Residual Chlorine Concentration Research Articles

Removal of Residual Chlorine from Stormwater Using Low-Cost Adsorbents and Phytoremediation

In recent decades, the pollution of water with micropollutants has become an increasing environmental concern. Since 2019, increased stormwater pollution from chlorine-based disinfectants has been recorded due to the COVID-19 pandemic. Runoff from disinfected areas and the residual chlorine present in stormwater are transported to surface water bodies, posing a risk to aquatic flora and fauna. The objectives of this study were (1) to evaluate the efficiency of different low-cost and recyclable filter materials in removing residual chlorine, and (2) to test plants’ ability to reduce residual chlorine concentrations through phytoremediation. Experiments were conducted in the laboratory (column and batch) and in the field (raised garden bed) to assess the efficiency of various filter materials (peat, wood chips, sawdust and the lightweight aggregates) in retaining residual chlorine to be implemented in green infrastructure. The best retainers of chlorine were sawdust (96%) and the LWA Leca (76%). No harmful effects of residual chlorine (changes in growth, color, leaf size, etc.) on plants (Tagetes patula or Pisum savitum) were observed and the residual chlorine in the leachate samples was below the equipment’s detection limit. Our research results will contribute to future studies aiming to remove various micropollutants from stormwater using remediation technologies.

Effects of Chlorinated Water on Neurite Length of Cultured Dorsal Root Ganglion Neurons and Semaphorin 3A Content of Cultured Epidermal Keratinocytes

The tap water that we normally use contains certain concentrations of free residual chlorine to kill microorganisms and viruses and make it safe for use. Water containing free residual chlorine not only dries out our hair and skin but can also cause irritation and itching in some people—especially those with sensitive skin or reduced skin barrier function. We investigated the effects of free residual chlorine on cultured dorsal root ganglion neurons and cultured epidermal keratinocytes. First, we measured neurite length in cultured rat dorsal root ganglion neurons. Next, to evaluate the effects of chlorine on semaphorin 3A (Sema3A) and nerve growth factor (NGF) levels in cultured human epidermal keratinocytes, we used an enzyme-linked immunosorbent assay to measure NGF in the supernatant and polymerase chain reaction and Western blot to determine Sema3A and NGF levels. Chlorine elongated the neurite length and increased the number of projections in cultured rat dorsal root ganglion neurons. Although there were no changes in NGF mRNA or protein levels in the supernatant of cultured human epidermal keratinocytes in the presence of chlorine, Sema3A mRNA and protein levels decreased, and the ratio of Sema3A to NGF was also reduced.

Reconsider the burn: The transient effect of a chlorine burn on controlling opportunistic pathogens in a full-scale chloraminated engineered water system

Nitrification is a serious water-quality issue in chloraminated engineered water systems (EWSs). Nitrification is often remediated by a chlorine burn (i.e., a free‑chlorine conversion), a short-term switch from chloramination to chlorination in EWSs. Opportunistic pathogens (OPs) are the dominant infectious agents in EWSs. However, the responses of OPs to a chlorine burn are unknown. This study for the first time assessed how a chlorine burn affected OPs in a full-scale EWS. We determined the impact of a 1.5-month chlorine burn on four dominant OPs (Legionella, Mycobacterium, Pseudomonas, and Vermamoeba vermiformis) in a representative full-scale chloraminated EWS in the United States. Legionella and Mycobacterium were the most abundant OPs. In the water main, the summed concentration of the four OPs during the chlorine burn [3.27 ± 1.58 log10(GCN·L−1); GCN: genome or gene copy number] was lower (p ≤ 0.001) than before the burn [4.83 ± 0.50 log10(GCN·L−1)]. After the burn, the summed concentration increased to 4.27 ± 0.68 log10(GCN·L−1), comparable to before the burn (p > 0.05), indicating a transient effect of the chlorine burn in the water main. At the residential sites, the summed concentrations of the four OPs were comparable (p > 0.05) at 5.50 ± 0.84, 5.27 ± 1.44, and 5.08 ± 0.71 log10(GCN·L−1) before, during, and after the chlorine burn, respectively. Therefore, the chlorine burn was less effective in suppressing OP (re)growth in the premise plumbing. The low effectiveness might be due to more significant water stagnation and disinfectant residual decay in the premise plumbing. Indeed, for the entire sampling period, the total chlorine residual concentration in the premise plumbing (1.8 mg Cl2·L−1) was lower than in the water main (2.4 mg Cl2·L−1). Consequently, for the entire sampling period, the summed concentration of the four OPs in the premise plumbing [5.26 ± 1.08 log10(GCN·L−1)] was significantly higher (p < 0.001) than in the water main [4.04 ± 1.25 log10(GCN·L−1)]. In addition, the chlorine burn substantially increased the levels of disinfection by-products (DBPs) in the water main. Altogether, a chlorine burn is transient or even ineffective in suppressing OP (re)growth but raises DBP concentrations in chloraminated EWSs. Therefore, the practice of chlorine burns to control nitrification should be optimized, reconsidered, or even replaced.

Deep learning for predicting the residual concentration of sodium hypochlorite in the cooling water OF nuclear power plants

Sea water is used as cooling water in several operating nuclear power plants, as well as in other industries such as petrochemicals. Biofouling is a common problem in systems that use sea water, causing corrosion in pipes and equipment, blockages, and loss of efficiency in heat exchangers. The use of sodium hypochlorite has proven to be effective in minimizing the damages caused by biofouling, provided that the residual chlorine concentration remains within a specific range, as low dosages will not protect the equipment and high dosages can cause environmental damage. With the aim of reducing maintenance and operation costs of equipment, and particularly ensuring the operability of systems related to the safety of nuclear power plants that use sea water, this study developed deep artificial neural networks (ANN) models capable of predicting the residual chlorine concentration in the main cooling and safety systems of a nuclear power plant. This prediction is based on environmental and process parameters that influence the residual chlorine concentration. Proposed model achieved average mean squared error of 0.002 and correlation coefficient (R2-Score) of 0.934, demonstrating to be a promising tool for predicting residual chlorine concentration.

The insightful water quality analysis and predictive model establishment via machine learning in dual-source drinking water distribution system

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.

District metered areas determination for water distribution networks using improved Girvan-Newman algorithm

In general, partitioning networks into district metered areas (DMA) is known as a practical method to manage the pressure and reduce water leakage in Water Distribution Networks (WDNs). Different methods such as engineering judgment, optimization techniques, and graph theory are proposed to determine the DMA. The graph theory is a traditional method for partitioning networks including several algorithms. One of these algorithms is the Girvan-Newman (GN) algorithm which is based on the mathematical parameters without considering the characteristics of networks. In other words, the hydraulic quality and quantity condition of WDNs are not used for DMA determination. Therefore, in this research, a new method is proposed to improve the performance of the GN algorithm for DMA determination considering the quantities of water networks. For this purpose, the average values of nodal pressure and residual chlorine concentration are calculated and used simultaneously for the junction’s weight determination. Then, the edge scores are calculated based on the junction’s weights, and the edges with the maximum scores are removed until to reach the desired number of DMAs. For comparison purposes, here, the Demand-Driven Simulation Method (DDSM) and Head-Driven Simulation Method (HDSM) analyses are used for the analysis of the Poulakis WDN, selected as a case study. A comparison of the results shows that by using the proposed method, the average pressure values are decreased and the average residual chlorine concentration values are increased. In other words, the pressure values are decreased from 16.32% to 26.23% and the average residual chlorine concentration values are increased from 2.5% to 18.37% in comparison with the results of the standard form of the GN algorithm using both DDSM and HDSM analyses. Furthermore, by using the proposed method, in all scenarios, the hybrid reliability values increase in comparison with the GN algorithm. The results indicate the unique performance of the proposed method in comparison with the standard form of the GN algorithm for DMA determination of WDNs.

Effect of peeling, cutting, or shredding of lettuce, carrot, or potato on the efficacy of chlorine disinfection.

Minimally processed vegetables are washed and subsequently disinfected by immersion in water solutions with antimicrobials which reduce the initial pathogenic or spoilage microbial load. Chlorine remains one of the most widely used disinfectants for vegetables and hence the importance of studying its properties. The aim of this study was to evaluate the effect of peeling, cutting, and shredding on the effectiveness of chlorine (200 ppm) as a disinfectant in lettuce, carrot, and potato. Three independent repetitions of each experiment were completed, and data was statistically analyzed. Results showed that the maintenance of the chlorine concentration in the disinfectant solution, over time, depended on the vegetables' preliminary processing technique (whole, peeled, cut, or shredded) (p < 0.05). In general, the disinfection treatments studied reduced Escherichia coli by 1-8 logs. The addition of chlorine in the disinfectant solution allowed greater reduction in E. coli than using water immersions (p < 0.05) and disinfection times longer than 5 min did not improve these microbiological reductions (p>0.05). The vegetables' subdivision (whole, peeled, cut, or shredded) can affect both E coli's reduction and the vegetables' residual chlorine concentration. No trend was observed in terms of sensory differences and their relationship to the vegetables' processing and disinfection. These results suggest that each facility must validate its disinfection processes, according to the conditions established on site and reduction goals related to initial microbial counts, vegetables' quality, processing operations, and other important aspects.

Effects of Massive Use of Disinfectants on the Plankton Communities in Lakes from Wuhan

The outbreak of COVID-19 led to the extensive use of disinfectants in urban areas. These disinfectants, along with disinfection by-products (DBPs), eventually enter waters and affected the aquatic organisms. But little information could be obtained on disinfectants threatening aquatic ecosystems. This study was conducted to obtain insight into the effects of massive use of disinfectants on freshwater ecosystems, DBPs, phytoplankton, and zooplankton in nine urban and two country lakes in Wuhan during the COVID-19 pandemic; in addition, the residual chlorine in the South Lake (one of the urban lakes), was investigated. The concentration of residual chlorine in the South Lake ranged from 0.000 mg L−1 to 0.427mg L−1, with an average concentration of 0.092 mg L−1. The total concentrations of DBPs (halogenated aliphatic DBPs and aromatic halogenated DBPs) detected in the urban and country lakes ranged from 4.22 μg L−1 to 16.59 μg L−1 and 5.92 μg L−1 to 7.84 μg L−1, respectively. There was no significant difference in DBPs content between urban lakes and country lakes (p &lt; 0.05). Mann–Whitney U tests showed no significant differences in plankton cell density, biomass, and alpha diversity indexes between urban and country lakes, except for the Shannon−Wiener diversity index of phytoplankton. Beta diversity demonstrated that plankton communities at different sampling stations in urban and country lakes were not significantly separated into two groups, but rather intersected each other. Variance partitioning analysis revealed that the composition of plankton communities was primarily influenced by other plankton organisms and community stability under the conditions of the investigated factors. Results indicated that the detected plankton communities in urban lakes from Wuhan were not significantly affected by the use of disinfectants. It could be deduced that the massive use of disinfectants in this outbreak had no significant impact on the plankton communities.

Machine-learning-assisted prediction and optimized kinetic modelling of residual chlorine decay for enhanced water quality management

The quality of water changes from source to tap, presenting challenges in maintaining consistent water quality across the system. Predicting water quality in distribution systems, including disinfectant residual loss and by-product formation, has been the subject of research since the early 1990s. Although numerous models have been proposed to predict residual chlorine decay, disputes exist among researchers and experts over the superiority of certain models. Accordingly, this study modified the existing process-based bulk decay models by replacing the initial Total Residual Chlorine (TRC) concentration parameter with TRC demand, leading to an improvement in the models' performance. The modification resulted in a 38.03%, 28.02%, 23.11%, and 33.29% average improvement in Mean Squared Error (MSE) values for the First Order Model (FOM), Parallel First Order Model (PFOM), Second Order Model (SOM), and Parallel Second Order Model (PSOM), respectively. The study also introduced an online predictive method based on a Machine Learning (ML) algorithm that predicts the first-order TRC bulk decay rate by using water quality parameters as inputs. A Gaussian Process Regression (GPR) model was used to predict the kinetic parameters in FOM, which accurately predicted the test sets for most of the cases. In addition, a new methodology was proposed in this study for predicting TRC in water distribution systems that incorporates the variability of source natural organic matter, operational actions, and water demands. This method seeks to develop high-fidelity and robust water quality predictions that provide operational decision support for optimized distribution system management. In conclusion, this study emphasizes the importance of understanding water quality changes from source to tap and the challenges of maintaining consistent water quality across the system. The study suggests modifying existing models and introducing a novel methodology for predicting residual chlorine in water distribution systems that can improve water quality management and, ultimately, better public health outcomes.

Modeling the Chlorine Series from the Treatment Plant of Drinking Water in Constanta, Romania.

Ensuring good drinking water quality, which does not damage the population's health, should be a priority of decision factors. Therefore, water treatment must be carried out to remove the contaminants. Chlorination is one of the most used treatment procedures. Modeling the free chlorine residual concentration series in the water distribution network provides the water supply managers with a tool for predicting residual chlorine concentration in the networks. With regard to this idea, this article proposes alternative models for the monthly free chlorine residual concentration series collected at the Palas Constanta Water Treatment Plant, in Romania, from January 2013 to December 2018. The forecasts based on the determined models are provided, and the best results are highlighted.

Formation of biosafety and greening of the production environment of food production with application of anolyte

Introduction: Electrochemically activated aqueous solutions are widely used in medical institutions, animal husbandry, as well as in large food processing facilities. This work presents a literature review on the use of electrochemically activated solutions in the food industry. These solutions can be used as a safe means for processing food raw materials, equipment, food containers, and surfaces of foodservice facilities. The question of using activated aqueous solutions in relation to foodservice establishments is not fully explored in the scientific literature.Purpose: The aim of the research is to study the possibility of using electrochemically activated aqueous solutions as safe and environmentally friendly means for treating objects in the production environment to eliminate the risk of contamination by microorganisms.Materials and Methods: Surfaces of the production environment were selected as the objects of study, including a table, scales, a knife, a slicer, and a gastronorm container. Sweet pepper fruits of the "Avangard" variety were chosen as non-heat-treated food products. An electrochemically activated aqueous solution of chlorine dioxide and hydrogen peroxide compounds with a concentration of active substances of 0.5 ± 0.05 g/L (Anolyte ANK-SUPER) was chosen as the treatment agent. Microbiological studies were conducted using XLD Agar medium and petrifilms. To assess the effectiveness of treatment, microbiological studies were conducted with the determination of Coliform Bacteria (BGKP) and Mesophilic Aerobic and Facultative Anaerobic Microorganisms (KMAFAnM). To achieve acceptable microorganism counts for counting purposes, the tenfold dilution method was used. The residual active chlorine concentration was determined using the iodometric method to determine the actual concentrations of working solutions.Results: The use of undiluted disinfectant with a concentration of 0.5 ± 0.05 g/L confirmed the effectiveness of electrochemically activated solutions against Coliform Bacteria (BGKP), with a significant reduction in the content of Mesophilic Aerobic and Facultative Anaerobic Microorganisms (KMAFAnM) compared to control samples. However, the results of the experiments suggest that parameter selection is required, specifically the concentration of the disinfectant and the duration of treatment depending on the object. The data obtained from preliminary experiments characterize the potential use of reduced concentrations of the working solution (no less than 0.05 ± 0.005 g/L of active chlorine), but do not confirm the possibility of full-scale application of the used concentrations due to the lack of quantitative data on the degree of contamination of the initial surfaces. The possibility of using the solutions as a single agent for treating both working surfaces and fruit and vegetable products has been demonstrated.Conclusion: As a result of the conducted research, the effectiveness of disinfectants based on electrochemically activated solutions against Escherichia coli bacteria (BGKP) and the reduction of Mesophilic Aerobic and Facultative Anaerobic Microorganisms (KMAFAnM) has been confirmed. In addition, among the possible prospects for research, determining the minimum concentrations and contact times for achieving stable disinfecting effects can be mentioned. The results obtained showed that the use of electrochemically activated solutions ensured the biological safety of treated production objects and surfaces of thermally untreated food products. An alternative method of using electrochemically activated solutions as a universal environmentally friendly agent has been proposed. Thus, in foodservice establishments, an effective "green" technology based on electrochemically activated solutions can be implemented.

Seismic resilience evaluation of water distribution systems considering hydraulic and water quality performance

The resilience evaluation of infrastructures provides an essential basis for resilience enhancement and disaster mitigation in urban communities. This study presents a seismic resilience evaluation framework of water distribution systems (WDSs) that integrates hydraulic with water quality simulation. The seismic damage scenarios of WDSs are generated according to the seismic fragility model of pipelines and Monte Carlo simulation, and the post-earthquake recovery of pipeline damages is simulated by a dynamic importance-based recovery sequence. The negative influence of earthquake hazards on water quality is investigated by the reduction of chlorine concentration in water, which depends on the changes in water age and supply path caused by pipeline damages. The water quality performance of WDSs is measured by the residual chlorine concentration at user nodes, and two water quality performance indexes, namely, the water supply that meets the quality threshold requirements and the water supply without quality deterioration after the earthquake. The proposed framework is implemented in three benchmark WDSs with different layouts and operational features. Application results show that the water quality resilience of WDSs tends to be lower than that of hydraulic services, and the relative difference in system resilience loss between the water quality and hydraulic service performance ranges from 17% to 286%, which varied according to the layout of WDSs. The water source redundancy, and not the loop pattern of the pipeline network, has crucial impacts on the water quality resilience of WDSs after an earthquake event.

Comprehensive comparison of water quality risk and microbial ecology between new and old cast iron pipe distribution systems

The effects of cast iron pipe corrosion on water quality risk and microbial ecology in drinking water distribution systems (DWDSs) were investigated. It was found that trihalomethane (THMs) concentration and antibiotic resistance genes (ARGs) increased sharply in the old DWDSs. Under the same residual chlorine concentration conditions, the adenosine triphosphate concentration in the effluent of old DWDSs (Eff-old) was significantly higher than that in the effluent of new DWDSs. Moreover, stronger bioflocculation ability and weaker hydrophobicity coexisted in the extracellular polymeric substances of Eff-old, meanwhile, iron particles could be well inserted into the structure of the biofilms to enhance the mechanical strength and stability of the biofilms, hence enhancing the formation of THMs. Old DWDSs significantly influenced the microbial community of bulk water and triggered stronger microbial antioxidant systems response, resulting in higher ARGs abundance. Corroded cast iron pipes induced a unique interaction system of biofilms, chlorine, and corrosion products. Therefore, as the age of cast iron pipes increases, the fluctuation of water quality and microbial ecology should be paid more attention to maintain the safety of tap water.

A Review of Traditional and Emerging Residual Chlorine Quenchers on Disinfection By-Products: Impact and Mechanisms.

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.

Study on the distribution characteristics and metabolic mechanism of chlorine-resistant bacteria in indoor water supply networks

The presence and attachment of chlorine-resistant bacteria on the surface of water distribution network will deteriorate water quality and threaten human health. Chlorination is critical in drinking water treatment to ensure the biosafety of drinking water. However, how disinfectants affect the structures of dominant flora during biofilm development and whether the changes are consistent with the free flora remain unclear. Therefore, we investigated changes in species diversity and relative abundance of different bacterial communities in planktonic and biofilm samples at different chlorine residual concentrations (blank, 0.3 mg/L, 0.8 mg/L, 2.0 mg/L and 4.0 mg/L), and the main reasons for the development of chlorine resistance in bacteria was also discussed. The results showed that the richness of microbial species in the biofilm was higher than that in planktonic microbial samples. In the planktonic samples, Proteobacteria and Actinobacteria were the dominant groups regardless of the chlorine residual concentration. For biofilm samples, the dominant position of Proteobacteria bacteria was gradually replaced by actinobacteria bacteria with the increase of chlorine residual concentration. In addition, at higher chlorine residual concentration, Gram-positive bacteria were more concentrated to form biofilms. There are three main reasons for the generation of chlorine resistance of bacteria: enhanced function of efflux system, activated bacterial self-repair system, and enhanced nutrient uptake capacity.

Spatial and socioeconomic inequalities in the access to safe drinking water in Peruvian households.

Access to safe drinking water has increased in Peru over the last decades, from 47% (2008) to 52% (2018). Nevertheless, such access would differ according to socioeconomic and regional factors. Thus, this study aimed to assess the socioeconomic inequality in the access to safe drinking water and identify its spatial distribution. We conducted a cross-sectional study based on the secondary data analysis of the 2021 Peruvian Demographic and Health Survey. Access to safe drinking water was a dummy variable categorised as safe if the residual chlorine concentration was ≥0.5 mg/L. Nationwide, 29.22% of households had access to safe drinking water. A pro-rich inequality in access to safe drinking water was observed. The spatial distribution was clustered. Significant hotspots were found in the south and centre of the country; however, cold spots were found in most areas. SaTScan analysis identified 32 and 63 significant clusters at high and low risks of having access to safe drinking water, respectively. In conclusion, approximately one out of four Peruvian households has access to safe drinking water, which was mostly concentrated among the wealthier households. Intra- and interdepartmental inequalities in access to safe drinking water were found, with several high-risk clusters.

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.

Decision-making for the hazard ranking of water distribution networks using the TOPSIS method

Abstract Owing to the consecutive occurrence of water quality incidents, such as the black and red water situation in Incheon, Mullae (Seoul), and Pohang, as well as the tap water larval events in Incheon and Jeju in 2020, South Korea's Ministry of Environment announced the new addition of Article 12-2 of the Enforcement Decree of the Water Supply and Waterworks Installation Act 2021. Additionally, the importance of intensive management for vulnerable areas where water quality incidents and complaints may occur repeatedly was specified. Therefore, this study suggests the hazard ranking of district-metered areas in the study area using the deterioration of the water distribution network, complaints (water quality, leakage), and modeling results (water pressure, residual chlorine concentration) to support the establishment of a reasonable water distribution network maintenance plan. The effective and sustainable maintenance of the water distribution network is possible if the hazard ranking is regularly determined using the suggested methodology proposed herein and the water distribution network intensive management area is managed.

Transcriptome reveals the immune and antioxidant effects of residual chlorine stress on Cyclina sinensis

Residual chlorine is a common by-product of warm drainage in coastal nuclear power plants. when accumulating to some limit, it may threaten marine ecosystem especially for benthic clam. However, there are few studies on the molecular mechanisms related to immunity and antioxidant of residual chlorine stress on clams. In this study, the clam (Cyclina sinensis) was exposed for 96 h at different concentrations (0, 50, 100, 150, 200, 250, 300, 350, 400, 450 and 500 mg/L) of residual chlorine to observe its mortality, measure the activity of antioxidant and immune-related enzymes, and analyses the gene expression level in the hepatopancreas by using the transcriptome sequencing. The results showed that the mortality rate increased with the increase of stress time and concentration, and the mortality rate in the 400, 450 and 500 mg/L groups reached 100% at 96 h. The tolerance to residual chlorine of C. sinensis decreased with the increase of chlorine dioxide concentration, and the LC50 of 96 h was 217.6 mg/L by linear regression method. After residual chlorine stress, the activity of antioxidant-related enzymes (T-AOC and SOD) in the hepatopancreas showed a trend of first increase and then decrease with the extension of stress time. The immune-related enzyme activities of AKP and LZM showed a downward trend between 0 and 96 h, while the ACP enzyme activity showed a trend of first rising and then decreasing. Transcriptome analysis showed that residual chlorine stress significantly changed the expression levels of immune-related molecules associated with signal transduction, prophenoloxidase cascade, cell apoptosis and pattern recognition protein/receptor. Moreover, glutathione S-transferase (GST), heat shock protein (HSP) and other antioxidant-related genes were significantly affected under residual chlorine stress. This study provided valuable information for understanding the effects of residual chlorine stress on survival, physiological metabolism and molecular mechanisms of immune and antioxidant functions of C. sinensis.

Biological stability of reclaimed greywater reused for flushing household toilets

Using reclaimed water to flush toilets could greatly reduce household water consumption. However, deterioration in the quality of reclaimed water, such as increased odor and bacterial growth, restrict its reuse. This study compared changes in residual chlorine, BOD5, NH3–N, turbidity, and color in four kinds of reclaimed water that can be reused for flushing household toilets. Bacterial reproduction and regeneration in the reclaimed water and the succession law of the microbial communities were also explored to reveal the influence of biofilm in the reclaimed water on the biological stability of the reclaimed water. The results show that under conditions of 30 °C after the addition of 6 mg/L of chlorine, no bacteria were discovered after 1 month, 20 days, or 10 days in the MBR effluent of greywater (GMBR), BAF effluent of greywater (GBAF), or reclaimed water of mixed domestic wastewater (MWR), respectively. The GMBR was similar to tap water, and was obviously better than the MWR. To control the proliferation of microorganisms, the recommend residual chlorine concentration should be no less than 0.80 mg/L at the point of use. However, when the concentration of residual chlorine in the reclaimed water was less than 0.9 mg/L (GMBR) or 1.5 mg/L (MWR), the chlorine could not kill the bacteria on the biofilm. Therefore, to control the biological stability of the reclaimed water, sufficient residual chlorine should be maintained, and the concentration of organic matter in the reclaimed water should be reduced as much as possible. Our findings showed that recycled greywater is more suitable for flushing household toilets than recycled mixed sewage.