Influence of module configuration and hydrodynamics in water clarification by immersed membrane systems.

Different immersed membrane systems were compared according to the module configuration. Filtering concentrated aqueous suspensions under constant permeate flux, the hydraulic performances of the systems were evaluated and compared through parameters such as critical permeate flux notion and trans-membrane pressure variation rates. Operational variables were membrane size and module fibre density, aeration inside or outside the fibre network, suspension concentration and physico-chemical conditioning. When using hollow fibres including a possible air injection inside the fibre network, results pointed out the positive role of the aeration on the fouling control. But too high a fibre density did not allow an optimal control when the aqueous suspension was very concentrated. On the other hand, when working with capillary membranes showing sufficient space between fibres, the major parameters were the transversal suspension flow circulation through the fibre network and the FeCl3 conditioning of the suspension. Experimental results show a possible working at a 0.07 m3 x m2 x h(1) permeate flow rate under low TMP evolutions, 0.02 Pa/s, even if the filtration was operated under high concentrated suspension, 5kgSS/m3.     

The significance of interactions between organic compounds on low pressure membrane fouling

Fouling of hollow fibre microfiltration and ultrafiltration membranes by solutions of pure organic compounds and mixtures of these compounds was studied with a backwashable membrane filtration apparatus. Small molecular weight compounds resulted in little fouling, while their polymeric analogues resulted in more severe fouling. Neutrally charged dextran resulted in minor, irreversible fouling, that was considered to be associated with blocking of small pores. Cationically charged chitosan produced gross fouling for which the extent of reversibility increased with salt addition. Anionically charged alginic acid resulted in gross irreversible fouling, except when being filtered by a hydrophilic membrane in the absence of calcium where a high degree of flux recovery was observed. Calcium addition to the alginic acid solutions resulted in gross fouling of all membranes and calcium bridging was considered to be responsible for this behaviour. Greater fouling occurred on the hydrophilic membrane compared to the hydrophobic membranes for bovine serum albumin (BSA) solutions, and this was considered to be due to physical blocking of pores, because addition of calcium resulted in lower flux declines. Addition of BSA and calcium to alginic acid solutions resulted in lower flux recoveries for the alginic acid system, consistent with the proposition that interactions between polysaccharide and other compounds are required for irreversible fouling on hydrophilic membranes.     

The membrane fouling simulator: a suitable tool for prediction and characterisation of membrane fouling.

A new tool--the Membrane Fouling Simulator (MFS)--is developed to measure membrane fouling (pressure drop increase) in a small and simple system, representative for spiral wound membranes applied in water treatment. With the MFS, fouling development can be monitored systematically by (i) pressure drop, (ii) in situ and non-destructive (visual) observations using the sight glass and (iii) analysis of coupons sampled from the membrane sheet in the MFS. A comparison study of the MFS with spiral wound membrane elements (test rigs and a full scale installation) showed the same fouling. The MFS provided reproducible data. The small size and low water and chemical use of the MFS facilitate to perform systematic parallel studies. With the MFS, fouling of membranes applied in water treatment can be characterised.     

Effect of bubble flow velocity on drag-force and shear stress working on submerged hollow fibre membrane.

This study is aimed at elucidating the mechanism by which rising air bubbles induce shear stress on hollow fibre membrane surfaces. Shear stress on hollow fibre membrane surfaces (laterally-set and vertically-set) caused by aeration was measured directly using a two-direction load sensor. In the laterally-set hollow fibre module, time-averaged upward-direction shear stress on the membrane surface was compared to theoretical shear stress values considering the effect of water flow on membrane surface. Measured time-average shear stress values were almost 200 times larger than theoretical values implying strong interactions between bubbles and solid surface. In the vertically-set membrane module, velocity measurement of bubble flow using laser Doppler velocimeter revealed that drag force working on membrane surface was closely related to upward-direction water velocity. Also fluctuation of drag force and shear force on membrane surface was found to be related to velocity fluctuation (turbulence).     

Analysis of shear stress and energy consumption in a tubular airlift membrane system

Application of a two-phase slug flow in side-stream membrane bioreactors (MBRs) has proven to increase the permeate flux and decrease fouling through a better control of the cake layer. Past literature has shown that the hydrodynamics near the membrane surface have an impact on the degree of fouling by imposing high shear stress near the surface of the membrane. Previously, shear stress histograms (SSH) have been introduced to summarize results from an experimental setup developed to investigate the shear stress imposed on the surface of a membrane under different two-phase flow conditions (gas and liquid) by varying the flow of each phase. Bimodal SSHs were observed, with peaks corresponding to the shear induced by the liquid and gas flow respectively. In this contribution, SSHs are modelled using simple empirical relationships. These are used to identify the two-phase flow conditions that optimize fouling control. Furthermore, the total energy consumption of the system was estimated based on the two-phase pressure drop. It was found that low liquid and high gas flow rates (ratio of approx. 4) balanced the peaks and minimized the energy consumption.     

Performance of individual fibers in a submerged hollow fiber bundle.

Hollow fiber membranes are popular as they have a high specific membrane area. To take advantage of this, it is necessary to pack the fibers into closely packed bundles. The fibers in different positions in the bundle behave differently as they are exposed to different hydrodynamic conditions. In this paper, a 'model' bundle of 9 fibers was tested in a setup which provides flow measurement from individual fibers and the same suction pressure in each fiber. The parameters studied were packing density, cross flow velocity, feed concentration and bubbling. It was found that a low cross flow velocities, high pressures and high feed concentrations, the surrounded (center) fiber performed very poorly compared to the fibers at the corner and the sides. Under these conditions, the overall performance of the bundle was much worse that of a single fiber.     

The role of fouling mechanisms in a membrane bioreactor.

The present study has aimed to quantify the role of pore blocking and cake layer in a laboratory scale hollow fibre membrane module in submerged configuration, The membrane reactor (MBR) was fed with raw wastewater, only screened with a 2-mm sieve, collected from the Palermo WWTP. The MBR was characterised by an operating volume of 190 L and equipped with an aeration system located on the bottom of the reactor. The MBR operated for 65 days. The permeate was extracted by imposing a constant flux through the membrane (21 Lh(-1) m(-2)). The results confirm the importance of pore blocking control during start-up. In particular, it provides a rapid irreversible fouling that takes place at the beginning of the filtration process, before the deposition mechanism. Therefore, low suspended solids concentration in the initial phase causes a fast irreversible fouling. This circumstance creates the need for more frequent chemical cleaning after start-up without inoculum. Finally, the results underline that the cake has a mainly reversible feature.     

Evaluation of membrane bioreactor performance via residence time distribution: effects of membrane configuration and mixing.

Unlike conventional wastewater treatment systems that have a single effluent discharge point, membrane bioreactors (MBR) may have multiple extraction points resulting from the location of the membrane element in the reactor. This leads to multiple residence time distributions for an MBR system. One method to characterise the mixing is based on the concept of residence time distribution (RTD). A set of RTDs were generated using the conservative tracer, lithium chloride, for pilot plant MBRs with capacity up to 300 m3/day. Flat sheet and hollow fibre pilot plant MBR systems were operated in parallel on primary effluent collected at the Bedok Water Reclamation Plant in the republic of Singapore. Analysis of the RTD profiles indicated that membrane geometry did not impact on the kinetic conversion associated with nitrification because both MBRs were in well mixed conditions. However, the energy required to achieve perfect mixing with a hollow fibre module MBR, as defined by the velocity gradient, was lower than that with a flat sheet module MBR. The implication is that energy input associated with reactor mixing will depend on the configuration of the membrane. The difference in energy requirements between flat sheets and hollow fibres is such that careful consideration should be given to membrane selection in larger municipal installations.     

Membrane bioreactor aeration: investigation of the velocity flow pattern.

Results of investigations concerning membrane bioreactor aeration are presented which were carried out at the Institute of Environmental Engineering of RWTH Aachen University (ISA) in cooperation with the Department of Chemical Engineering of the Technische Universit?t Berlin. In the field of industrial and municipal wastewater treatment the use of membrane bioreactors (MBR) is of increasing interest especially due to the high requirements on effluent quality nowadays. The design of aeration systems is a very important aspect of MBR development because it influences both cost of operation and filtration flux. The ISA has carried out tests concerning the velocity flow pattern in flat sheet membrane modules (developed by the A3 Water Solutions GmbH) to identify the effects of different aeration systems, aeration intensities and module constructions. The Department of Chemical Engineering is currently using the results obtained from the ADV to calibrate a numerical model which simulates two phase water and gas flow within an aerated membrane module. Optical investigations concerning the bubble distribution give a better understanding of the flow conditions in MBR. Developing a numerical tool for membrane module optimization concerning the hydrodynamics is the aim of the investigation of membrane bioreactor aeration.     

Optimal performance of an immersed membrane bioreactor equipped with a draft tube for domestic wastewater reclamation.

One of the options to prevent membrane fouling is to implement air lifting that can improve the cake removal from the membrane surface. This study presents the results of tests that were carried out at the Institutes for Desert Research, Kiryat Sde-Boker, Israel, and focused on the influence of hydrodynamic conditions on fouling in a pilot-scale immersed membrane bioreactor (IMBR) using a hollow fiber membrane module of ZW-10 (Zenon Environmental, Canada) under ambient conditions. In this system, the cross-flow velocities across the membrane surface were induced by one conical and four cylindrical draft-tubes. The relationship between the crossflow velocity and the aeration intensity, the influence of the crossflow on fouling rate under various hydrodynamic conditions were investigated and optimal operating conditions were obtained. Optimal operating conditions were reached during the long-term experiment period (70 days) for the treatment of domestic wastewater. The system was stable without external chemical cleaning. The results showed that the permeate was of high quality, and the removal of COD and BOD was 94.0% and 98.8%, respectively.The crossflow near the membrane surface reveals a major contribution for minimizing membrane fouling, and could offer guidelines for future design of similar systems.     

Principles of an enhanced MBR-process with mechanical cleaning

Up to date, different physical and chemical cleaning protocols are necessary to limit membrane fouling in membrane bioreactors. This paper deals with a mechanical cleaning process, which aims at the avoidance of hypochlorite and other critical chemicals in MBR with submerged flat sheet modules. The process basically consists of the addition of plastic particles into the loop circulation within submerged membrane modules. Investigations of two pilot plants are presented: Pilot plant 1 is equipped with a 10 m(2) membrane module and operated with a translucent model suspension; pilot plant 2 is equipped with four 50 m(2) membrane modules and operated with pretreated sewage. Results of pilot plant 1 show that the establishment of a fluidised bed with regular particle distribution is possible for a variety of particles. Particles with maximum densities of 1.05 g/cm(3) and between 3 and 5 mm diameter form a stable fluidised bed almost regardless of activated sludge concentration, viscosity and reactor geometry. Particles with densities between 1.05 g/cm(3) and 1.2 g/cm(3) form a stable fluidised bed, if the velocity at the reactor bottom is sufficiently high. Activities within pilot plant 2 focused on plant optimisation and the development of an adequate particle retention system.     

Membrane fouling and selectivity mechanisms in effluent ultrafiltration coupled with flocculation.

Membrane filtration is adequate for producing disinfected clear water suitable for various kinds of applications. However, fouling of membranes is the main limitation. The scope of the present study is to examine the effect of iron coagulation of primary wastewater effluent on membrane filtration, in parallel to fouling characterization of the iron itself. The fouling of ultrafiltration membranes by colloidal iron hydroxide-oxide has been studied by measuring the pore streaming potential of PES UF membrane. pH 5.5 (charge neutralization zone) provided better removal and lower fouling intensity than pH 7.8 (sweep coagulation zone), but the internal clogging at acidic pH was higher. Fouling of the membrane as measured by flux reduction was usually accompanied by a positive change in zeta potential and iso-electric point (IEP) of the membrane. An initially large change in zeta potential (without charge reversal) was seen even after relatively small amounts of iron solution were filtered through the membrane. A control experiment showed this is not due to iron adsorption equilibrium, but should probably be attributed to fouling. Change in zeta potential, can be used as an indicator for commencement of fouling even for small flux reductions. UF membrane critical flux after iron filtration can be evaluated more accurately by zeta potential than pressure drop or change in iron concentration.     

Optimising the operation of a MBR pilot plant by quantitative analysis of the membrane fouling mechanism.

In order to optimize some operational conditions of MBR systems, a MBR pilot plant equipped with a submerged hollow fibre membrane module was employed in this study. The pilot MBR was fed with real municipal wastewater and the filtration flux, backwashing interval, aeration frequency and temperature were varied. A filtration flux below 25 I/m2h is generally recommended, at below this flux, the MBR operated at sub-critical flux conditions, the filter cake was minimized and membrane fouling was mainly attributed to the membrane pore blocking. Moreover, the membrane fouling, at below 25 I/m2h, was more reversible to backwashing; above this value, backwashing became less efficient to clean the membrane. Less frequent backwashing (e.g. 600 s filtration/45 s backwashing) decreased the amount of fouling irreversible to backwashing and its performance was superior to that of frequent backwashing (e.g. 200 s filtration/15 s backwashing). The MBR suffered more fouling at low temperature conditions (e.g. at 13-14 degrees C) than at high temperature conditions (e.g. at 17-18 degrees C). A conceptual model was built up and successfully interpreted this temperature effect.     

一体式膜生物反应器膜污染防治理论研究

从理论上分析了一体式膜生物反应器内曝气量与污水流量、反应器设计高度、主副腔过流面积的关系,膜外污染错流临界流速与抽吸压力及主腔宽度的关系,膜管内污染的临界速度与膜管径、膜壁吸力的关系,并得出采用化学方法消除膜污染的原理。     

Comparison of membrane biofouling in nitrification and denitrification for the membrane bioreactor (MBR).

Batch filtration tests were conducted to compare the characteristics of membrane biofouling with regard to nitrification and denitrification. A Modified Fouling Index (MFI) was obtained using a stirred cell tester. The denitrification assays showed higher membrane fouling rates than the nitrification assays. The fouling became worse, not only due to pore blocking resistance, but also from cake layer resistance after denitrification. The Extracellular Polymeric Substances (EPS) concentration and relative hydrophobicity were decreased after denitrification, resulting in floc deterioration. The floc deterioration was assumed to have increased the cake layer resistance in the filtration test. The protein Soluble Microbial Products (SMP) concentration, portion of high molecular weight in carbohydrate SMP and relative hydrophobicity were increased after denitrification, which was assumed to cause membrane pore blocking. The changes in the EPS and SMP characteristics were the main fouling parameters in denitrification.     

Correlations between silt density index,turbidity and oxidation-reduction potential parameters in seawater reverse osmosis desalination

The reverse osmosis method is one of the most widely used methods of seawater desalination at present.Hydrophilic and desalting membranes in reverse osmosis systems are highly susceptible to the input pollutants.Various contaminants,including suspended organic and inorganic matter,result in membrane fouling and membrane degradation.Fundamental parameters such as the turbidity,the amount of chlorine injection,and silt density index(SDI)are the most predominant parameters of fouling control in the membranes.In this study,the operation system included a water intake unit,a pretreatment system,and an RO system.The pretreatment system encompassed a clarifier,a gravity sand filter,pressurized sand filters,and a cartridge filter.The correlation between the amount of chlorine injection in terms of the oxidation-reduction potential(ORP)and the SDI value of the input water was investigated at a specified site next to the Persian Gulf.The results showed that,at certain intervals of inlet turbidity,injection of a certain amount of chlorine into the raw water has a distinct effect on the decrease of SDI.     

Pulse shear stress for anaerobic membrane bioreactor fouling control

Increase of shear stress at membrane surfaces is a generally applied strategy to minimize membrane fouling. It has been reported that a two-phase flow, better known as slug flow, is an effective way to increase shear stress. Hence, slug flow was introduced into an anaerobic membrane bioreactor for membrane fouling control. Anaerobic suspended sludge was cultured in an anaerobic membrane bioreactor (AMBR) operated with a side stream inside-out tubular membrane unit applying sustainable flux flow regimes. The averaged particle diameter decreased from 20 to 5 microm during operation of the AMBR. However, the COD removal efficiency did not show any significant deterioration, whereas the specific methanogenic activity (SMA) increased from 0.16 to 0.41 gCOD/g VSS/day. Nevertheless, the imposed gas slug appeared to be insufficient for adequate fouling control, resulting in rapidly increasing trans membrane pressures (TMP) operating at a flux exceeding 16 L/m2/h. Addition of powdered activated carbon (PAC) enhanced the effect of slug flow on membrane fouling. However, the combined effect was still considered as not being significant. The tubular membrane was subsequently equipped with inert inserts for creating a locally increased shear stress for enhanced fouling control. Results show an increase in the membrane flux from 16 L/m2/h to 34 L/m2/h after the inserts were mounted in the membrane tube.     

Evaluation of innovative operation concept for flat sheet MBR filtration system.

One of the most limiting factors for the extension and acceptance of MBR filtration systems for municipal and industrial wastewater is the impact of membrane fouling on maintenance, operation and cleaning efforts. One field of action in the European Research Project "AMEDEUS" is the development and testing of MBR module concepts with innovative fouling-prevention technology from three European module manufacturers.This article deals with the performances of the flat-sheet modules by A3 Water Solutions GmbH in double-deck configuration evaluated over 10 months in Anjou Recherche under typical biological operation conditions for MBR systems (MLSS = 10 g/l; SRT = 25 days). By using a double-deck configuration, it is possible to operate with a net flux of 25.5 l/m2.h at 20 degrees C, a membrane air flow rate of 0.21 Nm3/h.m2 of membrane to achieve a stable permeability of around 500-600 l/m2.h.bar. Additionally, it was observed that it is possible to recover the membrane performance after biofouling during operation without intensive cleaning and to maintain stable permeability during peak flows.The evaluated concepts for equipping and operating MBR systems will be applied to several full-scale plants constructed by A3 Water Solutions GmbH.     

Tracking particle size distributions in a moving bed biofilm membrane reactor for treatment of municipal wastewater.

A study has been conducted to investigate the effect of loading rates on membrane fouling in a moving bed biofilm membrane reactor process for municipal wastewater treatment, especially analysing the fate of submicron colloidal particles and their influence on membrane fouling. Two operating conditions defined as low and high organic loading rates were tested where the development and fate of the particulate material was characterised analysing the particle size distributions throughout the process. Analysis of the membrane performance showed higher fouling rates for the high-rate conditions. The fraction of colloidal submicron particles was higher in the membrane reactor indicating that fouling by these particles was a dominant contribution to membrane fouling.     

Scale formation in NF/RO: mechanism and control.

Scale formation of soluble salts is one of the major factors limiting the application of nanofiltration (NF) and reverse osmosis (RO) membranes. This article reviews the scale formation mechanisms in membrane systems, methods to retard scale formation, and a new hybrid system consisting of MF-NF/RO. Two distinct mechanisms in NF/RO fouling by scale formation including surface and bulk crystallization have been identified and investigated. The hydrodynamic operating conditions as well as module geometry determines which fouling mechanism is dominant. An increase in solute concentration at the membrane surface by concentration polarization is responsible for surface crystallization. Conventional methods for scale control only retard the rate of scale formation and their performances are unpredictable. On the other hand, using a MF-NF/RO hybrid system for continuous removal of crystal particles from the retentate stream appears to be effective at high recovery of permeate. When applying the MF-NF/RO hybrid system, substantial improvement in flux is observed in spiral wound module, whereas it is negligible in case of the tubular module. This is because the microfilter could only removes crystals formed in the retentate through the bulk crystallization that is the dominant fouling mechanism in the spiral wound module.