REPORT OF THE PERFORMANCE OF NANOFILTRATION MEMBRANE ON TAP WATER

Objective: To investigate the performance of nanofiltration membrane on tap water.

Abstract: Membrane performances based on percentage rejection permeate flux, feed and

permeate conductivity of tap water were studied. A dead – end type membrane stirred cell for

nanofiltration test was used for the experiment. A Magnetic stirring system was applied to

overcome a pressure-induced concentration polarization occurred over a membrane surface in

the test cell. A high pressure N2 tank is used as a pressure source. The permeate flux patterns

for two different membranes were investigated. The effect of pressure in the rejection of

membranes was verified through a series of experiments using 1540-3 nanofiltration

membrane in which the rejections were measured under two applied pressures.

Keywords: Nanofiltration membrane, permeate flux, rejection rate, feed and permeate

conductivity.

Experimental Setup: On laboratory scale, using a stirred cell [memcom of project series

09097] at constant pressure and nanofiltration membrane, some filtration runs were carried

out by using ordinary tap water. For the entire test procedures performed, the model of the

cell take a volume of 1000ml of solution and it uses a membrane of 0.117m in diameter and it

effective area of 0.01075m². The stirrer cell is equipped with magnetic stir holder which

holds a stir bar to move the membrane surface. The magnetic stirrer mechanism was available

to control the concentration polarization or accumulation of macromolecules on the

membrane surface. The reference membranes were purchased from Israel. According to the

manufacturer’s, the membranes have the following advantages.

High chemical stability

Proven stability in hot and concentrated sulphuric acid

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High selectivity

Higher separation ratios

Increase in the throughput (flux)

Higher rejection rates

Experimental Procedure: Several runs of the experiment were conducted at constant

pressure of 20bar and four of the runs were conducted at different pressures of 18bar and

20bar. This was done in order to know if pressure has significant effect on the membrane

performance. High purity nitrogen gas was used to pressurise the stirred cell throughout the

experiment. The pressure for the filtration runs was set by precision regulator equipped with

digital pressure display. The precision pressure was integrated downstream from the nitrogen

pressure regulator screwed into the cylinder outlet.

In some of the filtrations run, permeate was collected at every 100ml in a beaker, while in

some, permeate was collected at every 15minutes interval in a beaker. In some filtrations run,

feed and permeate conductivity were measured at every 100ml while the feed and permeate

conductivity were measure at every 15 minutes interval in some filtrations run. This was done

in order to find the best method of running the remaining part of the experiments. The

permeate conductivity was measured using a conductivity meter. The operating speed of the

magnetic stirrer was 400rpm throughout the filtrations run. Permeate passing through the

membrane is drawn into a 1000ml beaker which was weight on a digital weighing balance.

The beaker was maintained on a weighing balance so that permeate could be measured

continuously. And the permeate conductivity was measured at every 100ml or every 15

minutes in a small container of 10ml. By the same time, the feed conductivity was also

measured.

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Findings/Observation: The operating parameters such as pressure, flowrate, the

concentration of the feed and in permeate have an important influence on the separation

performance. Permeate mass, flow have to be determined in this experiment. The membrane

water permeability is dependent on the size of the pores in the membrane, the porosity of the

membrane and the thickness of the membrane. The membrane water permeability is varies

over time. Because water is retained by the membrane, a concentration difference over the

membrane exists. At the concentrate side (which is the feed in the stirring cell) the

concentration is much higher than the concentration in the permeate.

RESULTS AND DISCUSSION

Flux rate: For the nanofiltration process, the membrane productivity is expressed as the

permeate flux through the membrane. The permeate flux was calculated using the formula

below.

Flux, J =

Vt⋅A

(1)

Where V is the volume of permeate, t is the permeate collection time and A is the area of the

membrane. The results of fig 1 and 2 show that concentration polarization exists in the

membrane separation process and have great influence of on the separation performance of

NF membranes. The impurity in tap water could induce clogging of the membrane when the

concentration polarization occurs over the membrane surface. These behaviours could be the

reason why the flux declined too much. The permeation flux falls with rising time. The

decline of flux was due to membrane compaction at operating pressure and membrane

fouling. The densification of the membrane under pressure reduced the flux through the

membrane.

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0 0.5 1 1.5 2 2.5 320.3

22.3

24.3

26.3

28.3

30.3

32.3

Permeate Flux (L/m²/hr)

Time (hr)

Perm

eate

flux

(L/m

²/hr

)

Fig 1: Permeate flux against time

0 0.5 1 1.5 2 2.5 321

22

23

24

25

26

27

28

29

30

Permeate Flux (L/m²/hr)

Time, (hr)

Perm

eate

Flu

x, (L

/m²/

hr)

Fig 2: Permeate flux against time

The permeate flux patterns for two different membranes were quantitatively similar as shown

in fig 3. The permeate flux declined subsequently and this was related to the boundary layer

near the membrane surface and the cake layer deposited on the membrane surface.

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0 0.5 1 1.5 2 2.5 3202224262830323436

Permeate Flux against using two different membrane at 20bar

Permeate Flux, 1540-3 NFPermeate Flux, 1530 NF

Time, hr

Perm

eate

Flu

x, L/

m²/

hr

Fig 3: Permeate flux against time

Permeate rejection: The degree to which material passes through the membrane is

generally evaluated in terms of rejection of permeate. The rejection can be calculated

through the equation 2.

R =

Feed−PermeateFeed

×100(2)

The effect of pressure in the rejection of membranes was verified through a series of

experiments using 1540-3 nanofiltration membrane in which the rejections were measured

under two applied pressures.

Permeate rejection relies on the ratio of the transport rate of feed to that of permeate.

Generally percentage rejection was found to increase with increasing water recovery. The

Fig: 4 illustrate the rejection behaviour for water upon pressure. Rejection was strongly

dependent of operational pressure: The increase in rejection at higher pressures is generally

explained by a shift in the transport mechanism across the membrane. Higher rejection at

higher pressure. At lower pressures a diffusive transport of salts occurs, which accounts for

the lower rejections and at higher pressures convective transport of salts through the

membrane becomes dominant.

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0 10 20 30 40 50 60 70 80 90404550556065707580

Permeate Rejection as a function of Water recovery at different Pressure

Permeate Rejection at 20 barPermeate Rejection at 18 bar

Water Recovery, %

Perm

eate

Rej

ectio

n, %

Fig 4: Permeate rejection as a function of water recovery at different pressure.

Feed and permeate conductivity: The concentration in feed and permeate is indirectly

deduced from electrical conductivity measurement. Feed conductivity measurements

indicate a change in water source, perhaps because of seasonal variations or surface water

influences-both of which require operation interface to ensure proper operation of the

membrane system. Fig 5 shows the feed and permeates conductivity as a function of

water recovery using 1540-6 NF membrane.

0 10 20 30 40 50 60 70 80 900

100200300400500600700800900

Feed and Permeate Conductivity as a function of Water Recovery at pressure 18 and 20 bar

Conductivity of feed at 18barConductivity of permeate at 18barConductivity of feed at 20barConductivity of permeate at 20bar

Water Recovery, %

Cond

uctiv

ity ,

µS/c

m

Fig 5: Feed and permeates conductivity as a function of water recovery

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Conclusions: The flow rate and pressure have significant influence on permeate flux. The

rejection rate in this work confirms the expectation of the Nano pro membranes which is

higher rejection rate. The percentage recovery of the membrane system feed water that

emerges from the system as product water/permeate is 80%.

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