurements, remained stable at 298 ± 55 ng ATP/g media over the trial period. N-nitrosodimethylamine (NDMA) and haloacetonitrile (HAN₄) formation potential (FP) tests conducted under uniform formation conditions (UFC) using extracted biofilm yielded 0.80 ± 0.27 ng NDMA/g media and 18.7 ± 3.3 ng dichloroacetonitrile (DCAN)/g media. Further analyses of extracted biofilm using fluorescence spectroscopy and liquid chromatography-organic carbon detection indicated the presence of proteins above 20 kDa and humic-like substances. Extracted proteins (93.5 ± 8.1 μg/g media) correlated well (R = 0.90) with UV 280 measurements, indicating that spectrophotometry may serve as a valuable tool to quantify proteins in extracted biofilms. While substances in biofilms can serve as NDMA and DCAN precursors, the full-scale cyclically-operated biofilter that was examined did not show release of NDMA precursors during start-up following stagnation periods of 6 h or more. These biofilters consistently removed 6.9 ± 4.3 ng/L of NDMA precursors; typical NDMA UFC-FP of biofilter effluent was 8.5 ± 2.6 ng/L.

Understanding the impacts of intermittent supply on the drinking water microbiome

Increasing access to piped water in low-income and middle-income countries combined with the many factors that threaten our drinking water supply infrastructure mean that intermittent water supply (IWS) will remain a common practice around the world. Common features of IWS include water stagnation, pipe drainage, intrusion, backflow, first flush events, and household storage. IWS has been shown to cause degradation as measured by traditional microbial water quality indicators. In this review, we build on new insights into the microbial ecology of continuous water supply systems revealed by sequencing methods to speculate about how intermittent supply conditions may further influence the drinking water microbiome, and identify priorities for future research.

The Water Microbiome Through a Pilot Scale Advanced Treatment Facility for Direct Potable Reuse

Advanced treatment facilities for potable water reuse of wastewater are designed to achieve high removal levels of specific pathogens, as well as many other constituents. However, changes to the microbial community throughout treatment, storage, and distribution of this water have not been well characterized. We applied high-throughput amplicon sequencing, read-based, assembly-based, and genome-resolved metagenomics, and flow cytometry to investigate the microbial communities present in a pilot-scale advanced water treatment facility. Advanced treatment of secondary-treated wastewater consisted of ozonation, chloramination, microfiltration, reverse osmosis (RO), advanced oxidation (UV/H₂O₂), granular activated carbon (GAC) filtration, and chlorination. Treated water was fed into bench-scale simulated distribution systems (SDS). Cell counts and microbial diversity in bulk water decreased until GAC filtration, and the bacterial communities were significantly different following each treatment step. Bacteria grew within GAC media and contributed to a consistent microbial community in the filtrate, which included members of the Rhizobiales and Mycobacteriaceae. After chlorination, some of the GAC filtrate community was maintained within the SDS, and community shifts were associated with stagnation. Putative antibiotic resistance genes and potential opportunistic pathogens were identified before RO and after advanced oxidation, although few if any members of the wastewater microbial community passed through these treatment steps. These findings can contribute to improved design of advanced treatment trains and management of microbial communities in post-treatment steps.

Using rapid quantification of adenosine triphosphate (ATP) as an indicator for early detection and treatment of cyanobacterial blooms

Early detection of harmful cyanobacterial blooms allows identification of potential risk and appropriate selection of treatment techniques to prevent exposure in recreational water bodies and drinking water supplies. Here, luminescence-based adenosine triphosphate (ATP) analysis was applied to monitor and treat cultured and naturally occurring cyanobacteria cells. When evaluating lab-cultured Microcystis aeruginosa, ATP concentrations (≤252,000 pg/mL) had improved sensitivity and correlated well (R² = 0.969) with optical density measurements at 730 nm (OD730; ≤0.297 cm^-1). Following one year of monitoring of a surface water supply, ATP concentrations (≤2000 pg/mL) correlated (R² = 0.791) with chlorophyll-a concentrations (≤50 μg/L). A preliminary early warning threshold of 175 pg ATP/mL corresponded with 5 μg/L chlorophyll-a to initiate increased monitoring (e.g., of cyanotoxins). Following oxidation processes (i.e., chlorine, chloramine, ozone, permanganate), ATP was demonstrated as an indicator of cell lysis and a threshold value of <100 pg/mL was recommended for complete release of intracellular cyanotoxins. ATP was also used to assess efficacy of copper (Cu(II)) treatment on cyanobacteria-laden surface water. While 24-h exposure to 2.5 mg Cu(II)/L did not impact chlorophyll-a, ATP decreased from 13,500 to 128 pg/mL indicating metabolic activity was minimized. Ultimately, ATP analysis holds promise for early detection and mitigation of potentially harmful algal blooms based on superior sensitivity, independence from cell morphology artifacts, rapid time for analysis (<10 min), and ease of deployment in the field compared to conventional methods.

Using discrete and online ATP measurements to evaluate regrowth potential following ozonation and (non)biological drinking water treatment

Water utilities must control microbial regrowth in the distribution system to protect public health. In this study, an adenosine triphosphate (ATP)-based biomass production potential test using indigenous bacterial communities were used to evaluate regrowth potential following ozonation with either biofiltration (BF) or sustained chlorination (SCl₂). Two full-scale water treatment plants with different upstream processes (i.e., WTP-BF: ozonation, coagulation/flocculation, biofiltration, UV irradiation, chlorination; and WTP-SCl₂: ozonation, chlorination, coagulation/flocculation, filtration, chlorination) were compared. Characterization of indigenous bacteria using 16S rRNA gene sequencing, qPCR, and cellular ATP (cATP) showed microbial diversity changes across treatment, biomass sloughing from biofilters (effluent cATP = 30 ± 1 ng/L), and disinfection by chlorine (cATP < 1 ng/L). For both WTPs, 14-day cumulative biomass production (CBP_t =  ∑t=0tATP(t)×Δt) was highest for ozonated water samples (CBP₁₄ = 1.2 × 10³-3.0 × 10³ d ngATP/L). CBP further increased with increasing ozone dose due to production of more biodegradable carbon. Growth promotion by carbon was confirmed from the consumption of ozonation byproducts (carboxylic acids, aldehydes) and the increase in CBP (9.5 × 10²-2.9 × 10³ d ngATP/L) after addition of 50-300 μgC/L acetate. Ozone followed by sustained chlorination (WTP-SCl₂) effectively controlled biomass growth across the treatment process (CBP₁₄ <10 d ngATP/L). In contrast, ozone followed by biofiltration (WTP-BF) reduced regrowth potential by 30% (biofilter influent CBP₁₄ = 1.3 × 10³ d ngATP/L; biofilter effluent CBP₁₄ = 9.3 × 10² d ngATP/L). After adding chlorine to the biofilter effluent, CBP₁₄ was reduced to <10 d ngATP/L. Lastly, online ATP measurements confirmed the discrete measurements and improved identification of the cATP peak and growth phases of indigenous bacteria.

Biofilter scaling procedures for organics removal: A potential alternative to piloting

To provide information for the design and improvement of full-scale biofilters, pilot-scale biofiltration studies are the current industry standard because they utilize the same filter media size and loading rate as the full-scale biofilters. In the current study, bench-scale biofilters were designed according to a biofilter scaling model from the literature, and the ability of the bench-scale biofilters to accurately represent the organics removal of pilot-scale biofilters was tested. To ensure similarity in effluent water quality between bench- and pilot- or full-scale biofilters at the same influent substrate concentration, the tested model requires that either mass transport resistance or biofilm shear loss takes primacy over the other. The potential primacy of mass transport resistance or biofilm shear loss was evaluated via water quality testing (dissolved organic carbon, specific ultraviolet absorbance, liquid chromatography - organic carbon detection, trihalomethane formation potential, and haloacetic acid formation potential). The biofilters also were characterized for adenosine triphosphate (ATP) content, enzyme activity, extracellular polymeric substances, and microbial community structure. The results of this study indicate that biofilm shear loss takes primacy over mass transport resistance for bench-scale biofilter design in this system; thus, bench-scale biofilters designed in this manner accurately represent organics removal in pilot-scale biofilters. Applying this scaling procedure can reduce filter media requirements from many kilograms to just a few grams and daily water requirements from thousands of liters to less than 10 L. This scaling procedure will allow future researchers to test alternative treatment designs and operating conditions without the need for expensive pilot-scale studies.

Response of microorganisms in biofilm to sulfadiazine and ciprofloxacin in drinking water distribution systems

Effects of sulfadiazine and ciprofloxacin on microorganisms in biofilm of drinking water distribution systems (DWDSs) were studied. The results verified that the increases of 16S rRNA for total bacteria and bacterial genus Hyphomicrobium were related to the promotion of antibiotic resistance genes (ARGs) and class 1 integrons (int1) in DWDSs with sulfadiazine and ciprofloxacin. Moreover, the bacteria showed higher enzymatic activities in DWDSs with sulfadiazine and ciprofloxacin, which resulted in more production of extracellular polymeric substances (EPS). The higher contents of EPS proteins and secondary structure β-sheet promoted bacterial aggregation and adsorption onto surface of pipelines to form biofilm. EPS can serve as a barrier for the microorganisms in biofilm. Therefore, the biofilm bacterial communities shifted and the 16S rRNA for total bacteria increased in DWDSs with antibiotics, which also drove the ARGs promotion. Furthermore, the two antibiotics exhibited stronger combined effects than that caused by sulfadiazine and ciprofloxacin alone.

Bacterial release from pipe biofilm in a full-scale drinking water distribution system

Safe drinking water is delivered to the consumer through kilometres of pipes. These pipes are lined with biofilm, which is thought to affect water quality by releasing bacteria into the drinking water. This study describes the number of cells released from this biofilm, their cellular characteristics, and their identity as they shaped a drinking water microbiome. Installation of ultrafiltration (UF) at full scale in Varberg, Sweden reduced the total cell count to 1.5 × 10³ ± 0.5 × 10³ cells mL⁻¹ in water leaving the treatment plant. This removed a limitation of both flow cytometry and 16S rRNA amplicon sequencing, which have difficulties in resolving small changes against a high background cell count. Following installation, 58% of the bacteria in the distributed water originated from the pipe biofilm, in contrast to before, when 99.5% of the cells originated from the treatment plant, showing that UF shifts the origin of the drinking water microbiome. The number of bacteria released from the biofilm into the distributed water was 2.1 × 10³ ± 1.3 × 10³ cells mL⁻¹ and the percentage of HNA (high nucleic acid) content bacteria and intact cells increased as it moved through the distribution system. DESeq2 analysis of 16S rRNA amplicon reads showed increases in 29 operational taxonomic units (OTUs), including genera identified as Sphingomonas, Nitrospira, Mycobacterium, and Hyphomicrobium. This study demonstrated that, due to the installation of UF, the bacteria entering a drinking water microbiome from a pipe biofilm could be both quantitated and described.

Enzyme-Identified Phosphorus Limitation Linked to More Rapid Headloss Accumulation in Drinking Water Biofilters

Drinking water biofilters can improve water quality by transforming contaminants or their precursors, but they also can develop headloss more rapidly than do abiotic filters. Phosphorus supplementation has been proposed as one strategy to lengthen biofilter run times, but the impact of this strategy in field tests has been mixed. The current bench-scale study found that severe phosphorus limitation, as indicated by a high phosphatase to total glycosidase activity ratio (PHO:GLY), led to 230% higher headloss accumulation rate when particles were loaded onto the biofilters as compared to the same experiment performed under a mild phosphorus limitation. Phosphorus limitation was associated with higher concentrations of extracellular polymeric substances, lower biomass concentrations, a more filamentous biofilm morphology, and increased relative abundance of Hyphomicrobiaceae (a family of stalked bacteria) on the biofilter media. These differences in the biofilm likely contributed to higher headloss. This work suggests that phosphorus supplementation could improve biofilter hydraulics in the field if the biofilter is severely phosphorus limited, which was indicated by a PHO:GLY greater than 154 under the conditions tested in this study.

Pilot-scale comparison of cyclically and continuously operated drinking water biofilters: Evaluation of biomass, biological activity and treated water quality

The objective of this pilot study was to evaluate the impact of cyclical (operated 8-12 h per day) and continuous biofilter operation with respect to biomass development, biological enzyme activity and treated water quality (in terms of organics, nutrients and disinfection by-product (DBP) formation potential). Continuously operated biofilters developed greater densities of biomass, as measured by ATP, when compared to cyclically operated filters; reducing the empty bed contact time (EBCT) increased biomass density under continuous flow conditions. However, once normalized to biomass, it was shown that cyclically operated filters exhibited higher enzyme activity, indicating that this method of operation may improve bacterial function. Reduction of organics was generally similar for both continuous and cyclical filters with the same EBCT, however, cyclical filters demonstrated higher variability during the first 4 h following start-up. Overall, HAA formation potential was better controlled by continuously operated filters, due to poor performance by the cyclical filters upon start-up while THM precursors were removed equally well by all filters. To understand the removal capacity for NDMA precursors through biological filters, both naturally occurring NDMA FP and NDMA FP resulting from spiked anthropogenic precursors was monitored through the filter depth. All the filters removed 90% of the naturally occurring NDMA FP within the first 45 cm; cyclical operation resulted in higher reduction of spiked anthropogenic NDMA precursors (50% higher than continuously operated) demonstrating the advantage of routine shut down on overall microbial activity. Tools to monitor and predict biofilter performance are in high demand. Here we present an "effective activity" term which combines enzyme activity with contact time (EBCT). Effective esterase activity was strongly correlated to DOC reduction as a function of filter operation (cyclical or continuous) and EBCT; effective phosphatase activity was indicative of phosphate removal. The results of this study indicate that routine shut down of the filters as this location improved enzyme activity without compromising control of chlorinated DBPs (THMs and HAAs) or NDMA derived from natural and anthropogenic precursors.

O3-BAC-Cl2: A multi-barrier process controlling the regrowth of opportunistic waterborne pathogens in drinking water distribution systems

Simulated drinking water distribution system (DWDS) treated with O₃-BAC-Cl₂ (ozone-biological activated carbon-chlorine) was constructed to study its effects on the regrowth of five typical opportunistic pathogens (OPs). It was found that O₃-BAC-Cl₂ could significantly reduce the regrowth of target OPs in the effluents of DWDS compared with Cl₂ and O₃-Cl₂ with the same residual chlorine levels. However, the effect of O₃-BAC-Cl₂ on the average numbers of target OPs gene markers in the biofilms of DWDS was not apparent, suggesting that OPs in the biofilms of DWDS were tolerant to the upstream disinfection process. The quantification of target OPs in the BAC-filter column demonstrated that OPs decreased with the increase of depth, which was likely due to the organic nutrient gradient and microbial competition inside the BAC-filter. Increase in the ozone dose could further reduce the OPs at the bottom of the BAC-filter. Spearman correlation analysis demonstrated that some significant correlations existed between target microorganisms, suggesting potential microbial ecological relationships. Overall, our results demonstrated that the BAC-filter may act as a "battlefield" suppressing the OPs through microbial competition. O₃-BAC-Cl₂ could be an effective multi-barrier process to suppress the proliferation of OPs in the bulk water of DWDS. However, OPs protected by the biofilms of DWDS should receive further attention because OPs may be detached and released from the biofilms.

The utility of flow cytometry for potable reuse

Protecting public health from pathogens is critical when treating wastewater to drinking water standards (i.e., planned water reuse). Viruses are a principal concern, yet real-time monitoring strategies do not currently measure virus removal through reuse processes. Flow cytometry (FCM) has enabled rapid and sensitive bacteria monitoring in water treatment applications, but methods for virus and protozoa monitoring remain immature. We discuss recent advances in the FCM field and FCM applications for quantifying microorganisms in water. We focus on flow virometry (FVM) developments, as virus enumeration methods show promise for water reuse applications. Ultimately, we propose FVM for near real-time monitoring across treatment to more accurately validate virus particle removal and for pilot studies to characterize removal through understudied unit processes.

Removal Efficacy of Opportunistic Pathogens and Bacterial Community Dynamics in Two Drinking Water Treatment Trains

Drinking water treatment processes (DWTPs) impact pathogen colonization and microbial communities in finished water; however, their efficacies against opportunistic pathogens are not fully understood. In this study, the effects of treatment steps on the removal of Legionella spp., Legionella pneumophila, nontuberculous mycobacteria, Mycobacterium avium, and two amoeba hosts (Vermamoeba vermiformis, Acanthamoeba) are evaluated in two parallel trains of DWTPs equipped with different pretreatment units. Quantitative polymerase chain reaction analysis demonstrates significantly reduced numbers of total bacteria, Legionella, and mycobacteria during ozonation, followed by a rebound in granular activated carbon (GAC) filtration, whereas sand filtration exerts an overarching effect in removing microorganisms in both treatment trains. V. vermiformis is more prevalent in biofilm (34%) than water samples (7.7%), while Acanthamoeba is not found in the two trains of DWTPs. Illumina sequencing of bacterial 16S rRNA genes reveals significant community shifts at different treatment steps, as well as distinct bacterial community structures in water and biofilm samples in parallel units (e.g., ozonation, GAC, sand filtration) between the two trains (analysis of similarities (ANOSIM), p < 0.05), implying the potential influence of different pretreatment steps in shaping the downstream microbiome. Overall, the results provide insights to mitigation of opportunistic pathogens and engineer approaches for managing bacterial communities in DWTPs.

Using upstream oxidants to minimize surface biofouling and improve hydraulic performance in GAC biofilters

The combination of biological growth and particle loading can adversely affect hydraulic performance in drinking water biofilters. In this study, upstream oxidant addition was used to distribute biologically-derived filter clogging in granular activated carbon (GAC) biofilters. Oxidant penetration was assessed during pilot-scale operation and backwashing of dual media (GAC/sand) and multimedia (GAC/anthracite/sand) biofilters. Influent chlorine (HOCl), monochloramine (NH₂Cl), and hydrogen peroxide (H₂O₂) residuals were optimized to react with the GAC surface in the upper portion of the filter media bed (depth < 0.5 m) to attenuate biomass development. As the oxidant residual was quenched by surface-mediated reaction with the filter media, biomass growth was promoted deeper in the filter bed (depth > 0.5 m). The oxidant-induced effects on biomass and hydraulic performance were monitored through measurements of adenosine triphosphate (ATP) and head loss accumulation at different media depths. Addition of oxidants (e.g., 0.6 mg Cl₂/L HOCl) could decrease terminal head loss by 20% in dual media filters and 40% in multimedia filters. These hydraulic benefits were achieved without significantly affecting removal of assimilable organic carbon (AOC), total organic carbon (TOC), turbidity, and particle counts. Oxidant type, residual concentration, media type, media age, and media depth influenced the passage of oxidant residuals and distribution of filter biomass. When oxidants were added during backwashing, oxidant residual was quenched through the bed depth from a combination of reactions with GAC media and biofilm degradation. This attenuation of residual oxidant may prevent the oxidant residual from penetrating the entire bed depth, potentially compromising backwashing objectives.

Changes in the Drinking Water Microbiome: Effects of Water Treatments Along the Flow of Two Drinking Water Treatment Plants in a Urbanized Area, Milan (Italy)

While safe and of high quality, drinking water can host an astounding biodiversity of microorganisms, dismantling the belief of its "biological simplicity." During the very few years, we are witnessing an exponential growth in scientific publications, exploring the ecology hidden in drinking water treatment plants (DWTPs) and drinking water distribution system (DWDS). We focused on what happens to the microbial communities from source water (groundwater) throughout the main steps of the potabilization process of a DWTP, located in an urbanized area in Northern Italy. Samples were processed by a stringent water filtration to retain even the smallest environmental bacteria and then analyzed with High-Throughput DNA Sequencing (HTS) techniques. We showed that carbon filters harbored a microbial community seeding and shaping water microbiota downstream, introducing a significant variation on incoming (groundwater) microbial community. Chlorination did not instantly affect the altered microbiota. We were also able to correctly predict (through machine learning analysis) samples belonging to groundwater (overall accuracy was 0.71), but the assignation was not reliable with carbon filter samples, which were incorrectly predicted as chlorination samples. The presence and abundance of specific microorganisms allowed us to hypothesize their role as indicators. In particular, Candidatus Adlerbacteria (Parcubacteria), together with microorganisms belonging to Alphaproteobacteria and Gammaproteobacteria, characterized treated water, but not raw water. An exception, confirming our hypothesis, is given by the samples downstream the filters renewal, which had a composition resembling groundwater. Volatility analysis illustrated how carbon filters represented an ecosystem that is stable over time, probably bearing the environmental conditions that promote the survival and growth of this peculiar microbial community.

Characterization of the bacterial community composition in water of drinking water production and distribution systems in Flanders, Belgium

The quality of drinking water is influenced by its chemical and microbial composition which in turn may be affected by the source water and the different processes applied in drinking water purification systems. In this study, we investigated the bacterial diversity in different water samples from the production and distribution chain of thirteen drinking water production and distribution systems from Flanders (Belgium) that use surface water or groundwater as source water. Water samples were collected over two seasons from the source water, the processed drinking water within the production facility and out of the tap in houses along its distribution network. 454‐pyrosequencing of 16S ribosomal RNA gene sequences revealed a total of 1,570 species‐level bacterial operational taxonomic units. Strong differences in community composition were found between processed drinking water samples originating from companies that use surface water and other that use groundwater as source water. Proteobacteria was the most abundant phylum in all samples. Yet, several phyla including Actinobacteria were significantly more abundant in surface water while Cyanobacteria were more abundant in surface water and processed water originating from surface water. Gallionella, Acinetobacter, and Pseudomonas were the three most abundant genera detected. Members of the Acinetobacter genus were even found at a relative read abundance of up to 47.5% in processed water samples, indicating a general occurrence of Acinetobacter in drinking water (systems).

Long-term spatial and temporal microbial community dynamics in a large-scale drinking water distribution system with multiple disinfectant regimes

Long-term spatial-temporal investigations of microbial dynamics in full-scale drinking water distribution systems are scarce. These investigations can reveal the process, infrastructure, and environmental factors that influence the microbial community, offering opportunities to re-think microbial management in drinking water systems. Often, these insights are missed or are unreliable in short-term studies, which are impacted by stochastic variabilities inherent to large full-scale systems. In this two-year study, we investigated the spatial and temporal dynamics of the microbial community in a large, full scale South African drinking water distribution system that uses three successive disinfection strategies (i.e. chlorination, chloramination and hypochlorination). Monthly bulk water samples were collected from the outlet of the treatment plant and from 17 points in the distribution system spanning nearly 150 km and the bacterial community composition was characterised by Illumina MiSeq sequencing of the V4 hypervariable region of the 16S rRNA gene. Like previous studies, Alpha- and Betaproteobacteria dominated the drinking water bacterial communities, with an increase in Betaproteobacteria post-chloramination. In contrast with previous reports, the observed richness, diversity, and evenness of the bacterial communities were higher in the winter months as opposed to the summer months in this study. In addition to temperature effects, the seasonal variations were also likely to be influenced by changes in average water age in the distribution system and corresponding changes in disinfectant residual concentrations. Spatial dynamics of the bacterial communities indicated distance decay, with bacterial communities becoming increasingly dissimilar with increasing distance between sampling locations. These spatial effects dampened the temporal changes in the bulk water community and were the dominant factor when considering the entire distribution system. However, temporal variations were consistently stronger as compared to spatial changes at individual sampling locations and demonstrated seasonality. This study emphasises the need for long-term studies to comprehensively understand the temporal patterns that would otherwise be missed in short-term investigations. Furthermore, systematic long-term investigations are particularly critical towards determining the impact of changes in source water quality, environmental conditions, and process operations on the changes in microbial community composition in the drinking water distribution system.

Monitoring biofilm function in new and matured full-scale slow sand filters using flow cytometric histogram image comparison (CHIC)

While slow sand filters (SSFs) have produced drinking water for more than a hundred years, understanding of their associated microbial communities is limited. In this study, bacteria in influent and effluent water from full-scale SSFs were explored using flow cytometry (FCM) with cytometric histogram image comparison (CHIC) analysis; and routine microbial counts for heterotrophs, total coliforms and Escherichia coli. To assess if FCM can monitor biofilm function, SSFs differing in age and sand composition were compared. FCM profiles from two established filters were indistinguishable. To examine biofilm in the deep sand bed, SSFs were monitored during a scraping event, when the top layer of sand and the schmutzdecke are removed to restore flow through the filter. The performance of an established SSF was stable: total organic carbon (TOC), pH, numbers of heterotrophs, coliforms, E. coli, and FCM bacterial profile were unaffected by scraping. However, the performance of two newly-built SSFs containing new and mixed sand was compromised: breakthrough of both microbial indicators and TOC occurred following scraping. The compromised performance of the new SSFs was reflected in distinct effluent bacterial communities; and, the presence of microbial indicators correlated to influent bacterial communities. This demonstrated that FCM can monitor SSF performance. Removal of the top layer of sand did not alter the effluent water from the established SSF, but did affect that of the SSFs containing new sand. This suggests that the impact of the surface biofilm on effluent water is greater when the deep sand bed biofilm is not established.

Spatiotemporal changes in bacterial community and microbial activity in a full-scale drinking water treatment plant

To gain insight into the bacterial dynamics present in drinking water treatment (DWT) systems, the microbial community and activity in a full-scale DWT plant (DWTP) in Guangzhou, South China, were investigated using Illumina Hiseq sequencing analyses combined with cultivation-based techniques during the wet and dry seasons. Illumina sequencing analysis of 16S rRNA genes revealed a large shift in the proportion of Actinobacteria, Proteobacteria and Firmicutes during the treatment process, with the proportion of Actinobacteria decreased sharply, whereas that of Proteobacteria and Firmicutes increased and predominated in treated water. Both microbial activity and bacterial diversity during the treatment process showed obvious spatial variation, with higher levels observed during the dry season and lower levels during the wet season. Clustering analysis and principal component analysis indicated dramatic shifts in the bacterial community after chlorination, suggesting that chlorination was highly effective at influencing the bacterial community. The bacterial community structure of finished water primarily comprised Pseudomonas, Citrobacter, and Acinetobacter, and interestingly showed high similarity to biofilms on granular activated carbon. Additionally, the abundance of bacterial communities was relatively stable in finished water and did not change with the season. A large number of unique operational taxonomic units were shared during treatment steps, indicating the presence of a diverse core microbiome throughout the treatment process. Opportunistic pathogens, including Pseudomonas, Acinetobacter, Citrobacter, Mycobacterium, Salmonella, Staphylococcus, Legionella, Streptococcus and Enterococcus, were detected in water including finished water, suggesting a potential threat to drinking-water safety. We also detected bacteria isolated from each treatment step using the pure-culture method. In particular, two isolates, identified as Mycobacterium sp. and Blastococcus sp., which belong to the phylum Actinobacteria, were obtained from finished water during the dry season. Together, these results provided evidence of spatial and temporal variations in DWTPs and contributed to the beneficial manipulation of the drinking water microbiome.

Identifying the underlying causes of biological instability in a full-scale drinking water supply system

Changes in bacterial concentration and composition in drinking water during distribution are often attributed to biological (in)stability. Here we assessed temporal biological stability in a full-scale distribution network (DN) supplied with different types of source water: treated and chlorinated surface water and chlorinated groundwater produced at three water treatment plants (WTP). Monitoring was performed weekly during 12 months in two locations in the DN. Flow cytometric total and intact cell concentration (ICC) measurements showed considerable seasonal fluctuations, which were different for two locations. ICC varied between 0.1-3.75 × 10⁵ cells mL^-1 and 0.69-4.37 × 10⁵ cells mL^-1 at two locations respectively, with ICC increases attributed to temperature-dependent bacterial growth during distribution. Chlorinated water from the different WTP was further analysed with a modified growth potential method, identifying primary and secondary growth limiting compounds. It was observed that bacterial growth in the surface water sample after chlorination was primarily inhibited by phosphorus limitation and secondly by organic carbon limitation, while carbon was limiting in the chlorinated groundwater samples. However, the ratio of available nutrients changed during distribution, and together with disinfection residual decay, this resulted in higher bacterial growth potential detected in the DN than at the WTP. In this study, bacterial growth was found to be higher (i) at higher water temperatures, (ii) in samples with lower chlorine residuals and (iii) in samples with less nutrient (carbon, phosphorus, nitrogen, iron) limitation, while this was significantly different between the samples of different origin. Thus drinking water microbiological quality and biological stability could change during different seasons, and the extent of these changes depends on water temperature, the water source and treatment. Furthermore, differences in primary growth limiting nutrients in different water sources could contribute to biological instability in the network, where mixing occurs.

Medium shapes the microbial community of water filters with implications for effluent quality

Little is known about the forces that determine the assembly of diverse bacterial communities inhabiting drinking water treatment filters and how this affects drinking water quality. Two contrasting ecological theories can help to understand how natural microbial communities assemble; niche theory and neutral theory, where environmental deterministic factors or stochastic factors predominate respectively. This study investigates the development of the microbial community on two common contrasting filter materials (quartz sand and granular activated carbon-GAC), to elucidate the main factors governing their assembly, through the evaluation of environmental (i.e. filter medium type) and stochastic forces (random deaths, births and immigration). Laboratory-scale filter columns were used to mimic a rapid gravity filter; the microbiome of the filter materials, and of the filter influent and effluent, was characterised using next generation 16S rRNA gene amplicon sequencing and flow-cytometry. Chemical parameters (i.e. dissolved organic carbon, trihalomethanes formation) were also monitored to assess the final effluent quality. The filter communities seemed to be strongly assembled by selection rather than neutral processes, with only 28% of those OTUs shared with the source water detected on the filter medium following predictions using a neutral community model. GAC hosted a phylogenetically more diverse community than sand. The two filter media communities seeded the effluent water, triggering differences in both water quality and community composition of the effluents. Overall, GAC proved to be better than sand in controlling microbial growth, by promoting higher bacterial decay rates and hosting less bacterial cells, and showed better performance for putative pathogen control by leaking less Legionella cells into the effluent water.

Evaluating the biosafety of conventional and O3-BAC process and its relationship with NOM characteristics

It is the priority to guarantee biosafety for drinking water treatment. The objective of this study was to evaluate the impact of widely applied conventional and ozone-biological activated carbon (O₃-BAC) advanced treatment technology on biosafety of drinking water. The items, including assimilable organic carbon (AOC), biodegradable dissolved organic carbon (BDOC), heterotrophic plate counts (HPCs) and the microorganism community structures, were used to evaluate the biosafety. Moreover, their relationships with molecular weights (MWs) and fluorescence intensity of dissolved organic matter were investigated. The results indicated that the technology provided a considerable gain in potable water quality by decreasing dissolved organic carbon (DOC, from 5.05 to 1.71 mg/L), AOC (from 298 to 131 μg/L), BDOC (from 1.39 to 0.24 mg/L) and HPCs (from 275 to 10 CFU/mL). Ozone brought an increase in DOC with low MW <1 kDa, which accompanies with an increase in AOC/BDOC concentration, which could be reduced effectively by subsequent BAC process. The formation of AOC/BDOC was closely related to DOC with low MWs and aromatic protein. Bacteria could be released from BAC filter, resulting in an increase in HPC and the presence of pathogenic bacteria in effluent, while the post sand filter could further guarantee the biosafety of finished water.

Comparative assessment of ceramic media for drinking water biofiltration

Media type is a critical design consideration when implementing biofiltration for drinking water treatment. Granular activated carbon (GAC) has been shown to provide superior performance when compared to a wide range of media types, largely due to its higher surface area. Engineered ceramic media is an attractive alternative to GAC as it has a similar surface area but at a lower cost. This pilot-scale biofiltration study compared the performance of GAC, anthracite and two different effective sizes of ceramic (CER) media (1.0 mm and 1.2 mm), in terms of dissolved organic carbon (DOC), head loss, turbidity, and disinfection by-product formation potential (DBPFP). Biological acclimation was monitored using adenosine tri-phosphate (ATP) measurements; biomass was further examined using laccase and esterase enzyme activity assays. When compared to other media types examined, biological GAC had higher (p > 0.05) removals of DOC (9.8 ± 3.8%), trihalomethane formation potential (THMFP, 26.3 ± 10.2%), and haloacetic acid formation potential (HAAFP, 27.2 ± 14.0%). CER media required 6-7 months to biologically acclimate, while filters containing GAC and anthracite were biologically active (>100 ng of ATP/g media) following 30-45 days of operation. Once acclimated, ATP values of 243 and 208 ng/g attained for CER 1.0 and 1.2, respectively, were statistically comparable to GAC (244 ng/g) and higher than anthracite (110 ng/g), however this did not translate into greater organics removal. Esterase and laccase enzyme kinetics were highest for GAC, while CER was shown to have greater biodegradation potential than anthracite. The four media types attained similar turbidity reduction (p > 0.05), however ceramic media filters were observed to have run times which were 1.5-2.3 times longer when compared to anthracite, which could represent potential cost savings in terms of energy for pumping and backwash requirements. Overall, ceramic media was shown to be a potential alternative to anthracite when considering biofiltration, especially during cold water conditions (T < 10 °C).

Metagenomic characterization of biofilter microbial communities in a full-scale drinking water treatment plant

Microorganisms inhabiting filtration media of a drinking water treatment plant can be beneficial, because they metabolize biodegradable organic matter from source waters and those formed during disinfection processes, leading to the production of biologically stable drinking water. However, which microbial consortia colonize filters and what metabolic capacity they possess remain to be investigated. To gain insights into these issues, we performed metagenome sequencing and analysis of microbial communities in three different filters of a full-scale drinking water treatment plant (DWTP). Filter communities were sampled from a rapid sand filter (RSF), granular activated carbon filter (GAC), and slow sand filter (SSF), and from the Schmutzdecke (SCM, a biologically active scum layer accumulated on top of SSF), respectively. Analysis of community phylogenetic structure revealed that the filter bacterial communities significantly differed from those in the source water and final effluent communities, respectively. Network analysis identified a filter-specific colonization pattern of bacterial groups. Bradyrhizobiaceae were abundant in GAC, whereas Nitrospira were enriched in the sand-associated filters (RSF, SCM, and SSF). The GAC community was enriched with functions associated with aromatics degradation, many of which were encoded by Rhizobiales (∼30% of the total GAC community). Predicting minimum generation time (MGT) of prokaryotic communities suggested that the GAC community potentially select fast-growers (<15 h of MGT) among the four filter communities, consistent with the highest dissolved organic matter removal rate by GAC. Our findings provide new insights into the community phylogenetic structure, colonization pattern, and metabolic capacity that potentially contributes to organic matter removal achieved in the biofiltration stages of the full-scale DWTP.

Microbial Communities Shaped by Treatment Processes in a Drinking Water Treatment Plant and Their Contribution and Threat to Drinking Water Safety

Bacteria play an important role in water purification in drinking water treatment systems. On one hand, bacteria present in the untreated water may help in its purification through biodegradation of the contaminants. On the other hand, some bacteria may be human pathogens and pose a threat to consumers. The present study investigated bacterial communities using Illumina MiSeq sequencing of 16S rRNA genes and their functions were predicted using PICRUSt in a treatment system, including the biofilms on sand filters and biological activated carbon (BAC) filters, in 4 months. In addition, quantitative analyses of specific bacterial populations were performed by real-time quantitative polymerase chain reaction (qPCR). The bacterial community composition of post-ozonation effluent, BAC effluent and disinfected water varied with sampling time. However, the bacterial community structures at other treatment steps were relatively stable, despite great variations of source water quality, resulting in stable treatment performance. Illumina MiSeq sequencing illustrated that Proteobacteria was dominant bacterial phylum. Chlorine disinfection significantly influenced the microbial community structure, while other treatment processes were synergetic. Bacterial communities in water and biofilms were distinct, and distinctions of bacterial communities also existed between different biofilms. By contrast, the functional composition of biofilms on different filters were similar. Some functional genes related to pollutant degradation were found widely distributed throughout the treatment processes. The distributions of Mycobacterium spp. and Legionella spp. in water and biofilms were revealed by real-time quantitative polymerase chain reaction (qPCR). Most bacteria, including potential pathogens, could be effectively removed by chlorine disinfection. However, some bacteria presented great resistance to chlorine. qPCRs showed that Mycobacterium spp. could not be effectively removed by chlorine. These resistant bacteria and, especially potential pathogens should receive more attention. Redundancy analysis (RDA) showed that turbidity, ammonia nitrogen and total organic carbon (TOC) exerted significant effects on community profiles. Overall, this study provides insight into variations of microbial communities in the treatment processes and aids the optimization of drinking water treatment plant design and operation for public health.

16S rRNA Gene Amplicon Sequencing Reveals Significant Changes in Microbial Compositions during Cyanobacteria-Laden Drinking Water Sludge Storage

This is the first study to systematically investigate the microbial community structure in cyanobacteria-laden drinking water sludge generated by different types of coagulants (including AlCl₃, FeCl₃, and polymeric aluminum ferric chloride (PAFC)) using Illumina 16S rRNA gene MiSeq sequencing. Results show that Cyanobacteria, Proteobacteria, Firmicutes, Bacteroidetes, Verrucomicrobia, and Planctomycetes were the most dominant phyla in sludge, and because of the toxicity of high Al and Fe level in AlCl₃ and FeCl₃ sludges, respectively, the PAFC sludge exhibited greater microbial richness than that in AlCl₃ and FeCl₃ sludges. Due to lack of light and oxygen in sludge, relative abundance of the dominant genera Microcystis, Rhodobacter, Phenylobacterium, and Hydrogenophaga clearly decreased, especially after 4 days storage, and the amounts of extracellular microcystin and organic matter rose. As a result, the relative abundance of microcystin and organic degradation bacteria increased significantly, including pathogens such as Bacillus cereus, in particular after 4 days storage. Hence, sludge should be disposed of within 4 days to prevent massive growth of pathogens. In addition, because the increase of extracellular microcystins, organic matter, and pathogens in AlCl₃ sludge was higher than that in FeCl₃ and PAFC sludges, FeCl₃ and PAFC may be ideal coagulants in drinking water treatment plants.

Heterotrophic monitoring at a drinking water treatment plant by matrix-assisted laser desorption/ionization-time of flight (MALDI-TOF) mass spectrometry after different drinking water treatments

The aim of this work was to assess the suitability of matrix-assisted laser desorption/ionization-time of flight mass spectrometry (MALDI-TOF MS) for routine heterotrophic monitoring in a drinking water treatment plant. Water samples were collected from raw surface water and after different treatments during two campaigns over a 1-year period. Heterotrophic bacteria were studied and isolates were identified by MALDI-TOF MS. Moreover, the diversity index and the coefficient of population similarity were also calculated using biochemical fingerprinting of the populations studied. MALDI-TOF MS enabled us to characterize and detect changes in the bacterial community composition throughout the water treatment plant. Raw water showed a large and diverse population which was slightly modified after initial treatment steps (sand filtration and ultrafiltration). Reverse osmosis had a significant impact on the microbial diversity, while the final chlorination step produced a shift in the composition of the bacterial community. Although MALDI-TOF MS could not identify all the isolates since the available MALDI-TOF MS database does not cover all the bacterial diversity in water, this technique could be used to monitor bacterial changes in drinking water treatment plants by creating a specific protein profile database for tracking purposes.

Microbial biotechnologies for potable water production

Sustainable Development Goal 6 requires the provision of safe drinking water to the world. We propose that increased exploitation of biological processes is fundamental to achieving this goal due to their low economic and energetic costs. Biological processes exist for the removal of most common contaminants, and biofiltration processes can establish a biologically stable product that retains high quality in distribution networks, minimizing opportunities for pathogen invasion.

Flow cytometric bacterial cell counts challenge conventional heterotrophic plate counts for routine microbiological drinking water monitoring

Drinking water utilities and researchers continue to rely on the century-old heterotrophic plate counts (HPC) method for routine assessment of general microbiological water quality. Bacterial cell counting with flow cytometry (FCM) is one of a number of alternative methods that challenge this status quo and provide an opportunity for improved water quality monitoring. After more than a decade of application in drinking water research, FCM methodology is optimised and established for routine application, supported by a considerable amount of data from multiple full-scale studies. Bacterial cell concentrations obtained by FCM enable quantification of the entire bacterial community instead of the minute fraction of cultivable bacteria detected with HPC (typically < 1% of all bacteria). FCM measurements are reproducible with relative standard deviations below 3% and can be available within 15 min of samples arriving in the laboratory. High throughput sample processing and complete automation are feasible and FCM analysis is arguably less expensive than HPC when measuring more than 15 water samples per day, depending on the laboratory and selected staining procedure(s). Moreover, many studies have shown FCM total (TCC) and intact (ICC) cell concentrations to be reliable and robust process variables, responsive to changes in the bacterial abundance and relevant for characterising and monitoring drinking water treatment and distribution systems. The purpose of this critical review is to initiate a constructive discussion on whether FCM could replace HPC in routine water quality monitoring. We argue that FCM provides a faster, more descriptive and more representative quantification of bacterial abundance in drinking water.

Changes in degrading ability, populations and metabolism of microbes in activated sludge in the treatment of phenol wastewater

With an increase in phenol concentration, biodegradable soluble microbial by-product-like matter in sludge EPS is gradually replaced by non-biodegradable tryptophan protein-like matter.