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Ammonia removal by sweep gas membrane distillation

Zongli Xie*, Tuan Duong, Manh Hoang, Cuong Nguyen, Brian Bolto

CSIRO Materials Science and Engineering, Private Bag 33, Clayton South, Vic. 3169, Australia

a r t i c l e i n f o

Article history:

Received 17 September 2008

Received in revised form

16 December 2008

Accepted 27 December 2008

Published online 18 January 2009

Keywords:

Membrane distillation

Sweep gas

Ammonia removal

* Corresponding author. Tel.: þ61 3 9545 293E-mail address: zongli.xie@csiro.au (Z. Xi

0043-1354/$ – see front matter Crown Copyrdoi:10.1016/j.watres.2008.12.052

a b s t r a c t

Wastewater containing low levels of ammonia (100 mg/L) has been simulated in experi-

ments with sweep gas membrane distillation at pH 11.5. The effects of feed temperature,

gas flow rate and feed flow rate on ammonia removal, permeate flux and selectivity were

investigated. The feed temperature is a crucial operating factor, with increasing feed

temperature increasing the permeate flux significantly, but reducing the selectivity. The

best-performing conditions of highest temperature and fastest gas flow rate resulted in

97% removal of the ammonia, to give a treated water containing only 3.3 mg/L of ammonia.

Crown Copyright ª 2009 Published by Elsevier Ltd. All rights reserved.

1. Introduction The MD process is a thermally driven process and only

Ammonia is a major pollutant in many industrial and agri-

cultural wastewaters (El-Bourawi et al., 2007). Its elimination

is essential in reusing wastewaters in industry. Ammonia is

often removed by conventional technologies such as packed

tower aeration, biological treatment or adsorption as ammo-

nium ion onto zeolites. The applicability of ammonia removal

technologies generally depends upon several factors such as

contamination level, plant safety and regulatory consider-

ation, and availability of a heating source and chemicals. In

some situations, conventional methods can be costly and

inefficient (Bonmatı́ and Flotats, 2003; Liao et al., 1995). There

is a continuing need for an alternative separation technique

for more efficient removal of ammonia from aqueous streams.

In recent years, membrane distillation (MD) has received

much attention for the removal of volatile compounds like

ammonia because of its potentially low energy requirement. It

has potential for recycling and reuse of industrial wastewater,

and can be especially beneficial for wastewater streams

having a high temperature but relatively low levels of volatile

organic compounds and ammonia.

8; fax: þ61 3 9544 1128.e).ight ª 2009 Published by

vapour molecules are transported through the microporous

hydrophobic membrane. The membrane acts as a barrier to

separate the feed solution (hot side) from the permeate (cool

side) which contains either a liquid or a gas phase (Banat and

Simandi, 1998). The hydrophobic nature of the microporous

membrane prevents liquid from entering its pores, and a fixed

interface is formed at the entrance to the pores. The concen-

tration and temperature difference at the pores’ entrance

produces a vapour pressure gradient, which is the driving

force for vapour molecules of more volatile compounds to

migrate from the feed to the permeate side of the membrane.

At the permeate side, migrated molecules are either

condensed or removed in the vapour phase, depending on the

configuration (El-Bourawi et al., 2006, 2007; Lawson and Lloyd,

1996, 1997).

There are generally four well-known MD configurations:

direct contact MD, air gap MD, sweep gas MD and vacuum MD.

Different MD configurations have been reviewed extensively

(Alklaibi and Lior, 2004; Burgoyne and Vahdati, 2000;

El-Bourawi et al., 2006; Lawson and Lloyd, 1997; Qin et al.,

1996). Other membrane technologies that have been tested for

Elsevier Ltd. All rights reserved.

PumpPreheater

Feedcontainer

Rotameter

Balance

TI

TITI Membranemodule

Absorbingbottle

Air

Vent

Fig. 1 – Sweep gas MD apparatus.

w a t e r r e s e a r c h 4 3 ( 2 0 0 9 ) 1 6 9 3 – 1 6 9 91694

ammonia removal are osmotic membrane distillation (Wang

et al., 2004) and pervaporation (Banat and Simandi, 1996).

Ammonia in aqueous solution exists in two forms:

volatile ammonia molecules NH3 and NH4þ ions. Only the

volatile ammonia molecules are removed by MD. The

amount of ammonia that can be removed depends largely on

the pH and temperature (El-Bourawi et al., 2007). Increasing

pH reverses the ammonia dissociation reaction to produce

more volatile ammonia in the aqueous solution. Increasing

temperature also favours production of volatile ammonia in

the aqueous solution. This is because the solubility of

ammonia decreases with increasing temperature, resulting

in a higher total vapour pressure. For a 10 wt% ammonia

solution, the total vapour pressure increases from 12.1 kPa at

20 �C to 48.3 kPa at 50 �C (Salavera et al., 2005). In addition,

increasing the ammonia concentration in water also

increases the total vapour pressure of the solution. At 20 �C,

the vapour pressure increases from 12.1 to 148.8 kPa, when

the ammonia concentration is increased from 10 to 40 wt%

(Salavera et al., 2005).

Early membrane work on ammonia extraction from

synthetic feed waters with a spiral wound polytetrafluoro-

ethylene (PTFE) gas separation membrane had sulphuric acid

as an ammonia absorbing liquor on the receiving side of the

system (Imai et al., 1982). There was no temperature differ-

ence across the membrane in this approach. An analogous MD

investigation, again using a PTFE membrane, has explored

ammonia recovery from livestock wastewater. It made use of

inorganic acids and also fumaric acid as absorption solutions

on the permeate side (Sato et al., 2006). When fumaric acid

was employed the ammonium fumarate obtained could be

utilised in enzymatic amino acid production.

In one study using various MD configurations, ammonia

(up to 39 g/L) was removed from water using flat sheet PTFE

membranes with a mean pore size of 0.1 or 0.2 mm and 60%

porosity (Ding et al., 2006). Higher ammonia concentrations

resulted in lower selectivity. The separation performance was

greatly affected by the membrane properties. Higher mass

transfer but lower selectivity were obtained with thinner

membranes of larger pore size. Both could be improved by

raising the pH. Vacuum MD showed the highest mass transfer,

but lowest selectivity. Direct contact MD gave the highest

selectivity and moderate mass transfer. The sweep gas mode

had moderate selectivity and the lowest mass transfer.

Vacuum MD of ammonia solutions (4.9–21 g/L) with PTFE

membranes has been studied under various operating condi-

tions (El-Bourawi et al., 2007). High feed temperatures, low

downstream pressures, high feed concentrations and high pH

enhanced ammonia removal, with pH being the dominant

factor. An ammonia removal efficiency of up to 90% and

a separation factor of 8 were reported. It was noted that the

resistance imposed by thermal and concentration boundary

layers adjacent to the membrane surface could contribute

significantly to the overall mass transfer resistance.

The removal of ammonia from a wastewater having

a relatively high ammonia concentration (500–10,000 mg/L)

has also been studied (Ding et al., 2006; Zhu et al., 2005). At

ammonia concentrations of up to 3200 mg/L the mass transfer

coefficient for sweep gas MD was found to be similar to that

obtained for vacuum MD; but the selectivity was 27–100%

higher (Ding et al., 2006). Membrane wetting caused by the

condensation of water droplets in the membrane pores is

minimised in sweep gas MD (Franken et al., 1987). Amongst

various MD operating modes, the sweep gas MD was found to

be the most suitable for the removal of volatile chemicals from

industrial wastewaters (Khayet et al., 2003; Rivier et al., 2002).

Despite extensive studies of ammonia removal from

aqueous streams by MD, little work has been published on the

removal of ammonia from wastewaters containing low

concentrations of ammonia by sweep gas MD. It is known that

the feed concentration has a significant influence on MD

performance and many industries discharge the wastewater

containing only low levels of contaminant. It is important to

gain an understanding of the operating parameters in the

performance of MD for scaled-up applications. This paper

presents a study simulating the polishing of industrial

wastewater that has a low level of ammonia (w100 mg/L) and

high pH (>11). Sweep gas MD was employed for all experi-

ments and the effects of operating parameters such as feed

flow rate, feed temperature, sweep gas flow rate and sweep

gas temperature on ammonia removal and separation

performance were investigated.

2. Experimental

2.1. Membrane distillation system

The experiments were conducted using commercial micro-

porous hydrophobic flat sheet PTFE membranes supplied by

Advantec MFS Inc. Membranes used in the study had a pore

size of 0.45 mm, 70% porosity and thickness of 100 and 200 mm.

Fig. 1 shows a schematic drawing of the sweep gas MD system.

The membrane module was made from stainless steel and

had a membrane surface area of 50 cm2. The feed solution was

pumped continuously through a pre-heater to reach the

required temperature prior to it entering the feed side of the

MD unit. A compressed air stream (4 bar) at room temperature

used as the sweep gas was introduced countercurrently to the

permeate side of the membrane. The air flow rate was

controlled by a mass flow controller (Smart-Trak� M100) with

an accuracy of �1%. Temperatures at the feed inlet, feed

outlet, air inlet and air outlet were monitored with K-type

0

20

40

60

80

100

0 50 100 150 200 250Time (min)

Am

mo

nia rem

oval (%

)

250 ml/min100 mL/min

59 mL/min

Fig. 2 – Effect of feed flow rate on ammonia removal (feed

temperature 65 8C, gas flow rate 3.0 L/min).

w a t e r r e s e a r c h 4 3 ( 2 0 0 9 ) 1 6 9 3 – 1 6 9 9 1695

thermocouples. The feed inlet temperature was maintained

constant with the aid of a temperature controller in the pre-

heater, with an accuracy of �1 �C. For all experiments

performed, the temperature difference along the membrane

on the feed side was found to be within 4 �C even at the low

feed flow rate. However, on the permeate side, there was

a large temperature rise from the air inlet to the air outlet,

generally >30 �C. It was found that the temperature drop on

the feed side and the temperature rise on the permeate side

were affected by the feed and sweep gas flow rate.

During the experiment, samples were taken periodically

for ammonia analysis. The weight loss from the feed side was

measured every 30 s with a digital balance (A&D model

GF-6000 with an accuracy �0.1 g), and recorded by a computer

interfaced to the system. At the outlet of the MD unit, the

sweeping air stream carrying ammonia and water vapour was

first introduced into an absorbing bottle before it was vented.

The absorbing bottle was placed in a cold trap filled with ice to

maximise condensation of the water vapour. Prior to the start

of each experiment, 100 mL of 0.1 M H2SO4 solution was added

to the absorbing bottle to maximise the absorbing efficiency of

ammonia. The weight gain and the ammonia concentration in

the absorbing bottle were measured at the end of the experi-

ment. Based on this, the weight of permeate collected and the

ammonia concentration in the permeate were then calcu-

lated. To ensure the accuracy of calculation of the flux and

selectivity, mass balance for both water and ammonia was

conducted for each experiment by using above measured

weight and concentration data. This was done on both the

feed side and the permeate side. The agreement on mass

balance from both sides was generally good, with a discrep-

ancy of �2%.

An ammonia concentration of 100 mg/L with a pH of 11.5

was selected as the feed solution in the study to simulate

industrial wastewaters with similar characteristics. The

aqueous ammonia feed solution was prepared from 25%

analytical grade ammonia solution (Chemsupply) and de-

ionised water. The pH of feed solution was adjusted to 11.5 by

the addition of NaOH solution prior to each experiment.

2.2. Ammonia analysis

A Hanna ammonia ion specific meter (HI 93733) was used to

measure the ammonia concentration in the solution. The

measurement is based on the ASTM Manual of Water and

Environmental Technology, D1426-92, Nessler method. The

reaction between ammonia and reagents used causes a yellow

tint in the sample which is subsequently detected by a diode

photocell.

2.3. Ammonia removal efficiency and separationperformance

The ammonia removal efficiency is determined from the

equation:

Ammonia removalð%Þ ¼ Cf � Ct

Cf� 100 (1)

where Cf is the ammonia concentration in the feed solution

(mg/L) and Ct is the ammonia concentration at time t.

The separation performance is usually discussed in terms

of flux and selectivity, and generally there is a trade off

between flux and selectivity. Permeate flux is determined by

the mass of permeate collected, the membrane area and the

time of the experimental run. Selectivity represents the

measure of the preferential transport of ammonia. Selectivity

is defined by the equation:

Selectivity ¼ y=ð1� yÞx=ð1� xÞ (2)

where x and y are the mass fractions of ammonia in the feed

and the permeate, respectively.

3. Results and discussion

3.1. Effect of feed flow rate

The effect of feed flow rate on ammonia removal at 65 �C and

with a sweep gas flow rate of 3 L/min is shown in Fig. 2. The

ammonia removal increased from 67 to 77% after 2 h as the

feed rate was increased from 59 to 100 mL/min. However, only

a slight improvement in the removal rate was found as the

feed rate was further increased from 100 to 250 mL/min. As

expected, the higher feed flow rate increased the turbulence

on the feed side and consequently promoted both heat and

mass transfer from the bulk feed to the membrane surface,

thus resulting in higher ammonia removal efficiencies.

Ammonia removal from aqueous solution by sweep gas

MD consists of three mass transfer resistances which exist on

the feed side, the membrane itself and the permeate side

(Ding et al., 2006). These include the diffusion of ammonia

from the bulk feed to the membrane interface, across the

gas-filled pore of the membrane and from the membrane-

permeate interface to the permeate bulk phase. The mass

transfer of ammonia is further complicated by the dissocia-

tion of ammonia.

For ammonia removal by the MD process, the driving force

for the transfer of ammonia across the membrane is the

difference in the partial pressure of ammonia on each side of

0.0

3.0

6.0

9.0

12.0

0 50 100 150 200 250 300Feed flow rate, mL/min

Selectivity

Fig. 4 – Effect of feed flow rate on ammonia selectivity (feed

temperature 65 8C, gas flow rate 3.0 L/min).

w a t e r r e s e a r c h 4 3 ( 2 0 0 9 ) 1 6 9 3 – 1 6 9 91696

the membrane (Cussler, 2000). Ammonia molecules can be

removed directly at the membrane interface, but ammonia

ions NH4þ must first react with hydroxide to form ammonia

molecules before they can be removed. The reaction of NH4þ

with hydroxide is a rapid chemical step; however, the diffu-

sion of hydroxide near the membrane surface is not always

fast. Thus the local absence of hydroxide may sometimes

inhibit ammonia removal (Semmens et al., 1990). The high

feed flow rate increases the diffusion rate of OH� towards the

membrane surface, thus improving the ammonia removal

efficiency. Once the diffusion rate of OH� and the dissociation

rate reach the optimum, unless there are changes in

membrane properties and operating conditions, the

maximum possible removal efficiency is reached, as can be

seen from Fig. 2.

The effect of feed flow rate on the permeate flux and

ammonia selectivity is shown in Figs. 3 and 4, respectively. As

the feed flow rate increased, the permeate flux was increased,

but the selectivity slightly decreased. This could be explained

by temperature and concentration polarisation. As the heat

transfer coefficient increases with feed flow rate, the

temperature difference between the bulk feed and the

membrane surface is reduced. At the same time, increased

turbulence caused by the higher feed flow rate also increases

the ammonia concentration and water vapour pressure on the

feed-membrane interface. The combined effect thus results in

an increase in the permeate flux, but not necessarily the

selectivity.

100

3.2. Effect of sweep gas flow rate

In sweep gas MD, the inert gas, normally operated at ambient

condition, removes vapour from the permeate side of the

membrane and condensation takes place outside the

membrane module. Sweep gas MD combines both the low

conductive heat loss of air gap MD and the reduced mass

transfer resistance of direct contact MD (El-Bourawi et al.,

2006). The gas is not stationary and sweeps over the

membrane surface, which in turn results in enhancing the

mass transfer coefficient, leading to a higher permeate flux

compared to air gap MD. In addition, the sweep gas MD

configuration allows a higher permeate flux and evaporation

0

4

8

12

16

0 50 100 150 200 250 300Feed flow rate, mL/min

Perm

eate flu

x, kg

/m

2 h

r

Fig. 3 – Effect of feed flow rate on permeate flux (feed

temperature 65 8C, gas flow rate 3.0 L/min).

efficiency in comparison to direct contact MD (Khayet et al.,

2003; Rivier et al., 2002). In sweep gas MD, the sweep gas flow

rate is more likely to control the MD process. A small change

in the sweep gas flow rate is expected to have significant

effects on the permeate flux.

Fig. 5 shows the effect of sweep gas flow rate on ammonia

removal with a feed temperature at 75 �C and a feed flow rate

of 250 mL/min. With the sweep gas flow rate increasing from

0.4 to 3 L/min, ammonia removal increased from 48 to 96%

after 2 h. This is because the driving force in MD is the vapour

pressure difference across the membrane. As the sweep gas

flow rate increases, the vapour pressure of both water and

ammonia on the permeate side decreases, resulting in an

increased driving force. In addition, an increase in sweep gas

flow rate reduces the mass transfer boundary layer resistance,

so the ammonia removal efficiency is increased. However, it

was noted that there was little increase in ammonia removal

efficiency as the flow rate was further increased to 5 L/min.

This is because the pressure of the sweep gas increases with

the increasing sweep gas flow rate, leading to an increase in

the resistance of the boundary layer. As a result, there is no

0

20

40

60

80

0 50 100 150 200Time, min

Am

mo

nia rem

oval, %

0.4 L/min2 L/min3 L/min5 L/min

Fig. 5 – Effect of sweep gas flow rate on ammonia removal

(feed temperature 75 8C, feed flow rate 250 mL/min).

0

3

6

9

12

0 1 2 3 4 5 6Sweep gas flow rate, L/min

Selectivity

50°C65°C75°C

Fig. 7 – Effect of sweep gas flow rate on ammonia

selectivity (feed flow rate 250 mL/min).

w a t e r r e s e a r c h 4 3 ( 2 0 0 9 ) 1 6 9 3 – 1 6 9 9 1697

further improvement when the sweep gas flow rate is

increased. Therefore, there is an optimal sweep gas flow rate

that needs to be identified in order to obtain a high ammonia

removal efficiency.

The effect of sweep gas rate on the permeate flux and

ammonia selectivity is shown in Figs. 6 and 7, respectively.

Under the conditions used in the study, the permeate flux

increased with increasing sweep gas flow rate. The enhancing

effect of sweep gas flow rate on the permeate flux has also

been observed earlier (Khayet et al., 2000, 2002; Ding et al.,

2006). The enhancement could be explained by the reduced

temperature polarisation effect and the increase of the sweep

gas temperature along the membrane module. This increases

the driving force and consequently the permeate flux. The

selectivity of ammonia was found to decrease slightly when

operated at 55 and 65 �C, but remains almost unchanged at

75 �C as the sweep gas flow rate is increased. This may be

related to the increased amount of water vapour transported

to the permeate side with the increased sweep gas flow rate

compared to the transfer of ammonia.

Temperature polarisation effects depend mainly on the

dynamic properties of the fluids adjoining the membrane. In

sweep gas MD, the temperature polarisation was found to be

localised at the permeate side and the air temperature polar-

isation coefficient becomes the dominant parameter (Khayet

et al., 2002). As the sweep gas flow rate increases, the Reynolds

number increases. The temperature at the membrane surface

and the temperature in the bulk permeate are almost the

same. As a result, the thermal boundary layer resistance is

reduced and heat transfer increases with minimised temper-

ature polarisation effects. Therefore the permeate flux

increases. In one study (Khayet et al., 2000), it was found that

the temperature polarisation coefficient increased with

increasing air velocity, indicating a decrease in the tempera-

ture polarisation effect.

As mentioned earlier, the extent of the sweep gas

temperature rise was more than that of the feed temperature

drop along the membrane. For example, at the sweep gas flow

rate of 3.0 L/min, the feed temperature was reduced from

75 �C at the inlet to 71 �C at the outlet. On the other hand, the

air temperature increased from 23 �C at the inlet to 66 �C at the

0

4

8

12

16

0 1 2 3 4 5 6Sweep gas flow rate, L/min

Perm

eate flu

x, kg

/m

2 h

r

50°C

65°C

75°C

Fig. 6 – Effect of sweep gas flow rate on permeate flux (feed

flow rate 250 mL/min).

outlet. When the sweep gas flow rate increases, the variation

of the increase of the sweep gas temperature along the

membrane might affect the local driving force and conse-

quently the permeate flux, as found in the previous study

(Khayet et al., 2000).

3.3. Effect of feed temperature

Fig. 8 shows the effect of feed temperature with a sweep gas

flow rate of 3.0 L/min and a feed flow rate of 250 mL/min. As

expected, higher feed water temperatures gave better

ammonia removal. A total ammonia removal of 97% after 2 h

at 75 �C resulted in a treated water with an ammonia

concentration of only 3.3 mg/L. High feed temperatures

enhance ammonia diffusion in both the membrane pores and

the sweep gas due to a higher mass transfer coefficient. In

addition, more volatile ammonia is present in the feed solu-

tion due to the endothermic nature of the dissociation of

ammonium ions.

The effect of feed temperature on the permeate flux and

ammonia selectivity is shown in Figs. 9 and 10, respectively.

0

20

40

60

80

100

120

0 50 100 150 200Time (min)

Am

mo

nia rem

oval (%

)

50°C

65°C75°C

Fig. 8 – Effect of feed temperature on ammonia removal

(feed flow rate 250 mL/min, gas flow rate 2 L/min).

0

4

8

12

16

40 50 60 70 80Temperature, °C

Pe

rm

ea

te

flu

x, k

g/m

2 h

r

2 L/min3 L/min5 L/min

Fig. 9 – Effect of feed temperature on permeate flux (feed

flow rate 250 mL/min).

0

10

20

30

40

50

60

70

80

90

0 20 40 60 80 100 120 140Time (min)

Am

mo

nia rem

oval (%

)

■ Air inlet 5°C♦ Air inlet 23°C

Fig. 11 – Effect of sweep gas inlet temperature on ammonia

removal (feed flow rate 250 mL/min, feed temperature

75 8C, gas flow rate 3.0 L/min).

w a t e r r e s e a r c h 4 3 ( 2 0 0 9 ) 1 6 9 3 – 1 6 9 91698

The permeate flux increased significantly with increasing feed

temperature, but the selectivity decreased. This is in agree-

ment with findings reported in the literature (Ding et al., 2006;

Khayet et al., 2002; Lawson and Lloyd 1996). As the tempera-

ture is increased, there is an exponential increase in the

vapour pressure of the feed solution which in turn increases

the transmembrane vapour pressure and the driving force.

However, since the water vapour pressure and diffusivity also

increased, the mass transfer of water vapour is intensified and

as a result, the ammonia selectivity is decreased.

3.4. Effect of sweep gas inlet temperature

The effect of sweep gas inlet temperature on ammonia

removal is presented in Fig. 11. The results indicate that the

sweep gas inlet temperature does not play important role in

ammonia removal efficiency. When the air inlet temperature

was increased from 5 to 23 �C, the ammonia removal

decreased slightly (<5%). This means that the effect of the

0

3

6

9

12

40 50 60 70 80Temperature,°C

Selectivity

2 L/min3 L/min5 L/min

Fig. 10 – Effect of feed temperature on ammonia selectivity

(feed flow rate 250 mL/min).

sweep gas inlet temperature on sweep gas MD for ammonia

removal is negligible or only marginal. The result is in agree-

ment with the finding reported by Basini et al. (1987) and Lee

and Hong (2001), but not with the finding reported by Khayet

et al. (2002). The negligible effect of sweep gas inlet tempera-

ture could be because the specific heat capacity of air is much

lower than that of water. As a result, the sweep gas temper-

ature increases very rapidly from the inlet to the outlet of the

module. As observed during the experiment, the sweep gas

outlet temperature remained almost the same (66� 1 �C),

whereas the gas inlet temperature increased from 5 to 23 �C.

That is, increasing the sweep gas inlet temperature had only

a marginal effect on the temperature gradient across the

membrane and hence the driving force. Therefore, the effect

of sweep gas inlet temperature on the ammonia removal and

the permeate flux is essentially negligible.

4. Conclusion

Sweep gas MD provides a potential application for very effi-

cient recycling of industrial process wastewaters containing

low levels (up to 100 mg/L) of ammonia. Up to 97% removal of

ammonia could be achieved, resulting in an ammonia

concentration in the treated water being as low as 3.3 mg/L.

Operating variables such as feed temperature, feed flow rate

and gas flow rate were found to have significant influences on

the efficiency of ammonia removal. Sweep gas inlet temper-

ature had a negligible influence on ammonia removal. A

higher feed temperature, feed flow rate and sweep gas flow

rate promoted ammonia removal efficiency and permeate

flux. The feed temperature was a crucial operating factor in

the process, as a balance between flux and selectivity needs to

be considered. The feed flow rate and gas flow rate have less

influence on ammonia selectivity.

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