► Hybrid Membrane Process (HMP) combines activated carbon with low-pressure membranes. ► Performances of the HMP are reviewed (water quality and membrane fouling). ► Activated carbon as adsorbent: ...high performance but higher costs. ► Biological activated carbon: promising results for ammonia and NOM removal. ► Optimal operating conditions in HMP have not been clearly delineated yet.
More stringent regulations on drinking water quality as well as an increased focus on emerging contaminants has favored the development of alternative technologies to the conventional process (clarification+filtration+chlorination). Over the last decade, low-pressure membrane filtration coupled with activated carbon has been emerging as a promising solution, often termed as the Hybrid Membrane Process. Combining activated carbon with membanes presents numerous challenges including membrane abrasion, membrane fouling, optimization of operating conditions, prediction of process performances and selection of the process configuration. This paper presents a review of the current knowledge concerning the use of the Hybrid Membrane Process, applied either under a biological or an adsorption mode, in view to produce drinking water. The paper reviews the alternative process layouts and discuss their expected performances with respect to water quality and membrane fouling.
Sodium hypochlorite is commonly used as a cleaning agent to remove adsorbed foulants from PVDF based micro/ultra filtration membranes in water and wastewater treatment applications. Although ...effective for fouling control, extended sodium hypochlorite exposure can affect the physical/chemical characteristics and hinder the treatment performance of these membranes. To assess these effects, PVDF based membranes were exposed to sodium hypochlorite at different concentrations for varying periods of time, and the physical/chemical characteristics of the virgin and sodium hypochlorite exposed membranes were compared. The membranes were characterized based on chemical composition (FTIR and NMR), mechanical strength (yield strength), surface hydrophilicity (contact angle), pore size and porosity (scanning electron microscopy and challenge test), and membrane resistance (clean water permeation test). The results indicated that exposure dose and concentration of the sodium hypochlorite used have significant influence on the membrane characteristics. The impact of sodium hypochlorite exposure on the parameters investigated could be most accurately and consistently correlated to an exposure dose relationship of the form Cnt (where, C = concentration and t = exposure time) rather than the Ct relationship commonly used to define the extent of exposure to cleaning agents. For all the parameters investigated, the power coefficient n was less than 1 indicating that time had a greater impact on the changes than did the concentration of the sodium hypochlorite. The results suggest that the use of sodium hypochlorite for chemical cleaning, at concentrations that are higher than those typically used for chemical cleaning would have less of an effect on the characteristics of the membrane materials. Changes in the characteristics were attributed to the oxidation of the hydrophilic additives (HA) present in blended PVDF membranes.
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•Exposure dose and concentration of NaOCl used affected the membrane characteristics.•The changes were attributed to the oxidation of the membrane's hydrophilic additives.•The changes could be correlated to an exposure dose relationship of the form Cnt.•Exposure time had a greater impact on the changes than did the concentration of NaOCl.•For a given exposure dose, lower concentration had greater impact on the changes.
Membrane filtration is a rapidly expanding choice for drinking water treatment. Unfortunately, there is limited data on long-term changes in the membranes’ performance as they age. The present ...research investigated changes in performance factors as well as chemical characteristics for hollow-fibre ultrafiltration membranes that ranged in age from 8 full-scale drinking water treatment plants. Membranes were harvested by plant operators regularly and analyzed using standardized laboratory tests. Approximately half of the membranes were a new PVDF-based chemistry. These were observed to have insignificant changes in performance factors and chemical characteristics since their beginning of operation. However, because these membranes were newer, only data for the first 5 years of operation was available. The other half of the membranes, with an older PVDF-based chemistry, were observed to have stable behaviour until approximately 5 years of operation; after this time, performance factors and chemical characteristics of the membranes began to change significantly. For these membranes, the clean water resistance and fouling rate increased after 5 years of operation. The mechanical properties of these membranes also deteriorated after 5 years of operation, suggesting that their susceptibility to breach is higher after prolonged use. These changes in performance factors paralleled, and were possibly caused by, the removal of hydrophilic additives from the membrane material. Clean water resistance was identified as a good benchmark for all the parameters studied, a finding that is useful for water treatment facilities in quickly assessing the status of their membranes. Finally, although cumulative exposure dose (C*t) was not used as a metric of membrane age, we observed that when higher doses of hypochlorite were applied, all metrics changed faster than expected based only on years of operation. Therefore, limiting the magnitude of the cumulative hypochlorite dose is essential in managing membrane deterioration. This research illuminates the knowledge gap between bench-scale ageing studies and operational water treatment plants.
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•Impacts of ultrafiltration membrane ageing are observed after 5 years of operation.•Ageing is evident in both membrane performance and chemical characteristics.•Changing resistance is a good indicator of changes in other membrane properties.•Membranes subjected to high hypochlorite dose have the most extensive ageing.
Groundwater (GW) is one of the most abundant water resources and around 1.5 billion people rely on GW as their main water supply. Manganese (Mn) and iron (Fe) are very common GW contaminants. Even ...though their presence is considered mainly as an organoleptic and operational nuisance, water with elevated Mn content may also lead to adverse health impacts. Amongst the most common treatment processes currently used to treat domestic GW supplies: catalytic filtration may lead to Mn leaching if improperly maintained; while ion exchange consumes a considerable amount of salt and produces a brine waste which pollutes the environment. Thus, it is proposed to design a simple, yet robust treatment system which can be implemented in small/remote communities or even domestic applications. To this end, the main objective of this investigation was to assess the potential application of novel outside-in sulfonated polyethersulfone thin-film composite hollow fiber nanofiltration (HFNF) membranes to remove dissolved Mn, Fe and natural organic matter (NOM) from domestic GW supplies. Of particular interest was the impact of GW matrix on performance of the HFNF membranes. Our experimental findings demonstrated that, in absence of hardness and the cumulative throughput of 1.9 L/m2, above 90% of Mn, Fe and NOM were retained by the examined HFNF membranes (MWCO ∼ 200 Da) regardless of their initial concentrations in the feed solution (250–1000 μ g/L). In contrast, increasing the hardness level reduced the removal of Mn and Fe ions. XPS analysis revealed that the surface properties of the HFNF membranes were altered when the membranes were exposed to calcium and magnesium salts. These observations were attributed to the propensity for Ca and Mg ions to bind to the sulfonic groups present on the surface of the HFNF membranes which, subsequently, weakens rejection by charge exclusion. On the other hand, in the absence of GW hardness, charge exclusion was mainly responsible for rejection of dissolved Mn and Fe. It was also found that GW hardness had no marked impact on the NOM rejection as the later was mostly removed by size exclusion.
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•HFNF removed above 90% of Mn, Fe and NOM from the model solutions.•No fouling was observed due to the smooth surface of the HFNF membranes and applied hydrodynamic conditions.•Formation of Ca-membrane and Mg-membrane complexes decreased Mn and Fe rejections.•Hardness level of the synthetic and natural GW solutions had no noted effect on NOM retention.•Sieving effect was the main rejection mechanism for NOM removal via HFNF membranes.
► Influence of fluid dynamics on fouling in MBRs. ► Review of experimental techniques and set-ups. ► Numerical and modelling approaches. ► Aspects of fluid dynamic membrane tank and module ...optimization.
The importance of the multiphase fluid dynamics for fouling mitigation in MBR systems has been widely acknowledged with air sparging having been applied commercially for about 20years. However, the effects of air scouring are still not fully understood since the transient orthogonal and parallel flows as well as turbulent eddies created by bubbling generate complex hydrodynamic flow fields in the vicinity of a membrane. There is no generally valid model that describes the relationship between fouling rate and fluid dynamics. So, a reliable and universally applicable model to optimize membrane module and tank geometries, air scouring and filtration cycles is still pending.
In addition to providing a discussion on the importance of multiphase fluid dynamics for fouling control, this review aims at developing guidelines to choose appropriate experimental and numerical methods for fluid dynamics investigations in MBR systems.
The presence of natural organic matter (NOM) in drinking water treatment presents many challenges. Integrated treatment processes combining oxidation and biofiltration have been demonstrated to be ...very effective at reducing NOM, specifically biodegradable organics. Laboratory bench-scale experiments were carried out to investigate the effect of oxidation by ozonation or UV/H2O2 on NOM. Specifically the rate of biodegradation was studied by performing bench-scale biodegradation experiments using acclimatized biological activated carbon (BAC). For the source water investigated, oxidation did not preferentially react with the biodegradable or non-biodegradable NOM. In addition, the type or dose of oxidation applied did not affect the observed rate of biodegradation. The rate kinetics for biodegradation were constant for all oxidation conditions investigated. Oxidation prior to biofiltration increased the overall removal of organic matter, but did not affect the rate of biodegradation of NOM.
Anionic exchange is an effective treatment option for the removal of natural organic matter from surface waters. However, the management of the spent brine regenerant often limits the adoption of ...this process. The current study reports one year of operation of ion exchange resins under biological mode (BIEX, i.e. without regeneration to promote biofilm growth on the media) compared to the performance of (i) ion exchange with weekly regeneration (IEX), (ii) granular activated carbon under biological mode (BAC) and (ii) granular activated carbon under adsorption mode (GAC). Four parallel pilot filters (GAC, BAC, IEX and BIEX) were fed with a colored and turbid river water without pretreatment. Although IEX provided the best performance (80% DOC removal) throughout the study, BIEX achieved a similar performance to IEX prior to DOC breakthrough (92 days) and subsequently achieved a mean DOC removal of 62% in warm water conditions. The GAC filter was rapidly exhausted (2 weeks) while the BAC filter only provided a 5% DOC reduction. Full nitrification was observed on both the BIEX and BAC filters under warm water conditions (>15 °C). After one year of operation, BIEX was successfully regenerated with brine. According to a mass balance, 69% of DOC removal in BIEX was due to ion exchange while we assume the remainder was biodegraded. Operation of ion exchange in biological mode is a promising option to reduce spent brine production while still achieving high DOC removal.
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•In the biological IEX filter, DOC breakthrough occurred after 60 days.•After DOC breakthrough, BIEX reduced DOC from 7 mg C/L to 2–3 mg C/L in warm water.•Nitrification in warm water was as efficient in BIEX filters as in BAC filters.•BIEX media was successfully regenerated after 331 days of operation.•NOM removal in BIEX was mostly (69%) due to ion exchange.
•Affinity of the aged (NaOCl exposure) membranes for foulants increased significantly.•NaOCl Concentration and the duration of exposure affected the filtration performance.•Ageing affected rate and ...extent of the permeability recovery during chemical cleaning.•A new non-destructive and continuous tool to evaluate membrane ageing is proposed.
In the present research, laboratory-scale tests were conducted to assess the impact of membrane ageing, defined as the cumulative exposure to sodium hypochlorite, on the filtration and cleaning performances of PVDF-based membranes. Filtration tests using virgin and laboratory aged (i.e., previously exposed to sodium hypochlorite) membranes were conducted using activated sludge as a model foulant solution. As membranes age, both the filtration and cleaning performances were negatively impacted. At a given exposure dose of sodium hypochlorite, the impact of ageing was greater at a lower exposure concentrations and larger exposure times, indicating that the relationship commonly used to quantify age and assess the cumulative impact of previous exposure to sodium hypochlorite (i.e., Ct value) on the state of membranes is not appropriate. A new non-destructive technique is proposed to quantify the state of a membrane, and potentially gain insight into the need for membrane replacement, based on the rate of permeability recovery during extended cleaning chemical.
Fouling and cleaning of water treatment ultrafiltration membranes are well understood, but long-term changes (i.e. ageing) have not been fully explored. Changes in physical and chemical ...characteristics, and changes in membrane performance, have been attributed to ageing; however, the tools to probe these effects and their causality are limited. Most membrane ageing research focuses on bench-scale ageing of membranes, with scant attention to full-scale ageing or how bench-scale ageing can be useful in understanding full-scale ageing.
The present research links membrane ageing changes observed at bench-scale with changes observed in previous full-scale studies. Two bench-scale ageing techniques were used to probe changes in performance and characteristics: soaking membranes in NaOCl as the cleaning agent, and cycling them with foulant and the cleaning agent. Changes in membrane chemistry were similar for both bench-scale and full-scale ageing, but performance differed. Membrane process complexity increases from bench-scale soaking, to bench-scale cycled operation, to full-scale operation. For some properties, the progression due to ageing differs depending on process complexity. By comparing the two bench-scale techniques with the full-scale ageing findings, it was established that irreversible foulants are critical components of full-scale ageing, and the impact of these foulants can be approximated to some extent by cycling membranes at bench-scale.
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•Experimental conditions for mimicking full-scale ageing at bench-scale were assessed.•Fouling causes higher resistance in full-scale than bench-scale aged membranes.•Membranes cycled with foulant/NaOCl perform more like full-scale than if only soaked.•Bench-scale membrane ageing effectively mimics hydrophilic additive removal.
Biological ion exchange (BIEX) has proved to remove natural organic matter (NOM) better than biological activated carbon (BAC). This raises the question if BIEX can be integrated into a full-scale ...drinking water treatment plant to remove NOM and ammonia. In this study, a pilot plant consisting of one BIEX filter, three GAC filters and one BAC filter was set up as second-stage filtration at the Sainte-Rose drinking water treatment plant (Laval, Canada). The pilot plant was operated for a period of nine months without regeneration of the ion exchange resins. The influent water showed low DOC (2.5 mg/L) and high sulfate concentrations (28.2 mg/L). Except of a short peak of DOC released at about 1 000 BV, the BIEX filter achieved a nearly constant removal of 29–36% over the whole study period. The DOC removals of GAC were similar to BIEX at < 8000 BV but then stabilized at 13–24% after 8 000 BV. Most DOC removal in the BIEX filter was achieved at the top 30 cm layer (81%) compared to 62–66% removal in the GAC/BAC filters in the same layer. After the rapid exhaustion of the primary ion exchange capacity (<1 000 BV), sulfate displaced the fraction of NOM with lower affinity than sulfate, corresponding to the initial DOC release in the BIEX filter. The fraction of NOM with higher affinity than sulfate can still replace sulfate, which explains the good long-term performance of the BIEX filter. BIEX released ammonia with an average of 15% in warm water condition, probably related to the small diameter of the column which limited backwash effectiveness.
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•BIEX filter was operated for nine months without regeneration of the resins.•BIEX filter achieved better NOM removal compared to GAC/BAC filters.•NOM removal in BIEX filter was mainly due to the secondary ion exchange.•BIEX filter released ammonia in warm water condition.
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