Identification of biofoulant of membrane bioreactors in soluble microbial products

To reveal primary biofoulant in soluble microbial products (SMP) and/or soluble extracellular polymeric substances (EPS), after removal of sludge particles, activated sludge samples were subjected to microfiltration tests in a submerged MBR. Filtration resistance directly correlates with the saccharide concentration. Saccharides in wastewater from several sources contained uronic acids, which increased the filtration resistance. When the microfiltration test liquids contained saccharides over 80 mg l⁻¹, a gelatinous mass remained on the membrane surface after filtration and contained concentrations of saccharides and uronic acids 50 times higher than the original test liquid while only trace amounts of these substances were contained in the filtrate. The gelatinous mass contained high molecular weight substances of 10⁶-10⁸ Da, suggesting the presence of polysaccharides. However, molecules of this size were calculated to be much smaller than the pore size of the membrane. Ethylenediaminetetraacetic acid decreased filtration resistance, suggesting that polysaccharides containing uronic acid units could undergo intermolecular or intramolecular ionic cross-linking by polyvalent cations and form the gel, thus clogging the membrane pores as an actual biofoulant.     

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.     

Growth, maintenance and product formation of autotrophs in activated sludge: taking the nitrite-oxidizing bacteria as an example

The autotrophs in activated sludge play an important role in biological wastewater treatment, especially in the nitrification process. Compared with the heterotrophs in activated sludge, information about the growth, maintenance, and product formation of the autotrophs is still sparse. In this work both experimental and modeling approaches are used to investigate the growth, nitrite inhibition, maintenance, and formation of extracellular polymeric substances (EPS) and soluble microbial products (SMP) of the autotrophs, with nitrite-oxidizing bacteria (NOB) in activated sludge as an example. The unified theory for EPS and SMP is integrated into our model to describe the microbial product formation of the NOB. Extensive experiments were carried out using the NOB-enriched in a sequencing batch reactor for the calibration and validation of the developed model. Results show that the NOB spend a considerable amount of energy on maintenance processes. Their apparent growth yield is estimated to be 0.044 mg COD biomass mg⁻¹ N. The model simulations reveal that the concentrations of EPS and SMP in the NOB-enriched culture initially increase, but later decrease gradually, and that the SMP formed in the nitrite oxidation process are biodegradable.     

Understanding soluble microbial products (SMP) as a component of effluent organic matter (EfOM)

Formation of soluble microbial products (SMP) during biological degradation of organic compounds in a sequencing batch reactor (SBR) was investigated using high performance liquid chromatography--size exclusion chromatography (HPSEC) as well as other organic matter characterization tools. Results showed that carbon compounds in a glucose feed solution were totally transformed to other organic products classified biomass-associated products (BAP). The SMP-BAP contained in the SBR effluent consisted mainly of high-molecular weight (MW) fractions of organic matter, possibly originating from cell lysis. These compounds exhibited a low specific ultraviolet absorbance (SUVA) and a hydrophilic character. In addition, the characteristics of bulk effluent organic matters (EfOM) samples from wastewater treatment facilities were studied. It was observed that EfOM consisted of humic-like and hydrophobic (HPO) compounds, derived from the corresponding drinking water source, in addition to SMP-BAP. A superimposition of SEC chromatograms of the SMP-BAP and humic-like compounds represented a fingerprint of EfOM.     

Spectrometric characterization of effluent organic matter of a sequencing batch reactor operated at three sludge retention times

Effluent organic matter (EfOM) from activated sludge systems is composed primarily of influent refractory compounds, residual degradable substrate, intermediate products and soluble microbial products (SMPs). Depending on operational conditions (hydraulic and sludge retention time (SRT)), the quantity and quality of EfOM significantly changes. The main objective of this research was to quantify and characterize the EfOM of a lab-scale activated sludge sequencing batch reactor (SBR), which was operated at three SRTs and fed glucose, an easily biodegradable substrate. EfOM was followed with two direct-quantification methods (chemical oxygen demand (COD) and dissolved organic carbon (DOC)), three spectrometric methods (ultraviolet absorbance at 254 nm (UVA₂₅₄), excitation-emission matrix (EEM) fluorescence and parallel factor analysis (PARAFAC)) and three organic matter (OM) indices (specific UVA₂₅₄ (SUVA), SUVA-COD, COD/DOC ratio). The significant increment of UVA₂₅₄ and OM indices after treatment indicated an accumulation of refractory high-molecular-weight humic-like compounds in the EfOM, which demonstrated that EfOM was composed mainly by SMPs and not glucose. On the other hand, as the SRT increased, the amount of EfOM decreased, but SUVA, SUVA-COD and fluorescence intensity increased; these trends indicated the accumulation of SMPs of increased molecular weight and aromaticity. Increasing SRT in the SBRs reduced the amount of EfOM, but increased its aromaticity and reactivity. Visual analysis of EfOM EEMs showed two protein- and one humic-like peak, which were attributed to SMPs generated within the SBRs. PARAFAC determined that a two-component model best represented EfOM EEMs. The two-components from PARAFAC were mathematically correlated to the visually identified protein- and humic-like SMPs peaks.     

2-fluorophenol degradation by aerobic granular sludge in a sequencing batch reactor

Aerobic granular sludge is extremely promising for the treatment of effluents containing toxic compounds, and it can economically compete with conventional activated sludge systems. A laboratory scale granular sequencing batch reactor (SBR) was established and operated during 444 days for the treatment of an aqueous stream containing a toxic compound, 2-fluorophenol (2-FP), in successive phases. Initially during ca. 3 months, the SBR was intermittently fed with 0.22 mM of 2-FP added to an acetate containing medium. No biodegradation of the target compound was observed. Bioaugmentation with a specialized bacterial strain able to degrade 2-FP was subsequently performed. The reactor was thereafter continuously fed with 0.22 and 0.44 mM of 2-FP and with 5.9 mM of acetate (used as co-substrate), for 15 months. Full degradation of the compound was reached with a stoichiometric fluoride release. The 2-FP degrading strain was successfully retained by aerobic granules, as shown through the recovering of the strain from the granular sludge at the end of the experiment. Overall, the granular SBR has shown to be robust, exhibiting a high performance after bioaugmentation with the 2-FP degrading strain. This study corroborates the fact that bioaugmentation is often needed in cases where biodegradation of highly recalcitrant compounds is targeted.     

Effect of solids retention time on structure and characteristics of sludge flocs in sequencing batch reactors

The effect of solids retention time (SRT) (4-20 d) on sludge floc structure, size distribution and morphology in laboratory-scale sequencing batch reactors receiving a glucose-based synthetic wastewater was studied using image analysis in a long-term experiment over one year. Floc size distribution (>10 microm) could be characterized by a log-normal model for no bulking situations, but a bi-modal distribution of floc size was observed for modest bulking situations. In each operating cycle of the SBRs, the variation in food /microorganisms ratio (0.03-1.0) had no significant influence on floc size distribution and morphology. The results from a long-term study over one year showed that no clear relationship existed between SRT and median floc size based on frequency. However, sludge flocs at the lower SRTs (4-9 d) were much more irregular and more variable in size with time than those at higher SRTs (16 and 20 d). The level of effluent-suspended solids at lower SRTs was higher than that at higher SRTs.     

Treatment of easily degradable wastewater in a stirred anaerobic sequencing batch biofilm reactor

The main objectives of this study were to evaluate the performance of an anaerobic sequencing batch reactor when subjected to a progressive increase of influent glucose concentration and to estimate the kinetic parameters of glucose degradation. The reactor was initially operated in 8-h cycles, treating glucose in concentrations of 500, 1000 and 2000 mg l(-1). No glucose was detected in the effluent under these three conditions. The reactor showed operating stability when treating a glucose concentration of approximately 500 mg l(-1), with filtered chemical oxygen demand (COD) removal efficiencies varying from 93% to 97%. Operational instability occurred in the operation with glucose concentrations of approximately 1000 and 2000 mg l(-1), caused mainly by a production of extracellular polymeric substances (EPS), which led to hydrodynamic and mass transfer problems in the reactor. The mean volatile acid concentration values in the effluent were approximately 159+/-72 and 374+/-92 mg l(-1), respectively. A first-order model was adjusted to glucose concentration profiles and a modified model, including a residual concentration of substrate, was adjusted to COD temporal profiles. To check the formation of EPS, the reactor was operated in 3-h cycles with concentrations of 500 and 1000 mg l(-1). The purpose of this step was to discover if the production of EPS resulted from the biomass's exposure to a low concentration of substrate over a long period of time. Thus, it was hypothesized that a reduction of the time cycle would also reduce the exposure to low concentrations. However, this hypothesis could not be confirmed because large amounts of EPS were formed already under the first operational condition, using approximately 500 mg l(-1) of glucose in the influent, thus indicating the fallacy of the hypothesis. The production of EPS proved to depend on the organic volumetric load applied to the reactor.     

Halophilic biological treatment of tannery soak liquor in a sequencing batch reactor

Hypersaline wastewater (i.e. wastewater containing more than 35 gl(-1) total dissolved solids (TDS)) is generated by various industrial activities. This wastewater, rich in both organic matter and TDS, is difficult to treat using conventional biological wastewater treatment processes. Among the industries generating hypersaline effluents, tanneries are prominent in India. In this study, tannery wastewater from soak pit was treated in a lab-scale SBR for the removal of organic matter. The characterisation of the soak liquor showed that this effluent is biodegradable, though not easily, and highly variable, depending on the origin and the nature of the hides. TDS was in the range of 21-57 gl(-1) and COD was in the range of 1.5-3.6 gl(-1). This soak liquor was biologically treated in an aerobic sequencing batch reactor seeded with halophilic bacteria, and the performance of the system was evaluated under different operating conditions with changes in hydraulic retention time, organic loading rate and salt concentration. The changes in salinity appeared to affect the removal of organic matter more than the changes in hydraulic retention time or organic loading rate. Despite the variations in the characteristics of the soak liquor, the reactor achieved proper removal of organic matter, once the acclimation of the microorganisms was achieved. Optimum removal efficiencies of 95%, 93%, 96% and 92% on COD, PO4 3-, TKN and SS, respectively, could be reached with 5 days hydraulic retention time (HRT), an organic loading rate (OLR) of 0.6 kg COD m(-3)d(-1) and 34 g NaCl l(-1). The organisms responsible for nitrogen removal appeared to be the most sensitive to the modifications of these parameters.     

Evaluation on the impacts of predators on biomass components and oxygen uptake in sequencing batch reactor and continuous systems

An expanded unified model for the biomass fractions, soluble-organic fractions, and oxygen-uptake rates considering extracellular polymeric substances (EPS), intracellular storage products (X_STO), and predators for activated sludge is used to study the impacts of predators on biomass components and oxygen uptake. The new model is applied to evaluate how predation affects the oxygen-uptake rate (OUR) and the different forms of biomass: active bacteria (X_H), X_EPS, and X_STO, under dynamic feast-and-famine and continuous conditions. For the dynamic conditions of a sequencing batch reactor (SBR), eliminating predators from the model increases X_H and X_EPS fractions significantly, and this causes the substantial increases in OUR and MLVSS once the famine period begins. An analysis of how the OUR is distributed among the several respiration processes shows that the predation of X_H is the most significant oxygen utilization rate process in the system under famine conditions of an SBR. Application of the model to simulate the long-term operation of an SBR indicates that predators reach their maximum fraction in the MLVSS (∼4% of MLVSS) at a solids retention time of about 13 days, but they are washed out at a solids retention time less than ∼3 days. Simulation for a continuous system indicates that predators take more time (about 800 h) to reach steady state and reach their maximum fraction (∼5.5%) at an SRT of ∼14 days. Comparison of SBR and continuous systems reveals that the predators have greater impact in the continuous system because the permanent near-famine condition accentuates predation processes.     

Characterization of the size-fractionated biomacromolecules: tracking their role and fate in a membrane bioreactor

This article presents a study aimed at the fractionation and characterization of what is thought to be one of the most complex organic mixtures produced by activated sludge: biomacromolecules (BMM). Photometric quantification combined with excitation-emission matrix (EEM) fluorescence spectroscopy and nuclear magnetic resonance (NMR) measurements were used to characterize BMM in a membrane bioreactor (MBR) from a chemical perspective. Overall, the BMM in sludge supernatant were mainly present in three fractions: colloidal BMM (BMM_c, >0.45 μm), biopolymeric BMM (BMM_b, 0.45 μm-100 kDa) and low molecular weight (MW) fraction (<5 kDa). The analysis of fluorescence regional integration (FRI) showed that the organics in membrane permeate and those in the low-MW fraction of sludge supernatant were of similar chemical composition. The characterization by NMR suggested that the BMM_c fraction had similar carbon content of proteins and polysaccharides. In contrast, the BMM_b and the low-MW BMM were proved to be carbonaceous and aromatics, respectively. Moreover, because of the high MW and gelling property, polysaccharides were found to have a high potential to accumulate on the membranes. In addition, the lipids present in the BMM_b of the sludge supernatant were demonstrated to be another important foulant due to their large size. Our results also indicated that aromatic proteins had a higher fouling propensity than tryptophan proteins though they were of similar size nature. This work could be useful for better understanding of the chemical nature of BMMs in MBRs.     

Application of multiway ICA for on-line process monitoring of a sequencing batch reactor

Multiway principal component analysis has been shown to be a powerful monitoring tool in many industrial batch processes. However, it has the shortcomings that all batch lengths should be equal, the measurement variables must be normally distributed and that future values of the current batch must be estimated to allow on-line monitoring. In this work, it is shown that multiway independent component analysis (MICA) can be used to overcome these drawbacks and obtain better monitoring performance. The on-line MICA monitoring of batch processes is based on a new unfolding method and independent component analysis (ICA). ICA provides better monitoring performance than PCA in cases with non-Gaussian data because it is not based on the assumption that the latent variables are normally distributed. The MICA algorithm does not require any estimation of future batch values and can also be applied to non-equal batch length data sets. This article describes the application of on-line MICA monitoring of a sequencing batch reactor (SBR). It is successfully applied to an 80L SBR for biological wastewater treatment, which is characterized by a variety of disturbance sources with non-Gaussian characteristics. The SBR poses an interesting challenge from the point of process monitoring characterized by non-stationary, batchwise, multiscale, and non-Gaussian characteristics. The results of the bench-scale SBR monitoring clearly showed the power and advantages of MICA monitoring in comparison to conventional monitoring methods.     

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.     

Recent advances in membrane bioreactors (MBRs): Membrane fouling and membrane material

Membrane bioreactors (MBRs) have been actively employed for municipal and industrial wastewater treatments. So far, membrane fouling and the high cost of membranes are main obstacles for wider application of MBRs. Over the past few years, considerable investigations have been performed to understand MBR fouling in detail and to develop high-flux or low-cost membranes. This review attempted to address the recent and current developments in MBRs on the basis of reported literature in order to provide more detailed information about MBRs. In this paper, the fouling behaviour, fouling factors and fouling control strategies were discussed. Recent developments in membrane materials including low-cost filters, membrane modification and dynamic membranes were also reviewed. Lastly, the future trends in membrane fouling research and membrane material development in the coming years were addressed.     

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.     

Quantification of the shear stresses in a microbial granular sludge reactor

Since a certain level of hydrodynamic shear force is needed in the formation of microbial granules for wastewater treatment, a method for quantifying the shear stresses in a microbial granular sludge reactor is highly desirable. In this work a novel energy-dissipation-based model was established and validated to quantitatively describe the shear stresses in a granular sludge sequencing batch reactor (SBR). With this model, the shear stress at the solid–liquid interface in an SBR was estimated and the relative magnitudes of shear stresses induced by fluid, gas bubble and collision on granules were evaluated. The results demonstrate that the effect of reactor geometry on the global shear stress was significant. Both the shear stress at the microbial granule surface and the biomass-loss rate increased with an increase in biomass concentration in the SBR. The gas bubble and the collision were found to be the main source for the shear stress at the granule surface.     

Roles of extracellular polymeric substances (EPS) in the migration and removal of sulfamethazine in activated sludge system

The occurrences, transformation of antibiotics in biological wastewater treatment plants have attracted increasing interests. However, roles of extracellular polymeric substances (EPS) of activated sludge on the fate of antibiotics are not clear. In this study, the roles of EPS in the migration and removal of one typical antibiotic, sulfamethazine (SMZ), in activated sludge process were investigated. The interaction between EPS and SMZ was explored through a combined use of fluorescence spectral analysis, laser light scattering and microcalorimetry techniques. Results show that SMZ interacted with the proteins in EPS mainly with a binding constant of 1.91 × 10⁵ L/mol. The binding process proceeded spontaneously, and the driving force was mainly from the hydrophobic interaction. After binding, the structure of EPS was expanded and became loose, which favored the mass transfer and pollution capture. The removal of SMZ was influenced by interaction with EPS. SMZ could be effectively adsorbed on EPS, which accounted for up to 61.8% of total SMZ adsorbed by sludge at the initial adsorption stage and declined to around 35.3% at the subsequent biodegradation stage. The enrichment of SMZ by EPS was beneficial for SMZ removal and acquisition by microbes at the subsequent biodegradation stage.     

Coupling of sequencing batch reactor and mesh filtration: operational parameters and wastewater treatment performance

Wastewater treatment performance of the combined process of sequencing batch reactor (SBR) and mesh filtration bio-reactor was investigated with a synthetic wastewater. In this system, the filtration was performed only by the water level difference between the reactor and the effluent port, with the help of a sludge layer which accumulated on the mesh filter.

A half volume of the mixed liquor was filtrated for ca. 1 h, and the filtration time was not affected by the initial pressure within the range of 0.5–2.0 m-H₂O. Since the mesh filter could effectively reject the biomasses in the reactor, the effluents contained SS of less than 1 mg/L and BOD of less than 10 mg/L under continuous or intermittent aeration conditions. Nitrogen was also removed effectively with the adjustment of aeration time under the intermittent aeration conditions.

The results obtained in this work indicate that mesh filtration could be effectively combined with SBR and improve the performance of SBR.

In addition, it was shown that the performance of the mesh filtration such as filtration time and solids separation was influenced significantly by the saccharide content in the exocellular polymer of the activated sludge.     

Simultaneous nitrification and p-cresol oxidation in a nitrifying sequencing batch reactor

The tolerance, kinetic behavior and oxidizing ability of a nitrifying sludge exposed to different initial concentrations of p-cresol (25-150mg/l) were evaluated in a sequencing batch reactor (SBR) fed with 200mg NH(4)(+)-N/ld. The nitrifying SBR operated up to 300mg/ld of p-cresol, achieving simultaneously the complete ammonium oxidation to nitrate and the total consumption of p-cresol and its transitory intermediates from the culture. p-Cresol induced a significant decrease in the values for specific rates of ammonium consumption, showing that the ammonium oxidation pathway was mainly inhibited. After 7 months of operation in SBR, the specific rates of NH(4)(+)-N oxidation, NO(3)(-)-N formation, and total organic carbon consumption were 0.6g NH(4)(+)-N/g microbial protein-Nh, 0.3g NO(3)(-)-N/g microbial protein-Nh, and 0.24g total organic carbon/g microbial protein h, respectively. The microbial growth rate was always low (maximum value of 12.2+/-0.4mg protein-N/ld) and settleability of the sludge was good with sludge volume index values lower than 21ml/g. The oxidation of p-cresol and its intermediates was carried out faster throughout the cycles and nitrification inhibition decreased with the number of cycles.     

Influence of mixed liquor properties and aeration intensity on membrane fouling in a submerged membrane bioreactor at high mixed liquor suspended solids concentrations

This paper presents the results of 195 days of pilot-scale submerged membrane bioreactor (SMBR) experiments on settled municipal wastewater. Short-term and long-term thickening experiments were performed at a constant membrane flux of 30L/(m(2)h) to determine the impact of the following mixed liquor properties: colloidal material, soluble COD, soluble microbial products, extracellular polymeric substances, and viscosity along with aeration intensity on membrane fouling at high mixed liquor suspended solids (MLSS) concentrations. The normalized permeability declined with increasing MLSS concentrations in all experiments and increasing the coarse bubble aeration intensity increased the permeability at a given MLSS concentration. Using a dynamic approach, this work demonstrates the importance of mixed liquor viscosity, which impacts the efficacy of the coarse bubble aeration, in sustaining membrane permeability. Over an extended thickening time period, a small increase in MLSS concentration and mixed liquor viscosity becomes more prevalent and leads to greater permeability decline at a given MLSS concentration.