New insights into membrane fouling in a submerged anaerobic membrane bioreactor based on characterization of cake sludge and bulk sludge

A laboratory-scale submerged anaerobic membrane bioreactor (SAnMBR) treating thermomechanical pulping whitewater was operated for over 7 months to investigate and compare the characteristics of cake sludge and bulk sludge during stable state operation period. Serial analysis showed that cake sludge had a smaller particle size distribution (PSD), much higher specific filtration resistance (1.34 × 10¹⁴ m/kg), 1.5 times higher bound EPS and significantly different microbial community as compared with bulk sludge. Further analysis indicated that small flocs, bound EPS and inorganic materials play important role in cake formation process. The formed cake layer was found to have a heterogeneous structure. The results obtained in this study indicated that cake formation process started from attachment of small flocs and/or specific bacterial clusters which colonize the surface of the membrane and provide enhanced conditions that allow for cake formation to progress.     

Membrane fouling in a membrane bioreactor (MBR): sludge cake formation and fouling characteristics

A submerged membrane bioreactor (MBR) with a working volume of 1.4 L and a hollow fiber microfiltration membrane was used to treat a contaminated raw water supply at a short hydraulic retention time (HRT) of approximately 1 h. Filtration flux tests were conducted regularly on the membrane to determine various fouling resistances, and confocal laser scanning microscopy (CLSM) and scanning electron microscopy (SEM) were employed to characterize the biofouling development and sludge cake formation on the membrane. The experimental results demonstrate that the MBR is highly effective in drinking water treatment for the removal of organic pollutants, ammonia, and UV absorbance. During the MBR operation, the fouling materials were not uniformly distributed on the entire surface of all of the membrane fibers. The membrane was covered partially by a static sludge cake that could not be removed by the shear force of aeration, and partially by a thin sludge film that was frequently washed away by aeration turbulence. The filtration resistance coefficients were 308.4 x 10(11) m(-1) on average for the sludge cake, 32.5 x 10(11) m(-1) on average for the dynamic sludge film, and increased from 10.5 x 10(11) to 59.7 x 10(11) m(-1) for the membrane pore fouling after 10 weeks of MBR operation at a filtration flux of 0.5 m3/m2 x d. Polysaccharides and other biopolymers were found to accumulate on the membrane, and hence decreased membrane permeability. More important, the adsorption of biopolymers on the membrane modified its surface property and led to easier biomass attachment and tighter sludge cake deposition, which resulted in a progressive sludge cake growth and serious membrane fouling. The sludge cake coverage on the membrane can be minimized by the separation, with adequate space, of the membrane filters, to which sufficient aeration turbulence can then be applied.     

Influence of elevated pH shocks on the performance of a submerged anaerobic membrane bioreactor

The effects of elevated pH shocks on the performance and membrane fouling of a submerged anaerobic membrane bioreactor (SAnMBR) treating thermomechanical pulping (TMP) whitewater was studied over a 120-day period. Changes in chemical oxygen demand (COD) removal, biogas production, sludge and cake layer properties, and their correlations to membrane fouling – before and after pH shocks – were systematically studied using various analytical tools. The results showed that a pH 8.0 shock had a minor impact, while pH 9.1 and 10.0 shocks exerted significant long-lasting negative impacts on COD removal, biogas production and membrane filtration performance of the SAnMBR. When the normal pH (7.0) was resumed, it took approximately 1, 6, and 30 days for the performance to recover for pH 8.0, 9.1 and 10.0 shocks, respectively. The elevated pH shocks induced the dispersion of sludge flocs and resulted in the accumulation of colloids and solutes or biopolymers in the sludge suspension, and thus deteriorated membrane performance. Statistical analysis showed that the ratio of proteins (PN) to polysaccharides (PS) in extracellular polymeric substances (EPS) had a strongly negative effect on the membrane fouling rate. There were smaller size particles deposited on the membrane surface and a more compact and denser cake layer was formed after being exposed to an alkaline shock at pH 10, resulting in higher membrane fouling rates.     

Impact of sludge retention time on MBR fouling: role of extracellular polymeric substances determined through membrane autopsy

The impact of sludge retention time (SRT) on the biofouling of a membrane bioreactor (MBR) by extracellular polymeric substances (EPS) was investigated. The MBR was operated at 60 and 20 d SRT. The gel layer (recovered through optimized membrane autopsy methods) and the cake layer were analyzed for their content and profile of EPS proteins and polysaccharides. The change to a shorter SRT led to decreased membrane filterability, concomitant with a higher expression of EPS proteins in the cake layer, which were identified as being mainly related with biosynthesis and stress functions. The gel layer was more substantial in internal fibers, with polysaccharides being the major component in this layer. With the decrease in SRT (and filterability decrease), the overall polysaccharide content and sugar variety increased. In conclusion, SRT impacted not only on the quantity but also the composition of EPS molecules, and both were shown to be important in biofouling.     

Tracking variations in fluorescent-dissolved organic matter in an aerobic submerged membrane bioreactor using excitation–emission matrix spectra combined with parallel factor analysis

In this study, the variations in the fluorescent components of dissolved organic matter (DOM) were tracked for an aerobic submerged membrane bioreactor (MBR) at three different operation stages (cake layer formation, condensation, and after cleaning). The fluorescent DOM was characterized using excitation–emission matrix (EEM) spectroscopy combined with parallel factor analysis (PARAFAC). Non-aromatic carbon structures appear to be actively involved in the membrane fouling for the cake layer formation stage as revealed by much higher UV-absorbing DOM per organic carbon found in the effluent versus those inside the reactor. Four fluorescent components were successfully identified from the reactor and the effluent DOMs by EEM-PARAFAC modeling. Among those in the reactor, microbial humic-like fluorescence was the most abundant component at the cake layer formation stage and tryptophan-like fluorescence at the condensation stage. In contrast to the reactor, relatively similar composition of the PARAFAC components was exhibited for the effluent at all three stages. Tryptophan-like fluorescence displayed the largest difference between the reactor and the effluent, suggesting that this component could be a good tracer for membrane fouling. It appears that the fluorescent DOM was involved in membrane fouling by cake layer formation rather than by internal pore adsorption because its difference between the reactor and the effluent was the highest among all the four components, even after the membrane cleaning. Our study provided an insight into the fate and the behavior fluorescent DOM components for an MBR system, which could be an indicator of the membrane fouling.     

Influence of polyaluminum chloride on microbial characteristics in anaerobic membrane bioreactors for sludge digestion

In this study, two parallel lab-scale anaerobic membrane bioreactors (AnMBRs), one of which was dosed with polyaluminum chloride (PAC) for membrane fouling control, were operated for treating excess activated sludge collected from a wastewater treatment plant (WWTP). The AnMBRs were inoculated with anaerobic digested sludge collected from an anaerobic digester of another WWTP. The microbial community of digested sludge and cake layer in AnMBRs, as well as that of excess sludge, was analyzed through polymerase chain reaction coupled with denaturing gradient gel electrophoresis (PCR-DGGE) and Illumina MiSeq. The dynamic variation of archaeal community in AnMBRs was not as obvious as that of bacterial community based on the PCR-DGGE results. Under the circumstance of stable operation, Cloacimonetes, Chloroflexi, Bacteroidetes, Proteobacteria, Firmicutes, and Ignavibacteriae were observed as the predominant phyla in digested sludge based on the Illumina results. In addition to that, the cake layer possessed similar predominant phyla with the digested sludge but owned a higher diversity. Furthermore, overlapping bacterial communities were discovered between the excess sludge and digested sludge. However, the abundance of aerobic bacteria was substantially reduced, while the abundance of anaerobic microorganisms like phylum Cloacimonetes and Smithella was enriched in digested sludge over time. Additional PAC dosing, on the one hand, affected the bioavailable substrate, thus further changing the microbial community structure; on the other hand, aluminum itself also affected specific microbial communities. Besides, PAC dosing indirectly influenced the bacterial diversity in AnMBR as well.

     

Comparison between a moving bed membrane bioreactor and a conventional membrane bioreactor on membrane fouling

A membrane bioreactor filled with carriers instead of activated sludge named a moving bed membrane bioreactor (MBMBR) was investigated to minimize the effect of suspended solids on membrane fouling. The MBMBR and a conventional membrane bioreactor (CMBR) were operated in parallel for about two months. Unexpectedly, the rate of membrane fouling in MBMBR was about three times of that in CMBR. MBMBR showed a higher cake layer resistance than CMBR due to plenty of filamentous bacteria inhabited in suspended solids in MBMBR. Protein and polysaccharide contents of soluble EPS in MBMBR were obviously larger than those in CMBR. It could be speculated that the overgrowth of filamentous bacteria in MBMBR resulted in severe cake layer and induced a large quantity of EPS, which deteriorated the membrane fouling.     

New insight into the effects of Ca(II) on cake layer structure in submerged membrane bioreactors

The effects of Ca(II) on the structure of the cake layer in submerged membrane bioreactors (SMBRs) were investigated in this study. Three parallel laboratory-scale SMBRs were operated with synthetic municipal wastewater with three Ca(II) levels (82, 208 and 410?mg?l^?1). As the Ca(II) concentration increased, the sludge floc size increased and the molecular weight of the soluble microbial products (SMP) in the bulk liquid decreased. These observations were attributed to the neutralization and bridging function of Ca(II). Furthermore, Ca(II) addition did not change the thickness of the cake layer, but inhibited the deposition of other elements, such as Al, Si, Mg, and Fe. As a result of Ca(II) addition, the cake layer became less compact and more porous. The interspaces among the flocs in the cake layer helped to reduce the membrane fouling potential.     

Decolorization of 1-amino-4-bromoanthraquinone-2-sulfonic acid in bioaugmented membrane bioreactor

Decolorization of 1-amino-4-bromoanthraquinone-2-sulfonic acid (ABAS) in a Sphingomonas xenophaga-augmented membrane bioreactor was investigated. The results showed that sequencing bioaugmented membrane bioreactor (SMBR) could run stably for 60 days. During the process, over 99% ABAS (570 mg/L) was decolorized and 60% COD was removed. Moreover, the yellow intermediate produced in continuous operation mode was not detected in SMBR. It was probably a dimer resulting from the autoxidation of 2-amino-3-hydroxy-5-bromobenzenesulfonic acid. Ribosomal intergenic sequence analysis (RISA) of the sludge in SMBR suggested that bioaugmentation resulted in large decrease or increase of certain population and Strain QYY can persist in SMBR. The biomass of membrane surface in SMBR could be further divided into a cake layer and a dense layer. The former is mainly composed of coccus, bacillus and filamentous bacteria, which is different from suspended sludge flocs with predominant bacillus.     

Comparison and Analysis of Membrane Fouling between Flocculent Sludge Membrane Bioreactor and Granular Sludge Membrane Bioreactor

The goal of this study is to investigate the effect of inoculating granules on reducing membrane fouling. In order to evaluate the differences in performance between flocculent sludge and aerobic granular sludge in membrane reactors (MBRs), two reactors were run in parallel and various parameters related to membrane fouling were measured. The results indicated that specific resistance to the fouling layer was five times greater than that of mixed liquor sludge in the granular MBR. The floc sludge more easily formed a compact layer on the membrane surface, and increased membrane resistance. Specifically, the floc sludge had a higher moisture content, extracellular polymeric substances concentration, and negative surface charge. In contrast, aerobic granules could improve structural integrity and strength, which contributed to the preferable permeate performance. Therefore, inoculating aerobic granules in a MBR presents an effective method of reducing the membrane fouling associated with floc sludge the perspective of from the morphological characteristics of microbial aggregates.     

Anaerobic submerged membrane bioreactor (AnSMBR) treating low-strength wastewater under psychrophilic temperature conditions

An anaerobic submerged membrane bioreactor (AnSMBR) treating low-strength wastewater was operated for 90 days under psychrophilic temperature conditions (20 °C). Besides biogas sparging, additional shear was created by circulating sludge to control membrane fouling. The critical flux concept was used to evaluate the effectiveness of this configuration. Biogas sparging with a gas velocity (U_G) of 62 m/h together with sludge circulation (94 m/h) led to a critical flux of 7 L/(m² h). Nevertheless, a further increase in the U_G only minimally enhanced the critical flux. A low fouling rate was observed under critical flux conditions. The cake layer represented the main fouling resistance after 85 days of operation. Distinctly different volatile fatty acid (VFA) concentrations in the reactor and in the permeate were always observed. This fact suggests that a biologically active part of the cake layer contributes to degrade a part of the daily organic load. Hence, chemical oxygen demand (COD) removal efficiencies of up to 94% were observed. Nevertheless, the biogas balance indicates that even considering the dissolved methane, the methane yield were always lower than the theoretical value, which indicates that the organic compounds were not completely degraded but physically retained by the membrane in the reactor.     

Enhancement effects of cationic contaminants from bacteria on cake layer formation and biofouling on an RO membrane

The model cationic molecule prodigiosin interacted with a polyamide/polysulfone composite reverse osmosis (RO) membrane, resulting in a reduction of the membrane permeation rate. Prodigiosin is an antibacterial agent produced by Serratia marcescens that is frequently isolated from activated sludge of domestic or industrial wastewater. Such molecules respectively secreted or leaked from live or dead cells are thought to affect membrane biofouling. In this study, a cell suspension containing prodigiosin-producing S. marcescens AS-1 wild-type or the non-producing AS-1ΔspnI strain was fed to the thin RO membrane to determine the occlusion ratio on the membrane. Cationic prodigiosin enhanced membrane biofouling by clogging the pores and enhanced the accumulation of the cake layer. The effects remarkably recovered the occlusion ratio after removing the cake layer by feeding with water. After temporary pressure relief, the occlusion ratios for AS-1 and AS-1ΔspnI were recovered to stable levels from approximately 70 to 49% and 23%, respectively. Zetapotential analysis supported the neutralization effects leading to the accumulation of bacterial cells under applied high pressure for RO membrane permeation.     

Microbial behaviors involved in cake fouling in membrane bioreactors under different solids retention times

Biomass characteristics and membrane performances in the MBRs operated at a high flux of 30 L/m² h under different SRTs (10, 30 days, and infinity) were monitored. Results showed that more serious cake-fouling happened in the SRT-infinity MBR, which correlated with the activated sludge characteristics such as smaller floc size and greater EPS amount. DGGE analysis indicated that the microbial community shifted in different ways under various SRTs, which also influenced EPS productions in the MBRs. Different microbial communities were developed on the membrane surfaces at various operating stages and SRTs. Possibly, the activated sludge characteristics (such as MLSS concentration, EPS properties) and hydrodynamic conditions influenced by the SRTs were associated with cake layer development and membrane fouling propensity. Insight into the EPS characteristics and deposition behaviors of bacterial flocs will be crucial to explore appropriate biofouling control strategies in MBRs.     

A mechanistic study of in situ chemical cleaning-in-place for a PTFE flat sheet membrane: fouling mitigation and membrane characterization

This study aimed at unfolding the role and mechanisms of chemically enhanced cleaning-in-place (CIP) regimes in fouling control of polytetrafluoroethylene (PTFE) made flat sheet (FS) membrane bio-reactors (MBRs). The trans-membrane pressure (TMP) was successfully maintained below 10 kPa using a daily CIP regime consisting of 100 to 600 mg l^?1 of NaOCl and cake layer resistance control was shown to be critical for effective high-flux MBR operation. In contrast, in the control unit without the CIP, the TMP exceeded 35 kPa at a flux of 40 LMH. The extracellular polymeric substances associated with proteins (EPS_protein) were also controlled effectively with a daily application of the CIP to the fouled membrane. Moreover, the CIP prompted a thinner and looser bio-cake layer on the membrane surface, suggesting that in situ CIP can be a favorable method to control FS membrane fouling at high-flux MBR operation.     

不同结构配比的鼠李糖脂乳化活性与油泥清洗效果

以大庆油田原油和含油污泥为对象,研究不同结构配比鼠李糖脂表面活性剂乳化活性及其对含油污泥清洗效果的影响,并优化清洗工艺参数。结果表明:单鼠李糖脂比例越高,其表面活性越好;双鼠李糖脂比例越高,其对原油的乳化能力越强;临界胶束浓度随着双鼠李糖脂比例的增加而逐渐加大;单、双鼠李糖脂配比不同的表活剂对油泥的清洗效果也不同,质量比为50∶50时清洗效率最高;鼠李糖脂浓度为1.0 g·L^-1、热洗时间为1.5h、热洗温度为65℃、洗脱强度为220 r·min^-1、固液质量比为1∶5条件下,油泥的清洗效率最高,可达81.3%;含油率为29.6%的落地油泥,经一级洗涤后油泥残油率降至5.5%,原油回收率达到87.3%,清洗出的原油无明显乳化,易于分离。由此可知,鼠李糖脂的单、双糖脂比例不同对其理化性质和清洗含油污泥的效果均有不同程度的影响。     

Biofouling development and rejection enhancement in long SRT MF membrane bioreactor

This study was conducted to investigate the development and extracellular polymeric substances (EPS) distribution in biofouling layer and biofouling effect on permeate quality. The experimental results suggested that formation of biofouling layer was started by the attachment of polysaccharides and formed a biogel like layer on top of membrane surface (adhesive attachment). It further induced the attachment of protein, polysaccharides and bioparticles, and formed cake layer (cohesive attachment). As evidenced in SEM photos and permeates quality, the formed biofouling layer had changed the properties of membrane surface such as the pore and porosity, and hence produce the better permeates quality. A great enhancement of rejection performance occurred at the early filtration period, and followed by a slight enhancement in rejection throughout the entire filtration. This enhancement of rejection performance by biofouling layer can be mathematically expressed by the logarithm function with the degree of membrane fouling.     

Comparison between a moving bed membrane bioreactor and a conventional membrane bioreactor on organic carbon and nitrogen removal

A membrane bioreactor filled with carriers instead of activated sludge named a moving bed membrane bioreactor (MBMBR) was investigated for simultaneously removing organic carbon and nitrogen in wastewater. Its performance was compared with a conventional membrane bioreactor (CMBR) at various influent COD/TN ratios of 8.9–22.1. The operational parameters were optimized to increase the treatment efficiency. COD removal efficiency averaged at 95.6% and 96.2%, respectively, for MBMBR and CMBR during the 4 months experimental period. The MBMBR system demonstrated good performance on nitrogen removal at different COD/TN ratios. When COD/TN was 8.9 and the total nitrogen (TN) load was 7.58 mg/l h, the TN and ammonium nitrogen removal efficiencies of the MBMBR were maintained over 70.0% and 80.0%, respectively, and the removed total nitrogen (TN) load reached to 5.31 mg/l h. Multifunctional microbial reactions in the carrier, such as simultaneous nitrification and denitrification (SND), play important roles in nitrogen removal. In comparison, the CMBR did not perform so well. Its TN removal was not stable, and the removed total nitrogen (TN) load was only 1.02 mg/l h at COD/TN ratio 8.9. The specific oxygen utilization rate (SOUR) showed that the biofilm has a better microbial activity than an activated sludge. Nevertheless, the membrane fouling behavior was more severe in the MBMBR than in the CMBR due to a thick and dense cake layer formed on the membrane surface, which was speculated to be caused by the filamentous bacteria in the MBMBR.     

Biofouling of reverse osmosis membranes: effects of cleaning on biofilm microbial communities,membrane performance,and adherence of extracellular polymeric substances

Laboratory-scale reverse osmosis (RO) flat-sheet systems were used with two parallel flow cells, one treated with cleaning agents and a control (ie undisturbed). The cleaning efforts increased the affinity of extracellular polymeric substances (EPS) to the RO membrane and altered the biofilm surface structure. Analysis of the membrane biofilm community composition revealed the dominance of Proteobacteria. However, within the phylum Proteobacteria, γ-Proteobacteria dominated the cleaned membrane biofilm, while β-Proteobacteria dominated the control biofilm. The composition of the fungal phyla was also altered by cleaning, with enhancement of Ascomycota and suppression of Basidiomycota. The results suggest that repeated cleaning cycles select for microbial groups that strongly attach to the RO membrane surface by producing rigid and adhesive EPS that hampers membrane performance.     

Rhamnolipid as new bio‐agent for cleaning of ultrafiltration membrane fouled by whey

In this work, rhamnolipid biosurfactant as an eco‐friendly and biodegradable cleaning agent was produced by Pseudomonas aeruginosa bacteria and was used to evaluate the chemical cleaning efficiency of whey fouled ultrafiltration membranes. Thin layer chromatography (TLC) and Fourier transform infrared spectroscopy (FTIR) confirmed the successful synthesis of rhamnolipid. The produced rhamnolipid was compared to chemical cleaners including sodium hydroxide (NaOH), sodium dodecyl sulfate (SDS) and Tween 20. Ultrafiltration membranes used for fouling and cleaning analysis were prepared using phase inversion via immersion precipitation technique. For studying the fouling mechanisms, Hermia's model adapted to cross‐flow was used. From the fouling mechanism experiments, it was found that the complete blocking and cake formation were the dominant fouling mechanisms. The highest values of cleaning efficiency were achieved using rhamnolipid and NaOH as cleaning agents with the flux recovery of 100%, but with considering the low concentration of the rhamnolipid used in the cleaning solution compared to NaOH (0.3 versus 4 g/L for NaOH), its application is preferred.     

Mechanistic modeling of the loss of protein sieving due to internal and external fouling of microfilters

Fed‐batch and perfusion cell culture processes used to produce therapeutic proteins can use microfilters for product harvest. In this study, new explicit mathematical models of sieving loss due to internal membrane fouling, external membrane fouling, or a combination of the two were generated. The models accounted for membrane and cake structures and hindered solute transport. Internal membrane fouling was assumed to occur due to the accumulation of foulant on either membrane pore walls (pore‐retention model) or membrane fibers (fiber‐retention model). External cake fouling was assumed to occur either by the growth of a single incompressible cake layer (cake‐growth) or by the accumulation of a number of independent cake layers (cake‐series). The pore‐retention model was combined with either the cake‐series or cake‐growth models to obtain models that describe internal and external fouling occurring either simultaneously or sequentially. The models were tested using well‐documented sieving decline data available in the literature. The sequential pore‐retention followed by cake‐growth model provided a good fit of sieving decline data during beer microfiltration. The cake‐series and cake‐growth models provided good fits of sieving decline data during the microfiltration of a perfusion cell culture. The new models provide insights into the mechanisms of fouling that result in the loss of product sieving. © 2017 American Institute of Chemical Engineers Biotechnol. Prog., 33:1323–1333, 2017