Increase In Transmembrane Pressure Research Articles

Application of pulsed UV-irradiation and pre-coagulation to control ultrafiltration membrane fouling in the treatment of micro-polluted surface water

A major cause of ultrafiltration (UF) membrane fouling is the accumulation of microorganisms and their associated soluble products. To mitigate fouling the application of pulsed short-wavelength ultraviolet (UVC) light (around 254 nm) within the membrane tank together with pre-coagulation was investigated. In mini-pilot-scale tests carried out in parallel with conventional pre-treatment (CUF), the impact of pulsed UV (CUF-UV) at different UV irradiances and fluxes on the increase of trans-membrane pressure (TMP) was evaluated and explained in terms of the quantity and nature of membrane deposits in the membrane cake layer and pores.The results indicated that at a flux of 20 L m−2 h−1, the pulsed UV (1 min within 31 min cycle) at 3.17 × 10−2 W/cm2 prevented any measureable increase in TMP over a period of 32 days, while there was a fourfold increase in TMP for the conventional pre-treatment. For the CUF-UV system the concentration of bacteria and soluble microbial products was much less than the conventional CUF system, and the cake layer was thinner and contained less biopolymers (proteins and polysaccharides). In addition, the pores of the CUF-UV membrane appeared to have less organic deposits, and particularly fractions with a high molecular weight (>10 kDa).At a lower UV irradiance (1.08 × 10−2 W/cm2), or higher flux (40 L m−2 h−1) with the same UV irradiance, there was a measurable increase in TMP, indicating some fouling of the CUF-UV membrane, but the rate of TMP development was significantly lower (∼50%) than the conventional CUF membrane system. Overall, the results show the potential advantages of applying intermittent (pulsed) UVC irradiation with coagulation to control UF membrane fouling.

Application of polyacrylamide flocculation with and without alum coagulation for mitigating ultrafiltration membrane fouling: Role of floc structure and bacterial activity

There is a growing interest in the use of ultrafiltration (UF) for the treatment of micro-polluted surface waters for drinking water supplies. Effective pretreatment is required to mitigate membrane fouling and in this paper we have evaluated the application of polyacrylamide (PAM) flocculation with alum coagulation. Bench scale tests were conducted over extended periods with two types of PAM (different molecular weights (MW)) applied with, and without alum coagulation, in order to investigate their impact on membrane fouling. The structure of the resulting flocs formed in the process and the activity of bacteria within the membrane tank were identified as two key factors influencing UF system performance. It was found that development of the cake layer and hydraulic resistance of the membrane were influenced by the floc properties, which were in turn related to the MW and dose of the PAM. Coagulation and flocculation using the larger MW PAM formed amorphous flocs with a lower fractal dimension, which contributed to a lower density of the cake layer and lower rate of increase in trans-membrane pressure. PAM flocculation without alum coagulation induced severe membrane fouling by forming a continuous gel-like layer on the membrane surface. By alum-PAM dosing it was found that the concentration of bacteria present in the membrane tank and adhering to the cake layer, was sufficient to remove nearly all of the ammonia and around 80% phosphorus in the raw water. These results demonstrate that the combination of a high MW PAM with alum as a pretreatment method of UF process can effectively improve the floc properties and cake layer structure for controlling membrane fouling and producing high quality treated water.

Effects of Quorum Quenching on the Microbial Community of Biofilm in an Anoxic/Oxic MBR for Wastewater Treatment.

Recently, bacterial quorum quenching (QQ) has been proven to have potential as an innovative approach for biofouling control in membrane bioreactors (MBRs) for advanced wastewater treatment. Although information regarding the microbial community is crucial for the development of QQ strategies, little information exists on the microbial ecology in QQ-MBRs. In this study, the microbial communities of biofilm were investigated in relation to the effect of QQ on anoxic/oxic MBRs. Two laboratory-scale MBRs were operated with and without QQ-beads (QQ-bacteria entrapped in beads). The transmembrane pressure increase in the QQ-MBRs was delayed by approximately 100-110% compared with conventional- and vacant-MBRs (beads without QQ-bacteria) at 45 kPa. In terms of the microbial community, QQ gradually favored the development of a diverse and even community. QQ had an effect on both the bacterial composition and change rate of the bacterial composition. Proteobacteria and Bacteroidetes were the most dominant phyla in the biofilm, and the average relative composition of Proteobacteria was low in the QQ-MBR. Thiothrix sp. was the dominant bacterium in the biofilm. The relative composition of Thiothrix sp. was low in the QQ-MBR. These findings provide useful information that can inform the development of a new QQ strategy.

Application of ceramic membrane for seawater desalination pretreatment

AbstractIn this study, a ceramic microfiltration membrane was examined as a pretreatment of reverse osmosis (RO) process for seawater desalination. The performance of ceramic membrane with and without coagulant were compared in terms of permeate qualities and transmembrane pressure (TMP). Various parameters were analyzed, such as turbidity, dissolved organic carbon (DOC), UV254, SDI15 (silt density index), and TMP. When ceramic membrane system was operated without coagulation at flux 2 m3/m2 d, SDI15 value and turbidity of permeate water was less than 3 and 0.1 NTU, respectively. However, SDI15 and turbidity were reduced to 0.9 and 0.076 NTU, respectively, when coagulant of 6 mg/L was applied. The comparison of TMP on different membrane flux was controlled flux 2 and 5, 10 m3/m2 d with and without coagulation. The increase of TMP was significantly mitigated from 90 to 13.8 kPa at flux 5 m3/m2 d, from 87.4 to 21.5 kPa at flux 10 m3/m2 d, respectively, by coagulation followed by filtration. Ceramic MF syste...

Optimization of protein fractionation by skim milk microfiltration: Choice of ceramic membrane pore size and filtration temperature

The objective of this study was to investigate how ceramic membrane pore size and filtration temperature influence the protein fractionation of skim milk by cross flow microfiltration (MF). Microfiltration was performed at a uniform transmembrane pressure with constant permeate flux to a volume concentration factor of 2.5. Three different membrane pore sizes, 0.05, 0.10, and 0.20µm, were used at a filtration temperature of 50°C. Furthermore, at pore size 0.10µm, 2 different filtration temperatures were investigated: 50 and 60°C. The transmission of proteins increased with increasing pore size, giving the permeate from MF with the 0.20-µm membrane a significantly higher concentration of native whey proteins compared with the permeates from the 0.05- and 0.10-µm membranes (0.50, 0.24, and 0.39%, respectively). Significant amounts of caseins permeated the 0.20-µm membrane (1.4%), giving a permeate with a whitish appearance and a casein distribution (αS2-CN: αS1-CN: κ-CN: β-CN) similar to that of skim milk. The 0.05- and 0.10-µm membranes were able to retain all caseins (only negligible amounts were detected). A permeate free from casein is beneficial in the production of native whey protein concentrates and in applications where transparency is an important functional characteristic. Microfiltration of skim milk at 50°C with the 0.10-µm membrane resulted in a permeate containing significantly more native whey proteins than the permeate from MF at 60°C. The more rapid increase in transmembrane pressure and the significantly lower concentration of caseins in the retentate at 60°C indicated that a higher concentration of caseins deposited on the membrane, and consequently reduced the native whey protein transmission. Optimal protein fractionation of skim milk into a casein-rich retentate and a permeate with native whey proteins were obtained by 0.10-µm MF at 50°C.

Treatment performance and membrane fouling characteristics of inclined-tube anoxic/aerobic membrane bioreactor applied to municipal solid waste leachate

This study investigated the performance of inclined tube anoxic/aerobic membrane bioreactor (itMBR) treating concentrated municipal solid waste leachate over 550 d at organic loading rates of 4.2–31.8 kg BOD/m3 d. At optimum loading of 18 kg BOD/m3 d, the itMBR could achieve high removals of 97% for biochemical oxygen demand, 94% for chemical oxygen demand, and 95% for total Kjeldahl nitrogen. Complete elimination of protein-like substances and partial removal of fulvic and humic acid-like substances were achieved. Major membrane foulants were found to be gel layers containing protein-like and polysaccharide-like substances and Ca-based inorganic scaling. Membrane cleaning using sodium hypochlorite followed by citric acid was found effective for the restoration of transmembrane pressure (TMP) while periodical chemical enhanced back-flushing at every 7–10 d could limit TMP increase rate and sustain the operation in long term.

Fouling propensity of a poly(vinylidene fluoride) microfiltration membrane to several model oil/water emulsions

Laboratory membrane fouling studies are often performed with a single foulant. However, studies comparing the behavior of different foulants using a single membrane are rarely reported. In this study, a poly(vinylidene fluoride) (PVDF) microfiltration membrane was challenged with a series of aqueous-based model fouling media, including a suspension of latex beads, as well as soybean, motor and crude oil emulsions, in constant permeate flux fouling experiments. The critical and threshold fluxes were determined for each membrane-foulant pair. Constant permeate flux crossflow fouling experiments were performed at both low and high fluxes. A direct comparison of the fouling propensity of the PVDF membrane to the four fouling media was made. The fouling propensity was evaluated based on threshold flux values and the extent of transmembrane pressure (TMP) increase during constant permeate flux fouling experiments. In this study, the zeta potential of various fouling media correlated with their fouling propensities. The higher the zeta potential, the lower the fouling propensity. The fouling propensity followed the order of: latex beads<soybean oil<crude oil<motor oil. A three-stage TMP profile was observed with high fouling media, such as motor and crude oil emulsions. The TMP increased slowly in the early stage, then increased abruptly, and eventually reached a new pseudo-steady state TMP.

Antibiofilm effect enhanced by modification of 1,2,3-triazole and palladium nanoparticles on polysulfone membranes.

Biofouling impedes the performance of membrane bioreactors. In this study, we investigated the antifouling effects of polysulfone membranes that were modified by 1,2,3-triazole and palladium (Pd) nanoparticles. The modified membranes were evaluated for antibacterial and antifouling efficacy in a monoculture species biofilm (i.e., drip flow biofilm reactor, DFR) and mixed species biofilm experiment (i.e., aerobic membrane reactor, AeMBR). 1,2,3-triazole and Pd nanoparticles inhibited growth of Pseudomonas aeruginosa in both aerobic and anaerobic conditions. The decrease in bacterial growth was observed along with a decrease in the amount of total polysaccharide within the monoculture species biofilm matrix. When the modified membranes were connected to AeMBR, the increase in transmembrane pressure was lower than that of the non-modified membranes. This was accompanied by a decrease in protein and polysaccharide concentrations within the mixed species biofilm matrix. Biomass amount in the biofilm layer was also lower in the presence of modified membranes, and there was no detrimental effect on the performance of the reactor as evaluated from the nutrient removal rates. 16S rRNA analysis further attributed the delay in membrane fouling to the decrease in relative abundance of selected bacterial groups. These observations collectively point to a lower fouling occurrence achieved by the modified membranes.

Evaluation of ferrate as a coagulant aid/oxidant pretreatment for mitigating submerged ultrafiltration membrane fouling in drinking water treatment

Although pre-coagulation can mitigate ultrafiltration (UF) membrane fouling in the treatment of surface waters for drinking water supply, biological activities (‘biofouling’) can induce a continuous increase in membrane fouling. To meet this challenge, potassium ferrate, K2FeO4, a combined oxidant and coagulant, was evaluated as a pre-treatment chemical for controlling submerged UF membrane fouling in water treatment. Ferrate use as an alternative to- (phase 1: ∼23days), and in combination with- (phase 2: ∼30days), conventional FeCl3, have been studied using parallel continuous bench-scale submerged membrane systems, using FeCl3 as the reference. The poorer performance of ferrate (alone) as a pre-treatment compared to FeCl3 (phase 1) was the result of a lower coagulation efficiency, which outweighed the beneficial impact of the ferrate on bacterial inactivation. The net reduction in pre-treatment performance led to an increase in the concentration of residual, active bacteria in the membrane tank, and bacteria associated large molecular weight (MW) organic substances, such as extracellular polymeric substances (EPS) or biopolymers, which were the principal cause of the higher rate of membrane fouling observed. In contrast, ferrate performed best as a coagulant aid/oxidant (FeCl3/K2FeO4) (phase 2), with the rate of membrane fouling (increase in transmembrane pressure) 4.5 times lower than conventional FeCl3 pre-treatment. This pre-treatment arrangement resulted in less bacteria (and EPS) and suspended solids in the membrane tank, and less accumulation of materials in the cake layer and within the membrane pores. The results indicated clearly the potential benefit of applying ferrate as a coagulant aid/oxidant with a coagulant, in UF pre-treatment, with the control of bacteria and EPS a key factor in reducing membrane fouling.

Removal of iron, manganese and ammonia from groundwater using a PAC-MBR system: The anti-pollution ability, microbial population and membrane fouling

Iron, manganese and ammonia are common inorganic pollutants in groundwater. In the current study, a powdered activated carbon-amended membrane bioreactor (PAC-MBR) was designed for the treatment of groundwater (approximately Mn2+1.2mg·L−1, Fe2+=15.0mg·L−1 and NH4+-N=2.2mg·L−1). Two lab-scale PAC-MBR systems were implemented to operate for 220 d to purify three influents that had different levels of pollutants, including raw water and the effluent of conventional bio-sand filters with different dissolved oxygen levels (i.e., 9mg·L−1 and 6mg·L−1) before filtration. The results showed that an excellent MBR effluent (iron<0.2mg·L−1, manganese<0.1mg·L−1 and ammonia<0.05mg·L−1) was obtained irrespective of the amount of pollutants, but the start-up period varied (20–49d). With respect to the trans-membrane pressure (TMP) evolution, system I (PVDF) exhibited good performance with a slight TMP increase from 7.0 to 17.0kPa, whereas system II (PVC) exhibited a sharp rise from 20.0 to 60.0kPa. In brief, a cake layer containing PAC and oxides was the main reason for membrane fouling. High-throughput sequencing analysis revealed that Mn-oxidizing bacteria (MnOB) (i.e., Leptothrix, Pseudomonas, Hyphomicrobium and Planctomyces) and ammonia-oxidizing bacteria (AOB) (i.e., Nitrospira, Nitrosomonas) played major roles in the PAC-MBR systems.

Coagulation and oxidation for controlling ultrafiltration membrane fouling in drinking water treatment: Application of ozone at low dose in submerged membrane tank

Coagulation prior to ultrafiltration (UF) is widely applied for treating contaminated surface water sources for potable supply. While beneficial, coagulation alone is unable to control membrane fouling effectively in many cases, and there is continuing interest in the use of additional, complementary methods such as oxidation in the pre-treatment of raw water prior to UF. In this study, the application of ozone at low dose in the membrane tank immediately following coagulation has been evaluated at laboratory-scale employing model raw water. In parallel tests with and without the application of ozone, the impact of applied ozone doses of 0.5 mg L−1 and 1.5 mg L−1 (approximately 0.18 mg L−1 and 0.54 mg L−1 consumed ozone, respectively) on the increase of trans-membrane pressure (TMP) was evaluated and correlated with the quantity and nature of membrane deposits, both as a cake layer and within membrane pores. The results showed that a dose of 0.5 mgO3 L−1 gave a membrane fouling rate that was substantially lower than without ozone addition, while a dose of 1.5 mgO3 L−1 was able to prevent fouling effects significantly (no increase in TMP). Ozone was found to decrease the concentration of bacteria (especially the concentration of bacteria per suspended solid) in the membrane tank, and to alter the nature of dissolved organic matter by increasing the proportion of hydrophilic substances. Ozone decreased the concentration of extracellular polymeric substances (EPS), such as polysaccharides and proteins, in the membrane cake layer; the reduced EPS and bacterial concentrations resulted in a much thinner cake layer, although the suspended solids concentration was much higher in the ozone added membrane tank. Ozone also decreased the accumulation and hydrophobicity of organic matter within the membrane pores, leading to minimal irreversible fouling. Therefore, the application of low-dose ozone within the UF membrane tank is a potentially important approach for fully mitigating membrane fouling.

Comparison of axial vibration membrane and submerged aeration membrane in microalgae harvesting

The submerged aeration membrane (SAM) system and axial vibration membrane (AVM) system can mitigate membrane fouling. In this study, both systems were investigated to compare the performance of filtration and the membrane fouling in algae filtration. In 5-h filtration, the transmembrane pressure (TMP) of SAM reached to 70.0kPa, while there was almost no increase in TMP for AVM. After continuous filtration, it could be found that there was hardly any algae cells on the membrane of AVM (0.11g/m2), which was about 32.4 times less than that of SAM (3.56g/m2). Compared with the SAM system, AVM had a lesser membrane fouling, regardless of the reversible fouling or irreversible fouling. By SEM, FTIR and EEM, it could be found there was less irreversible extracellular organic matter (EOM) on the membrane of AVM. By MW distribution, it could be observed that less EOM with high-MW adhered to membrane of AVM.

Use of nanofiltration to reject cobalt (II) from ammoniacal solutions involved in absorption of SO2/NOx

The retention behavior of cobalt(II) by nanofiltration from ammoniacal solutions used for the simultaneous absorption of SO2/NOx was investigated. The cobalt(II) rejections in aqueous ammoniacal solutions were higher than 99% by the membrane with alkaline stability and a molecular weight cut-off of ~200Da. The presence of small and mobile NH4+ in the feed solution improved the cobalt(II) retention. It was found that NH4NO3 had little effect on the permeation flux or cobalt(II) rejection while (NH4)2SO4 lowered both. The effects of initial cobalt(II) concentration (0.006–0.03mol/L) in the feed on cobalt(II) rejection depended on the compositions of the solution. An increase in transmembrane pressure (0.4–0.8MPa) led to higher cobalt(II) rejections for all solutions. Cobalt precipitates were generated during the filtration process due to the loss of chelating agent NH3·H2O. The decline in permeation flux and cobalt(II) rejection could be recovered by effective cleaning. Adding NH3·H2O during the filtration process could prevent the formation of cobalt precipitates so as to avoid deterioration in the membrane performance.

Development of a continuous membrane reactor process for enzyme-catalyzed lactulose synthesis

The study addresses three main issues which are usually encountered in membrane-assisted enzymatic reactions, such as membrane–enzyme electrostatic interaction, optimum operating conditions (with industrially relevant parameters) and maintenance of catalytic activity for prolonged synthesis. Considering Aspergillus oryzae β-galactosidase charge distribution at pH 4.6 was largely positive, a negatively charged PES membrane was not suited for the synthesis. With this, a rapid increase in transmembrane pressure occurred during continuous lactulose synthesis. The UFX membrane MWCO 10kDa is permanently hydrophilic and exhibited repulsion to the enzyme molecules and in turn, had an insignificant transmembrane pressure increase. Thus, this membrane was selected for further investigations. The operating conditions were optimized successively and were found to be: enzyme concentration=10U/mL, molar ratio of lactose to fructose=1/4, hydraulic residence time=9h and incubation temperature=40°C. With an inaccuracy of enzyme dosing protocol less than 5%, a long-term synthesis of lactulose was performed for 28 days with an average lactulose outlet concentration of 8.67g/L. The enzyme activity was maintained by addition of fresh enzyme every 48h. This is the longest continuous lactulose synthesis ever reported in fully automated membrane reactor.

Membrane Treatment of Secondary Effluent for Subsequent Use

Granular-media filtered secondary effluent from a full-scale plant was subsequently treated at pilot-plant scale by combinations of low- and high pressure membranes. The feedwater was split between microfiltration (MF) and ultrafiltration (UF) treatment trains; ferric chloride (4 mg/L) was added to the UF feedwater. Filtrate from each of these trains became the feedwater to three different types of high-pressure membranes operating in parallel, two reverse osmosis (RO) units and one nanofiltration (NF) unit; no chemicals (e.g., chlorine) were added ahead of the high-pressure membranes to control biofouling.Both the low- and high pressure membrane systems were operated at constant flux such that fouling was measured by an increase in transmembrane pressure (TMP). Membrane selectivity was measured by the rejection of total organic carbon (TOC), selected inorganic parameters, particle counts, turbidity and a virus challenge study. Specific flux was higher and the average TMP was lower for the UF system than for the MF system.The fouling rate of three types of high-pressure membranes, as measured by the initial specific flux after cleaning and by the subsequent decline in specific flux, was lower for those receiving pretreatment by UF than by MF. The choice of membrane pretreatment did not affect rejection efficiency of any of the three high-pressure membranes.This title belongs to WERF Research Report SeriesISBN: 9781843397595 (eBook)

Impact of module design on the performance of membrane bioreactors treating municipal wastewater

ABSTRACTMembranes are located in a membrane module that physically seals and isolates the feed stream from the permeate flux in membrane bioreactors (MBRs). Therefore, module type, structure, and geometrical configuration are critical design considerations affecting membrane performance in MBRs. In this study, impact of membrane module design on treatment and filtration performance of MBRs was investigated. For this purpose, two flat sheet membrane modules with different outlet structures and module geometries, including rectangular- and D-shaped, were tested. In addition to the differences in outlet structure and module geometry, size of circular structures which supported membranes in rectangular- and D-shaped modules differed from each other. Considering the results, permeate quality was not affected from the change in the module design. However, the most remarkable impact of the module design was observed on the transmembrane pressure (TMP) evolution and fouling potential. D-shaped membrane module including smaller circular structures resulted in a decrease in fouling potential and thus, this module could be operated longer time in comparison to rectangular-shaped membrane module without a severe TMP increase. The observed differences in TMP increase and fouling potential lead to the hypothesis that module design is a critical factor affecting filtration performance in MBRs.

Rotating Disk-Assisted Cross-Flow Ultrafiltration of Sugar Beet Juice

Rotating disk module (RDM)-assisted cross-flow ultrafiltration of sugar beet juice, obtained by cold pulsed electric field-assisted pressing, was studied in full recycling and concentration modes. Results of trans-membrane pressure (TMP) stepping tests with 10- and 50-kDa membranes showed that filtration flux increased with rotation speed and TMP until 1000 rpm and 4 bar. A slight variation of filtration purity was observed for a rotation speed range of 0–2500 rpm. A TMP increase resulted in purity reduction due to the formation of thicker fouling layer. A full recycling test study with a 10-kDa membrane confirmed the influence of TMP and rotation speed on filtration flux. A rotation speed of 1000 rpm and a TMP of 4 bar were selected for sugar beet juice purification, by concentrating juice from 13 to 1 L. The results of concentration mode tests revealed that, with a 10-kDa membrane, the filtrate purity could reach 95.6 ± 0.5 %, which was comparable to the purity of sugar beet juice purified by conventional liming-carbonation method. The flux decline in cross-flow filtration was still important, justifying more effective anti-fouling technologies investigation.

Application of nanofiltration membrane for treatment of chloride rich steel plant effluent

Composite charged nanofiltration membrane of polysulfone and zinc chloride was used to treat the effluent from TATA Steel, India. The effluent contained high amount of total solid and various ions, namely chloride (1000–1200mg/l), fluoride (140mg/l), nitrate (37mg/l) and phosphate (1730mg/l). The effluent was pre-treated by passing over granular activated carbon and total solid was reduced by 38%. The membrane containing 1wt% ZnCl2 was found to be optimized one. Chloride concentration was reduced below 800mg/l via charge–charge interaction by nanofiltration. Similarly fluoride, nitrate and phosphate ions were rejected around 32%, 27% and 70%, respectively. Effect of different operating conditions (transmembrane pressure, cross-flow rate and solution pH) was studied. Rejection of various ions and solid content was reduced with increase in transmembrane pressure and cross flow rate. Based on permeate flux and quality, 1380kPa and 80l/h cross flow rate were selected as optimum operating conditions. Simple in-situ washing of the membrane with tap water could recover permeate flux upto 97%. A resistance in series model was used to quantify the effects of polarization on system performance. This system can be scaled up in spiral wound configuration for final deployment.

Effect of operating conditions on biofouling in reverse osmosis membrane processes: Bacterial adhesion, biofilm formation, and permeate flux decrease

We systematically evaluated the effects of operating conditions on biofouling behaviors in reverse osmosis (RO) membrane processes. The biofouling experiment was performed by filtrating nutrient-containing feed water with a cross-flow cylindrical membrane cell through a circular polyamide RO membrane with pre-adhered Pseudomonas putida. The trans-membrane pressure (TMP) and stirring rates in the cell were controlled as operating conditions. The stirring rates brought about shear force and thus corresponded to cross-flow velocities on the membrane surfaces in commercial operations. Permeate flux was monitored during the filtration, and bacterial adhesion to the membrane surfaces was observed using a confocal laser scanning microscope. An increase in TMP increased both the reduction rate of permeate flux and the volume of adhered bacteria, thus facilitating biofouling. Osmolarity calculation on the membrane surface suggested that TMP affected bacterial growth by concentration polarization (CP) of nutrients. Higher stirring rate prevented reduction of permeate flux and bacterial growth on membrane surfaces. The hydrodynamic shear force generated by stirring effectively detached the adhered bacteria, while CP of nutrients was not remarkably affected by the range of stirring rates used in this study. These results suggest that lower TMP and higher stirring rates prevent biofilm formation by decreasing CP and promoting bacterial detachment.

Hemodiafiltration: Technical and Clinical Issues

Hemodiafiltration (HDF) seems to represent the gold standard in the field of replacement of renal function by dialysis. High convective fluxes have been correlated with better clinical outcomes. Sometimes, however, there are technical barriers to the achievement of high blood flows adequate to perform effective convective therapies. In spite of optimized procedures, the progressive increase in transmembrane pressure (TMP), the blood viscosity due to hemoconcentration and blood path resistance sometimes becomes inevitable. We propose two possible solutions that can be operated automatically via specific software in the dialysis machine: predilution on demand and backflush on demand. Predilution on demand consists in an automatic feedback of the machine, diverting part of the filtered dialysate into a predilution mode with an infusion of 200 ml in 30 s while the ultrafiltration pump stops. This produces a sudden hemodilution with a return of the parameters to acceptable values. The performance of the filter improves, and the pressure alterations are mitigated. Backflush on demand consists in an automatic feedback of the machine triggered by the TMP control, producing a positive pressure in the dialysate compartment due to a stop of filtration and rapid infusion of at least 100 ml of ultrapure dialysate into the hollow fiber. This not only produces a significant hemodilution, but also backflushes the membrane pores detaching protein layers and improving membrane permeability. These are two examples of how technology will permit to overcome technical barriers to a widespread diffusion of HDF and adequate convective dose delivery.