in ultrafiltration and powdered activated carbon adsorption

in ultrafiltration and powdered activated carbon adsorption

Desalination 255 (2010) 124–128 Contents lists available at ScienceDirect Desalination j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m...

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Desalination 255 (2010) 124–128

Contents lists available at ScienceDirect

Desalination j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / d e s a l

Removal of organic xenobiotics by combined out/in ultrafiltration and powdered activated carbon adsorption Ivana Ivancev-Tumbas a,⁎, Ralph Hobby b a b

Faculty of Sciences, University of Novi Sad, Trg Dositeja Obradovica 3, 21000 Novi Sad, Serbia University Duisburg-Essen, Faculty of Engineering Science, Institute of Energy and Environmental Engineering/Water Technology, Bismarckstraße 90, D-47057 Duisburg, Germany

a r t i c l e

i n f o

Article history: Received 16 September 2009 Received in revised form 23 December 2009 Accepted 7 January 2010 Available online 31 January 2010 Keywords: Organic micropollutants Xenobiotics UF/PAC Activated carbon Hybrid process Immersed membranes Ultrafiltration Carbamazepine

a b s t r a c t The combination of ultrafiltration and activated carbon adsorption is an attractive alternative for removal of organic xenobiotics from water even in comparison with GAC fixed bed adsorbers. Results from a lab-scale investigation of combined out/in ultrafiltration and powdered activated carbon adsorption (PAC) for removal of p-nitrophenol (c0 = 1 mg/L) are presented. The behavior of different types of carbons in conditions with and without air scouring was assessed. The most efficient carbon was the one with the smallest particle size and the fastest kinetics. Efficiency showed a decreasing trend with increasing carbon particle diameter and apparent density regardless of air scouring application. On the other hand, the effect of air scouring was different for different carbons. It was absent when the carbon with the fastest kinetics was applied. Activated carbons with slower rates of adsorption showed different behavior. Addition of coagulant in the PAC/UF system with smallest particles increased removal efficiency. Furthermore, the system was tested for the removal of carbamazepine (4 μg/L) and xenobiotics uptake by the membrane was assessed. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Low-pressure membrane technologies are recognized by the water industry as very attractive processes for producing drinking water [1], but also for the treatment of the wastewater in combination with coagulation and activated carbon adsorption [2], in MBR processess [3] or for the reuse of spent filter backwash water [4]. In 2003, more than 5 millions m3/d of drinking water was produced worldwide using lowpressure membranes, including MF and UF [5]. UF technology has been found to exceed current water regulations for turbidity, Giardia, and also virus removal. It has been used worldwide for treating various water sources. The technology has been optimized and is becoming competitive as compared to conventional processes for larger scale plant capacities. The capital and O&M costs of UF membrane technology are still expected to decrease. Whether it is used for drinking water treatment or for waste water treatment, it is possible to combine it with activated carbon which is an effective barrier regarding the removal of organic micropollutants (e.g. pesticides, pharmaceuticals and other organic contaminants). For this combination e. g. the CRISTAL® process is well known [1,6]. It seems to be attractive alternative for the removal of organic xenobiotics from water in comparison with GAC fixed bed adsorbers [7]. Besides the CRISTAL® process where an in/out membrane ⁎ Corresponding author. E-mail addresses: [email protected] (I. Ivancev-Tumbas), [email protected] (R. Hobby). 0011-9164/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2010.01.005

and PAC are combined, immersed out/in membranes can also be used in a hybrid PAC/UF process. Immersed membranes are attractive since no pressure vessels are necessary and the use of air scouring improves the tolerance to suspended solids [8]. An example of their use with PAC is a pilot study for the City of Racine, WI, USA [9] where this combination was studied for upgraded disinfection and removal of odorous compounds. The technique can easily be combined with coagulation and flocculation and give even broader possibilities to solve various problems related to water quality, without the additional time and space consuming sedimentation operation. The beneficial effects of PAC addition on NOM fouling in PAC/UF systems are well known. Systems with submerged membranes showed large performance dependency on particle size and distribution. The influence of PAC particle size and distribution on the performance of submerged and hollow fiber membranes (e.g. flux decline) was investigated [10]. Narrow distribution and a PAC size of approximately 100 times larger than the membrane pore helps to prevent blocking. It was decided that membrane surface properties as well as PAC properties should be investigated in order to develop a better understanding of the interactions between membrane and particles. The most recent developments in this area deal with submicrometre powdered activated carbon (0.6–0.8 μm) [11]. The beneficial effects of super-powdered activated carbon were proven for adsorptive removal of geosmin by ceramic membrane filtration. Jia et al [12] investigated the effect of air scouring on atrazine adsorption by PAC (c0 = 200 µg/L). Their experiments showed that

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the adsorption process could benefit from air scouring in the case of 6.3 µm PAC particles (D50) which were obtained by grinding the PAC. Furthermore, a hybrid powdered activated carbon submerged microfiltration hollow fiber membrane system with air scouring was modeled with two strategies of batch and continuous PAC dosing for trace organics removal [13]. Experimental verification of the model was carried out. It was shown that the adsorption rate during batch PAC dosing mode can be improved by increasing the scouring rate or operation flux. In batch dosing mode it is assumed that the reactor is fully mixed by air scouring once the total mass of PAC is added. This was explained by the ability of higher scouring rates to facilitate the adsorption kinetics, because the air scouring mixes the suspension and assists contact between the pollutant and the PAC particles. The diffusion resistance of the liquid film surrounding the particle surface can be reduced. The removal of 17β-estradiol (E2) by PAC submerged microfiltration hybrid process was studied at a low initial concentration of 100 ng/L [14]. It was concluded that it is possible to remove E2 by PAC layer deposited on the membrane, but to a lesser extent than on PAC in the bulk phase. Overall removal was largely dependent on the fraction of deposited PAC which is influenced by the operating parameters (flux, hydraulic retention time, mixing intensity). The goal of this study was to compare different powdered activated carbons in combination with out/in membrane filtration for removal of organic xenobiotics using lab-scale apparatus. The effects of air scouring and coagulant addition were tested. Paranitrophenol (PNP) at concentration of 1 mg/L was used as a suitable and cheap test substance for comparison since it can be measured immediately upon collection of samples by simple UV spectroscopy. In addition, an experiment was performed in order to obtain information about removal efficiency of pharmaceutical carbamazepine at a three orders of magnitude lower concentration than PNP (carbamazepine initial concentration 4 μg/L). Permeate quality was monitored and an estimation was made on how much organic xenobiotics were “taken up” by the membrane. 2. Experimental The out/in hybrid process (ultrafiltration (UF) with powdered activated carbon (PAC)) was investigated in a lab-scale apparatus with GE ZW-1 ZeeWeed module (General Electrics) with a non-ionic hydrophilic membrane immersed in 1.5 L of solution of organic xenobiotic in 2 L beaker (Fig. 1). The solution had continuous flow through the membrane with a flux of near 20 L/m2/h (i. e., 0.8–1.2 L/h). Prominent pump gamma/4-1 with pressure holder Prominent DHV-SDK 1–10 bar 6–12ff was used for pumping the solution into the beaker. A Watson Marlow SCIQ 323 pump was used for ultrafiltration. Pressure was controlled by TIF instruments manometer. After equalization of feed and permeate concentration, PAC was added as a single dose

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(7.5 mg or 15 mg for experiments with PNP, that is equivalent to 5 and 10 mg/L in the beaker, and 0.45 mg for the experiment with carbamazepine, that is equivalent to 0.3 mg/L in the beaker) directly into the solution where the module was immersed. The same amount of PAC was in contact with the water stream for 120–150 min. The average contact time of water and carbon was 1.5 h. Every 15 min a backpulse with permeate was applied for 15 s without carbon loss from the system. Slow mixing by magnetic stirrer was applied during the experiment. In experiments where coagulation was tested, Al-based coagulant was added in a single dose of 0.15 mmol during fast mixing phase (1 min) together with PAC into the reactor. Thus the initial concentration in the beaker was 0.1 mmol/L. After that, slow mixing was continued with continual water flow and without any addition of coagulant or carbon. Simple tests without membrane filtration were also performed, to find out how much PNP can be removed as a result of coagulation and activated carbon adsorption alone. In a glass beaker of the same dimensions as the reactor, coagulant was dosed (0.1 mmol/l) together with 10 mg/L of NORIT SA UF in three different ways: • Sample A: firstly carbon was added and after 10 min of adsorption coagulant was added during the fast mixing phase (1 min, 1000 min − 1 ). After that, 15 min slow mixing was applied (200 min− 1) and the sample was allowed to settle for an additional 15 min. • Sample B: coagulant and carbon were added together during the 1 min fast mixing phase, then slow mixing was applied for 15 min and the settling phase was 15 min. • Sample C: firstly coagulant was added during the fast mixing phase (1 min). Further slow mixing was applied for 15 min. During 5th minute of slow mixing carbon was added. Furthermore, one trial of membrane filtration with coagulant addition and without PAC addition was performed in order to assess the removal efficiency of the coagulant itself. When applied, air scouring was done continuously during the whole experiment by ASF Thomas pump (1.8 L/min) while investigating the PAC/UF process, whereas for the hybrid process coagulation/PAC/UF air scouring was performed only during the backpulse phase. 3. Materials Removal efficiency was tested for three different PAC types (NORIT SA UF, NORIT SA 2 and CHEMVIRON carbon PULSORB RD 90). Their characteristics are given in Table 1. Two different xenobiotics were tested, namely p-nitrophenol (1 mg/L) in conditions with and without air scouring and

Table 1 Characteristics of the activated carbons applied.

Fig. 1. Schematic of experimental set-up of out/in hybrid process.

Parameter

NORIT SA UF

NORIT SA 2

CHEMVIRON PULSORB RD 90

D50, µm For PULSORB RD 90 mean particle diameter Apparent density, kg/m3 Iodine number Particle size >400 μm Particle size >180 μm BET, m2/g Equilibrium concentration of PNP in bottle isotherm tests (24 h with two PAC doses: 5 mg/L/10 mg/L), mg/L KF, (g/kg)/(mg/L)n [7] n [7]

5

20

– 10

160 1100 Max 0.0 Max 0.1 1200 0.510/0.217

460 850

300 1100

950 0.536/0.240

0.1 1100 0.573/0.278

127.9 0.3

112.2 0.27

88.5 0.23

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carbamazepine (4 μg/L) with air scouring. All experiments were done in tap water (pH = 7.2, DOC = 0.5 mg/L). 4. Analysis For the PNP analysis, samples were acidified to pH 2 with sulphuric acid and the UV absorbance measured at 317 nm. No significant matrix interference was found for the tap water under the given conditions. The instrument detection limit was 0.014 mg/L. The accuracy of the measurements was determined at three concentration levels: the recovery for 0.2 mg/L was 93.1% ± 1.2%, at 0.5 mg/L the recovery was 98.9% ± 1.8% and at 1.0 mg/L it was 99.4% ± 0.15%. The reproducibility of the carbon dosing procedure was tested and the measured deviations for given PAC concentrations range were up to 3.3%. The equilibrium adsorption parameters and solid phase concentration dependence on concentration of the carbons are given elsewhere [7]. For the carbamazepine analysis, whole samples between backpulse intervals were collected (250 ml) and sent to the laboratory of IWW, Water Center for analysis by GC/MS. The carbamazepine recovery at a concentration level of 5 µg/L was measured as 76.2%. 5. Results and discussion A comparison of carbon efficiencies is given in Fig. 2. The flow of water was continuous through the system and PAC was applied at once in the beaker at the beginning of filtration. Experiments for investigating the removal efficiency of different carbons resulted in a kind of “breakthrough” behavior under given conditions which is in accordance with literature [13]. One can conclude that the order of carbon efficiency is NORIT SA UF > Chemviron PULSORB RD 90 > NORIT SA 2. Efficiency showed a decreasing trend with increasing carbon particle diameter and apparent density regardless of air scouring application. Better removal efficiency was achieved with PULSORB RD90 than with NORIT SA2 although in batch isotherm tests a higher adsorption capacity was measured for NORIT SA2 carbon. This means that the kinetic behavior of carbons has to be taken into consideration. Using KIN Version 3.0 software [15] adsorption kinetics were modeled for three different carbons assuming that intraparticle diffusion is the dominating mass transfer process described by linear driving force. Surface diffusion coefficients (kSaV) were calculated by another software package [16] based on the dimensions of carbon particles given by the manufacturers and adsorption equilibrium parameters (Table 1). A diffusion coefficient DL of 6.081 × 10− 10 m2/s (12 °C, 139 g/mol) and surface diffusion coefficients of 1.13 × 10− 3 s− 1,

Fig. 2. Comparison of different carbons efficiencies (NORIT SA 2, NORIT SA UF and CHEMVIRON PULSORB RD 90) for PNP removal in combination with ultrafiltration with and without air scouring (carbon dose 15 mg, c0 = 1 mg/L).

6.76 × 10− 4 s− 1 and 3 × 10− 4 s− 1 were calculated for NORIT SA UF, PULSORB RD 90 and NORIT SA 2 respectively and assumed to be constant. Adsorption kinetics were modeled based on those results together with KF, n and D50 values (Table 1), carbon concentration (5 mg/L) and initial concentration of the solution (1 mg/L). Results are presented in Fig. 3. Besides the modeled values, kinetics were measured for two carbons: NORIT SA UF and PULSORB RD 90. Very good agreement between the observed and modeled values was found for those two carbons (Fig. 3). NORIT SA UF has the fastest kinetics (Fig. 3). Comparing modeled kinetics for PUSLORB RD90 and NORIT SA2, a slightly faster rate of adsorption is expected with PULSORB RD 90, for which a lower adsorption capacity is observed (Table 1). This could explain better performance of this carbon presented in Fig 2. during the PAC/UF process.

5.1. Effect of air scouring One can conclude for Chemviron PULSORB RD 90 activated carbon (mean particle diameter 10 μm) that air scouring has a slightly positive effect on removal efficiency during the first 20 min (effect ≥5% observed in three measurements, Fig. 2). After 20 min of filtration, better removal efficiency was observed without air scouring application (≥5%). This negative effect of scouring might be caused by active layer disturbance. Values below 5% were assumed to be negligible due to experimental error. Such results are in accordance with the literature findings of Jia et al. [12] who reported that aeration improved the rate of atrazine adsorption in static tests (for D50 = 6.3 µm carbon particles) during the initial stage of adsorption (e.g. 60 min, c0 = 200 µg/L, PAC dose 5 mg/L). In experiments with carbon with larger particles (NORIT SA2, D50 = 20 μm) such an effect was not observed (Fig. 2). This might be explained by assuming PAC layer formation on the membrane surface from the beginning of the cycle since it is more readily formed for activated carbon with larger particles and higher apparent density because it is able to settle faster. For carbon with smaller particles (PULSORB RD 90) the positive effect of increasing the adsorption rate by air scouring seems to be possible during the initial phase when the layer is still not formed. Once the layer is formed scouring might disturb it and thus might lower removal efficiency. There are literature findings which support the hypothesis of formation of an active layer on the membrane surface [14]. Experiments were carried out for 17β-estradiol ( E2) removal (c0 = 100 ng/L) [14] and different concentrations of E2 were observed in a reactor where the membrane was immersed and in the permeate. After an initial drop the concentration of E2 in the reactor starts to increase, while in the permeate the concentration was constant due to the activity of the deposited PAC layer on the membrane. This actually proved activity of a small amount of PAC deposited on the membrane [14]. Measurements were made to investigate the influence of air scouring on the carbon with the smallest particles, NORIT SA UF

Fig. 3. Kinetic evaluation of activated carbons.

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(D50 = 5 μm). Measurements were done in triplicate without air scouring and in duplicate with air scouring. Their results are presented in Fig. 4. Better removal of PNP was achieved without air scouring in two of three cases. However, for those two cases (Fig. 4, samples without air scouring 1st and 2nd trial) it is characteristic that a regular membrane cleaning protocol with NaOCl solution was still not established before the experiments. This means that before the first trial 9 days elapsed since membrane disinfection while for the 2nd trial two weeks elapsed. Because of the non reproducible results shown in Fig. 4 (without air scouring 1st and 2nd trial), a special protocol was established where the membrane was cleaned intensively with NaOCl solution before each carbon type trial. It is important to mention that the protocol was applied for all other measurements presented in the paper (Figs. 2, 5 and 6). After cleaning saturation of the system with xenobiotic solution was performed until the feeding and permeate concentration were identical. After that PAC was added at the start of experiment. In between experiments with the same carbon type half an hour of air scouring with backwashing was applied to clean the membrane surface in clean water, and after that saturation with xenobiotic solution was performed again until the concentration in the permeate was the same as in the starting solution. After such intensive cleaning of the membrane, the results for filtration without air scouring 3rd trial were similar to those obtained with air scouring (1st and 2nd trial). Based on those results, it is possible to conclude that there were no significant differences between runs with and without air scouring when the membrane is used 1–2 days after intensive cleaning with NaOCl (differences ≤ 5%). One possible explanation might be the extremely fast kinetics of NORIT SA UF carbon in comparison with the other two carbons. In cases where a longer period elapsed after cleaning, performance is better without air scouring, most probably due to the additional adsorption of PNP on the fouling layer or biofilm formed on the membrane. Further investigation of these phenomena is needed as they cannot be fully assessed and explained on the results obtained to date. In general, part of the PAC in the reactor moves towards the membrane and accumulates on the surface due to continuous suction, despite mechanical steering and air scouring. This was visible on the membrane surface. It seems that a beneficial effect from air scouring on mixing and local mass transfer is to be expected with the carbon with slower kinetics, smaller particle diameter and apparent density (e.g. PAC type PULSORB RD 90, Fig. 2). For the carbon with a high adsorption rate (e.g. the extremely fast kinetics of NORIT SA UF) the effect was absent as well as for the carbon with larger particles and higher apparent density (NORIT SA 2) which settles very fast on the membrane surface.

Fig. 4. The effect of air scouring (carbon NORIT SA UF, dose 7.5 mg, PNP solution c0 = 1 mg/L).

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Fig. 5. PAC/UF process for PNP removal with coagulation aid (0.15 mmol Al): A — blank without carbon and without air scouring; B — NORIT SA UF without air scouring; C — NORIT SA 2 without air scouring; D — NORIT SA 2 with air scouring (PAC dose 15 mg, c0 = 1 mg/L).

5.2. Effects of coagulant addition Results of simple adsorption tests under various dosing conditions of PAC and coagulant are given in Table 2. The highest removal efficiency was achieved when PAC was added after coagulation, while the A and B patterns of dosing coagulant and PAC gave almost the same results. They were all smaller than the adsorption capacity of the carbon without coagulant (near 80%, see Table 1). This means the flocculation process influences the adsorption partially, for example, by embedding the activated carbon into the floccs. During membrane filtration PAC was added together with the coagulant (as in test procedure B). Air scouring when applied in this experiment was performed only during a short period of backpulse. The results are presented in Fig. 5. The better performance of NORIT SA UF was again confirmed. The results are even better than in the case when coagulant was not applied (Fig. 2, sample NORIT SA UF, with air, 15 mg). PNP removal by the carbon with the largest particles (NORIT SA 2) in combination with coagulant did not show any dependence on air scouring during the first 20 min of filtration (Fig. 5). After that time a slightly higher efficiency was observed for the filtration when the layer of settled PAC on the membrane was not disturbed by air scouring. The beneficial effect was 5–7%. Additionaly, the removal of carbamazepine was studied (Fig. 6). This was performed under a solute concentration three orders of magnitude lower than the previously studied PNP. The membrane was first saturated by carbamazepine solution until the concentrations of the feeding solution and permeate were the same. Then PAC NORIT SA UF was applied with continuous air scouring (single dose of 0.45 mg PAC which is equivalent to a concentration in the tank of 0.3 mg/L). The removal of carbamazepine was observed from the second cycle (total permeate of 250 ml was analyzed) and at the end of the fifth cycle. The concentration in the permeate was in the range

Fig. 6. Carbamazepine removal by PAC/UF (NORIT SA UF 0.45 mg, c0 = 4 μg/L).

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Table 2 Results from the static test of PNP removal during the combined adsorption coagulation process (0.1 mmol/L Al, 10 mg/L PAC NORIT SA UF and 1 mg/L PNP). Test procedure

Aa

Bb

Cc

PNP removal efficiency, %

25.5

25.2

30.7

a

A: firstly carbon was added and after 10 min of adsorption coagulant was added during the fast mixing phase (1 min, 1000 min− 1). After that, 15 min slow mixing was applied (200 min− 1) and the sample was allowed to settle for an additional 15 min. b B: coagulant and carbon were added together during the 1 min fast mixing phase, then slow mixing was applied for 15 min and the settling phase was 15 min. c C: firstly coagulant was added during the fast mixing phase (1 min). Further slow mixing was applied for 15 min. During 5th minute of slow mixing carbon was added.

2.3–2.7 μg/L. After one and a half hours still no increase in permeate concentration was observed. The profile of the curve obtained was different from the breakthrough curve obtained for the experiment with PNP (c0 = 1 mg/L). Calculations were performed to estimate the amounts of xenobiotics which were “taken up” by the membrane during saturation phase before carbon addition. It was shown that 106 mg/m2 of PNP (initial concentration of 1 mg/L) was enough to saturate the system, while for carbamazepine (initial concentration 4 μg/L) it was less than 255 µg/m2 under the same conditions of pressure and flow since it was confirmed that the concentrations in the feed and permeate were identical after 3 h of filtration without carbon. However, the actual quantity is even lower, since no samples of permeate were taken before 3 h. This result might be important when assessing the behavior of membranes during filtration related to possible changes of pollutant concentration in the feeding solution. 6. Conclusions In order to obtain information on the efficiency of different powdered activated carbons for organic xenobiotics removal in combination with out/in ultrafiltration, experiments were performed on lab-scale apparatus with p-nitrophenol (c0 = 1 mg/L). The most efficient carbon was the one with the smallest particles (NORIT SA UF). Efficiency showed a decreasing trend with increasing carbon particle diameter and apparent density, regardless of air scouring application. The influence of air scouring was different for the different carbons applied. In the case of carbon with the smallest particles and fastest kinetics no significant difference between runs with and without air scouring was noticed. It seems that air scouring can be expected to have a beneficial effect on mixing and local mass transfer for the carbon with a slower adsorption rate, smaller particle diameter and smaller apparent density (e.g. PULSORB RD 90). For the carbon with high adsorption rate (e.g. extremely fast kinetics for NORIT SA UF) the effect is absent as well as for the carbon with larger particles and higher apparent density most probably due to faster settling on the membrane surface (e.g. NORIT SA 2). The addition of coagulant in PAC/UF system with smallest particles increased removal

efficiency. The carbon with largest particles did not show any dependence on air scouring during the first 20 min of filtration/ coagulation. After that time a slightly higher efficiency was observed for filtration when the layer of settled PAC on the membrane was not disturbed by air scouring (beneficial effect was 5–7%). Furthermore, the system was tested for the removal of carbamazepine at a three orders of magnitude lower concentration (4 μg/L), and it was shown that it is possible to achieve near 40% removal after 30 min with carbon dosed at once (PAC concentration in the tank was 0.3 mg/L). A stable concentration in the permeate was observed for the next one hour with continuous water flow and without new carbon additions. Comparison of membrane uptake for two compounds gave a result of 106 mg/m2 for PNP and less than 255 µg/m2 for carbamazepine under a flux of 20 l/m2/h and concentrations of 1 mg/L and 4 μg/L, respectively.

Acknowledgement The research results presented here were attained with the assistance of the Alexander von Humboldt Foundation through Humboldt fellowship grant for Dr. I. Ivancev-Tumbas in the period 04/2005-03/2006. We would like to thank Prof. Dr.-Ing. Rolf Gimbel for hosting and supporting this research. Analytical measurements were performed by Laboratory for Organic analysis at Water Quality Department of IWW Water Center in Mülheim an der Ruhr, Germany. The authors wish to acknowledge the help of Mrs Miriam Sustrath for her great experimental work.

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