Influence of shear on the production of extracellular polymeric substances in membrane bioreactors

Shear is used to control fouling in membrane bioreactor (MBR) systems. However, shear also influences the physicochemical and biological properties of MBR biomass. The current study examines the relationship between the level of shear and extracellular polymeric substance (EPS) production in MBRs. Two identical MBRs were operated in parallel where the biomass in one reactor was exposed to seven times greater shear forces. The concentrations of floc-associated and soluble EPS were monitored for the duration of the experiment. The stickiness of extracted floc-associated EPS from each reactor was also characterized using atomic force microscopy. A mathematical model of floc-associated and soluble EPS production was applied to quantitatively describe changes in EPS production with shear. Biomass grown in a high shear environment has lower floc-associated EPS production compared to biomass grown in a lower shear environment. This decrease in floc-associated EPS production also corresponds to a decrease in soluble EPS production, which can be explained by both the lower concentration of floc-associated EPS and the production of stickier floc-associated EPS that is more erosion resistant in the high shear reactor. This research suggests that mechanical stresses can have a significant impact on the production rates of floc-associated and soluble EPS—key parameters governing membrane fouling in MBRs.     

The influence of aeration intensity on predation and EPS production in membrane bioreactors

Shear, in the form of vigorous aeration, is used to control fouling in membrane bioreactor (MBR) systems. However, shear also influences the physicochemical and biological properties of MBR biomass. The current study examines the relationship between the aeration intensity and extracellular polymeric substance (EPS) production in MBRs. Two identical submerged MBRs were operated in parallel but the aeration rate was three times greater in one of the MBRs. The concentrations of floc-associated and soluble EPS were monitored for the duration of the experiment. Microscopic images and floc-size measurements were also collected regularly. The membrane fouling potential of the biomass was quantified using the flux-step method. Increased aeration did not have a direct effect on soluble or floc-associated EPS production in the microfiltration MBRs. However, aeration intensity had a significant effect on predatory organisms. Large aquatic earthworms, Aeolosoma hemprichi, proliferated under lower shear conditions but were never observed in the high shear reactor. Predation by A. hemprichi resulted in increased floc-associated and soluble EPS production. Thus, the mixing conditions in the low shear MBR indirectly resulted in increased soluble EPS concentrations and higher fouling potential. This research suggests that predation can have a significant impact on the production rates of floc-associated and soluble EPS - key parameters driving membrane fouling in MBRs.     

Modelling the production and degradation of soluble microbial products (SMP) in membrane bioreactors (MBR)

MBR biochemical conditions have an effect on membrane fouling and SMP have been attributed to be the main MBR foulant. Thus, predicting the SMP concentration is essential for understanding and controlling MBR fouling. However, existing SMP models are mostly too complex and over-parameterized, resulting in inadequate or absent parameter estimation and validation. This study extends the existing activated sludge model No. 2d (ASM2d) to ASM2dSMP with introduction of only 4 additional SMP-related parameters. Dynamic batch experimental results were used for SMP parameter estimation leading to reasonable parameter confidence intervals. Finally, the ASM2dSMP model was used to predict the impact of operational parameters on SMP concentration. It would found that solid retention time (SRT) is the key parameter controlling the SMP concentration. A lower SRT increased the utilization associated products (UAP) concentration, but decreased the biomass associated products (BAP) concentration and vice versa. A SRT resulting in minimum total SMP concentration can be predicted, and is found to be a relatively low value in the MBR. If MBRs operate under dynamic conditions and biological nutrient removal is required, a moderate SRT condition should be applied.     

A unified theory for extracellular polymeric substances,soluble microbial products,and active and inert biomass

We present a critical review of the relationships among three microbial products: extracellular polymeric substances (EPS), soluble microbial products (SMP), and inert biomass. Up to now, two different "schools" of researchers have treated these products separately. The "EPS school" has considered active biomass and EPS, while the "SMP school" has considered active biomass, SMP, and inert biomass. Here, we provide a critical review of each of the microbial products. Then, we develop a unified theory that couples them and reconciles apparent contradictions. In our unified theory, cells use electrons from the electron-donor substrate to build active biomass, and they also produce bound EPS and utilization-associated products (UAP) at the same time and in proportion to substrate utilization. Bound EPS are hydrolyzed to biomass-associated products (BAP), while active biomass undergoes endogenous decay to form residual dead cells. Finally, UAP and BAP, being biodegradable, are utilized by active biomass as recycled electron-donors substrates. Our unified theory shows that the apparently distinct products from the SMP and EPS schools overlap each other. Soluble EPS is actually SMP, or the sum of UAP and BAP. Furthermore, active biomass, as defined by the SMP school, includes bound EPS, while inert biomass includes bound EPS and the residual dead cells.     

Non-steady state modeling of extracellular polymeric substances,soluble microbial products,and active and inert biomass

We present a modeling approach that quantifies the unified theory presented in the companion paper. In this approach, we use mathematical modeling to quantify the relationships among three solid species--bacteria, extracellular polymeric substances (EPS), and inert residual biomass-two soluble microbial products (SMP), original substrate, and an electron acceptor. According to the model, donor electrons are used for the synthesis of biomass, EPS, and utilization-associated products. Residual inert biomass and biomass-associated products are produced from the decay of active biomass and the hydrolysis of EPS, respectively. The model includes mass balance equations that consistently describe the flow of electrons among the components. It is solved with a set of parameters appropriate to the experimental study of Hsieh et al. (Biotech. Bioeng. 44 (1994) 219). Model outputs capture all trends observed in steady-state CSTR experiments and transient batch experiments. This agreement supports that the unified theory correctly captures the interconnections among SMP, EPS, and active and inert biomass.     

Characteristics and fates of soluble microbial products in ceramic membrane bioreactor at various sludge retention times

The formation and fate of soluble microbial products (SMP) in membrane bioreactor (MBR) was investigated at various sludge retention times (SRT) for 170 days. The SMP concentration was estimated by feeding glucose, which could be completely degraded, and by measuring the dissolved organic carbon (DOC) of the effluent from MBR. Under the conditions of SRT of 20 days, influent DOC of 112 mg/l and HRT of 6 h, the produced SMP was 4.7 mg DOC/l of which 57% was removed or retained by the membrane. DOC of MBR supernatant increased during 100 days and then gradually decreased. Specific UV absorbance showed that the accumulated compounds had a portion of larger, more aromatic, more hydrophobic and double-bond-rich organics, which originated from the decayed biomass. Molecular weight distributions of SMP in MBR supernatant showed that the acclimated microorganisms in a long SRT could decompose high molecular weight organics, it caused the shift of molecular weight distributions of SMP to a lower range. During the operation period, enumeration of active cells in the MBR showed that microbial inhibitions by accumulated SMP was not observed.     

Characterization of dissolved organic matter in a submerged membrane bioreactor by using three-dimensional excitation and emission matrix fluorescence spectroscopy

Three-dimensional excitation-emission matrix (EEM) fluorescence spectroscopy was employed to characterize dissolved organic matter (DOM) in a submerged membrane bioreactor (MBR). Three fluorescence peaks could be identified from the EEM fluorescence spectra of the DOM samples in the MBR. Two peaks were associated with the protein-like fluorophores, and the third was related to the visible humic acid-like fluorophores. Only two main peaks were observed in the EEM fluorescence spectra of the extracellular polymeric substance (EPS) samples, which were due to the fluorescence of protein-like and humic acid-like matters, respectively. However, the EEM fluorescence spectra of membrane foulants were observed to have three peaks. It was also found that the dominant fluorescence substances in membrane foulants were protein-like substances, which might be due to the retention of proteins in the DOM and/or EPS in the MBR by the fine pores of the membrane. Quantitative analysis of the fluorescence spectra including peak locations, fluorescence intensity, and different peak intensity ratios and the fluorescence regional integration (FRI) analysis were also carried out in order to better understand the similarities and differences among the EEM spectra of the DOM, EPS, and membrane foulant samples and to further provide an insight into membrane fouling caused by the fluorescence substances in the DOM in submerged MBRs.     

Soluble microbial products formation in anaerobic chemostats in the presence of toxic compounds

Anaerobic chemostats fed on glucose (approximately 10 g chemical oxygen demand (COD)/L) were used to investigate the effects of toxicity on soluble microbial product (SMP) formation. Addition of the toxic compounds chloroform and chromium increased the net accumulation of SMP, despite reducing the percentage of SMP in the effluent due to the overwhelming production of volatile fatty acids (VFAs). In the reactor spiked with chloroform the normalized accumulation of SMP (SMP/So) increased from 2% to 8%, whereas in the reactor spiked with Cr (VI) the SMP/So ratio reached as high as 20% after the spike, and in both cases SMP net accumulation was proportional to the concentration of toxicant. After the chloroform and chromium spikes biomass seemed to produce more extra cellular polymeric substances (EPS) suggesting that this might have helped them to cope with the stress. Chromatography results indicate that some of the high MW compounds present in the SMP might have been due to EPS release into the bulk solution, and that other compounds, probably released as a result of cell lysis, were also present. Hydrolysis of EPS did not seem to contribute to SMP accumulation in the presence of toxic compounds, and DNA analysis suggested that cell lysis products was an important contribution to SMP accumulation, in the presence of chromium.     

Soluble microbial products in membrane bioreactor operation: Behaviors, characteristics, and fouling potential

This paper presents an experimental study on soluble microbial products (SMP) in membrane bioreactor (MBR) operation at different sludge retention times (SRTs). A laboratory-scale MBR was operated at SRT of 10, 20, and 40 days for treatment of readily biodegradable synthetic wastewater. The accumulation, composition, characteristics, and fouling potential of SMP at each SRT were examined. It was found that accumulation of SMP in the MBR became more pronounced at short SRTs. Carbohydrates and proteins appeared to be the components of SMP prone to accumulate in the MBR compared with aromatic compounds. The proportions of SMP with large molecular weight in supernatants and in effluents were almost identical, implying that membrane sieving did not work for most SMP. In addition, the majority of SMP was found to be composed of hydrophobic components, whose proportion in total SMP gradually increased as SRT lengthened. However, fouling potentials of SMP were relatively low at long SRTs. The hydrophilic neutrals (e.g., carbohydrates) were most likely the main foulants responsible for high fouling potentials of SMP observed at short SRTs.     

The effect of calcium on the membrane biofouling in the membrane bioreactor (MBR)

A continuous bench-scale submerged membrane bioreactor (BSMBR) was operated in two modes; low (LC) and optimum calcium (OC) concentrations, to investigate the effect of calcium on membrane biofouling. Both the cake layer and pore blocking resistances were reduced in the operation under OC conditions. In order to find the causes of fouling, molecular weight fractionations of the extracellular polymeric substances (EPS) and soluble microbial products (SMP) were conducted, and the relative hydrophobicity of the sludge, EPS and SMP were also measured. The reduction in the cake layer resistance was assumed to be due to the decrease of filamentous bacteria, and better flocculation caused by the calcium bridges and the increased hydrophobicity of EPS in the operation under OC conditions. It was suggested that the pore blocking resistance was reduced due to the low SMP rejection of the membrane and the fewer hydrophobic reactions of the SMP in the operation under OC conditions. Fourier-transform infrared analysis of the fouled membrane surface supported that proteins and carbohydrates were the main foulants during the BSMBR operation.     

Accumulation of biopolymer clusters in a submerged membrane bioreactor and its effect on membrane fouling

A laboratory-scale submerged membrane bioreactor (SMBR) with a hollow-fibre membrane module was used to investigate membrane fouling under various operational conditions. The results showed that the sludge supernatant inside the SMBR had a consistently higher organic content than the MBR effluent. Detailed analysis revealed a pool of organic substances, classified as biopolymer clusters (BPC), in the SMBR system that were larger in size than the soluble microbial products (SMP). The BPC content in the MBR sludge mixture ranged from 0.7 to 18.8 mg/L in terms of the total organic carbon (TOC), with an average of 5.6+/-3.5mg/L, which was about twice the SMP concentration in the suspension. Under a fluorescent microscope and using DAPI staining, the BPC in the sludge supernatant after centrifugation were found to be particulate organic substances that were independent of the sludge flocs and had a size distribution up to 50 microm. The findings of the experiment suggest that BPC are an important foulant and have a profound effect on membrane fouling. The fouling rate in the reactor, as indicated by the increase in trans-membrane pressure (TMP), correlates to a certain extent with the BPC concentration in the sludge suspension under various conditions. It is argued that BPC are a special form of organic substances that are formed by the adsorption and affinity clustering of mainly SMP within the sludge layer deposited on the membrane surface. The BPC can be detached with the sludge from the membrane and returned to the suspension. The accumulation of BPC in the SMBR sludge mixture would facilitate the formation of a sludge fouling layer on the membrane surface, thus causing a serious fouling problem.     

Influence of organic and inorganic flocculants on physical-chemical properties of biomass and membrane-fouling rate

The effects of addition of six types of flocculants (aluminium sulfate, ferric chloride, polyaluminium chloride, polymeric ferric sulfate, Chitosan, polyacrylamide) on mitigation of membrane fouling in membrane bioreactors (MBR) were investigated respectively. The biomasses in various MBRs were characterized by morphological properties (mean floc size (d_p), fractal dimension (df)), physical parameters (surface charge, relative hydrophobicity (RH), dynamic viscosity) and the biochemical components of the mixed liquor (extracellular polymeric substances (EPS), soluble microbial products (SMP)). Statistical methods such as normalization, nondimensionalization and multiple linear regressions were used to identify the dominant membrane-fouling contributors and to simulate membrane-fouling rates. The results demonstrated that addition of flocculants had significant impact on sustainable filtration time and the key factors affecting membrane fouling varied in different flocculants added MBRs. For the organic flocculants added MBRs, membrane-fouling alleviation was mainly due to the decrease in SMP and df as well as the increase in d_p. For the inorganic flocculants added MBRs, the lower fouling rate could be mainly attributed to the decrease in SMP and surface charge as well as the increase in RH. For each type of flocculants, the empirical equations of sustainable filtration time (Γ₄₅) were simulated to predict membrane-fouling rates in different MBRs.     

Relation between EPS adherence, viscoelastic properties, and MBR operation: Biofouling study with QCM-D

Membrane fouling is one of the main constraints of the wide use of membrane bioreactor (MBR) technology. The biomass in MBR systems includes extracellular polymeric substances (EPS), metabolic products of active microbial secretion that adversely affect the membrane performance. Solids retention time (SRT) in the MBR is one of the most important parameters affecting membrane fouling in MBR systems, where fouling is minimized at optimal SRT. Among the operating parameters in MBR systems, SRT is known to strongly influence the ratio of proteins to polysaccharides in the EPS matrix. In this study, we have direct evidence for changes in EPS adherence and viscoelastic properties due to changes in the sludge removal rate that strongly correlate with the membrane fouling rate and EPS composition. EPS were extracted from a UF membrane in a hybrid growth MBR operated at sludge removal rates of 59, 35.4, 17.7, and 5.9 L day^-1 (corresponding SRT of 3, 5, 10, and 30 days, respectively). The EPS adherence and adsorption kinetics were carried out in a quartz crystal microbalance with dissipation monitoring (QCM-D) technology in several adsorption measurements to a gold sensor coated with Polyvinylidene Fluoride (PVDF). EPS adsorption to the sensor surface is characterized by a decrease of the oscillation frequency and an increase in the dissipation energy of the sensor during parallel flow of aqueous media, supplemented with EPS, above the sensor surface. The results from these experiments were further modeled using the Voigt based model, in which the thickness, shear modulus, and shear viscosity values of the adsorbed EPS layers on the PVDF crystal were calculated. The observations in the QCM-D suggested that the elevated fouling of the UF membrane is due to higher adherence of the EPS as well as reduction in viscosity and elasticity of the EPS adsorbed layer and elevation of the EPS fluidity. These results corroborate with confocal laser scanning microscopy (CLSM) image analysis showing thicker EPS in close proximity to the membrane surface operated at reactor conditions which induced more fouling at elevated sludge removal rates.     

Biomass effects on oxygen transfer in membrane bioreactors

Ten biomass samples from both municipal and industrial pilot and full scale submerged membrane bioreactors (MBRs) with mixed liquor suspended solids concentrations (MLSS) ranging from 7.2 to 30.2g L(-1) were studied at six air-flow rates (0.7, 1.3, 2.3, 3, 4.4 and 6m(3)m(-3)h(-1)). Statistical analyses were applied to identify the relative impacts of the various bulk biomass characteristics on oxygen transfer. Of the biomass characteristics studied, only solids concentration (correlated with viscosity), the carbohydrate fraction of the EPS (EPS(c)) and the chemical oxygen demand (COD) concentration of the SMP (SMP(COD)) were found to affect the oxygen transfer parameters k(L)a(20) (the oxygen transfer coefficient) and alpha-factor. The relative influence on k(L)a(20) was MLSS>aeration>EPS(c)>SMP(COD) and on alpha-factor was MLSS>SMP(COD)>EPS(c)>aeration. Both k(L)a(20) and alpha-factor increased with increasing aeration and EPS(c) and decreased with increasing MLSS and SMP(COD). MLSS was found to be the main parameter controlling the oxygen transfer.     

Impact of ambient conditions on SMP elimination and rejection in MBRs

The widespread application of the membrane-assisted activated sludge process is restricted by membrane fouling, which increases investment and operating costs. Soluble microbial products (SMPs) are currently considered as the major cause of membrane fouling in membrane bioreactors (MBRs). This study aims at elucidating and quantifying the effects of varying environmental conditions on SMP elimination and rejection based on findings in a pilot MBR and in well-defined lab trials. Several factors are thought to influence the concentration of SMP and their fouling propensity in one way or the other, but findings are often inconsistent or even contradictory. Here, SMP loading rate was found to have the greatest effect on SMP elimination and thus on concentration in the MBR. The degree of elimination decreased at very low DO and low nitrate concentrations. On average, 75% of influent SMP were eliminated in both pilot and lab trials, with the elimination of polysaccharides (PS) mostly above 80%. Rejection of SMP components by the used membrane (PAN, 37nm) ranged mainly from 20% to 70% for proteins and from 75% to 100% for PS. Especially protein rejection decreased at higher temperatures and higher nitrification activity. The increased fouling rates at lower temperatures might therefore partly be explained by this increased rejection. Apparently, mainly the nitrite-oxidising community is responsible for the formation for smaller SMP molecules that can pass the membrane.     

Activated sludge model (ASM) based modelling of membrane bioreactor (MBR) processes: A critical review with special regard to MBR specificities

Membrane bioreactors (MBRs) have been increasingly employed for municipal and industrial wastewater treatment in the last decade. The efforts for modelling of such wastewater treatment systems have always targeted either the biological processes (treatment quality target) as well as the various aspects of engineering (cost effective design and operation). The development of Activated Sludge Models (ASM) was an important evolution in the modelling of Conventional Activated Sludge (CAS) processes and their use is now very well established. However, although they were initially developed to describe CAS processes, they have simply been transferred and applied to MBR processes. Recent studies on MBR biological processes have reported several crucial specificities: medium to very high sludge retention times, high mixed liquor concentration, accumulation of soluble microbial products (SMP) rejected by the membrane filtration step, and high aeration rates for scouring purposes. These aspects raise the question as to what extent the ASM framework is applicable to MBR processes. Several studies highlighting some of the aforementioned issues are scattered through the literature. Hence, through a concise and structured overview of the past developments and current state-of-the-art in biological modelling of MBR, this review explores ASM-based modelling applied to MBR processes. The work aims to synthesize previous studies and differentiates between unmodified and modified applications of ASM to MBR. Particular emphasis is placed on influent fractionation, biokinetics, and soluble microbial products (SMPs)/exo-polymeric substances (EPS) modelling, and suggestions are put forward as to good modelling practice with regard to MBR modelling both for end-users and academia. A last section highlights shortcomings and future needs for improved biological modelling of MBR processes.     

MBR、MCR处理微污染水的膜污染比较

膜污染是影响膜反应器稳定运行的重要原因之一，为此考察了膜生物反应器(MBR)和膜混凝反应器(MCR)处理微污染地表水时的运行状况，并对膜比通量的变化进行了比较，发现MBR的膜污染情况比MCR的严重。MCR和MBR的膜组件经物理、化学清洗后膜比通量分别恢复至新膜比通量的99．7％和76．9％，物理清洗对此的贡献较大。经分析发现，MCR中无机污染占优势，主要污染元素是Fe；MBR中微生物和有机物是膜污染的主要组成，而无机污染物则主要是铁盐和磷酸盐。     

Chelating properties and molecular weight distribution of soluble microbial products from an aerobic membrane bioreactor

This paper presents a detailed study on soluble microbial products (SMPs) in an aerobic membrane bioreactor (MBR) treating synthetic wastewater simulating municipal wastewater. The concentration of SMP in the reactor conformed to a cyclical pattern of accumulation and reduction in relation to SRT. The molecular weight (MW) distribution of accumulated SMP was determined to vary from <1kD to >100kD. Copper chelating properties of various SMP fractions in the MBR were compared before and after copper addition to the feed. The conditional stability constant (LogcK), complexation capacity (Cc), and SMP-ligand concentration (CL) were evaluated to determine the impact of copper on the chelating properties. The results indicated that accumulated SMP in the aerobic MBRs without copper addition are moderate chelators with LogcK values of 7.6-8.3 mol(-1) for the moderate ligands and 6.3-6.8 mol(-1) for the relatively weaker ligands. SMPs with MW of 1-10 kD were found to have the highest complexation capacity among all SMP fractions. The complexation capacity of accumulated SMP after feeding copper was 0.11 micromol/mg of SMP, almost half of its value prior to feeding copper. The reduction of C(c) after feeding copper was a result of an increase in large molecular weight SMP (>100 kD).     

Correlation of EPS content in activated sludge at different sludge retention times with membrane fouling phenomena

In this study, activated sludge characteristics were studied with regard to membrane fouling in membrane bioreactors (MBRs) for two pilot plants and one full-scale plant treating municipal wastewater. For the full-scale MBR, concentrations of extracellular polymeric substances (EPS) bound to sludge flocs were shown to have seasonal variations from as low as 17mgg(-1) dry matter (DM) in summer up to 51mg(gDM)(-1) in winter, which correlated with an increased occurrence of filamentous bacteria in the colder season. Therefore, it was investigated at pilot-scale MBRs with different sludge retention times (SRTs) whether different EPS contents and corresponding sludge properties influence membrane fouling. Activated sludge from the pilot MBR with low SRT (23d) was found to have worse filterability, settleability and dewaterability. Photometric analysis of EPS extracts as well as LC-OCD measurements showed that it contained significantly higher concentrations of floc-bound EPS than sludge at higher SRT (40d) The formation of fouling layers on the membranes, characterised by SEM-EDX as well as photometric analysis of EPS extracts, was more distinct at lower SRT where concentrations of deposited EPS were 40-fold higher for proteins and 5-fold higher for carbohydrates compared with the membrane at higher SRT. Floc-bound EPS and metals were suggested to play a role in the fouling process at the full-scale MBR and this was confirmed by the pilot-scale study. However, despite the different sludge properties, the permeability of membranes was found to be similar.     

Changes in characteristics of soluble microbial products in membrane bioreactors associated with different solid retention times: Relation to membrane fouling

A membrane bioreactor (MBR) is a promising wastewater treatment technology, but there is a need for efficient control of membrane fouling, which increases operational and maintenance costs. Soluble microbial products (SMP) have been reported to act as major foulants in the operation of MBRs used for wastewater treatment. In this study, SMP in MBRs operated with different sludge retention times (SRTs) were investigated by means of various analytical techniques and their relations to the evolution of membrane fouling were considered. Bench-scale filtration experiments were carried out in a laboratory with synthetic wastewater to eliminate fluctuations that would occur with the use of real wastewater and that would lead to fluctuations in compositions of SMP. Three identical submerged MBRs were operated for about 50 days under the same conditions except for SRT (17, 51 and 102 days). Accumulation of SMP in the MBRs estimated by conventional analytical methods (i.e., the phenol-sulfuric acid method and the Lowry method) was significant in the cases of short SRTs. However, the degrees of membrane fouling in the MBRs were not directly related to the concentrations of SMP in the reactors estimated by the conventional analytical methods. Non-conventional analytical methods such as excitation-emission matrix (EEM) fluorescence spectroscopy revealed that characteristics of SMP in the three reactors considerably differed depending on SRT. Foulants were extracted from the fouled membranes at the end of the operation and were compared with SMP in each MBR. It was clearly shown that characteristics of the foulants were different depending on SRT, and similarities between SMP and the extracted foulants were recognized in each MBR on the basis of results of EEM measurements. However, such similarities were not found on the basis of results obtained by using the conventional methods for analysis of SMP. The results of this study suggest that the use of conventional methods for analysis of SMP is not appropriate for investigation of membrane fouling in MBRs.