Visualisation and characterisation of biopolymer clusters in a submerged membrane bioreactor

A laboratory wastewater treatment membrane bioreactor (MBR) with a submerged hollow-fibre membrane was used to investigate the major foulants in sludge mixtures. Confocal laser scanning microscopy (CLSM) with a triple fluorescent staining protocol, i.e., SYTO9 for microbial cells, ConA-TRITC lectin for polysaccharides and NanoOrange for proteins, was utilised to visualise the fouling materials. A pool of biopolymer clusters (BPCs) ranging from 2.5 to 60 μm in size was identified in the liquid phase of the MBR sludge and in the cake sludge on the membrane surface. According to the CLSM examination, BPC are free and independent organic solutes that are different from biomass flocs and extracellular polymeric substances (EPS) and much larger than soluble microbial products (SMP). Compared to EPS, BPC contain more polysaccharides and proteins and less humic substances. It is believed that BPC are an important foulant that interacts with biomass flocs to form the sludge fouling layer on the membrane. A filtration test observed with the CLSM shows that BPC are apparently formed by the adsorption and affinity clustering of SMP within the sludge deposited on the membrane surface. The cake sludge on the fouled membrane has a much higher BPC content (16.8 mg TOC/g SS) than the MBR bulk sludge (0.4 mg TOC/g SS). It is argued that BPC behave as a glue to facilitate the growth of an impermeable sludge cake on the membrane surface, thus resulting in serious MBR fouling. These CLSM findings provide the first direct evidence of the presence of BPC in MBR and illustrate their essential role in membrane fouling.     

Mitigated membrane fouling in a vertical submerged membrane bioreactor (VSMBR)

In a laboratory-scale study, characteristics of membrane fouling in an A/O (anoxic/oxic) series membrane bioreactor (MBR) and in a vertical submerged membrane bioreactor (VSMBR) treating synthetic wastewater were compared under the same operating conditions. Accordingly, fouling characteristics of a pilot-scale VSMBR treating municipal wastewater were studied under various operating conditions. Various physical, chemical, and biological factors were used to describe membrane resistances. As a result, it was concluded that high concentrations of extracellular polymeric substances (EPS), high viscosity and a high sludge volume index (SVI) corresponded to high membrane resistance indicating severe membrane fouling in both the laboratory-scale MBRs and the pilot-scale VSMBR. In addition, high fouling potential was observed in the pilot-scale VSMBR at 60-day sludge retention time (SRT). In this case, as hydraulic retention time (HRT) decreased from 10 to 4 h, EPS concentrations increased and the average particle size increased, leading to reduced settling of the sludge and increased membrane fouling. To mitigate fouling, two different methods using air bubble jets were adopted in the pilot-scale VSMBR. As a result, it was found that air backwashing was more efficient for fouling mitigation than was air scouring.     

The performance and degradation mechanism of sulfamethazine from wastewater using IFAS-MBR

Sulfamethazine(SMZ) is an important sulfonamide antibiotic.Although the concentration in the environment is small,it is harmful.The drug residues can be transferred,transformed or accumulated,affecting the growth of animals and plants.In this study,the integrated fixed-film activated sludge membrane bioreactor(IFAS-MB R) were constructed to investigate the performance and degradation mechanism of SMZ.The addition of SMZ had a significant impact on the removal of the chemical oxygen demand(COD) and ammonia nitrogen(NH_4^+-N).The optimal operating conditions were hydraulic retention time(HRT) at 10 h and solid retention time(SRT) at 80 d,respectively.On this basis,the effects of different SMZ concentrations on nutrient removal,degradation,and sludge characteristics were compared.The removal efficiency of SMZ increased with the increase of SMZ concentration.The maximum removal rate was as high as 87%.The SMZ dosage also had an obvious effect on sludge characteristics.As the SMZ concentration increased,the extracellular polymer substances(EPS) concentration and the membrane resistance both decreased,which were beneficial for the reduction of membrane fouling.Finally,seven kinds of SMZ biodegradation intermediates were identified,and the possible degradation pathways were speculated.The microbial community results showed that the microbial diversity and richness in the reactor decreased after adding SMZ to the influent.The relative abundance of Bacteroidetes,Actinobacteria,Saccharibacteria and Nitro spirae increased at the phylum level.Sphingobacteria and Betaproteobacteria became dominant species at the class level.The relative abundance of norankp-Saccharibacteria and Nitrospirae increased significantly,and norank-p-Saccharibacteria may be the dominant bacteria for SMZ degradation.     

Membrane fouling in a submerged membrane bioreactor (MBR) under sub-critical flux operation: Membrane foulant and gel layer characterization

Membrane foulants and gel layer formed on membrane surfaces were systematically characterized in a submerged membrane bioreactor (MBR) under sub-critical flux operation. The evaluation of mean oxidation state (MOS) of organic carbons and Fourier transform infrared (FT-IR) spectroscopy demonstrated that membrane foulants in gel layer were comprised of not only extracellular polymeric substances (EPS) (proteins, polysaccharides, etc.) but also other kinds of organic substances. It was also found that fine particles in mixed liquor had a strong deposit tendency on the membrane surfaces, and membrane foulants had much smaller size than mixed liquor in the MBR by particle size distribution (PSD) analysis. Gel filtration chromatography (GFC) analysis showed that membrane foulants and soluble microbial products (SMP) had much broader distributions of molecular weight (MW) and a larger weight-average molecular weight (M_w) compared with the influent wastewater and the membrane effluent. Scanning electron microscopy (SEM) and energy-diffusive X-ray (EDX) analysis indicated that membrane surfaces were covered with compact gel layer which was formed by organic substances and inorganic elements such as Mg, Al, Fe, Ca, Si, etc. The organic foulants coupled the inorganic precipitation enhanced the formation of gel layer and thus caused membrane fouling in the MBR.     

Membrane adsorption bioreactor (MABR) for treating slightly polluted surface water supplies: As compared to membrane bioreactor (MBR)

In this paper, a submerged membrane adsorption bioreactor (MABR) was evaluated for drinking water treatment at a hydraulic retention time (HRT) as short as 0.5 h. As powdered activated carbon (PAC) was added to the bioreactor at 8 mg/L raw water, the MABR achieved much higher removal efficiency for organic matter in the raw water than the parallel-operated membrane bioreactor (MBR). Moreover, the trans-membrane pressure (TMP) of MABR developed much lower than that of MBR, demonstrating PAC in MABR could mitigate membrane fouling. It was also identified here that the removal of dissolved organic matter (DOM) in MABR was accomplished through the combination of three unit effects: rejection by ultrafiltration (UF) membrane, biodegradation by microorganism, and adsorption by PAC; the last was of great importance. A sludge layer was observed on the membranes surface in both MABR and MBR and PAC particles themselves constituted a part of the cake layer and helped to intercept DOM in the mixed liquor by adsorption in MABR, especially for organic molecules of 5000–500 Da. The UF membrane together with the sludge layer and PAC layer in the MABR was able to reject hydrophobic bases (HoBs), hydrophobic neutrals, hydrophobic acids (HoAs), weakly hydrophobic acids (WHoAs) and hydrophilic matter (HiM) in the mixed liquor by 40.0%, 43.9%, 71.8%, 56.6% and 35.9%, respectively.     

Characteristics of aerobic biogranules from membrane bioreactor system

As a novel membrane bioreactor (MBR) system, membrane granular sludge bioreactor (called MGSBR) had not only good performance of pollutant degradation in synthetic wastewater, but also alleviated membrane fouling. The study on the stability of aerobic biogranules followed the investigation of MGSBR performance will pay more attraction and potential to this technology during the wastewater treatment in the future. Although the granules had smaller average diameter, poorer settleability and of a sort sludge activity with 2.0 mm of the average diameter, 70 ± 10 mL gSS⁻¹ of SVI and 0.83 of VSS/SS ratio, respectively, the results presented in this paper demonstrated that some characteristics of aerobic biogranules in MGSBR system were ultimately preserved. Changes in characteristics of aerobic biogranules were more or less associated with the overgrowth of filamentous bacteria (showed by SEM observation), which was a phenomenon occurred under the combined effects of continuous aeration mode, reduced DO concentration, long SRT and high EPS concentration in MGSBR. Much research should be performed in the future to control the overgrowth of filamentous bacteria in MGSBR system.     

Lab-scale testing of a low-loaded activated sludge process with membrane filtration

Two membrane bioreactors (MBRs; volume = 300 L) equipped with different types of immersed membrane modules were operated simultaneously under the same laboratory conditions as a low-loaded activated sludge process without any membrane regeneration and excess sludge uptake (sludge retention time SRT up to 170 d; activated sludge concentration MLSS up to 11 g L⁻¹). The aim was to verify the quality of treated water and to study the properties of "very old" activated sludge. Another aim was to compare different selected membrane types and choose the best one for further pilot-scale testing. Presented at the 35th International Conference of the Slovak Society of Chemical Engineering, Tatranské Matliare, 26–30 May 2008.     

Modelling of submerged membrane bioreactor: Conceptual study about link between activated slugde biokinetics,aeration and fouling process

A mathematical model was developed to simulate filtration process and aeration influence on submerged membrane bioreactor (SMBR) in aerobic conditions. The biological kinetics and the dynamic effect of the sludge attachment and detachment from the membrane, in relation to the filtration and a strong intermittent aeration, were included in the model. The model was established considering soluble microbial products (SMP) formation-degradation. The fouling components responsible of pore clogging, sludge cake growth, and temporal sludge film coverage were considered during calculation of the total membrane fouling resistance. The influence of SMP, transmembrane pressure, and mixed liquor suspended solids on specific filtration resistance of the sludge cake was also included. With this model, the membrane fouling under different SMBR operational conditions can be simulated. The influence of a larger number of very important process variables on fouling development can be well quantified. The model was developed for evaluating the influence on fouling control of an intermittent aeration of bubbles synchronized or not with the filtration cycles, taking into account the effects of shear intensity on sludge cake removal.     

Use of submerged anaerobic–anoxic–oxic membrane bioreactor to treat highly toxic coke wastewater with complete sludge retention

Coke wastewater is an extremely toxic industrial effluent that requires treatment before discharge. A bench-scale, anaerobic–anoxic–oxic membrane bioreactor (A₁/A₂/O-MBR) system was utilized to treat real coke wastewater with complete sludge retention. In a 160-d test, the A₁/A₂/O-MBR system stably removed 87.9 ± 1.6% of chemical oxygen demand, 99.4 ± 0.3% of turbidity, and 99.7 ± 3.5% of NH₄⁺-N from coke wastewater. The membrane rejected almost all suspended solids; hence, a low food-to-microorganism environment was created to degrade refractory substances and reduce sludge production rates. The microbial diversity in the MBR system declined over time; however, neither pollutant removal efficiency nor total biological activity was adversely affected. Membrane fouling, which occurred during the operation of the MBR system, was principally resulted from the colloidal fraction of supernatant in suspension. Physical cleaning removed initial deposits of particles; however, prolonged operation resulted in severe clogging that can only be removed by chemical cleaning. An A₁/A₂/O-MBR system with short intermittent physical cleaning was recommended for coke wastewater treatment.     

Influence of Extracellular Polymeric Substances on Membrane Fouling and Cleaning in a Submerged Membrane Bioreactor

There is little information available on the correlation between the concentration of extracellular polymeric substances (EPS) and membrane fouling as well as cleaning efficiency. In this study, two lab-scale flat submerged membrane bioreactors (SMBRs) at sludge retention times (SRTs) of 25 and 250 days were operated at a constant permeate flux (12.5 l m⁻² h⁻¹). Samples of activated sludge were tested to quantify the concentration of extractable EPS using cation exchange resin. Batch filtration tests were also performed to determine the specific cake resistances and the flux recoveries. The extractable EPS and protein concentrations were relatively low at the prolonged SRT, leading to cake layers easily removable by the physical manual cleaning or the de-ionized water backwashing and the chemical cleaning with sodium hypochlorite methods. The extent of flux recoveries (both in SMBRs and batch filtration tests) and macroscopic as well as microscopic images indicated that the chemical cleaning could enhance the effectiveness of cleaning. The membrane fouling and cleaning mechanisms were also discussed.__________From Kolloidnyi Zhurnal, Vol. 67, No. 3, 2005, pp. 392–397.Original English Text Copyright © 2005 by Chackrit Nuengjamnong, Ji Hyang Kweon, Jinwoo Cho, Kyu-Hong Ahn, Chongrak Polprasert.This article was submitted by the authors in English.     

Ammonium ion sensor based on immobilized nitrifying bacteria and a cation-exchange membrane

The ammonium ion sensor is based on nitrifying bacteria isolated from activated sludge. The sensor comprises a cation-exchange membrane, an alkaline solution layer (pH 10), a gas-permeable membrane, an immobilized microbial membrane, and an oxygen electrode. This novel combination provides accurate amperometric determinations of ammonium ions in aqueous solutions within 7 min in the range 10^-4– 4.5 × 10^-2 M. Volatile amines or other ions do not interfere. The relative error is within 4% and the sensor can be used continually for more than 10 days.     

Significance of transparent exopolymer particles derived from aquatic algae in membrane fouling

In recent years, Transparent exopolymer particles(TEPs) have been identified as significant contributors to membrane surface biofouling. Reported research on the effect of TEPs on membrane fouling has mainly focused on algae-derived TEPs in the ocean, and very limited investigations have been conducted on those in freshwater systems. In this study, we investigated the characteristics of TEPs derived from Microcystis aeruginosa and their influence on membrane fouling in an ultrafiltration (UF) system. The results indicated that bound TEPs could lead to more serious membrane fouling while free TEPs caused more serious irreversible membrane fouling. Further studies showed that in free TEP solutions, small-sized colloidal TEPs (c-TEPs) rather than large-sized particle TEPs (p-TEPs) showed a significantly positive correlation with irreversible membrane fouling. The presence of Ca²⁺ ions in influent water can reduce membrane fouling to some extent since a low concentration of Ca²⁺ ions (1 mM) can lead to the transformation of most free TEPs from the colloidal to particulate state. Both acidic and alkaline environments of free TEP solutions result in more serious membrane fouling compared to a neutral environment of free TEP solution. The negative impact of the acidic environment on membrane fouling was more significant than that of the alkaline environment. The abovementioned results show that when using a UF system to filter water with high algal content, greater attention should be paid to free TEPs, especially those in the colloidal state, because they can cause serious, irreversible membrane fouling.     

Formation of porous poly(vinylidene fluoride) membranes with symmetric or asymmetric morphology by immersion precipitation in the water/TEP/PVDF system

The phase equilibrium boundaries of the membrane forming system, water/triethyl phosphate (TEP)/PVDF, at 25 °C were determined experimentally using cloud-point and equilibrium absorption methods. Based on the phase diagram, appropriate dope and bath compositions were selected to prepare microporous membranes by means of the isothermal immersion-precipitation technique. As a metastable casting dope with respect to crystallization was adopted, the formed membranes exhibited a uniform cross-section composed of interlocked crystal elements coexisting with the network of continuous pores, as was revealed by high resolution FESEM imaging. Morphologies of the membranes’ top surfaces were found to depend heavily on the bath strength, which was controlled by the TEP content. By changing the bath gradually from pure water to 70% TEP, the top surface evolved from a dense skin (asymmetric membrane) to a totally porous morphology (symmetric membrane). Wide angle X-ray diffraction analysis indicated that PVDF crystallized into α-type structure for all of the synthesized membranes. The crystallinity as determined from diffraction peak deconvolution was ≈65%, which value was confirmed by Differential Scanning Calorimetry (DSC). The obtained thermograms also showed a similar melting peak temperature (T_m ≈ 169 °C) for all membranes. Furthermore, water fluxes and tensile strengths of the membranes were measured. The results were found to correlate with the morphologies of the membranes.     

Chlorine-induced permeability recovery for low-pressure membrane filtration of natural waters

Chlorine treatment is widely used by membrane filtration plants to recover the loss of membrane permeability encountered in low-pressure membrane (LPM) filtration of natural waters. However, there are few methodical studies in the literature addressing the efficacy of chlorine in cleaning membranes. Thus, the purpose of this study was to assess chlorine-induced permeability recovery (CIPR) of LPMs using the Ct concept (product of chlorine dose concentration and treatment time) commonly employed in the disinfection literature. The experimental work was conducted by evaluating the efficacy of CIPR for a membrane and water combination under variable Ct exposures and determining the presence of minimum effective Ct exposure and proper empirical models for the CIPR. The results showed that the efficacy of CIPR depended on both C and t. A minimum Ct exposure of approximately 2 × 10⁵ (min mg)/L was required for effective CIPR, and the relationship between the residual fouling and chlorine exposures was best fitted using a revised Chick–Watson model. These results may be explained by a conceptual model that considers CIPR as a sequential process of oxidation of organic foulants and diffusional detachment of the reaction products from membrane surfaces. Additional work is needed to validate the applicability of the model to other waters and membranes.     

Improving the Performance of Membrane Bioreactors by Adding a Metabolic Uncoupler, 4-Nitrophenol

Effects of 4-nitrophenol (4NP) on the overall performance of membrane bioreactors (MBRs) were investigated in two bench-scale submerged MBRs. Positive impacts of 4NP on activated sludge production and membrane fouling were demonstrated over 45 days of stable operational period. After addition of 4-nitrophenol, the sludge production could be reduced effectively, but only a slight reduction in chemical oxygen demand removal was obtained. The effluent NH₄ ⁺-N concentrations were almost the same in two MBRs. The transmembrane pressures (TMPs) and resistance R increased with increasing mixed liquor suspended solid concentration at each MBR. The average daily TMP increase rates in the control MBR reactor remained at about 0.23 kPa day^?1 and dropped to about 0.12 kPa day^?1 in the 4NP-MBR. Compared with the control MBR, a wider dispersion and lower peak of floc size, a lower zeta potential, and a lower extracellular polymeric substance concentration were observed in the 4NP-MBR.     

Comparison between filtrations at fixed transmembrane pressure and fixed permeate flux: application to a membrane bioreactor used for wastewater treatment

Membrane bioreactors for wastewater treatment must operate for long periods without chemical cleaning. This paper investigates the critical flux concept introduced by Field et al. as a means for achieving this goal. Experiments were conducted on a membrane bioreactor containing 600 l of activated sludge, equipped with a 0.25 m² ceramic membrane and located in Compiegne wastewater treatment plant. Hydraulic retention time was set at 24 h and sludge retention time at 60 days, so that suspended solids concentration stabilises at 10 g/l. We conducted two series of tests: at fixed transmembrane pressure (TMP) and at fixed permeate flux, set by a volumetric pump on the permeate. In both cases, velocity was varied from 1 to 5 m/s. In fixed flux tests, the flux was increased by 10 l/h m² increments and the TMP was observed to rise moderately first and then stabilise in about 15 min until a critical value of the flux is reached. Above this critical flux, the TMP rises rapidly and does not stabilise, as in dead-end filtration. The critical flux was found to increase approximately linearly with velocity, reaching about 115 l/h m² at 4 m/s. These data were reproducible at various dates between 30 and 120 days of continuous operation of the bioreactor and permit to know at which flux a membrane bioreactor must be operated. Comparison of constant pressure and constant flux tests under same conditions showed that the critical flux is almost identical to the limiting or pressure independent flux obtained in constant pressure. More generally, constant flux procedure below the critical flux avoids overfouling of the membrane in the initial stage and is more advantageous for membrane bioreactor operation.     

Crosslinking and stabilization of nanoparticle filled PMP nanocomposite membranes for gas separations

Poly(4-methyl-2-pentyne) (PMP) has been crosslinked using 4,4′-(hexafluoroisopropylidene) diphenyl azide (HFBAA) to improve its chemical and physical stability over time. Crosslinking PMP renders it insoluble in good solvents for the uncrosslinked polymer. Gas permeability and fractional free volume (FFV) decreased as crosslinker content increased, while gas sorption was unaffected by crosslinking. Therefore, the reduction in permeability upon crosslinking PMP was due to decrease in diffusion coefficient. Compared to the pure PMP membrane, the permeability of the crosslinked membrane is initially reduced for all gases tested due to the crosslinking. By adding nanoparticles (FS, TiO₂), the permeability is again increased; permeability reductions due to crosslinking could be offset by adding nanoparticles to the membranes. Increased selectivity is documented for the gas pairs O₂/N₂, H₂/N₂, CO₂/N₂, CO₂/CH₄ and H₂/CH₄ using crosslinking and addition of nanoparticles. Crosslinking is successful in maintaining the permeability and selectivity of PMP membranes and PMP/filler nanocomposites over time.     

Oxygen permeation of BSCF membrane with varying thickness and surface coating

The oxygen permeation of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) membranes was measured between 750 and 900 °C as a function of membrane thickness with or without La_0.7Sr_0.3CoO₃ (LSC) coating layer under controlled PO₂-gradient$P_{{\text{O}}_2}\text{-gradient}$ (Air/He). In order to see the relative effects of bulk diffusion and surface-exchange kinetics, the thickness of membrane was varied from 0.5 to 2.0 mm. The oxygen-permeation flux at 900 °C increased with LSC coating from that of uncoated membrane. For example, it increased ∼1.8 times for 1 mm-thick BSCF membrane. The characteristic membrane thickness (L_C) which divides the bulk-diffusion limit and surface-exchange kinetics limit was estimated using the modified Wagner equation. The L_C values were 0.55 and 1.10 mm at 900 °C for the coated and uncoated BSCF membranes, respectively, and decreased with decreasing temperature.     

Ionic salt permeabilities and membrane potentials of asymmetric polypeptide membrane

Asymmetric membrane potentials and transport properties of polypeptide membrane consisting of two layers with poly(L-glutamic acid) and poly (γ-methylL-glutamate) were studied in the pH range of 2 to 6 at 25 °C. Under the condition adopted, the poly (L-glutamic acid) layer of the membrane underwentα helix to coil transition which was confirmed by ATR-IR measurements. The membrane potentials of the asymmetric membrane between two identical solutions of KCl,Δψ _asym, which is called asymmetric potential, were observed.Δψ _asym values were effected by the structural transition of poly (L-glutamic acid) layer. As a result, the effective charge density of the membrane, which was derived by the asymmetric membrane potential measurements, had a maximum at the transition region of pH=4.8. On the other hand, the permeation coefficient of KCl,P _i, was higher in the direction from poly (L-glutamic acid) side to poly (γ-methylL-glutamate) side, “G →M direction”, than in the oppositeM →G direction. Furthermore,P _i inG →M direction was dependent on the solution pH, that is,P _i decreased when pH was increased to 4.8 and increased on further increasing of pH These membrane behaviors were described in terms of the competition between structural transition and variation of fixed charge density owing to the helix to coil transition of the asymmetric polypeptide membrane.     

Reversibility of fouling formed in activated sludge filtration

This paper investigates the reversibility of membrane fouling by activated sludge in a membrane bioreactor equipped with a 0.1 μm pore ceramic membrane. The membrane was submitted to a series of tests in which the permeate flux, the transmembrane pressure (TMP) or the circulation velocity were successively varied in cycles by step increments or decreases. When the permeate flux is set below the critical flux, the TMP remains stable and fouling is reversible. On the contrary, when the critical flux is exceeded, the TMP increases and does not stabilize, as in dead-end filtration. The fouling formed is partly irreversible when the flux is lowered again. When the TMP is first increased up to 400 kPa and then decreased back at constant velocity, no hysteresis is found on the flux–TMP graph, showing that fouling is reversible in this case. Velocity cycles were performed by first lowering the velocity from 5 to 1 m/s and raising it again to 5 m/s. In this case again, the fouling induced by reducing the velocity was found to be reversible. However, when the same pressure and velocity cycle tests were performed with activated sludge collected in the aeration tank of a classical wastewater treatment plant, fouling was found to be partly irreversible, showing that the cake formed in the absence of shearing is much more cohesive. In the final part of the paper, we tested a hydrodynamic method of fouling control consisting in alternating short periods of filtration (1–4 s) and short periods of washing (1 or 2 s) at low TMP and high velocity. This method yielded to a 20% permeate flux increase with a 10% reduction in hydraulic energy consumption for classical plant activated sludge.