Fouling characteristics of reverse osmosis membranes at different positions of a full-scale plant for municipal wastewater reclamation

Anaerobic biodegradation enables zero liquid discharge of two-stage nanofiltration system for microelectronic wastewater treatment

Nanofiltration (NF) is a promising technology in the treatment of microelectronic wastewater. However, the treatment of concentrate derived from NF system remains a substantial technical challenge, impeding the achievement of the zero liquid discharge (ZLD) goal in microelectronic wastewater industries. Herein, a ZLD system, coupling a two-stage NF technology with anaerobic biotechnology was proposed for the treatment of tetramethylammonium hydroxide (TMAH)-contained microelectronic wastewater. The two-stage NF system exhibited favorable efficacy in the removal of conductivity (96 %), total organic carbon (TOC, 90 %), and TMAH (96 %) from microelectronic wastewater. The membrane fouling of this system was dominated by organic fouling, with the second stage NF membrane experiencing a more serious fouling compared to the first stage membrane. The anaerobic biotechnology achieved a near-complete removal of TMAH and an 80 % reduction in TOC for the first stage NF concentrate. Methyloversatilis was the key genus involved in the anaerobic treatment of the microelectronic wastewater concentrate. Specific genes, including dmd-tmd, mtbA, mttB and mttC were identified as significant players in mediating the dehydrogenase and methyl transfer pathways during the process of TMAH biodegradation. This study highlights the potential of anaerobic biodegradation to achieve ZLD in the treatment of TMAH-contained microelectronic wastewater by NF system.

Evaluating the Impact of Cl2•- Generation on Antibiotic-Resistance Contamination Removal via UV/Peroxydisulfate

The removal of antibiotic-resistant bacteria (ARB) and antibiotic-resistance genes (ARGs) using sulfate anion radical (SO4•-)-based advanced oxidation processes has gained considerable attention recently. However, immense uncertainties persist in technology transfer. Particularly, the impact of dichlorine radical (Cl2•-) generation during SO4•--mediated disinfection on ARB/ARGs removal remains unclear, despite the Cl2•- concentration reaching levels notably higher than those of SO4•- in certain SO4•--based procedures applied to secondary effluents, hospital wastewaters, and marine waters. The experimental results of this study reveal a detrimental effect on the disinfection efficiency of tetracycline-resistant Escherichia coli (Tc-ARB) during SO4•--mediated treatment owing to Cl2•- generation. Through a comparative investigation of the distinct inactivation mechanisms of Tc-ARB in the Cl2•-- and SO4•--mediated disinfection processes, encompassing various perspectives, we confirm that Cl2•- is less effective in inducing cellular structural damage, perturbing cellular metabolic activity, disrupting antioxidant enzyme system, damaging genetic material, and inducing the viable but nonculturable state. Consequently, this diminishes the disinfection efficiency of SO4•--mediated treatment owing to Cl2•- generation. Importantly, the results indicate that Cl2•- generation increases the potential risk associated with the dark reactivation of Tc-ARB and the vertical gene transfer process of tetracycline-resistant genes following SO4•--mediated disinfection. This study underscores the undesired role of Cl2•- for ARB/ARGs removal during the SO4•--mediated disinfection process.

Critical minority fractions causing membrane fouling in reclaimed water: Fouling characteristics, mechanisms and control strategies

In regard to membrane-based technologies of wastewater reclamation, the reported key foulants were faced with dilemma that they could not be effectively separated and extracted from reclaimed water for thorough investigation. In this study, the crucial foulants were proposed as "critical minority fraction (F_CM)", representing the fraction with molecular weight (MW) > 100 kDa which could be easily separated by physical filtration using MW cut-off membrane of 100 kDa with fairly high recovery ratio. F_CM with low dissolved organic carbon (DOC) concentration (∼1 mg/L) accounted for less than 20% of the total DOC in reclaimed water, while contributed to more than 90% of the membrane fouling, and thus F_CM could be considered as a "perfect criminal" causing membrane fouling. Furthermore, pivotal fouling mechanism was attributed to the significant attractive force between F_CM and membranes, which led to severe fouling development due to the aggregation of F_CM on membrane surface. Fluorescent chromophores of F_CM were concentrated in regions of proteins and soluble microbial products, with proteins and polysaccharides accounted for 45.2% and 25.1% of the total DOC, specifically. F_CM was further fractionated into six fractions, among which hydrophobic acids and hydrophobic neutrals were the dominant components in terms of DOC content (∼80%) as well as fouling contribution. Regarding to these pronounced properties of F_CM, targeted fouling control strategies including ozonation and coagulation were applied and proved to achieve remarkable fouling control effect. High-performance size-exclusion chromatography results suggested that ozonation achieved distinct transformation of F_CM into low MW fractions, while coagulation removed F_CM directly, thus leading to effective fouling alleviation. Therefore, the investigation of the critical foulants was expected to help glean valuable insight into the fouling mechanism and develop targeted fouling control technologies in practical applications.

Fouling in reverse osmosis membranes: monitoring, characterization, mitigation strategies and future directions

Water scarcity has been a global challenge for many countries over the past decades, and as a result, reverse osmosis (RO) has emerged as a promising and cost-effective tool for water desalination and wastewater remediation. Currently, RO accounts for >65% of the worldwide desalination capacity; however, membrane fouling is a major issue in RO processes. Fouling reduces the membrane's lifespan and permeability, while also increases the operating pressure and chemical cleaning frequency. Overall, fouling reduces the quality and quantity of desalinated water, and thus hinders the sustainable application of RO membranes by disturbing its efficacy and economic aspects. Fouling arises from various physicochemical interactions between water pollutants and membrane materials leading to foulants' accumulation onto the membrane surfaces and/or inside the membrane pores. The current review illustrates the main types of particulates, organic, inorganic and biological foulants, along with the major factors affecting its formation and development. Moreover, the currently used monitoring methods, characterization techniques and the potential mitigation strategies of membrane fouling are reviewed. Further, the still-faced challenges and the future research on RO membrane fouling are addressed.

Fluorescence characteristics and source analysis of DOM in groundwater during the wet season in the CKDu zone of North Central Province, Sri Lanka

The dissolved organic matter (DOM) should be purified for safe drinking water due to disinfection by-products (DBPs) produced by disinfectants reaction with DOM. Current research on groundwater in the chronic kidney disease with unknown etiology (CKDu) zone of the North Central Province (NCP) in Sri Lanka has focused mainly on aquatic chemistry, with limited attention paid to the spatial distribution, compositional sources and factors of DOM. Therefore, the structure, composition, source and spatial distribution of the DOM of two kinds of groundwater samples collected from dug well and tube well in the NCP during the wet season were determined, compared and analyzed by analytical tools such as parallel factor analysis (PARAFAC). Results show that the average concentrations of TOC in these two groundwater samples are generally higher than 5.0 mg/L, and the concentration of TOC in the groundwater of the shallow weathered aquifer is higher than that of the deep hard rock aquifer, while its distribution of the two aquifers are on contrary. The DOM in the dug well has three types and four components, including humus-like component C1 (33.36%) and C2 (38.60%), protein-like component C3 (13.09%) and heterogeneous organic component C4 (14.95%). In the tube well, two types and two components of the DOM are determined, including humus-like component CⅠ (69.80%) widely existing in natural water and soluble microbial by-product CⅡ (30.20%) produced by microbial community activities. In the dug well, DOM is mainly exogenous input, the higher ion concentration in water affected the fluorescence intensity of humus and protein components. And in the tube well, DOM has obvious endogenous characteristics, and higher pH value may inhibit the production of protein like fluorescent substances to a certain extent.

Ionic fluid as a novel cleaning agent for the control of irreversible fouling in reverse osmosis membrane processes

While a variety of chemical cleaning strategies has been studied to control fouling in membrane-based water treatment processes, the removal of irreversible foulants strongly bound on membrane surfaces has not been successful. In this study, we firstly investigated the diluted aqueous solutions of ionic fluid (IF, 1-ethyl-3-methylimidazolium acetate) as a cleaning agent for three model organic foulants (humic acid, HA; bovine serum albumin, BSA; sodium alginate, SA). The real-time monitoring of cleaning progress by optical coherence tomography (OCT) showed that fouling layer was dramatically swelled by introducing IF solution and removed by shear force exerted during cleaning. This phenomenon was induced due to the pre-existing interactions between organic foulants were weakened by the intrusion of IF into the fouling layer, which was analyzed by the measurement of adhesion forces using atomic force microscopy (AFM). In the experiments with model foulants and wastewater effluent, IF was added to alkaline cleaning agents (NaOH) to verify the applicability to be supplemented in commercial cleaning agents, and resulted in the significantly enhanced control of irreversible membrane fouling. Implication of utilizing recyclable IF with negligible volatility is that environmental effects of membrane cleaning solutions could be minimized by decreasing usage of cleaning chemicals, while increasing the cleaning efficiency.

Fouling investigation of cartridge filter (CF) used as "firewall" in a nanofiltration drinking water plant

The cartridge filter (CF) as a "firewall" is crucial between pretreatment and nanofiltration (NF) units, but CF fouling with risk has received limited attention. The systematic autopsy for CFs (CF₁ and CF₂) applied in a NF drinking water plant was conducted to reveal CF fouling profile. Herein, scale blocks, irregular-shaped particles, and stacked-floc clusters were observed as the main morphologies of foulants. The major elements from foulants included Fe, Ca, Al, Mg, Na, P, and Si. The dissolved matters especially bioproducts resulted in the secondary pollution of permeated water. Biofouling was mainly caused by Proteobacteria phyla, and consisted of a large proportion of polysaccharides (11% and 25.1%), proteins (10.3% and 22.7%), lipids (21.7% and 22.4%), respectively. In addition, an obvious contrast was observed regarding the antifouling performance of CFs. The surface scaling degree of CF₁ with horizontal irregular loose-pleats was more serious than CF₂ with vertical regular compact-pleats, while the latter with high-density pleats appeared the higher fouling potential due to a greater capacity for organic foulants in the inner layers of "firewall" and better bio-diversity and bio-evenness of microbial communities. This study provides a deeper insight into CF fouling and contributes to the application of CFs.

Ultrafiltration significantly increased the scaling potential of municipal secondary effluent on reverse osmosis membranes

Ultrafiltration (UF) was often used as pretreatment in front of reverse osmosis (RO) unit because of its high rejection efficiency of microbes and particles. However, in some cases UF pretreatment might show adverse effects on the RO membrane flux. In this study, the effects of UF pretreatment on secondary effluent water quality and its RO membrane fouling characteristics were explored. There was almost no change of water quality after UF with different molecular weight cut-off (MWCO) membranes (100, 30 and 10 kDa), including total dissolved solid (TDS), alkalinity, conductivity, ion concentrations, etc., while pH increased a little and dissolved organic carbon (DOC) declined by about 1 mg/L. On the contrary, the RO membrane flux of UF permeates presented clear decline in comparison to the secondary effluent. The membrane fouling velocity and steady-state flux of secondary effluent was 0.052 and 0.656, while fouling velocity increased (0.077, 0.071, 0.067) and steady-state flux decreased significantly (0.397, 0.416, 0.448) after 100, 30, 10 kDa UF membrane pretreatment. Scanning electron microscope (SEM) images showed many crystals on the fouled membrane surfaces, which turned out to be CaCO₃ by Energy dispersive spectrometer (EDS) analysis and precipitation calculation. After the addition of UF retentates to UF permeates, scaling was prevented and crystals on the RO membrane almost disappeared, which implied the anti-scaling effect of the UF retentates with low concentration. According to anti-scaling performance experiments, the anti-scaling performance of 100 k, 30 k, 10 k retentates was 2.7%, 4.0% and 7.3%, respectively. Excitation emission matrix (EEM) and fourier transform infra-red (FTIR) results showed that these retentates retained by different MWCO membranes were similar and composed of protein-like substances and soluble microbial products. The effect of key minority components in RO system deserved further exploration.

Improving the Performance of the Reverse Osmosis Process with Fiber Filter and Ultrafiltration: Promoting Municipal Sewage Reclamation and Reuse for Industrial Processes

Wastewater reuse presents a promising solution to the growing need for the sustainable use of available water resources. The reclamation of municipal sewage through reverse osmosis can be applied for diverse uses to alleviate chronic water scarcity. In this study, a pilot plant was fabricated to measure the efficiency and the costs that are associated with pretreatment by the fiber filtration and ultrafiltration of secondary effluent from a water resource recovery facility in Taiwan. The results of this dual-membrane process meet the quantity and quality standards for industrial reuse. The pretreatment produced feedwater with a silt density index (SDI15) lower than 4.1, and with average turbidity removal rates of 42.7% (fiber filtration) and 99.2% (ultrafiltration). Following reverse osmosis, a 97.9% rejection of the electrolyte conductivity was achieved in the reclaimed water. The fouling of the membranes was controlled through the application of intensive backwash, chemically enhanced backflushing, and cleaning in place. The proposed system improves the feasibility, reliability, and economy of the dual-membrane process as a tertiary treatment for safe water reuse, and it thereby demonstrates that this technology has reached maturity for the full-scale implementation of sustainable water reuse.

Novel Quantitative Evaluation of Biotreatment Suitability of Wastewater

The development of wastewater treatment industry has gradually entered the high-standard period and the wastewater treatment technology needs to be refined for different types of wastewater. Traditional water quality indicators are not able to explain new problems encountered in the current wastewater treatment process, especially the potential of removing pollutants via biological methods. This research proposed a new method of evaluating the biological treatment process by measuring the oxygen consumption in the biodegradation of pollutants on-the-go and describing the complete biological oxygen consumption process. The biodegradability of wastewater from an actual textile wastewater treatment plant was quantitatively evaluated by analyzing the proportion of different organic pollutions. Results showed that the hydrolytic acidification can improve the biodegradability of textile wastewater by increasing the content of biodegradable organic matter (growth of 86.4%), and air flotation has little effect on the biodegradability of the wastewater. Moreover, the biodegradability of the textile wastewater could be improved by increasing the nitrogen and phosphorus content, which could come from urea and K2HPO4. Concretely, nitrogen source mainly increases organic matter of rapid bio-treated and organic matter of easy bio-treated by 14.94% and 70.79%, and phosphorus source mainly increases the organic matter of easy bio-treated by 143.75%. We found that the optimum concentration of additional N and P to the textile wastewater was 35 mg/L and 45 mg/L, respectively. This approach holds great application prospects such as risk control, optimizing treatment technology, and management, due to its characteristics of being simple, easy to use, and rapid online implement action.

Evolution of membrane fouling and cleaning strategy development in municipal wastewater reclamation by nanofiltration

Membrane fouling is a serious concern in municipal wastewater treatment using nanofiltration (NF). The variation of NF membrane fouling at different operational durations were monitored and evaluated to understand the evolution of fouling mechanism in the present study. Six representative cleaning methods were picked for tests to deconstruct the structure and composition of the fouling layers on membrane surface. Characteristics of membrane surface morphology and foulants were analysed using advanced methods to investigate the evolution of membrane fouling. Both the composition and concentration of foulants on membrane surface showed significant variations with time during the whole operational duration, indicating the necessity of clear and precise understanding for the evolution of membrane fouling to expound its mechanism in depth. The fouling layers were mainly composed of hydrophobic organics while most inorganic salts were deposited on the surface or embedded into the upper layer of the cake in a short duration. However, after a long duration, the cake layer became thinner with much higher proportion of hydrophilic organics. Meanwhile, more inorganic foulants penetrated deep into the cake layer or even directly absorbed on the membrane surface, which resulted in more complicated fouling scenarios. Moreover, more efficient and effective targeted cleaning strategies could be further established based on the fouling mechanism identified.

Fouling control in reverse osmosis for water desalination & reuse: Current practices & emerging environment-friendly technologies

Reverse Osmosis (RO) is becoming increasingly popular for seawater desalination and wastewater reclamation. However, fouling of the membranes adversely impacts the overall process efficiency and economics. To date, several strategies and approaches have been used in RO plants and investigated at the laboratory-scale for their effectiveness in the control of different fouling types. Amid growing concerns and stringent regulations for the conservation of environment, there is an increasing trend to identify technologies that are effective in fouling mitigation as well as friendly to the environment. The present review elaborates on the different types of environment-friendly technologies for membrane fouling control that are currently being used or under investigation. It commences with a brief introduction to the global water crisis and the potential of membrane-based processes in overcoming this problem. This is followed by a section on membrane fouling that briefly describes the major fouling types and their impact on the membrane performance. Section 3 discusses the predominant fouling control/prevention strategies including feedwater pretreatment, membrane and spacer surface modification and membrane cleaning. The currently employed techniques are discussed together with their drawbacks, with some light being shed on the emerging technologies that have the ability to overcome the current limitations. The penultimate section provides a detailed discussion on a variety of eco-friendly/chemical free techniques investigated to control different fouling types. These include both control and prevention strategies, for example, bioflocculation and electromagnetic fields, as well as remediation techniques such as osmotic backwashing and gas purging. In addition, quorum sensing has been specifically discussed for biofouling remediation. The promising findings from different studies are presented followed by a discussion on their drawbacks and limitations. The review concludes with a need for carrying out fundamental studies to develop better understanding of the eco-friendly processes discussed in the penultimate section and their optimization for possible integration into the RO plants.

Removal characteristics of dissolved organic matter and membrane fouling in ultrafiltration and reverse osmosis membrane combined processes treating the secondary effluent of wastewater treatment plant

The widespread implementation of municipal wastewater treatment and reuse must first ensure the safety of reused wastewater. The effluent of the municipal wastewater treatment plant contains a large amount of dissolved organic matter (DOM), which adversely affects the reuse of wastewater. In this study, the ultrafiltration (UF) + reverse osmosis (RO) process was used to treat the secondary effluent from wastewater treatment plants. The relationship between the removal performance, membrane fouling of the UF + RO process, and DOM removal characteristics of influent were studied. The results show that DOM can be removed effectively by UF + RO process. The UF mainly removes DOM with a molecular weight greater than 10 kDa, while RO has a significant removal effect on low-molecular-weight DOM, which mainly causes UF and RO membrane fouling. The UF + RO process has a significant removal rate on fulvic acid, humic acid, tyrosine, and tryptophan, and the order is humic acid > fulvic acid > tyrosine > tryptophan. Fulvic acid contributed the most to the UF membrane fouling, while fulvic acid and protein-like proteins contributed mainly to the RO membrane fouling.

Simulating and predicting the flux change of reverse osmosis membranes over time during wastewater reclamation caused by organic fouling

During the operation of the RO system, it's significant to predict the flux change over time. Previous research conducted detailed exploration on the dynamics of RO membrane fouling, and provided a solid database for modelling. In this study, a modified intermediate blocking model with two parameters was proposed to describe the flux change of RO membranes under a huge variety of conditions. Raw data reported by over 20 research groups from 11 different countries was used to validate the feasibility of this model. It proved applicable to describe the flux change of RO membranes fouled by pure organic matter or mixture and tertiary treated wastewater. In order to reveal the relationship between model parameters and foulant concentrations, RO membrane fouling behaviors of typical foulants (sodium alginate (SA), bovine serum albumin (BSA) and mixture) were further investigated. We found that the change of model parameters with SA concentrations was in accordance with Langmuir adsorption isotherm model. Therefore, the model parameters could be calculated by SA concentrations under certain optional conditions, and then the flux change could be predicted by this model. In this way, a novel time-course model was established, which could predict the flux change of RO membranes over time only with SA concentrations. Besides, the synergic effect between SA and BSA on RO membrane fouling was directly quantified.

Enhanced simultaneous removal of nitrogen, phosphorous, hardness, and methylisothiazolinone from reverse osmosis concentrate by suspended-solid phase cultivation of Scenedesmus sp. LX1

The disposal of reverse osmosis (RO) concentrate (ROC) is a critical challenge impeding the application of RO-based wastewater reclamation. Herein, we proposed an enhanced biotreatment approach for the simultaneous removal of nitrogen, phosphorous, hardness, and methylisothiazolinone (MIT) from ROC by suspended-solid phase cultivation of Scenedesmus sp. LX1. Repeated carrier addition, guided by the developed optimal carrier addition model, efficiently enhanced algal growth and contaminant removal through dynamically controlling the suspended algal density by cell attachment. The maximum algal growth rate (212.2 mg/(L∙d)) increased by 41% compared with the control, and the time needed for reaching the maximum algal biomass (906.7 mg/L) was shortened by 1 d, attributing to the mitigation of density restriction. 91.8% of nitrogen (30.2 mg/L) was removed with 5.5 mg/(L∙d) accelerating removal rate, and phosphate (3.7 mg/L) was completely removed within 1 d. Hardness precursors calcium and inorganic carbon were also removed in large amounts, 268.4 and 128.2 mg/L, respectively. Moreover, suspended-solid phase cultivation significantly mitigated the growth inhibition caused by MIT toxicity, enabled the algae to completely biodegrade MIT of extremely high concentrations (4.7 mg/L and 11.4 mg/L) in a short time. Our results demonstrate the feasibility of suspended-solid phase algal cultivation for simultaneously and effectively removing multiple main contaminants from ROC.

Membrane fouling potential of the denitrification filter effluent and the control mechanism by ozonation in the process of wastewater reclamation

A process of denitrification filter (DNF) coupled with ultrafiltration (UF) and ozonation (DNF-UF-O₃) has been widely applied to advanced nitrogen removal for wastewater reclamation. Despite of the effective removal of nitrogen by DNF, the influence of DNF stage on the operation of UF was still unclear. In this study, a laboratory filtration system was used to investigate the membrane fouling potential of DNF effluent and the fouling control of ozonation. The membrane fouling potential was proved to be increased significantly after DNF stage and alleviated with ozonation treatment. With the help of UV-vis, fluorescence spectroscopy, scanning electron microscopy (SEM) and molecular weight (MW) analysis, the change of DOM component characteristics was proved to be in accordance with the change of fouling potential. The water samples were further fractionated into six hydrophobic/hydrophilic acidic/basic/neutral fractions, among which hydrophobic acids (HOA) and hydrophobic neutrals (HON) dominated the membrane fouling potential of DNF effluent. Detailed study of each fraction revealed that higher MW components in HOA and HON played a crucial role in the fouling of UF membrane. The dominant component of membrane fouling could be degraded and removed by ozonation, and therefore significant fouling alleviation was achieved. These results indicated that in the process of wastewater reclamation, besides conventional water quality indexes, more detailed water features should also be taken into consideration to optimize the whole process. Moreover, the control effects by ozonation could be monitored simply according to the change of specific UV absorbance (SUVA) and fluorescence intensity as surrogates in engineering applications. According to these results, a modified DNF-O₃-UF process with O₃ dosage of 3 mg/L was proposed simply by reversing the sequence of UF and O₃ with no more infrastructure. This modified DNF-O₃-UF process was expected to enlarge the produce capacity of reclaimed water with much lower electricity costs and chemical consumption.

Fouling properties of reverse osmosis membranes along the feed channel in an industrial-scale system for wastewater reclamation

Membrane fouling is an inevitable disadvantage of the reverse osmosis (RO) process for wastewater reclamation. In order to clarify the development process of membrane fouling, all the fouled membranes along a feed channel of a two-stage industrial-scale RO system for wastewater reclamation (six elements in each stage) were autopsied and analyzed. The water flux and salt rejection efficiency of the fouled membranes at the head and tail were the lowest among 12 elements, thereby indicating more severe fouling on these membranes. In this RO system, most of the organic compounds deposited on the head elements of each stage were mainly composed of proteins, polysaccharides, and fulvic acid. The ATP concentrations of the foulants on the first and twelfth elements were much higher than those of the other elements, suggesting severe biofouling. Although microbes can cause organic fouling owing to extracellular polymeric substances production, no clear correlation was found between organic fouling and biofouling in this study. For example, the ATP concentrations on the second element and seventh element were similar (1.16 ng/cm² and 1.26 ng/cm², respectively), thereby suggesting a similar extent of biofouling, but organic fouling of the second element was relatively slight (DOC: 24.8 mg/m²) compared with that of the seventh element (DOC: 46.2 mg/m²). The seventh element (ATP: 1.26 ng/cm²) was more severely biofouled than the eighth element (ATP: 0.15 ng/cm²), but they suffered from the same level of organic fouling (DOC: 46.2 mg/m² and 47.1 mg/m², respectively). Approximately 70% of metallic elements, predominantly Fe, were deposited on the first element. Although the concentration of Fe in the feed water was much lower than those of Ca and Mg, the concentration of Fe on the first three elements was significantly higher than that of any other element, suggesting that Fe was more easily deposited on the RO membranes.

Analyzing and verifying the association of spiral-wound reverse osmosis membrane fouling with different secondary effluents: full-scale experiments

In order to analyze and verify the association of the reverse osmosis (RO) membrane fouling with water quality in full-scale plants, two RO systems (40, 000 m³/d and 20, 000 m³/d) treating different secondary effluents were operated in parallel. The quality of secondary effluents and the performance of RO systems were monitored over 12 months. Difference in foulants distribution and fouling layer composition between the two systems were evaluated by membrane autopsy and foulants characterization. Results verified that: 1) the secondary effluent from municipal sewage caused more serious membrane fouling; 2) more foulants deposited on the surface of leading membrane both in two systems (3.11 ± 0.15 g/m² and 2.93 ± 0.13 g/m²); 3) the microbial community on the RO membrane surface contained more colonizing bacteria in the system treating municipal sewage secondary effluent ; 4) organics in the secondary effluent facilitated biofouling while higher ion concentration restrained biofouling.

Integration of an anaerobic fluidized-bed membrane bioreactor (MBR) with zeolite adsorption and reverse osmosis (RO) for municipal wastewater reclamation: Comparison with an anoxic-aerobic MBR coupled with RO

This study compared the performance of an anaerobic fluidized bed membrane bioreactor (AFMBR)-zeolite adsorption-reverse osmosis (RO) system and an anoxic-aerobic MBR-RO system for municipal wastewater reclamation. Both MBR-RO systems were operated in parallel with the same operating conditions. The results showed that the MBR systems achieved excellent organic removals (>95%) and the anoxic-aerobic MBR could also remove ∼57% of soluble total nitrogen. Compared to the aerobic MBR, the AFMBR displayed better membrane performance with less energy consumption, attributed to effective membrane scouring by liquid-fluidized GAC particles. Furthermore, a zeolite column was employed to remove ammonia in the AFMBR permeate, which ensured comparable organic and nitrogen levels in the feeds to RO units in the two processes. Although less organic substances and microbial cells were accumulated on the RO membrane fed with AFMBR-zeolite column effluent, its fouling rate (∼6.5 ± 2.2 bar/day) was significantly greater than that fed with anoxic-aerobic MBR permeate (∼1.1 ± 1.5 bar/day). This may be associated with more severe inorganic colloidal fouling on the RO membrane, illustrated by an electrical impedance spectroscopy fouling monitoring system.

Fluorescence characterization of organic matter and fouling: Case study in a full-scale reverse osmosis membrane treatment plant

Membranes used for water treatment are subject to organic fouling, caused by organic matter in source water. Characterizing organic matter has the potential to improve fouling prediction since the development of an organic fouling layer on the membrane is dependent on the specific characteristics of the organic matter. A field study was performed at a full-scale reverse osmosis water treatment plant that treats secondary wastewater effluent for industrial reuse at a power plant. Samples were collected at various points within the treatment process and were analyzed for turbidity, total organic carbon (TOC), conductivity, and fluorescence Excitation Emission Matrices (EEM). Parallel factor analysis (PARAFAC) was used to generate representative fluorescence measurements of the organic matter. Results indicate that TOC and fluorescence measurements were effective in differentiating between two observed fouling periods at multiple locations within the treatment plant. However, none of the water quality measurements were effective in tracking treatability of organic matter throughout pretreatment. The results of this case study provide important information about the relationship between fluorescence NOM signals and membrane fouling that can be used in future online detection systems. PRACTITIONER POINTS: TOC and fluorescence measurements were effective in differentiating between the high fouling and low fouling periods. Water quality measurements were not effective in tracking changes in organic matter throughout pretreatment. Implementation of online fluorescence monitoring of fouling potential could be used for real-time process control.

Effect of the membrane exclusion mechanism on phosphate scaling during synthetic effluent desalination

Calcium phosphate scaling is one of the main limitations in effluent desalination using membranes. This may be overcome by tailoring membranes with lower rejection of the scalant ions. In this study, we systematically examined the use of negatively and positively charged membranes, rejecting ions mainly based on Donnan exclusion, as a low-scaling alternative to dielectric-exclusion-dominated polyamide NF membranes for effluent desalination. The two charged membranes exhibited a lower calcium and especially phosphate rejection than the polyamide membrane. Consequently, the calcium phosphate supersaturation and then the propensity to scaling of the charged membranes were much lower than the polyamide membrane. This also allowed filtering at a much higher recovery ratio with the charged membranes. It was also found that, despite the fact that the charged membranes had an opposite fixed charge, their scaling behavior was similar. Apparently, although these membranes showed opposite selectivity towards scalant ions (phosphate and calcium) in single salt solutions, the rejection pattern in mixed salt solutions resulted in similar saturation indices, much lower than for polyamide membrane. The scale formed on all three membranes was identified as amorphous calcium phosphate (ACP), although its saturation index was lower than its solubility factor. This was explained by concentration polarization which increases the saturation index in the solution adjacent to the membrane surface. Tests in absence of permeate flux showed a much slower precipitation that took a few days compared with filtration conditions (few hours). In addition, under these conditions, the effect of the scaling on the membrane permeability was generally reduced and the scale contained crystalline calcium phosphate products, different from ACP. The results indicate that the ion rejection and resulting polarization next to the membrane surface plays a crucial role in scaling. Thus, tuning ion selectivity of NF membranes towards scalant ions presents a promising alternative for scaling mitigation during effluent desalination.

A Pilot Study of an Electromagnetic Field for Control of Reverse Osmosis Membrane Fouling and Scaling During Brackish Groundwater Desalination

This study investigated the effects of an electromagnetic field (EMF) on control of membrane fouling and scaling during desalination of brackish groundwater using a pilot reverse osmosis (RO) skid. The groundwater was primarily CaSO4 type with a total dissolved solids concentration of 5850 mg/L and hardness of 2500 mg/L as CaCO3. Two EMF devices were installed in the pipeline before a cartridge filter and in the RO feed inlet to induce an electric signal of ±150 kHz to the groundwater. The effects of EMF on membrane scaling were evaluated under accelerated conditions, i.e., without pH adjustment and addition of antiscalant. Two-phase experiments were conducted: Phase 1 (376 h) with the EMF devices turned on after 150 h baseline operation; and Phase 2 (753 h) with the EMF devices turned on from the beginning of testing. The EMF significantly reduced membrane scaling and improved RO performance by 38.3% and 14.3% in terms of normalized water permeability decline rate after 150 h and 370 h operation, respectively. Membrane autopsy results indicated that the fouling layer formed under the influence of EMF was loose with a low density and was easily removed by hydraulic flushing.

Chlorine disinfection significantly aggravated the biofouling of reverse osmosis membrane used for municipal wastewater reclamation

In reverse osmosis (RO) system for wastewater reclamation, biofouling is an inevitable issue. Chlorine disinfection is commonly used in pretreatment to control biofouling. Some chlorine-resistant bacteria could survive after chlorine disinfection and the microbial community structure in feed water changes significantly, thus leading to the change of biofouling potential. In this study, the effect of chlorine disinfection on the biofouling of RO membrane was investigated using a laboratory cross-flow RO system. Chlorine disinfection inactivated most bacteria in feed water. However, during the operation of RO system, with the increase of chlorine dosage the flux decline became more severe after a period of operation. The final normalized flux after 21 days was 0.27, 0.26, 0.20, and 0.21 with 0, 1, 5, and 15 mg-Cl₂/L chlorine as pretreatment, respectively. After the operation, the numbers of active bacteria in the foulants on the fouled membrane were on the same level regardless of the chlorine dosage, whereas the thickness of the foulants increased with the chlorine dosage significantly. Additionally, the higher total organic carbon concentration indicated more extracellular polymeric substances (EPS) in foulants. Microbial community structure analysis showed that the abundance and the species number of chlorine-resistant bacteria increased significantly with the chlorine dosage. Typical chlorine-resistant bacteria, including Methylobacterium, Pseudomonas, Sphingomonas, and Acinetobacter, were identified as significantly distinctive genera in the foulants after the pretreatment by 15 mg-Cl₂/L chlorine. Compared with the bacteria without chlorine disinfection, these remaining bacteria produced more EPS with higher molecular weight, which could be the major contribution to more severe RO membrane fouling after chlorine disinfection.

Evaluating membrane fouling potentials of dissolved organic matter in brackish water

Isolating dissolved organic matter (DOM) is a preliminary step that improves the accuracy of its characterization. In this study, DOM in brackish water was clearly separated and evaluated by multiple characterization analyses. The sample was divided into three fractions by preparative high-performance liquid chromatography (preparative HPLC) according to molecular size. The homogeneity of each fraction was estimated by analytical size exclusion chromatography (SEC) and fluorescence excitation-emission matrix (FEEM). Pyrolysis gas chromatography-mass spectrometry (Py-GC/MS) and liquid chromatography-organic carbon detection (LC-OCD) were used to characterize the physicochemical properties of each fraction. Py-GC/MS revealed that Fraction 1 consisted of evenly distributed organic matter in order polysaccharides, proteins, polyhydroxy aromatics, lignins, and lipids. However, Fraction 2 was primarily composed of dominant lipids and low portion of proteins, and Fraction 3 was composed predominantly of lignins and lipids. The LC-OCD results showed that Fractions 1 and 2 had similar organic carbon (OC) compositions: a humic substance (ca. 37%), building blocks (ca. 10%), and neutrals (ca. 37%), whereas Fraction 3 contained a high proportion of neutrals (62%). In the fouling experiments, the distinct DOM characteristics in each fraction resulted in different declining flux behaviors, ranked as: Fraction 2 > Fraction 1 > Fraction 3.

Inorganic fouling control in reverse osmosis wastewater reclamation by purging carbon dioxide

Inorganic fouling on the membrane surface is one of the major prevalent issues affecting the performance and cost of reverse osmosis system. Chemical dosage is a widely adopted method for the inhibition of inorganic scale on the membrane surface. In this study, CO₂ was used to control inorganic scale formation on surface of reverse osmosis (RO) membrane in wastewater reclamation. The pH of influent could be lowered by purging CO₂. It caused an increase in solubility of inorganic salts in water resulting in discharge of principle ions in concentrate stream. A pilot plant study was conducted with four different RO modules including control, with dosage of antiscalant, with purging CO₂ and with co-addition of antiscalant and CO₂. The effectiveness of CO₂ purging was assessed on the basis of operational analysis, in-line analysis and morphological results. Ryznar stability index was used to determine the scaling potential of system. The examined data indicated that CO₂ purging was successful to inhibit scale formation on the membrane surface. Moreover, CO₂ was found more eco-friendly than antiscalant, as no by-products were generated in concentrate stream.

Nanofiltration performances after membrane bioreactor for hospital wastewater treatment: Fouling mechanisms and the quantitative link between stable fluxes and the water matrix

Treatment combining membrane bioreactors (MBR) and nanofiltration (NF) is becoming an emerging wastewater treatment strategy. The combined process is capable of producing high quality water potentially reusable; however, diverse compositions of MBR effluents induce several types and degrees of NF membrane fouling that impacts process productivity. Moreover, since MBR effluent composition for one type of wastewater source is variable depending on the MBR efficiency at different periods, downstream NF membrane fouling types and degrees may consequently change over time. In that context, the present paper aims at developing effective fouling control strategies of NF membrane in the case of the filtration of MBR effluents taken from a MBR system installed in a French hospital. These effluents were filtrated under various transmembrane pressures, and stable fluxes during these filtrations were determined. Several types and degrees of fouling mechanisms were then identified through surface morphology observation and the analysis of chemical compositions of fouled membranes. The diverse flux behaviour was further associated with the fouling mechanisms and foulant compositions. Based on the study of these mechanisms, the quantitative link between stable fluxes and calcium phosphate concentrations in MBR effluents has been established.

Tolerance and resistance characteristics of microalgae Scenedesmus sp. LX1 to methylisothiazolinone

Methylisothiazolinone (MIT) has been widely used to control bacterial growth in reverse osmosis (RO) systems. However, MIT's toxicity on microalgae should be determined because residual MIT is concentrated into RO concentrate (ROC) and might have a severe impact on microalgae-based ROC treatment. This study investigated the tolerance of Scenedesmus sp. LX1 to MIT and revealed the mechanism of algal growth inhibition and toxicity resistance. Scenedesmus sp. LX1 was inhibited by MIT with a half-maximal effective concentration at 72 h (72 h-EC₅₀) of 1.00 mg/L, but the strain recovered from the inhibition when its growth was not completely inhibited. It was observed that this inhibition's effect on subsequent growth was weak, and the removal of MIT was the primary reason for the recovery. Properly increasing the initial algal density significantly shortened the adaptation time for accelerated recovery in a MIT-containing culture. Photosynthesis damage by MIT was one of the primary reasons for growth inhibition, but microalgal cell respiration and adenosine triphosphate (ATP) synthesis were not completely inhibited, and the algae were still alive even when growth was completely inhibited, which was notably different from observations made with bacteria and fungi. The algae synthesized more chlorophyll, antioxidant enzymes of superoxide dismutase (SOD) and catalase (CAT), and small molecules, such as reduced glutathione (GSH), to resist MIT poisoning. The microalgae-based process could treat the MIT-containing ROC, since MIT was added for only several hours a week in municipal wastewater reclamation RO processes, and the MIT average concentration was considerably lower than the maximum concentration that algae could tolerate.

A review of reverse osmosis membrane fouling and control strategies

Reverse osmosis (RO) membrane technology is one of the most important technologies for water treatment. However, membrane fouling is an inevitable issue. Membrane fouling leads to higher operating pressure, flux decline, frequent chemical cleaning and shorter membrane life. This paper reviews membrane fouling types and fouling control strategies, with a focus on the latest developments. The fundamentals of fouling are discussed in detail, including biofouling, organic fouling, inorganic scaling and colloidal fouling. Furthermore, fouling mitigation technologies are also discussed comprehensively. Pretreatment is widely used in practice to reduce the burden for the following RO operation while real time monitoring of RO has the advantage and potential of providing support for effective and efficient cleaning. Surface modification could slow down membrane fouling by changing surface properties such as surface smoothness and hydrophilicity, while novel membrane materials and synthesis processes build a promising future for the next generation of RO membranes with big advancements in fouling resistance. Especially in this review paper, statistical analysis is conducted where appropriate to reveal the research interests in RO fouling and control.

Effects of chemical cleaning on RO membrane inorganic, organic and microbial foulant removal in a full-scale plant for municipal wastewater reclamation

Of all of the strategies for controlling reverse osmosis (RO) membrane fouling, chemical cleaning is indispensable. To study the effects of chemical cleaning on membrane foulant removal, a comparative analysis of RO membranes before and after common alkaline and acid cleaning was conducted by dissecting lead and terminal RO membranes in a full-scale municipal wastewater reclamation plant. Most foulants on the membranes were removed by chemical cleaning processes. Calcium was the major inorganic component of the foulants because of its highest concentration in the feed water. Aluminum and iron were also abundant elements on the membranes due to their high deposition ratios and low removal efficiencies. Hydrophilic neutrals (HIN) and hydrophobic neutrals (HON) were the two largest dissolved organic matter (DOM) fractions on the membranes before cleaning. HIN and hydrophilic acids (HIA) were not effectively removed. Chemical cleaning removed 94% and 90% of the total bacteria on the lead and tail membranes and considerably changed the structure of the microbial communities. Bacteria excessively producing extracellular polymeric substance (EPS), such as Pseudomonas and Zoogloea, were much more resistant to the chemical cleaning process. After cleaning, the membrane microbial community structures were more similar to those in the feed water than the structures on the membranes before cleaning. These results shed light on the effects of cleaning in a full-scale RO plant, improves our understanding of the removal of foulants and provides potential research directions for cleaning methods and RO pretreatment processes.

Advanced treatment of municipal secondary effluent by catalytic ozonation using Fe3O4-CeO2/MWCNTs as efficient catalyst

The advanced treatment of municipal secondary effluent was performed by catalytic ozonation using Fe₃O₄-CeO₂/MWCNTs as catalyst. The experimental results showed that in catalytic ozonation system, the removal efficiency of soluble COD was more than 46% after 30 min reaction, and about 36% of effluent organic matters (EfOMs) were mineralized, which was four times higher than that in single ozonation system. Moreover, proteins, humic acids, and UV₂₅₄ decreased obviously after 30 min reaction, but polysaccharides did not significantly decrease. In catalytic ozonation system, the ozone utilization increased, which is favorable for the degradation of EfOM. The organic compounds and alkalinity were the main hydroxyl radical consumers in municipal secondary effluent. The catalytic ozonation process was also effective for the degradation of two target micropollutants (sulfamethazine and carbamazepine). The catalyst could be stable after five-time reuse for catalytic ozonation of effluent.

Performance of electrodialysis reversal and reverse osmosis for reclaiming wastewater from high-tech industrial parks in Taiwan: A pilot-scale study

Wastewater reclamation is considered an absolute necessity in Taiwan, as numerous industrial parks experience water shortage. However, the water quality of secondary treated effluents from sewage treatment plants generally does not meet the requirements of industrial water use because of the high inorganic constituents. This paper reports experimental data from a pilot-plant study of two treatment processes-(i) fiber filtration (FF)-ultrafiltration (UF)-reverse osmosis (RO) and (ii) sand filtration (SF)-electrodialysis reversal (EDR)-for treating industrial high conductivity effluents from the Xianxi wastewater treatment plant in Taiwan. The results demonstrated that FF-UF was excellent for turbidity removal and it was a suitable pretreatment process for RO. The influence of two membrane materials on the operating characteristics and process stability of the UF process was determined. The treatment performance of FF-UF-RO was higher than that of SF-EDR with an average desalination rate of 97%, a permeate conductivity of 272.7 ± 32.0, turbidity of 0.183 ± 0.02 NTU and a chemical oxigen demand of <4.5 mg/L. The cost analysis for both processes in a water reclamation plant of 4000 m³/d capacity revealed that using FF-UF-RO had a lower treatment cost than using SF-EDR, which required activated carbon filtration as a post treatment process. On the basis of the results in this study, the FF-UF-RO system is recommended as a potential process for additional applications.

A review of polymeric membranes and processes for potable water reuse

Conventional water resources in many regions are insufficient to meet the water needs of growing populations, thus reuse is gaining acceptance as a method of water supply augmentation. Recent advancements in membrane technology have allowed for the reclamation of municipal wastewater for the production of drinking water, i.e., potable reuse. Although public perception can be a challenge, potable reuse is often the least energy-intensive method of providing additional drinking water to water stressed regions. A variety of membranes have been developed that can remove water contaminants ranging from particles and pathogens to dissolved organic compounds and salts. Typically, potable reuse treatment plants use polymeric membranes for microfiltration or ultrafiltration in conjunction with reverse osmosis and, in some cases, nanofiltration. Membrane properties, including pore size, wettability, surface charge, roughness, thermal resistance, chemical stability, permeability, thickness and mechanical strength, vary between membranes and applications. Advancements in membrane technology including new membrane materials, coatings, and manufacturing methods, as well as emerging membrane processes such as membrane bioreactors, electrodialysis, and forward osmosis have been developed to improve selectivity, energy consumption, fouling resistance, and/or capital cost. The purpose of this review is to provide a comprehensive summary of the role of polymeric membranes in the treatment of wastewater to potable water quality and highlight recent advancements in separation processes. Beyond membranes themselves, this review covers the background and history of potable reuse, and commonly used potable reuse process chains, pretreatment steps, and advanced oxidation processes. Key trends in membrane technology include novel configurations, materials and fouling prevention techniques. Challenges still facing membrane-based potable reuse applications, including chemical and biological contaminant removal, membrane fouling, and public perception, are highlighted as areas in need of further research and development.