Impact of Membrane Surface Modification on the Treatment of Surface Water

New polyethersulfone (PES) based membranes for ultrafiltration (UF) were developed by blending a surface-modifying macromolecule (SMM) in the casting solution, in an attempt to minimize the impact of fouling. Fouling was evaluated using concentrated Ottawa River water (CORW), either unfractionated or fractionated via UF. These membranes also included some polyvinylpyrrolidone (PVP), a pore forming additive. A statistical analysis was conducted to evaluate the impact of some variables on the treatment of the surface water. The independent variables included PVP/PES ratio in the casting solution, with and without SMM, and the nature of the feed CORW [low molecular weight (LMW) fraction, unfractionated, high molecular weight (HMW) fraction]. The performance variables studied were total organic carbon (TOC) removal, the foulant accumulation at the membrane surface after filtration, the flux reduction, and the final permeate flux. The most important variable was the feed water. Filtration of LMW had a higher final flux, less fouling, but slightly lower TOC removal. The SMM did not significantly impact the membrane performance. TOC removal was high, compared with results reported in the literature for UF membranes.     

Existence of Critical Recovery and Impacts of Operational Mode on Potable Water Microfiltration

Results from a potable water microfiltration (MF) pilot study employing untreated surface water are reported. The effects of filtrate flux and recovery on direct flow, outside-inside, hollow fiber MF fouling rates, and backwash effectiveness are presented. Constant flux experiments suggested the existence of a critical recovery below which MF fouling rates were low and effectiveness of backwashes was high and relatively independent of the recovery. However, in the range of experimental conditions investigated, fouling rates increased dramatically and backwash effectiveness decreased steeply when this critical recovery was exceeded regardless of the flux. In general, for a fixed recovery, specific flux profiles analyzed on the basis of volume filtered per unit membrane area were insensitive to filtrate flux. Fouling was accelerated by operating membranes at constant flux rather than at constant pressure, in part, because of membrane compaction and cake compression. Changing the mode of filtration between constant flux and constant pressure is shown to have no effect on MF filtrate water quality. For any given capacity, membrane area requirements are decreased, and power requirements are increased when membranes are operated at constant flux rather than at constant pressure.     

Pressure-Dependent Permeate Flux in Ultra- and Microfiltration

The effect of applied pressure on the permeate flux in cross-flow ultrafiltration (UF) and microfiltration (MF) was investigated both theoretically and experimentally. In UF and MF processes, the permeate fluxes are controlled by concentration polarization and cake formation over the membrane surface. As a better understanding of concentration polarization and cake formation becomes available, the permeate flux under any pressure can be theoretically predicted. Experiments were conducted in a ceramic tubular cross-flow filter with silica colloids of a narrow size distribution (model colloids). The pressure-dependent flux of the model colloidal suspension in cross-flow filtration was investigated under various experimental conditions. The experimental measurements were compared with the theoretical predictions, and the results showed that the pressure-dependent permeate flux in cross-flow filtration can be adequately predicted. Furthermore, theory and experiments demonstrated that the performance and operating state of UF and MF could be well characterized by the so-called “characteristic pressure” of the process.     

Membrane Fouling Test: Apparatus Evaluation

Flux decline with time is one of the most serious shortcomings of microfiltration and ultrafiltration membranes. It is highly desirable to have a membrane (fouling) testing procedure that is short in duration, utilizes a minimum amount of test solution, only requires a small membrane area, and is representative of the large-scale process. The objective of this study was to compare the results of the testing of a given membrane using a number of different test units (reverse osmosis, ultrafiltration, dead-end, and cross-flow cells) and testing procedures. It was of particular interest to determine if smaller cells used in the literature perform similarly to the Sepa CF cell, as it is a standard. During six-day runs the flux decline of the polyethersulfone membrane tested was mainly caused by membrane compaction and much less due to fouling. As various membrane materials compact to a different extent, studies into the fouling characteristics of different types of membranes should incorporate precompaction and pure water testing to quantify the contribution of membrane compaction and true fouling to the overall flux decline. The dead-end cell performed very differently from continuous cells, so their use is not recommended. The six-day continuous flow tests showed that the reverse osmosis (RO), ultrafiltration (UF), and cross-flow (CF) cells yielded very similar dissolved organic carbon removals and flux decline, despite UF and RO cells using membrane coupons eight times smaller than CF cells.     

NOM Accumulation at NF Membrane Surface: Impact of Chemistry and Shear

The effects of solution chemistry, surface shear, and composition of natural organic matter (NOM) were investigated for their impact on accumulation of foulant material at the surface of charged polymeric nanofiltration membranes. The source of NOM was the Suwannee River. A bench-scale, batch recycle system was used with 20 hollow fiber, nanofiltration membranes. Membrane flux decline and foulant accumulation increased at low pH and high ionic strength as a result of neutralization of charge, electric double layer compression, and the apparent shift in conformation of charged NOM macromolecules. The rate of NOM accumulation decreased with operating time, suggestive of an eventual steady state between adsorption and desorption. The effect of NOM composition on membrane fouling could not be discerned by a standard technique to isolate hydrophobic and hydrophilic NOM fractions, quite possibly because of the fractionation methodology's failure to recover a small but important fouling fraction or because of NOM interactions that are lost when individual fractions are separately tested. However, a greater percentage of the hydrophilic than hydrophobic fraction permeated the membrane, in agreement with prior observations by others. Increasing the cross flow velocity from 85 to 255 cm∕s reduced the extent of flux decline, presumably due to hydrodynamic disruption of cake layer formation.     

Effects of ultrafiltration on solute clearances in hollow fiber artificial kidneys

Although solute clearances in artificial kidney coils increase with ultrafiltration (UF), we have previously shown that increases are usually less than UF rate (most likely because of decreases in diffusive transport with UF coils and, for larger solutes, molecular sieving). The present studies demonstrate the effects of UF on clearances of Na and bromsulphalein (BSP) (mol. wt. 838) in hollow fiber dialyzers. Clearances were measured at increasing transmembrane hydrostatic pressures at perfusion rates of 200 and 500 ml. per minute. Fractions of total clearance attributable to diffusion as compared to solvent drag forces were calculated. Sieving coefficients were determined in studies where diffusion was minimized and clearance was primarily by solvent drag. Clearance increases were less than UF rate only for BSP; molecular sieving most likely accounts for the difference at high perfusion rates. Only at 200 ml. per minute was slight decrease of diffusion with UF suggested. Thus, in contrast to coils, there is minimal or no decrease in diffusion with UF in hollow fiber dialyzers.     

Evaluation of Apparatus for Membrane Cleaning Tests

Membrane cleaning is critical to the operation of membrane processes. This paper studies the impact of using four different types of bench-scale membrane systems to assess the effectiveness of different cleaning steps after the filtration of colored river water. The systems are a stirred ultrafiltration (UF) cell, a SEPA cell, a small cross-flow (CF) cell, and a six-CF-cell-in-parallel system. The effect of cleaning frequency was also investigated. The comparison was implemented in terms of flux recovery, solute removal, solute resistance removal, and changes of contact angles. The stirred UF cell was only reliable and comparable in terms of flux and flux recovery results. The six-cell-in-parallel system requires further development due to their much lower flux. For cleaning at 30-min intervals, the cleaning efficiency of membranes was similar for the three CF systems. For cleaning intervals of 2 and 4 h did not statistically affect the flux recovery for the stirred UF cell and SEPA cell. There was some irreversible fouling that could not be restored completely by clean-in-place method even with rigorous chemical treatment.     

Effects of Operating Parameters in Tubular Ultrafiltration

A parametric waste-specific study was conducted to develop a more mechanistic understanding of the tubular ultrafiltration system using a surrogate metalworking (MW) fluid as a model waste stream. An average gel layer concentration of 31% oil was calculated and the gel layer concentration was determined to be independent of transmembrane pressure and cross-flow velocity. The thin-film model adequately described limiting flux data collected in this study, due to the use of discrete cross-flow velocity∕MW fluid concentration experiments; thus, an improved mechanistic understanding was achieved. Mass transfer and thus the pressure-independent “limiting” permeate flux were generally comparable to values observed in a high-shear rotary UF system, for oil concentrations <26%. However, a decrease in net permeate flux was observed in the tubular ultrafiltration (UF) system at high feed oil concentrations; thus, a hybrid system (conventional tubular followed by high-shear rotary UF) is proposed for treatment applications where high concentrations are desired.     

Removal of Copper Ions from Waters Using Surfactant-Enhanced Hybrid PAC/MF Process

This paper deals with the removal of copper ions from aqueous solutions by using surfactant-enhanced powdered activated carbon (PAC)/microfiltration (MF) hybrid process, including the evaluation of process performance and fouling dynamics at various linear alkyl benzene sulfonate (LABS), PAC, and Cu2+ concentrations of feed solution. Although the use of surfactant as an additive material increased the adsorption efficiency in PAC/MF hybrid process, a considerable amount of the flux was lost for surfactant concentration above critical micelle concentration. The process could be employed with a performance of 74.7%, 97.2% and 87?L/m2?h for LABS rejection, Cu2+ rejection and permeate flux at the conditions of 2?g PAC/L, 5?mM LABS, 0.2?mM Cu2+, and 60-min process time. Cu2+ rejection, which increased with increasing of LABS, and PAC amounts decreased with the increase in Cu2+ concentration. It was understood that the increments in LABS, PAC, and Cu2+ concentrations being an indicator for the feed solution quality led to the occurrence of more fouling on the membrane. The analyses of dynamics concerning the fouling behaviors, which were carried out using single and combined pore blocking models, put forward that the cake formation was the main predominant mechanism in the process. It was also determined that the variation of feed contents deduced the presence of rather complex fouling behaviors as a simultaneous function of secondary membrane layer formation and clogging and narrowing of membrane pores by surfactants.     

Influence of cell concentration, temperature, and press performance on flow characteristics and disintegration in the freeze-pressing of Saccharomyces cerevisiae with the X-press

The pressure required for initiation of flow when freeze-pressing with the X-press is related to the phase boundaries of water, particularly those between ice I and liquid even at temperatures around -25 degrees C and lower. Widening the orifice of the pressure chamber to diameters larger than 2.5 mm leads to lower pressures and less extensive cell disintegration. Pressing Saccharomyces cerevisiae slowly with the aid of a manual hydraulic jack at -25 degrees C produces a disintegration of 60-75% irrespective of cell concentration. Pressing at -35 degrees C shows no clear differences. Pressing more rapidly with the aid of a motor-driven hydraulic press produces a similar extent of disruption of diluted cell suspensions (5.4 mg/g) as slow pressing. However, freeze-pressing a paste of baker's yeast (270 mg/g) increases the degree of disintegration. Under these conditions the disintegration is further enhanced by a lower temperature, -35 degrees C, and by a high velocity of flow through the orifice, such that more than 95% of the S. cerevisiae is disrupted by one pressing at less than 2 X 10(8) Pa. Mechanisms for flow through the X-press are suggested and discussed in relation to the phase diagram of water.     

Continuous biosorption of copper and lead in single and binary systems using Sphaerotilus natans cells confined by a membrane: experimental validation of dynamic models

Biosorption of heavy metals using membrane reactors as confining devise for free cells is an alternative process to remove these metallic pollutants from aqueous solution. In this paper, experimental data and modelling of heavy metal biosorption onto Sphaerotilus natans cells confined by a ultrafiltration/microfiltration (UF/MF) membrane reactor are reported. Biosorption tests using single and binary metallic solutions (Cu, Pb and Cu–Pb) denoted the biomass affinity (Pb>Cu), the competition among metals simultaneously present in the system, the filtrate flux decline and the change of metal retention coefficient on the membrane for pore plugging by cell fragments. Dynamic modelling is developed considering the unsteady mass balances of the metal in the system and the equilibrium parameters obtained by biosorption batch tests using Langmuir models. Experimental validation of the dynamic models denoted the importance of partial degradation of cells, which is specifically considered in modelling by introducing a time-depending profile for the biomass concentration.     

Impacts of Hydrophilic Membrane Additives on the Ultrafiltration of River Water

One of the most serious disadvantages of membrane applications in water treatment is the decreasing water permeation rate with time, which is often called fouling. This study investigates surface modification of polyethersulfone (PES) ultrafiltration membranes as a fouling reduction strategy for drinking water treatment applications. Surface modification was achieved through the addition of three different tailor-made hydrophilic surface modifying macromolecules (LSMM200, LSMM400, and LSMM600). Flat sheet membranes were prepared via a single-step casting procedure; their surface hydrophilicity was quantified via contact angle measurements. The incorporation of hydrophilic additives produced slightly more hydrophilic membranes (contact angle reduction of up to 8°) and improved membrane performance compared with the PES membrane without blending. In the treatment of highly colored river water, LSMM400- and LSMM600-modified membranes achieved up to 32% higher final fluxes. Surface modification resulted in significantly decreased flux reductions and natural organic matter accumulation. Dissolved organic carbon removals were approximately 70% for all the membranes studied. No clear correlation between membrane hydrophilicity and fouling reduction was observed.     

Recovery of Metal Working Fluids Using Chelation-Ultrafiltration Process

Metal working (MW) fluids are used to control friction and temperature, improve workpiece surface quality, and reduce tool wear. The batch-life of the MW fluid investigated was controlled primarily by the concentration of metallic soaps (complexes between the oil emulsifier and Al∕Mg. Ethylenediaminetetraacetic acid, a strong metal chelator, was added to the MW fluid to break the oil-Al∕Mg complexes. The aqueous-phase ethylenediaminetetraacetic acid-Al∕Mg complexes were then separated from the oil phase by ultrafiltration (UF). Al levels were reduced between 39 and 49%, and Mg levels were reduced between 67 and 77%. Al transfer from the oil phase to the aqueous phase was slow and increased during UF concentration. Rinsing the concentrated MW fluid with deionized water decreased the ash content but had a lesser impact on Al∕Mg removal. Al mass balances produced errors of 2.8% for run 3 and 6.2% for run 4. The UF system (0.11-μm ceramic membrane) effectively separated the oil and aqueous phases. Permeate turbidity was generally <1 nephelometric turbidity unit and permeate flux ranged from 390 to 220 L∕m2-h (230 to 130 gal.∕ft2-day) depending on the degree of concentration.     

Effect of veno-venous ultrafiltration on myocardial performance immediately after cardiac surgery in children. A prospective randomized study

OBJECTIVES: This study sought to evaluate the effects of veno-venous ultrafiltration on myocardial contractility in children undergoing cardiopulmonary bypass (CPB) for repair of congenital heart defects. BACKGROUND: Ultrafiltration (UF) is currently used to diminish postoperative fluid accumulation following CPB in children. Previous reports indicate improvement in hemodynamics immediately after UF, but the mechanism of its action is unknown. METHODS: Twenty-three patients (ages 2 months to 9.1 years; 13 males, 10 females) underwent UF for 10 min after CPB. Twelve patients underwent UF immediately after CPB (Group A). They were studied: (1) before and (2) after CPB, (3) after UF, and (4) 10 min after UF. Eleven patients underwent UF 10 min after CPB (Group B). They were studied: (1) before and (2) after CPB, (3) after a 10-min delay before UF, and (4) after UF. Contractility was determined by the difference in the observed and predicted velocity of circumferential fiber shortening for the measured wall stress, using transesophageal echocardiography. Left ventricular wall thickness was also measured. RESULTS: There was significant improvement in contractility after UF in both groups (mean+/-SD, Group A: -0.28+/-0.13 to -0.01+/-0.21 circ/s, p < 0.05; Group B: -0.26+/-0.16 to -0.11+/-0.17 circ/s, p < 0.05). Myocardial thickness to cavity dimension decreased in both groups following UF (Group A: 0.19+/-0.04 to 0.14+/-0.03, p < 0.05; Group B: 0.18+/-0.04 to 0.14+/-0.03, p < 0.05). CONCLUSIONS: UF improves hemodynamics by improving contractility and possibly by reducing myocardial edema in children following cardiac surgery. Enhanced patient outcome after ultrafiltration may in part be due to these changes.     

Dehydrogenation of 3-phenoxybenzyl alcohol in isolated perfused rabbit skin, skin homogenate and purified dehydrogenases

The formation of 3-phenoxybenzoic acid from 3-phenoxybenzyl alcohol was determined in (a) rabbit ears, single-pass perfused with a protein-free buffer, pH 7.4; (b) the microsomal fraction and its supernatant from homogenized rabbit skin; and (c) purified alcohol dehydrogenase from horse liver and baker's yeast. The inhibition of product formation in (a) was about 60% by various 4-methylpyrazole concentrations, but metyrapone had no effect. Following ultracentrifugation, only the supernatant of homogenized skin showed product formation (apparent Vmay: 32 pmol/min per cm2 skin; apparent Km: 64 microM). 3-Phenoxybenzyl alcohol and ethanol dehydrogenation was similar by alcohol dehydrogenase from horse liver (apparent Km: 0.7 vs. 0.4 mM; apparent Vmax: 0.3 vs. 0.2 U/ microg protein). In baker's yeast, the apparent Km of 3-phenoxybenzoic acid formation was several times larger than that for ethanol dehydrogenation. The KI of 4-methylpyrazole for alcohol dehydrogenase from horse liver was 0.6 (3-phenoxybenzyl alcohol) vs. 0.04 microM (ethanol). The KI for ethanol in baker's yeast was 470 microM. In conclusion dehydrogenation is an important metabolic pathway in the skin for xenobiotics with an aliphatic alcohol at a side chain.     

Carbon dioxide inhibition of yeast growth in biomass production

Saccharomyces cerevisiae was grown under aerobic and substrate-limiting conditions for efficient biomass production. Under these conditions, where the sugar substrate was fed incrementally, the growth pattern of the yeast cells was found to be uniform, as indicated by a constant respiratory quotient during the entire growing period. The effect of carbon dioxide was investigated by replacing portions of the nitrogen in the air stream with carbon dioxide, while maintaining the oxygen content at the normal 20% level, so that identical oxygen transfer rate and atmospheric pressure were maintained for all experiments with different partial pressures of carbon dioxide. Inhibition of yeast growth was negligible below 20% CO2 in the aeration mixture. Slight inhibition was noted at the 40% CO2 level and significant inhibition was noted above the 50% CO2 level, corresponding to 1.6 X 10(-2)M of dissolved CO2 in the fermentor broth. High carbon dioxide content in the gas phase also inhibited the fermentation activity of baker's yeast.     

Crossflow microfiltration of yeast suspensions in tubular filters

Crossflow microfiltration experiments were performed on yeast suspensions through 0.2-microns pore size ceramic and polypropylene tubes at various operating conditions. The initial transient flux decline follows dead-end filtration theory, with the membrane resistance determined from the initial flux and the specific cake resistance determined from the rate of flux decline due to cake buildup. For long times, the observed fluxes reach steady or nearly steady values, presumably as a result of the cake growth being arrested by the shear exerted at its surface. The steady-state fluxes increase with increasing shear rate and decreasing feed concentration, and they are nearly independent of transmembrane pressure. The steady-state fluxes for unwashed yeast in deionized water or fermentation media are typically 2-4 times lower than those predicted by a model based on the properties of nonadhesive, rigid spheres undergoing shear-induced back-diffusion. In contrast, the steady-state fluxes observed for washed yeast cells in deionized water are only 10-30% below the predicted values. The washed yeast cells also exhibited specific cake resistances that are an order of magnitude lower than those for the unwashed yeast. The differences are due to the presence of extracellular proteins and other macromolecules in the unwashed yeast suspensions. These biopolymers cause higher cell adhesion and resistance in the cake layer, so that the cells at the top edge are not free to diffuse away. This is manifested as a concentration jump from the edge of the cake layer to the sheared suspension adjacent to it.(ABSTRACT TRUNCATED AT 250 WORDS)     

Icodextrin use in CCPD patients during peritonitis: ultrafiltration and serum disaccharide concentrations

BACKGROUND AND METHODS: In a randomized study on the biocompatibility of icodextrin (I) versus glucose (G) in CCPD we used icodextrin or glucose for the long daytime dwell. During the night-time dwells glucose was used in all patients. In case of peritonitis icodextrin was continued. In all patients ultrafiltration (UF) was recorded and serum icodextrin metabolites were determined every 3 months and during peritonitis in I-users when available. RESULTS: Thirty-eight patients ( 19 G, 19 I) entered the study and suffered 30 peritonitis episodes (16 G, 14 I). During peritonitis (P), daytime dwell UF decreased significantly in G (P=0.001), but remained stable in I patients compared to non-peritonitis (NP) episodes. Total 24-h UF decreased in G (P=0.001) and in I patients (P=0.04), as the result of a decreased daytime UF and night-time UF, respectively. There was no difference in the used glucose concentrations during the P versus NP episodes. In five I-patients serum disaccharides increased from 0.05+/-0.01 to 1.26+/-0.23mg/ml during follow up. During peritonitis serum disaccharide concentrations did not increase further (1.47+/-0.24 mg/ml, P= 0.56). In I patients total carbohydrate minus glucose rose to 5.72 +/- 1.2 mg/ml during follow up, and to 6.63 +/- 1.04 mg/ml during peritonitis (P=0.7). These concentrations are comparable to CAPD patients despite the longer dwelltime in CCPD (8-10 versus 14-16 h, respectively). Adverse reactions attributable to icodextrin were not encountered. CONCLUSIONS: In contrast to glucose, icodextrin preserved the daytime dwell ultrafiltration during peritonitis. Serum icodextrin metabolites increased during icodextrin use, but remained stable during peritonitis. Adverse effects were not observed.     

Ultrafiltration after cardiopulmonary bypass in children: effects on hemodynamics, cytokines and complement

OBJECTIVES: The purpose of the study was to evaluate the clinical and hemodynamic effect of intraoperative extracorporeal ultrafiltration (UF) and its potential in reducing the plasma concentration of circulating cytokines and complement activation products following open heart surgery in children. METHODS: Eighteen children with congenital heart disease were prospectively randomized into a control group (n = 9) and a group who underwent UF (n = 9). Serial plasma samples for measurements of circulating cytokines (interleukin 6 (IL-6), tumor necrosis factor alpha (TNF), and its soluble receptor (sTNF receptor)), and complement factors (C3 activation products (C3a and C3bc) and terminal complement complex (TCC)) were obtained before, during and up to 48 h after cardiopulmonary bypass (CPB). A pulmonary artery thermodilution catheter was introduced preoperatively for hemodynamic monitoring. RESULTS: Postoperative hemodynamics were similar in both groups. Plasma levels of IL-6, sTNF receptors, C3a, C3bc and TCC increased significantly perioperatively (P < 0.01) in both groups. TNF was detected transiently in 16 patients perioperatively and in 4 of the 9 ultrafiltrate samples in concentrations similar to the plasma levels. Complement activating products were not detected in the ultrafiltration samples except for small amounts of C3a in two cases. Compared to the control group the plasma levels of C3a, C3bc and TCC were unaffected by the ultrafiltration procedure. The level of IL-6 and sTNF receptors increased significantly after 15 min of UF but there was no significant difference between the two groups postoperatively. CONCLUSIONS: In this study no clinical or hemodynamic effect was registered after UF. TNF and C3a were occasionally detected in the ultrafiltrate but we were unable to demonstrate reduction of these or any of the other markers tested in the group subjected to ultrafiltration.