PROJECT FINAL REPORT

Grant Agreement number: 605641

Project acronym: O-WAR

Project title: An Intergrated Membrane Process for Oily Wastewater Treatment, Water Reuse and Valuable By-Products Recovery

Funding Scheme:

Period covered: from 01.01.2014 to 31.01.2016

Name of the scientific representative of the projec t's co-ordinator 1, Title and Organisation:

Johnny Marcher, LiqTech International A/S

Tel: +45 44 98 60 00

Fax: +45 44 98 60 61

E-mail: jom@liqtech.com

Project website Fejl! Bogmærke er ikke defineret. address: http://www.o-war.eu

1 Usually the contact person of the coordinator as specified in Art. 8.1. of the Grant Agreement.

1. EXECUTIVE ABSTRACT

This report describes the characterization of various oily wastewaters, as well as

the characterization of different membranes developed in the scope of the O-WaR

project for processing these wastewaters.

Six oily wastewater samples were received from four project partners: two olive

mill wastewaters (from Sovena and Adventech), one sunflower refining wastewater

(Sovena), one produced water (LiqTech) and two metal processing industry wastewater

(Venture). The samples were physically and chemically characterized. Parameters such

as total solids, suspended solids, volatile solids, pH, conductivity, viscosity and particle

size distribution were measured. Chemical oxygen demand, total organic carbon and

several trace elements were quantified.

The silicon carbide surface properties of a substrate and coated membranes

developed by LiqTech (1st and 2nd generation ceramic membranes) were characterized

by scanning electron microscopy and contact angle measurements. Novel materials

comprising titanium dioxide (TiO2), silicon dioxide (SiO2) and silicon carbide (SiC)

semiconductors as well as a Fe-dopant, were also deposited over silicon-carbide

substrates aiming to develop membranes with higher effectiveness and antifouling

properties. After this characterization, different oily wastewaters were processed by

ultrafiltration using the second generation silicon carbide (SiC) ceramic membranes

developed. The filtration/rejection behavior of the membranes developed was

characterized and the optimal flux maintenance strategies and cleaning procedures

reported.

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2. INTRODUCTION

Oily wastewaters are one of the main pollutants of the aquatic environment

(Cheryan and Rajagopalan 1998). A large volume of these wastewaters is generated

from various process industries, such as food processing, metal processing and

petrochemical, and needs to be treated before being discharged in the aquatic

environment.

Conventional physical and chemical treatment approaches include gravity

separation and skimming, coagulation, flocculation, sedimentation, and flotation.

However, these methods present disadvantages such as low efficiency in the treatment

of stable emulsions, high sludge production, high operation costs, and need of chemical

addition. All of these disadvantages have promoted the development of new processes

for oily wastewater treatment such as membrane processes.

Membrane separation processes stand out as alternatives to conventional

processes for the chemical, pharmaceutical, biotechnological and food industries.

Membranes act as a selective barrier, allowing the passage of certain compounds and

the retention of others from a determined mixture. The separation performance of a

membrane is influenced by its surface as well as feed composition, flow rate,

temperature, and pressure. In general, the most important characteristics of

membranes are the material, thickness, and porosity. Other important characteristics

are the thermal, chemical, and mechanical resistance of the membrane (Coutinho et al.

2009).

The application of membrane technologies has expanded during recent decades

for the treatment of oily wastewaters, as a result of increasingly stringent regulations in

wastewater discharge and continuing improvements in membrane technology.

Membrane filtration is able to remove small oil droplets and emulsions that are

not effectively removed by the conventional technologies such as coagulation,

flocculation, air flotation, and gravity separation (Cheryan and Rajagopalan 1998).

Membrane processes such as microfiltration, ultrafiltration, nanofiltration, and reverse

osmosis are increasingly being applied as alternatives for treating oily wastewater. The

main advantages of these methods are the high efficiency in treating stable emulsions,

high chemical oxygen demand (COD) removal, high quality of permeate produced, small

amount of solids requiring disposal, no need of chemical addition, smaller footprint,

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ease of operation, and easy combination with other treatment processes (Ashaghi et al.

2007, Mondal and Wickramasinghe 2008).

Wide acceptance of membrane processes by industries is limited by membrane

fouling, a critical factor in the treatment of highly emulsified oily wastewaters. Fouling

is caused by the accumulation of rejected oil, suspended solids and other components

of the wastewaters on the membrane surface and intrapore structure. Fouling results in

flux decline and low membrane lifetime due to frequent cleanings (Hofs et al. 2011).

Significant progress should therefore be made to reduce membrane fouling. High costs

related with pre-treatment and regular membrane cleanings are major setbacks related

with fouling that should be minimized for the future application of membrane

technologies.

Membrane fouling can be reduced through the optimization of filtration

conditions, the application of a pre-treatment, different membrane cleaning processes,

and/or the improvement of membrane hydrophilicity.

Membrane performance, especially in oily wastewater treatment and oil-in-

water emulsions, is affected by surface hydrophilicity of the membrane. Hydrophobic

solutes in feed water, such as emulsified oils, readily foul such membranes via strong

hydrophobic interactions (Miller et al. 2014). Therefore by improving the hydrophilicity

of the membrane it is possible to reduce the fouling.

Several studies describe ways to increase the hydrophobicity of the membrane

by modifying its structure, focusing on the use of nanoparticles. The use of nanoparticles

enables the production of desired membrane structures and functionalities that allow a

high degree of control over membrane fouling and achieving a high quality of permeate

(Kim and Van der Bruggen 2010).

This report describes the characterization of various oily wastewaters, as well as

the characterization of different membranes developed in the scope of the O-WaR

project for processing these wastewaters. Several studies were then performed to (a)

optimize the permeate flux using two different flux maintenance strategies (backpulse

and backwash); (b) characterize the membrane filtration effectiveness; and (c) optimize

the chemical cleaning procedures. A final concentration study was conducted under real

conditions and several volatile compounds that may be present in the different

wastewaters identified.

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3. CHARACTERIZATION OF TARGET WASTEWATERS

Six samples provided by different project partners were characterized: two samples

of olive mill wastewater (Sovena and Adventech), one sample of sunflower oil refining

wastewater (Sovena), one sample of produced water (LiqTech), and two samples of

metal processing industry wastewater (Venture). Images of these samples are presented

in Figure 1.

Figure 1: Wastewater samples characterized: (i) Olive oil Sovena, (ii) sunflower oil Sovena, (iii) olive oil Adventech,

(iv) produced water, (v) metal processing Rocklin P-05, (vi) metal processing Cimstar 606

These samples were characterized in terms of several parameters, using well

established methods (Lenore et al. 2005): total solids (Standard Method 2540B), total

suspended solids (Standard Method 2540D), total organic carbon (Standard Method

5310B), pH (Standard Method 4500 H+), and conductivity (Standard Method 2510).

Furthermore, chemical oxygen demand was determined according to ISO 15705,

viscosity was measured using a Brookfield digital viscometer LVTDV-II, particle size

distribution was determined by Dinamic Light Scattering, using a Malvern Zetasizer Nano

ZS equipment, and the quantification of trace elements was obtained by inductively

coupled plasma-atomic emission spectrometry. Trace elements analyzed for each

wastewater sample included copper, manganese, strontium, zinc, calcium, iron,

magnesium, aluminium, barium, chromium, nickel, lead, titanium and vanadium.

The concentration of total solids varied between 754 and 19080 mg/L. The

sample of produced water was the one with the lowest concentration, while the metal

processing wastewater sample Cimstar 606 was the one with the highest content of total

solids. A much lower concentration was obtained for the other sample from metal

(i) (ii) (iii) (iv) (v) (vi)

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processing industry. Noteworthy differences were also found for the two olive oil

wastewaters provided: 4740 mg/L (Sovena) and 14408 mg/L (Adventech).

Produced water was also the sample containing the lowest concentration of total

suspended solids (1.07 mg/L) while sunflower oil wastewater was the sample presenting

the highest value (3926 mg/L). Regarding the olive mill wastewater samples, the levels

quantified in the samples provided by Sovena and Adventech were very different (342

mg/L vs 2681 mg/L), as also observed for total solids. This feature may be explained by

differences in the production process. Overall, the comparison of the concentrations of

total solids and suspended solids showed that total suspended solids are a minor

fraction of total solids (0.1-28%).

Rocklin P-05 and Cimstar 606 samples from the metal processing industry

presented the lowest (1940 mg/L) and highest (59800 mg/L) values of chemical oxygen

demand, respectively. Similar chemical oxygen demand levels were attained for

sunflower oil, produced water, and olive mill wastewater from Sovena (20000-22125

mg/L). However, the value determined for olive oil sample from Adventech was

approximately the double. Once more, noteworthy differences were observed when the

two samples of olive mill wastewater are compared, in line with data attained for total

solids and total suspended solids.

As expected, the trend observed for total organic carbon was very similar to the

chemical oxygen demand trend. Metal processing industry wastewater samples

presented again the lowest (642 mg/L) and highest concentrations (16171 mg/L). Values

between 1880 mg/L (sunflower oil) and 11513 mg/L (olive oil from Adventech) were

attained for the other wastewaters. Overall, very low values of inorganic carbon (3.6-

397.5 mg/L) were obtained when compared with total organic carbon (642-16171

mg/L). As expected, the fraction of inorganic carbon was much higher in samples from

the metal processing industry than in the other samples, especially in the Cimstar 606

sample. On the other hand, the lowest inorganic carbon was determined for olive oil

from Adventech.

The pH of wastewater samples varied significantly. Samples of olive oil, produced

water, and Rocklin P-05 from the metal processing industry presented an acidic pH (3.6-

5.5) while a basic pH was determined for the other samples (8.7-9.5). Despite their origin

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in metal processing industry, Rocklin P-05 and Cimstar 606 samples presented the

lowest and highest pH values, respectively.

Conductivity values between 0.5 mS/cm (produced water) and 8.5 mS/cm

(sunflower oil) were determined. Noteworthy differences regarding this parameter

were not attained among the other wastewater samples. Similar viscosity values were

determined for all samples (1.1-1.7 cps).

Since wastewater samples are very heterogeneous, dynamic light scattering

analysis does not seem to be the most appropriate method to study particle size

distribution of this type of samples. Nevertheless, the results obtained showed the

existence of particles with sizes up to 7 µm in all samples analyzed. One of the

wastewaters from metal processing (Cimstar 606) presented the smaller particles

detected (35 nm).

A broader range of trace elements were detected in the samples from the metal

processing industry comparatively with the other samples. Regarding metal processing

samples, the concentration of some elements (aluminium, copper, and lead) was

particularly high in the Cimstar 606 sample with concentrations up to 3.8 mg/L.

Conversely, phosphorous was significantly higher in the Rocklin P-05 (1840 mg/L)

comparatively with Cimstar 606 sample (23.9 mg/L). For the other elements in these

samples, similar concentrations up to 13.0 mg/L were determined, where iron, silicium,

and zinc stood out due to their higher concentration. In olive oil, sunflower oil, and

produced water samples, copper, manganese, strontium, and zinc were found at

concentrations up to 1.1 mg/L while higher concentrations were obtained for calcium,

iron, and magnesium (up to 78.1 mg/L).

The characterized wastewaters showed very different compositions. Since the

olive oil and metal industries wastewaters proved to be the most challenging in terms

of total solids, total organic carbon, chemical oxygen demand and trace elements,

subsequent studies focused on these matrices.

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4. MEMBRANE DEVELOPMENT AND CHARACTERIZATION

LiqTech developed silicon carbide first and second generation tubular

membranes with anti-fouling behavior. The first generation membranes consist of a SiC

substrate, which is prepared by extrusion on the top of which two subsequent layers are

applied by dip-coating. The first membrane layer is a 1 μm layer that is sintered on the

substrate by high temperature thermal treatment (T > 2000 °C) in Ar atmosphere

followed by and oxidation step. Subsequently, the final selective layer (0.04 μm) is

coated on top of the 1μm layer and a second sintering is taking place (T > 1800 °C) to

achieve the desired pore size. In the second generation membranes, a new final layer

(0.04 μm) was applied directly to the substrate without the intermediate 1 μm

membrane layer. This procedure has the advantage that one firing step can be

eliminated in the production process reducing significantly both the production time

and the manufacturing costs. Moreover, the microstructure and the surface of the

substrate is better controlled, since the whole process has one less high temperature

firing, thus making the final layer smoother and with less defects.

Figure 2 and Figure 3 present scanning electron microscopy (SEM) images of the

1st and 2nd generation membranes developed by LiqTech.

Figure 2: Cross-section of 1st generation LiqTech SiC membrane consisting of substrate (bottom), intermediate

layer (middle) and final selective layer (top)

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Figure 3: Cross-section of 2nd generation LiqTech SiC membrane consisting of substrate (bottom) and final selective

layer (top)

These membranes were compared in terms of flux and retention using produced

water to evaluate the impact of the new manufacturing process on membrane quality.

Figure 4 shows pictures of the samples taken before and after filtration tests

where turbidity levels below 2NTU were achieved. It can be seen clearly that filtration

with the second generation membranes resulted in less colored and less turbid

permeate, which is a strong indication that higher oil removal was achieved.

Figure 4: Samples of the feed and permeate taken during the filtration tests with various membranes. A shows feed

and permeate from 2nd generation membrane. B shows feed and permeate of 1st generation membrane

These assays showed that the permeate samples produced with 2nd generation

membranes were markedly less turbid and less colored than those produced with 1st

generation membranes. Moreover, when the LabBrain unit was used to evaluate the

permeability values for 1st and 2nd generation membranes, higher permeability levels

were obtained for the second generation membrane (2083 Lh-1m-2bar-1) compared to

the first generation membrane (1677 Lh-1m-2bar-1).

B

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Novel materials comprising titanium dioxide (TiO2), silicon dioxide (SiO2) and

silicon carbide (SiC) semiconductors as well as a Fe-dopant, were also deposited over

silicon-carbide substrates aiming to develop membranes with higher effectiveness and

antifouling properties. Silicon carbide membranes were therefore modified with: (i) an

additional silicon carbide layer; (ii) a mixture of sol-gel titania with Degussa P25 with the

additional silicon carbide layer; (iii) a mixture of sol-gel titania with Degussa P25; (iv)

titania obtained by the sol-gel procedure; and (v) a mixture of sol-gel titania and silicon

dioxide.

The surface of the modified layers developed was characterized by scanning

electron microscopy, showing that all the immobilizations were carried out successfully

whereas the contact angle measurements revealed antifouling properties.

The water contact angles of the membrane surfaces (substrate, 1st, 2nd

generation and all the modified membranes) were measured using a goniometer (KSV

Instruments LTD, CAM 100, Finland) with the software KSV CAM 100. Four

measurements were carried out in different areas of the SiC membranes. Twenty frames

were attained for each measurement, with a frame interval of 100 ms.

The contact angle of the substrate was determined as 85.0 ± 5.3, showing that it

is rather hydrophobic. Figure 5 shows an example of a frame obtained during the

measurement.

Figure 5: An example of a frame attained during the measurement of the contact angle of the SiC

substrate.

For the modified membranes as well as the 1st and 2nd generation membranes, a

stable contact angle could not be determined because the coating of the membrane is

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extremely hydrophilic and the drop was readily absorbed by the membrane, which

makes impossible to attain a stable drop during the measurement. A short video

showing one of the measurements carried out may be found in the following hyperlink.

All the modified membranes showed a lower contact angle compared with the

control, with the membrane coated using a mixture of sol-gel titania and Degussa P25

presenting the lowest value. The results of the first angle measurement suggest that the

2nd generation membrane has a higher hydrophilicity.

The pore size was found to be considerably reduced using the different

nanoparticles in the flat membranes and these were thus considered promising results

to increase the membrane effectiveness. However, when tubular membranes were

modified and tested, the modified membranes exhibited much lower permeabilities

compared to the second generation membranes while the rejections of total solids,

chemical oxygen demand and total organic carbon of the two membranes remained

similar. The membrane filtration process was therefore optimized using the second

generation membranes developed by LiqTech without any further nanoparticles

modification.

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5. OPTIMIZATION OF LABBRAIN UNIT CONDITIONS TO TREAT OILY WASTEWATERS

Two different olive mill wastewaters samples, provided by Sovena and

Adventech, previously pre-treated by Adventech, were processed in a pilot scale

membrane filtration unit (LabBrain Unit depicted in Figure 6) using second generation

silicon carbide (SiC) membranes provided by LiqTech with an area of 0.09 m2 (described

in Table 1).

Figure 6: LabBrain pilot scale unit with cleaning devices

Table 1: Characterization of membranes used in the membrane filtration assays

Wastewater Matrix

provided by

SiC membrane used Hydraulic permeability

(Lh-1m-2bar-1)

Sovena 209485 2000

Adventech 209497 2100

A preliminary study of the best flux (flowrate per membrane area) conditions to

use in the filtration assays was carried out using the two different pre-treated

wastewater samples by assessing transmembrane pressure (TMP) variations under

different constant permeate flowrates set during five minute intervals. The chosen

flowrate to conduct the experiments was the one at which no significant TMP variation

was observed.

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For the pre-treated wastewaters provided by Sovena and Adventech, a permeate

flowrate of 8 Lh-1 and 6 Lh-1, respectively were chosen to initiate the tests since a low

TMP increase was observed at these flowrates.

The two different pre-treated wastewaters were then filtered during 24 h assays

with a crossflow velocity set at 2ms-1 (as proposed by LiqTech) and the determined

permeate flowrate values fixed (8 Lh-1 and 6 Lh-1 for Sovena and Adventech

wastewaters, respectively). The variation of TMP was followed and the effect of

backpulse (every 10 minutes) and backwash (every 2 hours) as flux maintenance

strategies was studied over the 24 h. The flowrate and pressure data acquisition in the

LabBrain unit was automatically stored. When the flux maintenance strategies applied

caused a low pressure variation, new assays were conducted with doubled permeate

flowrate levels to increase the water production.

In order to evaluate the efficiency of the silicon carbide membranes in the

removal of total solids, total suspended solids, chemical oxygen demand, total organic

carbon and oil and grease, these parameters were characterized in samples of feed

collected in the beginning of the assays (t = 0h) and samples of feed, retentate and

permeate collected in the end of the filtration assay (t=24h).

Even though a low change in TMP was detected over 5 minutes in the critical flux

tests, after 24 hours the TMP increased about 0.5 bars in the assay conducted without

backpulse (test 1). Another assay was thus conducted (test 2) using frequent backpulses

(every 10 minutes). This strategy proved to be very effective at maintaining a stable

TMP. To increase the water production rate two more assays were then conducted using

the double permeate flowrate of 16 Lh-1 (test 3 and 4 detailed in Table 2).

Table 2: Tests performed with the wastewater provided by Sovena

Test Test 1 Test 2 Test 3 Test 4

Permeate

Flowrate (Lh-1

) 8 8 16 16

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Flux maintenance

strategy No

Backpulse each 10 min

(duration:0.8s; TMP=-

3bar)

Backpulse each 10 min

(duration:0.8s; TMP=-3bar)

Backpulse each 10 min +

Backwash each 2h

(duration:2s)

Table 3: Tests performed with the wastewater provided by Adventech

Test Test 1 Test 2 Test 3 Test 4

Permeate

Flowrate (L h-1) 6 6 6 6

Flux

maintenance

strategy

No

Backpulse each 10 min

(duration:0.8s; TMP=-

3bar)

Backpulse each 10 min +

Backwash each 2h

(duration:2s)

Backpulse each 10 min +

Backwash each 2h

(duration:10s)

The use of backpulse every 10 minutes was not enough to maintain a constant

TMP during the 24 hours assays. On the other hand, results showed that the use of

backpulse in combination with backwash every 2 hours will help maintain flux and

minimize membrane cleanings even when higher permeate flowrate levels are used.

The effectiveness of the membrane filtration assays was evaluated by determining the

percent adsorption to the silicon carbide membranes and the water quality produced in terms

of the different parameters analysed (total solids, total suspended solids, chemical oxygen

demand, total carbon and oil and grease). An extremely high percent removal of total

suspended solids (92 to 100%) and oil and grease (69 to 99%) present in the feed

samples was removed. Removal percentages lower than 50% were obtained for the

other parameters (total solids, chemical oxygen demand and total carbon). For all the

parameters except total suspended solids, almost all the removal achieved was

essentially due to adsorption of the compounds to the membrane.

In order to find out the best cleaning strategy to recover the permeability of the

membrane, the effect of using different cleaning solutions and temperatures was

studied. Solutions of NaOH (varying between 1 and 4%), citric acid (2%) as well as Ultrasil

(0.5 – 1%) were tested. The recovery of the permeability achieved in each cleaning step

was determined to understand the efficiency of each cleaning. The permeability of the

membrane was considered to be restored when 90% of its hydraulic permeability was

recovered. Results indicate that to recover the membrane permeability acidic (citric acid

2%) and basic (NaOH 4%) cleaning solutions should be employed in sequence at high

temperature.

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In order to test conditions closer to reality, a concentration test was performed

in the same unit. 58L of a pretreated olive mill wastewater were filtered with total

recirculation of the retentate and recovery of the permeate. Samples were collected

during the assay to evaluate the effectiveness of membrane filtration.

The results obtained in terms of rejection were consistent with the results

previously obtained in the membrane filtration assays with total recirculation. The

silicon carbide membranes used are expected to ensure high removals of total

suspended solids (99.5%) and oil and grease (81%).

6. DETECTION OF VOLATILE COMPOUNDS

Thirty different volatile compounds were determined by solid phase

microextraction (SPME) gas chromatography mass spectrometry (GC/MS) in four

different olive oil wastewater samples collected after different membrane treatments.

Most of the compounds identified in this study have been previously reported in

olive oil by several authors (e.g. Angerosa, 2002; Haiyan et al, 2007; Kalua et al, 2007;

Reboredo-Rodríguez et al,2012; Tanouti et al, 2012).

The volatile compounds detected may be recovered and used by different

industries as valuable by-products.

7. CONCLUSIONS

Although wastewaters from several industries were characterized, studies

focused on the treatment of the olive oil and metal industries wastewaters that proved

to be the most challenging in terms of total solids, total organic carbon, chemical oxygen

demand and trace elements.

Even though several membrane modifications were tested, the second

generation membranes, due to their high permeabilities and rejection results as well as

reduced production time and manufacturing costs were addressed as the solution for

the O-War project. The best controlled permeate flux conditions as well as the different

flux maintenance strategies and membrane cleaning conditions to keep fouling at

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reduced and controlled levels were optimized for olive oil wastewaters provided by

Sovena and Adventech.

The main conclusions of this work are the following:

• The combination of backpulse every 10 minutes and backwash every 1 or

2 hours helps minimizing fouling (maintaining flux), increases the percent

rejection (improved permeate quality) and facilitates membrane cleaning

(less chemicals used and higher membrane lifetime),

• Backpulse and backwash did not increase the energy demand costs,

• To recover the membrane permeability acid and basic cleaning solutions

should be employed in sequence at controlled temperature,

• The silicon carbide membranes used ensure high removals of total

suspended solids and oil and grease,

• The high removals were maintained during a 7h concentration study,

• Removals of TS, COD, TC (in Sovena wastewater) and oil and grease (in

the Sovena and Adventech wastewaters) were essentially due to

adsoption of the compounds on the surface of the membrane,

• Thirty volatile compounds were identified in the wastewater samples

that may be used by different industries as valuable by-products.

8. ACKNOWLEDGEMENTS

Financial support from the EU FP7/SME theme [SME-2013-1] through the project

O-WaR (grant agreement no: 605641) is gratefully acknowledged.

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