Occurrence and composition of extracellular lipids and polysaccharides in a full-scale membrane bioreactor

Impacts of bio-carriers on the characteristics of cake layer and membrane fouling in a novel hybrid membrane bioreactor for treating mariculture wastewater

Membrane fouling is generally considered as a major bottleneck to the wide application of membrane bioreactor (MBR) for high saline mariculture wastewater treatment. Though numerous researches have investigated the membrane fouling of MBR combined with bio-carriers, few studies reveal the impacts of bio-carriers on the characteristics of cake layer and the mechanism of bio-carriers alleviating membrane fouling. In this study, two systems, namely carriers-enhanced MBR (R1) and conventional MBR (R2) were parallel operated, drawing a conclusion that bio-carriers effectively improved the characteristics of cake layer, thus mitigating membrane fouling. Fluorescence excitation emission matrix (EEM) analysis indicated that bio-carriers reduced the adhesion of proteins and humic acid-like materials on membrane surface. Molecular weight (Mw) distribution suggested that soluble microbial products (SMP) with small Mw (6-20 kDa) and biopolymers in extracellular polymeric substances (EPS) (50-300 kDa) was easier to accumulate on membrane surface in R2. The above results indicated that the presence of bio-carriers could effectively reduce the attachment of these organics on membrane surface, contributing to a larger porosity of cake layer and thus mitigating membrane fouling. Meanwhile, gas chromatography-mass spectrometry (GC-MS) clarified that more components were present in R2 than R1. Moreover, the majority of compounds in the SMP were present in both systems, while only 14 compounds in the EPS were the same between R1 and R2. Noticeably, certain aromatics only existed in R2, suggesting that bio-carriers effectively reduced the accumulation of recalcitrant materials, especially aromatics. These results revealed that bio-carriers shifted the precise composition of cake layers.

Extracellular biopolymers recovered as raw biomaterials from waste granular sludge and potential applications: A critical review

Granular sludge (GS) is a special self-aggregation biofilm. Extracellular polymeric substances (EPS) are mainly associated with the architectural structure, rheological behaviour and functional stability of fine granules, given that their significance to the physicochemical features of the biomass catalysing the biological purification process. This review targets the EPS excretion from GS and introduces newly identified EPS components, EPS distribution in different granules, how to effectively extract and recover EPS from granules, key parameters affecting EPS production, and the potential applications of EPS-based biomaterials. GS-based EPS components are highly diverse and a series of new contents are highlighted. Due to high diversity, emerging extraction standards are proposed and recovery process is capturing particular attention. The major components of EPS are found to be polysaccharides and proteins, which manifest a larger diversity of relative abundance, structures, physical and chemical characteristics, leading to the possibility to sustainably recover raw materials. EPS-based biomaterials not only act as alternatives to synthetic polymers in several applications but also figure in innovative industrial/environmental applications, including gel-forming materials for paper industry, biosorbents, cement curing materials, and flame retardant materials. In the upcoming years, it is foreseen that productions of EPS-based biomaterials from renewable origins would make a significant contribution to the advancement of the circular economy.

Microbial vanadate reduction coupled to co-metabolic phenanthrene biodegradation in groundwater

Vanadate [V(V)] and phenanthrene (PHE) commonly coexist in groundwater aquifer, posing potential threats to ecological environment and public health. However, little is known about the complicated biogeochemical processes involving microbial V(V) reduction coupled with co-metabolic PHE biodegradation. Herein we demonstrated that synchronous removal of V(V) and PHE could be realized under anaerobic condition. Complete V(V) removal and PHE degradation efficiency of 82.0 ± 0.8% were achieved in 7-d operation in batch experiment. 250-d continuous column experiment implied that hydrochemical condition affected V(V) and PHE removals. V(V) was reduced to insoluble vanadium (IV) and PHE was degraded into small molecule organics (e.g. salicylic acid). Geobacter and Acetobacterium used methanol and intermediates from PHE degradation as electron donors for V(V) reduction. PHE was decomposed by Mycobacterium and Clostridium with methanol as co-metabolic substrate and V(V) as electron acceptor. Genes encoding proteins for V(V) reduction (omcA, omcB and mtrC) and PHE degradation (phnAc) were upregulated. Cytochrome c and nicotinamide adenine dinucleotide promoted electron transfer for V(V) and PHE detoxification. Extracellular polymeric substances could bind V(V) and improve the bioavailability of PHE. Our findings provide a robust strategy for remediation of V(V) and PHE co-contaminated groundwater.

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.

Oil spill dispersants induce formation of marine snow by phytoplankton-associated bacteria

Unusually large amounts of marine snow, including Extracellular Polymeric Substances (EPS), were formed during the 2010 Deepwater Horizon oil spill. The marine snow settled with oil and clay minerals as an oily sludge layer on the deep sea floor. This study tested the hypothesis that the unprecedented amount of chemical dispersants applied during high phytoplankton densities in the Gulf of Mexico induced high EPS formation. Two marine phytoplankton species (Dunaliella tertiolecta and Phaeodactylum tricornutum) produced EPS within days when exposed to the dispersant Corexit 9500. Phytoplankton-associated bacteria were shown to be responsible for the formation. The EPS consisted of proteins and to lesser extent polysaccharides. This study reveals an unexpected consequence of the presence of phytoplankton. This emphasizes the need to test the action of dispersants under realistic field conditions, which may seriously alter the fate of oil in the environment via increased marine snow formation.

Evaluation of herbicide (persistent pollutant) removal mechanisms through hybrid membrane bioreactors

A laboratory-scale membrane bioreactor (MBR) combined with ultraviolet (UV) disinfection and granular activated carbon (GAC) adsorption was researched for over seven months to evaluate the removal efficiencies and mechanisms of a moderately persistent s-triazine herbicide (Ametryn), which is commonly used in Australian sugarcane farmlands. Long-term experiments showed that MBR alone (15h hydraulic retention time (HRT)) can remove 65% of Ametryn from its influent which had a concentration of 1-2mg/L. A batch study was carried out to assess the mechanisms of removal of Ametryn through MBR and found that 0.1186mg of Ametryn/g-VSS is adsorbed onto sludge particles when 1mg/L of Ametryn is added to the mixed liquor and showed a 64% removal after 12h. This experiment confirmed that 99%, 92% and 83% removal of Ametryn could be achieved only from biodegradation, if the MBR maintains a HRT of 7.5, 2.5 and 1.5days respectively.

Application of glyco-blotting for identification of structures of polysaccharides causing membrane fouling in a pilot-scale membrane bioreactor treating municipal wastewater

A new approach for the analysis of polysaccharides in membrane bioreactor (MBR) is proposed in this study. Enrichment of polysaccharides by glyco-blotting, in which polysaccharides are specifically collected via interactions between the aldehydes in the polysaccharides and aminooxy groups on glycoblotting beads, enabled MALDI-TOF/MS analysis at a high resolution. Structures of polysaccharides extracted from fouled membranes used in a pilot-scale MBR treating municipal wastewater and those in the supernatant of the mixed liquor suspension in the MBR were investigated. It was found that the overlap between polysaccharides found in the supernatants and those extracted from the fouled membrane was rather limited, suggesting that polysaccharides that dominate in supernatants may not be important in membrane fouling in MBRs. Analysis using a bacterial carbohydrate database suggested that capsular polysaccharides (CPS) and/or lipo-polysaccharides (LPS) produced by gram-negative bacteria are key players in the evolution of membrane fouling in MBRs.

Pre-treatment for ultrafiltration: effect of pre-chlorination on membrane fouling

Microbial effects are believed to be a major contributor to membrane fouling in drinking water treatment. Sodium hypochlorite (NaClO) is commonly applied in membrane cleaning, but its potential use as a pretreatment for controlling operational fouling has received little attention. In this study, the effect of adding a continuous low dose of NaClO (1 mg/l as active Cl) in combination with alum, before ultrafiltration, was compared with only alum as pretreatment. The results showed that the addition of NaClO substantially reduced membrane fouling both in terms of the rate of TMP development and the properties of the membrane cake layer. Although the size of nano-scale primary coagulant flocs changed little by the addition of NaClO, the cake layer on the membrane had a greater porosity and a substantially reduced thickness. NaClO was found to inactivate bacteria in the influent flow, which reduced both microbial proliferation and the production of proteins and polysaccharides in the cake layer and contributed significantly to improving the overall ultrafiltration performance. NaClO dosing had no adverse impact on the formation of currently regulated disinfection by-product compounds (THMs and HAAs).

Effects of suspended titanium dioxide nanoparticles on cake layer formation in submerged membrane bioreactor

Effects of the suspended titanium dioxide nanoparticles (TiO2 NPs, 50 mg/L) on the cake layer formation in a submerged MBR were systematically investigated. With nanometer sizes, TiO2 NPs were found to aggravate membrane pore blocking but postpone cake layer fouling. TiO2 NPs showed obvious effects on the structure and the distribution of the organic and the inorganic compounds in cake layer. Concentrations of fatty acids and cholesterol in the cake layer increased due to the acute response of bacteria to the toxicity of TiO2 NPs. Line-analysis and dot map of energy-dispersive X-ray were also carried out. Since TiO2 NPs inhibited the interactions between the inorganic and the organic compounds, the inorganic compounds (especially SiO2) were prevented from depositing onto the membrane surface. Thus, the postponed cake layer fouling was due to the changing features of the complexes on the membrane surface caused by TiO2 NPs.

Improvement on the modified Lowry method against interference of divalent cations in soluble protein measurement

This paper systematically investigated the interference of calcium and magnesium in protein measurement with a modified Lowry method first proposed by Frølund et al. (Appl Microbiol Biotechnol 43:755-761, 1995). This interference has in the past been largely ignored resulting in variable and unreliable results when applied to natural water matrices. We discovered significant formation of calcium and magnesium precipitates that lead to a decline in light absorbance at 750 nm during protein determination. Underestimation of protein concentration (sometimes even yielding negative concentrations) and low experiment reproducibility were demonstrated at high concentrations of divalent cations (e.g., [Ca(2+)] over 1 mmol L(-1)). To eliminate interference from calcium and magnesium, two pretreatment strategies were established based on cation exchange and dialysis. These pretreatments were convenient and were found to be highly effective in removing calcium and magnesium in protein samples. By using the modified Lowry method with these pretreatments, proteins in standard solutions and in wastewater samples were successfully quantified with good reliability and reproducibility. In addition, we demonstrated that simultaneous quantification of humic substances with the modified Lowry method was not affected by the two pretreatments. These approaches are expected to be applicable to protein and humic substance determination in different research fields, in cases where the modified Lowry method is sensitive to divalent cation concentrations.

Microbial transformation of biomacromolecules in a membrane bioreactor: implications for membrane fouling investigation

BACKGROUND

The complex characteristics and unclear biological fate of biomacromolecules (BMM), including colloidal and soluble microbial products (SMP), extracellular polymeric substances (EPS) and membrane surface foulants (MSF), are crucial factors that limit our understanding of membrane fouling in membrane bioreactors (MBRs).

FINDINGS

In this study, the microbial transformation of BMM was investigated in a lab-scale MBR by well-controlled bioassay tests. The results of experimental measurements and mathematical modeling show that SMP, EPS, and MSF had different biodegradation behaviors and kinetic models. Based on the multi-exponential G models, SMP were mainly composed of slowly biodegradable polysaccharides (PS), proteins (PN), and non-biodegradable humic substances (HS). In contrast, EPS contained a large number of readily biodegradable PN, slowly biodegradable PS and HS. MSF were dominated by slowly biodegradable PS, which had a degradation rate constant similar to that of SMP-PS, while degradation behaviors of MSF-PN and MSF-HS were much more similar to those of EPS-PN and EPS-HS, respectively. In addition, the large-molecular weight (MW) compounds (>100 kDa) in BMM were found to have a faster microbial transformation rate compared to the small-MW compounds (<5 kDa). The parallel factor (PARAFAC) modeling of three-dimensional fluorescence excitation-emission matrix (EEM) spectra showed that the tryptophan-like PN were one of the major fractions in the BMM and they were more readily biodegradable than the HS. Besides microbial mineralization, humification and hydrolysis could be viewed as two important biotransformation mechanisms of large-MW compounds during the biodegradation process.

SIGNIFICANCE

The results of this work can aid in tracking the origin of membrane foulants from the perspective of the biotransformation behaviors of SMP, EPS, and MSF.

Formation of distinct soluble microbial products by activated sludge: kinetic analysis and quantitative determination

Soluble microbial products (SMP) released by microorganisms in bioreactors are classified into two distinct groups according to their different chemical and degradation kinetics: utilization-associated products (UAP) and biomass-associated products (BAP). SMP are responsible for effluent chemical oxygen demand or for membrane fouling of membrane bioreactor. Here an effective and convenient approach, other than the complicated chemical methods or complex models, is developed to quantify the formation of UAP and BAP together with their kinetics in activated sludge process. In this approach, an integrated substrate utilization equation is developed and used to determine UAP and their production kinetics. On the basis of total SMP measurements, BAP formation is determined with an integrated BAP formation equation. The fraction of substrate electrons diverted to UAP, and the content of BAP derived from biomass can then be calculated. Dynamic quantification data are obtained for UAP and BAP separately and conveniently. The obtained kinetic parameters are found to be reasonable as they are generally bounded and comparable to the literature values. The validity of this approach is confirmed by independent SMP production tests in six different activated sludge systems, which demonstrates its applicability in a wide range of engineered system regarding SMP production. This work provides a widely applied approach to determine the formation of UAP and BAP conveniently, which may offer engineers with basis to optimize bioreactor operation to avoid a high effluent soluble organics from SMP or SMP-based membrane fouling in membrane bioreactors.

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 (BMMc, >0.45 μm), biopolymeric BMM (BMMb, 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 BMMc fraction had similar carbon content of proteins and polysaccharides. In contrast, the BMMb 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 BMMb 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.

Change in the fouling propensity of sludge in membrane bioreactors (MBR) in relation to the accumulation of biopolymer clusters

A membrane bioreactor (MBR) and an activated sludge process (ASP) were operated side by side to evaluate the change of sludge supernatant characteristics and the evolution of the sludge fouling propensity. The MBR sludge had a higher organic concentration and more biopolymer clusters (BPC) in the supernatant compared with ASP. BPC increased in both concentration and size in the MBR. The results show that the change in the liquid-phase property had a profound effect on the sludge fouling propensity. MBR operation transformed typical activated sludge to MBR sludge with a higher fouling propensity. Distinct from the ASP, membrane filtration retained soluble microbial products (SMP) within the MBR, and the vast membrane surface provided a unique environment for the transformation of SMP to large size BPC, leading to further sludge deposition on the membrane surface. Thus, membrane filtration is the crucial cause of the inevitable fouling problem in submerged MBRs.

Use of ozonation to mitigate fouling in a long-term membrane bioreactor

In order to confirm the effect of ozonation on fouling mitigation in long-term membrane bioreactors (MBRs), parallel operation of two MBRs with and without ozonation was repeated in triplicate with different dosing modes. A number of mixed liquor properties and removal of pollutants in both MBRs were investigated. Membrane fouling was effectively retarded by ozonation in long-term MBRs. The best dosage was 0.25 mg g(-1)-SS at 1 day intervals. A slow formation of a gel layer on membrane surface was the result of the low organics in the supernatant and the low extractable extra-cellular polymeric substances in the MBR-ozone system. Ozonation also enlarged suspended flocs by reducing zeta-potential and increasing hydrophobicity, thus enhancing flocculability of the particles in the mixed liquor. The microbial activity was somewhat inhibited, however, the effluent quality of the system was not affected. These findings indicate the feasibility of using ozone to improve MBR efficiency.

Fractionation of proteins and carbohydrates of extracellular polymeric substances in a membrane bioreactor system

The major operational problem associated with membrane bioreactors (MBR) is membrane fouling, for which extracellular polymeric substances (EPS) are primarily responsible. In this work both the soluble and bound EPS (i.e. SMP and EPS) produced in an MBR system operating under sludge retention times (SRT) of 10, 15, 20 and 33 days were fractionized by means of membranes having variable molecular weight cutoffs (300 kDa, 100 kDa, 10 kDa & 1 kDa). The results show that increasing the SRT leads to a reduction of SMP and EPS and that these reductions are more pronounced for the SRTs in the range 10-20 days. This reduction is more significant for carbohydrates than for proteins. The decrease of SMP and EPS with increasing SRT from 10 to 20 days led to a significant decrease of the level of fouling. The further increase of SRT to 33 days did not significantly impact on the level of fouling as the SMP and EPS concentrations did not change much. Under the examined operating conditions, EPS were found to be composed mainly of large macromolecules having a size of 0.45 microm-300 kDa and to a lower extent of very small molecules (<1 kDa) that are not easily decomposed by the biomass activity. The majority of SMP is composed of very small molecules (<1 kDa), while some macromolecules in the range of 0.45 microm-300 kDa are present. Consequently, both EPS and SMP were found to have a bimodal character.