Comparison of membrane biofouling in nitrification and denitrification for the membrane bioreactor (MBR).

Batch filtration tests were conducted to compare the characteristics of membrane biofouling with regard to nitrification and denitrification. A Modified Fouling Index (MFI) was obtained using a stirred cell tester. The denitrification assays showed higher membrane fouling rates than the nitrification assays. The fouling became worse, not only due to pore blocking resistance, but also from cake layer resistance after denitrification. The Extracellular Polymeric Substances (EPS) concentration and relative hydrophobicity were decreased after denitrification, resulting in floc deterioration. The floc deterioration was assumed to have increased the cake layer resistance in the filtration test. The protein Soluble Microbial Products (SMP) concentration, portion of high molecular weight in carbohydrate SMP and relative hydrophobicity were increased after denitrification, which was assumed to cause membrane pore blocking. The changes in the EPS and SMP characteristics were the main fouling parameters in denitrification.     

Impact of sludge retention time on sludge characteristics and microbial community in MBR

In this study, the impact of sludge retention time (SRT) on sludge characteristics and microbial community and the effect on membrane fouling in membrane bioreactor (MBR) was investigated. The results show that MBR with longer SRT has less fouling propensity, in agreement with other studies, despite the fact that the MBR with longer SRT contained higher MLSS and smaller particle size. However, much more soluble microbial products (SMPs) were released in MBR with shorter SRT. More slime on the membrane surface was observed in MBR with shorter SRT while sludge cakes formed on the membrane surface in MBR with longer SRT. The results show that SMP contributes to the severe fouling observed in MBR with shorter SRT, which is in agreement with other studies showing that SMPs were the major foulants in MBR. Under different SRTs of operation, the bacterial community structures of the sludge obtained by use of polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) were almost identical, but those on the membrane surface differed substantially. It suggests that, although SRT has impact on sludge characteristics, it doesn't affect the microbial community in the suspension.     

Identification of the effect of extracellular polymeric substances on bacterial adhesion to the membrane surface in a membrane bioreactor using Pseudomonas fluorescens.

In a membrane bioreactor (MBR) process containing a variety of bacteria, the bacterial adhesion to the membrane surface, prior to cake formation, causes an increased filtration resistance. In this study, Pseudomonas fluorescens, commonly found in the municipal wastewater treatment process with activated sludge, was used to show the effects of extracellular polymeric substances (EPS) on bacterial adhesion to the membrane surface in the MBR. Of the various roles of EPS in promoting membrane fouling, the adhesion of bacteria to the membrane surface was calculated using the specific cake resistance (alpha, m/kg). Although the amount of EPS binding with bacteria was increased by the addition of Ca2+, there was no significant effect on the bacterial growth. The results of the particle size distribution showed that the addition of Ca2+ increased flocculation, allowing the formation of a complex with the bacteria and EPS. In order to identify the effects of the addition of Ca2+ on the hydrophobicity, the contact angle was also measured. The result showed that the addition of Ca2+ showed no significant differences in the hydrophobicity, even though there was an increase in flocculation. With the bacteria containing a higher EPS concentration, a higher specific cake resistance was observed. From the results of the adhesion experiment, which was conducted with various EPS levels, displayed as the COD and TOC concentration, an increased EPS concentration was shown to promote bacterial adhesion to the membrane surface.     

Correlation between membrane fouling and soluble/colloidal organic substances in membrane bioreactors for municipal wastewater treatment.

Two similar membrane bioreactors of 2 m3 each were operated in parallel over two years under the same operational conditions, fed with the same municipal wastewater. The only process and operational difference between both pilot plants was the position of the denitrification zone (pre-denitrification in pilot 1 and post-denitrification in pilot 2). Despite parallel operation, the two MBRs exhibited different fouling rates and decreases in permeability. These differences could not be accounted for by MLSS concentrations, loading rates, or filtration flux. In a one-year investigation, soluble and colloidal organic material in the activated sludge of both MBR was regularly analysed by spectrophotometric and Size Exclusion Chromatography (SEC) methods. The larger organic molecules present in the sludge water phase (i.e. polysaccharides, proteins and organic colloids) originating from microbial activity (extracellular polymeric substances) were found to impact on the fouling and to explain the difference in membrane performance between the two MBR units. In both pilot plants, a linear relationship could be clearly demonstrated between the fouling rate of the membrane and the concentration of polysaccharides in the sludge water phase during a 5 month operational period at an SRT of 8 days.     

Characteristics of membrane fouling in submerged membrane bioreactor under sub-critical flux operation.

Recently, the membrane bioreactor (MBR) process has become one of the novel technologies to enhance the performance of biological treatment of wastewater. Membrane bioreactor process uses the membrane unit to replace a sediment tank, and this can greatly enhance treatment performance. However, membrane fouling in MBR restricts its widespread application because it leads to permeate flux decline, making more frequent membrane cleaning and replacement necessary, which then increases operating and maintenance costs. This study investigated the sludge characteristics in membrane fouling under sub-critical flux operation and also assessed the effect of shear stress on membrane fouling. Membrane fouling was slow under sub-critical flux operation. However, as filamentous microbes became dominant in the reactor, membrane fouling increased dramatically due to the increased viscosity and polysaccharides. A close link was found between membrane fouling and the amount of polysaccharides in soluble EPS. The predominant resistance was the cake resistance which could be minimized by increasing the shear stress. However, the resistance of colloids and solutes was not apparently reduced by increasing shear stress. Therefore, smaller particles such as macromolecules (e.g. polysaccharides) may play an important role in membrane fouling under sub-critical flux operation.     

The role of carbohydrate and protein parts of extracellular polymeric substances on the dewaterability of biological sludges.

There are many factors affecting the biological sludge dewaterability such as particle size distribution, floc structure, extracellular polymeric substances (EPS), etc. In this research, the role of the protein and carbohydrate parts of EPS (EPS(carbohydrate), and EPS(protein)) on the dewaterability of biological sludges was investigated. The sludge EPS composition was altered by feeding the sludges of same origin, in different reactors, with synthetic media having carbon to nitrogen (C/N) ratios of 8, 19 and 30 (in terms of COD/NH3-N), respectively. EPS in sludge samples were extracted by a cation exchange resin (CER). The characteristics of EPS were investigated by analytical methods and by using FT-IR spectroscopy. The dewaterability of the sludges was determined in terms of filterability and compactibility. Filterability, as filterability constant (X), and compactibility, as cake solids concentration, of sludges were determined by using the capillary suction time (CST) test and the centrifugation, respectively. The floc structure of sludge samples was also observed microscopically. Filterability and compactibility of the sludge samples were improved considerably with the increasing carbohydrate part and the decreasing protein part of the sludge EPS. EPS(protein) was inversely related to the cake solids concentration, which might be explained by the water holding capacity of EPS(protein). Filterability and compactibility of sludges improved by the increase of the size and strength of the flocs.     

Evaluation of the importance of various operating and sludge property parameters to the fouling of membrane bioreactors

A single-fibre microfiltration system was employed to investigate the importance of various operating and sludge property parameters to the membrane fouling during sludge filtration. The sludge was obtained from a submerged membrane bioreactor (SMBR). A series of comparative and correlative filtration and fouling tests were conducted on the influence of the operating variables, sludge properties and the liquid-phase organic substances on the membrane fouling development. The test results were analysed statistically with Pearson's correlation coefficients and the stepwise multivariable linear regression. According to the statistical evaluation, the membrane fouling rate has a positive correlation with the biopolymer cluster (BPC) concentration, sludge concentration (mixed liquor suspended solids, MLSS), filtration flux and viscosity, a negative correlation with the cross-flow velocity, and a weak correlation with the extracellular polymeric substances and soluble microbial products. BPC appear to be the most important factor to membrane fouling development during the sludge filtration, followed by the filtration flux and MLSS concentration. The cross-flow rate also is important to the fouling control. It is argued that, during membrane filtration of SMBR sludge, BPC interact with sludge flocs at the membrane surface to facilitate the deposition of the sludge cake layer, leading to serious membrane fouling.     

Characterization of sludge structure and activity in submerged membrane bioreactor.

Sludge characteristics of a submerged membrane bioreactor (MBR) and an activated sludge process (AS) were compared, during a first phase at the same operating conditions (low MLSS and conventional SRT) and in a second phase with a high sludge retention time (SRT) in the membrane bioreactor. During the first phase, a bimodal flocs size distribution was observed in the MBR with simultaneously a macro-flocs population (240 microm) bigger than the flocs of activated sludge due to the absence of recirculating pump, and also more microflocs (1 to 15 microm) and free suspended cells retained by the membrane. It is shown that the membrane leads to an accumulation of proteins and polysaccharides in the sludge supernatant which is probably responsible for the high fouling propensity of the sludge during the starting period of MBR. These compounds are partially degraded after 50 to 60 days of operation. In the first phase respirometric experiments didn't demonstrate a significant difference in the maximal removal rates of either MBR or AS biomass (with excess substrate), except in the dynamic period during which the membrane retention gave an advantage by increasing the biomass activity. On the other hand, the respirometry shows that the half saturation constant for nitrification was significantly higher in the MBR process, suggesting higher substrate transfer limitation. During the last phase, it is shown that an increase of SRT from 9 to 106 days leads to a diminution of average macro-flocs size in the MBR from about 240 to 70 microm. With the SRT increase, modification in the organic compounds is also observed (proteins, polysaccharides and COD) in the sludge supernatant. Increasing the SRT from 9 to 40 days seems to slightly reduce the level of organic compounds (probable biodegradation), but the concentrations increased when SRT changes from 40 days to 106 days (probable accumulation of non biodegradable compounds).     

Performance of membrane-coupled organic acid fermentor for the resources recovery form municipal sewage sludge.

This study focused on the treatment performance of membrane-coupled organic acid fermentor (MOF) with intermittent reciprocal air/ozone backwashing for the keeping of high permeation flux as well as for the effective recovery of dissolved organics from municipal sewage sludge. Intermittent reciprocal air/ozone backwashing was effective for membrane fouling reduction. When MOF was operated under the conditions of pH 5.5, hydraulic retention time (HRT) of 2 days and 20 days of solids retention time (SRT), most favourable fermentation efficiency was attained. Great inhibition for acid producing by intermittent reciprocal air/ozone backwashing was not observed during long-term operation. MOF with intermittent reciprocal air/ozone backwashing is believed to be an effective system for the recovery of organic matter from municipal sewage sludge and membrane fouling reduction.     

Effects of solid retention time on the performance of submerged anoxic/oxic membrane bioreactor.

In this study, four similar bench-scale submerged Anoxic/Oxic Membrane Bioreactors (MBR) were used simultaneously to investigate the effects of solids retention time (SRT) on organic and nitrogen removal in MBR for treating domestic wastewater. COD removal efficiencies in all reactors were consistently above 94% under steady state conditions. Complete conversion of NH(4+)-N to NO(3-)-N was readily achieved over a feed NH(4+)-N concentration range of 30 to 50 mg/L. It was also observed that SRT did not significantly affect the nitrification in the MBR systems investigated. The average denitrification efficiencies for the 3, 5, 10 and 20 days SRT operations were 43.9, 32.6, 47.5 and 66.5%, respectively. In general, the average effluent nitrogen concentrations, which were mainly nitrate, were about 22.2, 27.6, 21.7 and 13.9 mg/L for the 3, 5, 10 and 20 days SRT systems, respectively. The rate of membrane fouling at 3 days SRT operation was more rapid than that observed at 5 days SRT. No fouling was noted in the 10 days and 20 days SRT systems during the entire period of study.     

磁性悬浮颗粒减缓MBR中膜污染的研究

向MBR中投加磁性悬浮颗粒,通过改善污泥混合液特性来控制膜污染,试验结果表明:最佳吸附剂为四氧化三铁/硅藻土复合颗粒,吸附剂的最佳投加量为0.1 g/L;投加复合颗粒的MBR出水的COD、氨氮和硝态氮均优于参比MBR,说明复合颗粒能改善膜出水水质;投加颗粒的MBR膜表面的EPS各组分均低于参比MBR,说明四氧化三铁/硅藻土复合颗粒能有效的吸附污泥悬浮液中的EPS,加强MBR的通量稳定性,从而延缓膜污染,减少膜清洗次数,降低运行成本。     

Modeling of extracellular polymeric substances and soluble microbial products production in a submerged membrane bioreactor at various SRTs.

Extracelluar polymeric substances (EPSs) and soluble microbial products (SMPs) produced by microbial cultures involved in membrane biofouling have been widely investigated. A mathematical model of EPS and SMP formation and degradation was established based on the activated sludge model no. 1 (ASM1) and was applied to the membrane bioreactor sludge with different sludge retention times (SRTs). The unified theory that the distinct products from the EPS and SMP overlapped each other was integrated into the ASM1. Two components, five processes and eight parameters were newly added to set up the model. To increase the accuracy of model simulation, microbial kinetic parameters were determined by respirometry method and applied to the model instead of microbial kinetic constant offered in ASM1. From the respirometry result, both of heterotroph and autotroph showed different yield value, growth rate and decay rate from activated sludge. There was no significant effect of SRT on SMP production and the experimental results showed good agreement with the predicted values by the model simulation. With the developed unified EPS and SMP model, EPS and SMP production could be simulated so well that it can be applied for the membrane biofouling control.     

Occurrence of EPS in activated sludge from a membrane bioreactor treating municipal wastewater.

EPS are supposed to be among the causes of membrane fouling in membrane bioreactors (MBR). In this work they are measured as total proteins and total polysaccharides. Theoretical and empirical considerations of biomass membrane filtration lead to the conclusion that EPS in the water phase is decisive for the filterability of activated sludge. In this study therefore different ways of separating the water phase from the biomass are investigated, where a simple filtration over a paper filter turned out to be sufficient. Subsequently, a simple batch test set up was used to investigate the influence of substrate conditions on the amount of EPS in the water phase. Dilution of the biomass does not result in changes. Dilution together with substrate addition leads to an increase both in proteins and polysaccharides. Replacement of the water phase leads to no significant changes in protein concentration, but polysaccharide concentration may vary considerably. This phenomenon is more pronounced after replacement of the water phase and substrate addition.     

Comparison of the behaviour of selected micropollutants in a membrane bioreactor and a conventional wastewater treatment plant.

Micropollutants as pharmaceutical active compounds (PhACs), residuals of personal care products or endocrine disrupting chemicals are of increasing interest in water pollution control. In this context the removal efficiencies of sewage treatment plants (STPs) are of importance, as their effluents are important point sources for the release of those substances into the aquatic environment. Activated sludge based wastewater treatment is the worldwide prevalently used treatment technique. In conventional plants the separation of treated wastewater and sludge occurs via sedimentation. A new development is the application of membrane technology for this separation step. The studies focus on the influence of the solids retention time (SRT) on the removal efficiency, as the SRT is the most important parameter in the design of STPs. A conventional activated sludge plant (CASP) and a membrane bioreactor (MBR) were operated at different SRTs. The substances selected are the antiepileptic carbamazepine, the analgesics diclofenac and ibuprofen, the lipid regulator bezafibrate, the polycyclic musks tonalide and galaxolide and the contraceptive 17alpha-ethinylestradiole. No significant differences in the removal efficiency were detected. Due to the absence of suspended solids in the MBR effluent, substances with high adsorption potential could be retained to slightly higher amounts.     

A pilot study on accelerated sludge degradation by a high-concentration membrane bioreactor coupled with sludge pretreatment.

A new sludge treatment process combining a high MLSS membrane bioreactor with sludge pretreatment techniques was studied in pilot-scale experiments. The membrane bioreactor (MBR) was adopted for high efficiency aerobic digestion. The combination of alkaline-ozone treatment of the mixed liquor in the MBR reactor accelerated the biodegradation process by enhancing biodegradability of the sludge. The hydraulic retention time (HRT) of the reactor was set as 3.1 days and the DO level was 1 mg/L on average. After 5 months of operation, the accumulative total solids reduction was more than 70%. Removal efficiency of volatile solids and non-volatile solids were 76% and 54%, respectively. It was found that a considerable portion of the non-volatile solids was dissolved into ions and then flushed out with the effluent. Also, about 41% and 28% of T-N and T-P in the raw sludge were removed although no biological nutrient removal process was adopted. The experiment was run smoothly without significant membrane fouling, even at the relatively high levels of MLSS concentration (11,000-25,000 mg/L). It is concluded that the newly proposed process can significantly increase the sludge reduction efficiency with much shorter retention times.     

Impact of dissolved oxygen concentration on membrane filtering resistance and soluble organic matter characteristics in membrane bioreactors

This study investigated the impact of dissolved oxygen (DO) concentration on membrane filtering resistance, soluble organic matter (SOM) and extracellular polymeric substance (EPS) characteristics in a membrane bioreactor (MBR). A laboratory-scale MBR was operated under DO limited (0.2 mg L(-1) DO) and fully aerobic (3.7 and 5.4 mg L(-1) DO) conditions. Membrane filtering resistance was determined for the mixed liquor suspended solids (MLSS) and for resuspended microbial biomass after removing SOM. Regardless of the DO concentration, the cake resistance (Rc) was approximately 95 percent of the total resistance (Rt). The membrane cake resistance was found to decrease significantly after removing the SOM. The total resistance caused by the resuspended biomass was 29 percent of that caused by the MLSS under DO limited conditions, while the total resistance caused by resuspended biomass was 41 to 48 percent of that caused by the MLSS under fully aerobic conditions. Under DO limited conditions, SOM in the MLSS contained a larger amount of high molecular weight compounds, leading to higher cake resistance than under fully aerobic conditions. There was significant variation in the molecular weight fractions of the EPS, with no clear relationship with DO concentration. There was also no distinct relationship between membrane filtering resistance and molecular weight fraction of the EPS.     

The assessment of different operating strategies for minimising activated sludge deflocculation under temperature transient conditions.

Three operating strategies were tested for decreasing activated sludge deflocculation due to temperature shifts from 30 degrees to 45 degrees C: magnesium sludge enrichment, increased sludge retention time (33 d), and spikes of an easily degradable substrate (methanol). The temperature shifts were conducted sequentially in 4 parallel lab-scale sequencing batch reactors (SBRs) treating kraft pulp mill effluent. Three SBRs operated at an SRT = 20 days, and in one of them the sludge was not manipulated, thus, serving as a reference SBR. The temperature shift was associated with decreased soluble chemical oxygen demand (SCOD) removals, decreased sludge settleability and substrate removal capacity, and increased effluent suspended solids (ESS) and turbidity levels. The shift also increased the sludge specific respiration rates and reduced the sludge substrate removal capacity. Sludge deflocculation was assessed as floc solubilisation (increased effluent SCOD levels) and floc fragmentation (increase in effluent solids smaller than 50 microm). Mg enrichment of the sludge and methanol spikes reduced the ESS levels (in 9 and 25%), and the three operating strategies decreased effluent turbidity (in 22-35%) compared to the maximum levels from the non-manipulated reactor (44 mg ESS/L). The stronger sludge floc structure achieved by magnesium enrichment and a high sludge age of 33 days was unsuccessful in significantly decreasing deflocculation. The mechanisms involved in sludge deflocculation require further fundamental research.     

Two-stage biofilm-MBR for nitrogen removal and enhanced membrane performance

A two-stage biofilm-membrane bioreactor (MBR) was developed in this study. High total nitrogen removal (maximum: 81.4% with recycle ratio = 3.5) was observed by recycling the suspension liquid between an anaerobic FBBR and an aerobic MBBR. Very low (less than 60 mg/L) suspended solids was kept in the membrane unit, which could improve the membrane filtration performance. Membrane fouling was further reduced by increasing the recycle ratio. When influent organic load increased, the membrane fouling rate increased, coupling with higher FCOD and suspended solids (SS) values in the feed solution around the membrane.     

Effect of a suspended carrier on membrane fouling in a submerged membrane bioreactor.

In an attempt at membrane fouling control, a kind of cylindrical plastic suspended carrier was added in a submerged membrane bioreactor (SMBR) and its effect was investigated in this study. According to the transmembrane pressure (TMP) profiles and the sludge characteristics in comparative runs with and without suspended carriers, it was found that the suspended carriers added in SMBR had two effects on membrane fouling: one was the positive effect of mechanically scouring the membrane surface and the other was the negative effect of breaking up sludge flocs. Sludge particle size distribution change was mainly responsible. It was suggested to apply the suspended carrier at higher MLSS concentration and lower carrier dose based on the consideration for retarding sludge breakage caused by the carrier. The experiment was conducted under higher MLSS (8 gL(-1)) and lower carrier dose (carrier volume/total volume = 10/). The TMP increase was effectively retarded by added suspended carriers compared to the system without addition of the carriers. The effect of suspended carriers on membrane fouling at high MLSS concentration was verified.