Cake Layer Resistance Research Articles

Influence of surface hydrophilicity on polytetrafluoroethylene flat sheet membrane fouling in a submerged membrane bioreactor using two activated sludges with different characteristics

This study focused on how membrane surface hydrophilicity affects fouling in a membrane bioreactor. A lab-scale membrane bioreactor incorporating immersed polytetrafluoroethylene flat-sheet membrane modules with different contact angles (approximately 70° and 130°) was operated using the inocula activated sludge taken from a lab reactor and from a municipal wastewater treatment plant (WWTP). The reactor also contained a polyvinylidene difluoride flat-sheet membrane module as a reference. The hydrophobicity of the WWTP sludge was higher than that of the lab-reactor sludge. Results showed that when lab-reactor sludge was used, increases in transmembrane pressure (TMP) for the hydrophobic membrane with a high contact angle occurred more rapidly than for the hydrophilic membrane with a low contact angle. In contrast, when the WWTP sludge was used, TMP increases for the hydrophilic membrane occurred more rapidly than for the hydrophobic membrane. These results indicate that the effects of surface hydrophilicity on membrane fouling depend on the hydrophobicity of the activated sludge. When the WWTP sludge was used, the hydrophobic membrane exhibited much higher cake layer resistance than did the hydrophilic membrane. Formation of a dense cake layer on hydrophobic polytetrafluoroethylene membranes may deter adhesion of foulants on membrane surfaces.

A comparison study on membrane fouling in a sponge-submerged membrane bioreactor and a conventional membrane bioreactor

This study compared membrane fouling in a sponge-submerged membrane bioreactor (SSMBR) and a conventional membrane bioreactor (CMBR) based on sludge properties when treating synthetic domestic wastewater. In the CMBR, soluble microbial products (SMP) in activated sludge were a major contributor for initial membrane fouling and presented higher concentration in membrane cake layer. Afterwards, membrane fouling was mainly governed by bound extracellular polymeric substances (EPS) in activated sludge, containing lower proteins but significantly higher polysaccharides. Sponge addition could prevent cake formation on membrane surface and pore blocking inside membrane, thereby alleviating membrane fouling. The SSMBR exhibited not only less growth of the biomass and filamentous bacteria, but also lower cake layer and pore blocking resistance due to lower bound EPS concentrations in activated sludge. Less membrane fouling in SSMBR were also attributed to larger particle size, higher zeta potential and relative hydrophobicity of sludge flocs.

Analysis of Membrane and Cake Layer Resistances in Coagulation: Constant Flux Dead-End Microfiltration of NOM

Fouling analysis of tank coagulation (TC)/inline coagulation (IC) followed by dead-end microfiltration (MF) treating natural organic matter (NOM) was assessed using the blocking laws and resistance-in-series models. Different process conditions were found to have no significance on the overall treatment efficiency but did contribute to aspects of membrane operation, that is, membrane fouling. Blocking laws were found to be inadequate in explaining the fouling phenomenon, but may provide insight to the intrinsic membrane resistance behavior. The resistance-in-series model showed that cake compressibility was different in each process configuration tested and may therefore assist in determining the most suitable operating conditions. Floc properties resulting from TC and IC were different, impacting the cake compressibility behavior observed, which further could explain the differences in trans membrane pressure (TMP) increase and reversibility of membrane fouling.

Flux enhancement by air dispersion in cross-flow microfiltration of a colloidal system through spiral wound module

Enhancement of the permeate flux due to reduction of cake layer resistance by air-liquid twophase flow in a cross flow spiral wound microfiltration membrane has been investigated. Experiments were carried out with two different suspension systems, namely baker’s yeast suspension with a dry mass concentration of 5.4 kg m-3 and the colloidal suspension of starch and bacteria having a dry mass concentration of 3.41 kg m-3. An air-liquid two-phase flow was generated by injecting the air at the inlet of the membrane module and was fed into a horizontally mounted assembly of a spiral wound microfiltration membrane. The effect of air dispersion was studied on flux enhancement, specific cake layer resistance and loading of cake mass on the membrane surface. This study shows that by air dispersion, the permeate flux can be enhanced up to 60 percent for the colloidal suspension of starch and bacteria while a flux enhancement of 40 percent was achieved for yeast suspension. The specific cake layer resistance reduced by a factor of 10 for both of the suspensions when air was injected into the feed stream. The results of this study depict that the technique of air dispersion is effective in increasing the permeate flux for the suspension systems containing below and above micron sized particulates.

Impact of nitrogen loading rates on treatment performance of domestic wastewater and fouling propensity in submerged membrane bioreactor (MBR)

In this study, performance of laboratory-scale membrane bioreactor (MBR) was evaluated in treating high strength domestic wastewater under two nitrogen loading rates (NLR) i.e., 0.15 and 0.30 kg/m(3)/d in condition 1 and 2, respectively, while organic loading rate (OLR) was constant at 3 kg/m(3)/d in both conditions. Removal efficiencies of COD were above 95.0% under both NLR conditions. Average removal efficiencies of ammonium nitrogen (NH₄(+)-N), total nitrogen (TN) and total phosphorus (TP) were found to be higher in condition 1 (90.5%, 74.0%, and 38.0%, respectively) as compared to that in Condition 2 (89.3%, 35.0%, and 14.0%, respectively). With increasing NLR, particle size distribution shifted from narrow (67-133 μm) towards broader distribution (3-300 μm) inferring lower cake layer porosity over membrane fibers. Soluble extracellular polymer substance (sEPS) concentration increased at higher NLR due to biopolymers released from broken flocs. Higher cake layer resistance (Rc) contributed towards shorter filtration runs during condition 2.

Role of pre-oxidation, using potassium permanganate, for mitigating membrane fouling by natural organic matter in an ultrafiltration system

An ultrafiltration (UF) system was used as an advanced treatment, following the conventional sand filter process, to purify raw water from the Yangtze River. Potassium permanganate was used as a pre-oxidant to oxidize the UF system influent for controlling the membrane fouling. The process was implemented by directly dosing KMnO4 into the UF system influent, i.e. the sand filter effluent. The optimal dosage of KMnO4 was 0.4mg/L in terms of both the trans-membrane pressure (TMP) and product water quality. Compared to the results obtained when using a UF system without KMnO4, the preoxidation of the feed water by KMnO4 was found to result in a lower cake layer resistance. The pretreatment of the feed water by KMnO4 also indicated that some advantages in terms of mitigating TMPs were achieved by transforming molecular weight distribution of organic pollutants, which were mainly from hydrophilic and hydrophobic organic matter. The analysis of attenuated total reflection-Fourier transform infrared spectra of the cake layer showed that KMnO4 oxidized macromolecules of hydrophobic natural organic matter (NOM) to lower-molecular-weight hydrophilic organic matter, which mitigated the membrane fouling caused by the organic matter. Scanning electron microscopy indicated that loose fragments were formed on the filtration cake in the KMnO4/UF system, which was easily removed by hydraulic washing.

A new insight into membrane fouling mechanism in submerged membrane bioreactor: Osmotic pressure during cake layer filtration

Big gap between experimental filtration resistance of cake layer formed on membrane surface and the hydraulic resistance calculated through the Carman–Kozeny equation, suggested the existence of a new membrane fouling mechanism: osmotic pressure during cake layer filtration in SMBR system. An osmotic pressure model based on chemical potential difference was then proposed. Simulation of the model showed that osmotic pressure accounted for the major fraction of total operation pressure, and pH, applied pressure and ionic strength were the key determining factors for osmosis effect. It was found that, variations of osmotic pressure with pH, applied pressure and added ionic strength were well coincident with perditions of model's simulation, providing the first direct evidences of the real occurrence of osmosis mechanism and the feasibility of the proposed model. These findings illustrate the essential role of osmotic pressure in filtration resistance, and improve fundamental understanding on membrane fouling in SMBR systems.

Performance optimization of a 5 nm TiO2 ceramic membrane with respect to beverage production wastewater

A 5nm TiO2 ceramic ultrafiltration membrane was evaluated with respect to beverage production wastewater. The influence of operating parameters such as crossflow velocity (CFV), transmembrane pressure (TMP) and particle size distribution on filtration performance was investigated. Results show that certain combinations of higher CFVs and TMPs provide significant improvements in permeate flux and contaminant rejection rates. These improvements are attributed to the turbulent flow regime experienced by suspended particles during ultrafiltration. At higher CFVs of 3.1 and 3.6ms−1, the majority of the particles present in the wastewater are swept away from the membrane surface resulting in reduced concentration polarization (cake layer) resistance and improved permeate flux. The steady-state permeate fluxes measured at CFVs of 3.1 and 3.6ms−1 increased by 22% and 53% respectively when TMP was increased to 200kPa. Increasing the TMP further to 300kPa also increased the steady-state permeate fluxes by 16% and 4% respectively. Similarly, the reduction in concentration polarization resistances at these CFVs also reduced particle infiltration into the membrane pores resulting in better filtrate quality in terms of turbidity and COD. A CFV of 3.1ms−1 and a TMP of 300kPa were identified as optimal operating conditions in terms of energy consumption and permeate flow rate.

Relationship between feed water quality and membrane performance during the filtration of real secondary effluent

Water quality of feed stream certainly has a close relationship with membrane fouling during the microfiltration of secondary effluent. The current study tested the ceramic membrane performance using secondary effluents from two wastewater treatment plants and a pilot scale A2/O treatment equipment. Reversible and irreversible fouling were discussed using different water quality parameters. The water quality was characterized using general parameters, as well as particle size distribution (PSD) and fluorescence spectrum of different organic compositions. Higher particle concentration in feed water leads to a higher reversible fouling. Wider PSD results in lower porosity of the cake layer and higher cake layer resistance. Hydraulic pressure above 50 kPa results in the deformation of suspended particles and a more compact cake layer; thus, more dissolved or colloidal materials are retained. Based upon the humic substance content in secondary effluent from different wastewater treatment plants, higher humic substance concentration might contribute greatly to the irreversible fouling in the ceramic microfiltration of secondary effluent.

Study of antifouling modified ultrafiltration membrane based on the secondary treated water of urban sewage

Mixtures of polyvinylidene fluoride (PVDF) and polyvinyl alcohol (PVA) containing hydrophilic ultrafiltration membranes were prepared by adding PVA (5 to 30%) to PVDF by the phase inversion method. The hydrophilic contact angle (CA), equilibrium water content, pure water flux and bovine serum albumin retention were studied to assess the membrane performance. The anti-fouling performance of modified membrane to the secondary treated water was evaluated by flux decline, washing recovery rate and fouling resistance analysis. Scanning electron microscopy showed that the cross-section structure of the membranes had finger-like pores, which were well developed and uniformly distributed, and the sub-layer structure was looser and more porous with the increasing content of PVA. The CA gradually decreased. The steady flux was 800 L/m(2) h from P15 to P30, and the BSA retention sharply declined. The ultrafiltration tests for secondary treated water indicated that the main fouling source of the modified membrane was the concentration polarization and cake layer resistance. After physical flushing, the flux recovery ratio of the membrane could reach 100% when the PVA content was 5-15%, which shows excellent anti-pollution performance and good prospects for use in processing wastewater from urban sewage.

Fouling control of submerged hollow fibre membranes by vibrations

In this study, we examine the improvement of fouling control of hollow fibre membranes with mechanical vibrations in the dead-end filtration of an inorganic suspension. Hollow fibres with diameters of 1.7mm and 2mm vibrating at moderate frequencies (0–15Hz) and small amplitudes (0–12mm) were submerged vertically in a 4g/L Bentonite solution. Experiments were then conducted at both constant permeate flux and constant suction pressure conditions. The results showed that the membrane performance can be greatly improved when the vibration frequency or the vibration amplitude increases beyond a threshold magnitude. For example, over 90% reduction in the membrane fouling rate was achieved at 8mm amplitude and 8Hz frequency vibration compared to no vibration. Experiments were also conducted with 1% and 2% fibre looseness. The results showed that a small looseness can reduce the membrane fouling and increase the permeate flux under vibrations, which can be mainly attributed to the additional lateral movement of the fibres induced by the looseness. A comparison of vibrating the hollow fibres with and without the holding frame was also carried out to determine the effects of turbulence generated by the vibrating holding frame used in the experimental setup. Particle Image Velocimetry (PIV) measurements were performed to quantify the associated turbulence inside the membrane reactors. It was confirmed that the turbulence generated by the vibrating frame was more obvious at a high vibration frequency. However, it had little influence on the membrane filtration performance. Overall, the results from the present study confirm that at moderate frequencies, the cake layer resistance can be reduced substantially by vibration due to the dynamic shear enhancement on the membrane surface.

The role of EPS concentration on membrane fouling control: Comparison analysis of hybrid membrane bioreactor and conventional membrane bioreactor

A hybrid membrane bioreactor (HMBR) was developed by adding biofilm carriers into a conventional membrane bioreactor (CMBR), and comparison experiments were conducted to investigate the role of extracellular polymeric substances (EPS) on membrane fouling control in HMBR and CMBR for municipal wastewater treatment. Results showed that the concentrations of soluble EPS (S-EPS), loosely bond EPS (LB-EPS) and tightly bond EPS (TB-EPS) in the HMBR were decreased by 42.9%, 41.6% and 1.5%, respectively, compared with the CMBR. The physical properties of the activated sludge such as supernatant turbidity (ST), sludge volume index (SVI) and particle structure were found to be strongly influenced by LB-EPS concentration. In addition, major components of EPS could be detected as proteins, carbohydrates and humus, influence level of different components on sludge physical properties is polysaccharides>proteins>humus. After the same operational period, the cake layer resistance Rc and pore resistance Rp in the HMBR accounted for 43% and 56% of that in the CMBR, respectively. It resulted the duration of transmembrane pressure (TMP) to reach the prescribed maximum of 20 kPa from 57days in the CMBR prolonged to 97days in the HMBR. HMBR operation could control the LB-EPS concentration, and further controled the membrane fouling effectively.

Removal of humic acids from water by hybrid titanium-based electrocoagulation with ultrafiltration membrane processes

Coagulation imposes major impact on the removal of humic acids (HAs) and the reduction of ultrafiltration (UF) membrane fouling. In this study, titanium was used instead of iron and aluminum as a novel alternative sacrificial anode, and the removal of HAs from water by titanium-based electrocoagulation with submerged flat sheet ultrafiltration membrane was investigated. Results clearly indicate that the current intensity did have an apparent effect on the size and fractal structure of flocs. A combination of electrocoagulation with ultrafiltration can increase HAs rejection, reduce membrane fouling and decrease transmembrane pressure. Membrane permeability and the specific resistance of cake layer were controlled directly by coagulated floc structure, which was affected strongly by the applied current intensity. Hydrous titanium dioxide (TiO2) flocs were formed by anodizing of titanium and one-step hydrolysis process with titanium salt solution. TiO2 nanoparticles with large specific surface area produced by the processes showed high photocatalytic activity. Thus, the processes are not only efficient in the removal of HAs from water but also economical in sludge recycling and reduction. It may offer a novel solution to many economic and environmental problems associated with handling iron and aluminum salt coagulation.

Effect of shear conditions on floc properties and membrane fouling in coagulation/ultrafiltration hybrid process—The significance of Alb species

Impacts of increased shear during coagulation on ultrafiltration permeate flux in coagulation–ultrafiltration (C–UF) hybrid process were investigated in this study. Three Al-based coagulants, alum, PACl and PACl–Alb, were applied to explore the role of Alb species in the C–UF process. Floc characteristics, including floc size, strength, re-formation ability and fractal structure under different coagulation conditions, were studied using a laser diffraction particle sizing device. Additionally, resistance analyses were conducted to investigate the membrane fouling mechanisms. The results indicated that alum generated the largest flocs with loosely bonded structures before breakage, which were, however, the weakest and with poor re-growth abilities. PACl–Alb contributed to the strongest flocs with the best re-growth abilities and the highest compact degree. The results of ultrafiltration experiments showed that for conventional coagulation without breakage, alum led to a flux reduction of 34%, while the reduction for PACl and PACl–Alb was 39% and 49% respectively. However, under the increasing breaking shears, the adsorption and cake layer resistances for alum coagulation increased and fluxes were markedly aggravated; PACl–Alb coagulation effluent displayed the least resistances and flux variations. An extension in breakage time led to much severer flux decline than short breakage period. PACl–Alb led to the smallest changes in flux declines.

Characteristics of extracellular polymeric substances and bacterial communities in an anaerobic membrane bioreactor coupled with online ultrasound equipment

Two parallel anaerobic membrane bioreactors (MBRs), integrated with or without ultrasound equipment for online membrane fouling control (US-AnMBR, or AnMBR) were established to digest waste activated sludge (WAS). The characteristics of bound extracellular polymeric substances (EPS) and bacterial communities in the systems were investigated for further understanding of the membrane fouling mechanisms. Ultrasound was an effective method for reducing cake layer resistance. A relatively high amount of bound EPS were found in the cake layer, especially for the US-AnMBR, by responding to the external forces (i.e. cross flow and ultrasound). High-throughput pyrosequencing and denaturing gradient gel electrophoresis (DGGE) were applied to analyze the bacterial diversity. Some bacterial populations contributing to membrane fouling were identified to accumulate in the cake layer, such as Peptococcaceae, Bacteroides and Syntrophobacterales. Since the ultrasounded retentate was recirculated back to the reactor, the bacterial community in the digested sludge was affected.

Influence of biofilm carriers on membrane fouling propensity in moving biofilm membrane bioreactor

In moving biofilm membrane bioreactor (MB-MBR) sponge carriers for biofilm growth were coupled with conventional submerged membrane bioreactor (C-MBR). This study compared the fouling propensity of C-MBR with MB-MBR and investigated factors affecting fouling variations in both the systems. Membrane fouling tendencies were monitored in terms of trans-membrane pressure (TMP) and the fouling characterization included membrane fouling resistances in situ and specific cake resistance (SCR) in batch filtration cell. Comparison of TMP profiles depicted prolong filtration periods in MB-MBR. Cake layer resistance (Rc), pore blocking resistance (Rp) as well as SCR were higher in C-MBR. The study reveals that hybrid biomass in MB-MBR creates relatively more porous cake structure in the absence of filamentous bacteria which were found in abundance in C-MBR. Filamentous bacteria were also responsible for the release of high concentration of carbohydrates in the form of soluble extra polymeric substance (EPS) contributing to higher Rp in C-MBR.

Sludge Residence Time and Membrane Fouling: What is the Connection?

Nowadays, waste activated sludge is more and more regarded as an energy source (through subsequent anaerobic digestion) or even as a potential source of fine chemicals (through subsequent fermentation processes). To this end, the organic content of the activated sludge should remain as high as possible implying short sludge residence times (SRT). However, the settling characteristics of the resulting sludge are most often very poor. This fact is overcome by performing the water-sludge separation by low pressure membrane filtration instead of sedimentation. This contribution focuses on the impact of the sludge retention time (SRT) on membrane fouling in such low pressure membrane filtration processes. While the disadvantage of a high SRT is indeed the mineralization of the sludge and the higher biomass concentrations in the reactor, often impairing an efficient oxygen transfer, lower membrane fouling rates are observed at higher SRTs. Indeed, a recent review on membrane fouling in MBRs pinpoints the sludge retention time as an important factor influencing membrane fouling (Drews, 2010). At higher SRTs, the activated sludge appears to be more robust and fouling rates are less influenced by variations in temperature or in polysaccharide fractions of soluble microbial products. Apart from the SRT, also the particle size distribution is known to have a major influence on membrane fouling (Meng and Yang, 2007; Van den Broeck et al., 2010, 2011). Foulants with smaller sizes than the membrane pores may enter the pores and pore blocking can occur. However, MBR activated sludge flocs and free bacteria are considerably larger than membrane pores of membranes used in MBRs, i.e., 10-50 ?m and 1-2 ?m (Jiang et al., 2003), respectively, compared to 0.01-0.5 ?m. The effect of activated sludge particle size distribution can thus be expected at the level of the cake layer resistance rather than pore blocking. Combining the above findings, this contribution focuses on the effect that the SRT has on the flocs’ particle size distribution, and, hence, on membrane fouling.

Influences of polysilicic acid in Al13 species on floc properties and membrane fouling in coagulation/ultrafiltration hybrid process

Effect of polysilicic acid (pSi) on pre-formed Al13 polycation ([Al13O4(OH)24(H2O)12]7+) was investigated regarding the coagulation efficiency, floc properties and subsequent impact on membrane fouling in coagulation–ultrafiltration (C–UF) hybrid process. Also, the effect of pSi on the commonly used coagulant, polyaluminum chloride (PACl) was studied for comparison. Characteristics of aggregates pre-coagulated by different coagulants were investigated using a laser diffraction particle sizing device. And membrane fouling was investigated using a dead-end batch ultrafiltration unit. The results of this study indicated that the introduction of pSi into pre-hydrolyzed Al coagulants could obviously increase the coagulation efficiency, especially when the Si/Al ratio was 0.05. Al13 produced stronger flocs than PACl, while pSi enhanced the floc strength factors for both Al13 and PACl. However, Al13–Si and PACl–Si led to smaller recovery factors (Rf) of aggregates than Al13 and PACl. Additionally, the flocs formed by Al13 presented compacter degree (Df=2.34) than those formed by PACl (Df=2.22); while both Al13–Si and PACl–Si gave rise to much loosely constructed flocs with similar Df values of 2.15 and 2.12, respectively. The results of ultrafiltration experiments implied that Al13 produced less membrane fouling than PACl. Incorporation of pSi into Al coagulants could significantly alleviate the cake layer and adsorption resistance and thus the membrane fouling. The effects of Al13–Si and PACl–Si on flux declines and filtration resistances were similar.

Fouling characteristics of microfiltration membranes during the filtration of jet loop membrane bioreactor (JLMBR) activated sludge

The main objective of this study was to characterize the activated sludge of jet loop membrane bioreactor (JLMBR) in order to determine its effects on the membrane biofouling. The sludge characteristics were evaluated in terms of MLSS (mixed liquor suspended solid), EPS (extracellular polymeric substance), SMP (soluble microbial product), relative hydrophicity (RH) and viscosity. The membrane filtration tests were performed at a cross-flow microfiltration system by using cellulose acetate (CA) and cellulose nitrate (CN) filters with 0.2 and 0.45 μm of pore sizes. The jet loop bioreactor was operated at a batch mode for 36 d with an organic load of 3 kg COD m-3d -1. The COD treatment efficiency was achieved at 95%. It was found that the sludge properties changed with MLSS concentration. The viscosity of sludge increased and the RH of sludge decreased with increase in MLSS concentration. The EPS concentration of the sludge was much higher than SMP concentration. The carbohydrate contents of EPS and SMP were higher than the protein contents. The flux decline models and the resistance analysis were used to investigate the biofouling mechanism. In the resistance analysis, it was found that the pore blocking resistance (R p ) of CA membrane with pore size of 0.45 μm (CA045) was higher than the cake layer resistance (R c ), (3.56 and 1.02 × 1012 m-1). R c was more higher than R p for the other membranes. R p of membranes were found to be 33.6%, 77.1%, 23.4% and 21.9% of the total resistances (R t ) for CA02, CA045, CN02 and CN045, respectively. It was concluded that the high EPS content in JLMBR was the main fouling parameter for membranes. It was also found that the affinity of membrane types against EPS varied.

Mechanisms for the enhancement of ultrafiltration and membrane cleaning by different ultrasonic frequencies

The mechanisms for ultrasounds (USs) at the frequencies of 28, 45 and 100 kHz on ultrafltration (UF) and membrane cleaning were investigated in dead-end UF cell with dextran aqueous solution. Permeate flux and resistances at different US frequencies were recorded and calculated in terms of resistance-in-series model, respectively. The flux enhancements by US at the frequencies of 28 and 45 kHz were significant, whereas no noticeable effect at 100 kHz was found. US irradiation primarily affected the concentration polarization and cake layer resistance, leading to a reduction of the total resistance and an enhancement of the permeate flux. Recovery of flux after the membrane cleaning by US with low frequency of 28 or 45 kHz was higher than that by 100 kHz. Experimental results also indicated that the flux enhancement by US at 28 kHz was stronger than that at 45 kHz at low pressure, whereas increasing the pressure, particularly to 1.2 bar, the enhancement induced by US at these two frequencies US was quite similar. In conclusion, low frequency US, such as 28 kHz can significantly enhance the flux of UF and the membrane cleaning process which could effectively decrease the concentration polarization and the cake layer resistance.