Cake Layer Resistance Research Articles

Membrane fouling mitigation in different biofilm membrane bioreactors with pre-anoxic tanks for treating mariculture wastewater

This study compared the membrane fouling mitigation in two novel types of biofilm membrane bioreactor coupled with a pre-anoxic tank (BF-AO-MBR)—namely a fixed biofilm membrane bioreactor (FB-MBR) with fiber bundle bio-carriers and a moving-bed biofilm membrane bioreactor (MB-MBR) with suspended bio-carriers—relative to an anoxic/oxic MBR (AO-MBR), at salinities ranging from zero to 60 g/L. The results showed that the FB-MBR mitigated membrane fouling to a greater degree than the MB-MBR and AO-MBR. During operation, the FB-MBR exhibited the lowest fouling development, with three membrane filtration cycles, while the AO-MBR and MB-MBR had 22 and nine cycles, respectively. The key fouling factor in all reactors was cake layer resistance (RC), which contributed to 89.61, 62.20, and 83.17% of the total fouling resistance (RT) in AO-MBR, FB-MBR and MB-MBR, respectively. Additionally, in the FB-MBR, the pore blocking resistance (30.07%) was also an important cause of fouling. Fiber bundle bio-carriers and suspended bio-carriers reduced the RT by 37.68% and 21.24% (mainly the RC) compared to that of AO-MBR. Furthermore, FB-MBR and MB-MBR caused a decrease of suspended biomass (80.14 and 15.90%, respectively), and the latter exhibited a higher sludge particle size than AO-MBR, possibly resulting in the cake layer decline. The studied BF-AO-MBRs further alleviated the fouling propensity by reducing the amount of soluble microbial product (SMP) and extracellular polymeric substances (EPS) under all salinity levels, especially the FB-MBR. Among the protein components, the amounts of tryptophan protein-like substance and aromatic protein-like substance were significantly lower in the FB-MBR compared to the AO-MBR and MB-MBR. Additionally, at 60 g/L salinity, the structure of the microbial community in the FB-MBR had a lower abundance of Bacteroidetes and more biomacromolecule degraders, which may have contributed to the moderation of membrane fouling.

Effects of a novel bimetallic catalytic biofilter-based pretreatment technique on the form of ultrafiltration membrane fouling

Abstract Pretreatments of influents using bimetallic catalytic biofilter (BC-biofilter) can help reduce transmembrane pressures. For ultrafiltration membranes coupled with a conventional biofilter pretreatment, the cake layer resistance accounts for 25.0% of the total resistance. However, for those coupled with BC-biofilter pretreatment, the cake layer resistance accounts only for 12.5% of the total resistance. Confocal laser scanning microscopy is employed to determine the porosity of cake layer. It is found that ultrafiltration membranes with BC-biofilter pretreatment show a cake layer porosity of up to 0.56 or greater, whereas those with a conventional biofilter pretreatment exhibit a cake layer porosity of only 0.36. This is because micro-flocculation occurs in the effluents of BC-biofilter. The flocs generated through flocculation deposit on membrane surfaces to create highly porous cake layer. Moreover, catalytic reduction can increase the zeta potentials of the biofilter effluents. This makes the deposition of colloidal particles and flocs on membrane surfaces difficult under electrostatic repulsion. Simultaneously, micro-flocculation after BC-biofilter pretreatment can remove colloidal particles with particle sizes of 200–350 nm in water. This can effectively prevent the blockage of ultrafiltration membrane pores. Furthermore, compared to conventional biofilter, BC-biofilter pretreatment can more effectively reduce the number of colloidal particles and the van der Waals forces of ultrafiltration membranes. They can also change the action directions of electric double layers and thereby mitigate ultrafiltration membrane fouling.

Light conducting photocatalytic membrane for chemical-free fouling control in water treatment

This work shows for the first time the convenient in-situ UV illumination of a titanium dioxide (TiO2) photocatalytic membrane via a sintered porous glass substrate for effective fouling reduction during water treatment. By directing light through the light conducting glass substrate, this concept overcomes the current challenge of photocatalytic membranes that direct light through turbid, light obstructing feed waters. Fouling was tested by filtering model solutions of humic acid (HA), bovine serum albumin (BSA) or sodium alginate (SA). Photocatalysis initiated by simply directing light via the permeate side through the porous glass substrate led to significant reductions in trans-membrane pressure (TMP) rise rates between backwashes and all model organic fouling compounds. Specifically, the UV-light exposed membranes showed a 3.0-fold and 2.4-fold reduction in total filtration resistance for BSA and SA solutions, respectively, which also showed 2.7-fold and 4.2-fold reductions in the irreversible fouling indices. Analysis by SEM coupled with fouling modelling showed the beneficial anti-fouling effects stemmed from reduced intrusion of organic material inside the TiO2 membrane pores, as well as reduced cake layer resistance. The novel, convenient light conducting photocatalytic membranes concept could be used for sustainable, low-chemical membrane filtration of polluted water.

Mechanism of Membrane Fouling Control by HMBR: Effect of Microbial Community on EPS.

A hybrid membrane bioreactor (HMBR) employing activated sludge and biofilm simultaneously is proved to represent a good performance on membrane fouling control compared to conventional membrane bioreactor (CMBR) by reducing extracellular polymeric substances (EPS), especially bound EPS (B-EPS). In order to better understand the mechanism of membrane fouling control by the HMBR in regard of microbial community composition, a pilot scale HMBR operated to treat domestic wastewater for six months, and a CMBR operated at the same time as control group. Results showed that HMBR can effectively control membrane fouling. When transmembrane pressure reached 0.1 MPa, the membrane module in the HMBR operated for about 26.7% longer than that in the CMBR. In the HMBR, the quantity of EPS was significantly lower than that in the CMBR. In this paper, soluble EPS was also found to have a close relationship with cake layer resistance. The species richness and diversity in the HMBR were higher than those in the CMBR, and a certain difference between the compositions of microbial communities in the two reactors was confirmed. Therefore, the difference in microbial community compositions may be the direct reason why EPS in the HMBR was lower than that in the CMBR.

Antifouling behaviour of a photocatalytic modified membrane in a moving bed bioreactor for wastewater treatment

Membrane fouling is a major bottleneck in the application of moving bed-membrane bioreactors. A novel moving bed-UV-photocatalytic-modified membrane bioreactor (MB-UVPMBR) was proposed for efficient wastewater treatment and membrane fouling mitigation. Herein, the modified membrane served as photocatalysis and membrane processes simultaneously. Compared with the conventional moving bed-membrane bioreactor (MBMBR) and MB-UVPMBR without UV light irradiation, the MB-UVPMBR exhibited good effluent quality and anti-fouling ability with UV light irradiation. The cake layer resistance was reduced by 83%, and the pore blockage resistance was reduced by 88%, thus reducing the total membrane filtration resistance. The hydrophilicity of the modified membrane helped mitigate foulant deposition and cake layer formation on the membrane surface. The modified membrane effectively reduced the contents of soluble microbial products and foulants deposited on the membrane surface by photocatalysis. Additionally, the fouled membrane could be effectively regenerated by self-cleaning.

Membrane fouling quantification by specific cake resistance and flux enhancement using helical cleaners

Membrane fouling in a cross-flow ultrafiltration (UF) is investigated using a tubular membrane for three different feed solutions containing oil droplets and particles. The initial build-up of a highly concentrated fouled cake layer on the membrane surface results in sharp flux decline which acting as secondary filtration layer over the membrane surface. In the present work, the fouled cake layer resistance is quantified in terms of specific cake resistance (α), which depends strongly on the operating parameters. Cake resistance, α, calculated from experimental data using different models, is used as an operational indicator to quantify membrane fouling. Two sets of experiments are performed, first by operating tubular membrane as it is, and later by introducing a static helical turbulence promoter. Results showed that the influence of pressure and feed concentration on α is more important than that of feed temperature and cross-flow velocity. Oil droplets sizes, which vary mainly with temperature, play an important role in the interpretation of results. Droplets coalescence and flocculation of suspended particles and the aggregation of droplets on particles explain these variations. The cake layer is significantly reduced by using a novel static helical type turbulence promoter inside the tubular membrane module allowing operation at low pressure and low cross-flow velocity, thus, aid in reducing membrane fouling without significantly increasing energy consumption. Computational Fluid Dynamics (CFD) calculations of full Navier-Stokes equations are also performed to gain insight into flow hydrodynamics and for estimating shear stresses when the turbulent promoter is deployed. The result indicates that roughly 4–5 times higher shear stress on the membrane surface is generated depending on the applied cross-flow velocity. It potentially explains significant cake layer alteration with variations in specific resistance and corresponding flux enhancement as observed in experiments.

Microalgae Filtration Using an Electrochemically Reactive Ceramic Membrane: Filtration Performances, Fouling Kinetics, and Foulant Layer Characteristics.

Electrochemical membrane filtration has proven to be successful for microbial removal and separation from water. In addition, membrane fouling could be mitigated by electrochemical reactions and electrostatic repulsion on a reactive membrane surface. This study assessed the filtration performances and fouling characteristics of electrochemically reactive ceramic membranes (a Magneli phase suboxide of TiO2) when filtering algal suspension under different dc currents to achieve anodic or cathodic polarization. The critical flux results indicate that when applying positive or negative dc currents (e.g., 1.25-2.5 mA·cm-2) to the membrane, both significantly mitigated membrane fouling and thus maintained higher critical fluxes (up to 14.6 × 10-5·m3·m-2·s-1 or 526 LMH) compared to the critical flux without dc currents. Moreover, applying dc currents also enhanced membrane defouling processes and recovered high permeate flux better than hydraulic and chemical backwash methods. Moreover, fouling kinetics and the cake layer formation were further analyzed with a resistance-in-series model that revealed many important but underexamined parameters (e.g., cake layer resistance and cake layer thickness). The cake layer structures (e.g., compressibility) were shown to vary with the electrochemical activity, which provide new insight into the biofouling mechanisms. Finally, the algogenic odor, geosmin, was shown to be effectively removed by this reactive membrane under positive dc currents (2.5 mA·cm-2), which highlights the multifunctional capabilities of electrochemically reactive membrane filtration in biomass separation, fouling prevention, and pollutant degradation.

Characterization of activated sludge flocs in membrane bioreactor: stable and unstable flocs.

In this study, the properties of unstable and stable flocs were investigated under the steady operation of a membrane bioreactor (MBR). The extracellular polymeric substances (EPS) composition, surface charge, and hydrophobicity of unstable and stable flocs were examined and compared. Interfacial interactions of the membrane with unstable flocs, unstable flocs themselves, and unstable and stable flocs were assessed using the extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) models. Cake layer resistance was found to contribute more than 80% of total resistance under steady operating conditions. Compared with stable flocs, unstable flocs possessed a higher level of EPS, more diverse protein, more negative charge, weaker hydrophobicity, and higher fouling potential. Thermodynamic analyses showed that unstable flocs had a higher adhesive strength (- 63.4mJ/m2) with the membrane, lower self-cohesive strength (- 18.3mJ/m2), and higher cohesive strength (- 54.3mJ/m2) with stable flocs. Therefore, some unstable flocs remained on the membrane surface to form the cake layer due to their poor cohesion strength.

Biocarriers facilitated gravity-driven membrane (GDM) reactor for wastewater reclamation: Effect of intermittent aeration cycle

This study investigated the performances of gravity-driven membrane (GDM) reactors integrated with granule activated carbon (GAC) biofilm process for wastewater treatment under different intermittent aeration cycles (intensity and frequency). The results showed the removal efficiencies of dissolved organic carbon, total nitrogen, ammonia were significantly improved under intermittent aeration conditions (~86–87%, ~29–37%, and ~83–99%, respectively) compared to non-aeration condition (~72% and ~18%, and ~17%, respectively). In addition, it was found that the intermittent aeration significantly reduced the cake layer resistance and therefore improved ~130–300% the permeate flux compared to control without aeration. Microbial community analysis indicated that prokaryotic and eukaryotic compositions in the cake layer biofilm were significantly influenced by aeration condition. Lastly, energy consumption analysis revealed that GAC + GDM with shorter aeration period and low aeration intensity could be promising as a decentralized wastewater treatment process in terms of water quality and operating energy.

UV-LED/PMS preoxidation to control fouling caused by harmful marine algae in the UF pretreatment of seawater desalination

Preoxidation is gaining spotlight in the mitigation of ultrafiltration (UF) membrane fouling caused by algal organic matter (AOM). Although it is known to be beneficial in freshwater, its applicability in seawater treatment has barely been explored. This study first evaluated the effect of UV/permonosulfate (PMS) oxidation for UF process against marine harmful algal blooms (HAB). The transition of AOM released from Pseudo Nitzchia was investigated which produces a neurotoxin, domoic acid (DA). Specifically, UV light emitting diodes (LED) were employed as UV source. The results indicated UV-LED/PMS treatment effectively reduces the fouling potential of AOM. However, the participation of abundant chloride ions generated free chlorine and disinfection byproducts, thus, PMS addition below 0.5 mM was suggested for seawater applications. With respect to PMS 0.5 mM, TOC and cake layer resistance were reduced by 10% and 85%, respectively, and this was followed by significant improvements in water permeability, flux reversibility and permeate quality of UF process. Furthermore, PMS exhibited significant reactivity for decomposing DA achieving rapid disappearance of 1 ppm of DA within a minute. The aforementioned results encourage the use of PMS as a bulk oxidant in seawater treatment to simultaneously mitigate membrane fouling and improve permeate quality.

Exploring Nanosilver-Coated Hollow Fiber Microfiltration to Mitigate Biofouling for High Loading Membrane Bioreactor.

For the first time, a nanosilver-coated hollow fiber microfiltration (MF) was fabricated by a simple chemical reduction method, then tested for membrane biofouling mitigation study under extreme high mixed liquor suspended solid (MLSS) concentration for long term. This study presents a simple and novel technique to modify a commercially available MF membrane using silver nanoparticles (AgNPs) followed by an investigation of mitigating membrane biofouling potentials using this modified membrane to compare with an unmodified membrane for 60-day operation period. The modified membranes showed that AgNPs was attached to the MF-membrane successfully with a high density of 119.85 ± 5.42 mg/m2. After long-term testing of 60 days in membrane bioreactor with a MLSS concentration of 11,000 mg/L, specific flux of the AgNPs coated MF (AgNPs-MF) decreased 59.7%, while the specific flux of the unmodified membrane dropped 81.8%, resulted from the increase of transmembrane vacuum pressure for the AgNPs-MF was lower than that of the unmodified one. The resistance-in-series model was used to calculate the resistance coefficients of membrane modules, and the result showed that the cake layer resistance coefficient of the unmodified membrane was 2.7 times higher than that of the AgNPs-MF after the 60-day operation, confirming that AgNPs displayed great antimicrobial properties to mitigate membrane biofouling under such high MLSS.

Membrane fouling in a hybrid process of enhanced coagulation at high coagulant dosage and cross-flow ultrafiltration for deinking wastewater tertiary treatment

A hybrid process of enhanced coagulation at a high dosage of polyaluminum chloride and flat-sheet cross-flow ultrafiltration was used for the advanced treatment of the effluent from a biological treatment process of deinking wastewater in a paper mill. The experimental results obtained by a small-scale continuous laboratory experimental device showed that the absorption resistance was significantly lower than the membrane intrinsic resistance, the concentration polarization resistance, the pore blocking resistance, and the cake layer resistance and so it can be ignored. With increasing polyaluminum chloride dosage from 0 to 2,000 mgL-1, the total membrane filtration resistance decreased and the membrane flux increased, but the concentration polarization resistance increased. With the increase in cross flow velocity, the total membrane filtration resistance, the pore blocking resistance, the layer resistance, and the concentration polarization resistance all decreased, and consequently, the membrane flux increased. With regard to the concentration polarization resistance, the cross flow velocity had a more significant effect than the trans-membrane pressure. But the trans-membrane pressure had a more significant effect on the layer resistance, the pore blocking resistance, and the total membrane filtration resistance than the cross flow velocity. The filtration resistance at the polyaluminium chloride dosage of <1,000 mgL-1 could be ranked as follows: the membrane intrinsic resistance > the pore blocking resistance > the layer resistance > the concentration polarization resistance> the absorption resistance. But it was the membrane intrinsic resistance > the concentration polarization resistance > the pore blocking resistance > the layer resistance > the absorption resistance at the dosage of ≥1,000 mgL-1. The results are helpful for the membrane fouling control and can promote the application of flat-sheet cross-flow membrane process, resultantly improving the reclaim and reuse of deinking wastewater, and reducing the emission of contaminants in deinking wastewater.

In-situ characterization of cake layer fouling during crossflow microfiltration of oil-in-water emulsion

In view of membrane-filtration being a promising means for treating the large volumes of oily wastewater, much efforts have been dedicated to understanding membrane fouling by oil emulsions. However, the understanding on cake fouling, which is the governing form of fouling throughout filtration, by oil emulsions remain poorly understood. To bridge this gap, this study focused on characterizing the evolution of the oil-emulsion cake layer characteristics on hollow fiber (HF) membranes in real time using the direct observation through the membrane (DOTM) technique. The more well-studied latex particles were studied alongside to provide a benchmark for comparison. Despite the same feed concentration and crossflow velocity used, the transmembrane pressure (TMP) and cake layer resistance magnitudes were greater for oil emulsion than for latex particles at the same permeate flux. DOTM images revealed that this was linked to the following behaviors: (i) oil tended to deposit as a moving cake layer, whereas latex as scattered deposits; and (ii) oil gave a thinner yet denser cake layer. Moreover, it was observed that the cake thickness and porosity fluctuated significantly with respect to permeate volume for the oil emulsions, because the moving cake layer gave greater variations of cake thicknesses along the membrane. Finally, the interfacial interaction energy predicted by the XDLVO model gave higher oil-membrane and oil-oil affinities compared to that of latex, which underlie respectively the more extensive fouling and denser cake exhibited by oil. The unique characteristics of the cake layer of oil emulsion vis-à-vis the well-studied particulate foulant (i.e., latex) shown here provides new perspectives on the monitoring and control of oil fouling.

Recycling rainwater by submerged gravity-driven membrane (GDM) reactors: Effect of hydraulic retention time and periodic backwash

Rainwater recycling has been considered as an alternative cost-effective decentralized water supply. The low cost and effective gravity-driven membrane (GDM) filtration technology has been introduced to treat the rainwater prior use. In this study, we investigated the effects of hydraulic retention time (HRT; 27 h, 51 h, and 156 h) and periodic backwash durations (2 min, 5 min, 10 min, and 30 min per 2–3 days' filtration) on the permeate quality, flux and fouling mechanism in lab-scale submerged GDM reactors. Compared to the performance at HRT of 51 h (40% of DOC removal and ~2.9 L/m2 h), better permeate quality and higher membrane flux were achieved at HRT of 27 h (51% of DOC removal and ~4.2 L/m2 h) and 156 h (48% of DOC removal and ~5.0 L/m2 h). Although the hydraulically reversible resistance was predominant (up to 90% of the total fouling resistance), the permeate flux could not be fully recovered by periodic backwash, regardless of the backwash durations. After several filtration-backwash cycles, the stabilized flux of GDM reactor with backwash was even worse than those without backwash. However, no correlation can be established between the stabilized flux (i.e., cake layer resistance) and the soluble organics and microbial cells in the cake layer of the GDM system during rainwater treatment.

Effect of metabolic uncoupler, 2,4‑dinitrophenol (DNP) on sludge properties and fouling potential in ultrafiltration membrane process

Energy uncoupling technology was applied to the membrane process to control the problem of bio-fouling. Different dosages of uncoupler (2,4‑dinitrophenol, DNP) were added to the activated sludge, and a short-term ultrafiltration test was systematically investigated for analyzing membrane fouling potential and underlying mechanisms. Ultrafiltration membrane was used and made of polyether-sulfone with a molecular weight cut off (MWCO) of 150 kDa. Results indicated that low DNP concentration (15–30 mg/g VSS) aggravated membrane fouling because more soluble microbial products were released and then rejected by the membrane, which significantly increased cake layer resistance compared with the control. Conversely, a high dosage of DNP (45 mg/g VSS) retarded membrane fouling owing to the high inhibition of extracellular polymeric substances (proteins and polysaccharides) of the sludge, which effectively prevented the formation of cake layer on the membrane surface. Furthermore, analyses of fouling model revealed that a high dosage of DNP delayed the fouling model from pore blocking transition to cake filtration, whereas this transition process was accelerated in the low dosage scenario.

Evaluation of fouling in nanofiltration for desalination using a resistance-in-series model and optical coherence tomography

Resistance-in-series models have been applied to investigate fouling behavior. However, it is difficult to model the influence of morphology on fouling behavior because resistance is indirectly calculated from the water flux and transmembrane pressure. In this study, optical coherence tomography (OCT) was applied to evaluate the resistance of the fouling layer based on fouling morphology. Sodium alginate, humic acid, and bovine serum albumin (BSA) with high salts concentrations (conductivity: 23 mS/cm) were used as model foulants. At the same total fouling resistance, BSA showed the highest cake layer thickness (BSA (114.5 μm) > humic acid (53.5 μm) > sodium alginate (20.0 μm)). However, a different order was found for the cake layer resistance (BSA > sodium alginate > humic acid). This indicates that fouling thickness is not correlated with cake layer resistance. According to the Carman–Kozeny equation, fouling layer porosity decreased in the following order: humic acid (0.30) > BSA (0.21) > sodium alginate (0.20). In addition, we provided a specific value that was calculated using the ratio between the fouling thickness and cake layer resistance. The results show that alginic acid induced a stronger cake layer resistance, despite its thin fouling layer, whereas BSA showed a relatively low potential for inducing cake layer resistance. The results obtained in this study could be used for estimating and predicting fouling behavior.

Analysis of flux decline using sequential fouling mechanisms during concentration of Syzygium cumini (L.) leaf extract

Experimental studies were performed in a stirred cell for concentration of MF-pretreated extracts from jamun (Syzygium cumini L.) leaves using nanofiltration over a wide range of operating conditions. Performance of nanofiltration process was evaluated in terms of permeate flux, flux decline and extract quality (total polyphenol content, total flavonoid content and antioxidant activity). A total of ten phenolic compounds including six phenolic acids and four flavonoids were identified and quantified in jamun leaf extract using HPLC. A combined sequential model consisting of Hermia’s complete pore blocking and cake layer formation based on resistance-in-series theory was considered to explain the flux decline behavior with time. The model calculations were shown to be in excellent agreement with experimental flux data (R2>0.90; SD<2.5%). Experimental results showed that the operating conditions had significant effect on the onset of cake layer formation. The pore blocking resistance was found to be comparable with membrane resistance, while cake layer resistance was 20–40% of membrane resistance. In addition, the permeate flux for MF-pretreated extract was always higher than that for the untreated extract. The present study with the estimated model parameters provided important understandings into the underlying mechanisms of membrane fouling phenomena during concentration of jamun leaf extract.

Probing the fouling process and mechanisms of submerged ceramic membrane ultrafiltration during algal harvesting under sub- and super-critical fluxes

Abstract Membrane filtration is a promising approach for harvesting microalgae for production of biofuel and high-value products. Membrane fouling is the most limiting factor for filtration operation as it causes low flow flux, high transmembrane pressure and high maintenance cost associated with membrane backwash and replacement. In the present study, the harvesting efficiency and the membrane fouling properties of a bench-scale ultrafiltration system with a submerged flat-sheet ceramic membrane were investigated for harvesting green microalgae, Chlorella vulgaris, under sub- and super-critical flux. The fouling mechanism (cake layer, gel layer and membrane pore blocking) was systematically analyzed to illustrate the possible fouling profiles at sub and super-critical flux. The results showed that the productivity for harvesting microalgal cells under super-critical flux condition was three times higher than sub-critical flux condition, with the fouling rates 8 times higher as well. The increase in irreversible membrane fouling was attributed as the main cause of the membrane resistance increase at the super-critical flux condition although the resistance of cake layer was the highest for both conditions. SMPs, aromatic protein and humic-like organics of microalgae broth contribute to the irreversible membrane fouling.

In situ coagulation versus pre-coagulation for gravity-driven membrane bioreactor during decentralized sewage treatment: Permeability stabilization, fouling layer formation and biological activity

Gravity-driven membrane filtration systems are promising for decentralized sewage treatment due to their low energy consumption and low maintenance. However, the low stable permeability/flux is currently limiting their wider application. With the ultimate goal of increasing permeability, the aim of this study was to evaluate the effect of coagulation (in situ coagulation and pre-coagulation) on the performance of a gravity-driven membrane bioreactor (GDMBR) during treatment of synthetic sewage. Results show that in situ coagulation significantly increased permeability (more than two-fold); however, no stabilization of permeability occurred over the whole operation, when non-coagulated and pre-coagulated reactors were compared. The high permeability observed was attributed to the accumulated aluminium floc in the reactor, which prevented formation of fluorescent microbial metabolites (aromatic and tryptophan proteins, as well as fulvic acids), and further avoided membrane pore blocking. In addition, the surface porosity of the fouling layer was improved (from 11.2% to 32.4% for non-coagulated and in situ coagulated reactors). The unstable permeability was possibly associated with lower biological processes within the fouling layer. These might include lower adenosine triphosphate (ATP) content and lower fluorescent metabolites from the extracellular polymeric substances (EPS) caused by the accumulated Al (compared with the control). On the other hand, pre-coagulation improved the level of stable permeability compared with the control (80 versus 40 L/m2 h bar), mainly because pre-coagulation decreased the EPS content and also maintained high ATP content of the fouling layer. In addition, both coagulation processes reduced the total filtration resistance, mainly the hydraulically reversible resistance and cake layer resistance, which could lower the cleaning frequency. Overall, coagulation could greatly increase the removal efficiency and improve the GDMBR permeability, which would make the process suitable for decentralized wastewater treatment.

Study on the mechanical lifting process for membrane fouling control

ABSTRACTIn order to reduce energy consumption and control membrane fouling, a novel submerged membrane separation process was developed in this paper. In this design, the traditional aeration device was replaced by a low-power motor. Through the traction force produced by the motor, the membrane module can move up and down in the water. In this case, friction will be created between the membrane surface and the water, which can prevent the deposition of pollutants on the membrane surfaces and then control membrane fouling. After a 64-min filtration operation, compared with the aeration process, the cake-layer resistance of the lifting process reduced by about 13.6%. For a 120-min filtration operation, the transmembrane pressure (TMP) of the lifting process was 11.1% lower than that of the aeration process at the end of filtration. According to the long-time operation, the stable running time was increased from 352 min of the aeration process to 408 min of the lifting process. All these suggest that the novel lifting process can control membrane fouling more effectively than the aeration process. Therefore, the lifting process is an efficient and feasible membrane separation technology.