A comparative study of techniques used for porous membrane characterization: pore characterization

A range of commerical UF membranes have been characterized by thermoporometry, biliquid permporometry and molecular weight cut-off experiments. A comparison of results from these three independent techniques for the same types of membrane shows an indication of the strength and weakness of the methods. MWCO values determined from actual rejection values using PEG and dextran were significantly lower than the manufacturer supplied data. The data obtained using the biliquid permporometry and solute rejection tests produced contrasting results for Amicon polysulfone (PM30) and regenerated cellulose (YM30) membranes. While MWCO determination resulted in sharper cut-off curves, the biliquid permporometry offered a broader size distribution with the PM30 and vice versa with the YM30. The pore sizes obtained by thermoporometry were significantly larger than those by the biliquid permporometry. The biliquid permporometry and thermoporometry give significantly higher values than the MWCO method. The closest comparison is obtained between the EM values and the MWCO method. This suggests that the controlling pore dimension for separation is the surface skin dimension

Review of the Metropolitan Water Plan: Final Report

This report was commissioned by the NSW Cabinet Office to review the Metropolitan Water Plan 2004 (DIPNR, 2004a), and was undertaken by the Institute for Sustainable Futures at the University of Technology, Sydney and ACIL Tasman with technical advice from SMEC Australia. In February 2006, our interim review report (ISF, 2006) showed how the supply-demand balance in 2015 could be met with rain-fed supply and a suite of demand management initiatives, and how Sydneys water needs could be secured against the risk of severe drought by having the capacity to deploy groundwater and desalination

Succession of biofilm communities responsible for biofouling of membrane bioreactors (MBRs)

© 2017 Luo et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Biofilm formation is one of the main factors associated with membrane biofouling in membrane bioreactors (MBRs). As such, it is important to identify the responsible organisms to develop targeted strategies to control biofouling. This study investigated the composition and changes in the microbial communities fouling MBR membranes over time and correlated those changes with an increase in transmembrane pressure (TMP). Based on qPCR data, bacteria were the dominant taxa of the biofilm (92.9–98.4%) relative to fungi (1.5–6.9%) and archaea (0.03–0.07%). NMDS analysis indicated that during the initial stages of operation, the biofilm communities were indistinguishable from those found in the sludge. However, the biofilm community significantly diverged from the sludge over time and ultimately showed a unique biofilm profile. This suggested that there was strong selection for a group of organisms that were biofilm specialists. This pattern of succession and selection was correlated with the rapid increase in TMP, where bacteria including Rhodospirillales, Sphingomonadales and Rhizobiales dominated the biofilm at this time. While most of the identified fungal OTUs matched Candida sp., the majority of fungal communities were unclassified by 18S rRNA gene sequencing. Collectively, the data suggests that bacteria, primarily, along with fungi may play an important role in the rapid TMP increase and loss of system performance

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

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

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