Effect of backwash on the performance of submerged membrane filtration

Particles with a mean diameter of 5 μm were filtered by a ceramic tubular membrane to study the effects of backwash on the performance of submerged membrane filtration. A periodic backwash can completely remove the formed cake, diminishing a part of membrane internal fouling, and, therefore, recover the filtration flux. In a membrane-blocking/cake formation comparable filtration system, the filtration resistance due to membrane-internal fouling is over twice as high as that due to cake formation. The irreversible filtration resistance increases progressively during operation, and it can be regressed to a power-type empirical relationship. Filtration period data were analyzed using blocking models. Membrane blocking occurs in the early filtration periods and is followed by cake filtration. The filtration flux can be simulated by employing blocking models and empirical equations for filtration resistance. The backwash effectiveness was examined by comparing filtrate productivity and washing efficiency. The calculated results of productivity under various backwash durations agree well with experimental data. An increase in backwash flux or duration leads to higher productivity, when the duration is shorter than 2 min; however, the productivity may be decreased with an increase of backwash duration due to the back pumping of more filtrate. A longer filtration time in each cycle results in higher backwash efficiency since a formed cake may efficiently prevent further membrane pore clogging and is more easily removed by a backwash. The optimal backwash conditions can be determined appropriately by the proposed method, with respect to both backwash efficiency and filtrate productivity.     

Impact of ultrafiltration operating conditions on membrane irreversible fouling

The main limitation of the ultrafiltration (UF) process identified in drinking water treatment is membrane fouling. Although adsorption of natural organic matter (NOM) is known to cause irreversible fouling, operating conditions also impact the degree of irreversible fouling. This study examined the impact of several operating parameters on fouling including flux, concentrate velocity in hollow fibers, backwash frequency, and transmembrane pressure. A hydrophilic cellulose derivative membrane and a hydrophobic acrylic polymer membrane were used to conduct these tests. Pilot testing showed that when short-term reversible fouling was limited during a filtration cycle by increasing the concentrate velocity, reducing the flux, and increasing the backwash frequency, the evolution of the membrane toward irreversible fouling could be controlled. It appeared that operating parameters should be adjusted to maintain the increase of transmembrane pressure below a certain limit, determined to be approximately 0.85 to 1.0 bar for the tested UF membrane, in order to minimize the rate of irreversible fouling. This threshold for transmembrane pressure was confirmed empirically by compiling data from over 36 pilot studies. Other testing results demonstrated that hydraulic backwash effectiveness decreased as the transmembrane pressure applied in the previous filtration cycle increased. Backwash efficiency in terms of membrane flux recovery after hydraulic backwash was reduced by 50% when the transmembrane pressure was increased from 0.4 bar to 1.4 bar.     

Optimisation of dead-end filtration conditions for an immersed anoxic membrane bioreactor

The optimisation of the energy demand in the application of dead-end filtration in an immersed membrane bioreactor applied to groundwater denitrification has been studied. Filtration cycle length was varied at a set flux to control the amount of foulant deposited at the membrane surface. Physical cleans comprising a simultaneous backflush and gas injection were subsequently instigated and the reversibility of the deposit determined by the residual resistance, R_res. Examination of R_res versus flux and cycle length variation indicated an operational envelope where limited fouling occurred. The transition from limited fouling to extensive fouling was indicated by a parameter based on the critical accumulated mass, indicating incipient deposit consolidation. The transition between regions became less severe when the solids retention time was increased from 10 to 25 and 40 days. This was apparently related to a shift in bulk physical characteristics. Nevertheless, low residual fouling was observed during long-term filtration when operating below the critical mass, resulting in a 20× reduction in energy demand over that of constant gas injection.     

Coagulation and ultrafiltration: Understanding of the key parameters of the hybrid process

A hybrid coagulation–ultrafiltration process has been investigated to understand membrane performance. Coagulation prior to ultrafiltration is suspected to reduce fouling by decreasing cake resistance, limiting pore blockage and increasing backwash efficiency. Coagulation followed by tangential ultrafiltration should gather the beneficial effects of particle growth and cross-flow velocity. Our study aims at determining the key parameters to improve membrane performance, by describing floc behaviour during the hollow fibre ultrafiltration process. Flocs encounter a wide range of shear stresses that are reproduced through the utilization of different coagulation reactors. Performing a Jar-test enables the formation of flocs under soft conditions, whereas Taylor-Couette reactors can create the same shear stresses occurring in the hollow fibres or in the pump. Synthetic raw water was made by adding bentonite into tap water. Five organic coagulants (cationic polyelectrolytes) and ferric chloride were selected. Floc growth was thoroughly monitored in the different reactors by laser granulometry. Coagulation–ultrafiltration experiments revealed different process performance. The effect on the permeate flux depended on the coagulant used: some coagulants have no influence on permeate flux, another enables a 20% increase in permeate flux whereas another coagulant leads to a decrease of 50%. Flocs formed with ferric chloride do not resist shear stress and consequently have no influence on permeate flux. These results show the necessity to create large flocs, but the size is not sufficient to explain membrane performance. Even if flocs show a good resistance to shear stress, a high compactness (D_f = 3) will lead to a dramatic decrease of permeate flux by increasing the mass transfer resistance of the cake. On the contrary, flocs less resistant to shear stress, then smaller and also more open have no effect on permeate flux. An optimum was quantified for large flocs, resistant enough to shear stress facilitating flow between aggregates.     

Flux decline behaviors in dead-end microfiltration of activated sludge and its supernatant

The properties of dead-end microfiltration were explored under constant pressure using two types of activated sludge controlled under the condition of different air flow rates. The activated sludge cultured at the air flow rate of 0.15 L min⁻¹ (the anaerobic condition) exhibited a significant flux decline compared with the case of the air flow rate of 2.33 L min⁻¹ (the aerobic condition). It was found from the results of microfiltration of the supernatant separated by centrifugation that the constituents in the supernatant caused a major cake resistance in microfiltration of the activated sludge. The average specific filtration resistance for filtration of the activated sludge was closely consistent with that for filtration of the supernatant at low pressure (49 kPa). However, the cake resistance of the microbial floc in microfiltration of the activated sludge became substantial with increasing filtration pressure because of high compressibility of the microbial floc. Moreover, the foulant and the fouling mechanism in microfiltration of the supernatant were evaluated from both microfiltration test of the supernatant and microfiltration test of the filtrate collected thereby. As a result, the effects of the pore size and material of the microfiltration membrane on the flux decline behaviors in dead-end microfiltration were reasonably elucidated.     

A systematic insight into fouling propensity of soluble microbial products in membrane bioreactors based on hydrophobic interaction and size exclusion

Soluble microbial products (SMPs) contained in membrane bioreactor (MBR) supernatant have been proved to be main foulants. To obtain a comprehensive understanding of the fouling potential of SMPs on the basis of both hydrophilic/hydrophobic properties and molecular size, MBR supernatant of a pilot-scaled system treating municipal wastewater was partitioned into different hydrophilic/hydrophobic fractions by DAX-8 resins, with joint size partition of hydrophilic fraction also undertaken. A series of stirred dead-end filtration tests were conducted to investigate the flux decline. Hydrophilic fraction was found the dominant foulant responsible for flux deterioration, which was mainly attributed to the subclass of molecular weight above 100 kDa. The molecular weight distribution and atomic force microscopy images indicated that large molecules in hydrophilic fraction plugged the membrane pores. The backwash tests showed the flux decline caused by hydrophilic fraction was much less recoverable by hydraulic cleaning. It can be inferred that steric factor, i.e. size exclusion was the primary cause in the initial stage of fouling, while the role of hydrophobic interaction was of less significance. Additional modeling work indicates that the main fouling mechanism was complete blocking, further confirming the predominance of size exclusion contributing to membrane fouling by SMPs in MBR supernatant.     

含聚浓度对轴向动态反冲洗法过滤和反冲洗效能影响*

利用轴向动态反冲洗法处理含聚污水,研究考察了聚合物浓度对滤层阻力损失、油和悬浮物去除效能、反冲洗强度、反冲洗历时、及滤料反洗再生效能的影响规律。结果表明,聚合物浓度可以减少滤层成熟时间,过滤后期滤层阻力增长率大。含聚浓度对过滤效能影响较小,油和悬浮物去除率达到95.67 %和80.87%以上。聚合物浓度对反冲洗过程影响较大,通过提高反冲洗强度和增加反冲洗历时可以消除聚合物浓度对反冲洗过程的影响,反冲洗强度为10.5L/s.m₂和反冲洗历时22.0~25.0min时滤料可以获得较好的反洗再生效果,反冲洗后核桃壳滤料油量为0.21~1.34 mg(油)/g(干核桃壳)。     

Flux decline in crossflow microfiltration and ultrafiltration: experimental verification of fouling dynamics

The theory of fouling dynamics in crossflow membrane filtration is compared with ultrafiltration experiments with suspensions of 0.12 μm silica colloids. It has been experimentally verified that colloidal fouling in crossflow filtration is a dynamics process from non-equilibrium to equilibrium and that the steady state flux is the limiting flux. With the cake concentration c_g identified from an independent experiment and the specific cake resistance calculated by Carman–Kozeny equation, the time-dependent flux and the time to reach steady state in the experiments of this study are correctly predicted with the theory of fouling dynamics.     

Qualitative investigations of the photocatalytic dye destruction by TiO<Subscript>2</Subscript>-coated polyester fabrics

The preparation of TiO₂-coated polyester fabrics for purposes of photocatalytic water purification requires coating agents with crystalline TiO₂ particles preferably in the anatase modification. The resulting coatings should exhibit a high water resistance and high photocatalytic activity according to reaction with structurally different dyestuffs. For this, the synthesis of anatase sols by hydrolysis of tetraisopropyltitanate in acidic medium under reflux was optimized. By precoating or addition of polymeric epoxysilanes a good adhesion on the polyester support could be realized. The photocatalytic activity was tested with different dyestuffs as: Methylene blue, Rhodamine B and the azo dyes AcidOrange 7 and C.I. Reactive red 158. The rate of photodestruction depends strongly on the type of used dye and its structure. Surprisingly, no differences in photodegradation were found in case of investigations with Rhodamine B, if the photoreaction is performed under exposure with UV or with visible light. A possible explanation of the similar behavior of photoreaction under different light sources could be a photodestruction by electron transfer from Rhodamine B to TiO₂. Therefore, Rhodamine B seems to be generally not suitable for the evaluation of the photoactivity of TiO₂ under irradiation with visible light.     

Kinetic adhesion of bacterial cells to sand: cell surface properties and adhesion rate

Correlation between microbial surface thermodynamics using the extended DLVO (XDLVO) theory and kinetic adhesion of various bacterial cells to sand was investigated. Two experimental setups were utilized. Adhesion tests were conducted in batch reactors with slow agitation. Also, bacteria were circulated through small sand columns in a closed loop and the results were analyzed with a simple model which accounted for the rate of the adhesion phenomena (omega in h(-1)) and adhesion percentage. Cells surface properties were derived from contact angle measurements. The wicking method was utilized to characterize the sand. Zeta potentials were measured for the sand and the cells. Kinetic of bacterial retention by the porous media was largely influenced by the electrostatic interactions which are correlated with omega from the model (R(2)=0.71). Negative zeta potentials resulted in electrostatic repulsions occurring between the sand and the bacterial cells which in result delayed bacterial adhesion. While no correlation was found between the adhesion percentage and the total interaction energy calculated with the XDLVO theory the respective behavior of hydrophobic and hydrophilic bacteria as well as the importance of electrostatic interactions was evidenced. All the bacterial strains studied adhered more in the column experiments than in the adhesion tests, presumably due to enhanced collision efficiency and wedging in porous media, while filtration could be ignored except for the larger Bacillus strains. Approximate XDLVO calculations due to solid surface nanoscale roughness, retention in a secondary minimum and population heterogeneity are discussed. Our results obtained with a large variety of different physicochemical bacterial strains highlights the influence of both surface thermodynamics and porous media related effects as well as the limits of using the XDLVO theory for evaluating bacterial retention through porous media.     

Modeling of the permeate flux decline during MF and UF cross-flow filtration of soy sauce lees

Cross-flow ultrafiltration and microfiltration have been used to recover refined soy sauce from soy sauce lees for over 25 years. The precise mechanism which dominated the permeate flux during batch cross-flow filtration has not been clarified. In the present study, we proposed a modified analytical method incorporated with the concept of deadend filtration to determine the initial flux of cross-flow filtration and carried out the permeate recycle and batch cross-flow filtration experiments using soy sauce lees. We used UF and MF flat membrane (0.006 m² polysulfone) module under different transmembrane pressures (TMP) and cross-flow velocities. The modified analysis provided an accurate prediction of permeate flux during the filtration of soy sauce lees, because this model can consider the change in J₀ at initial stage of filtration which was caused by the pore constriction and plugging inside membrane, and these changes may not proceed when the cake was formed on the membrane surface. Mean specific resistance of the cake increased with TMP due to the compaction of the cake and decreased with cross-flow velocity due to the change of deposited particle size, but less depended on the membrane in the present study. These results indicate that the value of J₀ determined by modified method was relevant to exclude the effects of the initial membrane fouling by pore constriction due to protein adsorption and plugging with small particles. The modified analytical method for the cake filtration developed in the present study was considered to be capable of selecting an appropriate operating conditions for many cross-flow filtration systems with UF, MF membranes.     

Self-forming dynamic membrane for ultrafiltration of pineapple juice

The formation of self-forming dynamic membrane on a porous ceramic support was studied. Pineapple juice of 12° Brix concentration was used in the experiments which were carried out at 25°C by circulating the pineapple juice at the applied pressure of 100, 200, and 300 kPa and at cross-flow velocities of 1.30–2.95 m s⁻¹ through the ceramic membrane module for 1 h. The experimental data of flux and rejections showed that the dynamic membrane was well-formed after 30 min of circulation under the applied pressure of 300 kPa and at a cross-flow velocity of 2.0 m s⁻¹ in which the steady values of flux and rejections of macromolecules and sugars obtained from the filtration mode were 6.0×10⁻³ m³/m² h, 84–87% and 6%, respectively. The corresponding values for ultrafiltration by alumina membrane of MW cut-off 50,000, using equivalent conditions, were 15.8×10⁻³ m³/m² h, 91% and 10.5%. Ultrafiltration was found to be more promising. The stability of the self-forming dynamic membrane was acceptable when subjected to change of filtration conditions. The permeation flux increased with cross-flow velocity and decreased when the applied pressure was reduced. The resistances for filtration by dynamic membrane and by ultrafiltration were calculated. For a porous support of large pore sizes, an in-pore blockage of solutes which were smaller than the membrane pores reduced the pore volume and induced fouling. Internal fouling resistance (R_f) was, therefore significant and responsible for the values of flux and rejection and was approximately 70% of total resistance. While in ultrafiltration, in which membrane with a smaller pore diameter was used, R_f was only 20% but R_p, the polarized layer resistance, was as high as 60% of total resistance.     

Polypyrrole modified solvent resistant nanofiltration membranes

New solvent resistant nanofiltration (SRNF) membranes with polypyrrole (PPy) modified toplayer were prepared on different types support by in situ pyrrole polymerization. The morphology of the membranes was studied by SEM. The PPy modified membranes were applied in the filtration of organic solvents. All the PPy modified membranes showed a very high retention of the negatively charged RB in different solvent systems, comparable to those of the MPF-50 and STARMEM 122 commercial membranes, but at much higher flux. The extended filtration experiment in strong aprotic DMF of PPy modified membranes showed a clearly stable permeability and retention over 30 h. In addition, the PPy modified membranes showed a much higher flux in THF systems than for earlier reported crosslinked poly(imide) membranes.     

Bacterial attachment on reactive ceramic ultrafiltration membranes

Bacterial attachment is an initial stage in biofilm formation that leads to flux decline in membrane water filtration. This study compares bacterial attachment among three photocatalytic ceramic ultrafiltration membranes for the prevention of biofilm formation. Zirconia ceramic ultrafiltration membranes were dip-coated with anatase and mixed phase titanium dioxide photocatalysts to prevent biofilm growth. The membrane surface was characterized in terms of roughness, hydrophobicity, bacterial cell adhesion, and attached cell viability, all of which are important factors in biofilm formation. The titanium dioxide coatings had minimal impact on the membrane roughness, reduced the hydrophobicity of membranes, prevented Pseudomonas putida attachment, and reduced P. putida viability. Degussa P25 is a particularly promising reactive coating because of its ease of preparation, diminished cell attachment and viability in solutions with low and high organic carbon concentrations, and reduced flux decline. These reactive membranes offer a promising strategy for fouling resistance in water filtration systems.     

Xylose recovery by nanofiltration from different hemicellulose hydrolyzate feeds

Xylose is an intermediate product in xylitol production. Nanofiltration could simplify and enhance this separation step conventionally done by chromatographic methods. Here different hemicellulose hydrolyzate feeds were nanofiltered to recover xylose into the permeate.Two different batches of hemicellulose hydrolyzate were prepared: the hydrolyzate as such and modified with crystalline xylose addition. Both feed solutions were diluted to a total dry solids (TDS) content of approximately 21 wt.% and the xylose contents were 48.7% and 59.1% of the TDS_f (total dry solids in feed). The filtration experiments were made at 40, 50 and 60 °C in total reflux mode for approximately 30 min at each pressure of 20, 25, 30, 35 and 40 bar. In addition, a 20-h filtration was made at 50 °C and 30 bar. A DDS LabStak M20-filter was used and it was equipped with Desal-5 DK, Desal-5 DL and NF270 membranes.In short-term filtrations, the nanofiltered permeate of the original hydrolyzate had 78–82% xylose of the TDS_p (total dry solids in permeate) and the modified hydrolyzate 86–88% xylose of the TDS_p. Thus, considerable xylose purification was obtained. The addition of crystalline xylose into the hemicellulose hydrolyzate gave a notable increase in permeate fluxes. The 20-h filtration showed fouling and compaction effects as a flux decrease of approximately 10–25% was detected in the retention integrity test. According to the results, xylose purification from hemicellulose hydrolyzate could be enhanced by nanofiltration.     

Molecular (osmotic and electro-osmotic) backwash of cellulose acetate hyperfiltration membranes

A technique using osmosis and/or electro-osmosis was developed to clean and possibly decompact contaminated modified (asymmetric)cellulose acetate membranes. p]The rejuvenation technique developed is called “molecular backwash”. When contaminated with ferric hydroxide, the membranes (cured at 92°C) exhibited considerable flux decrease. After molecular backwashing, the hyperfiltration flux increased again. In some cases, molecular backwash also partially restored the salt rejection loss which had occurred during contamination. Flux loss recovered by molecular backwashing varied from 30% to over 100% (i.e. the flux of the compacted membrane was greater than before contamination, but not greater than the flux of a clean uncompacted membrane).     

Evaluation of crossflow filtration models based on shear-induced diffusion and particle adhesion: Complications induced by feed suspension polydispersivity

Specific flux data were obtained during the transient period of flux decline in laminar crossflow filtration. Effects of hydrodynamics on cake parameters such as specific resistance, mass and particle size distribution were studied experimentally. An evaluation of crossflow filtration models suggests that a model based on shear-induced diffusion [1] is a better predictor of specific flux decline than a particle adhesion model [2]. Even for relatively narrowly distributed suspensions, polydispersivity complicates analyses in a manner that is not adequately addressed by these models. Changes in experimental specific cake resistances with module hydrodynamics coupled to the inadequacy of these models for accurately predicting time-dependent specific flux profiles, cake specific resistances, and mass suggests that cake morphology is a key variable that needs to be incorporated in future modeling efforts.     

Impact of gel layer formation on colloid retention in membrane filtration processes

Colloidal particles in the feed streams of membrane filtration processes control membrane fouling rate in many instances. In this study, the non-gelling colloidal Na-alginate and the gelling colloidal Ca-alginate are employed to investigate the significance of gel layer formation in membrane filtration processes in terms of contribution to membrane fouling and supplementary impurity removal. The results show that contribution of colloidal particles to membrane fouling depends on the gelling propensity of the colloids and the operational mode (constant pressure or constant flux) implemented. A small dose of either Na-alginate or Ca-alginate was found to greatly increase membrane fouling rate during constant pressure filtration. Both the resistance to removal by application of shear and the lower susceptibility of the concentration polarization layer to shear resulted in more severe fouling during constant flux filtration in the presence of Ca-alginate assemblages than in the presence of Na-alginate. Apparent channel sizes of the Ca-alginate gel layer were calculated from the material properties of the fouling layer. Incomplete catalase retention highlighted the likely heterogeneity in size of liquid transport pathways. Adsorption also contributed to the trapping of colloidal particles according to the retention behaviour of BSA by the Ca-alginate gel layer. Gel layer formation propensity should be seriously considered for the operation of membrane filtration processes. Two simple methods based on (i) a permeability recovery experiment and (ii) comparison of dead-end filtration behaviour with and without shear application are proposed for evaluation of the gelling propensity of colloidal dispersions.     

The effect of fibre diameter on filtration and flux distribution — relevance to submerged hollow fibre modules

This paper examines the effect of fibre diameter on filtration and flux distribution with inter-fibre two-phase flow for conditions relevant to submerged bioreactors (SMBR). Hollow fibres of different diameters fixed in a specially designed holder providing shell-side feed were tested for a model biomass feed with pumping and submerged systems. The experimental results showed that the effect of the fibre diameter on filtration increased with the increase in turbulence around the fibres. For filtration with two-phase flow, the performance was sensitive to changes in fibre diameter and significantly lower flux declines were obtained with smaller fibres. On the other hand, a theoretical analysis of flux distribution along the fibre in a submerged system showed the smaller fibres to be disadvantaged. The theoretical model based on the simplified Navier–Stokes equations and filtration equations revealed that the flux distribution along the fibre depends on fibre inner diameter, length and fibre permeability. The effect of these factors can be related to a dimensionless coefficient ζ=4lR_i^−3/2R_m^−1/2. Sensitivity analysis demonstrated that for ζ>2, the maximum flux along the fibre can be approximately estimated by J_max=ζJ_mi. Although, the effect of high initial local flux on performance of filtration is still to be assessed, the flux distribution model is useful in design and operation of the SMBR system.