Secondary Water Supply System Research Articles

Achieving zero fouling in the ultrafiltration for secondary water supply systems in the absence of residual chlorine

Ultrafiltration (UF) technology is widely used in secondary water supply systems (SWSS) to provide high-quality drinking water. However, the challenge of severe membrane fouling, which leads to frequent cleaning requirements, makes UF maintenance intensive. In this study, we tried to validate the feasibility of achieving zero fouling without the need for cleaning in the UF for SWSS, i.e., the fouling resistance can be maintained for a very long time without any increase. We operated dead-end UF systems at different fluxes, both with and without residual chlorine, and monitored the formation of fouling layers during filtration. The results demonstrated the successful achievement of zero fouling under a flux of 10 L/(m2 h) in the absence of chlorine, evidenced by no increase in transmembrane pressure for three months. This zero-fouling phenomenon was attributed to the formation of a self-regulating biofouling layer. This biofouling layer could degrade the deposited foulants and featured a loose morphology, facilitated by microbial activities in the cake layer. Although residual chlorine reduced the fouling rate by half at a flux of 30 L/(m2 h), it hindered the achievement of zero fouling at the lower flux of 10 L/(m2 h), due to its inhibitory effect on microbial activity. Intermittent operation of UF was effective in achieving zero fouling at higher fluxes (e.g., 30 L/(m2 h)). This benefit was primarily ascribed to the biodegradation of accumulated foulants and the expansion of biofouling layer during the pause of the intermittent filtration, which prompted the formation of biofouling layers with loose structure and balanced composition. To the best of our knowledge, this study is the first attempt to achieve zero fouling in UF for SWSS, and the findings may offer valuable insights for the development of cleaning-free and low-maintenance membrane processes.

Drinking water quality & health risk assessment of secondary water supply systems in residential neighborhoods

Drinking water quality & health risk assessment of secondary water supply systems in residential neighborhoods

Unraveling bacterial and eukaryotic communities in secondary water supply systems: Dynamics, assembly, and health implications

Secondary water supply systems (SWSSs) are crucial water supply infrastructures for high-rise buildings in metropolitan cities. In recent years, they have garnered public attention due to increased microbial risks. However, our understanding of SWSS microbial ecology, particularly concerning the composition of eukaryotes and the underlying mechanisms driving microbial dynamics and assembly in SWSSs, remains elusive. Herein, we conducted a comprehensive investigation on both eukaryotes and bacteria along the water transportation pathway and across various microbial habitats (water, biofilm, and sediment) in SWSSs. Sequencing results revealed that eukaryotes within SWSSs predominantly consist of protists (average abundance: 31.23%) and metazoans (20.91%), while amoebae accounted for 4.71% of the total. During water transportation from the distribution mains to taps, both bacterial and eukaryotic communities exhibited significant community shifts, and higher degrees of variation were observed for eukaryotic community among different locations within SWSSs. The normalized stochasticity ratio (NST) analysis demonstrated that bacterial community assembly was governed by stochastic processes, while eukaryotic community assembly was primarily shaped by deterministic processes. Within SWSS tanks, bacterial communities significantly varied across water, biofilm, and sediment, whereas eukaryotic communities showed minor differences among these habitats. The co-occurrence networks analysis revealed that tank biofilm and sediment harbored more eukaryote-bacterium linkages than water, suggesting biofilm and sediment might be hotspots for inter-kingdom interactions. We also applied FEAST analysis to track the source of tap water microbiota, results of which showed that household-tap bacteria mainly originated from tank water. In contrast, tank biofilm was identified as the primary microbial source to eukaryotes in household tap water. Additionally, engineering factors such as tank materials significantly affected amoeba community, and the SWSS configuration was found to influence Legionella and Mycobacterium abundances in SWSSs. Overall, results of our study shed light on the microbial ecology in SWSS and provide insights into SWSS management and health risk control.

Hidden risks: Simulated leakage of domestic sewage to secondary water supply systems poses serious microbiological risks

There are continuous reports about the pollution of the secondary water supply systems (SWSSs), among which domestic sewage leakage is the most serious. In this study, a pilot experiment lasting 70 days was conducted to explore the changes in physicochemical water quality and the microbial profiles in SWSSs polluted by different doses of domestic sewage through qPCR and high-throughput sequencing methods. The results showed that when domestic sewage entered the simulated water storage tank, a large amount of organic matter brought by domestic sewage quickly consumed chlorine disinfectants. High pollution levels (pollution index ≥ 1/1000) were accompanied by significant increases in turbidity and ammonia nitrogen concentration (p < 0.05) and by abnormal changes in sensory properties. Although different microbial community structures were found only at high pollution levels, qPCR results showed that the abundance of the bacterial 16S rRNA gene and some pathogenic gene markers in the polluted tank increased with the pollution level, and the specific gene marker of pathogens could be detected even at imperceptible pollution levels. In particular, the high detection frequency and abundance of Escherichia coli and Enterococcus faecails in polluted tank water samples demonstrated that they can be used for early warning. Moreover, it seems that the microorganisms that came with the domestic sewage lost their cultivability soon after entering SWSSs but could recover their activities during stagnation. In addition, the biofilm biomass in the polluted tank with high pollution levels increased faster at the initial stage, while after a longer contact time, it tended to remain at the same level as the control tank. This study emphasized the high microbial risk introduced by sewage water leakage even at imperceptible levels and could provide scientific suggestions for early warning and prevention of pollution to SWSSs.

Study on Gamma selection in the optimal operation of secondary water supply system based on deep Q-learning network

Abstract When the requirements for water pressure and quantity of drinking water for residents and industrial buildings exceed the capacity of the urban water distribution system, a secondary water supply system (SWSS) is supplied to users by pipelines through storage, pressurization, and other facilities. In China, SWSS has been installed in 97% of residential buildings and the operation of SWSS is directly related to the water pressure and water quality of the users' tap water. In this paper, the operation optimization objectives for the SWSS with storage facilities were proposed, and deep Q-learning network (DQN) was applied to optimize the control of SWSS. In this study, the pressure, the water age in the roof water tank, and the power consumption of the pumps were selected as the optimization objectives. The equation for the qualitative selection of the key hyperparameter (Gamma) was proposed and verified by the experiments in a community of City S in East China. The results indicated that with the decrease in the volume of the water tank, the larger Gamma value was recommended, and the more future conditions were considered. It is hoped that the result can be used as a reference in SWSS operation optimization.

To close or open the tank input water valve: Secondary water-supply systems with double tanks will induce a higher microbial risk

Secondary water supply systems (SWSSs) are widely used to supply water to high-rise households in urban residential buildings. A special mode of double tanks with one used while another was spared was noted in SWSSs, which would facilitate microbial growth due to longer water stagnation in the spare tank. There are limited studies on the microbial risk of water samples in such SWSSs. In this study, the input water valves of the operational SWSSs consisting of double tanks were artificially closed and opened on time. Propidium monoazide-qPCR and high-throughput sequencing were performed to systematically investigate the microbial risks in water samples. After closing the tank input water valve, it may take several weeks to replace the bulk water in the spare tank. The residual chlorine concentration in the spare tank decreased by up to 85 % within 2–3 days compared with that in the input water. The microbial communities in the spare and used tank water samples clustered separately. High bacterial 16S rRNA gene abundance and pathogens-like sequences were detected in the spare tanks. Most antibiotic-resistant genes (11/15) in the spare tanks showed an increase in their relative abundance. Moreover, when both tanks within one SWSS were in use, the water quality of the used tank water samples deteriorated to varying degrees. Overall, running SWSSs with double tanks will reduce the replacement rate of water in one storage tank, and consumers who use taps served by the presented SWSSs may have a higher microbial risk.

Nitrifier Assemblages and Dynamics in Secondary Water Supply Systems: Predominance of Comammox Nitrospira in Tank Biofilm and Sediment

Nitrifiers in chloraminated drinking water distribution systems can trigger severe nitrification, resulting in subsequent water quality deteriorations. However, the occurrence and dynamics of nitrifiers in secondary water supply systems (SWSSs), an important water supply component beyond the distribution mains, remain largely unexplored. This study investigated the density, distribution, and diversities of nitrifiers in different microbial habitats (water, biofilm, and sediment) in SWSSs with different characteristics. Quantitative polymerase chain reaction analysis indicated higher gene copy numbers of ammonia-oxidizing archaea, ammonia-oxidizing bacteria, comammox clade A, and strict nitrite-oxidizing bacteria in SWSSs relative to the distribution mains (1.3–2.1 log10GC/mL increase, P < 0.05). Comammox were preferentially enriched in biofilm and sediments of SWSS tanks, highlighting biofilm and sediment as important ecological niches for comammox. Illumina sequencing of 16S rRNA genes revealed the presence of diverse nitrifiers (e.g., Nitrospira, Nitrosomonas, Nitrotoga, and Nitrobacter) in SWSSs, with distinct bacterial nitrifier communities noted in SWSS water compared to the distribution main water (analysis of similarity (ANOSIM), P < 0.05). Different bacterial nitrifier communities were noted between water and sediments in SWSS tanks, indicating niche segregations of nitrifiers (ANOSIM, P < 0.05). The results highlight SWSS as an important reservoir of nitrifiers and provide insights into nitrification monitoring and control strategies in SWSSs.

A Large Acute Gastroenteritis Outbreak Associated with Both Campylobacter coli and Human Sapovirus - Beijing Municipality, China, 2021.

Campylobacter is a significant foodborne pathogen that leads to global outbreaks of acute gastroenteritis (AGE) usually affecting less than 30 individuals. Human sapovirus (HuSaV) is an enteric virus responsible for sporadic cases and outbreaks of AGE worldwide. In a study conducted in Beijing, HuSaV detection ranked second after norovirus. We present a discussion of the first large-scale outbreak of AGE caused by both Campylobacter coli (C. coli) and HuSaV. The outbreak involved a total of 996 patients and exhibited two distinct peaks over a period of 17 days. Through case-control studies, we identified exposure to raw water from a secondary water supply system as a significant risk factor. Among 83 patients, 49 samples tested positive for C. coli, 39 samples tested positive for HuSaV, and 27 samples tested positive for both pathogens using real-time polymerase chain reaction detection. Furthermore, whole-genome sequencing of 17 C. coli isolates obtained from 17 patients revealed that all isolates belonged to a highly clonal strain of C. coli. Outbreaks of AGE resulting from multiple pathogen infections warrant increased attention. This report emphasizes the significance of ensuring the safety of drinking water, particularly in secondary supply systems.

CFD simulation of spatiotemporal distribution of residual chlorine in secondary water supply tanks

The mixing, transportation, and reaction of chlorine in a secondary water supply system were investigated by a computation fluid dynamics model coupling with reaction kinetics.

UV pretreatment reduced biofouling of ultrafiltration and controlled opportunistic pathogens in secondary water supply systems

Secondary water supply systems (SWSSs) with low residual chlorine may pose a risk to consumers due to the regrowth of some pathogenic microbes. This study investigated how UV pretreatment would affect biofouling of UF membrane when it was used to remove opportunistic pathogens (OPs) in SWSSs. Our results indicated that UV pretreatment inactivated OPs effectively through OH. and O2.−, when residual chlorine was present in water from SWSSs. After UV-UF treatment, Legionella spp., Legionella pneumophila, Mycobacterium spp., and Acanthameoba spp. were undetected in the water. After UV pretreatment, the extracellular polymeric substances (EPS) production and associated β-sheet in the protein of EPS were reduced due to the inhibition of related functional genes. Foulant analysis indicated that biofilms formed on the UF membrane after UV pretreatment had visible crevices and small pores, a lower bacterial volumetric fraction and lower roughness than the untreated UF membrane. This structure improved the UF flux and reduced the biofouling. The transmembrane pressure (TMP) of UF membrane gradually decreased from 74 KPa to 60 KPa after 70 days running. Therefore, UV pretreatment could be used to reduce the potential microbial risk of water from SWSSs, and mitigate biofouling of UF membranes.

Pitting corrosion of 304 stainless steel in secondary water supply system

The effect of NaClO solution on pitting behavior of 304 stainless steel was investigated by electrochemical tests and composition analysis. The results show that the open circuit potential (OCP) shifts to the positive direction with the increase of [ClO−], indicating that the passivation film formed on the sample surface is faster, but, has no effect on pitting potential (Eb) in the first stage (< 24 h). After 24 h immersion, the pitting occurs because Cl− is detected in the wake of the decomposition of ClO− for its thermodynamics instability. In the later stage, the [Cl−] increases with the increase of [ClO−], correspondingly resulting in lower pitting resistance. X-ray photoelectron spectroscopy (XPS) reveals that the Cr/Fe atomic ratio in the passivation film decreases with the increase of [ClO−], because Cr(III) is oxidized to Cr(VI) and dissolves in the solution, which further weakens the pitting resistance of 304 stainless steel. The electrochemical impedance spectroscopy (EIS), combined with the Mott-Schottky analysis (MSA) was employed to verify the properties of the passivation film related to pitting.

Modeling of Water Distribution System Based on Ten-Minute Accuracy Remote Smart Demand Meters

In the process of water distribution system modeling, nodal demands have been deemed to be the most significant input parameters causing uncertainties. Conventionally, users of the same type are assigned the same demand pattern, which does not reflect the variability in demand patterns. In Hefei City, a lot of remote smart demand meters were installed in recent years, providing real-time water consumption data for different water user sites. In this study, the water consumption data based on 10 min accuracy were collected and used in the established EPANET model of the study area. A 48 h simulation, including workdays and weekends, was carried out, and each node’s base demand and demand pattern were accurately calculated according to the remote data. The results show that the accuracy of the model has met the modeling criteria (less than 2 m for all pressure monitoring points), and there is no need to calibrate the nodal demand. The established offline hydraulic model can better reflect the water consumption characteristics of each type of user and has revealed the significant influence of secondary water supply systems.

A survey on heavy metal concentrations in residential neighborhoods: The influence of secondary water supply systems

Water quality deterioration often occurs in secondary water supply systems (SWSSs), and increased heavy metal concentrations can be a serious problem. In this survey, twelve residential neighborhoods were selected to investigate the influence of SWSSs on the seasonal changes in heavy metal concentrations from input water to tank and tap water. The concentrations of nine evaluated heavy metals in all groups of water samples were found to be far below the specified standard levels in China. The concentrations of Fe, Mn, and Zn increased significantly from the input water samples to the tank and tap water samples in spring and summer (p < 0.05), especially for the water samples that had been stagnant for a long time. Negative correlations were found between most of the heavy metals and residual chlorine (Fe, Cu, Zn, and As, r = -0.186 to -0.519, p < 0.05). In particular, a high negative correlation was observed between Fe and residual chlorine (r = -0.489 to -0.519, p < 0.01) in spring and summer. Fe and Mn displayed positive correlations with turbidity (r = 0.672 and 0.328, respectively; p < 0.05). In addition, Cr and As were found to be positively associated with some nutrients (NO3−, TN, and SO42−; r = 0.420-0.786, p < 0.01). The material of the storage tanks had little influence on the difference in heavy metal concentrations. Overall, this survey illustrated that SWSSs may pose a chronic threat to water quality and could provide useful information for practitioners.

UV Reduced Membrane Biofouling of Ultrafiltration for Controlling Opportunistic Pathogens in Secondary Water Supply Systems

UV Reduced Membrane Biofouling of Ultrafiltration for Controlling Opportunistic Pathogens in Secondary Water Supply Systems

A comprehensive investigation of the microbial risk of secondary water supply systems in residential neighborhoods in a large city

Secondary water supply systems (SWSSs) are characterized by long water stagnation and low levels of chlorine residuals, which may pose a high microbial risk to terminal users. In this study, the SWSSs of 12 residential neighborhoods in a metropolitan area of 5 million people in southeastern China were seasonally investigated to assess their microbial risks by determining more than 30 physicochemical and biological parameters. Although the microbiological quality of SWSS water met the requirements of the standards for drinking water quality of China, it did deteriorate in various aspects. The heterotrophic plate counts with R2A media were high (> 100 CFU/mL) in some SWSS tank and tap water samples. Propidium monoazide (PMA)-qPCR revealed a one magnitude higher abundance of viable bacteria in the tank and tap water samples (average 103.63±1.10 and 103.65±1.25 gene copies/mL, respectively) compared with the input water samples, and Enterococcus, Acanthamoeba, and Hartmannella vermiformis were only detected in the tanks. In particular, the high detection frequency of Legionella in 35% tank and 21% tap water samples suggested it is a supplementary microbial safety indicator in SWSSs. The microbial regrowth potential was more obvious in summer, and Illumina sequencing also demonstrated distinct seasonal changes in the relative abundance of bacterial gene sequences at the genus level. Turbidity and residual chlorine were closely connected with total bacterial biomass, and the latter seemed responsible for microbial community structure alteration. The extremely low chlorine residuals associated with a high abundance of total bacteria (as high as 106.48 gene copies/mL) and Legionella (as high as 106.71 gene copies/100 mL) in the closed valve tanks highlighted the high microbial risk increased by mishandling the operation of SWSSs. This study found that SWSSs possessed a higher microbial risk than the drinking water network, which suggested that the frequency and scope of monitoring the microbial risk of SWSSs in megacities should be strengthened for the purpose of waterborne epidemic disease prevention and control.

Control of maximum water age based on total chlorine decay in secondary water supply system

97% of residential buildings are installed with secondary water supply system (SWSS) in China. In order to meet the water pressure demand, the SWSS has become a key solution to store and transport drinking water. The water age of the SWSS directly determines the quality of tap water, while total chlorine is a key indicator to evaluate the quality and safety of the water supply network. This study revealed the relationship between total chlorine and water age controlled by adjusting the liquid level of the secondary water supply tank. Models governing water age and the total chlorine concentration were developed based on the variation of the liquid level and the attenuation rate of the total chlorine in the SWSS. Furthermore, the validation was performed through case studies. The developed models can gain effective insights for determining the longest water age while guaranteeing the concentration of total chlorine meets the demand of the lower standard in SWSS. The secondary chlorine dosage would be quantified and added to the pipe network. The integration of the SWSS would be guided by water age in some old communities. The taste of tap water for direct drinking water could be improved by adjusting of water age using this model. The optimization method is easy to use for identifying efficient solutions for SWSS operation.

Characterization of the synergistic relationships between nitrification and microbial regrowth in the chloraminated drinking water supply system

Deterioration of water quality is commonly found in secondary water supply systems (SWSSs), especially the growth of microbes. To explore the metabolic mechanism for rapid microbial regrowth in SWSSs, a regrowth potential assessment, flow cytometry, and quantitative PCR were conducted. Metagenomic and 16S rRNA gene sequencing were performed to better understand the microbial communities and metabolism. It was found that the increased biomass in the SWSS was significantly higher than that in the drinking water distribution system (DWDS). Statistical analysis revealed that ammonia oxidation was the dominant driver of increased biomass in the SWSS. The abundances of ammonia oxidation bacteria, concentration of nitrogen species, and related enzymes demonstrated that ammonia oxidation in the SWSS was more vigorous than that in the DWDS. In the SWSS, the metabolism of the ammonia oxidation cluster was more vigorous, and ammonia-oxidizing bacteria (AOB) were the dominant nitrifying bacteria. Incomplete nitrification products were involved in the metabolism of heterotrophic bacteria and promoted the growth of heterotrophic bacteria in the SWSS. More attention should be given to controlling incomplete nitrification to improve tap water quality.

Formation of disinfection by-products in a UV-activated mixed chlorine/chloramine system

In recent years, ultraviolet (UV) irradiation coupled with chlor(am)ination process is ubiquitous in secondary water supply systems in many cities of China. However, the disinfection by-products (DBPs) formation in a UV-activated mixed chlorine/chloramine system (MCCS) still remains unclear. In this study, the DBPs formation in a UV-activated MCCS was systematically investigated, considering influencing factors including the mass ratios of free chlorine to NH2Cl, UV irradiation, pH values, NOM types, Br− concentration and toxicity of the DBPs. Results indicated that DBPs formation decreased remarkably as mass ratio of free chlorine to NH2Cl changed from 5:0 to 0:5. The DBPs formation in humic acid (HA)-containing water was the highest, followed by those in fulvic acid (FA) and algal organic matter (AOM). Besides, better control of the DBP-related calculated toxicity can be achieved in acidic conditions regardless of the UV irradiation. Furthermore, in the presence of Br−, a significant reduction of DBPs formation could be achieved in a UV-activated MCCS. The findings also demonstrated that DBPs formation in real water can be effectively reduced at high UV fluence in a MCCS.

Response of bacterial regrowth, abundant and rare bacteria and potential pathogens to secondary chlorination in secondary water supply system

This study investigated the effects of secondary chlorination on bacterial regrowth, microbial communities (abundant and rare taxa) and bacterial functions of pipe wall biofilm and bulk water in simulated secondary water supply system (SWSS). Continuous secondary chlorination was more effective than short-term secondary chlorination to control the bacterial regrowth in both biofilm and water samples. Bacterial diversity slightly reduced after continuous secondary chlorination, and 19.27% of the total operational taxonomic units (OTUs) were shared by biofilm and water samples, with Bacillus as the dominant genus. Abundant and rare taxa exhibited different community structures. Proteobacteria and candidate division WPS-1 predominated in abundant and rare phyla were sensitive to chlorine, while Firmicutes, Acidobacteria and Bacteroidetes, exhibited relative strong chlorine resistance. The abundant genera in control sample (e.g., Bosea, Sphingobium and Gemmata) exhibited poor tolerance to chlorine, while Bacillus in biofilm and Defluviimonas in water were the main chlorine-resistant genera. Moreover, the composition of rare genera in each sample was obviously different. Furthermore, a total of 18 potential pathogens were detected with Pseudomonas as the dominant genus, most of which were significantly reduced after disinfection. There were mainly positive interactions among potential pathogenic bacteria, with Enterococcus, Legionella and Vibrio as the hub genera as revealed by network analysis. Similar bacterial functions in both biofilm and water were observed with metabolism as the predominant bacterial function, while, human disease function only accounted for 1.07% of bacterial functions. These results highlighted the importance of continuous secondary chlorination for controlling biosafety of SWSS and identified the dissimilar responses of abundant and rare bacteria to the disinfection, as well as the co-occurrence patterns among potential pathogens, improving our understanding of bacterial communities in SWSS.

Effects of urban forms on energy consumption of water supply in China

More than half of the energy consumed for urban water utilities is for the water supply system (including water distribution and secondary water supply systems). Urban form (defined as city shapes and population distribution) is found to have significant impacts on energy used by the water supply system. This study aimed to explore the integrated impacts of urban forms on energy consumption for water supply by employing empirical analysis methods and virtual city modeling. In total, three types of city shapes (i.e. radial, grid and satellite) and three types of population distributions (i.e. uniform, monocentric and polycentric) were identified. The results reveal that population distribution has more significant impacts on the energy for water distribution and secondary water supply than city shapes. Energy consumed by secondary water supply accounts for more than 80% of total energy consumption. It is found that radial-uniform cities consume the least energy for water supply (up to 89% lower). The findings of this study shed light on enhancing overall sustainability in the urban water supply system. For future urban planning and governance, radial/monocentric urban form is encouraged when secondary water supply is not widely applied or needed, radial/uniform urban form is recommended when secondary water supply is required, and the entirely pressurized pumping system is suggested for secondary water supply in order to reduce energy use.