Simulated Drinking Water Distribution Systems Research Articles

Plasmid-mediated transfer of antibiotic resistance genes and biofilm formation in a simulated drinking water distribution system under chlorine pressure

The effects of disinfectants and plasmid-based antibiotic resistance genes (ARGs) on the growth of microorganisms and the plasmid-mediated transfer of ARGs in the water and biofilm of the drinking water distribution system under simulated conditions were explored. The heterotrophic plate count of the water in reactors with 0.1 mg/L NaClO and NH2Cl was higher than in the control groups. There was no similar phenomenon in biofilm. In the water of reactors containing NaClO, the aphA and bla genes were lower than in the antibiotic resistant bacteria group, while both genes were higher in the water of reactors with NH2Cl than in the control group. Chloramine may promote the transfer of ARGs in the water phase. Both genes in the biofilm of the reactors containing chlorine were lower than the control group. Correlation analysis between ARGs and water quality parameters revealed that the copy numbers of the aphA gene were significantly positively correlated with the copy numbers of the bla gene in water and significantly negatively correlated in biofilm (p < 0.05). The results of the sequencing assay showed that bacteria in the biofilm, in the presence of disinfectant, were primarily Gram-negative. 1.0 mg/L chlorine decreased the diversity of the community in the biofilm. The relative abundance of some bacteria that may undergo transfer increased in the biofilm of the reactor containing 0.1 mg/L chlorine.

Effect of booster disinfection on the prevalence of microbial antibiotic resistance and bacterial community in a simulated drinking water distribution system

Booster disinfection was often applied to control the microorganism's growth in long-distance water supply systems. The effect of booster disinfection on the changing patterns of antibiotic resistance and bacterial community was investigated by a simulated water distribution system (SWDS). The results showed that the antibiotic resistance bacteria (ARB) and antibiotic resistance genes (ARGs) were initially removed after dosing disinfectants (chlorine and chloramine), but then increased with the increasing water age. However, the relative abundance of ARGs significantly increased after booster disinfection both in buck water and biofilm, then decreased along the pipeline. The pipe materials and disinfectant type also affected the antibiotic resistance. Chlorine was more efficient in controlling microbes and ARGs than chloramine. Compared with UPVC and PE pipes, SS pipes had the lowest total bacteria, ARB concentration, and ARB percentage, mainly due to higher disinfectant residuals and a smoother surface. The significant correlation (rs = 0.77, p < 0.001) of the 16S rRNA genes was observed between buck water and biofilm, while the correlations of targeted ARGs were found to be weak. Bray-Curtis similarity index indicated that booster disinfection significantly changed the biofilm bacterial community and the disinfectant type also had a marked impact on the bacterial community. At the genus level, the relative abundance of Pseudomonas, Sphingomonas, and Methylobacterium significantly increased after booster disinfection. Mycobacterium increased after chloramination while decreased after chlorination, indicating Mycobacterium might resist chloramine. Pseudomonas, Methylobacterium, and Phreatobacter were found to correlate well with the relative abundance of ARGs. These results highlighted antibiotic resistance shift and bacterial community alteration after booster disinfection, which may be helpful in controlling potential microbial risk in drinking water.

Realism-based assessment of the efficacy of potassium peroxymonosulphate on Stenotrophomonas maltophilia biofilm control

The potential of pentapotassium bis(peroxymonosulphate) bis(sulphate) (OXONE) to control biofilms in drinking water distribution systems (DWDS) was evaluated and compared to chlorine disinfection. Mature biofilms of drinking water (DW)-isolated Stenotrophomonas maltophilia were formed using a simulated DWDS with a rotating cylinder reactor (RCR). After 30 min of exposure, OXONE at 10 × minimum bactericidal concentration (MBC) caused a significant 4 log reduction of biofilm culturability in comparison to the unexposed biofilms and a decrease in the number of non-damaged cells below the detection limit (4.8 log cells/cm2). The effects of free chlorine were restricted to approximately 1 log reduction in both biofilm culturability and non-damaged cells. OXONE in synthetic tap water (STW) at 25 ºC was more stable over 40 days than free chlorine in the same conditions. OXONE solution exhibited a disinfectant decrease of about 10% of the initial concentration during the first 9 days, and after this time the values remained stable. Whereas possible reaction of chlorine with inorganic and organic substances in STW contributed to free chlorine depletion of approximately 48% of the initial concentration. Electron paramagnetic resonance (EPR) spectroscopy studies confirmed the presence of singlet oxygen and other free radicals during S. maltophilia disinfection with OXONE. Overall, OXONE constitutes a relevant alternative to conventional DW disinfection for effective biofilm control in DWDS.

Effect of Pipe Materials on Bacterial Community, Redox Reaction, and Functional Genes

In the present study, the effect of pipe materials on water quality as well as the microbial community was researched with static devices as well as dynamic ones. Five kinds of pipe materials (SP: steel plastic composite pipe, SS: stainless steel pipe, DI: ductile iron pipe, CI: cast iron pipe, GS: galvanized steel pipe) were chosen, and the soaking experiment was carried out with bench-scale devices. To further investigate the performance of pipe materials over a long term, a pilot-scale simulated drinking water distribution system was constructed, and the water quality parameters were monitored for six months. The pipe materials were ranked as SP, DI, and CI by the order of increasing turbidity, CODMn, and NH3-N. Furthermore, the biofilm samples were analyzed via pyrosequencing and COG functional categories. The DI biofilm possessed the highest bacterial diversity with a Shannon index of 3.56, followed by SP (3.14) and CI (0.77). The presence of nitrate-reducing bacteria (NRB), iron-oxidizing bacteria (IOB), iron-reducing bacteria (IRB), and sulfate-reducing bacteria (SRB)was identified, and NRB composed the largest share in all pipe materials (13.0%–17.2%), with other redox bacteria making up a minor proportion (0.02%–1.52%). NRB and IRB inhibited the corrosion process while IOB and SRB enhanced it. Most dominant genera present in samples were derived firstly from soil or active sludge, indicating a turbidity problem due to soil contamination in the distribution network.

Influence of chlorinated disinfection by-products on transmission of antibiotic resistance genes in biofilms and water of a simulated drinking water distribution system

The spread of antibiotic resistance genes (ARGs) caused by biofilm growth in drinking water distribution systems (DWDSs) poses a serious threat to human health. The influence of disinfection by-products (DBPs) on the distribution of ARGs and mobile genetic elements (MGEs) in different phases (biofilms and water) of DWDSs is unclear. In this study, the characteristics of microbial indices, trihalomethanes (THMs), haloacetic acids (HAAs), ARGs and MGEs in biofilms and effluents of simulated DWDS reactors were detected during 150 d of operation. During the mature biofilm periods of 90 d or 120 d, for both biofilms and effluents, seven categories of ARGs (except tetracycline) and MGEs were at their highest and lowest relative abundance in the entire operation cycle, respectively. The influence of DBPs on ARGs and MGEs in the DWDS was different in different phases. In effluents, DBPs showed a stronger relationship with ARGs compared with MGEs. THMs and HAAs had positive correlations with various ARGs, which indicated that DBPs affected the distribution of resistance genes. In biofilms, the influence of MGEs on ARGs was stronger than that of DBPs on ARGs. Thus, DBPs might be an important contributor to the enrichment and spread of ARGs in water.

Inhibiting the increase of antibiotic resistance genes during drinking water distribution by superior microbial interface using Fe modified granular activated carbon

The effects of biological activated carbon treatment using Fe2O3-modified granular activated carbon in inhibiting antibiotic resistance genes in simulated drinking water distribution systems was compared with unmodified granular activated carbon as a reference. Fe2O3-modified biofiltration resulted in a sustained inhibition of resistance genes during drinking water chlorination and distribution (relative abundance in simulated tap water after the modified and unmodified filtration was 1.31% and 9.40%, respectively). A new electron transfer pathway occurring in attached biofilms on Fe2O3-modified granular activated carbon surface, which was identified using X-ray photoelectron spectroscopy and the phenanthroline spectrophotometric method, enhanced the extracellular electron transfer rate and weakened the pressure of organic micropollutants on microorganisms. Hence, the relative abundance of resistance genes (36.32%) and integron (8.79%) on modified carbon was considerably lower than that on unmodified carbon (115.59% and 13.85%, respectively). Meanwhile, the secreted extracellular polymeric substances on modified carbon presented higher flocculating efficiency and better mechanical stability, resulting in the suspended extracellular polymeric substances in downstream water exhibiting stronger electrostatic repulsion. The particle-attached biofilms in downstream distribution systems consistently failed to form larger aggregates, inhibiting horizontal gene transfer, and overall microbial metabolism. Based on network analysis, 11 OTUs in the water samples from raw water to simulated tap water formed an extremely interrelated module with no links to target resistance genes and integron. Therefore, a range of microbial variations triggered by the microbial interface on modified carbon successfully controlled the transfer of antibiotic resistance genes-associated risk from biological activated carbon effluent to tap water. Our findings revealed that enhancing the microbial interface using Fe2O3-modified granular activated carbon is a promising option for inhibiting the antibiotic resistance genes increase in tap water.

Enhancing inhibition of disinfection byproducts formation and opportunistic pathogens growth during drinking water distribution by Fe2O3/Coconut shell activated carbon.

The effects of biological activated carbon treatment using Fe2O3 modified coconut shell-based activated carbon (Fe/CAC) were investigated on the occurrence of opportunistic pathogens (OPs) and formation of disinfection by-products (DBPs) in simulated drinking water distribution systems (DWDSs) with unmodified CAC as a reference. In the effluent of annular reactor (AR) with Fe/CAC, the OPs growth and DBPs formation were inhibited greatly. Based on the differential pulse voltammetry and dehydrogenase activity tests, it was verified that extracellular electron transfer was enhanced in the attached biofilms of Fe/CAC, hence improving the microbial metabolic activity and biological removal of organic matter especially DBPs precursors. Meanwhile, the extracellular polymeric substances (EPS) on the surface of Fe/CAC exhibited stronger viscosity, higher flocculating efficiency and better mechanical stability, avoiding bacteria or small-scale biofilms falling off into the water. Consequently, the microbial biomass and EPS substances amount decreased markedly in the effluent of Fe/CAC filter. More importantly, Fe/CAC did significantly enhance the shaping role on microbial community of downstream DWDSs, continuously excluding OPs advantage and inhibiting EPS production. The weakening of EPS in DWDSs resulted in decrease of microbial chlorine-resistance ability and EPS-derived DBPs precursors supply. Therefore, the deterioration of water quality in DWDSs was inhibited greatly, sustainably maintaining the safety of tap water. Our findings indicated that optimizing biological activated carbon treatment by interface modification is a promising method for improving water quality in DWDSs.

Effects of bacterial community composition and structure in drinking water distribution systems on biofilm formation and chlorine resistance

Community-intrinsic properties affect the composition and function of a microbial community. Understanding the microbial community-intrinsic properties in drinking water distribution systems (DWDS) could help to select disinfection strategies and aid in the prevention of waterborne infectious diseases. In this study, we investigated the formation of multi-species biofilms in six groups, each consisting of four or five mixed bacterial strains isolated from a simulated DWDS, at different incubation times (24, 48, and 72 h). We then evaluated the chlorine resistance of the 72-h multi-species biofilms in the presence of 0.3, 0.6, 1, 2, 4, and 10 mg/L residual chlorine. Microbacterium laevaniformans inhibited the formation of multi-species biofilms, Sphingomonas sp., Acinetobacter sp. and A. deluvii had the effect of promoting their growth, and B. cereus has little effect on the growth of multi-species biofilms. However, these inhibition and promotion effects were weak and inadequate to completely control the growth of multi-species biofilms. All multi-species produced strong biofilms after 72 h incubation, which could be due to microbial community-intrinsic properties. Community-intrinsic properties could maintain high EPS production and cell-to-cell connections in multi-species biofilms, and could affect the formation of multi-species biofilms. The chlorine resistance of multi-species biofilms was significantly improved by B. cereus, but significantly reduced by M. laevaniformans. These results indicated that the microbial community-intrinsic properties were influenced by the environment. At a relatively low disinfectant concentration (<2 mg/L residual chlorine), the community-intrinsic properties were maintained; however, when the disinfectant concentration was increased to 2–4 mg/L residual chlorine, the community-intrinsic properties weakened, and significantly affected the resistance of the microbial communities to the disinfectant. With further increases in concentration, to >4 mg/L residual chlorine, no significant difference was observed in the disinfectant resistance of the microbial community.

Effects of interspecific interactions on biofilm formation potential and chlorine resistance: Evaluation of dual-species biofilm observed in drinking water distribution systems

Interspecific interactions could influence microbial resistance to disinfectants in drinking water distribution systems (DWDS). Understanding the effects of interspecific interactions on biofilm formation potential and chlorine resistance could improve future disinfection strategies and prevent the spread of infectious water-borne diseases. The present study first investigated biofilm formation of 10 bacterial groups with two mixed bacterial strains isolated from a simulated DWDS over different incubation times (24 h, 48 h, and 72 h), and then evaluated chlorine resistance of the 72 h incubated biofilms under residual 0.3, 0.6, 1, 2, 4, and 10-mg/L chlorine concentrations. The Acidovorax + Acinetobacter, Sphingomonas + Bacillus, Sphingomonas + Acidovorax, and Acidovorax + Microbacterium groups exhibited high biofilm formation potential, which could be attributed to synergism interactions. The Acinetobacter + Microbacterium, Sphingomonas + Microbacterium, and Sphingomonas + Acinetobacter groups exhibited weak or moderate biofilm formation potential, which could be attributed to competition interactions. The Acidovorax + Bacillus, Bacillus + Acinetobacter, and Bacillus + Microbacterium groups exhibited moderate biofilm formation potential, which could be attributed to the neutralism interactions. The chlorine resistance of most of the dual-species biofilms associated with Sphingomonas sp. and Bacillus cereus was higher than those of the corresponding single-species biofilms. Considering the interspecific interactions between the Microbacterium laevaniforms and four other species were mainly either neutralism or competition interactions, the chlorine resistance of the group associated with M. laevaniforms could be inhibited to some extent. The specific structures (e.g., extracellular polymeric substances and spores) of dual-species biofilm had a greater influence on chlorine resistance than interspecific interactions. There were diverse interspecific interactions. Synergism could promote chlorine resistance in dual-species biofilm, neutralism had minimal effect on the chlorine resistance, while competition could reduce the chlorine resistance, and the reduction in chlorine resistance could be mitigated if there were specific morphological structures in the biofilm such as spores.

Biofilm formation potential and chlorine resistance of typical bacteria isolated from drinking water distribution systems.

Biofilms are the main carrier of microbial communities throughout drinking water distribution systems (DWDSs), and strongly affect the safety of drinking water. Understanding biofilm formation potential and chlorine resistance is necessary for exploring future disinfection strategies and preventing water-borne diseases. This study investigated biofilm formation of five bacterial strains isolated from a simulated DWDS at different incubation times (24 h, 48 h, and 72 h), then evaluated chlorine resistance of 72 h incubated biofilms under chlorine concentrations of 0.3, 0.6, 1, 2, 4, and 10 mg L−1. All five bacterial strains had biofilm formation potential when incubated for 72 h. The biofilm formation potential of Acinetobacter sp. was stronger than that of Bacillus cereus, Microbacterium sp. and Sphingomonas sp. were moderate, and that of Acidovorax sp. was weak. In contrast, the order of chlorine resistance was Bacillus sp. > Sphingomonas sp. > Microbacterium sp. > Acidovorax sp. > Acinetobacter sp. Thus, the chlorine resistance of a single-species biofilm has little relation with the biofilm formation potential. The biofilm biomass is not a major factor affecting chlorine resistance. Moreover, the chlorine resistance of a single-species biofilm is highly related to the physiological state of bacterial cells, such as their ability to form spores or secrete extracellular polymeric substances, which could reduce the sensitivity of the single-species biofilm to a disinfectant or otherwise protect the biofilm.

Effect of sequential UV/free chlorine disinfection on opportunistic pathogens and microbial community structure in simulated drinking water distribution systems

Drinking water distribution systems (DWDS) may be a “Trojan Horse” for some waterborne diseases caused by opportunistic pathogens (OPs). In this study, two simulated DWDS inoculated with groundwater were treated with chlorine (Cl2) and ultraviolet/chlorine (UV/Cl2) respectively to compare their effects on the OPs distributed in four different phases (bulk water, biofilms, corrosion products, and loose deposits) of DWDS. 16S rRNA genes sequencing and qPCR were used to profile microbial community and quantify target genes of OPs, respectively. Results showed that UV/Cl2 was more effective than single Cl2 to control the regrowth of OPs in the water with the same residual chlorine concentration. However, the OPs inhabiting the biofilms, corrosion products, and loose deposits seemed to be tolerant to UV/Cl2 and Cl2, demonstrating that OPs residing in these phases were resistant to the disinfection processes. Some significant microbial correlations between OPs and Acanthamoeba were found by Spearman correlative analysis (p < 0.05), demonstrating that the ecological interactions may exist in the DWDS. 16S rRNA genes sequencing of water samples revealed a significant different microbial community structure between UV/Cl2 and Cl2. This study may give some implications for controlling the OPs in the DWDS disinfected with UV/Cl2.

Selective antibiotic resistance genes in multiphase samples during biofilm growth in a simulated drinking water distribution system: Occurrence, correlation and low-pressure ultraviolet removal

The aim of this study was to gain comprehensive insights into the characteristics of antibiotic resistance genes (ARGs) in multiphase samples from drinking water distribution pipelines using a simulated biofilm reactor. During 120 d of continuous operation, common parameters and six ARGs (ermA, ermB, aphA2, ampC, sulII, and tetO) in samples of three phases (water, particle, and biofilm) from the reactor were investigated, which demonstrated secondary contamination by ARGs. Abundances of the six ARGs in the reactor effluent increased gradually, and in the 120 d effluent, the relative abundances of aphA2 and sulII were the highest, at 9.9 × 10−4 and 1.3 × 10−3, respectively, with a 1.5-fold and 2.8-fold increase, compared with those in the influent. The relative abundances of the six ARGs in the biofilm phase increased significantly (P < 0.05) at 120 d, which was caused by robust bacteria in biofilm that was newly exposed following the detachment of a large piece of aging biofilm. In the particle phase, four of the ARGs did not change significantly during the 120 d period. The six ARGs in the samples of three phases showed a negative correlation with residual chlorine in the pipe water, which demonstrated that low abundance of ARGs in the samples of three phases was related to the improvement of residual chlorine. The proportion of cultivable bacteria illustrated that the robust and active bacteria were negatively correlated with the six ARGs in the biofilm. Total organic carbon (TOC) in the pipeline showed a positive correlation with the proportion of cultivable bacteria in both the water and biofilm phases, which indicated that a TOC reduction in the pipeline contributed to low abundance of ARGs. With low-pressure ultraviolet (LP-UV) irradiation of 20 mJ/cm2, ARGs in the samples of three phases were efficiently controlled, which showed that LP-UV can be used for ARG removal in terminal water for supplemental bactericidal treatment of pipeline effluent.

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.

Sulfadiazine/ciprofloxacin promote opportunistic pathogens occurrence in bulk water of drinking water distribution systems.

Effects of sulfadiazine and ciprofloxacin on the occurrence of free-living and particle-associated opportunistic pathogens in bulk water of simulated drinking water distribution systems (DWDSs) were investigated. It was found that sulfadiazine and ciprofloxacin greatly promoted the occurrence of opportunistic pathogens including Pseudomonas aeruginosa, Legionella pneumophila, Mycobacterium avium and its broader genus Mycobacterium spp., as well as the amoebae Acanthamoeba spp. and Hartmanella vermiformis, in bulk water of DWDSs. Moreover, sulfadiazine and ciprofloxacin exhibited much stronger combined effects on the increase of these opportunistic pathogens. Based on the analysis of the antibiotic resistance genes (ARGs) and extracellular polymeric substances (EPS), it was verified that EPS production was increased by the antibiotic resistant bacteria arising from the effects of sulfadiazine/ciprofloxacin. The combined effects of sulfadiazine and ciprofloxacin induced the greatest increase of EPS production in DWDSs. Furthermore, the increased EPS with higher contents of proteins and secondary structure β-sheet led to greater bacterial aggregation and adsorption. Meanwhile, large numbers of suspended particles were formed, increasing the chlorine-resistance capability, which was responsible for the enhancement of the particle-associated opportunistic pathogens in bulk water of DWDSs with sulfadiazine/ciprofloxacin. Therefore, sulfadiazine and ciprofloxacin promoted the occurrence of particle-associated opportunistic pathogens in bulk water of DWDSs due to the role of EPS produced by the bacteria with ARGs.

Characterization of bacterial community and iron corrosion in drinking water distribution systems with O3-biological activated carbon treatment

Bacterial community structure and iron corrosion were investigated for simulated drinking water distribution systems (DWDSs) composed of annular reactors incorporating three different treatments: ozone, biologically activated carbon and chlorination (O3-BAC-Cl2); ozone and chlorination (O3-Cl2); or chlorination alone (Cl2). The lowest corrosion rate and iron release, along with more Fe3O4 formation, occurred in DWDSs with O3-BAC-Cl2 compared to those without a BAC filter. It was verified that O3-BAC influenced the bacterial community greatly to promote the relative advantage of nitrate-reducing bacteria (NRB) in DWDSs. Moreover, the advantaged NRB induced active Fe(III) reduction coupled to Fe(II) oxidation, enhancing Fe3O4 formation and inhibiting corrosion. In addition, O3-BAC pretreatment could reduce high-molecular-weight fractions of dissolved organic carbon effectively to promote iron particle aggregation and inhibit further iron release. Our findings indicated that the O3-BAC treatment, besides removing organic pollutants in water, was also a good approach for controlling cast iron corrosion and iron release in DWDSs.

Pyrosequencing analysis of source water switch and sulfate-induced bacterial community transformation in simulated drinking water distribution pipes.

Inter-basin water transfer and source water switching will be increasingly launched due to significant population increase and the shortage of the local water resources in cities around the world. Source water switch may cause physiochemical and microbiological de-stabilization of pipe material, biofilms, and loose deposits in drinking water distribution system (DWDS). Great sulfate alteration during source water switch had been deemed as the main cause of a red water case that occurred in a northern China city. To ascertain the relationship between water quality changing and bacterial communities of biofilms in DWDS and possible bacteria risk in a red water case, water quality changing experiments in simulated DWDSs were conducted for approximately 2years. Twenty-five corrosion scale samples and eight water samples collected from pipe harvest sites or during experimental periods were analyzed for their bacterial community composition by 454-pyrosequencing technology. Taxonomy results together with redundancy analysis (RDA) or canonical correspondence analysis (CCA) and hierarchical cluster analysis all indicated that bacterial community of samples with groundwater (GW) or surface water (SW) supply history and their variations under high sulfate water were rather different owing to different water source histories and the original pipe scale characteristics. Potential opportunistic pathogens: Burkholderia, Escherichia-Shigella, Mycobacterium, Serratia, Ralstonia, Novosphingobium, Flavobacterium, Sphingomonas, and Sphingopyxis were observed in scale or water samples.

Impacts of bacteria and corrosion on removal of natural organic matter and disinfection byproducts in different drinking water distribution systems

Impacts of bacteria and corrosion on removal of natural organic matter (NOM) and disinfection byproducts (DBPs) were studied in simulated drinking water distribution systems (DWDSs) using two annular reactors (ARs) with different disinfection. The results verified that the removal of NOM and DBPs was related very well to the corrosion and bacterial regrowth in both ARs. The initial stages (before 50 days) at which the rate of corrosion is higher in both ARs is considered as stage I. At stage I, UV controlled bacterial regrowth and more chlorine reacted with iron to promote corrosion rate, inducing the higher removal of NOM and DBPs in AR treated with UV/Cl2 than that treated with Cl2 alone. At stage II (from 50 days to 250 days), comparing with Cl2 disinfection alone, UV/Cl2 decreased the number of the total bacteria in effluents and the dehalogenation related bacteria in biofilms effectively. Moreover, UV/Cl2 induced the bacterial genus Dechloromonas dominant in the biofilms, resulting in the lower corrosion rate. The lower bacterial regrowth and corrosion rate caused the less removal of NOM and DBPs. These results are very helpful for the control of NOM and DBPs in DWDSs with different disinfection.

Resilience of microbial communities in a simulated drinking water distribution system subjected to disturbances: role of conditionally rare taxa and potential implications for antibiotic-resistant bacteria

CIRCOS plots representing the pan-genome and resistome of waterborne resistant bacteria.

Effects of microbial cycling of Fe(II)/Fe(III) and Fe/N on cast iron corrosion in simulated drinking water distribution systems

Effects of microbial cycling of Fe(II)/Fe(III) and Fe/N, as driven by the bacterium Dechloromonas hortensis, on iron corrosion were studied in simulated drinking water distribution systems (DWDSs) with different nitrate concentrations. Fe/N redox cycling accelerated Fe(II) oxidation and α-FeOOH formation in DWDSs with 22.0mg/L initial NO3−-N. The greatest amount of Fe3O4 was formed through Fe(II)/Fe(III) redox cycling in DWDSs with 2.2mg/L initial NO3−-N. This resulted in the lowest corrosion rate and iron release. Therefore, the redox cycling of Fe(II)/Fe(III) driven by D. hortensis was favourable to the formation of more stable corrosion scales in DWDSs.

Effects of Ni2+ on aluminum hydroxide scale formation and transformation on a simulated drinking water distribution system

Observations of aluminum containing sediments/scales formed within the distribution pipes have been reported for several decades. In this study, the effect of Ni2+ on the formation and transformation processes of aluminum hydroxide sediment in a simulated drinking water distribution system were investigated using X-ray diffraction spectrum (XRD), Fourier transform infrared spectrum (FT-IR), scanning electron microscope (SEM), and thermodynamic calculation methods. It was determined that the existence of Ni2+ had notable effects on the formation of bayerite. In the system without Ni2+ addition, there was no X-ray diffraction signal observed after 400d of aging. The presence of Ni2+, however, even when present in small amounts (Ni/Al=1:100) the formation of bayerite would occur in as little as 3d at pH 8.5. As the molar ratio of Ni/Al increase from 1:100 to 1:10, the amount of bayerite formed on the pipeline increased further; meanwhile, the specific area of the pipe scale decreased from 160 to 122m2g−1. In the system with Ni/Al molar ratio at 1:3, the diffraction spectrum strength of bayerite became weaker, and disappeared when Ni/Al molar ratios increased above 1:1. At these highs Ni/Al molar ratios, Ni5Al4O11⋅18H2O was determined to be the major component of the pipe scale. Further study indicated that the presence of Ni2+ promoted the formation of bayerite and Ni5Al4O11⋅18H2O under basic conditions. At lower pH (6.5) however, the existence of Ni2+ had little effect on the formation of bayerite and Ni5Al4O11⋅18H2O, rather the adsorption of amorphous Al(OH)3 for Ni2+ promoted the formation of crystal Ni(OH)2.