The application of nitric oxide to control biofouling of membrane bioreactors

A novel strategy to control membrane bioreactor (MBR) biofouling using the nitric oxide (NO) donor compound PROLI NONOate was examined. When the biofilm was pre‐established on membranes at transmembrane pressure (TMP) of 88–90 kPa, backwashing of the membrane module with 80 μM PROLI NONOate for 45 min once daily for 37 days reduced the fouling resistance (R_f) by 56%. Similarly, a daily, 1 h exposure of the membrane to 80 μM PROLI NONOate from the commencement of MBR operation for 85 days resulted in reduction of the TMP and R_f by 32.3% and 28.2%. The microbial community in the control MBR was observed to change from days 71 to 85, which correlates with the rapid TMP increase. Interestingly, NO‐treated biofilms at 85 days had a higher similarity with the control biofilms at 71 days relative to the control biofilms at 85 days, indicating that the NO treatment delayed the development of biofilm bacterial community. Despite this difference, sequence analysis indicated that NO treatment did not result in a significant shift in the dominant fouling species. Confocal microscopy revealed that the biomass of biopolymers and microorganisms in biofilms were all reduced on the PROLI NONOate‐treated membranes, where there were reductions of 37.7% for proteins and 66.7% for microbial cells, which correlates with the reduction in TMP. These results suggest that NO treatment could be a promising strategy to control biofouling in MBRs.     

Improving the performance of an aerobic membrane bioreactor (MBR) treating pharmaceutical wastewater with powdered activated carbon (PAC) addition

In this study, the effects of organic loading rate (OLR) and the addition of powdered activated carbon (PAC) on the performance and membrane fouling of MBR were conducted to treat real pharmaceutical process wastewater. Over 145 days of operation, the MBR system was operated at OLRs ranging from 1 to 2 kg COD m^?3 day^?1 without sludge wasting. The addition of PAC provided an improvement in the flux, despite an increase in the OLR:PAC ratio. The results demonstrated that the hybrid PAC-MBR system maintained a reduced amount of membrane fouling and steadily increased the removal performance of etodolac. PAC addition reduced the deposition of extracellular polymeric substance and organic matter on the membrane surface and resulted an increase in COD removal even at higher OLRs with low PAC addition. Membrane fouling mechanisms were investigated using combined adsorption fouling models. Modified fouling index values and normalized mass transfer coefficient values indicated that predominant fouling mechanism was cake adsorption.     

Influence of solids retention time on membrane fouling: characterization of extracellular polymeric substances and soluble microbial products

The objective of this study was to investigate the influence of solids retention time (SRT) on membrane fouling and the characteristics of biomacromolecules. Four identical laboratory-scale membrane bioreactors (MBRs) were operated with SRTs for 10, 20, 40 and 80 days. The results indicated that membrane fouling occurred faster and more readily under short SRTs. Fouling resistance was the primary source of filtration resistance. The modified fouling index (MFI) results suggested that the more ready fouling at short SRTs could be attributed to higher concentrations of soluble microbial products (SMP). Fourier transform infrared (FTIR) spectra indicated that the SRT had a weak influence on the functional groups of the total extracellular polymeric substances (TEPS) and SMP. However, the MBR under a short SRT had more low-molecular-weight (MW) compounds (<1 kDa) and fewer high-MW compounds (>100 kDa). Aromatic protein and tryptophan protein-like substances were the dominant groups in the TEPS and SMP, respectively.     

Pretreatment with alum or powdered activated carbon reduces bacterial predation-associated irreversible fouling of membranes

This study evaluated the co-application of bacterial predation by Bdellovibrio bacteriovorus and either alum coagulation or powdered activated carbon adsorption to reduce fouling caused by Escherichia coli rich feed solutions in dead-end microfiltration tests. The flux increased when the samples were predated upon or treated with 100 ppm alum or PAC, but co-treatment with alum and predation gave the best flux results. The total membrane resistance caused by the predated sample was reduced six-fold when treated with 100 ppm PAC, from 11.8 to 1.98 × 10¹¹ m^?1, while irreversible fouling (R_p) was 2.7-fold lower. Treatment with 100 ppm alum reduced the total resistance 14.9-fold (11.8 to 0.79 × 10¹¹ m^?1) while the R_p decreased 4.25-fold. SEM imaging confirmed this, with less obvious fouling of the membrane after the combined process. This study illustrates that the combination of bacterial predation and the subsequent removal of debris using coagulation or adsorption mitigates membrane biofouling and improves membrane performance.     

Effects of low-concentration Cr(VI) on the performance and the membrane fouling of a submerged membrane bioreactor in the treatment of municipal wastewater

The effects of low-concentration Cr(VI) (0.4 mg?l^?1) on the performance of a submerged membrane bioreactor (SMBR) in the treatment of municipal wastewater, as well as membrane fouling were investigated. Compared with the SMBR for control municipal wastewater, the SMBR for Cr(VI)-containing municipal wastewater had a higher concentration of soluble microbial products (SMP) with lower molecular weights, and smaller sludge particle sizes. Furthermore, low-concentration Cr(VI) induced membrane fouling, especially irreversible membrane pore blocking, which markedly shortened the service life of the membrane.     

Performance of an anaerobic membrane bioreactor in which granular sludge and dynamic filtration are integrated

To alleviate the fouling of a filter, simple substrates, dynamic filtration, and granular sludge were applied in an anaerobic membrane bioreactor (AnMBR). The results showed that under a transmembrane pressure < 20 kPa, the filter flux ranged between 15 and 20 l (m^?2 h)^?1 for a period of 30 days. The flux was higher than the typical flux of AnMBRs with conventional membranes and most current dynamic filters. In addition, the low cost of the filter avoided the need for a higher flux. Moreover, a stable granular sludge bed, which consumed all volatile fatty acids, was maintained. A compact fouling/filtration layer formed on the filter, which contributed to low effluent chemical oxygen demand concentrations and turbidity. In addition, substrate scarcity in the filtration zone resulted in the evolution of diverse bacteria on the filter.     

Microbial community developments and biomass characteristics in membrane bioreactors under different organic loadings

Microbial community developments and biomass characteristics (concentration, particle size, extracellular polymeric substances (EPS), and membrane fouling propensity) were compared when three MBRs were fed with the synthetic wastewater at different organic loadings. Results showed that the bacterial communities dynamically shifted in different ways and the EPS displayed dissimilar profiles under various organic loadings, which were associated with the ratios of food to microorganism and dissolved oxygen levels in the MBRs. The membrane fouling tendency of biomass in the low-loading MBR (0.57 g COD/L day) was insignificantly different from that in the medium-loading MBR (1.14 g COD/L day), which was apparently lower than that in the high-loading MBR (2.28 g COD/L day). The membrane fouling propensity of biomass was strongly correlated with their bound EPS contents, indicating cake layer fouling (i.e., deposition of microbial flocs) was predominant in membrane fouling at a high flux of 30 L/m² h.     

Enrichment of ANME‐2 dominated anaerobic methanotrophy from cold seep sediment in an external ultrafiltration membrane bioreactor

Anaerobic oxidation of methane (AOM) coupled to sulfate reduction is a microbially mediated unique natural phenomenon with an ecological relevance in the global carbon balance and potential application in biotechnology. This study aimed to enrich an AOM performing microbial community with the main focus on anaerobic methanotrophic archaea (ANME) present in sediments from the Ginsburg mud volcano (Gulf of Cadiz), a known site for AOM, in a membrane bioreactor (MBR) for 726 days at 22 (± 3)°C and at ambient pressure. The MBR was equipped with a cylindrical external ultrafiltration membrane, fed a defined medium containing artificial seawater and operated at a cross flow velocity of 0.02 m/min. Sulfide production with simultaneous sulfate reduction was in equimolar ratio between days 480 and 585 of MBR operation, whereas methane consumption was in oscillating trend. At the end of the MBR operation (day 726), the enriched biomass was incubated with ¹³C labeled methane, ¹³C labeled inorganic carbon was produced and the AOM rate based on ¹³C‐inorganic carbon was 1.2 μmol/(g_dw d). Microbial analysis of the enriched biomass at 400 and 726 days of MBR operation showed that ANME‐2 and Desulfosarcina type sulfate reducing bacteria were enriched in the MBR, which formed closely associated aggregates. The major relevance of this study is the enrichment of an AOM consortium in a MBR system which can assist to explore the ecophysiology of ANME and provides an opportunity to explore the potential application of AOM.     

Prediction of membrane fouling in MBR systems using empirically estimated specific cake resistance

The focus of this study was to empirically estimate the specific cake resistance (SCR) by the variation in shear intensity (G) in four laboratory-scale MBRs. The control reactor (MBR₀) was operated with aeration only while other MBRs (MBR₁₅₀, MBR₃₀₀ and MBR₄₅₀) were operated with aeration and mechanical mixing intensities of 150, 300 and 450 rpm, respectively. It was found that the SCR was strongly correlated (R² = 0.99) with the fouling rates in the MBRs. Moreover, the contribution of cake resistance (R_c) to the total hydraulic resistance (R_t) was predominant compared to the irreversible fouling resistance (R_f). On this basis, the cake filtration model was selected as a predictive tool for membrane fouling. This model was modified by replacing the SCR with its empirical shear intensity relationship. The modified model can predict the fouling rate for a given shear intensity (G) within 80 and 250 s⁻¹ in a MBR system.     

Application of zinc oxide and sodium alginate for biofouling mitigation in a membrane bioreactor treating urban wastewater

Abstract

This research aimed to mitigate fouling in membrane bioreactors (MBR) through concurrent usage of zinc oxide as an antibacterial agent (A) and sodium alginate as a hydrophilic agent (H) within a polyacrylonitrile membrane (PM) structure. The antibacterial polymeric membranes (APM) and antibacterial hydrophilic polymeric membranes (AHPM) synthesized showed a higher porosity, mechanical strength and bacterial inhibition zone, and a lower contact angle in comparison with PM membranes. EDS, SEM and AFM analyses were used to characterize the chemical, structural, and morphological properties of PM, APM, and AHPM. The flux of PM, APM, and AHPM in MBR was 37, 48, and 51?l m^?2 h^?1 and COD removal was 81, 93.5, and 96.7%, respectively. After MBR operation for 35?days in an urban wastewater treatment, only 50% of the flux of PM was recovered, while the antibacterial and hydrophilic agents yielded a flux recovery of 72.7 and 100% for APM and AHPM, respectively.     

Quorum sensing: an emerging link between temperature and membrane biofouling in membrane bioreactors

Lab-scale membrane bioreactors (MBRs) were investigated at 12, 18, and 25?°C to identify the correlation between quorum sensing (QS) and biofouling at different temperatures. The lower the reactor temperature, the more severe the membrane biofouling measured in terms of the transmembrane pressure (TMP) during filtration. More extracellular polymeric substances (EPSs) that cause biofouling were produced at 18?°C than at 25?°C, particularly polysaccharides, closely associated with QS via the production of N-acyl homoserine lactone (AHL). However, at 12?°C, AHL production decreased, but the release of EPSs due to deflocculation increased the soluble EPS concentration. To confirm the temperature effect related to QS, bacteria producing AHL were isolated from MBR sludge and identified as Aeromonas sp., Leclercia sp., and Enterobacter sp. through a 16S rDNA sequencing analysis. Batch assays at 18 and 25?°C showed that there was a positive correlation between QS through AHL and biofilm formation in that temperature range.     

Membrane fouling control and enhanced phosphorus removal in an aerated submerged membrane bioreactor using modified green bioflocculant

This study aims at developing a modified green bioflocculant (GBF) for membrane fouling control and enhanced phosphorus removal in a conventional aerated submerged membrane bioreactor (SMBR) to treat a high strength domestic wastewater (primary sewage treated effluent) for reuse. The GBF was evaluated based on long-term operation of a lab-scale SMBR. These results showed that SMBR system could achieve nearly zero membrane fouling at a very low dose of GBF addition (500 mg/day) with less backwash frequency (2 times/day with 2-min duration). The transmembrane pressure only increased by 2.5 kPa after 70 days of operation. The SMBR could also remove more than 95% and 99.5% dissolved organic carbon and total phosphorus, respectively. From the respiration tests, it was evident that GBF not only had no negative impact on biomass but also led to high oxygen uptake rate (OUR) (>30 mg O₂/L h) and stable specific oxygen uptake rate (SOUR). These results also indicated that GBF had no effect on nitrogen removal and nitrification process.     

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.     

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.     

Comparison of the performance of submerged membrane bioreactor (SMBR) and submerged membrane adsorption bioreactor (SMABR)

This study focuses on comparing the performance of submerged membrane bioreactor (SMBR) and submerged membrane adsorption bioreactor (SMABR) over a period of 20 days at a hydraulic retention time (HRT) of 3.1h. The effects of PAC on critical flux and membrane fouling were also investigated. The SMABR exhibited better results in terms of mixed liquor suspended solids (MLSS) growth, DOC removal (over 96%), COD removal (over 95%), transmembrane pressure (TMP) and oxygen uptake rate. Nearly 100% of bacteria and 100% of total coliforms were removed in both systems. The addition of PAC could maintain the critical flux at a lower TMP value (7.5 kPa), while irreversible fouling caused by PAC occurred when the filtration flux exceeded critical flux.     

Separation of BSA through FAU-type zeolite ceramic composite membrane formed on tubular ceramic support: Optimization of process parameters by hybrid response surface methodology and biobjective genetic algorithm

In this study, Faujasite (FAU) zeolite was coated on low-cost tubular ceramic support as a separating layer through hydrothermal route. The mixture of silicate and aluminate solutions was used to create a zeolitic separation layer on the support. The prepared zeolite ceramic composite membrane was characterized using X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), particle size distribution (PSD), field emission scanning electron microscopy (FESEM), and zeta potential measurements. The porosity of ceramic support (53%) was reduced by the deposition of FAU (43%) zeolite layer. The pore size and water permeability of the membrane were evaluated as 0.179?µm and 1.62?×?10^?7?m³/m²?s?kPa, respectively, which are lower than that of the support (pore size of 0.309?µm and water permeability of 5.93?×?10^?7?m³/m²?s?kPa). The permeate flux and rejection potential of the prepared membrane were evaluated by microfiltration of bovine serum albumin (BSA). To study the influences of three independent variables such as operating pressure (68.94–275.79?kPa), concentration of BSA (100–500?ppm), and solution pH (2–4) on permeate flux and percentage of rejection, the response surface methodology (RSM) was used. The predicted models for permeate flux and rejection were further subjected to biobjective genetic algorithm (GA). The hybrid RSM-GA approach resulted in a maximum permeate flux of 2.66?×?10^?5?m³/m²?s and BSA rejection of 88.02%, at which the optimum conditions were attained as 100?ppm BSA concentration, 2 pH solution, and 275.79?kPa applied pressure. In addition, the separation efficiency was compared with other membranes applied for BSA separation to know the potential of the fabricated FAU zeolite ceramic composite membrane.     

Virus harvesting in perfusion culture: Choosing the right type of hollow fiber membrane

The use of bioreactors coupled to membrane-based perfusion systems enables very high cell and product concentrations in vaccine and viral vector manufacturing. Many virus particles, however, are not stable and either lose their infectivity or physically degrade resulting in significant product losses if not harvested continuously. Even hollow fiber membranes with a nominal pore size of 0.2 µm can retain much smaller virions within a bioreactor. Here, we report on a systematic study to characterize structural and physicochemical membrane properties with respect to filter fouling and harvesting of yellow fever virus (YFV; ~50 nm). In tangential flow filtration perfusion experiments, we observed that YFV retention was only marginally determined by nominal but by effective pore sizes depending on filter fouling. Evaluation of scanning electron microscope images indicated that filter fouling can be reduced significantly by choosing membranes with (i) a flat inner surface (low boundary layer thickness), (ii) a smooth material structure (reduced deposition), (iii) a high porosity (high transmembrane flux), (iv) a distinct pore size distribution (well-defined pore selectivity), and (v) an increased fiber wall thickness (larger effective surface area). Lowest filter fouling was observed with polysulfone (PS) membranes. While the use of a small-pore PS membrane (0.08 µm) allowed to fully retain YFV within the bioreactor, continuous product harvesting was achieved with the large-pore PS membrane (0.34 µm). Due to the low protein rejection of the latter, this membrane type could also be of interest for other applications, that is, recombinant protein production in perfusion cultures.     

Enhanced COD and nitrogen removals for the treatment of swine wastewater by combining submerged membrane bioreactor (MBR) and anaerobic upflow bed filter (AUBF) reactor

In order to enhance performances of organics removal and nitrification for the treatment of swine wastewater containing high concentration of organic solids and nitrogen than conventional biological nitrogen removal process, a submerged membrane bioreactor (MBR) was followed by an anaerobic upflow bed filter (AUBF) reactor in this research (AUBF–MBR process). The AUBF reactor is a hybrid reactor, which is the combination of an anoxic filter for denitrification and upflow anaerobic sludge blanket (UASB) for acid fermentation. In the AUBF–MBR process, it showed a considerable enhancement of the effluent quality in terms of COD removal and nitrification. The submerged MBR could maintain more than 14,000 mg VSS/L of the biomass concentration. Total nitrogen (T-N) removal efficiency represented 60% when internal recycle ratio was three times of flow-rate (Q), although the nitrification occurred completely. Although the volatile fatty acids produced in AUBF reactor can enhance denitrification rate, but the AUBF–MBR process showed reduction of overall removal efficiency of the nitrogen due to the reduction of carbon source by methane production in the AUBF reactor compared to that of theoretical nitrogen removal efficiency.

Long-term operation of the submerged MBR showed that the throughputs of the submerged MBR were respectively 74, 63, and 31 days at 10, 15, and 30 L/m² h (LMH) of permeate flux. Resistance to filtration by rejected solid is the primary cause of fouling, however the priority of cake resistance (R_c) and fouling resistance (R_f) with respect to filtration phenomenon was different according to the amount of permeate flux. The submerged MBR, here, achieved a steady-state flux of 15 LMH at 0.4 atm. of trans-membrane pressure (TMP) but the flux can be enhanced in the future because shear force by tangential flow will be greater when multi-layer sheets of membrane were used.     

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.