Transmembrane Pressure Rise Research Articles

Effects of SRT and HRT on Treatment Performance of MBR and Membrane Fouling

40L of hollow fiber membrane bioreactor with solids retention times (SRT) of 30, 15 and 4 days were setup for treating synthetic wastewater at hydraulic retention times (HRT) of 12, 8 and 4 hours. The objectives of the study were to investigate the effects of SRT and HRT on membrane fouling. A comparative analysis was carried out for physiochemical quality parameters (turbidity, suspended solids, COD, NH3-N and PO43-). Scanning electron microscopy (SEM), energy diffusive X-ray (EDX) analyzer and particle size distribution (PSD) were used to characterize the membrane fouling properties. The influence of SRT on the quality of effluent, activated sludge quality, and membrane fouling were also correlated. Lower membrane fouling and slower rise in trans-membrane pressure (TMP) were noticed at the longest SRT and HRT of 30d and 12h, respectively. Increasing SRT results in noticeable reduction of dissolved organic matters. The best removal efficiencies of COD, TSS, NH3-N and PO43- were 93%, 98%, 80% and 30% respectively. The high HRT with shorter SRT induced faster fouling rate. The main fouling resistance was cake layer. The most severe membrane fouling was observed at SRT and HRT of 4 and 12, respectively with thickness cake layer of 17mm as reflected by higher TMP, lower effluent removal and thick sludge cake layer.

Tracing biofouling to the structure of the microbial community and its metabolic products: A study of the three-stage MBR process

The biofouling characteristics of a sequential anoxic/aerobic-membrane bioreactor (A/O MBR) were analyzed during the three-stage process (fast-slow-fast transmembrane pressure (TMP) increasing). The results indicated: during the stage 1 (before day 1), the microbial communities in the activated sludge (AS), cake sludge (CS) and biofilm (BF) were similar to each other, and the adsorption of microbes and the metabolic products was the main factor that led to TMP increase; during the stage 2 (between day 1 and day 7), the cake layer begun to form and the TMP continued to rise gradually at a reduced rate compared to stage 1, at this point a characteristic microbial community colonized the CS with microorganisms such as Saprospiraceae and Comamonadaceae thriving on the membrane surface (BF) probably due to greater nutrient availability, and the predominance of these species in the microbial population led to the accumulation of biofouling metabolic products in the CS, which resulted in membrane fouling and the associated rise in TMP; during the final stage (after day 7), the biofilm had matured, and the activity of anaerobes stimulated cake compaction. The statistical analysis showed a correlation between the TMP changing rate and the carbonhydrates of soluble microbial products (SMPc) content in the CS. When the SMPc concentration rose slowly there was a low level of biofouling. However, when the SMPc accumulating rate was greater, it resulted in the more severe biofouling associated with the TMP jump. Furthermore, the correlation coefficient for the TMP increase and protein concentrations of extracellular polymeric substances (EPSp) in the CS was highly significant. The cluster analysis suggested that the AS microbial community remained stable during the three TMP change stages, while the CS and BF community were changed accompanied with the TMP change. The interaction between the microbial communities and the metabolic products lead to the significant correlation between them. The EPSp in conjunction with the SMPc were the main factors that accelerate the membrane fouling. The rapid rise of SMPc triggered a sudden increase in the TMP, while the accumulation of EPSp caused the sustained rise in TMP.

New insights into membrane fouling in submerged MBR under sub-critical flux condition

This study investigated the membrane fouling in MBRs under sub-critical flux condition. Results showed membrane fouling at subcritical flux evolved through a three-stage process: a slow linear increase in transmembrane pressure (TMP) (stage I), followed by an exponential increase in TMP (stage II), and finally a rapid linear TMP rise was observed at stage III. It was found that bound EPS would play a significant role in fouling development at stage I, while SMPs appeared to be the major contributor to self-accelerating fouling phenomena observed at stage II. At stage III, the entire membrane was covered by a cake layer of flocs, as the result, the fouling rate was likely determined by floc characteristics. This study offers new insights into the fouling development under sub-critical flux condition.

Dynamics of biofilm formation under different nutrient levels and the effect on biofouling of a reverse osmosis membrane system

Pseudomonas aeruginosa PAO1 wild type and a mucoid derivative (FRD1) which over produces alginate were used to foul reverse osmosis (RO) membranes. When operated at a constant flux, biofilm formation on the RO membrane resulted in a slow rise in transmembrane pressure (TMP) of 22% for the initial four days of operation, followed by a sharp increase of 159% over the following two days. The initial slow increase in TMP was probably due to the formation of a biofilm on the membrane surface, which then accelerated the rate of biofouling through the effect of concentration polarization. At later stages of operation, most of the bacterial biomass consisted of dead cells. The amount of extracellular polymeric substances appeared to correlate positively with the number of dead cells. The results indicate that prolonging the initial stage of slow TMP increase and avoiding the latter stage of accelerated TMP increase would provide a sustainable operation of the RO system. These results suggest that nutrient limitation could reduce biofilm accumulation and delay the increase in TMP.

The influence of operating conditions on the filtration behavior of actual extracellular polymeric substances (EPS) using dead-end membrane filtration cell

Abstract The EPS solution extracted from the activated sludge of sequencing batch reactors (SBRs) by the formaldehyde–NaOH extraction method was filtered in dead-end cell with 0.1 μm PVDF micro filtration membrane under various operating conditions, and the filtration behaviors of actual EPS solution were investigated. The experimental results show that: firstly, the membrane filtration mechanism is governed by cake filtration, and the cake is compressible; secondly, the cake specific resistance increased with the increase of transmembrane pressure (TMP) and decreased as the concentration increased; thirdly, all operating conditions had a significant influence on the cumulative filtrate volume (CFV) of actual EPS solution; finally, the CFV increased with the rise of temperature and TMP, but decreased as EPS concentration increased. The sequence of influence degree of operating conditions is the temperature (38.1%) > the TMP (34.8%) > the EPS concentration (27.1%). A quantitative regression relationship be...

The impact of flux and spacers on biofilm development on reverse osmosis membranes

Abstract Biofouling has been traditionally described as biofilm development on the membrane surface and leads to an increase in the required trans-membrane pressure (TMP). This study focuses on the impact of the feed channel spacer on the extent and nature of biofouling on the membrane. Experiments were conducted under conditions of constant flux with different hydrodynamics and biofouling was measured by TMP rise. In addition, biofilm development in the spacer-filled channel was monitored by confocal laser scanning microscope (CLSM) to both correlate the increase in TMP with the amount of biofilm development as well as to compare biofilm development on the feed channel spacer and the membrane. Faster TMP rise was observed under higher flux, lower crossflow, and higher salinity conditions. The presence of the feed channel spacer had a positive impact in terms of reduction of TMP rise, and no significant increase in channel pressure drop was observed during the experiments. In order to reconcile these observations with other studies that emphasize the role of spacer fouling, the scenarios likely to lead to predominance of membrane fouling or spacer fouling are discussed.

Performance of a stainless steel membrane in membrane bioreactor process

Stainless steel membrane has recently emerged as a durable membrane for microfiltration. An aerobic membrane bioreactor (MBR) equipped with a tubular stainless steel membrane of pore size 0.3μm was submerged in a wastewater reactor to treat municipal wastewater of about 1,000 mg/L COD. The membrane operational performance was tested at three different permeate flux (7.5, 10 and 15 L/m2 h [LMH]) for 2 days each. In all cases, the rate of transmembrane pressure (TMP) rise was very high at 30, 45 and 80 kPa for 7.5, 10 and 15 LMH, respectively. Different analytical techniques i.e. particle size distribution, UV spectrometry, fluorescent spectrometry and size exclusion chromatography were used to study the nature of mixed liquor and the fouling deposited on the membrane surface. Rapid rise of TMP and decrease in permeate flux was observed during the experiment. Analysis of the fouling indicated a negligible difference in nature of organics between it and the mixed liquor. Filtration flux test showed a high sludge cake and pore-blocking resistance of 4.4 × 1019 and 2.8 × 1016 m-1, respectively, compared to a clean membrane resistance 5.4 × 1012 m-1. The similar nature of organics in the mixed liquor and the foulant and recovery of flux after removal of the foulant after gentle washing in water, indicated a rapid sludge accumulation rather than the irreversible fouling.

Combined effect of air and mechanical scouring of membranes for fouling reduction in submerged membrane reactor

This study investigated the combined effect of air flow and use of granular support medium in suspension in a submerged membrane reactor to reduce membrane fouling. Lower membrane fouling and a slower rise in transmembrane pressure (TMP) were noticed when a higher air flow rate was used for membrane scouring. Further fouling reduction was achieved by adding a granular medium in the reactor. The results showed that in the absence of the granular medium, when air flow was tripled (from 600 to 1800L/h/m2), the TMP development was decreased by 60%. TMP further dropped to 85% with the addition of granular medium (for the same air flow rate). The doubling of the air flow rate (from 600 to 1200L/h/m2), without granular medium, led to a 32% reduction in TMP development at 10L/m2.h. The same result was obtained at a lower air flow rate of 600L/h/m2 with the granular medium. This result shows that the same reduction of TMP can be obtained by adding granular medium instead of doubling air flow rate. Therefore adding granular medium in the suspension (mechanical scouring) with air flow (air scouring) could be a sustainable alternative to applying high air flow in submerged membrane systems.

Application of air flow for mitigation of particle deposition in submerged membrane microfiltration

This study investigates the effect of microfiltration operating conditions on membrane fouling of colloidal particles of kaolin clay. Experiments were conducted with a flat sheet membrane submerged in a suspension prepared from kaolin clay powder of size varying from 0.1 to 4 &mgr;m (Sigma) with a mean particle size 2.10 &mgr;m. The particle size distribution of clay was unimodal and the concentration of kaolin clay was similar to the biomass concentration in a membrane bioreactor (10 g/L). The effects of scouring and permeate flux rates were studied in terms of the membrane fouling rate. A linear relationship between the transmembrane pressure (TMP) and particle deposition was established for different air flow rates and permeate flow rates. Air scouring was more effective at a low permeate flux. There was only a minor change in the mean particle size of deposited colloidal particles on the membrane at a given flux under varying air flows and at the beginning all had a similar rise in TMP. However, at the later stages as particles accumulated on the membrane surface there was a significant rise in TMP. 15 LMH flux was observed as critical flux beyond which a rise in the permeate flux showed a sharp rise in the TMP which varied with air flow rates and particle deposition. The sharp TMP rise that occurred during the initial few hours of operation indicated that air flow for fouling mitigation strategies should target this period to optimise the membrane process. The study showed that air flow and flux rates are the two major governing factors for particle deposition on the membrane surface.

Influence of aeration and permeate flux on deposition of particulates on membrane surface

In microfiltration, a deposit of foulant tends to form on the membrane surface and this usually controls the performance of the filtration process. This paper discusses the influence of physical parameters such as aeration and permeates flux on migration and deposition of above micron particles on the membrane surface. Kaolin clay suspension of particle 3.7–8 μm with mean particle diameter 4.1 μm was used in this study. Equal amount of mass of deposited particles on the membrane surface created different transmembrane pressure (TMP) when operated at different aeration rates and permeate flux showing that there is a composite effect. The particle deposition rate at the beginning at lower flux was almost linear which changed to a sharp logarithamic rise at higher flux. The difference in TMP rise for the same amount of deposit demonstrated the selective nature of particle deposition. The mass of the particle deposition on the membrane surface could be described by two parameters: maximum deposition and time using a simple empirical logarithamic equation y=k/[1+exp(b−at)], where k, a, and b are constant; y is the particulate mass deposit (g/m2) and t is the time. The maximum particle mass deposition growth could be described by the equation dy/dt=1/4ka.

Characterisation of initial fouling in aerobic submerged membrane bioreactors in relation to physico-chemical characteristics under different flux conditions

The initial fouling characteristics of aerobic submerged membrane bioreactors (MBRs) were analysed under different flux conditions. Physico-chemical analyses of the mixed liquor hinted that carbohydrates were more important to membrane fouling than proteins. However, this contrasted with the characterisation of foulants on the membrane surfaces. Micro-structural analyses of the foulants on the membrane surfaces showed that the dominant foulants were different under different flux conditions. Membrane fouling occurred through a biofilm-dominated process under lower flux conditions, but the mechanism shifted towards a non-biofilm, organic fouling process as the flux was increased. In spite of the differences in fouling mechanisms, it was found that the protein fraction on the membrane surfaces, in the initial stages of MBR operations, had the greatest impact in the rise of transmembrane pressure.

Subcritical fouling behaviour modelling of membrane bioreactors for municipal wastewater treatment: The prediction of the time to reach critical operating condition

After the advent of critical flux concept, studies on constant subcritical flux operation of membrane bioreactors (MBRs) gained interest, presumably, because of preferred least fouling operational strategy; however, such a chosen operational condition has demonstrated the continuous slow and irreversible fouling followed by a sudden rise in transmembrane pressure (TMP). Such peculiar fouling behaviour has been explained to be evolved due to extracellular polymeric substances (EPS) originated through the course of biological processes inherent to MBRs. A subcritical flux fouling model based on the role of two forms of EPS, viz. bound and free, has been presented here. The model, studied for MBR treating municipal wastewater, could successfully predict the time at which a sudden change in the TMP–time profile occurs; the predicted time of 200 h by the model has shown good agreement with the observed time of approximately 190 h. Although the fouling problem is one of the major bottlenecks for the MBR technology to become widely applicable as an efficient municipal wastewater treatment technology, from the operational point of view, prediction of the time after which severe fouling occurs (a sudden jump in the TMP–time profile) during subcritical operation could be an important step. Such a prediction would help in developing an efficient and well regulated cleaning protocol for better management of MBR treatment plant operations.

Application of local material properties to prediction of constant flux filtration behaviour of compressible matter

Difficulties with existing approaches to the modelling of filtration of compressible materials under constant flux conditions are addressed in this paper and a new approach to modelling simultaneous consolidation and cake build-up based on dewatering theory is presented. Flocculated yeast is used as the test system with the predicted trans-membrane pressure rise as a function of time under constant flux conditions compared with experimental data. Good agreement is observed between model and experimental trends. The close correspondence between experimental and predicted results also suggests that it may be possible to predict trans-membrane pressure rise during constant flux filtration on the basis of material properties determined through simple constant pressure steady-state experiments.

Membrane bioreactor with bubble‐size transformer: Design and fouling control

AbstractBubbling requirements for membrane bioreactors (MBRs) are typically split into fine bubbles for aeration and larger coarse bubbles for fouling control. This article examines the possibility of reducing air requirements by using the same air supply to achieve both tasks for a submerged flat‐sheet MBR. A bubble‐size transformer (BST) comprising converging channels and a tube‐bank unit was used to coalesce fine bubbles into coarse bubbles. The BST was located under the flat‐sheet membrane modules and generated coarse bubbles to scour the membrane surfaces. Approximately 65% of the fine bubbles produced from fine ceramic air diffusers were transformed into coarse bubbles, and 35% of the fine bubbles were used for biological activities and to prevent the settlement of mixed liquor suspended solid (MLSS) in the bioreactor. Both bubble diameters and bubbling velocities were found increased after the transformations. The BST increased bubble diameters from 0.5–2 mm to 9–18 mm. The coarse bubbles after transformation were bigger than the gaps between the flat‐sheet modules at 5 mm so that slug flow probably occurred in the module gaps. Long‐term MBR experiments at air flowrates of 3 and 5 L/min with permeate fluxes of 22.5 and 35 L/m2.h were used to evaluate the bubble‐size transformation technique for the MBR. The benefit from the BST can be seen by comparing the “sustainable” flux period before a rapid rise in transmembrane pressure. © 2006 American Institute of Chemical Engineers AIChE J, 2007

Effects of sludge retention time on membrane fouling and microbial community structure in a membrane bioreactor

In this study, the effect of sludge retention time (SRT) on membrane bio-fouling was investigated in a membrane bioreactor (MBR) equipped with a sequential anoxic/anaerobic reactor. Specific cake resistance ( α), trans-membrane pressure (TMP), mixed liquor suspended solids (MLSS), particle sizes, extracellular polymer substances bound in sludge (bound-EPS) and their correlations with membrane bio-fouling were studied at different SRTs. As SRT decreased to 20 days, the bound-EPS per unit of biomass increased, and consequently, the value of α increased, which resulted in the rise of TMP. However, the reduction of the bound-EPS content was relatively small as compared to a significant decrease in the value of α at longer SRTs (above 60 days). These observations suggest that colloidal particles significantly contribute to membrane bio-fouling. In addition, the diversity of the microbial community structure of activated sludge in the MBRs was observed using the respiratory quinone profile. The ubiquinone species containing UQ-8, belonging to the class β- Proteobacteria type were the major constituents of the microbial community structure. The mole fraction of menaquinone MK-6, -7 and -8(H2) increased as SRT increased. Thus, the results of this study indicate that growth of microorganisms belonging to the δ- and ɛ-subclass of Proteobacteria as well as the members of the Cytophaga– Flavobacterium cluster increased at longer SRTs.

Factors affecting the membrane performance in submerged membrane bioreactors

Submerged membrane bioreactors (SMBRs) operate at constant imposed flux usually with the membranes in direct contact with the mixed liquor. Membrane fouling is evident as a rise in the required transmembrane pressure (TMP). In this paper we describe the rise in TMP as a three stage process. Stage 1 occurs in a period of a few hours and involves abrupt TMP rise due to ‘conditioning’, presumably by pore blockage and closure. Stage 2 is a prolonged period of slow TMP rise, which we ascribe to accumulation of extracellular polymeric substances (EPS) and other products of bioactivity, either deposited from the bulk liquor or produced in biofilms on the membrane surface. Polysaccharides are predominant in the supernatant EPS, and appear to be major membrane foulants deposited by the filtration process. Stage 3 is a sudden rise in TMP, which rapidly leads to inoperability of the membranes. This stage could have several causes, which we summarize, and which are all driven by the self-accelerating nature of fouling under constant flux operation. In order to understand the complex nature of fouling in submerged MBRs we have developed the MBR ‘roadmap’ that shows the relationship between the many operational and design characteristics of the MBR and the generic ‘fouling factors’, i.e. the nature of the feed (to the membrane), the membrane properties, and the hydrodynamic environment. We also present a MBR ‘fouling mechanism map’ which depicts the three stages of fouling and the multiple fouling mechanisms that could occur. Experimental observations are provided in support of many of the proposed fouling mechanisms.

Operation of Membrane Bioreactor with Powdered Activated Carbon Addition

The effect of powdered activated carbon (PAC) addition to the activated sludge (AS) in a membrane bioreactor (MBR) has been investigated. The long term nature of the tests allowed the PAC to gradually incorporate into the biofloc forming biologically activated carbon (BAC). One series of tests involved 4 bench scale (2 L) MBRs operated at sludge retention times (SRTs) of 30 days with PAC inventories of 0, 1, 3 and 5 g/L and steady state biomass concentrations of 12.0±1.0 g/L. The characteristics of the mixed liquors (MLSS) from the 4 reactors were compared. Short term filtration tests, including measurement of specific cake resistance (SCR), flux decline profile, and irreversible fouling resistance in an unstirred cell and “sustainable” flux (by monitoring transmembrane pressure (TMP) rise) in a crossflow cell all showed better filtration performance for the MLSS with BAC compared with the AS alone. In terms of SCR and flux decline profile the 1 g/L PAC addition performed best, but in terms of minimizing irreversible membrane fouling and maximizing “sustainable” flux the 5 g/L PAC was best. All 4 systems showed lower total organic carbon (TOC) in the permeate compared to the bioreactors, but the lowest permeate TOC (and the best removal) was for the highest PAC loading. The benefit of PAC addition was confirmed in a second series of tests with two 20 L MBRs with submerged hollow fibers, one operated without PAC, the MBR(AS), and the other with 5 g/L PAC, the MBR(BAC). For an SRT of 30 days (which involved 3.3% sludge wastage per day and 3.3% new PAC addition per day) and a fixed flux of 21 L/m2hr the MBR(AS) showed a TMP rise of about 2.4 kPa/day whereas the MBR(BAC) showed a rise of only 0.8 kPa/day. However when the MBRs were operated without wastage the performance of the MBR(BAC) was worse than the MBR(AS). Thus the improved performance of the MBR(BAC) requires regular replenishment of aged BAC with fresh PAC.

Factors affecting the performance of a submerged hollow fiber bundle

This study examines the performance of submerged hollow fibers with and without bubbling. The mechanisms of fouling were investigated with a model submerged hollow fiber bundle of nine fibers arranged in a 3 × 3 array. Parameters varied included the axial flow velocity, the bubble size and bubble frequency. An experimental setup was devised that could maintain a constant average flux across the bundle, yet allow measurement of the individual flux provided by each fiber. It was found that the standard deviations of the fluxes of the individual fibers tend to rise due to fouling. The standard deviation can be used as a characteristic indicator of uneven fouling inside the bundle and to predict an imminent increase in the rise in trans-membrane pressure (TMP). ‘Hydrodynamic resistance’ is a performance measure which takes into account the effect of the other hollow fibers on the apparent resistance experienced by an individual hollow fiber in a bundle. The ‘hydrodynamic resistance’ has two components, one is the ‘bundle resistance’ caused by lateral flow through the bundle, the other is ‘permeate competition’ from surrounding fibers; the ‘permeate competition’ effect can be dominant. It was found that the standard deviation of the fluxes that preceded TMP rise was close to the standard deviation predicted by incorporating the hydrodynamic resistance. One outcome of these observations is that it is important to arrange the bubbling configuration so that the agitation is homogeneous within the bundle, to ensure that the fibers in the bundle foul evenly. The hydrodynamics within the fiber bundle were investigated by the particle image velocimetry (PIV) technique. To facilitate observation with PIV, optical fibers were used in place of the hollow fibers, and the refractive index of the fluid was matched to that of the optical fibers. It was found that the axial velocities within the hollow fiber module could be up to 10 times lower than the velocities outside the fiber bundle, which indicated a strong sheltering effect. The standard deviation of the velocities within the hollow fiber bundle had a much larger effect on the performance of the bundle compared to the velocity per se. We have also compared the fouling control effects of bubbles of different sizes and report preliminary evidence that many small bubbles can be more effective than a few large bubbles at the same volumetric flow rate.

Evolution of fouling during crossflow filtration of model EPS solutions

Extracellular polymeric substances (EPS) are a major fouling component in membrane bioreactor (MBR) systems. For a better understanding of the fouling mechanisms of EPS, the evolution of fouling of sodium alginate, a microbial polysaccharide was studied during crossflow ultrafiltration and microfiltration. Incremental flux-stepping experiments and long-term subcritical flux filtration were carried out. A two-stage of transmembrane pressure (TMP) profile reported during long-term filtration of MBR effluents was also observed for alginate solutions. An initial slow and gradual TMP increase was followed by a sudden transition to a rapid TMP rise. Alginate transmission and deposition pattern supported the concept of flux redistribution among open pores as a cause of two steps of fouling profile. A desorption method with sodium hypochlorite was developed to characterise the temporal and spatial distribution of alginate along the membrane surface. The TMP rise was increased, not only due to the increase of alginate loading on the membrane (from 52 to 252 mg m −2 in days 1 and 6, respectively), but also because of the temporal increase of the specific cake resistance (from 5.9 to 16.1 × 10 15 m kg −1 within 6 days). The fouling layer formed in the long-term subcritical flux operation appeared irreversible, while fouling layers formed in the short-term dead-end constant pressure or flux-stepping experiment showed greater reversibility. Membrane autopsy using FESEM technique also confirmed the mostly irreversible nature of the fouling layer during the long-term subcritical flux operation.

Fouling phenomena in a MBR: transmembrane pressure transients and the role of EPS (extracellular polymeric substances)

A membrane bioreactor (MBR), comprising an anaerobic reactor followed by crossflow MF, was operated at a fixed flux substantially lower than the nominal “critical flux” of the feed. A slow rise in transmembrane pressure (TMP) was observed which increased with imposed flux and organic loading to the bioreactor. After extended operation there was a sudden transition to a rapid TMP rise. This TMP profile was reproducible and depended on the imposed flux. The initial and gradual TMP rise is believed to be caused by the deposition of extracellular polymeric substances (EPS). The sudden rise in TMP is believed to be caused by local flux in regions of the membrane increasing (to maintain the fixed permeate flow) and exceeding the critical flux of the dominant foulant (biomass). Membrane autopsy revealed a significant and uneven distribution of fouling by EPS.