Transmembrane Pressure Variation Research Articles

Electro living membrane bioreactor for highly efficient wastewater treatment and fouling mitigation: Influence of current density on process performances

The next generation of the self-forming dynamic membrane, referred to in this study as the “Living Membrane (LM)”, is a new patented technology based on an encapsulated biological layer that self-forms on a designed coarse-pore size support material during wastewater treatment and acts as a natural membrane filter. Integrating electrochemical processes with wastewater treatment using the LM approach has also been recently studied (the reactor is referred to as the Electro-Living Membrane Bioreactor or e-LMBR). This study investigated the effects of varying current densities, i.e., 0.3, 0.5, and 0.9 mA/cm2, on the performance of an e-LMBR. The results were also compared with those of the Living Membrane Bioreactor or LMBR (without applied current density).Higher pollutant removals were observed in the presence of the electric field. However, the effect of varying applied current densities on the COD (98–99 %), NH3-N (97–99 %), and PO43−P (100 %) removals was not statistically significant. The more prominent differences (p < 0.05) were observed in the decrease of NO3−-N concentrations from mixed liquor to effluent, with increasing current density resulting in lower mean NO3−-N effluent concentrations (0.3 mA/cm2: 6.13 mg/L; 0.5 mA/cm2: 4.38 mg/L; 0.9 mA/cm2: 3.70 mg/L). The reduction of NO3−-N concentrations as wastewater permeated through the LM layer also confirmed its role in removing nitrogen-containing compounds.Higher current densities resulted in lower concentrations of fouling substances, particularly those of microbial extracellular polymeric substances (EPS) and transparent exopolymer particles (TEPs). The average values of the temporal variation of transmembrane pressure (d(TMP)/d(t)) in the e-LMBR were extremely low, in the range of 0.013–0.041 kPa/day, throughout the operation period. The highest (d(TMP)/d(t)) was observed for the highest current density. However, the TMP values remained below 2 kPa in all the e-LMBR runs even after the initial LM formation stage.

Forecasting multicycle hollow fiber ultrafiltration fouling using time series analysis

This study proposes the application of time series analysis and exponential smoothing (ETS) to predict multicycle membrane fouling with high accuracy and great interpretability. First, lab-scale filtration tests were performed with foulant surrogates to assess the potential of ETS models in predicting multicycle membrane fouling. Two models, ETS (A,A,A) and ETS (A,A,M), were found to be most suitable for describing the variation of transmembrane pressure. The ETS (A,A,A) model showed a slight edge over the ETS (A,A,M) model when reversible fouling was dominant. In contrast, the ETS (A,A,M) performed better when irreversible fouling was prevalent. Then, the effectiveness of ETS models in multicycle forecasting during the filtration of natural water resources was evaluated. Residual mean squared errors between 0.02 bar and 0.15 bar were obtained for horizons going from 2 up to 10 filtration cycles. Finally, ETS models were applied to predict the variation in permeability of drinking water treatment plant membrane skid. A three-month dataset was utilized to predict the next 41 days of permeability variation, yielding an average error of 3.2 %. Along with that, a web application designed to assist membrane users in utilizing ETS was developed and presented at the end. The web application can be accessed through the following link: https://fouling-ets.shinyapps.io/shiny-ets-forecasting/.

Influence of Solid Retention Time on Membrane Fouling and Biogas Recovery in Anerobic Membrane Bioreactor Treating Sugarcane Industry Wastewater in Sahelian Climate.

Sugarcane industries produce wastewater loaded with various pollutants. For reuse of treated wastewater and valorization of biogas in a Sahelian climatic context, the performance of an anaerobic membrane bioreactor was studied for two solid retention times (40 days and infinity). The pilot was fed with real wastewater from a sugarcane operation with an organic load ranging from 15 to 22 gCOD/L/d for 353 days. The temperature in the reactor was maintained at 35 °C. Acclimatization was the first stage during which suspended solids (SS) and volatile suspended solids (VSS) evolved from 9 to 13 g/L and from 5 to 10 g/L respectively, with a VSS/SS ratio of about 80%. While operating the pilot at a solid retention time (SRT) of 40 days, the chemical oxygen demand (COD) removal efficiency reached 85%, and the (VSS)/(TSS) ratio was 94% in the reactor. At infinity solid retention time, these values were 96% and 80%, respectively. The 40-day solid retention time resulted in a change in transmembrane pressure (TMP) from 0.0812 to 2.18 bar, with a maximum methane production of 0.21 L/gCOD removed. These values are lower than those observed at an infinite solid retention time, at which the maximum methane production of 0.29 L/gCOD was achieved, with a corresponding transmembrane pressure variation of up to 3.1 bar. At a shorter solid retention time, the fouling seemed to decrease with biogas production. However, we note interesting retention rates of over 95% for turbidity.

Artificial intelligence modeling to predict transmembrane pressure in anaerobic membrane bioreactor-sequencing batch reactor during biohydrogen production

The complex nature of wastewater treatment has led to search for alternative strategies such as different artificial intelligence (AI) techniques to model the various operational parameters. The present work is aimed at predicting the transmembrane pressure (TMP) as a key operational parameter in the case of anaerobic membrane bioreactor-sequencing batch reactor (AnMBR-SBR) during biohydrogen production using the adaptive neuro-fuzzy inference systems (ANFIS) and artificial neural network (ANN). In both the models, organic loading rates (OLR) ranging from 0.5 to 8.0 g COD/L/d, effluent pH (3.6–6.9), mixed liquor suspended solid (4.6–21.5 g/L) and mixed liquor volatile suspended solid (3.7–15.5 g/L) were used as the input parameters to test TMP as an output parameter. The ANFIS model was trained using the hybrid algorithms for TMP prediction. The higher prediction performance was obtained by using the Gauss membership function with four membership numbers. A back-propagation algorithm was also employed for the feed forward training of ANN model; the best structure was a Levenberg-Marquardt training algorithm with nine neurons in the hidden layer. By employing ANFIS and ANN models, relatively a good prediction of TMP was obtained with the R2 values of 0.93 and 0.88, respectively while the calculated mean square error for TMP in the ANFIS model (7.3 × 10−3) was lower than that of ANN model (8.02 × 10−3). The higher R2 and lower MSE values for the ANFIS model exhibited a better TMP prediction performance than the ANN model. Finally, it was observed that in the sensitivity analysis of ANN model, OLR was the most important input parameter on the variation of TMP.

An Investigation of the Membrane Fouling Behaviors in Constant Flux Mode

Studying the effect of different operation conditions (bovine serum albumin (BSA) concentration, flux and stirring speed) on the membrane fouling behaviors (the variation of transmembrane pressure (TMP), cake resistance (Rc), pore blocking resistance (Rc ) and available membrane area ratio (A/A0 ) is of paramount importance in the membrane bioreactor (MBR) process. The constant flux filtration experiment was conducted to investigate the fouling behaviors of BSA solution, which provides a guidance for controlling the membrane fouling. The intermediate blocking-cake combined model was established and it was described mathematically with good results. The results showed that the influence sequence of operating conditions on the TMP was obtained as flux > concentration > stirring speed. Meanwhile, Rc decreased with the increase of stirring speed, and the A/A0 decreased with the increase of BSA concentration and the flux.

Membrane Fouling Due to Protein-Polysaccharide Mixtures in Dead-End Ultrafiltration; the Effect of Permeation Flux on Fouling Resistance.

Significant gaps exist in our knowledge of ultrafiltration (UF) membrane fouling, due to mixtures of poly-saccharides and proteins, despite a fair amount of related research. To get new insights into fouling layer characteristics, experiments were performed under constant-flux, within the range of practical interest (15–90 L/m2h), with typical polysaccharides (sodium alginate, SA), proteins (bovine serum albumin, BSA) as well as their mixtures in various proportions (1:3, 1:1, 3:1), and total organic matter concentration of 30 mg/L. The feed-water salinity and calcium ion concentration were 2000 mg/L NaCl and 2 mM, respectively. The temporal evolution of such fouling layers on flat-sheet membranes was monitored by recording the trans-membrane pressure variation. The results show that the specific fouling resistance α is strongly affected by flux, and the fouling propensity of polysaccharide-protein mixtures is significantly enhanced compared to single foulants, i.e., when BSA and SA are alone. The fouling layers are compressible and their resistance α tends to increase with the mass ratio of alginate in the mixture, particularly at high fluxes. To quantify these effects, correlations are presented of the initial fouling resistance αi with permeate flux J and of the evolution of α. R&D priorities are suggested on this topic of mixed foulants.

New insights into the rapid formation of initial membrane fouling after in-situ cleaning in a membrane bioreactor

Abstract In-situ chemical cleaning with sodium hypochlorite (NaClO) is often employed for maintaining constant permeability in a membrane bioreactor (MBR). In this study, the effect of NaClO shock on the rapid formation of initial membrane fouling was investigated over the course of the in-situ cleaning of a MBR. Variations of trans-membrane pressure (TMP) in the different runs indicated that the bulk sludge after NaClO-shock promotes the rapid formation of initial membrane fouling. After in-situ cleaning, the sludge had a high protein content in different extracellular polymeric substance layers and a substantial deterioration in sludge filterability compared to raw sludge. Based on the extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory, NaClO shock significantly changes the surface properties of raw sludge. It reduces the surface electron donor component ( γ − ) and increases surface hydrophobicity. The sludge after NaClO-shock exhibited a stronger adhesive energy with the membrane and higher self-cohesive ability, resulting in severe membrane fouling.

Gas-liquid membrane contactors: Modeling study of non-uniform membrane wetting

Current available models for simulation of the separation process in gas-liquid membrane contactor (MC) systems consider a uniform partial membrane wetting along the membrane length (Lmem). However, transmembrane pressure changes along the Lmem. Additionally, the available models usually consider a simplified uniform pore size to represent the micro-porous MC. Additionally, not all pores in a MC have the same size. In this study, a pore-scale network model is developed to simulate the physical separation of H2S using MCs by taking to account for (i) transmembrane pressure variation and (ii) pore size distribution of MC. The model results are compared with the experimental results of H2S separation. Modeling results indicate that membrane wetting is non-uniformly distributed along Lmem. The membrane wetting ratio is maximum at the inlet of the liquid phase, and it decreases toward the liquid outlet. Also, increasing the inlet gas pressure reduces the membrane wetting ratio, and leads to higher H2S separation. Moreover, the model results indicate that shifting of the pore size distribution of MC towards smaller pore size declines the membrane wetting ratio and increases the H2S separation efficiency.

The processing of used cooking oil (yellow grease) using combination of adsorption and ultrafiltration membrane processes

Yellow grease is used cooking oil whose quality has degraded due to the oxidation, polymerization, or hydrolysis process. In previous studies, yellow grease refining had been conducted either by adsorption or by using membrane. In this study, adsorption process using adsorbent from bagasse activated with H3PO4 12.5%, and ultrafiltration using Polyethersulfone (PES) membrane were combined. In adsorption stage, several variation of bagasse mass was fed into 200 ml of yellow grease and stirred for 60 minutes at 60 rpm. Yellow grease produced from adsorption with best condition was then processed using ultrafiltration membran that is PES membran with concentration by 15 wt % with transmembrane pressure variation by 0.5; 1; 1.5; 2; and 2.5 Bar. Analysis of yellow grease characteristics before refined showed its acid number, peroxide number, iodine number, and water content respectively by 2.68 mgKOH/Kg; 5.97 Meq/Kg; 51,48; and 1.29%. Characteristics of yellow grease after adsorption at its best condition on the parameters of acid number, peroxide number, iodine number, and water content are respectively by 2.55 mgKOH/Kg; 4.19 Meq/Kg; 40,02; and 0.27%. Characteristics of yellow grease after ultrafiltration at its best condition on the parameters of acid number, peroxide number, iodine number, and water content are respectively by 1.12 mgKOH/Kg; 1.8 Meq/Kg; 41,36; and 0.02%. Combination of adsorption and ultrafiltration processes for yellow grease processing showed decreasing value on the parameters of acid number, peroxide number, and water content that conforms to the SNI quality standard, but has not been able to increase the iodine number.

Critical flux investigation in treating o/w emulsion by TiO2/Al2O3-PVDF UF membrane.

A standard transmembrane pressure (TMP) step method has been used in membrane fouling assessment in tube ultrafiltration (UF) membrane system treating oil water (o/w) emulsion operated at constant TMP. Three flux reduction curve with different o/w concentration based on TMP variation were concluded by experiment, then, to describe fouling behavior and identify the occurrence of fouling in the so-called critical flux. Furthermore, sub-critical and super-critical flux experiment with a long time was determined, and zero rate of flux reduction (dF/dt) was never found during the whole trial period, indicating that critical flux in o/w UF process with its strict definition could not be defined in this paper. However, quasi-critical flux exists, under which the pollution rate was very slow. Moreover, a high-efficiency four steps cleaning method: mechanic scraping, pure water wash, pure water reverse wash, and dosing cleaning, was explored. It concluded that critical flux in real o/w UF system determined by TMP-step method can be used to predict long-term critical behavior with useful data on fouling propensity.

A Comparison Between Coagulation and Ultrafiltration Processes for Biodiesel Wastewater Treatment

The progressive increasing on biodiesel production has been generating a large amount of wastewater due to the biodiesel purification process. The present work aims to develop and evaluate different processes to improve the quality of biodiesel wastewater to achieve legislation requirements for its final disposal. In order to do that membrane filtration and coagulation processes were tested. Micro and ultrafiltration ceramic membranes obtained by isostatic pressing were used for the cross-flow filtration process and tannin coagulant (Tanfloc SG) was employed in the coagulation process. A pre-treatment for grease removal was accomplished by adding sulfuric acid the wastewater until pH 2.5. Ultrafiltration membrane (alumina-zirconia and sintered at 1,500 °C) showed better results for color, turbidity and Chemical Oxygen Demand (COD) removal. These tests were conducted at a transmembrane pressure of 2 bar and keeping constant the other operational parameters. Once the best membrane was determined, the variation of transmembrane pressure was investigated by increasing it to 3 bar. The alumina zirconia ultrafiltration membrane, using the pre-treated wastewater, had a COD, color and turbidity removal of 19.5, 74.1 and 100%, respectively. The biodiesel wastewater was submitted to coagulation experiments for different pH values and tannin concentration. Through the coagulation tests, the pre-treated effluent with the higher tannin concentration, at pH of 8, had better results when compared to the untreated wastewater. The best coagulation results showed a COD, color and turbidity removal of 73, 69 and 75%, respectively. Both ultrafiltration and coagulation presented good efficiency for organic matter removal, but the coagulation process reached higher COD removal.

Application of electrochemical processes to membrane bioreactors for improving nutrient removal and fouling control.

Membrane bioreactor (MBR) technology is becoming increasingly popular as wastewater treatment due to the unique advantages it offers. However, membrane fouling is being given a great deal of attention so as to improve the performance of this type of technology. Recent studies have proven that the application of electrochemical processes to MBR represents a promising technological approach for membrane fouling control. In this work, two intermittent voltage gradients of 1 and 3V/cm were applied between two cylindrical perforated electrodes, immersed around a membrane module, at laboratory scale with the aim of investigating the treatment performance and membrane fouling formation. For comparison purposes, the reactor also operated as a conventional MBR. Mechanisms of nutrient removal were studied and membrane fouling formation evaluated in terms of transmembrane pressure variation over time and sludge relative hydrophobicity. Furthermore, the impact of electrochemical processes on transparent exopolymeric particles (TEP), proposed as a new membrane fouling precursor, was investigated in addition to conventional fouling precursors such as bound extracellular polymeric substances (bEPS) and soluble microbial products (SMP). All the results indicate that the integration of electrochemical processes into a MBR has the advantage of improving the treatment performance especially in terms of nutrient removal, with an enhancement of orthophosphate (PO4-P) and ammonia nitrogen (NH4-N) removal efficiencies up to 96.06 and 69.34%, respectively. A reduction of membrane fouling was also observed with an increase of floc hydrophobicity to 71.72%, a decrease of membrane fouling precursor concentrations, and, thus, of membrane fouling rates up to 54.33%. The relationship found between TEP concentration and membrane fouling rate after the application of electrochemical processes confirms the applicability of this parameter as a new membrane fouling indicator.

Sequential pressure-driven membrane operations to recover and fractionate polyphenols and polysaccharides from second racking wine lees

Abstract Wine lees from red wine production are important sources for the recovery of antioxidant compounds, namely polyphenols and polysaccharides. In this work a set of ultrafiltration and nanofiltration membranes was investigated aiming the fractionation of polyphenols and polysaccharides present in wine lees. Permeation fluxes and solute rejection coefficients were evaluated in terms of transmembrane pressure variation. In all the evaluated pressure range (3–15 bar), NF membrane presented a linear increase in permeation fluxes such that the ratio between permeabilities (feed solution/water) stayed constant at 0.72. In contrast, for the UF membranes this ratio decreases by increasing the pressure and the membrane’s MWCO. For membranes with MWCO of 1000 and 10,000 Da, the permeabilities’ ratio is reduced from 0.34 to 0.23 and from 0.29 to 0.17, respectively, as the pressure is increased from 5 to 15 bar. Ultrafiltration has proven to be effective to separate the polysaccharides of the polyphenols, so that the polysaccharides mainly stayed in the retentate stream, while polyphenols preferentially permeated these membranes. Sequentially, considering that the NF270 membrane presented polyphenols rejections higher than 92%, the polyphenols that permeated the ultrafiltration membrane may be concentrated by nanofiltration, and therefore, it can be achieved a concentrated solution having higher antioxidant activity. Thus, the integration of different membrane separation processes emerges as an alternative for the recovery of polyphenols and polysaccharides.

Model-based analysis of the effect of different operating conditions on fouling mechanisms in a membrane bioreactor

This study proposes a model-based evaluation of the effect of different operating conditions with and without pre-denitrification treatment and applying three different solids retention times on the fouling mechanisms involved in membrane bioreactors (MBRs). A total of 11 fouling models obtained from literature were used to fit the transmembrane pressure variations measured in a pilot-scale MBR treating real wastewater for more than 1year. The results showed that all the models represent reasonable descriptions of the fouling processes in the MBR tested. The model-based analysis confirmed that membrane fouling started by pore blocking (complete blocking model) and by a reduction of the pore diameter (standard blocking) while cake filtration became the dominant fouling mechanism over long-term operation. However, the different fouling mechanisms occurred almost simultaneously making it rather difficult to identify each one. The membrane "history" (i.e. age, lifespan, etc.) seems the most important factor affecting the fouling mechanism more than the applied operating conditions. Nonlinear regression of the most complex models (combined models) evaluated in this study sometimes demonstrated unreliable parameter estimates suggesting that the four basic fouling models (complete, standard, intermediate blocking and cake filtration) contain enough details to represent a reasonable description of the main fouling processes occurring in MBRs.

Membrane fouling of hybrid submerged membrane bioreactor (hMBR) in treating municipal wastewater

A hybrid submerged membrane bioreactor (hMBR) with porous polyvinylchloride carriers was used to treat the municipal wastewater, and a parallel MBR (pMBR) without the porous polyvinylchloride carriers was run as a contrast. The results of the chemical oxygen demand and nitrogen removal demonstrated that the hMBR process was more efficient than pMBR. Membrane fouling was analyzed by variations of trans-membrane pressure. It is believed that the carriers served to scrub the membrane and adsorb extracellular polymeric substance of the biofilm, therefore, extending the operation time of the hMBR membrane fouling. The analysis of molecular-based methods showed that the two MBRs had the same kinds of gene express forms, but the pMBR had more total bacteria than the hMBR to form biofilm on the membrane, which suggested that more serious membrane fouling in the pMBR than in the hMBR.

Impact of operating conditions on the flux changing rate during dead-end microfiltration process

Dead-end microfiltration experiments with the activated sludge suspension from submerged membrane bioreactors were performed to determine the critical flux, which is a very significant factor to indicate the membrane fouling in microfiltration process. In this study, the critical permeation fluxes were measured by successive variations of transmembrane pressure (step by step technique) under different operating conditions (temperatures, stirring rate and concentrations of the mixed liquor suspended solid [MLSS]) using polyvinylidene fluoride membranes with a pore size of 0.1 μm. A study of the impact of various operating conditions on the fouling rate when the critical flux is achieved was especially made. The experiments revealed that: the increase in stirring rate leads to a decrease in the rate of fouling; the fouling rate decreased with the increase in temperature when it was less than a certain values, after that, it increased with the continuous increase in temperature, while the variation of the concentration of the MLSS has a negative influence on the membrane fouling. The results obtained from the experiments can provide reliable information to the optimization of operating conditions to alleviate membrane fouling.

Application of a Hybrid Process Combining Ozone-Coagulation-Microfiltration for Drinking Water Production

The study concerns on drinking water treatment with combining ozonation process, coagulation and Microfiltration (MF) of Poly-vinylidene Fluoride (PVDF) in a dead-end filtration. The pilot-scale process consists of several parts: pre-ozonation, coagulation, sedimentation and microfiltration membrane separation. Ozone of 0.5mg/l and alum of 10 mg/l were added into raw water. The experience focuses on investigation of permeate water quality and variation of trans-membrane pressure (TMP). The results of the study showed that good permeate quality were fulfilled. Turbidity of permeate was consistently below 0.1 NTU. The MF membrane removed iron perfectly, which was below 0.3 mg/l. The reduction of manganese and CODMn met the China Drinking Water Regulations. Additionally, TMP was stable during filtration for a long time and ozone could delay membrane fouling. Chemical cleaning of membrane could decrease TMP to initial level, and chemical analysis for chemical cleaning solution shown that organic matters and manganese caused membrane fouling.

Impact of Protein Interactions and Transmembrane Pressure on Physical Properties of Filter Cakes Formed during Filtrations of Skim Milk

Microfiltration (0,1μm) was used to fractionate casein (d50,3 = 180nm) micelles from whey proteins (2 - 6nm). The casein fraction forms a deposited layer on the membrane surface. Little is known about the structure of these layers and how transmembrane pressure affects their structure. In order to assess the properties, casein micelle deposits were characterized by dead-end microfiltrations. Casein micelle deposits of constant composition were formed following a standardized layer build up (permeate mass, pressure, pH). Then protein free milk serum was filtered through the deposit. Thus, the impact of compressive forces and protein-protein interactions on physical cake properties during constant solid height filtrations could be determined by stepwise variations of transmembrane pressure and pH without further deposition of proteins. It was found that the casein micelle deposits became more compactable, when their surface charge was lower. Specific cake properties were related to hydrophilic repulsion between casein micelles, whereas the influence of electrostatic interactions between micelles was negligible. The observed cake material properties obtained from dead-end filtrations provide valuable insights into deposit layer build-up and structure.Results obtained for dead-end filtrations were used to describe cross-flow filtrations. For this purpose a new method was developed to assess the specific cake resistances by the evaluation of the kinetic of flux decrease due to deposit layer build-up at the start of filtrations. For the first time it could be shown that deposits consisting of casein micelles during cross-flow filtration were thin layers (1-3μm), resulting in high specific resistances (up to 20•10-15 m/kg). The specific cake resistance was pressure dependent and increased when the hydrophilic repulsion between casein micelles was reduced at a pH beyond the isoelectric point.

Modeling of Membrane Fouling Based on Extracellular Polymers in Submerged MBR

To offer an overall conceptual understanding and extend the knowledge of combined submerged MBR process, a modified membrane fouling model was developed. The model was constructed based on Extracellular Polymers (EPS) which increased with biomass growth in submerged MBR system. Due to density of EPS accumulated on the membrane surface caused trans-membrane pressure increase which finally results in flux decrease, the membrane fouling model adopted EPS concentration in mixed liquor and EPS density on membrane surface as main influence factors for trans-membrane pressure variation. Simulation results show that the fouling model could well predict membrane fouling in submerged MBR which three zones as anoxic, aerobic and anaerobic, it could offer operational and design parameters decision and optimal operational conditions.

The impact of in-line coagulant addition on fouling potential of secondary effluent at a pilot-scale immersed ultrafiltration plant

The impact of in-line coagulation pre-treatment of secondary effluent on the operation of an immersed hollow-fibre ultrafiltration membrane pilot was evaluated as part of a larger study on optimising phosphorus removal. The efficacy of alum and ferric chloride was investigated, with an emphasis on alum use. Both coagulants were found to shift the particle-size distribution of organic matter in the feed towards larger fractions, with a notable reduction in colloidal matter. This was reflected in a reduction of both average daily transmembrane pressure increases, as well as a reduction of transmembrane pressure increases within backpulse intervals. Fouling reduction was observed with both lower and higher membrane packing density modules (membrane surface areas of 55.7 and 62.7 m 2/module). The results of one-way analysis of variance (ANOVA) testing indicate that for this pilot system, chemical pre-treatment and solids concentrations in the feed water played a statistically significant role in determining transmembrane pressure variations. Membrane packing density and membrane production method did not exhibit a statistically significant effect on transmembrane pressure under the conditions of this study.