Evaluation of chemical cleaning to control fouling on nanofiltration and reverse osmosis membranes after desalination of MBR effluent

Evaluation of chemical cleaning to control fouling on nanofiltration and reverse osmosis membranes after desalination of MBR effluent

Desalination 466 (2019) 44–51 Contents lists available at ScienceDirect Desalination journal homepage: www.elsevier.com/locate/desal Evaluation of ...

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Desalination 466 (2019) 44–51

Contents lists available at ScienceDirect

Desalination journal homepage: www.elsevier.com/locate/desal

Evaluation of chemical cleaning to control fouling on nanofiltration and reverse osmosis membranes after desalination of MBR effluent

T

Mert Can Hacıfazlıoğlua,c, İlker Parlara, Taylan Ö. Pekb, Nalan Kabaya,



a

Ege University, Faculty of Engineering, Chemical Engineering Department, Izmir, Turkey İTOB Organized Industrial Zone, Tekeli-Menderes, Izmir, Turkey c Istanbul Technical University, Faculty of Engineering, Chemical Engineering Department, Istanbul, Turkey b

GRAPHICAL ABSTRACT

ARTICLE INFO

ABSTRACT

Keywords: Desalination Fouling Membrane cleaning Nanofiltration (NF) Reverse osmosis (RO) Wastewater reuse

In this study, the wastewater treated by the membrane bioreactor (MBR) process at the wastewater treatment plant of ITOB Organized Industrial Zone was used as the feed solution for further desalination by nanofiltration (NF90) and reverse osmosis (BW30) membranes. The effect of chemical cleaning to control the membrane fouling during long-term repeated uses of both NF and RO membranes were investigated. The water permeability values were obtained using deionized water as the feed solution after five cycles of desalination-membrane cleaning-permeability tests. Water permeability values changed from 3.93 L/m2·h·bar to 1.35 L/m2·h·bar for BW30 and 7.76 L/m2·h·bar to 2.65 L/m2·h·bar for NF90. The permeability tests were also carried out by using MBR effluent as the feed solution for better evaluation of fouling effects for MBR treated wastewater on desalination performance of NF and RO membranes. An optimal cleaning strategy was suggested throughout the study.

1. Introduction WHO stated that 50% of the world population is or will be living under water-stressed regions by 2025 [1]. Some countries –especially the ones located in water-poor Middle-East region– are already forced



to find a solution to water scarcity problems and desalination via membrane processes is the predominant solution today. New desalination plants are installed every year to recover water from various water resources including wastewater for its later use in irrigation, industry or for drinking purposes [2].

Corresponding author. E-mail address: [email protected] (N. Kabay).

https://doi.org/10.1016/j.desal.2019.05.003 Received 10 February 2019; Received in revised form 27 April 2019; Accepted 3 May 2019 Available online 21 May 2019 0011-9164/ © 2019 Elsevier B.V. All rights reserved.

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Industrial wastewaters are mostly treated by conventional activated sludge (CAS) process or membrane bioreactor (MBR) process. MBR process is simply an integrated treatment system of microfiltration (MF) or ultrafiltration (UF) membranes with a biological treatment reactor. It is usually used for the treatment of industrial wastewaters to produce low or medium quality water production [3]. Secondary treatment effluents –e.g. MBR effluents– often include high concentrations of dissolved matter, pesticides, pathogens and heavy metals [4]. This makes a further treatment necessity to reclaim water for reuse. Nanofiltration (NF) and reverse osmosis (RO) membranes can be used to reduce salinity and organic content in treated wastewater and they can solve the problem with secondary effluents. On the other hand, it must be noted that especially NF membranes may not reject some of the metal ions completely, hence metal content of secondary effluent should be monitored carefully and they should be below discharge standards [5]. Concentration polarization in high-pressure membrane applications is an important problem for RO membranes. Rejected salts tend to form a bulk phase on the membrane surface, hence increasing the concentration difference between two sides of the membrane. This situation results in lower permeate flux with lower quality [6]. Polymeric NF and RO membranes are made mostly from polyamide, polysulphone or polycarbonate. Their permeate fluxes and water recoveries are higher than those of inorganic membranes but they are not resistant to extreme conditions and fouling. Commonly used pre-treatment methods, MF or UF may not eliminate all fouling agents from feed solution. This causes membrane fouling in NF or RO membranes used after pre-treatment steps [7]. Some researchers try to solve the membrane fouling problem by adding such materials as zeolite [8–10] or carbon nanotubes [11–12] onto the surface of the membrane. The aim of these applications is to be able to ignore the trade-off between permeate flux and rejection percentage of membranes [13]. However, most of those solutions are not cost-effective. Optimization of chemical cleaning is important to reduce operating cost. Pre-treatment alone can reduce fouling and increase lifespan of the membrane but inevitably membranes need chemical cleaning. Insufficient chemical dosage or usage of ineffective chemicals will decrease the lifespan of the membrane and increase membrane replacement costs. On the other hand, using chemicals at excess amount will also damage the membrane since most cleaning agents are harmful for the membrane structure at high concentrations [7,14]. Various researches were performed on the mechanism of membrane fouling, effect of chemical cleaning method to remove fouling, comparison between organic and inorganic cleanings and cleaning agents in literature. Consensus of membrane researchers on cleaning strategies is to clean membranes frequently with an acid solution for cleaning inorganic foulants and with alkaline solution for cleaning organic foulants and finally using disinfectants to reduce fouling caused by microorganisms. Cleaning strategies (flushing time, backwashing time, backwashing repeating frequency, chemical dosage amounts, used chemicals, chemical cleaning times and chemical cleaning repeating frequency) depend on the quality of feed water, membrane types, membrane structures, operating time, and system configurations. Therefore, cleaning strategies must be separately developed according to the conditions of treatment plants [15–21]. Aim of this research is to solve the membrane fouling problem of mini pilot-scale NF and RO system installed at wastewater treatment plant of ITOB Industrial Organized Zone located in Izmir/Turkey. A cleaning procedure was applied for NF and RO membranes employed for desalination of MBR effluent discharged from wastewater treatment plant in order to develop a strategy to increase the lifespan of membranes as long as possible.

Table 1 Properties of NF/RO membranes employed. Membrane

Producer

Active membrane area (m2)

pH interval

Maximum temperature (°C)

Maximum pressure (bar)

NF90 (NF) BW30 (RO)

Dow Dow

2.6 2.6

2–11 2–11

45 45

41 41

2. Material and methods 2.1. Experimental setup Experiments were performed by using a mini pilot system including BW30-RO and NF90 membranes. Each membrane has 2.6 m2 of active membrane area. The system included two cartridge filters and one sand filter prior to collection of the feed (MBR effluent) into the feed tank and one cartridge filter after feeding water from a tank to the mini pilot system. The properties of spiral-wound NF and RO membranes are summarized in Table 1. Flow scheme of the treatment system is depicted in Fig. 1. 2.2. Evaluation of membrane performance There are various transport models to calculate permeate flux and solute flux. The simplest model for high pressure membranes is to relate permeate flux to a constant and transmembrane pressure difference. Permeate flux (Jw) is calculated by Eq. (1) and it is also equal to permeate flow rate per unit area [22]:

Jw = LP

( P

)

(1)

2

where Lp (L/(m ·h·bar)) is water permeability. When pure water (or water with very low salinity) is used as the feed solution, osmotic pressure becomes near zero and water flux can be calculated by Eq. (2) while Lp becomes pure water permeability. These equations can be used to understand the fouling on membranes since water permeability and pure water permeability will decrease with operating time due to the membrane fouling occurred [23].

Jw = L p

(2)

P

All permeate fluxes are normalized at 25 °C by using Eq. (3)

Jwadj =

Jw 1.03(T

25)

(3)

where Padj is the adjusted flux normalized at 25 °C and T is temperature of permeate sample at which the tests are performed [24]. Water recovery (WR) is calculated for each measurement of permeate and concentrate flow rates by using Eq. (4):

WR (%) =

( ) Q ( )

Qp

L min

f

L min

(4)

where WR is the water recovery and the feed flow rate (Qf) is the sum of permeate flow rate and concentrate flow rate. Membrane rejection percentage (R) is calculated by Eq. (5):

R (%) =

Cf

Cp Cf

(5)

where Cf and Cp are concentrations of any selected parameter in feed and permeate stream, respectively. 2.3. Membrane cleaning after long term membrane operations First step of all experimental cycles was 24 h of continuous

45

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Fig. 1. Flow scheme of the mini pilot system used in permeability tests. Table 2 Average properties of MBR permeate.

Table 3 Average properties of NF90 and BW30 permeates.

Parameter

Unit

Parameter value

Parameter

Unit

NF 90 permeate

BW 30 permeate

EC TDS NO3-N Total N PO4-P HCO3 Cl SO4 Ca Mg K Na TOC Free chlorine TSS pH Turbidity

mS/cm g/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L – NTU

3.96 2.05 8.43 6.06 < 0.1 70.6 940 209 160 32.0 81.1 707 6.72 < 0.5 1.5 6.93 1.51

EC TDS NO3-N Total N PO4-P HCO3 Cl SO4 Ca Mg K Na TOC Free chlorine TSS pH Turbidity

μS/cm g/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L – NTU

120.7 57.1 1.04 1.44 < 0.1 5.78 47.9 1.58 4.73 0.03 2.93 26.5 1.46 < 0.5 0.50 6.61 0.63

228.1 109.4 1.21 1.25 < 0.10 0.80 21.0 4.12 4.97 0.03 2.14 46.7 0.91 < 0.50 0.50 6.83 0.43

experiments under 15 bar of applied pressure for NF90 membrane and 20 bar for BW30-RO membrane. Second step was water permeability tests, first performed by using deionized water as the feed solution and then by using MBR effluent as the feed solution. Applied pressure for permeability tests changed between 8 and 32 bar with 2 bar of pressure differences while the water recovery was kept around 50% with a 1% of error margin. Pressure interval differed in some experiments due to some system constraints because of the high pressure pump employed. Starting from the end of 2nd cycle, permeability tests were performed three times for each cycle before cleaning, after alkaline cleaning and after acid cleaning to see whether organic or inorganic pollutants are more problematic for membrane fouling. Alkaline cleaning was conducted by using NaOH and NaOCl

solutions while acid cleaning was conducted by using citric acid solution. For alkaline cleaning, 2.5–3.0 ppm·h NaOH and 5.3–5.8 ppm·h NaOCl were applied to membranes by adding calculated amounts of those chemicals to the cleaning tank filled with 300 L of deionized water. Acid cleaning operation was performed with 300 L of citric acid solution at a concentration of 1000 mg/L (kept around 650–700 ppm·h). The citric acid solution with a 1000 mg/L of concentration was not enough to recover much of the permeate flux before the fifth (final) cycle, hence citric acid solution of 2000 mg/L (around 1350–1400 ppm·h) was used after the final long term membrane operation with MBR effluent. After 1st cleaning, cleaning period was increased from 35 min (15 min for alkaline cleaning and 20 min for acid cleaning) to 70 min (30 min alkaline cleaning and 40 min for acid cleaning). 46

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Normalized Flux (L/m2.h.bar)

250 NF90 - Fresh

y = 7.764x R² = 0.983

200 y = 6.486x R² = 0.981

150

NF90 - After 1st Cleaning y = 5.517x R² = 0.887

NF90 - After 2nd Cleaning

y = 4.944x R² = 0.735 y = 3.653x R² = 0.954

100

NF90 - After 3rd Cleaning

NF90 - After 4th Cleaning

y = 2.653x R² = 0.985

50

NF90 - After 5th Cleaning

0 0

10

20 Pressure (bar)

30

40

Fig. 2. Water permeability test results for NF90 by using deionized water.

180

y = 4.927x -1.40 R² = 0.996

NF90 - Fresh

Normalized Flux (L/m2.h.bar)

160 140

y = 3.697x + 1.63 R² = 0.985

y = 5.579x - 3.49 R² = 0.997

120

y = 3.881x - 3.56 R² = 0.993

100

NF90 - After 2nd Cleaning

y = 3.204x - 4.36 R² = 0.987

80 60

NF90 - After 1st Cleaning

NF90 - After 3rd Cleaning

y = 2.180x - 2.72 R² = 0.998

NF90 - After 4th Cleaning

40 20

NF90 - After 5th Cleaning

0 0

10

20 Pressure (bar)

30

40

Fig. 3. Water permeability test results for NF90 by using MBR permeate.

2.4. Feed water characteristics and sample measurements

3. Results and discussion

The feed solution for all experiments was the permeate of MBR system operated at wastewater treatment plant of ITOB (Izmir Chamber of Commerce) Organized Industrial Zone located at Tekeli, MenderesIzmir, Turkey. Average properties of the MBR permeate are summarized in Table 2. EC (electrical conductivity), TDS (total dissolved solid matter) and pH were measured online via a conductometer and a pH meter. Sampling was done at the beginning and at the end of each cycle for laboratory analysis. Table 2 shows average measurement results. Nitrate-N, total-N, phosphate-P, free chlorine, turbidity and TSS (total suspended solid) were measured by using chemical measurement kits and a DR 3900 benchtop VIS model spectrophotometer. Bicarbonate concentration was measured by titration with 0.05 M HCl solution while using methyl orange as indicator. Anion concentrations (Cl− and SO42−) were measured by ion chromatography (Shimadzu Prominence HIC-SP Model) while cation concentrations (Ca2+, Mg2+, K+ and Na+) were measured by an atomic absorption spectrometer (Shimadzu AA7000 Model).

Chemical cleaning for membrane processes –especially for RO– are being developed for a long period in literature. However, the correlation between long-term experimentation and cleaning procedures is not well established. This study also emphasizes on long-term experimentation (24 h for each cycle) by using MBR effluent of the wastewater treatment plant of an industrial organized zone as the feed solution. Permeate flow rate and permeate temperature were measured during experiments and normalized permeate flux was calculated to eliminate the possible effects of temperature on results. Calculated normalized permeate flux (at y-axis) was put in a graph against applied pressure (at x-axis). Since salinity and hence the osmotic pressure of deionized water (RO permeate was used as deionized water in experiments) is very low, intercept of linear regression lines were taken as zero. However, intercept of linear regression line cannot be taken as zero for MBR permeate since electrical conductivity of the MBR permeate is around 4 mS/cm. Hence, the intercept is also considered for permeability tests by using MBR permeate while calculating slopes. 47

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120 Normalized Flux (L/m2.h.bar)

BW30 - Fresh

y = 3.405x R² = 0.99

y = 3.932x R² = 0.982

100

y = 3.250x R² = 0.979 y = 3.342x R² = 0.975

80

BW30 - After 2nd Cleaning

60

BW30 - After 3rd Cleaning

y = 1.315x R² = 0.994

40

BW30 - After 1st Cleaning

BW30 - After 4th Cleaning

y = 1.354x R² = 0.987

20

BW30 - After 5th Cleaning

0

0

10

20 Pressure (bar)

30

40

Fig. 4. Water permeability test results for BW30 by using deionized water.

Normalized Flux (L/m2.h.bar)

120 BW30 - Fresh

y = 3.159x - 3.15 R² = 0.998

100

y = 2.840x - 1.79 R² = 0.996

80

y = 2.848x - 2.71 R² = 0.994 y = 2.710x - 2.28 R² = 0.993

60

BW30 - After 1st Cleaning

BW30 - After 2nd Cleaning

BW30 - After 3rd Cleaning

y = 1.231x - 0.25 R² = 0.9923

40

BW30 - After 4th Cleaning y = 1.614x - 9.66 R² = 0.972

20

BW30 - After 5th Cleaning

0 0

10

20 Pressure (bar)

30

40

Fig. 5. Water permeability test results for BW30 by using MBR permeate. Table 4 Normalized flux values (L/m2·h) obtained by BW30 and NF90 membranes. Membrane

BW30 (Applied pressure: 20 bar)

NF90 (Applied pressure: 15 bar)

Feed water

Deionized water

MBR permeate

Deionized water

MBR permeate

Fresh After 1st cleaning After 2nd experiment After 2nd organic cleaninga After 2nd inorganic cleaningb After 3rd experiment After 3rd organic cleaning After 3rd inorganic cleaning After 4th experiment After 4th organic cleaning After 4th inorganic cleaning After 5th experiment After 5th cleaningc

78.7 68.9 67.2 67.7 68.1 57.7 58.5 66.5 17.6 18.1 26.2 2.7 28.3

60.6 54.3 53.3 53.3 54.2 47.3 48.2 53.3 15.0 15.2 24.2 2.1 24.9

155.4 129.1 110.9 113.6 113.6 95.0 95.9 107.2 66.4 73.8 75.6 24.4 55.3

110.1 99.6 75.2 101.2 101.9 75.3 79.0 96.1 49.4 60.5 61.4 21.8 46.0

a

Organic cleaning: by alkaline solution. Inorganic cleaning: by acid solution. c Flux was nearly zero after 5th experiment for BW30. It was very hard to measure permeate flow rate even after the organic cleaning. Hence, data was obtained after both organic and inorganic cleanings are applied. b

48

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Table 5 Flux drops and recoveries obtained during 5 cycles of operations. After Cycle No

Flux Drop (%)

Flux Recovery (%)

BW30

1 2 3 4 5

NF90

BW30

NF90

Deionized Water

MBR Permeate

Deionized Water

MBR Permeate

Deionized Water

MBR Permeate

Deionized Water

MBR Permeate

Not measured 1.32 4.59 32.8 90.5

Not measured 1.69 11.3 38.0 91.6

Not measured 2.38 11.4 12.1 55.9

Not measured 26.2 21.6 19.5 52.6

Not measured 1.32 4.59 32.8 90.5

Not measured 1.69 11.3 38.0 91.6

Not measured 2.38 11.4 12.1 55.9

Not measured 26.2 21.6 19.5 52.6

200 RO - Feed: MBR Effluent

Normalized Flux (L/m2.h)

180 160

140

NF - Feed:Deionized Water

120 100

NF - Feed: MBR Effluent

80 60 RO - Feed: Deionized Water

40 20 00 0

1

2

3 4 Cycle No

5

6

Fig. 6. Flux drop after each cycle. Table 6 Permeability values during five cycles of operations. Membrane

BW30

NF90

Feed water

Deionized water

MBR permeate

Deionized water

MBR permeate

Fresh After 1st cleaning After 2nd experiment After 2nd organic cleaning After 2nd inorganic cleaning After 3rd experiment After 3rd organic cleaning After 3rd inorganic cleaning After 4th experiment After 4th organic cleaning After 4th inorganic cleaning After 5th experiment After 5th cleaning

3.932 3.405 3.257 3.264 3.250 2.775 2.842 3.342 0.821 0.823 1.315 0.115 1.354

3.159 2.840 2.696 2.739 2.848 2.762 2.802 2.710 0.933 0.993 1.508 0.199 1.667

7.764 6.486 5.489 5.512 5.517 4.475 4.520 4.944 3.241 3.619 3.653 1.209 2.653

5.579 3.881 4.202 4.699 4.927 3.889 3.923 3.697 2.347 2.835 3.204 1.178 2.180

Since MBR process removes most of the organic foulants but incapable of removing inorganic foulants, it is considered that acid cleaning was the prominent subject when MBR effluent is used as the feed solution. Initial acid concentration was selected much higher than base concentration based on this knowledge. Some researchers also pointed out that strong acids and high acid concentration may cause degradation of RO membranes since RO membrane structure is sensitive to pH level [25]. Other researchers used various types of acid solutions (hydrochloric, nitric, phosphoric, sulphamic and citric acids) for chemical cleaning of RO membrane and the highest flux recovery was obtained by citric acid [17,26]. In this study, citric acid is used in cleaning process with an initial concentration of 1 g/L taking into consideration the information given in the literature.

Dual-step chemical cleaning (acid cleaning followed by alkaline cleaning) proposed by researchers [7] was applied in this research to prevent competing cleaning mechanisms and preventing acid-base reaction. Initial cleaning periods were decided by considering a research about membrane cleaning [18]. This research showed that cleaning efficiency increased with increasing cleaning (contact) time with chemicals. By considering points explained above, an initial cleaning strategy is developed and few changes were made later to optimize cleaning performance. Table 3 shows the lowest quality obtained (in final 24 h experiments) by NF90 and BW30 membranes. It is seen that BW30 permeate quality is affected much more significantly than NF90 permeate quality. This also fits the permeability data where permeability of RO membrane (BW30) decreased sharply and more drastically than NF90. 49

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Normalized Flux (L/m2.h.bar)

160

y = 4.944x R² = 0.735

140

BW30 - After 3rd Experiment

y = 4.520x R² = 0.934

BW30 - After Organic Cleaning

y = 4.475x R² = 0.943

120

BW30 - After Inorganic Cleaning

y = 3.342x R² = 0.975 y = 2.926x R² = 0.986

100 80

NF90 - After 3rd Experiment

y = 2.857x R² = 0.990

60

NF90 - After Organic Cleaning

40 20

NF90 - After Inorganic Cleaning

0 0

10

20 Pressure (bar)

30

40

Fig. 7. Effects of organic and inorganic cleanings on permeabilities measured by deionized water for NF90 and BW30 membranes.

The results of water permeability tests for NF90 by using deionized water are given in Fig. 2. Total flux drop for NF90 (with deionized water used as the feed solution) was 65.8% after 120 h of experimental time. The value of R2 for most lines is above 95% which means that the linear regression with zero intercept assumption is correct. Water permeability test results for NF90 by using MBR permeate are given in Fig. 3. Similar results were observed while using MBR permeate as the feed solution. Total flux drop was 60.9% after 120 h of experimental time. High R2 values (above 98%) indicated that the linear regression assumptions of flux equation for MBR permeate are correct. Except one of the lines (after 3rd cleaning line), all lines had negative intercepts which was expected because osmotic pressure decreased the permeate flux. Results showed that the flux after 1st cleaning was much lower than after 2nd and 3rd cleanings, probably due to insufficient time of chemical cleaning in the first one. Water permeability test results for BW30 by using deionized water are given in Fig. 4. Total flux drop was 65.6% after 120 h of experimental time. This flux drop percentage is similar to results of NF90 membrane indicating that fouling behaviours of two membranes are similar too. Gradual decline was not observed for BW30 membranes, instead, a very low flux decline for first 72 h, then a sharp decline for next 48 h was observed. Water permeability test results for BW30 by using MBR permeate is given in Fig. 5. Total flux drop was 52.8% after 120 h of experimental time. A very low flux decline for the first 72 h and then a sharp decline for the next 48 h were observed instead of a gradual decline for BW30 again indicating this is the behaviour of this membrane. After a very sharp flux decline after fourth 24 h-cycle, concentration of citric acid was increased from 1000 to 2000 mg/L. This increase proved itself significant when the flux is increased to a value higher than before both for MBR permeate and deionized water. All flux results are given in Table 4 while flux drops after each cycle and flux recoveries obtained with chemical cleanings after each cycle are given in Table 5. Flux drop after each cycle and cleaning is visualized in Fig. 6. Percent flux recovery values were calculated by the difference between the permeate flux obtained after 24 h of experiment and permeate flux obtained after chemical cleaning. Low flux recovery obtained for both membranes after second and third cycles are due to low flux drops during those experiments. Fourth cycle ended with very high flux losses but flux recovery was very low (around 35% for RO and 15% for NF). Increased citric acid concentration after fifth cycle also

increased the flux recovery with around 90% for RO and 55% for NF. Flux drops of the RO membrane were much higher than of the NF membrane probably caused by the non-porous structure of the RO membrane. Permeability test results are summarized in Table 6, unit of all permeability values are L/(m2·h·bar). It is observed that acid cleaning by using citric acid was more effective than alkaline cleaning by looking at summarized results in Table 6. Fig. 7 shows the effects of alkaline and acid cleanings on water permeabilities and supports this idea. Fig. 7 shows those parameters only before and after third cycle although other cycles gave similar results. When the flux drop after fourth cycle was too high, the decision of increasing citric acid concentration was made depending on this observation. A sixth cycle (120–144 h of experimental time) by using BW30 was impossible since the flux after fifth cycle was nearly zero while the amount of NF90 permeate was also drastically decreased, experiments were ended. Optimum cleaning strategy would be frequent acid cleaning (every 24 h for first 72 h and every 12 h after 72 h) with at least 2000 mg/L of citric acid solution alongside with less frequent alkaline cleaning by using a mixture of NaOH and NaOCl solutions. By this proper chemical cleaning procedure, it may be possible to extend the membrane usage period. 4. Conclusions Different conditions depending on feed water, membrane type, cleaning agents and cleaning intervals will require different cleaning strategies. Hence, new strategies can be designed for different feed waters. In this case, our experimental results indicated that main fouling sources for NF and RO membranes employed for treatment of MBR effluent are inorganic foulants such as insoluble salts of divalent cations. Occasional organic cleaning (by using low concentrations of NaOH and HOCl solutions) accompanying with frequent acid cleaning (by using at least 2000 mg/L of citric acid or less concentration of a stronger acid) is necessary to increase the lifetime of membranes used. In order to further increase the lifespan of membranes, pre-treatment for MBR effluent using UF prior to NF and RO processes is also suggested. Acknowledgements TUBITAK (Project no: 114Y500) is acknowledged for the financial 50

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support to the project and scholarship given to M. Hacıfazlıoğlu and İ. Parlar. Acknowledgement is also given to ITOB Organized Industrial zone for their support.

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