Fouling effects on rejection in the membrane filtration of natural waters

Membrane processes in drinking water applications are micro- (MF), ultra- (UF) and nanofiltration (NF). These processes remove turbidity and bacteria (MF), viruses and macromolecules (UF) and small molecules and hardness (NF). Of particular concern in water treatment is the removal of natural organic matter (NOM) which contains potential disinfection by-product precursors. The presence of colloids, multivalent ions and organics in surface waters may cause substantial fouling of membranes. A study was carried out which looked at the rejection abilities of a range of membranes targeting hematite colloids (40–500 nm), NOM and cations, fouling conditions and cost of treatment of these processes with consideration of chemical pretreatment with ferric chloride [1]. In this paper the effect of membrane fouling on rejection is presented. The study was based on experiments with two MF membranes (GVWP, GVHP, 0.22 μm, Millipore), six UF membranes (1, 3, 5, 10, 30, 100 kDa, regenerated cellulose, Millipore), and four organic NF membranes (TFC-SR, TFC-S, TFC-ULP, CA-UF, Fluid Systems, US). Three different types of organics (IHSS humic acid, IHSS fulvic acid and an Australian concentrated NOM) in a carbonate buffer containing calcium chloride and a background electrolyte were used. Experiments were carried out in perspex (MF, UF) and stainless steel (NF) stirred cells of a volume of 110–185 mL and a membrane area of 15.2–21.2×10−4 m2 at transmembrane pressures of 1, 1–3, and 5 bar for MF, UF, and NF, respectively. UF removes 10–95% of NOM depending on the molecular weight cut-off (MWCO) of the membrane. Pore sizes of <6 nm are required to remove about 80% of NOM, where a 6 nm pore size corresponds to a MWCO of about 10 kDa. Colloids are fully rejected. NF removes NOM effectively (70–95% as dissolved organic carbon (DOC) and 85–98% as UV absorbance). Cation rejection is very membrane dependent and varies for the investigated membrane types between 13 and 96% for calcium and 10–87% for sodium. Fouling was also dependent on pore size and was caused by large colloids (250 nm) or coagulant flocs in MF, small colloids, organic-calcium flocs and aggregates with a dense structure (formed slowly) in UF, and by a calcium-organic precipitate in NF. The fouling influenced the rejection of colloids in MF and that of NOM in UF and NF. If a highly charged layer was deposited on the NF membranes, cation rejection was also influenced. The characterisation of permeate organics revealed that low molecular weight acids passed through the NF membranes and that the rejection of these acids was also dependent on the deposit on the membrane. The mechanisms which can explain such an increase in rejection are different for the three membrane processes. In MF, pore plugging and cake formation was found responsible for fouling. This reduces the pore size and increases rejection. In UF, internal pore adsorption of calcium-organic flocs reduces the internal pore diameter and subsequently increases rejection. In NF, the key factor appears to the charge of the deposit. This was investigated with the deposition of a ferric chloride precipitate. If the precipitate was of high positive charge, the rejection of cations increased and that of negatively charged low molecular weight acids decreased compared to more neutral or negative precipitates. In essence, the rejection characteristics of membranes depend more on the fouling state of the membranes and the nature of the foulants than on the initial membrane characteristics

Nanofiltration of Natural Organic Matter: Removal, Fouling and the Influence of Multivalent Ions

The presence of calcium and humic substances or natural organic matter (NOM) in surface waters can cause severe fouling of nanofiltration (NF) membranes. Conditions of fouling were studied using a stainless steel stirred cell of volume 185 mL and a membrane area of 21.2*10-4m2 at a transmembrane pressure of 5 bar. Deposition of organic matter was determined by mass balance in feed and concentrate samples. Electron microscopy and X-ray photoelectron spectroscopy (XPS) were used to study the morphology and composition of the fouling layer. During permeate recycle experiments, which were used for fouling studies, it was found that calcium concentration (as a representative of multivalent ions) and the type of organic play a major role in fouling. The calcium forms complexes with some of the organics and deposits on the membrane surface. Depending on the solution conditions the organic or calcite (on which organics adsorb) precipitate. Factors, which influence the concentration of organics and ions at the membrane surface such as stirring, flux and transmembrane pressure, influenced the deposition of organic matter significantly. Irreversible fouling occurred with all membranes at high calcium concentrations, although the cellulose acetate membrane showed an overall better performance, possibly due to its low salt rejection and smooth surface. IHSS humic acid is the organic which deposits most easily and comparison of UV absorbance and DOC data showed that the fraction which absorbs UV strongest, and is more hydrophobic, deposits preferentially on the membranes. These substances also have the lowest solubility stressing the importance of concentration polarisation effects

Chemical Addition prior to Membrane Processes for Natural Organic Matter (NOM) Removal

Membrane processes for surface water treatment include microfiltration (MF), ultrafiltration (UF) and nanofiltration (NF), depending on the target material to be removed and the limiting process economics. MF will remove turbidity, but no dissolved compounds, unless associated with colloids, UF will, depending on the molecular weight cut off (MWCO), partially remove NOM, and NF will remove NOM almost completely, but for a price often considered as uneconomic due to energy costs. Chemical addition prior to MF or UF may enhance the NOM removal capacity of these processes to a comparable range as achieved with NF. In this work the improvement of NOM removal by MF with chemical pretreatment was investigated using FeCl3 and hematite (a-Fe2O3) addition. The results achieved with the addition of ferric chloride as a coagulant prior to MF showed that 95% removal of NOM can be achieved at a dosage of 25 mgL-1. The flocs form a gelatinous deposit on the membranes and cause flux decline, however the resulting flux is still high compared to UF and NF. Higher dosage of 100 mgL-1 resulted in a very high flux decline. The addition of hematite synthesised as monodispersed, spherical colloids in the sizes 75, 250 and 500 nm showed the importance of colloid size on MF flux. Small colloids (75 nm) are not retained by the membrane when stabilised due to the adsorption of organics, but also adsorb larger amounts of NOM than do larger hematite particles. Aggregation of these colloids increased colloid rejection with a concomitant increase (to about 20% at a low dosage of 10 mgL- 1 Hematite) in removal of adsorbed organic matter. Aggregation of small colloids increases the adsorbant surface area significantly versus larger primary colloids. The structure of the aggregates was found to be important for membrane flux. Alternatively, tighter membranes can be used. UF membranes showed a NOM removal of 10 to 90% for a MWCO of 30 to 1 kDa (five membranes were investigated), respectively. NF removed > 95% of organics, independent of solution chemistry and could remove a large fraction of multivalent ions. The study shows that if no salt rejection (softening) but very high NOM removal (> 90%) are required in a water treatment application, hybrid processes of MF with chemical pretreatment may be a very attractive alternative to UF or NF

Ultrafiltration of Natural Organic Matter

Increasingly stringent regulations for drinking water quality have stimulated the application of ultrafiltration to water treatment. In addition to removing particulate materials from water (including microorganisms, bacteria and viruses), the use of membrane treatment also meets purification requirements. However, irreversible fouling curtails the economic viability of such a process. Experiments in stirred-cells were conducted to evaluate the effects of surface water composition on rejection and fouling of two ultrafiltration membranes with different molecular weight cut-offs (10kDa and 100kDa). Experimental solutions consisted of natural organic matter or humic substances in a background electrolyte. The effect of calcium concentration decreased rejection of humic acid under certain circumstances. This is believed due to reduced molecular size with an initial increase in calcium concentration. However, at about 2.5mM CaCl2, IHSS humic acid aggregates. This aggregation increased rejection, and also caused irreversible fouling of the 100kDa membrane, presumably as a result of pore size reduction due to internal deposition of aggregates. This was confirmed by blocking law analysis. The variation of transmembrane pressure indicated the importance of a ‘critical flux’ effect. The organics and their various fractions showed differences both in rejection and flux decline. The larger and more UV-absorbing fraction of humic acid was shown to be responsible for irreversible pore adsorption and plugging. The fulvic acid and the hydrophilic fraction showed a smaller and mostly reversible flux decline

Adsorption of the Endocrine-Active Compound Estrone on Microfiltration Hollow Fiber Membranes

Results of studies reported here show that adsorption could result in considerable accumulation of hormones on hydrophobic hollow fibre membrane surfaces during filtration of trace-hormone containing feed solutions with a linear adsorption isotherm applicable over the majority of the estrone concentration range examined (2.6 to 154 ng/L). Models based on both diffusion and surface reaction limitation were used to describe the kinetics of estrone adsorption to the membranes tested. Results indicate that the rate of adsorption of estrone to the hollow fibre membranes was limited principally by surface reaction rate rather than the rate of diffusive transport to membrane surface sites. Both adsorption and desorption kinetics were satisfactorily described by pseudo-first order expressions. These results are of environmental significance, especially in drinking water applications, where contaminants such as natural and synthetic hormones may accumulate on the membranes and desorb during backwashing and membrane cleaning

Binding of Estrone to Microfiltration Hollow Fibre Membranes in Filtration of Solutions Containing Trace Estrone

Increased concern is being paid to the health and environmental risk caused by trace natural and synthetic hormones discharged from sewage treatment plant (STPs). This study, which is part of a larger project on investigation of hybrid membrane processes for trace hormones removal, focuses on binding of hormones to microfiltration hollow fibre membranes in filtration of solutions containing trace hormones. The adsorption capacity of the membrane, kinetics of adsorption and desorption of to/from microfiltration hollow fibre membranes, and the factors affecting adsorption to the membranes have been studied using estrone as model solute. The experiments showed that the adsorption of estrone to the microfiltration membranes could result in high retention of estrone molecules. However, since the mechanism of retention is adsorption rather than sieving, the retention decreases with increase in the amount of estrone accumulated on the membrane and breakthrough occurs when the accumulated estrone on the membrane reaches an equilibrium concentration corresponding to the feed concentration. For long-term operation, although membranes could be saturated by hormone molecules the membrane processes could still have a function of buffer for instantaneous high hormones concentration because the saturation surface concentration increased with the increase in the feed concentration. In addition, the experiments also indicated that the adsorption capacity of the membranes for hormones could be affected by membrane types, pH, affinity of hormones to water, as well as the presence of other organics. These results are of relevance to the potential release of trace hormones from water treatment systems where microfiltration membranes are used as a process barrier

Charge Effects in the Fractionation of Natural Organics using Ultrafiltration

Comparison of two commonly used techniques for molecular weight determination of natural organics, UF fractionation and high performance size exclusion chromatography (SEC), show that neither technique gives absolute measures of molecular weight. Investigations of both International Humic Substances Society standard humic and fulvic acids as well as natural organic matter concentrated from surface freshwaters show that charge effects and solution conditions are important in both SEC and UF fractionation with various components of the natural organics being affected differently. Membranes with a smaller molecular weight cut-off produce permeates with a lower UV/DOC ratio suggesting that the more aromatic components of natural organics are removed by the lower molecular weight cut-off membranes. Variation in ionic strength has little effect on the rejection of humic acid fractions but does significantly influence the rejection of low molecular weight acids. pH and organic concentration do not affect DOC rejection significantly over the pH range 4.5 to 10 and the DOC concentration range of 15 to 60 mgL-1. These results indicate that UF should not be applied for quantitative “size” analysis unless operated under welldefined conditions. If operated under conditions appropriate to water treatment, UF fractionation can give information of direct applicability to treatment such as the MWCO required to achieve significant organics removal

Adsorption of trace steroid estrogens to hydrophobic hollow fibre membranes

This paper discusses adsorption of estrone to microfiltration hollow fibre membrane from aqueous phase using estrone as the model compounds. The partitioning of estrone between membrane and aqueous phase at equilibrium state, the concept of membrane retention towards estrone caused by adsorption, adsorption kinetics, and the application potential of membrane adsorption have been assessed through batch adsorption and dead-end filtration of solution containing trace estrone. The results show that adsorption could result in significant accumulation of estrone on membrane surface. The partition of estrone between membranes and aqueous phase can be characterized by the Freundlich equation. The microfiltration membrane could exhibit high retention to estrone due to adsorption but the retention decrease with the increase in estrone amount accumulated on the membrane surface. Implication of this study is of an important nature, especially in drinking water applications. Contaminants such as natural and synthetic hormones may accumulate on the membranes and desorb during backwash or membrane cleaning. Further studies are needed to address risk issues involved