Dual-source drinking water distribution systems (DWDS) over single-source water supply systems are becoming more practical in providing water for megacities. However, the more complex water supply problems are also generated, especially at the hydraulic junction. Herein, we have sampled for a one-year and analyzed the water quality at the hydraulic junction of a dual-source DWDS. The results show that visible changes in drinking water quality, including turbidity, pH, UV254, DOC, residual chlorine, and trihalomethanes (TMHs), are observed at the sample point between 10 and 12 km to one drinking water plant. The average concentration of residual chlorine decreases from 0.74 ± 0.05 mg/L to 0.31 ± 0.11 mg/L during the water supplied from 0 to 10 km and then increases to 0.75 ± 0.05 mg/L at the end of 22 km. Whereas the THMs shows an opposite trend, the concentration reaches to a peak level at hydraulic junction area (10-12 km). According to parallel factor (PARAFAC) and high-performance size-exclusion chromatography (HPSEC) analysis, organic matters vary significantly during water distribution, and tryptophan-like substances and amino acids are closely related to the level of THMs. The hydraulic junction area is confirmed to be located at 10-12 km based on the water quality variation. Furthermore, data-driven models are established by machine learning (ML) with test R2 higher than 0.8 for THMs prediction. And the SHAP analysis explains the model results and identifies the positive (water temperature and water supply distance) and negative (residual chlorine and pH) key factors influencing the THMs formation. This study conducts a deep understanding of water quality at the hydraulic junction areas and establishes predictive models for THMs formation in dual-sources DWDS.

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.

Microorganisms and sediments in reclaimed water adhere to the inner walls of pipes or water tanks, forming biofilms that support the continuous growth of microorganisms. These biofilms provide a protective barrier, shielding bacteria from disinfectants. This study investigated the impact of biofilms on bacterial growth and reproduction in reclaimed water and the factors limiting bacterial growth in reclaimed graywater (GMR) and reclaimed mixed wastewater (MWR). The results revealed that biofilm biomass gradually increased and reached a maximum value on Days 20-25, and the biomass of organisms continued to decrease after 40 days. Biofilms serve as a source of bacteria, continuously releasing them into reclaimed water systems. The presence of biofilms reduced the biological stability of the reclaimed water, leading to water quality deterioration. The concentration of assimilable organic carbon in the reclaimed water showed a positive correlation with the heterotrophic bacterial count and Escherichia coli levels in both the reclaimed water and biofilms. The threshold value of chlorine for inhibiting biofilms in reclaimed water was no <2 mg/L. High concentrations of free chlorine delayed the growth of biofilms but did not reduce the final biomass generated by the biofilms. Carbon was the limiting factor for the biological stability of reclaimed water, while nitrogen, phosphorus, and inorganic salts were not limiting factors. Thus, minimizing the concentration of organic matter in reclaimed water can reduce the nutrient sources available for biofilm formation. This study provides support for advancements in the wastewater reuse industry.

The study identifies potential carcinogenic health risk-zone of Chattogram city for the occurrence of trihalomethanes (THMs) at its water distribution network. The EPANET-THMs simulation model along with an empirical model have been adopted in the study to predict THMs content of supply water of the distribution network of the city\'s Karnaphuli service area. The empirical model has estimated THMs level of supply water based on influential water quality parameters, and few of these have been used as pre-set values for subsequent EPANET simulation. The simulation (R2= 0.7) shows that THMs\' concentrations throughout the network vary from 33 to 486 μg/L. Around 60% of total junctions showed THMs concentrations above 150 μg/L, while that is above 50 μg/L for most (99%) of the junctions. Residual Free chlorine, one of the precursors for the THMs formation in distribution line, has also been simulated by EPANET considering varying applied chlorine dose at the water purification unit and wall (Kw) and bulk (Kb) decay constants. The simulated free residual chlorine peaks are found to be closer to the actual values with chlorine dose of 2 mg/L, and decay constants, Kw = 1 d-1 and Kb = 1 d-1. A mean lifetime total risk of cancer due to the presence of THMs has been found to be very high. Spatial distribution of carcinogenic risk shows that the central zone of the service area is the most vulnerable zone, followed by the western and northern zone. The first ever zone wise risk identification could be used as baseline data for operational and regulatory purposes and may raise awareness among the city\'s inhabitants. Furthermore, the application of EPANET in combination with an empirical model could be an effective tool for predicting THMs\' concentration in water distribution networks in developing countries like Bangladesh to minimize the expenses of measuring THMs.

The occurrence of viable but non-culturable (VBNC) bacteria in drinking water may result in significant underestimation of viable cell counts detected by culture-based method, thus raising microbiological safety concern. Chlorine disinfection has been widely used in drinking water treatment to ensure microbiological safety. However, the effect of residual chlorine on inducing bacteria in biofilms into a VBNC state remains unclear. We determined cell numbers of Pseudomonas fluorescence in different physiological states (culturable, viable, dead) by heterotrophic plate count method and flow cytometer in a flow cell system under 0, 0.1, 0.5, 1.0 mg/L chlorine treatment. Numbers of culturable cells were 4.66 ± 0.47 Log10, 2.82 ± 0.76 Log10, 2.30 ± 1.23 Log10 (CFU/112.5 mm3) in each chlorine treatment group. However, viable cell numbers remained at 6.32 ± 0.05 Log10, 6.11 ± 0.24 Log10, 5.08 ± 0.81 Log10 (cells/112.5 mm3). Significant difference between numbers of viable and culturable cells demonstrated chlorine could induce bacteria in biofilms into a VBNC state. In this study, flow cells combination with Optical Coherence Tomography (OCT) were applied to construct an Automated experimental Platform for replicate Biofilm cultivation and structural Monitoring (APBM) system. The OCT imaging results demonstrated that changes of biofilm structure under chlorine treatment were closely related to their inherent characteristics. Biofilms with low thickness and high roughness coefficient or porosity were easier to be removed from the substratum. Biofilm with high rigid properties were more resistant to chlorine treatment. Even though >95 % bacteria in biofilms entered a VBNC state, the biofilm physical structure was still remained. This study revealed the possibility of bacteria to enter a VBNC state in drinking water biofilms and changes of biofilm structure with different characteristics under chlorine treatment, which provide reference for biofilms control in drinking water distribution systems.

Disinfection by-products (DBPs) are the most common organic contaminants in tap water and are of wide concern because of their highly developmental toxic, cytotoxic, and carcinogenic properties. Typically, to control the proliferation of pathogenic microorganisms, a certain concentration of residual chlorine is retained in the factory water, which reacts with the natural organic matter and the disinfection by-products that have been formed, thus affecting the determination of DBPs. Therefore, to obtain an accurate concentration, residual chlorine in tap water needs to be quenched prior to treatment. Currently, the most commonly used quenching agents are ascorbic acid, sodium thiosulfate, ammonium chloride, sodium sulfite, and sodium arsenite, but these quenching agents can cause varying degrees of DBPs degradation. Therefore, in recent years, researchers have attempted to find emerging chlorine quenchers. However, no studies have been conducted to systematically review the effects of traditional quenchers and new ones on DBPs, as well as their advantages, disadvantages, and scope of application. For inorganic DBPs (bromate, chlorate, and chlorite), sodium sulfite has been proven to be the ideal chlorine quencher. For organic DBPs, although ascorbic acid caused the degradation of some DBPs, it remains the ideal quenching agent for most known DBPs. Among the studied emerging chlorine quenchers, n-acetylcysteine (NAC), glutathione (GSH), and 1,3,5-trimethoxybenzene are promising for their application as the ideal chlorine quencher of organic DBPs. The dehalogenation of trichloronitromethane, trichloroacetonitrile, trichloroacetamide, and bromochlorophenol by sodium sulfite is caused by nucleophilic substitution reaction. This paper takes the understanding of DBPs and traditional and emerging chlorine quenchers as a starting point to comprehensively summarize their effects on different types of DBPs, and to provide assistance in understanding and selecting the most suitable residual chlorine quenchers during DBPs research.

Hydrodynamic cavitation (HC) was a kind of advanced oxidation mode. There were defects in the common HC devices, such as high energy consumption, low efficiency, and easy plugging. In order to effectively utilize HC, it was urgent to research new HC devices and used them together with other traditional water treatment methods. Ozone was widely used as a water treatment agent that does not produce harmful by-products. Sodium hypochlorite (NaClO) was efficient and cheap, but too much chlorine will be harmful to water. The combination of ozone and NaClO with the HC device of propeller orifice plate can improve the dissolution and utilization rate of ozone in wastewater, reduce the use of NaClO, and avoid the generation of residual chlorine. The degradation rate reached 99.9% when the mole ratio γ of NaClO to ammonia nitrogen (NH3-N) was 1.5 and the residual chlorine was near zero. As for the degradation rate of NH3-N or COD of actual river water and real wastewater after biological treatment, the ideal mole ratio γ was also 1.5 and the ideal O3 flow rates were 1.0 L/min. The combined method has been preliminarily applied to actual water treatment and was expected to be used in more and more scenarios.

Legionella is a pathogen that colonizes soils, freshwater, and building water systems. People who are most affected are those with immunodeficiencies, so it is necessary to monitor its presence in hospitals. The purpose of this study was to evaluate the presence of Legionella in water samples collected from hospitals in the Campania region, Southern Italy. A total of 3365 water samples were collected from January 2018 to December 2022 twice a year in hospital wards from taps and showers, tank bottoms, and air-treatment units. Microbiological analysis was conducted in accordance with the UNI EN ISO 11731:2017, and the correlations between the presence of Legionella and water temperature and residual chlorine were investigated. In total, 708 samples (21.0%) tested positive. The most represented species was L. pneumophila 2-14 (70.9%). The serogroups isolated were 1 (27.7%), 6 (24.5%), 8 (23.3%), 3 (18.9%), 5 (3.1%), and 10 (1.1%). Non-pneumophila Legionella spp. represented 1.4% of the total. Regarding temperature, the majority of Legionella positive samples were found in the temperature range of 26.0-40.9 °C. An influence of residual chlorine on the presence of the bacterium was observed, confirming that chlorine disinfection is effective for controlling contamination. The positivity for serogroups other than serogroup 1 suggested the need to continue environmental monitoring of Legionella and to focus on the clinical diagnosis of other serogroups.

This study applied a method for estimating chlorine decay constant (k) in urban water distribution systems using fluorescence excitation-emission matrix spectroscopy-parallel factor analysis (FEEM-PARAFAC), considering that it accounts for the influence of organic matter in the target area. The simultaneous impacts of seasonal variations on chlorine consumption and dissolved organic matter (DOM) composition were investigated for a year in three full-scale water distribution systems in I city (areas S, K, and G). Bulk decay constants (kb) were obtained through bottle tests, and the kb value was observed to differ by season and significantly affected by temperature. It exhibited its highest value, 0.794 d-1, in summer at area G. As a result of analyses through F-EEM-PARAFAC, it was determined that the components of the target raw water were humic-like and tryptophan-like. The quantitative analysis of organic substances through PARAFAC revealed that area G had the highest score (C1+C2) than other areas. 11.568, 10.578, and 11.771 in summer at areas S, K, and G, respectively. The model equations were derived such that the significant (R2 = 0.85-0.95) correlation between the C1 and C2 model scores and total chlorine decay constants (kt) verified via PARAFAC analysis of the target raw water was considered. Furthermore, a method for obtaining the wall decay constants at a target point based on the correlation equation was investigated.

The demand for multi-point water quality monitoring is increasing to solve the global problem of safe drinking water supply and environmental water contamination by industries. Therefore, compact devices are needed for on-site water quality analysis. On-site devices require low cost and high durability because they are placed outdoors, exposing them to strong ultraviolet rays and a wide range of temperatures. Our previous study reported on a compact and low-cost water quality meter that uses microfluidic devices with resin to monitor chemicals. In this study, we extended the fabrication range of the glass molding method to fabricate a glass microfluidic device with a 300 µm deep channel on a 50 mm in diameter substrate for constructing a low-cost and high-durability device. Finally, we developed a low-cost, highly robust glass device with a diamond-like carbon-coated channel surface to measure residual chlorine. The experimental results indicated that this device can endure outdoor conditions and be attached to small internet of things devices for analyzing chemical substances, such as residual chlorine.