Periodic air/water cleaning for control of biofouling in spiral wound membrane elements

The main problem during the operation of nanofiltration or reverse osmosis membrane plants is fouling of feed spacers in membrane elements due to biofouling and particulate fouling. In order to control biofouling and particulate fouling in membrane elements, both daily air/water cleaning (AWC) and daily copper sulphate dosing (CSD) were investigated and compared to a reference without daily cleaning. A pilot study was carried out for 110 days with three parallel spiral wound membrane elements; AWC, CSD and the reference which were fed by tap water enriched with a biodegradable compound (100 μg acetate-C/L). The CSD element, which combined daily copper sulphate dosing and sporadically air/water cleaning, performed best with an increase in pressure drop of 18% and a biomass concentration of 8000 pg ATP/cm² within 110 days. This was followed by the AWC element with a pressure increase of 37% and biomass concentration of 20,000 pg ATP/cm² within 110 days. The reference element showed a pressure increase of 120% within 21 days. The presented approach is considered very successful in controlling particulate fouling and biofouling, especially when air/water cleaning is combined with copper sulphate dosing.     

Membrane fouling and cleaning in ultrafiltration of wastewater from banknote printing works

The hollow fiber blend membrane, fouled by plant wastewater from banknote printing works, was characterized with SEM and the fouling elements were investigated by EDX. Based on analysis results, fouling process in ultrafiltration was simulated by using the model substances, which exist in the wastewater from banknote printing works, such as Turkey red oil, sodium hydroxide and calcium ion. It is observed that the reaction between Turkey red oil and calcium ion forms sediments, which leads to the beginning of membrane fouling. Furthermore, a four-step cleaning method, including de-ionized water cleaning, hydrochloric acid (0.1N) aqueous solution cleaning, second de-ionized water cleaning and sodium hydroxide (1 wt.%) aqueous solution cleaning, was used to clean the seriously fouled membrane in both lab and plant scale (membrane areas were 0.0157 and 80 m², respectively) experiments. The results show that the cleaning method is effective. The membrane surface after cleaning was also analyzed by SEM/EDX and the foulants in the cleaning solutions were identified by TOC and ICP. According to these experimental results, the mechanisms of membrane fouling and cleaning were proposed. The four-step cleaning method has been widely used in the ultrafiltration of wastewater from banknote printing works.     

The cleaning of ultrafiltration membranes fouled by protein

The relationship between membrane fouling and cleaning was investigated in terms of flow conditions, transmembrane pressures, pH, membrane properties, and cleaning agents using a stirred batch cell and aqueous albumin solution. Fouling was less at the pH extremes than at the isoelectric point for both retentive and partially permeable membranes. Membranes with partial permeability showed a greater tendency for fouling and were less responsive to cleaning than retentive membranes. The results in the stirred cell were shown to be similar to those for a crossflow module under similar operating conditions.     

Forward Osmosis Membranes for Water Reclamation

This review addressed the fundamental principles, advantages and challenges of forward osmosis (FO) membrane processes. FO is receiving more and more research attractions because it can concurrently produce clean water with low energy input and generate hydraulic energy (pressure retarded osmosis). FO typically requires zero or low hydraulic driving pressure, therefore the fouling potential of the FO membranes is much lower than conventional pressure-driven membrane processes. However, concentration polarization (CP), especially the internal CP significantly reduces the effective osmotic pressure across the FO membrane, the major driving force for the filtration process. As a result, innovative FO membrane materials like electrospun nanofibers have been explored to make low tortuosity, high porosity, and thin FO membranes with a high rejection rate of solutes and low or zero diffusion of the draw solute. The orientation of the FO membrane with active layer-facing-feed solution has less fouling than active layer-facing-draw solution. In addition, to further decrease the fouling potential, a hydrophilic and more negatively charged membrane is preferred when filtration of natural organic matter (NOM) or alginate in the absence of multivalent cations.     

Critical flux measurement for model colloids

The conventional operating membrane of a laboratory membrane filtration process is to apply controlled transmembrane pressures to the retentate side of the membrane, with the permeate side open ended. Often the minimum transmembrane pressure available is sufficient to cause membrane fouling in a given system. A membrane rig has been built which monitors transmembrane pressure in increments of 0.001 bar and by pumping permeate at a specified rate controls the flux to be constant. The technique used allows sensitive detection of trace fouling. Under a variety of low flux conditions fouling was not observed and it was found to be useful to produce an experimentally related definition of two types of critical flux. In the first definition a `strong form' of critical flux exists if the flux of a suspension is identical to the flux of clean water at the same transmembrane pressure. In the second definition a `weak form' of the critical flux exists if the relationship between transmembrane pressure and flux is linear, but the slope of the line differs from that for clean water. This paper describes how the use of this operating mode led to the successful experimental measurements of critical fluxes for two colloidal silica suspensions, BSA solution and a baker's yeast suspension with a 50k MWCO membrane. These measurements could not be made successfully in constant-pressure mode. The paper also reports experimental evidence in support of a `strong form' of the critical flux for the filtration of X30 silica suspension. Finally, we report the effect of membrane pore size on critical flux measurements for the three types of feed fluids.     

Synergetic cleaning procedure for a ceramic membrane fouled by beer microfiltration

Apart from considerations for hygienic operation, membrane cleaning is essential to maintain consistent permeability and selectivity of membrane systems for clarifying beer and beverages where balanced fractionation of particles/macromolecules is necessary. Experiments involved formulating and optimising chemical cleaning methods for a ceramic microfiltration membrane, which had been severely fouled during clarification of a commercial beer. The cleaning processes employed NaOH, HNO₃, H₂O₂, and Ultrasil 11 as the chemical cleaning agents. The cleaning ability and cleaning kinetics of the processes were evaluated in parallel with the study of the fouling mechanism, formation and strength so as to elucidate the synergetic relationship between fouling and cleaning. A three-step cleaning mechanism was postulated. This led to the development of a fast and effective combined simultaneous caustic cleaning and oxidation method (CSCCO), which was able to restore 87% of the original membrane's water permeability within 8 min. Analysis suggested the concept of a cleaning energy barrier E_c and a cleaning rate constant k_c0. This study confirmed the existence of a synergetic relationship between the prior fouling and optimum formulation of cleaner and optimal cleaning condition. The study varied beer filtration conditions. Transmembrane pressure (TMP) and crossflow velocity during fouling appeared to have a minimal effect on the membrane's subsequent cleanability, especially when the powerful CSCCO process was employed. The number of previous fouling/cleaning cycles was influential. A complete removal of the residual fouling, formed on the virgin membrane's surface proved beyond the means of the harsh chemical cleaning used under any conditions. The degree of residual fouling eventually reached a plateau and a level of 87% of the original water flux could be restored repeatedly.     

Critical flux in NF of high molar mass polysaccharides and effluents from the paper industry

High molar mass polysaccharides (locust bean gum and karaya gum) and effluents from a mechanical pulp mill and a paper mill were nanofiltered with commercially available nanofiltration (NF) membranes. The effect of the filtration conditions on the flux (critical flux), retention, and the fouling of the membranes was studied. The experiments were conducted by increasing and decreasing the pressure and measuring the flux thus obtained.

The critical flux was observed to increase with increasing flow velocity and decreasing concentration. An increase in pH increased the electrostatic repulsion between the membrane and the dissociated (charged) components in the paper mill effluents. As a result, a higher critical flux was obtained and also the retentions of the charged substances improved. Only a weak form of the critical flux was observed with the mill effluents. The permeate flux deviated from the pure water flux even at the lowest pressure, but increased linearly with pressure until the weak form of the critical flux was exceeded. The small decrease in flux immediately after filtration was started was probably caused by the plugging of the free spaces in the membranes or by the adsorption of foulants onto the membrane surface.

In the filtrations with the high molar mass polysaccharides, a strong form of the critical flux as well as a weak form were observed. The significant irreversible fouling of the most hydrophobic membrane was due to adsorption of the model substances by hydrophobic interaction. A precleaning of the membranes with an alkaline cleaning agent improved the pure water fluxes by up to 30%, but it had only a small effect on the critical or the limiting flux. The pure water fluxes of precleaned membranes after filtration were still higher than the pure water fluxes of the untreated membranes before filtration.     

Membrane performance with a pulp mill effluent: Relative contributions of fouling mechanisms

Severe flux decline was observed during ultrafiltration of a pulp mill effluent. Membrane fouling was the result of varying combinations of adsorption, pore plugging and concentration polarization or gel layer formation. A wide range of membrane materials and pore sizes were evaluated, showing the relationship between the membrane material, pore size and the relative contribution of the different fouling mechanisms. Individual resistances were evaluated for adsorption, R_a, pore plugging, R_pp, and concentration polarization, R_cp, using a series resistance model. These were based on the pure water flux for (1) the new membrane, J_i, (2) after static adsorption with the mill effluent, J_a, (3) the product rate when ultrafiltering the effluent, J_v, and (4) the pure water permeability with the fouled membrane, J_f. These resistances were shown to be misleading in terms of the observed flux loss for cases with significant adsorptive fouling. Adsorptive fouling was underestimated and concentration polarization overestimated. An alternative method, which we shall call flux loss ratios, is proposed, which is based on the flux decline due to a particular mechanisms as a fraction of the overall flux decline. These new measures more accurately reflect the flux decline attributable to each fouling mechanism.     

Chemical and physical aspects of organic fouling of forward osmosis membranes

The growing attention to forward osmosis (FO) membrane processes from various disciplines raises the demand for systematic research on FO membrane fouling. This study investigates the role of various physical and chemical interactions, such as intermolecular adhesion forces, calcium binding, initial permeate flux, and membrane orientation, in organic fouling of forward osmosis membranes. Alginate, bovine serum albumin (BSA), and Aldrich humic acid (AHA) were chosen as model organic foulants. Atomic force microscopy (AFM) was used to quantify the intermolecular adhesion forces between the foulant and the clean or fouled membrane in order to better understand the fouling mechanisms. A strong correlation between organic fouling and intermolecular adhesion was observed, indicating that foulant–foulant interaction plays an important role in determining the rate and extent of organic fouling. The fouling data showed that FO fouling is governed by the coupled influence of chemical and hydrodynamic interactions. Calcium binding, permeation drag, and hydrodynamic shear force are the major factors governing the development of a fouling layer on the membrane surface. However, the dominating factors controlling membrane fouling vary from foulant to foulant. With stronger intermolecular adhesion forces, hydrodynamic conditions for favorable foulant deposition leading to cake formation are more readily attained. Before a compact cake layer is formed, the fouling rate is affected by both the intermolecular adhesion forces and hydrodynamic conditions. However, once the cake layer forms, all three foulants have very similar flux decline rates, and further changes in hydrodynamic conditions do not influence fouling behavior.     

A new normalization method for determination of colloidal fouling potential in membrane processes

Normalization of permeate flux data is widely used to characterize membrane fouling under different experimental conditions. The main intention of normalization is to allow a fair comparison of feed water fouling potentials by eliminating the effects of different operational parameters used in the experiments, such as net driving pressure and clean-membrane resistance. However, it was demonstrated that the commonly used intuitive normalization methods usually could not serve their intended purpose. In this study, a new normalization method was proposed for characterizing water-fouling potential based on fundamental principles of membrane fouling. The intention of this normalization method was to define a fouling potential for feed water that was independent of, or at least, not strongly affected by operational conditions. Laboratory-scale ultrafiltration fouling tests were conducted under different colloid sizes, concentrations, and driving pressures. The experiments showed that the fouling potentials defined by the newly proposed normalization method were linearly related to the colloid concentration of the feed water and that the effect of operational conditions used in the fouling experiments on the fouling potential was minimal.     

Statistical analysis of data from accelerated ageing tests of PES UF membranes

In this research, membrane life-time was evaluated by means of accelerated ageing experiments. A pressure pulse unit was used to perform the ageing experiments in an accelerated way. An experimental design has been set up and four ageing factors were varied at two levels. The four ageing factors studied were: fouling status of the membrane, cleaning agent concentration, magnitude of the back pulse and number of applied back pulses. The integrity of the membrane modules was evaluated by means of permeability testing, pressure decay tests and bubble tests. Also tensile tests were performed to investigate the mechanical properties of the membrane modules. The collected data was used for an analysis of variance to determine which ageing factors and which combination of ageing factors influence membrane life time. The analysis showed that the fouling status in combination with the number of applied pressure pulses were significant ageing factors. Additional tensile tests confirmed these results.     

Ultrasonic time-domain reflectometry as a non-destructive instrumental visualization technique to monitor inorganic fouling and cleaning on reverse osmosis membranes

Fouling is readily acknowledged as one of the most critical problems limiting the wider application of membranes in liquid separation processes. A better understanding of fouling layer formation and its monitoring is needed in order to improve on existing cleaning techniques. The overall objective of this research was to develop a non-destructive, real-time, in situ visualization technique or device for fouling layer monitoring. Ultrasonic time-domain reflectometry (UTDR) was employed as a visualization technique to provide real-time characterization of the fouling layer. The fouling experiment was carried out with 2 g/l calcium carbonate solution. Results confirmed that there is a correspondence between the flux decline behavior and the UTDR response from membranes in reverse osmosis (RO) modules. The ultrasonic technique could effectively detect fouling layer initiation and growth on the membrane in real-time at different axial velocities. In addition to the measurement of fouling, the ultrasonic technique was also successfully employed for monitoring membrane cleaning. The UTDR technique, due to its extremely powerful capabilities and its use in monitoring devices, can be of great significance in the membrane industry.     

Application of heat-activated peroxydisulfate process for the chemical cleaning of fouled ultrafiltration membranes

Na Cl O has been widely used to restore membrane flux in practical membrane cleaning processes, which would induce the formation of toxic halogenated byproducts. In this study, we proposed a novel heatactivated peroxydisulfate(heat/PDS) process to clean the membrane fouling derived from humic acid(HA). The results show that the combination of heat and PDS can achieve almost 100% recovery of permeate flux after soaking the HA-fouled membrane in 1 mmol/L PDS solution at 50 °C for 2 h, which is att...     

Controlled flux behaviour of membrane processes

Controlling ultrafiltration (UF) and microfiltration (MF) membrane fluxes at or around a region where fouling is minimal can provide an interesting and economic operating regime. Selectivity may be enhanced and cleaning may be easier. For a given flux it is sometimes possible to filter a product suspension at the same trans-membrane pressure (TMP) as for pure water (PWP), but this can require a lot of energy input to maintain cross-flow or high shear in other ways if high fluxes are required. The critical flux is the flux above which one starts to observe fouling. By operating at lower cross-flow velocities and just above the critical flux, and thus, with lower TMPs, periodic cleaning can be effected by temporarily stopping permeation. A change in feed rate demands a change in flux which is obtained by temporarily increasing energy inputs. Controlled flux improves macromolecular fractionation. As flux increases the rejection of high molecular weight materials decreases whilst that of lower molecular weight materials decreases. This paper discusses the causes of fouling and the use controlled flux operation to mitigate its effects.     

NaClO原位清洗对膜生物反应器生物膜污染影响研究进展

膜生物反应器(MBR)的膜污染问题严重制约了该工艺进一步快速的商业化推广,全面认识NaClO原位氧化清洗对MBR生物膜污染的影响,对于开发新型膜清洗技术及MBR工程优化具有重要意义。本文从微生物胞外关键组分空间分布角度综述了NaClO原位清洗对生物膜污染及生物絮凝的影响,并探讨了生物絮体重构机制及强化生物絮凝的相关措施。最后,本文从减缓膜污染的角度,对该领域未来的研究方向进行了论述。     

Irreversible membrane fouling in microfiltration membranes filtering coagulated surface water

Chemical coagulation has been widely used as a method to mitigate membrane fouling in MF/UF membranes used for drinking water treatment. Optimization of coagulation as pre-treatment of membrane processes has not been achieved yet: the optimum condition of coagulation for conventional treatment systems is not necessarily applicable to membrane-based treatment systems. This study investigated (physically) irreversible membrane fouling in an MF membrane used with pre-coagulation by aluminum salt. In a series of bench-scale filtration tests, feed water containing commercially available humic acid or organic matter isolated from surface water was coagulated with polyaluminum chloride (PACl) under various conditions and subsequently filtered with an MF membrane with the nominal pore size of 0.1 μm. It was found that coagulation conditions had great impacts on the degree of physically irreversible fouling. Acidic conditions improved the quality of treated water but generally caused greater physically irreversible fouling than did neutral or alkaline conditions. Also, dosage of coagulant was found to be influential on the degree of membrane fouling: high dosage of coagulant frequently caused more severe irreversible fouling. Sizes of flocs seemed to become small under acidic conditions in this study, which was indicated by high concentrations of aluminum in the permeate under acidic conditions. It is thought that small flocs produced under acidic conditions could migrate into micropores of the membrane and caused physically irreversible fouling by plugging or adsorption. These findings obtained in the bench-scale tests were verified in a long-term pilot-scale test.     

One‐step electrochemical deposition to achieve superhydrophobic cobalt incorporated amorphous carbon‐based film with self‐cleaning and anti‐corrosion

Exploiting a superhydrophobic surface is very significant due to its excellent water repellency which has many practical applications in various fields. In this work, the cobalt incorporated amorphous carbon‐based (Co/a‐C:H) film was prepared successfully on Si substrate via a simple 1‐step electrochemical deposition where electrochemical deposition technology was using cobalt (II) acetylacetonate methanol solution as electrolyte under high voltage, atmospheric pressure, and low temperature. Surprisingly, the as‐prepared film showed a superior superhydrophobic surface with a water contact angle of 153 ± 1° and a sliding angle of 7.6° without any further modification of low surface energy materials. Especially, the tape adhesive, corrosion resistance, and self‐cleaning tests demonstrated that the as‐prepared carbon‐based film could possess fairly well adhesion, superior anti‐corrosion resistance, and self‐cleaning ability, respectively. It indicated that the superhydrophobic Co/a‐C:H film might have potential promising applications in the field of anti‐fouling, anti‐corrosion, and drag resistance, such as the above‐deck structures on icebreaker vessels, ship hulls, and offshore wind turbine blades.     

In situ monitoring techniques for concentration polarization and fouling phenomena in membrane filtration

Membrane fouling and subsequent permeate flux decline are inevitably associated with pressure-driven membrane processes. Despite the myriad of studies on membrane fouling and related phenomena--concentration polarization, cake formation and pore plugging--the fundamental mechanisms and processes involved are still not fully understood. A key to breakthroughs in understanding of fouling phenomena is the development of novel, non-invasive, in situ quantification of physico-chemical processes occurring during membrane filtration. State-of-the-art in situ monitoring techniques for concentration polarization, cake formation and fouling phenomena in pressure-driven membrane filtration are critically reviewed in this paper. The review addresses the physical principles and applications of the techniques as well as their strengths and deficiencies. Emphasis is given to techniques relevant to fouling phenomena where particles and solutes accumulate on the membrane surface such that pore plugging is negligible. The relevance of the techniques to specific processes and mechanisms involved in membrane fouling is also elaborated and discussed.     

Enzymatic cleaning of ultrafiltration membranes fouled by protein mixture solutions

Compared to other typical cleaning agents, application of enzyme in cleaning of membranes fouled with protein solution promised the high cleaning efficiencies with lower environmental impact. This paper is focused on the mechanisms of protein removal by enzyme cleaning agent from the membrane surface by analysis hydraulic resistance, total protein removal using Lowry method, and membrane surface analysis using MALDI-MS and gel electrophoresis to estimate the foulant composition. Using single and binary protein solutions of bovine serum albumin and beta-lactoglobulin as the feed solution for filtration process, the experimental results indicate that optimum cleaning time and cleaning agent concentration is due to the competition between foulant removal and deposition of enzymes on the membrane during the cleaning process. The removal rate of different protein species in the fouling layer is varied, indicating that cleaning strategies can be tailor-made for fouling layer with different protein compositions.     

Modeling of concentration polarization and depolarization with high-frequency backpulsing

Rapid backpulsing to reduce membrane fouling during crossflow microfiltration and ultrafiltration is studied by solving the convection-diffusion equation for concentration polarization and depolarization during cyclic operation with transmembrane pressure reversal. For a fixed duration of reverse filtration, there is a critical duration of forward filtration which must not be exceeded if the formation of a cake or gel layer on the membrane surface is to be avoided. The theory also predicts an optimum duration of forward filtration which maximizes the net flux, since backpulsing at too high of frequency does not allow for adequate permeate collection during forward filtration relative to that lost during reverse filtration, whereas backpulsing at too low of frequency results in significant flux decline due to cake or gel buildup during each period of forward filtration. In general, short backpulse durations, low feed concentrations, high shear rates, and high forward transmembrane pressures give the highest net fluxes, whereas the magnitude of the reverse transmembrane pressure has a relatively small effect.Rapid backpulsing experiments with yeast suspended in deionized water performed with a flat-sheet crossflow microfiltration module and cellulose acetate membranes with 0.07 μm average pore diameter. The optimum forward filtration times were found to be 1.5, 3, and 5 s, respectively, for backpulse durations of 0.1, 0.2, and 0.3 s. Both theory and experiment gave net fluxes with backpulsing of about 85% of the clean membrane flux (0.022 cm/s = 790 l/m² h), whereas the long-term flux in the absence of backpulsing is an order-of-magnitude lower (0.0026 cm/s = 94 l/m² h).