Effect of a nanofiltration combined process on the treatment of high-hardness and micropolluted water

Combining nanofiltration and electrooxidation for complete removal of nanoplastics from water

Nanoplastics (NPs) have emerged as significant water contaminants, attracting increasing attention due to their potential impacts on aquatic ecosystems and human health. In addressing the environmental and health hazards posed by NPs in water, this new study explores a combined nanofiltration (NF) and electrooxidation (EO) approach. The proposed process begins with NF to concentrate the NPs in the water, followed by EO to degrade the NPs in the NF rejection. The results indicated that the employed NF system could completely eliminate NPs at different transmembrane pressures and times. The study also highlighted the influence of NP concentrations on recovery rates, showing a reduction in recovery at higher concentrations. Moreover, following the NF process, the EO process was examined for its efficiency in removing NPs over time and at various initial NP concentrations. The results revealed that the most effective durations were 20, 30, and 40 min for NP concentrations of 10, 22.5, and 35 mg/L, respectively. As a kinetic study, the rate of NPs degradation by the EO process was modeled using Langmuir-Hinshelwood (L-H) as well as power law models. The comparison between the models' predictions and the experimental data demonstrated that the power law and L-H models had good predictability for NP concentrations exceeding 10 mg/L and 2 mg/L, respectively. At concentrations below the 2 mg/L, deviations from the model were observed, likely due to changes in the reaction mechanism. It can be concluded from these results that, at low concentrations, the surface reactions were no longer the rate-determining step.

Membrane Treatment to Improve Water Recycling in an Italian Textile District

The textile district of Prato (Italy) has developed a wastewater recycling system of considerable scale. The reclaimed wastewater is characterized by high levels of hardness (32 °F on average), which precludes its direct reuse in numerous wet textile processes (e.g., textile dyeing). Consequently, these companies utilize ion exchange resins for water softening. However, the regeneration of the resins results in an increased concentration of chlorides in the reclaimed wastewater that exceeds the limit set by Italian regulations for the reuse of water for irrigation purposes. The objective of this study is to investigate the potential of membrane filtration as an alternative method for removing hardness from water. Therefore, an industrial-scale ultrafiltration-nanofiltration (UF-NF) pilot plant was installed to test the rejection of hardness from the reclaimed wastewater. The experiment employed two types of NF membranes and three permeate fluxes (27, 35, and 38 L·m-2·h-1) for testing. The results demonstrated that the system could remove hardness with efficiencies exceeding 98% under all conditions tested. The experimental findings indicate that the UF-NF system has the potential to be employed as a post-treatment step to render the reclaimed wastewater suitable for all textile finishing processes and to expand the scope for reuse.

Global perspective of municipal solid waste and landfill leachate: generation, composition, eco-toxicity, and sustainable management strategies

Globally, more than 2 billion tonnes of municipal solid waste (MSW) are generated each year, with that amount anticipated to reach around 3.5 billion tonnes by 2050. On a worldwide scale, food and green waste contribute the major proportion of MSW, which accounts for 44% of global waste, followed by recycling waste (38%), which includes plastic, glass, cardboard, and paper, and 18% of other materials. Population growth, urbanization, and industrial expansion are the principal drivers of the ever-increasing production of MSW across the world. Among the different practices employed for the management of waste, landfill disposal has been the most popular and easiest method across the world. Waste management practices differ significantly depending on the income level. In high-income nations, only 2% of waste is dumped, whereas in low-income nations, approximately 93% of waste is burned or dumped. However, the unscientific disposal of waste in landfills causes the generation of gases, heat, and leachate and results in a variety of ecotoxicological problems, including global warming, water pollution, fire hazards, and health effects that are hazardous to both the environment and public health. Therefore, sustainable management of MSW and landfill leachate is critical, necessitating the use of more advanced techniques to lessen waste production and maximize recycling to assure environmental sustainability. The present review provides an updated overview of the global perspective of municipal waste generation, composition, landfill heat and leachate formation, and ecotoxicological effects, and also discusses integrated-waste management approaches for the sustainable management of municipal waste and landfill leachate.

Advanced oxidation processes for synchronizing harmful microcystis blooms control with algal metabolites removal: From the laboratory to practical applications

Harmful algal blooms (HABs) in freshwater systems have become a global epidemic, leading to a series of problems related to cyanobacterial outbreaks and toxicity. Studies are needed to improve the technology used for the simultaneous removal of harmful cyanobacteria and algal metabolites. In this review, widely reported advanced oxidation processes (AOPs) strategies for removing major species Microcystis aeruginosa (M. aeruginosa) and microcystins (MCs) were screened through bibliometrics, such as photocatalysis, activated persulfate, H2O2, Ozone oxidation, ultrasonic oxidation, and electrochemical oxidation, etc. AOPs generate kinds of reactive oxygen species (ROS) to inactivate cyanobacteria and degrade cyanotoxins. A series of responses occurs in algal cells to resist the damaging effects of ROS generated by AOPs. Specifically, we reviewed laboratory research, mechanisms, practical applications, and challenges of HABs treatments in AOPs. Problems common to these technologies include the impact of algal response and metabolites, and environmental factors. This information provides guidance for future research on the removal of harmful cyanobacteria and treatment of algal metabolites using AOPs.

Study on the operation performance and floc adhesion mechanism of dissolved air flotation equipment

As a critical air dissolving system, the performance of air flotation equipment directly determines the adhesion efficiency and pollutant removal efficiency of air flotation processes. The factors affecting the performance of air flotation equipment and the relationships between equipment performance and pollution removal efficiency were studied. The results show that when the dissolved gas pressure was 0.4 MPa and the air intake rate was 24 mL/min, the dissolved gas efficiency of the equipment reached its highest value of 55%, the average particle size of bubbles was maintained at 24 µm, and the dissolved oxygen (DO) content significantly increased. When the dissolved gas pressure was 0.4 MPa, the air intake rate was 24 mL/min, and the coagulant dose was 6 mg/L; the removal rates for turbidity, chlorophyll-a, total organic carbon (TOC), and UV absorbance at 254 nm (UV₂₅₄) reached 95.76%, 96.41%, 34.21%, and 65.96%, respectively. The degree of pollutant removal was positively correlated with changes to the equipment performance parameters. Microbubbles (MBs) showed good removal of high-molecular weight, strongly hydrophobic organic matter and showed some removal of the trihalomethane formation potential (THMFP) of the water. The removal mechanism mainly depended on the hydrophobic interactions of the MBs with algae and organic matter. The flocs and MBs collided and adhered to form air-entrained flocs. The separation of air-entrained flocs depended on the relationship between the surface load and the rising velocity. The surface load has to be lower than the rising velocity of the minimum air-entrained flocs to ensure good effluent outcomes.

Denitrification performance of nitrate-dependent ferrous (Fe2+) oxidizing Aquabacterium sp. XL4: Adsorption mechanisms of bio-precipitation of phenol and estradiol

In this study, a nitrate-dependent ferrous (Fe²⁺) oxidizing strain under anaerobic conditions was selected and identified as XL4, which belongs to Aquabacterium. The Box-Behnken design (BBD) was used to optimize the growth conditions of strain XL4, and the nitrate removal efficiency of strain XL4 (with 10% inoculation dosage, v/v) could reach 91.41% under the conditions of 30.34 ℃, pH of 6.91, and Fe²⁺ concentration of 19.69 mg L^-1. The results of Fluorescence excitation-emission matrix spectra (EEM) revealed that the intensity of soluble microbial products (SMP), aromatic proteins and the fulvic-like materials were obvious difference under different Fe²⁺ concentration, pH, and temperature. X-ray diffraction (XRD) data confirmed that the main components of bio-precipitation were Fe₃O₄ and FeO(OH), which were believed to be responsible for the adsorption of phenol and estradiol. Furthermore, the maximum adsorption capacity of bio-precipitation for phenol and estradiol under the optimal conditions were 192.6 and 65.4 mg g^-1, respectively. On the other hand, the adsorption process of phenol and estradiol by bio-precipitation confirmed to the pseudo-second-order and Langmuir model, which shows that the adsorption process is chemical adsorption and occurs on the uniform surface.

Insights into metal-organic frameworks HKUST-1 adsorption performance for natural organic matter removal from aqueous solution

Natural organic matter (NOM) has rich halogenation reactive sites, therefore acts as the main precursor of disinfection byproducts (DBPs) in the chlorine disinfection process during drinking water treatment. In this research, high-quality metal-organic framework HKUST-1 is rapidly synthesized by a solvothermal method, and we are the first to report adsorption of aqueous humic acid (HA), representing NOM, and its adsorption behavior, influencing factors, and recycling capability. The crystalline HKUST-1 possessed a microporous framework with a high 1385 m²/g specific surface area, and three-dimensional structure as characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscope (SEM). 99% removal of 5 mg/L HA was observed at pH 5.8, room temperature, and 0.6 g/L HKUST-1. The maximum capacity was 14.42 mg HA/g HKUST-1 at room temperature. The Langmuir adsorption isotherm, quasi-second-order kinetic model, and thermodynamic parameters accurately describe the spontaneous and disorderly endothermic adsorption of HA by HKUST-1. The desorption regeneration process was accomplished by washing HKUST-1 with NaOH and calcination; it showed that HKUST-1 was viable in three regeneration cycles. The mechanism of HA adsorption by HKUST-1 is electrostatic and synergistic interaction between π-π bonding, and hydrogen bonding. HKUST-1 is a potential treatment strategy to remove NOM.

A Review on the Use of Membrane Technology Systems in Developing Countries

Fulfilling the demand of clean potable water to the general public has long been a challenging task in most developing countries due to various reasons. Large-scale membrane water treatment systems have proven to be successful in many advanced countries in the past two decades. This paves the way for developing countries to study the feasibility and adopt the utilization of membrane technology in water treatment. There are still many challenges to overcome, particularly on the much higher capital and operational cost of membrane technology compared to the conventional water treatment system. This review aims to delve into the progress of membrane technology for water treatment systems, particularly in developing countries. It first concentrates on membrane classification and its application in water treatment, including membrane technology progress for large-scale water treatment systems. Then, the fouling issue and ways to mitigate the fouling will be discussed. The feasibility of membrane technologies in developing countries was then evaluated, followed by a discussion on the challenges and opportunities of the membrane technology implementation. Finally, the current trend of membrane research was highlighted to address future perspectives of the membrane technologies for clean water production.

Nanofiltration for drinking water treatment: a review

In recent decades, nanofiltration (NF) is considered as a promising separation technique to produce drinking water from different types of water source. In this paper, we comprehensively reviewed the progress of NF-based drinking water treatment, through summarizing the development of materials/fabrication and applications of NF membranes in various scenarios including surface water treatment, groundwater treatment, water reuse, brackish water treatment, and point of use applications. We not only summarized the removal of target major pollutants (e.g., hardness, pathogen, and natural organic matter), but also paid attention to the removal of micropollutants of major concern (e.g., disinfection byproducts, per- and polyfluoroalkyl substances, and arsenic). We highlighted that, for different applications, fit-for-purpose design is needed to improve the separation capability for target compounds of NF membranes in addition to their removal of salts. Outlook and perspectives on membrane fouling control, chlorine resistance, integrity, and selectivity are also discussed to provide potential insights for future development of high-efficiency NF membranes for stable and reliable drinking water treatment.

Chemicals-free approach control interface characteristics of nanofiltration membrane: Feasibility and mechanism insight into CEM electrolysis

The combined fouling effect prevalent in the nanofiltration (NF) process severely limits its use. In this study, cation exchange membrane (CEM) electrolysis was performed to alleviate NF membrane fouling by controlling interface characteristics. The results revealed that CEM electrolysis (hydraulic retention time with 0.24 or 0.36 h) effectively improved NF membrane permeability by 201%-211% and achieved a stability of > 8 LMH/bar. The divalent cations were removed through CEM electrolysis, with a decrease in Ca²⁺ and Mg²⁺ by approximately 68.8% and 30.9%, respectively, which was related to scaling potential reduction. This softening function reduced the possibility of bridging of organics with divalent cations, which contributed to the lower molecular weight of organic matter (mainly humic substances) distributed in 1.4-23 kDa. The improved organic indicators of the NF membrane permeate quality implied that the membrane interface characteristics improved. The foulant layer on the NF membrane dominated humic substances, and biopolymers exhibited hydrophobic, smooth, and porous characteristics. The self-aggregation of foulants on the NF membrane surface stimulated the interface characteristics with high water permeability. Energy consumption confirmed the feasibility of CEM electrolysis on NF application. Thus, CEM electrolysis as a chemical-free approach that can be combined with NF and can provide guidance for NF membrane fouling in urban water treatment and water reclamation.

Pure and Ce-doped spinel CuFe2O4 photocatalysts for efficient rhodamine B degradation

Wastewater management is becoming a serious issue worldwide. To enhance the reuse of wastewater, one has to remove toxic pollutants present in it. High amount of dye is present in wastewater, and to remove these dyes is the large scope of this research. Herein, we report production of pure and Ce-doped copper ferrite via hydrothermal route. The synthesized nanoparticles were collected and analyzed by basic characterization techniques. The bandgap energy calculated for pure, 1% Ce, and 2% Ce-doped CuFe₂O₄ was found to be 2.77, 2.57, and 2.36eV, respectively. Reduction in bandgap was attributed to the doping element. The shape and size of pure and Ce-doped products were investigated using a scanning electron microscope. Agglomeration was observed in the pure copper ferrite sample. In the Ce-doped sample, agglomeration was clearly reduced and the 2% Ce-doped CuFe₂O₄ sample showed growth of small nanoparticles. They showed complete growth and were arranged in a uniform manner without agglomeration. The surface area of the 2% Ce-CuFe₂O₄ sample was found to be 65.89 m²/g with 7.02 nm pore diameter. The photocatalytic activity of the prepared material was observed for rhodamine B degradation. The pure and catalyst-added dye was exposed under visible light. The samples were tested for UV. The efficiency obtained for pure dye solution, pristine CuFe₂O₄-added, and 1% Ce and 2% Ce-doped CuFe₂O₄-added dye solutions were 48%, 50%, 66%, and 88% within 2 h of irradiation. The 2% Ce-doped CuFe₂O₄ sample showed excellent photocatalytic activity as the bandgap and morphology were enhanced by doping an appropriate ratio of Ce ions.

An electro-oxidation reactor for treatment of nanofiltration concentrate towards zero liquid discharge

Nanofiltration (NF) concentrate generated from the secondary wastewater treatment contains high concentration of ammonium nitrogen and refractory organics, thus having great environmental risks. In this study, an electro-oxidation (EO) reactor built up with a boron-doped diamond (BDD) anode is utilized to treat the NF concentrate. To reach "zero liquid discharge", a mixture of the electrolytic effluent and the raw secondary wastewater was collected and transported back to the NF module. Results show that under the current density of 30 mA·cm^-2, most of ammonia nitrogen was decomposed into N-gases within 30 min due to the active chlorine radicals generated in the electrochemical process. Moreover, the EO reactor completely eliminated antibiotics, humic acids and bacteria in the NF concentrate under long electrolysis time of 60 min. In particular, the organic pollutants removal rate was kept at a stable value in the EO reactor for a long-term operation of up to 120 h. In addition, the NF membrane remained a constant permeate flux without being affected by the membrane biofouling caused by organic components in wastewater. Our study highlights the potential of the NF-EO process as a "zero liquid discharge" approach for treatment of the secondary wastewater.

The inactivation of bacteriophage MS2 by sodium hypochlorite in the presence of particles

The inactivation of bacteriophage MS2 by sodium hypochlorite was investigated to understand the effect of solution chemistry on the disinfection efficacy in the presence of particles. Kaolinite and Microcystis aeruginosa (M. aeruginosa) were used as the models of inorganic and organic particles to simulate high turbidity and algal cells, respectively, in drinking water sources. In both particle-containing solutions, lower pH, the presence of cations (di-valent Ca²⁺) and natural organic matters (NOM) were regarded as the main factors to influence the aggregation and inactivation of MS2. The results showed that MS2 aggregated in all solutions at pH 3.0, protecting the inner viruses. At pH 7.0, the presence of Na⁺ cations (0-200 mmol/L) did not affect the inactivation efficacy of MS2, which always followed the order of particles-free ≈ kaolinite > M. aeruginosa. The inactivation efficacy of MS2 in the presence of Ca²⁺ cations followed the order of kaolinite > particles-free > M. aeruginosa at 0-50 mmol/L Ca²⁺ cations, while the inactivation efficacy remained almost constant in the range of 100-200 mmol/L Ca²⁺ cations. By contrast, kaolinite offered not enough protection to adsorbed MS2, but MS2 aggregation decreased disinfection efficacy at a high concentration of Ca²⁺ cations. Moreover, the presence of humic acid as NOM decreased the inactivation of MS2 more significantly than M. aeruginosa due to the more consumption of free chlorine from humic acids. Therefore, the co-existence of NOM and di-valent Ca²⁺ cations are potential challenges for the inactivation of viruses by sodium hypochlorite in safe drinking water.

Selective electrosorption of Ca2+ by MXene cathodes coupled with NiAl-LMO anodes through ion intercalation

Capacitive deionization (CDI), or electrosorption, is a desalination technology that exhibits significant potential; however, its major technical requirement of selective ion separation poses a challenge for its further practical application. Herein, a titanium carbide (MXene)-layered electrosorption electrode with high selectivity for Ca²⁺ was fabricated. The prepared MXene electrode had many surface hydroxyl functional groups that serve as adsorption sites for Ca²⁺. Ca²⁺ was successfully inserted into the interlayers of the MXene cathode and formed a strong interaction with [Ti-O] bonds during the capacitive deionization process. When a Ni-Al layered metal oxide anion intercalation electrode was employed as the counter electrode, Ca²⁺ adsorption by the MXene electrode was significantly enhanced due to the valence compensation balance effect. The maximum Ca²⁺ electrosorption capacity of the MXene electrode reached 1011.82 mg per gram effective MXene material, which is 6.3 times higher than that of Na⁺ based on the Langmuir adsorption isotherm model. The MXene electrode exhibited prominent selectivity for Ca²⁺ ions in the presence of Na⁺ and Mg²⁺. The Ca²⁺/Mg²⁺ selectivity factor for electrosorption reached 2.63, and Ca²⁺/Na⁺ selectivity factor could achieve 9.84, respectively. After five electrosorption/desorption cycles, the Ca²⁺ removal rate only decreased from 46.96% to 45.34%, suggesting that the MXene electrode has excellent stability. Our study demonstrated a novel CDI electrode and technical approach for softening water.

Insight into the influence of humic acid and sodium alginate fractions on membrane fouling in coagulation-ultrafiltration combined system

Membrane fouling has become the one of main obstacles for the widespread application of membrane technology in water treatment processes. Coagulation as pretreatment is proven to be effective for the alleviation of membrane fouling. In this study, the influence of humic acid (HA)/sodium alginate (SA) fractions in the structure and resistance of cake layer on the membrane surface was investigated. The presence of SA at an appropriate fraction could facilitate the formation of large and loosely branched flocs and thereby form a more permeable cake layer on the membrane surface due to good bridging and charge neutralization abilities of SA molecules. The particle image velocimetry (PIV) technique was employed for monitoring the dynamic formation process of cake layer under different HA/SA fractions. The cake layer with a higher thickness was observed to be rapidly formed on the membrane surface at the presence of SA in water. According to the theoretical analysis, the membrane fouling in coagulation-ultrafiltration (UF) combined system demonstrated to be highly dependent on the size and intra-porosity of flocs. The fractal dimension of flocs might have an impact on the resistance of cake layer through affecting the porosity of aggregated flocs. The SA molecules could be used as the coagulant aid for effective alleviation of membrane fouling and the improvement of filtration performance in a coagulation-UF combined system.