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).     

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.     

Influence of the size of membrane foulants on the filterability of WWTP-effluent.

The application of membrane filtration for the polishing of wastewater treatment plant (wwtp) effluent is more and more widely used. However, fouling problems are still not well understood. In order to investigate the role of particles in dead-end ultrafiltration of wwtp-effluent, Roorda studied the influence of pre-filtrated wwtp-effluent on the filterability. In addition this research investigates the relation between the filterability of the different fractions in the wwtp-effluent and the amount of organic foulants in these fractions. The results show a relation between the filterability and supra-dissolved particles in the fractions < 0.45 and < 0.2 microm. The amount of COD, colour, proteins and polysaccharides in these fractions are found of minor importance. A significant increase in filterability is found between the fractions < 0.2 and < 0.1 microm as well as a significant decrease in amount of polysaccharides. It is suggested that an essential feature in understanding membrane fouling during ultrafiltration of wwtp-effluent is to investigate the influence of supra-dissolved particles on the filterability as well as the influence of the amount of organic foulants (total organic carbon, proteins, polysaccharides, colour and humic substances) on the reversibility.     

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.     

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.     

Side effects of flux enhancing chemicals in membrane bioreactors (MBRs): study on their biological toxicity and their residual fouling propensity

Soluble and colloidal materials like soluble microbial products (SMP) or extracellular polymeric substances (EPS) are considered to be major foulants in membrane bioreactors (MBRs). Removing these fouling causing substances is thus thought to reduce the fouling of the membrane in general.In addition to traditional strategies for fouling prevention which mostly try to remedy the effects of fouling by air scour, etc., the new and promising method of adding chemicals is being investigated here. Previous tests with 30 different substances have shown that several of these reduce SMP concentration in the supernatant and enhance filtration. Nevertheless, additive dosing might have unknown side effects in filtration systems. Results presented in this study indicate that these additives may themselves cause severe fouling on different membranes if they remain unbound in the liquid phase. Therefore, the thorough control of the dosing rate of these chemicals will be of paramount importance in full scale applications. Biological toxicity of additives was measured in terms of respiration. OUR tests did not show inhibiting effects for most additives. Chitosan even showed an enhanced OUR due to biodegradability. Oxygen transfer could be enhanced for 25% with the addition of a polymer.     

Dynamics of dewaterability and bacterial populations in activated sludge

Relationships of bacterial populations and extracellular polymer substances (EPS) to dewaterability of activated sludge were studied on three laboratory-scale activated sludge reactors fed with synthetic wastewater. Dewaterability of activated sludge was evaluated by a novel method developed by the authors, in which small amount of sludge was centrifugally dewatered, and its water content was measured. Bacterial populations during the reactor operation were analyzed by the polymerase chain reaction/terminal-restriction fragment length polymorphism (PCR/T-RFLP) targeted at a partial 16S rRNA gene. Extracellular polymeric substances (EPS) were extracted using cation exchange resin (CER), and polysaccharides and total protein in EPS were determined. Some of the dominant terminal-restriction fragments (T-RFs) were observed to have significant relationships with dewaterability of sludge, and it was suggested that bacterial species corresponding to those peaks significantly affected dewaterability. On the other hand, significant relationships were not found between EPS concentration and dewaterability of sludge.     

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.     

Adsorption of Cd(II), Zn(II) by extracellular polymeric substances extracted from waste activated sludge

Adsorption of Cd(II) and Zn(II) ions in single solutions using extracellular polymeric substances (EPS) from activated sludge was investigated. Langmuir and Freundlich models were applied to describe metal adsorption. The results showed that EPS was an effective adsorbent for the zinc and cadmium ions from aqueous solution. The equilibrium metal uptake was increased with increasing the initial concentration of metal ion. Constants calculated from isotherms model showed that the maximum uptake capacity of cadmium was estimated to be 45 mg/g of Cd(II) and 80 mg/g of Zn(II). Both Langmuir and Freundlich isotherms were suitable for describing adsorption of Cd(II) by EPS, while the Langmuir isotherm equation fit the date of Zn(II) adsorption better, indicating that EPS adsorb Cd(II) and Zn(II) by different mechanisms.Analysis of FTIR spectra demonstrated that C-O-C of polysaccharides at 1,150-1,030 cm(-1), group of the amide(I), CH(2) group of the lipids, carboxyl and -OH groups of proteins and polysaccharides were involved in cadmium and zinc binding, of which the -OH groups and the C-O-C group of polysaccharides.     

The relationship between the structure of activated sludge flocs and the sorption of hydrophobic pollutants

The relationships between the structure of minimally perturbed activated sludge flocs and the sorption of organic contaminants were studied. Sorption, settling velocity, size distributions, floc structure and EPS composition were all examined. The results show significant removal of selected halogenated hydrocarbons and polycyclic aromatic hydrocarbons by biosorption to activated sludge flocs. However, statistically significant effects on the settling or size of the flocs caused by this sorption were not observed. The addition of chromium (Cr(III)) metal ions to the biomass caused observable changes in the floc structure and decreased ruthenium red binding to the acidic polysaccharides of the floc matrix. At low concentrations (0.6 mg/1), chromium caused an increase in the sorption of organic compounds to flocs, suggesting that changes in the floc structure can be induced, which can have an impact on the sorption of pollutants to the flocs.     

Environmental Factors Controlling Physiological Changes in Cladophora in Lake Erie

Several physiological characteristics of Cladophora glomerata from eastern Lake Erie were monitored frequently during the summer of 1977. Soluble reactive phosphorus and nitrate-nitrogen in the lake water were also measured.Cladophora biomass generally increased throughout June, although there were periodic declines. In mid-July, biomass sharply decreased to a low level. Chlorophyll content tended to be high during periods of increasing biomass but was lower after each biomass decline. Changes in biomass did not correlate with changes in either phosphate-phosphorus or nitrate-nitrogen in the lake water. Cellular nitrogen levels fluctuated independent of lake water nitrate-nitrogen concentrations. Cellular nitrogen levels remained above the critical concentration, 1.1%-N. Luxury phosphorus levels were observed to increase following pulses of soluble orthophosphate in the lake water. Measurements of cellular total phosphorus indicate that levels remain well above the critical concentration of 0.06%-P. It is concluded that neither nitrogen nor phosphorus is limiting the growth of Cladophora at the sites sampled. The mid-summer die-off does not appear to be due to a simple nutrient deficiency.     

Effects of extracellular polymeric substances on granulation of anoxic sludge in sequencing batch reactor

Variations of extracellular polymeric substances (EPS) and its components with sludge granulation were examined in a lab-scale sequencing batch reactor (SBR) which was fed with sodium nitrate and sodium acetate. Ultrasonication plus cation exchange resin (CER) were used as the EPS extraction method. Results showed that after approximately 90 d cultivation, the sludge in the reactor was almost granulated. The content of extracellular polysaccharides increased from 10.36 mg/g-VSS (volatile suspended solids) at start-up with flocculent sludge to 23.18 mg/g-VSS at 91 d with matured granular sludge, while the content of extracellular proteins were almost unchanged. Polysaccharides were the major components of EPS in anoxic granular sludge, accounting for about 70.6-79.0%, while proteins and DNA accounted for about 16.5-18.9% and 4.6-9.9%, respectively. It is proposed that EPS play a positive role in anoxic sludge granulation and polysaccharides might be strongly involved in aggregation of flocs into granules.     

Influence of extracellular polymeric substances on nitrogen removal in an intermittently-aerated membrane bioreactor.

Influence of EPS on fouling of intermittent aeration MBR reactor (denitrification MBR) was investigated changing intermittent aeration cycle (10 minute-cycle and 120 minute-cycle) in laboratory-scale reactors using synthetic wastewater. EPS were extracted from bacterial cells using cation resin method and molecular weight fractioning of EPS was conducted using gel chromatography. In both of the reactors, nitrogen removal rate was almost 100% after 50th day although DO concentration was not very high during the aerated phase because of accumulation of nitrifying bacteria in the reactors. In the 120 minutes-cycle reactor, trans-membrane pressure increased more rapidly than in the 10 minutes-cycle reactor. The reason might be that EPS of more than 1000 kDa, which are the main fouling substances, are produced more rapidly in the 120 minute-cycle condition. It was also found that three peaks at around 100 kDa, 500 kDa and 2000 kDa are prominent in EPS in intermittent-aeration MBR irrespective of cycle and higher molecular weight EPS are decomposed to smaller molecular weight EPS on membrane surface.     

Fouling cake layer in a submerged anaerobic membrane bioreactor treating saline wastewaters: curse or a blessing?

The treatment of inhibitory (saline) wastewaters is known to produce considerable amounts of soluble microbial products (SMPs), and this has been implicated in membrane fouling; the fate of these SMPs was of considerable interest in this work. This study also investigated the contribution of SMPs to membrane fouling of the; (a) cake layer/biofilm layer, (b) the compounds below the biofilm/cake layer and strongly attached to the surface of the membrane, (c) the compounds in the inner pores of the membrane, and (d) the membrane. It was found that the cake/biofilm layer was the main reason for fouling of the membrane. Interestingly, the bacteria attached to the cake/biofilm layer showed higher biodegradation rates compared with the bacteria in suspension. Moreover, the bacteria attached to the cake layer showed higher amounts of attached extracellular polysaccharides (EPS) compared with the bacteria in suspension, possibly due to accumulation of the released EPS from suspended biomass in the cake/biofilm layer. Molecular weight (MW) analysis of the effluent and reactor bulk showed that the cake layer can retain a large fraction of the SMPs in the reactor and prevent them from being released into the effluent. Hence, while cake layers lead to lower fluxes in submerged anaerobic membrane bioreactors (SAMBRS), and hence higher costs, they can improve the quality of the reactor effluent.     

Preparation, characterization and performance of a novel PVDF/PMMA/TPU blend hollow fiber membrane for wastewater treatment

Polyvinylidene fluoride (PVDF)/polymethylmethacrylate (PMMA)/thermoplastic polyurethane (TPU) blend hollow fiber membranes were successfully prepared by the wet-spinning method with the loading of PMMA and TPU in a range of polymer concentrations varying from 0 to 20 wt% and at a total polymer concentration of 16 wt%. The influence of the addition of PMMA and TPU on the morphologies and the properties of such prepared membranes was investigated through FTIR-ATR, SEM, viscosity measurements, UF experiments and mechanical strength tests. Based on the experimental results, the compatibility of the PVDF, PMMA and TPU blend was best under the conditions of the PVDF-rich phase. The elongation at break of the membrane increased to a maximum of 146% with increase in the TPU concentration to 20 wt% in dope solution. The addition of PMMA increased the water permeation flux from 120 to 195 L/(m(2) h) initially. The flux then decreased when PMMA concentration was increased to over 10 wt%. The membranes obtained at optimized blending ratio were applied to the dyeing process wastewater filtration. During continuous filtration for 8 h, the flux was stabilized at about 20 L/(m(2) h) at 0.1 MPa. The reduction in COD(Cr), turbidity and color were about 63, 84 and 63% respectively.     

Hydrogen-dependent denitrification in an alternating anoxic-aerobic SBR membrane bioreactor.

A novel hydrogenotrophic denitrification system, which consisted of a sequencing batch membrane bioreactor, was evaluated for simultaneous removal of nitrate and soluble microbial products (SMP) from a synthetic groundwater feed. A hollow fiber membrane diffuser was used for bubble-less diffusion of hydrogen into the bioreactor under anoxic condition followed by aerobic SMP removal and biomass filtration. During the anoxic period, the nitrate loading of 0.328 kg N m(-3) d(-1) was completely denitrified to below detectable levels. A denitrification rate of 0.8 kg N m(-3) d(-1) was obtained at steady state biomass concentrations of 1,162 mg I(-1). During the aerobic period when biomass filtration was performed, 81% of SMP produced within the anoxic phase was retained by the membrane, 9% was biologically removed, 5% was passed through the membrane and 5% was discharged during the wasting of mixed liquor. The aerobic cycle was instrumental as it allowed for effective biomass filtration via membrane scouring and assisted in further reduction of effluent organic matter.     

A distillery by-product as an external carbon source for enhancing denitrification in mainstream and sidestream treatment processes

The use of fusel oil as an 'alternative' carbon source for denitrification in the mainstream and sidestream treatment processes was studied. Research comprised two kinds of batch experiments as well as acclimation of process biomass to external carbon sources. In the conventional nitrate utilization rate (NUR) measurements (one-phase experiments with non-acclimated biomass), the NUR with fusel oil was 1.4-1.7 g N/(kg VSS·h which was comparable to NUR with ethanol and with slowly biodegradable fraction of the settled wastewater. When fusel oil was added at the beginning of the anoxic phase, preceded by an anaerobic phase (in two-phase experiments with non-acclimated biomass), the NURs of 2.5-2.9 g N/(kg VSS·h) were comparable to the tests without the addition of any external carbon sources. The addition of fusel oil and ethanol resulted in a significant enhancement of the denitrification efficiency in lab-scale sequencing batch reactors treating sludge reject water. The NURs continuously increased from below 1 g N/(kg VSS·h) to over 10 g N/(kg VSS·h) over the entire 4-week operational period, indicating gradual acclimation to the substrate. The overall total N removal efficiency reached ～90%.     

Photosynthetic bacteria production from food processing wastewater in sequencing batch and membrane photo-bioreactors

Application of photosynthetic process could be highly efficient and surpass anaerobic treatment in releasing less greenhouse gas and odor while the biomass produced can be utilized. The combination of photosynthetic process with membrane separation is possibly effective for water reclamation and biomass production. In this study, cultivation of mixed culture photosynthetic bacteria from food processing wastewater was investigated in a sequencing batch reactor (SBR) and a membrane bioreactor (MBR) supplied with infrared light. Both photo-bioreactors were operated at a hydraulic retention time (HRT) of 10 days. Higher MLSS concentration achieved in the MBR through complete retention of biomass resulted in a slightly improved performance. When the system was operated with MLSS controlled by occasional sludge withdrawal, total biomass production of MBR and SBR photo-bioreactor was almost equal. However, 64.5% of total biomass production was washed out with the effluent in SBR system. Consequently, the higher biomass could be recovered for utilization in MBR.