Activated Sludge Filtration Research Articles

Potential positive effects of natural sunlight on a biological system for landfill leachate treatment

AbstractActivated sludge filtration performance is a significant mean to evaluate membrane fouling trend for wastewater treatment. Here the impact of sunlight irradiation on activated sludge filtration performance and microbial communities in sequencing batch reactors (SBRs) when treating raw landfill leachate were studied. The sludge in photic SBR (i.e., SBR exposed to natural sunlight) exhibited better filtration performance compared to dark SBR (i.e., SBR unexposed to sunlight). The removal efficiency of COD and NH4+‐N in the photic SBR were slightly higher than those in the dark SBR. The contents of the extracellular polymeric substances (EPSs) of sludge in both SBRs initially increased and then decreased. However, in the later period, the sludge filtration performance worsened due to the fungal activity in the dark SBR. Natural sunlight irradiation promoted sludge filtration performance by affecting the microorganism structure in the photic SBR. The bacterial genus Thauera was dominant in the photic SBR (39.35%), whereas Planktosalinus and Ottowia were dominant in the dark SBR (16.84% and 12.55%, respectively). Natural sunlight irradiation had a prominent effect on the fungal diversity in the system, and filamentous bulking caused by the fungi genus Trichosporon’s proliferation was observed in the dark SBR but not in the photic SBR, which also increased the polysaccharide content.

Effect of surface-patterned topographies of ceramic membranes on the filtration of activated sludge and their interaction with different particle sizes

The main benefits of surface patterning on filtration membranes are two-fold: (1) surface patterns increase the effective filtration area, and (2) enhance the back diffusion of foulants to minimise membrane fouling. Although the influence of pattern sizes has been revealed, the relationship between pattern sizes and foulant particle sizes has not been systematically studied. In this study, we fabricated ceramic membranes with line patterns of various designed topographies through 3D printing and investigated their differences in fouling alleviation with activated sludge and 5 latex particle sizes with narrow size distributions. Combinations of three pattern heights ( h = 80, 120, 250 μm) and three pattern spacings ( s = 200, 500, 1000 μm) were prepared in this study. Membrane fouling rates were affected by the topography, even after accounting for differences in the effective surface areas. Structures with high height (Pattern HM) performed poorly and had the highest fouling rate in activated sludge filtration compared to the rest of the patterns. The average rate of transmembrane pressure increase for HM was 2.48 ± 0.07 kPa/min, 3 times higher than that of the best performing design (Pattern MM). The same phenomenon was observed for monodispersed latex microspheres. Moreover, we found that the fouling rate varied with h , s and particle size ( a ) and increased with a/(s/h) ratio. For the activated sludge and range of latex particles tested, a combination of h = 120 μm and s = 500 μm (Pattern MM) was the overall best design for fouling alleviation. Our findings provide an effective pathway to achieve maximum benefits of the surface-patterned ceramic membrane according to particle sizes, e.g., the s/h ratio must be designed to increase for the treatment of larger particles. • Appropriate surface patterning on ceramic membrane reduced membrane fouling. • Topographies of line patterns were important considerations for design. • Particle diameter ( a ), pattern height ( h ) and pattern spacing ( s ) studied. • Fouling rate increased with a/(s/h) ratio. • Combination of h = 120 μm and s = 500 μm was best for fouling alleviation.

The Use of Verbascum Thapsus L as a Biomembrane for Activated Sludge Filtration

Membrane technology is a green technology, but it still faces a pressing problem related to the effect of fabrication materials on the environment. The plant Verbascum thapsus L (VTL) was utilized as a biomembrane to reduce chemicals. In this study, VTL was successfully utilized as a membrane for activated sludge separation. The membrane was characterized via scanning electron microscopy (SEM)-EDX, Fourier transform infrared (FTIR), and contact angle measurement. Additionally, the effects of pressures on the fluxes and the rejection ability were studied. The permeability of the bio-based membrane reached 581 L/m2 .h.bar. The VTL membrane was examined for the removal of chemical oxygen demand (COD), protein, and carbohydrate. Accordingly, the maximum COD removal was obtained at a transmembrane pressure of 2.5 bar and reached up to 57%. The protein and carbohydrates rejections raised from 80% and 84% at 0.5 bar to 90% and 98% at 2.5 bar, respectively. The total resistance increased from 87% at a pressure of 0.5 bar to 96% at 2.5 bar. The flux recovery ratio (FRR) for the membrane at working pressures (0.5-2.5 bar) was 96% for 0.5 bar and 70% for 2.5 bar. The physical cleaning showed a flux recovery after three operation cycles. At the end of the filtration experiments, the pressure variation along streamlines over the membrane cross-section was simulated. As a result of this study, the use of a naturally-derived membrane is considered a green technology. The plant-based membrane reduces the use of non-green chemicals. Moreover, VTL has no commercial value and is recognized as an invasive plant species. All of the previous issues made the study worthwhile.

Filterability of Polysulfone Membrane in a Tilted Panel System for Activated Sludge Filtration

Membrane bioreactors (MBRs) are established technology for treatment of domestic and industrial wastewater because they offer a small footprint and high quality of effluent, in addition to lower excess sludge. However, their widespread applications are still limited by higher expenditure for compensating for membrane fouling. In this study, polysulfone (PSF)-based ultrafiltration membranes were developed and integrated with a tilted panel system for fouling control in activated sludge filtration. The results show an enhanced performance of filtration system thanks to the mutual advantage of the tilted panel system and the membrane properties. Both membranes showed a clear trend of higher permeability with respect to the tilted panel parameters, namely, higher tilting angle, higher aeration rate, and shorter intermittent/switching period. PSF-1 (1 wt% polyethylene glycol (PEG) additive) shows significantly better performance than PSF-3 (3 wt% PEG additive) although their mean flow pore size, structural properties, and contact angle do not differ significantly. PSF-1 shows superior filterability performance of about 45% for panel tilting angles of 20° at an aeration rate of 1.8 L·min−1, and 11% for a switching period of 1 min compared with PSF-3. The key property enhancing the performance of the PSF-1 is its narrower distribution of pore size. Overall results suggest that an optimum system could be achieved by optimizing both the filtration system and the membrane material properties.

MBR공정의 혼합액 슬러지 성분에 따른 기질저감과 막오염특성

This study aims to investigate the effects of sludge components and agitation rate on contaminant removal, membrane filtration, and fouling in a membrane bioreactor (MBR) by using a dead-end stirrer cell. The real mixed liquor used for this study was pretreated to segregate three sludge components, namely activated sludge (AS), suspended solids (SS), and supernatant (SUP) including colloids and solutes. In reducing the contaminants, MBR filtration of AS and SS at agitation rates of 0 - 1000 rpm achieved higher efficiencies (20-30%) than those of achieved SUP filtration. In particular, the removal efficiencies of COD and T-P by the MBR were about three times higher than that of T-N. In reducing total organic carbon (TOC) and dissolved organic carbon (DOC), the MBR filtration of AS and SS achieved 1.7 times higher efficiency than that of SUP, whereas the MBR filtration of SUP in reducing ultraviolet at 254nm (UV₂₅₄) achieved 1.4 and 4.7 times higher efficiencies, respectively than those of AS and SS. On the other hand, the profiles of TMP build-up within 30 min of filtration time were larger, corresponding to AS > SS > SUP. This reveals that the solid level in developing TMP has a greater effects than dissolved level in mixed liquor. This result also indicates that both membrane filtration to lower TMP and an appropriate agitation rate (< 400rpm) are required to minimize membrane fouling by creating a shear force near the membrane surface, in order to prevent both micro-flocs generation and cake layer formation.

Assessment of patterned membrane in a tilted panel filtration system for fouling control in activated sludge filtration

Membrane bioreactor (MBR) is a combination between advanced membrane filtration system and activated sludge biological treatment for sewage effluents treatment. Despite demonstrating a remarkable advantage over the conventional activated sludge process, MBR is still limited by the occurrence of membrane fouling problem, which reduces the performance and lifespan of the membrane causing higher maintenance and operation costs. Finding sustainable membrane fouling mitigation strategies has become the main concern in MBR operation. Hence, air bubbling method had been proven as one of the membrane fouling mitigation strategies. Ineffective air bubbling contributes to higher energy consumption for MBR operation. It must be pumped at high rates as low rates air bubbles only show poor contacts with the membrane surface in which limits its efficacy to scour-off the foulant. In this study, the performance of a patterned membrane in a tilted panel system was systematically evaluated to enhance air bubble effectiveness for membrane fouling control in activated sludge filtration. The patterned surface topography in a combination of the tilted panel is expected to enhance air bubbles contacts and increase their fouling control roles. An imprinting step was included into the fabrication procedure in order to create membrane with patterned surface. Results showed that the patterned outperform flat membrane in both tilted and vertical systems. Such an advantage can be ascribed by both the difference in membrane properties (effective surface area and pore size) and the nature of the surface (flat vs patterned). The patterned membrane yields higher permeance in the tilted panel over the vertical one in all tests, being the highest at shortest filtration/relaxation cycle of 5 min, shortest switching period of 2.5 min and highest aeration rate of 1.5 L/min corresponding to permeances of 465 vs 409, 369 vs 318 and 447 vs 381 L/(m2h.bar), respectively. However, since operation with tilting only allows one-sided membrane, the patterned membrane offers better panel productivity when operated in a vertical panel. Operation with switching (intermittent aeration) only offers the slight advantage of panel productivity over the vertical one (546-738 vs 440-632 L/m2h.bar).

Patterned Membrane in an Energy-Efficient Tilted Panel Filtration System for Fouling Control in Activated Sludge Filtration.

A membrane bioreactor enhances the overall biological performance of a conventional activated sludge system for wastewater treatment by producing high-quality effluent suitable for reuse. However, membrane fouling hinders the widespread application of membrane bioreactors by reducing the hydraulic performance, shortening membrane lifespan, and increasing the operational costs for membrane fouling management. This study assesses the combined effect of membrane surface corrugation and a tilted panel in enhancing the impact of air bubbling for membrane fouling control in activated sludge filtration, applicable for membrane bioreactors. The filterability performance of such a system was further tested under variable parameters: Filtration cycle, aeration rate, and intermittent aeration. Results show that a combination of surface corrugation and panel tilting enhances the impact of aeration and leads to 87% permeance increment. The results of the parametric study shows that the highest permeance was achieved under short filtration–relaxation cycle of 5 min, high aeration rate of 1.5 L/min, and short switching period of 2.5 min, to yield the permeances of 465 ± 18, 447 ± 2, and 369 ± 9 L/(m2h bar), respectively. The high permeances lead to higher operational flux that helps to lower the membrane area as well as energy consumption. Initial estimation of the fully aerated system yields the energy input of 0.152 kWh/m3, much lower than data from the full-scale references of <0.4 kWh/m3. Further energy savings and a lower system footprint can still be achieved by applying the two-sided panel with a switching system, which will be addressed in the future.

Review of effective tools for monitoring the performance of membrane bioreactors

The production of high quality wastewater after treatment plants using a membrane bioreactor (MBR) probably requires less control than traditional processes of activated sludge. However, the use of MBR leads to additional problems associated with the activated sludge filtration, the formation of a sediment layer on the surface of the membrane and membrane fouling. Operation engineers require simple tools for controlling and monitoring the operation of treatment plants that can provide timely information about the biological state of the activated sludge and the filterability of the waste fluid, as well as the risks of membrane contamination. To this end, effective means of monitoring MBR and methods for determining biological activity are considered. For a broader characterization of the wastewater composition, the use of total nitrogen and total carbon analyzers is considered, which have significant advantages over the analysis of chemical and biological oxygen consumption (COD, BOD).These tools can be used regularly to ensure timely intervention and more stable monitoring of the operation of MBR systems.

Polycitrate-para-aminobenzoate alumoxane nanoparticles as a novel nanofiller for enhancement performance of electrospun PAN membranes

In the present study, the novel high flux and antifouling polycitrate-para-aminobenzoate alumoxane (PC-PABA) nanoparticles embedded polyacrylonitrile (PAN) microfiltration membranes were prepared by electrospinning method. PC-PABA nanoparticles are modified carboxylate alumoxanes with boehmite core containing extra hydroxyl, carboxylate and citrate alumoxane groups on their surface, which introduced in this study for the first time. The influence of the PC-PABA nanoparticles on morphology and efficiency of the prepared electrospun membranes was studied. The FTIR spectra of the PC-PABA nanoparticles showed that the citrate alumoxane, carboxylate, and hydroxyl groups were successfully formed on the surface of PC-PABA nanoparticles. The morphology studies by field emission scanning electron microscopy (FESEM) revealed that at low concentration of the PC-PABA nanoparticles, the nanofibers at the surface of the membranes was smooth. Nevertheless, at higher PC-PABA nanoparticle concentrations, the nanoparticles were decorated on the nanofibers due to agglomeration of the nanoparticles. The hydrophilicity and permeation flux of the membranes were enhanced by embedding the PC-PABA nanoparticles. The activated sludge filtration results showed that by adding a low amount of PC-PABA nanoparticles, the flux recovery ratio of the membranes was increased. The 3 wt% PC-PABA/PAN membrane, exhibited the highest flux recovery ratio (FRR) of 98.1% and considerable reusability during long-term filtration.

Magnetic micro-particle conditioning–pressurized vertical electro-osmotic dewatering (MPEOD) of activated sludge: Role and behavior of moisture and organics

In this study, a magnetic micro-particle conditioning–pressurized vertical electro-osmotic dewatering (MPEOD) process with magnetic micro-particle conditioning–drainage under gravity–mechanical compression–electrical compression (MMPC–DG–MC–EC) stages was established to study the distribution and migration of water, extracellular polymeric substances (EPS), and other organic matter in the activated sludge (AS) matrix at each stage. Results showed that the MPEOD process could attain 53.52% water content (WC) in dewatered AS with bound water (BW) and free water (FW) reduction rates of 82.97% and 99.67%, respectively. The coagulation and time-delayed magnetic field effects of magnetic micro-particles (MMPs) along the MMPC–DG–MC stages initiated the transformation of partial BW to FW in AS. EC had a coupling driving effect of electro-osmosis and pressure on BW, and the changes in pH and temperature at EC stage induced the aggregation of AS flocs and the release of partial BW. Additionally, MMPs dosing further improved the dewatering performance of AS by acting as skeleton builders to provide water passages. Meanwhile, MMPs could disintegrate sludge cells and EPS fractions, thereby reducing tryptophan-like protein and byproduct-like material concentrations in LB-EPS as well as protein/polysaccharide ratio in AS matrix, which could improve AS filterability. At EC stage, the former four Ex/Em regions of fluorescence regional integration analysis for EPS were obviously reduced, especially the protein-like substances in LB- and TB-EPS, which contributed to improvement of AS dewaterability.

Biofouling in membrane bioreactors: nexus between polyacrylonitrile surface charge and community composition

The influence of membrane surface charge on biofouling community composition during activated sludge filtration in a membrane bioreactor was investigated in this study using polyacrylonitrile-based membranes. Membranes with different surface properties were synthesized by phase inversion followed by a layer-by-layer modification. Various characterization results showed that the membranes differed only in their surface chemical composition and charge, ie two of them were negative, one neutral and one positive. Membrane fouling experiments were performed for 40 days and the biofouling communities were analyzed. PCR-DGGE fingerprinting indicated selective enrichment of bacterial populations from the sludge suspension within the biofilms at any time point. The biofilm community composition seemed to change with time. However, no difference was observed between the biofilm community of differently charged membranes at specific time points. It could be concluded that membrane charges do not play a decisive role in the long-term selection of the key bacterial foulants.

The effect of the solids and hydraulic retention time on moving bed membrane bioreactor performance

The aim of the present paper was to investigate the effect of solids (SRT) and hydraulic (HRT) retention time on Integrated Fixed Film Activated Sludge (IFAS) University of Cape Town (UCT) membrane Bioreactor (MBR). In particular, three different pairs of SRT and HRT values were analysed, namely, Phase I 56 d/30 h, Phase II 31 d/15 h and Phase III 7 d/13 h. The short-term effect of these three SRT/HRT conditions was assessed by analysing several system performance indicators: organic carbon and biological nutrient (nitrogen and phosphorus) removal, biomass respiratory activity, activated sludge filtration properties and membrane fouling. The results showed that the decrease of SRT/HRT had a positive influence on system performance. Specifically, the IFAS-UCT-MBR showed excellent removal of organic matter - highest value (99%) at the shortest SRT/HRT (7 d/13 h). Also, the increase in organic loading rate resulting from the decrease of SRT and HRT led to improved nitrogen removal due to higher N removal by sludge wasting requiring less N removal (as N2) by denitrification. Complete nitrification of influent ammonia was achieved at all three SRT/HRT phases, guaranteed by the presence of biofilm carriers in the aerobic reactor, which ensured a higher media SRT than suspended biomass SRT. The increase of the organic loading rate and decrease in SRT led also to a higher heterotrophic activity as demonstrated by the respirometric batch tests, which is due to the increasing active biomass fraction of the volatile suspended solids as SRT decreases. The SRT/HRT decrease over the three phases resulted in an overall increase of the Extracellular Polymeric Substance concentration, which caused an increase in membrane fouling.

Study of PVDF asymmetric membranes in a high-throughput membrane bioreactor (HT-MBR): Influence of phase inversion parameters and filtration performance

Abstract Porous polyvinylidene fluoride (PVDF) membranes with different morphology were prepared via phase inversion followed by comprehensive screening for use in activated sludge filtration applying new type of lab-scale high-throughput membrane reactor (HT-MBR). First, the validation of the HT-MBR was performed involving multiple filtration tests of PVDF membrane samples, either in the same or in different positions in the reactor to verify reproducibility of the measurements in the HT-MBR and to check the homogeneity of the feed liquor hydrodynamics in the reactor. The validated set-up was then used for an accelerated screening of a systematic set of membranes with varying structure. Sixteen PVDF-membranes with mean pore size ranging from 0.05 to 0.37 μm and surface porosity from 2.8% to 13.9% were prepared via non-solvent induced phase separation by varying some phase-inversion parameters. The results showed that lowering the polymer concentration in the membrane casting solution, increasing the exposure time to the air of the cast membrane before solidification, and the use of additives (polyvinylpyrrolidone, PVP) to the casting solution resulted in more porous membranes, larger average pore sizes and higher surface porosities, thus offering higher clean water permeability. Activated sludge filtrations were evaluated implementing two different tests: a flux-stepping and a long-term filtration test. The more porous membranes were superior over the less porous ones, as higher critical fluxes and (slightly) higher permeance was maintained during the 37-day long-term test, although a sharp permeance decrease was observed in the first 7 days. However, for long-term application, these porous membranes should still be improved in order to have more sustained high permeability.

Unique performance of poly(p-phenylene terephthamide) hollow fiber membranes

Poly(p-phenylene terephthamide) (PPTA) hollow fiber membrane with outstanding thermal and chemical stability (endures above 60 °C and almost any organic solvents) has been fabricated by dry–wet spinning method for the first time. The heat and solvent resistance as well as anti-fouling performances was studied in this paper. The properties of thermal stability were characterized by pure water flux, mechanical strength, FTIR, and pore size distribution, and the properties of chemical resistance were characterized by organic solvent flux. The results showed that PPTA hollow fiber membrane was a kind of inner and outer dual-skin layer structure. The flux was stable in hot water and organic solvents, which indicated that the membrane structure hardly changed. The average pore size was slightly increased during high-temperature experiment. Moreover, the properties of anti-fouling were characterized by simulated activated sludge filtration. The fouled PPTA membranes were cleaned by ultrasonic, citric acid, and alkali treatments, and the membrane surface was detected by energy-dispersive X-ray spectroscopy. The results showed that the effect of the citric acid treatment was more preferable to the other ways, which indicated that the inorganic substance was easily adsorbed on the membrane surface compared with organic substance. Therefore, PPTA hollow fiber membrane exhibited the excellent anti-fouling performance by the reason of strong hydrophilicity and electronegativity.

Ribbed PVC–silica mixed matrix membranes for membrane bioreactors

The feasibility of ribbed-PVC–silica mixed matrix membranes (MMM) in a membrane bioreactor (MBR) application was investigated. Three operational aspects were evaluated: inclusion of vibration, use of relaxation (cycle time), and optimization of the cleaning procedure. Results show that the specific ribs on the membranes give a 52% higher surface area compared to the corresponding flat membranes, offering two main benefits: i.e. a higher clean water permeability (increased by 70%) and an increased critical flux (increased by 30%) for activated sludge filtration. Applying vibration increased the critical fluxes about 30–50%. In aeration mode, a filtration time of 8min and a relaxation time of 1min are found to keep both reversible and residual fouling minimal. Despite these advantages, the increase of the critical flux and the filtration performance (i.e. fouling resistance) are not completely in proportion with the increase of the surface area. Furthermore, a lower cleaning efficiency for the ribbed membrane was found. These results indicate that the ribs on the membrane of the current system are still not optimized to additionally work to promote local shear near the membrane surface and as turbulence promoter in the vibration mode.

An experimental study on the impact of bioflocculation on activated sludge separation techniques

Membrane bioreactors have become a well-established alternative for conventional activated sludge systems with a filtration instead of a sedimentation step to separate the sludge from the purified water. Both separation processes, however, depend on good bioflocculation for their performance. Small activated sludge constituents are known to be detrimental in activated sludge filtration, while poor settling can often be attributed to improper floc formation. Therefore, the aim of this paper is to further unravel the relationship between bioflocculation and activated sludge separation processes. To this end, synthetic influents with different mono-over-polyvalent (M/P) cationic compositions are used to create activated sludges with different morphologies and bioflocculation states during a 178days experiment. The activated sludges were characterized by microscopic image analysis, relative hydrophobicity, extracellular polymeric substances, and were subjected to sedimentation, dead-end and submerged crossflow filtration tests. The deflocculating effect of a high M/P ratio is affirmed, inducing the release of colloidal material that caused severe fouling. Sludge flocs became empty and did not settle well. In the low M/P reactor, overall a better separation performance was observed but a non-expected filamentous bulking episode caused bad settleability and bad crossflow filterability, while dead-end filterability was good since a porous, well structured fouling layer was formed. Hence, it is concluded that dead-end filtration tests cannot asses sludge filterability in MBRs and that big activated flocs cannot contribute to the cake layer and cannot ensure its porosity in this way. Hydrophobicity was generally higher in the low M/P reactor but this might be attributed to an abundance of filaments during part of the testing period. No conclusive information could be extracted regarding the role of EPS, thus the use of EPS as a fouling or bioflocculation indicator is (again) questioned.

Surface modification of PVDF membranes by sputtered TiO2: fouling reduction potential in membrane bioreactors

This study investigates activated sludge filterability of a commercial poly(vinylidene fluoride) microfiltration membrane with and without titanium dioxide (TiO2) coating. Deposition of a TiO2 thin film on the membrane surface was made by direct current reactive sputtering from a high purity Ti target in Ar/O2 atmosphere. The modified membrane was characterized by X-ray photoelectron spectroscopy analysis, water contact angle measurement, scanning electron microscopy, and activated sludge filtration measurement. The results showed about 52.5% contact angle reduction of the modified membrane. The modified membrane exhibited less flux decline in activated sludge filtration due to the improvement of surface physical properties of the membrane. A comparison of filtration index (I40) between the modified and the neat membranes showed about twofold increase due to the TiO2 coating. The loss of nano-TiO2 particles was revealed by energy dispersion of X-ray analysis after an ultrasonic washing test, and this influenced stability of the modified membrane.

Impact of suspended solids concentration on sludge filterability in full-scale membrane bioreactors

The relation between activated sludge filterability and mixed liquor suspended solids (MLSS) concentration in membrane bioreactors (MBRs) is framed in a single hypothesis, explaining results seemingly contradictory. A total of 44 activated sludge samples were collected and analyzed on a variety of parameters, i.e. filterability, MLSS concentration, soluble microbial products (SMP) concentrations and particle size distribution in the range of 2–100μm and of 0.4–5.0μm. The sludge filterability was assessed by using the Delft Filtration Characterization method (DFCm). In order to investigate the impact of MLSS concentration, identical samples were diluted with permeate. Results showed that dilution of the samples led to an increased activated sludge filterability, but only when the starting MLSS concentration was below the apparent critical value of 10.5g/L. As opposed, the filterability of sludge with MLSS concentrations above 10.5g/L, and which was characterized by a moderate to good filtration quality, i.e. ΔR20<1 [×1012m−1], worsened when diluted. The specific resistance times the particle concentration of a cake layer obtained when filtrating sludge of moderate to good filterability and MLSS concentration above the apparent critical value was 5.5 times smaller compared to the cake layer of sludge with MLSS concentration below the critical value. Results from SMP assessment and particle counting in the range 2–100µm showed that reduction in sludge mass and de-flocculation occurred, upon dilution of all samples. However, when diluting sludge samples with MLSS concentrations exceeding 10.5g/L and which were characterized by a moderate to good filtration characteristics, there was also release of particles below 0.4µm, opposite to dilutions of samples with MLSS concentrations below 10.5g/L. We postulate that sludge, which is characterized by a moderate to good filterability, having an MLSS concentration above the apparent critical value of 10.5g/L, is likely to retain particles smaller than 0.4μm in its mass, as opposed to sludge with MLSS concentration below the apparent critical value. Our work indicates that there are optimal MLSS concentration ranges in MBR technology, to promote good filterable sludge quality in order to avoid fouling.

Impact of sludge cation distribution pattern on its filterability in membrane bioreactor

The distributions of cations of various valences (Na+, Ca2+ and Fe3+) in the outer layers of extracellular polymeric substances (EPSs) to pellet have a significant impact on the stratification structure of polysaccharides (PS) or proteins (PN) in activated sludge. Comparison with the control showed that the monovalent Na+ reduced flocculability slightly (about 9.75%), whereas Ca2+ and Fe3+ increased flocculability significantly. The modified fouling index (MFI) had a significant correlation with PN in the supernatant (rp=0.8593), slime (rp=0.7218) and loosely bound EPS (LB, rp=0.8012). However, it had a moderate correlation with PS in supernatant (rp=0.5842), and weak correlation to slime (rp=0.3785) or LB (rp=0.3219). There was an ignored correlation with PN or PS in the tightly bound EPS (TB) or pellet. The lower amount of PN or PS in the supernatant would have positive impact on improving the activated sludge filterability.

Mechanisms of action of particles used for fouling mitigation in membrane bioreactors

Adding chemicals to the biofluid is an option to mitigate membrane fouling in membrane bioreactors. In particular, previous studies have shown that the addition of particles could enhance activated sludge filterability. Nevertheless, the mechanisms responsible for the improved filtration performance when particles are added are still unclear. Two main mechanisms might occur: soluble organic matter adsorption onto the particles and/or cake structure modification. To date, no studies have clearly dissociated the impact of these two phenomena as a method was needed for the in-line characterization of the cake structure during filtration. The objective of this study was thus to apply, for the first time, an optical method for in-situ, non-invasive, characterization of cake structure during filtration of a real biofluid in presence of particles. This method was firstly used to study local cake compressibility during the biofluid filtration. It was found that the first layers of the cake were incompressible whereas the cake appeared to be compressible at global scale. This questions the global scale analysis generally used to study cake compressibility and highlights the interest of coupling local characterization with overall process performance analysis. Secondly, the impact of adding submicronic melamine particles into the biofluid was studied. It appears that particles added into the biofluid strongly influence the cake properties, making it thicker and more permeable. Furthermore, by using liquid chromatography with an organic carbon detector to determine the detailed characteristics of the feed and permeate, it was shown that the modification of cake structure also affected the retention of soluble organic compounds by the membrane and thus the cake composition. Simultaneous use of a method for in-situ characterization of the cake structure with a detailed analysis of the fluid composition and monitoring of the global performance is thus a powerful method for evaluating cake structure and composition and their impact on global process performance. The use of this methodology should allow “cake engineering” to be developed so that cake properties (structure, composition) can be controlled and process performance optimized.