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Abstract Global climate change is intensifying flood damage in urban rivers. Notably, most small and medium‐sized urban rivers have a brief concentration period and are highly vulnerable to sudden heavy rains that lead to a rapid increase in water levels. Therefore, rapid flood forecasting must be performed through accurate flood occurrence and timing prediction. In this study, a flooding time nomograph (FTN) was proposed to predict the flood occurrence time according to rainfall conditions, such as intensity, time distribution, and duration. In addition, rainfall–runoff simulations were performed by establishing different virtual rainfall scenarios using Huff's quartile rainfall time distribution. The simulation results were used to formulate the relationship between the rainfall intensity and flood occurrence time to generate the FTN. The applicability of this tool was verified through a comparison with the observed flood occurrence time for an actual rainfall event, which was highly accurate in the target watershed, with a correlation coefficient >0.8 and Nash–Sutcliffe efficiency >0.6. Therefore, the proposed FTN can be used to reasonably predict the occurrence of floods and the time of flood occurrence using only predicted rainfall information.

Understanding the energy efficiency of direct contact membrane distillation (DCMD) is important for the widespread application and practical implementation of the process. This study analyzed the available energy, known as exergy, in a DCMD system using computational fluid dynamics (CFD). A CFD model was developed to investigate the hydrodynamic and thermal conditions in a DCMD module. After the CFD model was verified, it was used to calculate the temperature polarization coefficient (TPC) and exergy destruction magnitudes under various operating conditions. The results revealed that slight decreases and increases in the TPC occurred with distance from the inlet in the module. The TPC was found to increase as the feed temperature was reduced and the feed and permeate flow rates were increased. The exergy destruction phenomenon was more significant under higher feed temperatures and higher flux conditions. Although the most significant exergy destruction in the permeate occurred near the feed inlet, the effect became less influential closer to the feed outlet. An analysis of exergy flows revealed that the efficiency loss in the permeate side corresponded to 32.9–45.3% of total exergy destruction.

Membrane distillation (MD), which uses hydrophobic porous membranes with a temperature gradient to produce pure water, has the potential to treat high-salinity wastewater. However, it cannot directly treat wastewater containing surfactants, which lower the surface tension and thus result in membrane wetting. To overcome this limitation, this study proposed a hybrid process consisting of powdered activated carbon (PAC) adsorption and MD, where PAC removes the surfactants in the wastewater to alleviate the wetting of the MD membranes. A bench-scale vacuum-assisted air gap MD (V-AGMD) equipment was adopted for the treatment of synthetic wastewater containing inorganic salts and surfactants. The conductivity of the permeate from V-AGMD was continuously monitored to detect membrane wetting. Without the use of PAC, the MD membrane was wetted within a short period, which decreased as the surfactant concentration increased. On the other hand, the addition of PAC retards the onset of wetting even at higher surfactant concentrations. The effectiveness of the PAC addition to the MD system on wetting control was examined under various conditions to elucidate its mechanism. [ABSTRACT FROM AUTHOR]

A multistage submerged vacuum membrane distillation system (SVMD) offers a novel approach to increase thermal energy efficiency by recovering the latent heat in the vapor from the membrane. However, most studies have focused on single-stage direct contact membrane distillation systems with lower thermal efficiency. This study investigated the feasibility of a multistage SVMD system for seawater desalination to provide insight into the design of MD systems. A theoretical model to predict the performance of the multistage SVMD was developed. A bench-scale experimental device was fabricated to experimentally evaluate the efficiency of the multistage SVMD system and verify the theoretical model. A hollow fiber MD membrane made of polyvinylidene fluoride was used. The experimental results show that the three-stage SVMD can significantly reduce the thermal energy requirement with a slight trade-off of the overall flux. Moreover, the theoretical model is useful in predicting the performance of the multistage SVMD system at different operating conditions. [ABSTRACT FROM AUTHOR]

Hollow fiber microporous membranes have been widely used in microfiltration (MF) for water and wastewater treatment as a pressure-driven separation process. Among them, hydrophobic membranes have been also considered in membrane distillation (MD) for seawater desalination as a thermally-driven separation process. In both cases, membrane fouling is a major bottleneck limiting their widespread applications. This study focused on the control of membrane fouling in MF and MD processes by directly vibrating the modules. Experiments were carried out using a laboratory-scale setup in both MF and MD. Synthetic feed water containing NaCl of 35,000 mg/L and CaSO4 of 2,000 mg/L was used for fouling tests. Factors affecting flux were examined, including vibration, operation type. The frequency of vibration feed was adjusted from 100 to 200 Hz. Results showed that the effect of the vibration was more important in the MD process than in the MF process. In MD, the vibration was more effective in the inside-out operation mode than in the outside-in operation mode. The blocking of the membrane surface by the foulants occurs less severely with the vibration than without the vibration. [ABSTRACT FROM AUTHOR]

Microporous membranes have been widely used in various water treatment systems, including pressure-driven separation processes such as microfiltration (MF) and ultrafiltration (UF) and thermally-driven separation processes such as membrane distillation (MD). However, control of membrane fouling is key to the successful design and operation of these processes. In this study, the effect of aeration on the fouling control was investigated in MF and MD processes, which have different driving forces. Experiments were carried out using the same microporous membranes in MF and MD processes to compare the difference in fouling propensity and the antifouling effect by the aeration. The reversibility of the foulants deposited on the membrane was also compared. Hermia’s fouling models were applied to analyze the aeration effect in both MF and MD but failed to fit the experimental data in most cases. Accordingly, a novel fouling model was suggested and used to interpret the experimental results. The specific fouling potential (φ) was introduced to show how the fouling potential changes during the operation of the membrane processes. It was confirmed from this analysis that the aeration effect was different between MF and MD, which is attributed to the inherent difference in the driving forces. [ABSTRACT FROM AUTHOR]

The occurrence of natural disasters such as floods, droughts, and earthquakes may cause water supply to stop working, which may also lead to social disasters and economic damages. Therefore, it is essential to prepare an emergency response management system to prepare disasters in water treatment infrastructures. In this study, a decision support system (DSS) responding to an emergency was proposed to systematically address possible problems in water treatment plants affected by various disasters. The DSS consists of several modules in the stages for planning, disaster sensing, and response and each module was developed based on a different approach depending on its function and purpose. The paper illustrates each module in detail and discusses the application in the proposed situation. [ABSTRACT FROM AUTHOR]

Recently, a hybrid system consisting of forward osmosis (FO) and membrane distillation (MD) has come into the spotlight. In the FO-MD hybrid system, freshwater is obtained from feed water through the FO process and the diluted draw solution from the FO process is re-concentrated by the MD process. However, relatively little information is available on the optimization of the hybrid system of FO and MD and their combination in a single module. Accordingly, this study focused on the development of an integrated module for the FO-MD hybrid system. The module was designed, fabricated, and tested in a bench-scale system. The effect of design and operating parameters, including flow velocity, the temperature of solutions, and membrane areas, on the overall performance of the module and system, was investigated. With the control of the temperature of the draw solution, the integrated FO-MD module enabled the constant flux operation, which is essential for the practical implementation of FO technology. [ABSTRACT FROM AUTHOR]

This paper presents the experimental results on the effect of hydraulic pressure on the wetting of membrane distillation (MD) membranes. Using CaSO4 as a model foulant, a series of MD experiments were carried out by adjusting the feed and distillate pressure between 0.1 and 0.7 bar. The flux, electric conductivity of the distillate, rejection, and recovery was measured in each experiment. The rate of the conductivity increase (dEc/dt) and the liquid entry pressure (LEP) were also estimated as quantitative measures for wetting propensity. Results showed that there was no wetting due to scale formation without the application of the external hydraulic pressure. As the feed pressure exceeds the distillate pressure, the wetting was accelerated. The recovery was higher when the distillate pressure was higher than the feed pressure. The rejection maintained high when the feed pressure was equal or less than the distillate pressure. The wetting by net pressure lower than the LEP is attributed to the existence of relatively large pores in the MD membrane. [ABSTRACT FROM AUTHOR]

This study focuses on ultrasonic-assisted cleaning of MD membranes for the treatment of high-TDS feed waters. The conditions for the ultrasonic application were explored to increase the cleaning efficiency and to minimize physical damage to the membrane. MD fouling tests were performed using synthetic feed waters with high scaling potential. After the fouling tests, cleaning experiments were performed by immersing the membrane cell in an ultrasonic generator and operated with a citric acid solution. The recovery of membrane permeability, liquid entry pressure, and ion rejection were measured, and scanning electron microscopy was used to visually analyze the membrane surface. Results indicated that the ultrasonic irradiation at low frequencies (28 and 45 kHz) led to either structural damage or wetting of the MD membranes. However, the ultrasonic irradiation at a higher frequency (72 kHz) exhibited a high cleaning efficiency without the structural damage and wetting. Compared with physical and chemical cleaning techniques, the ultrasonic-assisted cleaning resulted in higher flux recovery and foulant removal with a reduction in cleaning chemical consumption. [ABSTRACT FROM AUTHOR]

This study focused on the comparison of inside-out or outside-in membrane distillation (MD) modules under the conditions where fouling due to scale formation occurs. Experiments were carried out using a laboratory-scale direct contact MD setup. Synthetic feed water containing NaCl of 200,000 mg/L and CaSO4 of 2,000 mg/L was used for accelerated fouling tests. In-house MD mini modules were fabricated and operated either in the inside-out or outside-in modes. Results showed that the initial flux of the inside-out MD module was slightly higher than the outside-in MD module under no fouling conditions. The temperature difference was maintained at 40°C (feed inlet: 60°C; distillate: 20°C). The flow rate of the feed was the same as that of the distillate, which was 0.4 L/min. The average flux values of the inside-out and outside-in modes were, approximately, 1.56 and 1.42 kg/m²h, respectively. However, a more rapid decline of flux due to scale formation occurred in the inside-out MD modules compared with the outside-in MD modules under the same operating condition. The blocking coefficient was initially very low and suddenly increased after the volume of concentration factor (VCF) of 1.35 in the case of the inside-out mode. The blocking coefficient started to increase after the VCF of 1.75 in the case of the outside-in mode. Different mechanisms for scale formation in the inside-out and outside-in modes were suggested to elucidate the results of fouling tests, which were also supported by the scanning electron microscope analysis. [ABSTRACT FROM AUTHOR]

One of the challenges of membrane distillation (MD) technology is the wetting of the membranes. However, relatively little information is available on membrane wetting induced by fouling in MD process. Accordingly, this study examines the MD membrane wetting caused by fouling under various conditions. Various feed solutions containing NaCl, CaSO4, humic acid, alginate, and SDS were compared using polyvinylidene fluoride and polytetrafluoroethylene membranes in a series of MD experiments. Scanning electron microscope (SEM), liquid entry pressure and dynamic contact angle were used to interpret the results from the MD wetting experiments. Results showed that wetting was induced by severe fouling in the cases of 200,000 mg L-1 NaCl solution and 200,000 mg L-1 NaCl and 2,000 mg L-1 CaSO4 solution. The organic matters (50 mg L-1 humic acid and 50 mg L-1 alginate) did not cause neither fouling nor wetting by themselves. On the other hand, the addition of 50 mg L-1 SDS accelerated wetting. Based on these experimental results, the wetting potentials for various combinations of foulants are presented. [ABSTRACT FROM AUTHOR]

Although membrane distillation (MD) has drawn attention as a promising thermal separation technique to produce freshwater from seawater or concentrated wastewater, membrane fouling is a critical issue to be addressed for its applications. In this study, physical cleaning techniques to mitigate fouling of MD membranes were compared, including temperature reversal, air sparging, and ultrasonication. Experiments were conducted using flat sheet polyvinylidene fluoride membranes in a bench-scale direct contact membrane distillation system. Model foulants such as silica, humic acid, and CaSO4 were used to prepare feed solutions. The optimum condition for each physical cleaning technique was explored. Results showed that the air sparging was the most effective to recover flux among these three methods. The temperature reversal also showed potential to retard the progress of fouling. However, the ultrasonication does not seem to be applicable under the conditions considered in this work, because it led to severe damage of the membrane. [ABSTRACT FROM AUTHOR]

In this study, a model for the progress of membrane fouling due to scale formation in nanofiltration systems is presented. The model was developed based on the surface blockage equation to estimate flux as a function of volume concentration factor (VCF). Experiments were carried out to apply the model using CaSO4 saturated solution. Humic acid was used as a background organic matter that may interfere with the scale formation. The model could match the experimental data well with the overall R² value of 0.95 for more than 1,000 data points. The addition of the humic acid affected the changes in flux with VCF. The model parameters were different for different humic acid concentration. It is likely that the humic acid retards the scale formation rate by changing the crystallization kinetics. The rate of flux decline and VCF increase were also estimated as a function of the humic acid concentration. [ABSTRACT FROM AUTHOR]

This study examined fouling characteristics of a commercial grade electrospun nanofiber membrane, which has the nominal pore size of 0.23 µm and the thickness of 165 µm. A set of filtration experiments were carried out in a bench-scale equipment using real wastewater with the mixed liquor suspended solid of 6,500 mg/L. Critical flux was experimentally determined and analyzed using mathematical models such as pore blocking, pore constriction, and cake formation equations. Techniques for physical cleaning of the membrane were applied and compared, including periodic pump stop and backwash. The efficiency of chemical cleaning was measured using NaOCl solutions of different concentrations. Scanning electron microscopy was applied to examine the membrane surfaces before and after chemical cleaning. Results showed that the electrospun membrane has potential for high fouling resistance and effective cleaning. The critical flux seems to exist between 20 and 30 L/m2 h and the cake formation model resulted in the best fit to the experimental data. The transmembrane pressure increase was reduced by 88% after the application of backwash and the efficiency of chemical cleaning ranged from 84% to 91%. [ABSTRACT FROM AUTHOR]

Membrane distillation (MD) is a promising technology in the production of pure water from saline water or wastewater. Although MD has various attractive advantages compared with traditional desalination techniques, fouling is one of the main issues that should be resolved for long-term operation and its commercialization. Therefore, this study intends to understand the behaviors of colloidal silica fouling. Using model solution containing colloidal silica in the presence of NaCl, a series of experiments were carried out. Based on response surface methodology, second-order polynomial model, which can predict fouling ratios (J/J0 ), was developed and the effect of each factor on fouling was analyzed with flux patterns and scanning electron microscopic images. Severe silica cake layers were formed on the membrane surfaces in high ionic strength conditions and various flux tendencies were observed according to the change of factors. [ABSTRACT FROM AUTHOR]

Although membrane distillation (MD) can treat high-salinity feedwater such as reverse osmosis (RO) brine and produced water, the control of fouling resulted from scale formation is mandatory in such applications. As one of the attempts to alleviate fouling and scaling, this study applied an in-line air injection into the feed inlet of the membrane module in MD systems. Experiments were carried out in a bench-scale direct contact membrane distillation systems using a saturated CaSO4 solution as the model feedwater. It was revealed that the air injection was effective to retard flux decline due to CaSO4 scale formation. As the air flow rate and feed flow velocity increased, the flux reduction volume concentration factor increased from 1.17 to a maximum of 1.27. The critical concentration factor (CCF) that is defined as the concentration factor of rapid flux decline increased by the aeration. Since the MD fouling due to scale formation was attributed to the bulk crystallization, it is likely that the aeration can control the deposition of crystal particles on the membrane surface. In addition, the feed flow velocity was found to be an important factor affecting the effectiveness of aeration for MD scale control. [ABSTRACT FROM AUTHOR]

We study voter models defined on large finite sets. Through a perspective emphasizing the martingale property of voter density processes, we prove that in general, their convergence to the Wright-Fisher diffusion only involves certain averages of the voter models over a small number of spatial locations. This enables us to identify suitable mixing conditions on the underlying voting kernels, one of which may just depend on their eigenvalues in some contexts, to obtain the convergence of density processes. We show by examples that these conditions are satisfied by a large class of voter models on growing finite graphs. [ABSTRACT FROM AUTHOR]