Humic Acid Retention Research Articles

BiOCl as the role of cleaner for the fabrication of self-cleaning PES membranes based on the RTIPS method

Due to its cost-effectiveness and productivity, membrane separation technology is widely utilized in the treatment of wastewater, but a major barrier to its further development is organic membrane fouling, which is mainly caused by humic acid (HA). To address this issue, BiOCl/polyvinylpyrrolidone (PVP) composites were produced using the ambient stirring method, while polyethersulfone (PES)/BiOCl/PVP self-cleaning membranes were manufactured using the reverse thermally induced phase separation (RTIPS) method. Considering the pure water flux of 3566.67 L m−2 h−1 and HA retention of 87.24 %, the 0.3 wt% PES/BiOCl/PVP (PB2–65) composite membrane had excellent permeability. The flux recovery of the composite membrane was 76.19 % after three photocatalytic cycles, and cycling experiments proved that the PB2–65 self-cleaning mechanism was stable. The performance of PB2–65 was not compromised when the use cycle was noticeably prolonged under light irradiation. The membranes in this study offer beneficial strategies and solutions for membrane contamination issues.

Copper ferrite modified catalytic ceramic membrane for effective pollutant degradation and membrane fouling alleviation: The coupling of peroxymonosulfate activation and membrane filtration

In this study, a copper ferrite decorated ceramic membrane (CFCM) for peroxymonosulfate (PMS) activation was synthesized, and the dual function of catalytic oxidation and membrane filtration on organics removal and fouling mitigation was systematically investigated. Characterization results demonstrated that the catalyst particles were uniformly anchored throughout the whole membrane. The catalytic membrane exhibited significantly improved bisphenol A (BPA) degradation and humic acid (HA) retention compared to the conventional CM filtration. SO4− and 1O2 were considered to be the main reactive species for BPA removal in the CFCM/PMS system by quenching experiments and EPR tests, and two possible degradation pathways were proposed. CFCM/PMS filtration showed superior antifouling property with reduced reversible and irreversible fouling resistances. Membrane performance evaluation in BPA-HA removal showed that the competition of HA for reactive radicals and the coverage of catalytic active sites by HA could lead to the decrease of BPA removal while the cake layer formed on membrane surface would increase HA retention. In addition, stable catalytic activity after 8 cycles, high removal efficiency for various organic pollutants and low metals leaching revealed the excellent applicability of CFCM in wastewater treatments.

Water and humic acid transport in graphene-derived membrane: Mechanisms and implications to functional membrane design

Graphene-derived membranes (GDM) have drawn broad attentions due to their superb water permeation and contaminant retention capabilities. In this paper, the transport of model humic acid (HA) and water through GDM is studied using the molecular dynamics simulation approach. By systematically varying the positions of functional groups and membrane pore dimensions, we find that the introduction of functional groups at the slit edge, the basal plane or the interlayer of bilayer graphene nanosheets reduces water permeation through the membranes to significantly different extents. Comparatively, -COO- at the slit edge of GE nanosheet produces the strongest electrostatic interaction for enhanced HA retention and greater water permeability than graphene oxide membranes. Furthermore, HA adsorbed inside GDM may cause membrane fouling due to pore blocking at the slit entrance or pore constriction inside the slit or the interlayer. Meanwhile, the reversibility of membrane fouling depends upon HA desorption, which is easier for HA anchored via functional groups than stacked via π-π interaction onto graphene surfaces. Our findings emphasize that the performance of functional GDM for water treatment is highly dependent upon the specific position of chemical modification, which has been commonly overlooked in previous studies.

Effects of Hydrogen Peroxide and Sodium Hypochlorite Aging on Properties and Performance of Polyethersulfone Ultrafiltration Membrane.

Chemical reaction of main polymer and additive with oxidative cleaning agents plays an important role in aging of polymeric membrane for water and wastewater treatment. As a green and powerful oxidant, hydrogen peroxide (H2O2) can achieve good cleaning efficacy under alkaline condition, but its influence on membrane aging was poorly understood. In this study, degradation of polyethersulfone (PES) membrane due to H2O2 exposure under alkaline condition (pH 9 and 11) was holistically investigated by humic acid (HA) filtration experiments and multiple membrane characterization techniques, with sodium hypochlorite (NaClO) aging examined as a comparison. Membrane permeability and HA retention rate was hardly changed by H2O2 aging at an exposure dose of 500 g·h/L, whereas NaClO aging led to substantial increase of membrane permeability and significant decrease of retention ability. Meanwhile, H2O2 aging slightly increased fouling propensity with HA filtration, while NaClO aging resulted in more serious fouling. ATR-FTIR and XPS analysis revealed much less degradation of PES and hydrophilic additive by H2O2 than that by NaClO, and membrane morphology and surface properties were characterized to explain the variation of filtration performance. Overall, compared with cleaning with NaClO, membrane degradation can be minimized by cleaning with H2O2.

Studies of Humic Acid Removal from Aqueous Systems by Using Polymeric Membrane Ultrafiltration Process

This paper proposes to investigate in detail the behavior of polymeric membranes ultrafiltration process uses in the retention of humic acid from aqueous systems. Nowadays, the aqueous systems (contaminated water and/or wastewater) resulting from various domestic and industrial activities contain many contaminants, some of them increasingly dangerous to the environment. Laboratory testing of membranes in the ultrafiltration process as well as their experimentation with different chemical compounds is essential because it is important to know how they can be eliminated before they cause a negative impact. Testing of polymeric membranes consists in carrying out the process of ultrafiltration of aqueous systems containing humic acid over a set time, following the variation in time of the volume of permeate obtained as well as the influence of process parameters. Following the investigation of the ultrafiltration process of the water through the polymeric membrane, it was found that the formation of the humic acid layer on the surface of the membrane that favors fouling is delayed, after many hours of operation, due to both the internal fiber structure which offers advantages to this type of membranes as well as high efficiency of humic acid retention. Experimental results showed that the humic acid retention rate increased by up to 37% as a result of the double decrease of the conductivity value in the permeate samples from about 600 mS/cm to 373 mS/cm and the temperature being continuously increasing, from 22.5 at 25oC over 5 h influenced the entire ultrafiltration process. The full process approach as well as the expression and interpretation of the results will be detailed in this paper.

Enhanced removal of humic acid from aqueous solution by novel stabilized nano-amorphous calcium phosphate: Behaviors and mechanisms

Stabilized nano-amorphous calcium phosphate (nACP) was prepared using polyethylene glycol as stabilizer to obtain a nanosized amorphous adsorbent. The produced nACP was evaluated by using XRD, FTIR, SEM and X-ray photoelectron spectroscopy (XPS). The sedimentation test demonstrated that nACP exhibited better stability than crystallized hydroxyapatite. The adsorption efficiency of the nACP material for aqueous humic acid (HA) was evaluated from the point of view of medium pH, adsorption time, temperature, and ionic strength, as well as the presences of metal ions. The results of the study showed very good adsorption performance towards aqueous HA. The Sips modeling results revealed that the stabilized nACP adsorbent had a considerably high adsorption capacity (248.3mg/g) for HA at 298K. The adsorption data fitted well into pseudo-second order and Elovich kinetic models. XPS analyses indicated that HA retention on nACP material might be due to the surface complexation reaction between oxygen-containing group and calcium of HA and nACP, respectively. Moreover, the HA adsorption capacity of nACP could still keep more than 86% after four adsorption-desorption cycles. By taking into account all results it was concluded that the nACP adsorbent leveraged its stability in combination with its high uptake capacity to offer a great promise for HA adsorption from water.

Surface photochemical graft polymerization of acrylic acid onto polyamide thin film composite membranes

ABSTRACTSurface modification is an effective approach to enhance the properties of polymeric membranes. In this work, the UV‐photo‐induced graft polymerization of acrylic acid (AA) onto the surfaces of polyamide thin film composite (TFC‐PA) membranes was carried out using an immersion method performed under ambient conditions. The experimental results indicate that the membrane surface becomes more hydrophilic because of the appearance of new carboxylic groups on the surface after the modification. This reduces the water contact angle and increases the water permeability compared with the unmodified membrane. The membrane surface is relatively compact and smooth due to the formation of the polymeric AA‐grafted layer. The separation performance of the modified membrane is improved with enhancements of the permeate flux and the retention of humic acid from aqueous feed solutions compared with those of the unmodified membrane. The fouling resistance of the membrane is also improved because of the higher maintained flux ratios and the lower irreversible fouling factors for the removal of various organic compounds from feed solutions. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 44418.

Effect of granular activated carbon on percent retention of humic acid and permeate flux in GAC/UF membrane process

In this study, an attempt has been made to produce portable water having low concentrations of natural organic matter. First, the adsorption parameters of granular activated carbon (GAC) were determined by batch experiments. Fixed bed experiments were performed for the determination of GAC column parameters. To evaluate the effect of GAC filter on membrane parameters, the GAC filter was connected with ultrafiltration (UF) membrane pilot plant. Higher percent retentions and permeate fluxes were observed for GAC/UF hybrid process as compared to UF process alone. With GAC about 17.5%, high retention of humic acid was observed. UF membrane system was operated in dead-end mode with transmembrane pressure of 0.8 bar. The back washing duration for GAC/UF hybrid process was lower than that of UF operation alone.

Development of a nanofiltration membrane for humic acid removal through the formation of polyelectrolyte multilayers that contain nanoparticles

Poly(sodium-4-styrenesulphonate) (PSS) and poly(diallyldimethylammonium chloride) have been employed to construct polyelectrolyte multilayers on a polyethersulphone substrate for use in the nanofiltration of humic acid. Self-synthesised Ag2O and commercial ZnO nanoparticles were incorporated into the multilayers separately, showing increases in the permeability from 7.53±1.83 to 8.39±1.37 and 8.62±1.03 L m−2 h−1 bar−1, respectively, with no significant leaching observed in the permeates. All polyelectrolyte-modified membranes exhibited good film-formation stabilities and high humic acid retention capabilities, which ranged from 93.14±2.43 to 95.57 ± 3.87%. Less humic acid solutes were deposited onto the surface of the polyelectrolyte-modified membrane, which was confirmed through field emission scanning electron microscopy images. The contact angle was reduced from 44.00 ± 3.46° to 39.10 ± 3.47° when the membrane surface was hydrophilised with PSS as the terminating layer.

Transport and retention of humic acid through natural quartz sand: Influence of the ionic strength and the nature of divalent cation

In order to elucidate the environmental problems arising from the mobility enhancement of metallic contaminants due to the presence of humic acid (HA) in water, transport and retention experiments of HA, in the presence and in the absence of divalent ions Me2+ (Ca2+, Cu2+ and Pb2+), through natural quartz sand (NQS) porous medium, were carried out. The breakthrough curve and the HA adsorbed amount were measured in each experiment, indicating colloid retention was highly dependent on the suspension ionic strength, and the nature of divalent cation present in the aqueous phase. The data show that in the absence of divalent cations, the adsorbed HA amount at solid–water interface was found to increase with increasing the ionic strength, whereas the flow rate does not affect the retained HA amount on quartz sand. Further, in the presence of divalent cations, the adsorbed HA amount was found to be function, not only of the ionic strength, but also to depend on the divalent cation affinity toward the HA macromolecule. Thus, cations having lower affinity for HA, lead to highest adsorbed HA, whereas cations having higher affinity for HA, give the lowest adsorbed HA on the NQS surface. It is shown that the adsorption of HA, or its complex form with divalent cations (HA–Me2+), on the mineral–water interface, is an irreversible process which is limited by the blocking effect.

Deactivation of nanoscale zero-valent iron by humic acid and by retention in water

The effects of the deactivation of nanoscale zero-valent iron (NZVI), induced by humic acid (HA) and by the retention of NZVI in water, on nitrate reduction were investigated using a kinetic study. Both the nitrate removal and generation of ammonia were significantly inhibited as the HA adsorption amount and retention time were increased. However, HA removal was greatly enhanced when the NZVI was used after 1 d or 25 d of retention in water. The results are caused by the formation of iron oxides/hydroxides, which increased the specific surface area and the degree of NZVI aggregation which was observed by transmission electron microscopy (TEM). However, the nitrate reduction was greater at the beginning of reaction in the presence of HA when fresh NZVI was used, because of the enhanced electron transfer by the HA in bulk phase and on NZVI surface as train sequences. The pseudo second order adsorption kinetic equation incorporating deactivation and a Langmuir-Hinshelwood (LH) type kinetic equation provided accurate descriptions of the nitrate removal and ammonia generation, respectively. The deactivation constant and the reaction rate constant of the LH type kinetic equation were strongly correlated with the HA amount accumulated on NZVI. These results suggest that the HA accumulation on the NZVI surface reactive sites plays the dominant role in the inhibition and the inhibition can be described successfully using the deactivation model. The HA accumulation on NZVI was verified using TEM.

Organic fouling and floc transport in capillaries

Coagulation and adsorption are two widely recommended pretreatments to ultrafiltration (UF) membranes. Optimal concentrations of ferric chloride and powdered activated carbon (PAC) are reducing membrane fouling and increasing the retention of the organic matter. Intuitive pretreatment may result in either increased fouling or in low retention of organic matter. In addition, the inside-out capillaries might become partially plugged by the formed flocs. The understanding of the fouling mechanisms in inside-out capillaries is therefore a challenging and much practically needed task. We investigated the fouling using an unique setup with separate measurements of flux and retention of organic matter in five segments of a 1.5-m long multibore capillary. The high coagulant doses resulted in average 90% humic acid retention and in severe fouling. The use of powdered activated carbon at higher pH also contributed to severe increases in membrane resistance. As the filtration progressed, the dead-end part of the capillary (far from the feed) gradually became clogged with flocs.

Surfactant modified zeolite as adsorbent for removal of humic acid from water

Coal fly ash is a solid waste generated in large amounts worldwide; therefore, development of new techniques for its productive reuse is important. Two zeolites synthesized from coal fly ashes (ZFAs) were prepared and then modified using hexadecyltrimethylammonium (HDTMA). Both ZFAs and surfactant modified ZFAs (SMZFAs) were investigated for their ability to adsorb humic acid (HA) from water. The results indicated that, while ZFA had little affinity for HA, SMZFA showed greatly enhanced adsorption capacity. The adsorption isotherm was well fitted to the Langmuir model. The uptake of HA was substantially influenced by pH, increasing under acidic pH conditions. The SMZFA, which had a higher BET area and higher amount of loaded HDTMA, showed greater retention of HA. Uptake of HA by SMZFA was improved in the presence of NaCl. The retention of HA by SMZFA was primarily attributed to HDTMA loaded onto the zeolite surface. However, because modification of ZFA by HDTMA resulted in an increase in Zeta potential, iron oxide in SMZFA also contributed to HA adsorption via a ligand exchange mechanism.

Morphological and separation performance study of polysulfone/titanium dioxide (PSF/TiO 2) ultrafiltration membranes for humic acid removal

In this study, polysulfone (PSF) hollow fiber membranes with enhanced performance for humic acid removal were prepared from a dope solution containing PSF/DMAc/PVP/TiO 2. The main reason for adding titanium oxide during dope solution preparation was to enhance the antifouling properties of membranes prepared. In the spinning process, air gap distance was varied in order to produce different properties of the hollow fiber membranes. Characterizations were conducted to determine membrane properties such as pure water flux, molecular weight cut off (MWCO), humic acid (HA) rejection and resistance to fouling tendency. The results indicated that the pure water flux and MWCO of membranes increased with an increase in air gap distance while HA retention decreased significantly with increasing air gap. Due to this, it is found that the PSF/TiO 2 membrane spun at zero air gap was the best amongst the membranes produced and demonstrated > 90% HA rejection. Analytical results from FESEM and AFM also provided supporting evidence to the experimental results obtained. Based on the anti-fouling performance investigation, it was found that membranes with the addition of TiO 2 were excellent in mitigating fouling particularly in reducing the fouling resistances due to concentration polarization, cake layer formation and absorption.

Impact of speciation on removal of manganese and organic matter by nanofiltration

Research Article| March 01 2010 Impact of speciation on removal of manganese and organic matter by nanofiltration Annalisa De Munari; Annalisa De Munari 1School of Engineering, University of Edinburgh, Edinburgh EH9 3JL, UK Tel.: +44 (0) 131 650 7860 ; Fax: +44 (0) 131 650 6781; E-mail: A.De-Munari@ed.ac.uk Search for other works by this author on: This Site PubMed Google Scholar Andrea I. Schäfer Andrea I. Schäfer 1School of Engineering, University of Edinburgh, Edinburgh EH9 3JL, UK Search for other works by this author on: This Site PubMed Google Scholar Journal of Water Supply: Research and Technology-Aqua (2010) 59 (2-3): 152–163. https://doi.org/10.2166/aqua.2010.067 Article history Received: July 01 2009 Accepted: October 04 2009 Views Icon Views Article contents Figures & tables Video Audio Supplementary Data Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Cite Icon Cite Permissions Search Site Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentAll JournalsThis Journal Search Advanced Search Citation Annalisa De Munari, Andrea I. Schäfer; Impact of speciation on removal of manganese and organic matter by nanofiltration. Journal of Water Supply: Research and Technology-Aqua 1 March 2010; 59 (2-3): 152–163. doi: https://doi.org/10.2166/aqua.2010.067 Download citation file: Ris (Zotero) Reference Manager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex The removal of manganese and humic acid (HA) by two nanofiltration membranes, TFC-SR2 and TFC-SR3, was investigated in order to highlight the influence of speciation on manganese and HA retention. Manganese speciation and complexation with HA were modelled to understand how speciation could affect NF removal mechanisms. The behaviour of the two membranes was dramatically different in terms of manganese retention. Manganese retention for TFC-SR3 was higher and dominated by size exclusion. Manganese retention for TFC-SR2 varied with pH. At pH 7 (i.e. the pH of most natural waters) manganese retention for TFC-SR2 was about 45% versus 90% for TFC-SR3, with fluxes of about 75 and 25 l m−2 h−1, respectively. Both membranes showed very high retention of HA (about 80%); therefore they are suitable for surface waters where no salt removal is required. Manganese deposit on both membranes was generally low (<10%), but increased at pH 10 and 12 as manganese deposited as precipitated MnCO3, and the membranes showed a yellow-brownish layer. humic acids, manganese, nanofiltration, speciation This content is only available as a PDF. © IWA Publishing 2010 You do not currently have access to this content.

Analysis of polysaccharide, protein and humic acid retention by microfiltration membranes using Thomas’ dynamic adsorption model

Thomas’ model has been applied in this study for the quantitative description and parameterization of the dynamic adsorption of organic foulants to membranes by analysis of the permeate concentration as a function of filtration time (or effluent volume). A hydrophilically enhanced polyvinylidene fluoride (PVDF) membrane was used with dextran (DEX), bovine serum albumin (BSA) and Aldrich humic acid (HA) that were used as model compounds representative of polysaccharides, proteins and humic substances, respectively. Thomas’ model stemming from Langmuirian adsorption kinetics was found to give a good description of physically irreversible membrane fouling as well as the corresponding adsorption constants. The simplification of Langmuirian kinetics to Linear kinetics was found to be reasonable though with no further mathematical simplifications or approximations valid for the case of fouling of a hydrophilically enhanced PVDF membrane. Ka, i.e. the product of the equilibrium constant and the total adsorption capacity, is considered to be a more practicable measure of adsorption affinity than K. The Ka values of 2.09, 2.83 and 3.44 for DEX, BSA and HA, respectively, confirm that the affinity exhibited by foulants typically follows the order: humic substances > proteins > polysaccharides for hydrophilically enhanced PVDF membranes. However, the final adsorbed amount (i.e. the fouling extent) is dependent upon not only the membrane-binding affinity but also the concentration of foulant in the solution.

Characterization and retention of NF membranes using PEG, HS and polyelectrolytes

Nanofiltration (NF) membranes retention depends on charge repulsion and size exclusion, combining the properties of ultrafiltration and reverse osmosis membranes. The molecular weight cut-off of two NF membranes was determined using cross-flow filtration of different molecular weights of non-ionized polyethylene glycol (PEG). Their retention of humic acid (HA), fulvic acid (FA), diallyldimethylammonium chloride and copolymer of dimethyl aminoethyl acrylate was also determined. NF270 membrane's pure water flux was higher than NF90. NF270 also produced lower retention compared to NF90. Observed retention of PEG using NF270 was lower than NF90, but real retention of PEG was higher for NF270 than NF90. This unexpected result is due to the inconsistency of the concentration polarization model. According to the concentration polarization model, real retention determination is dependent on permeate flux (Jv) instead of taking account of membrane flux reduction. The two membranes are supposed to operate at similar flux to overcome this inconsistency. Incomplete retention of poly diallyldimethylammonium chloride (PDADMAC) and copolymer of dimethyl aminoethyl acrylate (CoAA) is due to the presence of their monomers. HA retention is higher than FA because of its higher molecular weight range. NF270 membrane retention of HA and FA is lower than NF90 because of its higher pore size and porosity. Solute retention is constant with increasing pressure due to the competition between concentration polarization and dilution effects at the studied range of trans-membrane pressure (TMP).

Characterization and retention of UF membranes using PEG, HS and polyelectrolytes

Molecular weight cut-off (MWCO) of two different types of membranes (polyether sulfone (PES) and regenerated cellulose (RC) ultrafiltration (UF) membranes) has been investigated using 2, 4 and 6 kDa polyethylene glycol (PEG) solutes. The retention results of humic acid (HA), fulvic acid (FA), copolymers of dimethyl aminoethyl acrylate (CoAA) and poly dially dimethylammonium chloride (PDADMAC) is also presented in this paper. Retention of PEG solutions produced MWCO lower than stated by the manufacturers. The MWCO of P005F, C005F and P020F was 3.75, 3.35 and 12.1 kDa, respectively. The manufacturer values were 5 kDa for P005F and C005F and 20 kDa for P020F. The rejection of PDADMAC and CoAA was found to be lower than expected with respect to the MW specified by the supplier. The MW of PDADMAC and CoAA is 60–200 kDa and 10–12 intrinsic viscosity, respectively. The retention of such high MW substances should be ≫90%, but the results show a maximum retention of 91% and the other results are <90%. Nevertheless, the retention of HA and FA was as expected. The humic substances (HA and FA) were extracted from soil, which has a low MW i.e. <6 kDa. The maximum rejection of HA and FA was ~93 and ~84%, respectively. The lower rejection of FA is attributed to the lower MW of FA compared to HA. The observed rejection of PDADMAC and CoAA decreases with increasing the pressure, but the real rejection is constant. This is attributed to the increase in concentration at the wall of the membrane, hence increasing the mass transfer coefficient. The same trend is achieved when filtering HA and FA. The rejection of P005F is always higher than the rejection of C005F for all substances. Although the manufacturer stated the same MWCO of both membranes, the P005F MWCO was higher than C005F producing a higher rejection.

Retention of humic acid by ultrafiltration with polyaluminium coagulant

The chemical and technological aspects for the retention of humic acid (HA) by combining coagulation and ultrafiltration (UF) are discussed in this paper. Coagulation was performed with polyaluminium chloride (PACl) in three operating modes: in-line 30 s coagulation, coagulation with 25 min flocculation, and coagulation/flocculation with 3 h sedimentation. UF capillaries with a molecular weight cut-off (MWCO) of about 200 kDa, made of polyethersulfone (PES), were operated in inside-out mode with constant positive pressure. The applied experimental scheme posed additional demands to the HA-PACl flocs, limiting their size and density. The optimal floc was achieved at low [Al T ]/DOC mass-to-mass ratio of 0.4. The obtained flocs were big enough to provide sufficient retention of HA by UF, small enough to minimize clogging, and with moderate density to minimize fouling. The optimal floc provided a highest 70% HA retention with minimal fouling. Combining coagulation with flocculation we were able to achieve an accumulation of 5 mg HA per capillary not compromising the duration of the filter run. Alteration of pH and Al dose to achieve ultimate ‘charge neutralization’ or ‘sweep coagulation’ yielded virtually similar results in terms of HA retention.

Separation of humic acid from a model surface water with PSU/SPEEK blend UF/NF membranes

Membranes made from blends of polysulphone and sulphonated poly(ether ether ketone) (PSU/SPEEK) have high porosity, high charge and a pore size at the boundary between nanofiltration (NF) and ultrafiltration (UF). Their application to the treatment of surface water for drinking purposes has been investigated through study of their ability to remove humic acid (HA) from a model water, in comparison with two commercial membranes. The PSU/SPEEK membranes, in planar or tubular form, showed excellent retention of HA and very high flux. Their high charges gave rise to critical fluxes for deposition. As an additional benefit, HA deposits forming on the charged membranes above the critical fluxes had a loose structure, as visualised by atomic force microscopy (AFM), and were consequently efficiently removed by rinsing. Three multivalent ions, Fe 3+, Al 3+ and Mn 2+, at their concentrations in a natural water did not affect the membrane process significantly.