Feasibility study of moving-fiber biofilm membrane bioreactor for wastewater treatment: process control

Non-biodegradable solid wastes of non-intact membrane fibres/flatsheets and modules disposed from membrane bioreactor (MBR) plants are in a great concern for environmental impact. Estimated cumulative amount of the module solid wastes from European countries in the next five years should be larger than 1000 tons in which a proper management strategy and reuse for the disposed solid waste are urgently required. This article was aimed to propose an alternative to make uses of the non-intact membrane fibres for the aerobic biofilm supports and to study the feasibility on process operation of novel moving-fiber biofilm MBR. A system of moving-fiber biofilm membrane bioreactor was designed and evaluated experimentally, including an upflow anaerobic sludge reactor, an aerobic moving-fiber biofilm reactor, and a submerged membrane filtration unit. Start-up method and operating conditions to control the biofilms growing on the moving fibers were investigated. Organic removal rates, optimum operating conditions for the system, and membrane fouling rates at various membrane aeration rates and permeate fluxes were monitored to evaluate the performance of the proposed BF-MBR process.     

Investigation of biocide efficacy by photoacoustic biofilm monitoring

The undesired growth of biofilms on solid surfaces is often termed biofouling. Biofilms consist mainly of water and microbial cells which are embedded in a biopolymer matrix. Biofouling lowers the water quality and increases the frictional resistance in tubes. Further, biofilms increase the pressure differences in membrane processes and can clog filtration membranes, valves, and nozzles. For investigation and improvement of biocide efficacy and anti-fouling strategies, on-line and in situ monitoring of the biofilm is necessary. In this study, photoacoustic spectroscopy (PAS) was employed for biofilm monitoring. PAS allows the depth-resolved investigation of growth and detachment processes of biofilms. Strategies based on the oxidant hydrogen peroxide were compared to popular isothiazolinone biocides. Hydrogen peroxide allowed a very fast and efficient removal of attached biofilms, whereas no effect on the biofilm matrix was observed in most cases when isothiazolinone biocides were used.     

Bacteriophage MS-2 removal by submerged membrane bioreactor

A membrane bioreactor (MBR) may serve as a pre-disinfection or disinfection unit, in addition to its solid/liquid separation and biological conversion functions, to produce sewage effluent of high quality. This bench-scale pilot study focuses on investigating the performance of a submerged MBR in pathogen removal and the factors affecting the removal, using a 0.4-microm hollow-fiber membrane module submerged in an aeration tank and bacteriophage MS-2 as the indicator organism. Removal of the MS-2 phage was found to be contributed by physical filtration by the membrane itself, biomass activity in the aeration tank and bio-filtration achieved by the biofilm developed on the membrane surface. The membrane alone gave poor virus removal (0.4+/-0.1 log) but the overall removal increased substantially with the presence of biomass and the membrane-surface-attached biofilm. The contributions of the suspended biomass and attached biofilm to the phage removal are dependent on the inter-related parameters including the concentration of mixed liquor suspended solids (MLSS), the sludge retention time (SRT) and the food to mass (F/M) ratio. The correlations between effluent flux/trans-membrane pressure and virus removal give evidence that phage removal in the MBR is most likely susceptible to both biological and physical factors including the quantity and property of the biomass and the biofilm and the membrane pore size reduction.     

Novel magnetically induced membrane vibration (MMV) for fouling control in membrane bioreactors

Conventional submerged membrane bioreactors (MBRs) rely on the coarse bubbles aeration to generate shear at the liquid-membrane interface to limit membrane fouling. Unfortunately, it is a very energy consuming method, still often resulting in a rapid decrease of membrane permeability and consequently in higher expenses. In this paper, the feasibility of a novel magnetically induced membrane vibration (MMV) system was studied in a lab-scale MBR treating synthetic wastewater. The effects on membrane fouling of applied electrical power of different operation strategies, of membrane flux and of the presence of multiple membranes on one vibrating engine on membrane fouling were investigated. The filtration performance was evaluated by determining the filtration resistance profiles and critical flux. The results showed clear advantages of the vibrating system over conventional MBR processes by ensuring higher fluxes at lower fouling rates. Intermittent vibration was found a promising strategy for both efficient fouling control and significant energy saving. The optimised MMV system is presumed to lead to significant energy and cost reduction in up-scaled MBR operations.     

Study of a membrane bioreactor with glass fiber flat grille modules and the modules' optimization based on the local critical flux theory

A novel flat grille membrane module using inorganic glass fibers as filter media is proposed for use in a membrane bioreactor for wastewater treatment. A model which integrates the concepts of back transport velocity, spatial local critical flux and temporal variation of the local flux has been developed. The membrane module was optimized based on experimental results and calculations using the model. The optimized parameters include the volume ratio of membrane solution for the surface modification of glass fibers, the fiber inner diameter and fiber length. The optimal values were 1:2 and 5 mm respectively but the length had little effect on the performance of the module. The critical time was then calculated with the model and an equation developed. The result was in very good agreement with the observed one. Finally, the performance of the glass fiber MBR was monitored. The effluent quality and stability of the system were comparable to that of conventional MBRs. This MBR will be a promising technique for wastewater treatment given its low cost, high strength and good effluent quality.     

Effect of DO concentration on biofilm structure and membrane filterability in submerged membrane bioreactor

The structures of biofilms deposited on the membrane surface under different dissolved oxygen (DO) conditions were characterized to identify its relation to membrane filterability in membrane bioreactors (MBR). The rate of membrane fouling for the low DO (LDO) reactor was 7.5 times faster than that for the high DO (HDO) reactor. Even though the biofilm deposited on the membrane surface in the HDO was thicker than in the LDO at the operating terminated (TMP reached 30 Kpa), biofilm resistance in both reactors were similar. Exactly, specific cake resistance of the HDO was lower than that of the LDO. Difference in biofilm characteristics as a result of different DO level was main factor affecting biofouling for both MBRs. The number of small particles ranging from 2-5mum in the biofilm as well as in the bulk solution for the LDO was greater than those for the HDO. The small particles in the bulk solution of the LDO more preferentially deposited on the membrane surface than those of HDO did. Hence, the biofilm porosity in the LDO (0.65) was smaller than that in HDO (0.85). The reduced porosity of LDO biofilm resulted in lower filterability than the HDO. The porosity data obtained from analysis of images of biofilm using confocal scanning laser microscopy (CLSM) was verified in terms of specific cake resistance (alpha) by comparing the experimentally measured values with the semi-theoretically computed values.     

Do biological-based strategies hold promise to biofouling control in MBRs?

Biofouling in membrane bioreactors (MBRs) remains a primary challenge for their wider application, despite the growing acceptance of MBRs worldwide. Research studies on membrane fouling are extensive in the literature, with more than 200 publications on MBR fouling in the last 3 years; yet, improvements in practice on biofouling control and management have been remarkably slow. Commonly applied cleaning methods are only partially effective and membrane replacement often becomes frequent. The reason for the slow advancement in successful control of biofouling is largely attributed to the complex interactions of involved biological compounds and the lack of representative-for-practice experimental approaches to evaluate potential effective control strategies. Biofouling is driven by microorganisms and their associated extra-cellular polymeric substances (EPS) and microbial products. Microorganisms and their products convene together to form matrices that are commonly treated as a black box in conventional control approaches. Biological-based antifouling strategies seem to be a promising constituent of an effective integrated control approach since they target the essence of biofouling problems. However, biological-based strategies are in their developmental phase and several questions should be addressed to set a roadmap for translating existing and new information into sustainable and effective control techniques. This paper investigates membrane biofouling in MBRs from the microbiological perspective to evaluate the potential of biological-based strategies in offering viable control alternatives. Limitations of available control methods highlight the importance of an integrated anti-fouling approach including biological strategies. Successful development of these strategies requires detailed characterization of microorganisms and EPS through the proper selection of analytical tools and assembly of results. Existing microbiological/EPS studies reveal a number of implications as well as knowledge gaps, warranting future targeted research. Systematic and representative microbiological studies, complementary utilization of molecular and biofilm characterization tools, standardized experimental methods and validation of successful biological-based antifouling strategies for MBR applications are needed. Specifically, in addition, linking these studies to relevant operational conditions in MBRs is an essential step to ultimately develop a better understanding and more effective and directed control strategy for biofouling.     

Bacterial community composition and structure of biofilms developing on nanofiltration membranes applied to wastewater treatment

The structure and microbial communities of biofilms developing on cross-flow nanofiltration (NF) membranes at different temperatures (20, 25 or 34 degrees C) and operation lengths (8h-24days) were studied. Feedwater comprised tertiary quality wastewater effluent or synthetic media mimicking effluents of intermediate quality. After each run, the membranes were autopsied for bacterial enumeration, bacterial community composition and microscopy visualization (SEM, CLSM and AFM/NSOM). Community composition was analyzed by polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) coupled with sequence analysis of 16S rRNA gene fragments from dominant bands. Deposition of polysaccharides and initial bacterial colonization were observed within 8h, whereas developed biofilms markedly affecting membrane permeability were evident from days 2-3 onwards. Regardless of applied conditions, the heterotrophic plate counts in the biofilm were 3-4x10(6)CFU/cm(2) and the thickness of the biofouling layer was 20-30microm. From a total of 22 sequences obtained from 14 independent experiments, most species identified were Gram negative (19 of 22 sequences). Proteobacteria were found to be a prevalent group in all cases (16 of 22 sequences) and among it, the beta-subclass was the most predominant (8 sequences), followed by the gamma-subclass (5 sequences). Pseudomonas/Burkholderia, Ralstonia, Bacteroidetes and Sphingomonas were the dominant groups found in most cases. Even though the microbial population might be important with respect to biofouling patterns, membrane permeability decline seems to be more substantially influenced by the formation and accumulation of exopolymeric substances (EPS).     

Impact of microfiltration treatment of secondary wastewater effluent on biofouling of reverse osmosis membranes

The effects of microfiltration (MF) as pretreatment for reverse osmosis (RO) on biofouling of RO membranes were analyzed with secondary wastewater effluents. MF pretreatment reduced permeate flux decline two- to three-fold, while increasing salt rejection. Additionally, the oxygen uptake rate (OUR) in the biofouling layer of the RO membrane was higher for an RO system that received pretreated secondary wastewater effluent compared to a control RO system that received untreated secondary effluent, likely due to the removal of inert particulate/colloidal matter during MF. A higher cell viability in the RO biofilm was observed close to the membrane surface irrespective of pretreatment, which is consistent with the biofilm-enhanced concentration polarization effect. Bacterial 16S rRNA gene clone library analysis revealed dominant biofilm communities of Proteobacteria and Bacteroidetes under all conditions. The Cramer-von Mises test statistic showed that MF pretreatment did not significantly change the bacterial community structure of RO membrane biofilms, though it affected bacterial community structure of non-membrane-associated biofilms (collected from the feed tank wall). The finding that the biofilm community developed on the RO membrane was not influenced by MF pretreatment may imply that RO membranes select for a conserved biofilm community.     

Biofilm loss during sample preparation for scanning electron microscopy

Biofilm morphology and mass are two of the important characteristics governing the kinetics of substrate removal by biofilms. Many previous studies showed only sparse biofilm on attaching media. However, one of the reasons which might cause a sparse-biofilm specimen in scanning electron microscopy (SEM) is that the treatment of a biofilm sample is harsh and can cause various artifacts. This study was conducted to evaluate the loss of biofilm during a conventional preparation procedure. Biofilm lost during preparation stages was collected on membrane filters. A different technique—the membrane filter method—was devised to carry a biofilm sample through the preparation stage with reduced losses. The sample was sandwiched between membrane filters during preparation so that the loss was minimized and the lost biofilm could be saved for qualitative and quantitative evaluations. SEM examination of biofilms and membrane filters showed that biofilm loss was much more significant with conventional treatment.     

新型填料曝气生物滤池去除氮化合物的效果

采用聚丙烯腈中空纤维膜段作为上流式曝气生物滤池的填料,就系统启动期间生物膜的形成以及对生活污水中氮化合物的去除规律进行了研究,并考察了稳定运行阶段的气水比、水力停留时间等对硝化和反硝化的影响.试验发现:在载体的外表面进行硝化反应的同时,其内部还发生了反硝化反应;中空纤维膜段的形状结构和表面特点使其适宜作为生物膜的载体.     

新型膜生物反应器的设计及其性能研究

结合无泡曝气原理和气体反冲洗方法,设计了一种新型的一体式膜生物反应器(MBR),并将其与传统MBR进行了对比.结果表明,当处理黄麻生物脱胶废水时,在相同的运行条件下,新型MBR中混合液的DO含量明显高于传统MBR,对COD的去除效果也明显好于传统MBR;另外,与传统MBR相比,新型MBR具有膜过滤压力小、膜污染轻、能耗低等优点.     

Visualisation of transient processes in biofilms by optical coherence tomography

Biofilms occur in natural and engineered water systems. In technical processes, biofouling lowers the water quality and increases the frictional resistance in tubes. In wastewater treatment plants, biofilms are used for the removal of organic and inorganic pollutants. For improvement of antifouling strategies and for process optimisation in wastewater treatment plants, analytical techniques for online monitoring of biofilms are needed. Optical coherence tomography (OCT) is a non-invasive optical tomography technique, which is increasingly applied in medical diagnostics. It reveals photon-reflecting structures in tissue with lateral and axial resolution in the range of 10 microm. In this paper, we demonstrate the capabilities of OCT for the monitoring of biofilm structures and their detachment. OCT is able to reveal spatially resolved structural information on biofilm without staining. A main focus of this work is set on the ability of OCT to monitor transient processes with temporal resolution in a second to minute scale. These key features of OCT allow online monitoring of biofilm growth and detachment in a flow channel. Three-dimensional (3D) imaging quality, spatial resolution, and temporally resolved profiling of biofilms are demonstrated. The results give rise to the hope that OCT may evolve to a standard tool for monitoring of biofilm density.     

Membrane coupled high-performance compact reactor: a new MBR system for advanced wastewater treatment

This study examined the potential and limitations of a new submerged membrane system coupled with a High-performance Compact Reactor (HCR) to take advantages of both systems. The configuration and installation position in the HCR of the membrane module were thoroughly investigated for the optimum design of a submerged membrane coupled with HCR, e.g., MHCR. Inside the draft tube proved to be better location rather than outside the draft tube or in the degas tank and an open-type configuration of a membrane module has an advantage over a fixed-type configuration in terms of membrane fouling. Comparison of the innovative MHCR with a conventional membrane bioreactor (MBR) was made to identify and prove the superiority of MHCR to MBR with respect to the membrane performance. The MHCR has shown the great potential, particularly for the treatment of wastewater of high organic strength.     

Effect of mechanical cleaning with granular material on the permeability of submerged membranes in the MBR process

The research on fouling reduction and permeability loss in membrane bioreactors (MBRs) was carried out at two MBR pilot plants with synthetic and real wastewater. On the one hand, the effect of mechanical cleaning with an abrasive granular material on the performance of a submerged MBR process was tested. Additionally, scanning electron microscopy (SEM) measurements and integrity tests were conducted to check whether the membrane material was damaged by the granulate.The results indicate that the fouling layer formation was significantly reduced by abrasion using the granular material. This technique allowed a long-term operation of more than 600 days at a flux up to 40 L/(m² h) without chemical cleaning of the membranes. Moreover, it was demonstrated that the membrane bioreactor (MBR) with granulate could be operated with more than 20% higher flux compared to a conventional MBR operation. SEM images and integrity tests showed that in consequence of abrasive cleaning, the granular material left brush marks on the membrane surface, however, the membrane function was not affected.In a parallel experimental set up, the impact of the operationally defined “truly soluble fraction” <0.04 μm from wastewater and activated sludge on the ultrafiltration membrane fouling characteristics was investigated. It was shown that the permeability loss was caused predominantly by the colloidal fraction >0.04 μm rather than by the dissolved fraction of wastewater and activated sludge.     

Effect of disintegrated sludge recycling on membrane permeability in a membrane bioreactor combined with a turbulent jet flow ozone contactor

We have combined a turbulent jet flow ozone contactor (TJC) with a membrane bioreactor (MBR) to establish a zero-discharge system in terms of excess sludge in the MBR. The TJC-MBR system was compared with the conventional MBR (Control-MBR) with respect to i) the size and zeta potential of the sludge particles, ii) the loosely bound extra-cellular polymeric substances (EPSs) and tightly bound EPS of the microbial flocs, iii) the porosity and biovolume of the bio-cake accumulated on the membrane, and iv) the membrane permeability. The TJC system generated the ozonated sludge with a negligible amount of loosely bound EPS and a positive zeta potential. As a result, when such ozonated sludge was recycled, the average size of the sludge particles (e.g., microbial flocs) increased in the TJC-MBR. Consequently the bio-cake formed in the TJC-MBR had greater porosity than that in the Control-MBR, giving rise to higher membrane permeability in the TJC-MBR.     

Peak flux performance and microbial removal by selected membrane bioreactor systems

A pilot study was conducted over a period of 18 months at the Point Loma Wastewater Treatment Plant (PLWWTP) in San Diego, CA to evaluate the operational and water quality performance of six selected membrane bioreactor (MBR) systems at average and peak flux operation. Each of these systems was operated at peak flux for 4 h a day for six consecutive days to assess peak flux performance. Virus seeding studies were also conducted during peak flux operation to assess the capability of these systems to reject MS-2 coliphage. When operating at steady state, these MBR systems achieved an effluent BOD concentration of <2 mg/L and a turbidity of <0.1 NTU. Peak flux for the MBR systems ranged from 56 to 76 L/m²/h (liters per square meter per hour) with peaking factors in the range of 1.5-3.2. When switching from average to peak flux operation, a reversible drop of 22-32% in temperature-corrected permeability was observed for all submerged MBR systems. The percent drop in permeability increased as MLSS concentration in the membrane tank increased from 11,100 mg/L to 15,300 mg/L and was observed to be highest for the system operating at highest MLSS concentration. Such trends were not observed with an external MBR system. Each MBR system was able to sustain a 4-h-a-day peak flow for six consecutive days with only moderate membrane fouling. The membrane fouling was quantified by measuring the drop in temperature-corrected permeability. This drop ranged from 13 to 33% over six days for different submerged MBR systems. The MBR systems achieved microbial removal in the range of 5.8-6.9 logs for total coliform bacteria, >5.5 to >6.0 logs for fecal coliform bacteria and 2.6 to >3.4 logs for indigenous MS-2 coliphages. When operating at peak flux, seeded MS-2 coliphage removal ranged from 1.0 to 4.4 logs, respectively. The higher log removal values (LRVs) for indigenous MS-2 coliphage among different MBR systems were probably the result of particle association of indigenous coliphage. Differences in membrane pore size (0.04-0.2 μm) amongst the MBR systems evaluated did not have a substantial impact on indigenous MS-2 coliphage removal, but seeded MS-2 coliphage removal varied among the different MBR systems.     

Nonlinear deflection of polypropylene hollow fiber membranes in transverse flow

The mass transfer coefficient (K_L) for oxygen transfer in a hollow fiber module with liquid flow normal to the fiber is much higher than for modules with flow parallel to the fibers. In the transverse flow modules studied, individually sealed fibers are mounted vertically at the bottom of a channel with water flow horizontally past the fibers. As a result, the free standing fibers deflect in the flow, like vertical cantilever beams subjected to uniform horizontal force. With this module configuration, a high mass transfer coefficient can be achieved at low liquid flow rates resulting in a more energy efficient process. Previous studies have shown that heat and mass transfer coefficients strongly depend on the orientation of the fibers with respect to the flow direction. The objective of this paper is to present a model for predicting the deflected shape of hollow fibers in a transverse flow module based on the flexural strength of the membrane material and the loading conditions. The modulus of elasticity (E) of the fiber material, measured by conducting conventional stress-strain tests, was correlated to the deflected shape of the fiber. The experimental value of E was verified by allowing the fibers to bend under their own weight. The predicted deflection of the fibers showed close agreement with the observed profile. Experiments were also performed to investigate the deflection of the fibers in the flowing water. The observed curvature of the fibers matched the predicted deflection well.     

一体式中空纤维微滤膜膜生物反应器设计要点

以中空纤维微滤膜为核心膜组件的一体式膜生物反应器系统处理生活污水为例,介绍了该类系统与板框式膜组件及分体式(管式)构型比较;一体式中空纤维微滤膜膜生物反应器设计范围、设计步骤。最后列举设计案例,介绍了工程设计过程和设计要点。     

Fate of antibiotics in activated sludge followed by ultrafiltration (CAS-UF) and in a membrane bioreactor (MBR)

The fates of several macrolide, sulphonamide, and trimethoprim antibiotics contained in the raw sewage of the Tel-Aviv wastewater treatment plant (WWTP) were investigated after the sewage was treated using either a full-scale conventional activated sludge (CAS) system coupled with a subsequent ultrafiltration (UF) step or a pilot membrane bioreactor (MBR) system. Antibiotics removal in the MBR system, once it achieved stable operation, was 15-42% higher than that of the CAS system. This advantage was reduced to a maximum of 20% when a UF was added to the CAS. It was hypothesized that the contribution of membrane separation (in both systems) to antibiotics removal was due either to sorption to biomass (rather than improvement in biodegradation) or to enmeshment in the membrane biofilm (since UF membrane pores are significantly larger than the contaminant molecules). Batch experiments with MBR biomass showed a markedly high potential for sorption of the tested antibiotics onto the biomass. Moreover, methanol extraction of MBR biomass released significant amounts of sorbed antibiotics. This finding implies that more attention must be devoted to the management of excess sludge.