Membrane Technology in Water Treatment Applications

7/29/2019 Membrane Technology in Water Treatment Applications

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MEMBRANE TECHNOLOGY IN WATER TREATMENTAPPLICATIONS

Wolfgang M. Samhaber, Institute of Process Engineering,Johannes Kepler University Linz; A-4060 Leonding, Welser Strae 42

Bernhard Haschek, EVN Water, A- 2344 Maria Enzersdorf, EVN-Platz

Abstract

Drinking water and pure water are an increasingly limited resource and billions ofpeople are living already today with shortages or with an inadequate access to cleanwater [1]. In many countries available water sources can not be used straight fromsurface water or from sub-surface water or ground water.

In contrast to those regions the quality of fresh water resources in Austria exhibits anexcellent standard, because most of the water is based on deep ground watersources. Beside of this great extend of best quality water sources there are certainareas, which have some requirements of treatment to get the accustomed highquality drinking water.

The aim of the field test investigation was to evaluate the conceptional design of aground water treatment process, which can be operated on a low energy basiswithout a need of antiscalants and frequent acidic flushing. The ground water sourcefor this study was characterized by a high degree of hardness and nitrate contentclose to the upper regulation limit. The requirement therefore is a strictly controlleddegree of the hardness on the retentate side which should not exceed a maximum

limit of 80 dH.

The combination of NF and RO membrane types has proven advantageous in thefield tests concerning the given targets. The NF membrane contributes in thesoftening and in a major flow yield parallel to a small nitrate reduction. The ROmembrane again takes care among others of nitrate reduction and of course to acomplete hardness reduction too. The combination of both membrane types showsan appropriate way to produce high quality water.In a field test study practical data and long-term test experiences have been collectedand some of it will be presented in this paper. The field tests were established and

focused at the water sources of EVN Water in Bisamberg, located north-west ofVienna.

KEYWORDSWater treatment, nanofiltration, reverse osmosis, EVN Bisamberg


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1. Introduction

Since the beginning of the membrane technique RO membranes and later also NFmembranes are used to produce high quality water from different sources like seawater, and brackish water [2], surface water, secondary treated effluent and in certain

aereas and local situation sub-surface or ground water.

Water contains beside of the necessary dissolved matters suspended and dissolvedimpurities. The water sources often bear disease causing bacteria, viruses, andparasites and this water must be treated and purified to meet human needs.Chemical disinfection is in wide use to treat water but disinfection forms againdisinfection by-products (DBP). To avoid the generation of such DBPs it is necessaryto remove natural organic matters efficiently prior to disinfection. Pesticides andnitrates in ground water is a growing problem in agricultural areas [3].

Beside of flocculatio, coagulation and clarification membranes have demonstrated

excellent results in natural organic matter separation and have gained a highpotential in future in the production of drinking water.

0,1 nm 1 nm 10 nm 0,1 m 1 m 10 m 100 m

suspended solidscolloidal

Viruses

Ca, Mg,

carbonate

protozoasNOMs

ions, salts

RO

NF

UF- MF

microorganisms

dead-end

filtration

Fig. 1: Membrane application in the different water treatment areas

The major fields of membrane applications are in the treatment of different sources ofwater and are shown in Fig.1. Membrane and membrane processes are generallyalternative ways to conventional techniques and it should be noted to choose theclassical way when ever this is technically and economically possible. Membraneprocesses are combined with a relevant demand for electrical energy. Theminimization of the energy demand of water treatment systems is an important taskfacing our future water resources [4]. Membrane processes need hydraulic pressureto force water through a semi-permeable membrane which needs energy and if weare speaking of energy, we have to face the mass of CO2 which is emitted thereforeto get water in defined quality.If we count the quantity of CO2 which we will emit to produce softwater out of ground

water, we have to realize that we are emitting between 200 to 500 g CO2 perproduced 1000 l of product water. Its a relatively wide CO2 emission range which is aresult of different process parameters and of the feed water quality.


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However membranes are applicable for water sources which cannot be treatedanymore successfully by e.g. convetional techniques combined with disinfectionchemicals. Membranes exhibit the ability to reject most contaminants and haveattained for these tasks a high acceptance in the last 20 to 30 years and manymembrane applications in water treatment have been realized therefore and

devepment of economically and ecologically processes is needed [5].

In the water industry there have been collected parallel to this developmentexperienced ways of operations in the treatment of different water sources. The scaleof application of membrane plants has grown as well and the sizes of membraneplants will have further more a rising demand for developments to eneable thepurifying water in areas where water is available but the quality for humanconsumption is not given. The challenge to apply membrane processes for thedifferent raw water qualities is to make these techniques applicable in respect tospecific plant and energy costs. Certain plant concepts and process combinations areinevitable to meet these designated targets and some of them will be presented in

this paper and practical experiences from field tests discussed.Variations of this technology include reverse osmosis (RO), nanofilitration or lowpressure RO, ultrafiltration and microfiltration. As membrane plants need energy it iscrucial to minimize the energy consumption for this fast growing demand in watertreatment applications also in view of climate change emission reduction.

2. Aim of field test investigations

The purification of water with less energy costs and minimum use of chemicals andminimizing the impact on the environment have been specified for objective targetsparallel to a major reduction of nitrate content with an accompanying softeningprocess. The treatment plant is foreseen to be built in Bisamberg in 2012 and thetotal final capacity of available ground water treatment will be about 1000 m3/h.

2.1 Raw water quality

The feed water quality is to be seen from Table 1 which contains the total hardness(GH) instead of mg/l in degree of German Hardness [dH].

Content mg/l Ca Mg GH Na K Cl NO3 SO4 HCO3

Averages 129 71 35 35 8 90 53 184 395

Tab.1: Raw water composition of the feed well water

2.2 Pilot plant

The field tests were carried out form July 2009 until August 2010. During this time thepilot plant was operated 24 hours/d and 7 days per week continuousely. To avoid apossible fouling a Bio-Cel filter was used at the beginning, but by-passed after aperiod of 4 months to find out if cartridge filters (5m and/or 1m) are appropriate

and operable for taking care of suspended solids in the up-stream side of the pilotplant. The replacement of cartridge filter period was about 3 months due to smallamounts of iron in the feed, which was acceptable for pre-treatment.


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ML1

m3/h bar Yield% %

l/h

% l/h

l/h

Feed

Line 1 l/h

bar

P 1.1 Retentat

l/h

Drain

PDISHL

1.626

TIRSHL

1.624 SIR

1.625

FIR

1.622PIRSH 0,9

1.630

293

789

5,7

37

467

322

5,3

bar

1.632 174

1.620

FIRCSHL

41

1.500

1.628

FICR

4,863

FIR

M

M

Fig. 2: Flow sheet of one membrane separation line of the pilot plant

The membrane separationpart of the pilot plantconsists of two identicallyand separated lines, eachline equipped with two4040 elements in series.Therefore the treatment

was done in two steps perline.As it can be seen from theflow sheet from Fig. 2, thepermeate lines of each4040 element wereseparated and the sampleswere taken from eachpermeate line to enable theevaluation of the combinedperformance and that for

observing the specificcontribution of the differentmembranes within theestablished separationconcept.The instrumentation allowto collect all hydraulic dataand they were stored every10 minutes and it facilitatesfurther the fully automatedprocess.

Fig. 4: Pilot plant mounted within a air-conditionedcontainer with all necessary process control items for

a fully automated and remote controlled operation


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2.3 Membranes

From the previous laboratory studies the AG (Desal-GE) and NF90 (Dow-FilmTec)were selected, which were well performing in the small scale studies. For that reasonit was decided to test on the one hand the membrane types of the lab studies and on

the other hand to take over the ESNA1-LF2 (Hydranautics), HL (GE), and the XLE(Dow-FilmTec) types of membrane, which were recommended additionally for thefield tests. The element size was 4040 and the membrane area about 85 ft

2(7,9 m

2).

3. Results of the pilot tests

3.1 Lessons learned with scaling

During the the first weeks of field tests the combination of the evaluated membraneswere experienced together with parallel simulation on the basis of the received data.

The relatively high hardness brought some uncontrolled scaling problems at thebeginning and made shortly certain operational modes necessary to avoidovershooting the CaCO3 solubility limits.

HL4040 ML1.2

10

20

30

40

50

60

70

80

90

31.07.

09

31.07.

12

31.07.

14

31.07.

16

31.07.

19

31.07.

21

01.08.

00

01.08.

02

01.08.

04

Ti me

Flux [l/h bar element] Yield in %

HL 4040 ML1.2 (no circulation)

10

20

30

40

50

60

70

80

90

03.08.

07

03.08.

09

03.08.

12

03.08.

14

03.08.

16

03.08.

19

03.08.

21

04.08.

00

04.08.

02

t i m e

Spec.Flux [l/h bar element] Yield [%]

Fig. 3: Description of the 2nd element (HL) of ML1 with and almost without scaling

In Fig. 3 the rapid scaling is demonstrated which was induced by a steady increase ofthe yield on the left side and on the right by keeping the yield low not to exceed the

solubility limit to attain stable conditions.The optimal operating concept could be obtained by a combination of a NF first and aRO type membrane on the second place. With this concept it was possible to achieve65 to 70 % yield depending on the hardness in the raw water, which varied between32 and 42 dH. This result was obtained without using any antiscalant in a long-termcontinuous operation. The crucial hardness which was absolutely to observe was aupper limit in the retentate of 82% based on this specific feed water.

A second scaling experience should be additional described, to demonstrate thesensitivity and the dynamic of that system and the affect of available cristallizationseeds.


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It was started with a linear increas of the flow yield which raised the concentrationand the hardness during the 22

ndof April 10 (see Fig. 4).

HL4040N-Flux vers Yield

0,10

0,20

0,30

0,40

0,50

0,60

0,70

22.04.10 29.04.10 06.05.10 13.05.10 20.05.10 27.05.10

YIELD

[/]

2,00

2,50

3,00

3,50

4,00

4,50

5,00

Flux

[l/mhbar

Yield Flux

Fig. 4: Flux effects trough yield increase due to on-going carbonat scaling and

regeneration through complete or incomplete acidic flushing

After acidic wash, which was very short and uncompleted it can bee seen that theflux is again decreasing despite of a significant reduction of the yield. After a properflushing the flux rate was recovered and back at the previous level. During the entiretime of the field tests the membranes never have seen any antiscalings.

5,0

5,5

6,0

6,5

7,0

7,5

8,0

8,5

9,0

9,5

10,0

16.05.10 06.06.10 27.06.10 18.07.10 08.08.10 29.08.10

Time

SpecificFlux[l/m2hbar]

Fig. 5: Permeate flux of ESNA with a mean flow yield of 41 % (Pos.: ML1.1)

In Fig. 5 the specific permeate flux of the ESNA in ML1.1 is shown as a perfectconstant flux over the period between May and August. The slight increase of fluxover time can be explained by temperature elevation in ground water from 12,5 inMay to 14 C with the end of August 10.


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3.2 Membrane performance comparison

On the collected experiences of the field test the ESNA in connection with the XLE inseries have proven its operability for this task. The ESNA is an open NF membranewhich exhibits still some percentage points in nitrate retention beside of a very high

permability for monovalent ions to pass to the product water, where those ions areneeded for the water quality.A comparison of the retention of ESNA and HL for mono- and divalent ions can bedrawn on the basis of the calculated data, which were collected during the field testsand are illustrated in Fig. 6. ESNA and HL exhibit almost similar separationperformances regarding the depicted ion retentions. The HL membrane indicates justfor Mg a higher retention; the other retention values can be considered as a little bithigher of the HL membrane.Beside of the retention coefficients the obtained specific fluxes are determiningadditional the separation performance.

Fig. 6: Retention of Ca-, Mg-, HCO3-, SO4- and NO3 -ions of measured during thefields of the HL and ESNA membrane

The decision between HL and ESNA could be done based on the flux comparison inFig.7, where the different data of these membranes are plotted.

It is evident, that ESNA is better working on a low pressure basis with still quite a

significant rejection for divalent species to contribute to softening. The ESNA elementhas shown a very smooth operation over the whole period of field test time and stableand of course higher fluxes. In connection with the RO step the ESNA typemembrane provided a flow yield of about 40 % and hardness reduction down to20dH without a relevant retention for the monovalent ions.

In the combination NF membrane which makes about 40 % yield must be followed bya high selective low pressure RO which produces a proper permeate quality forblending with the NF permeate to get the wanted final quality of the treated water.


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250

300

350

400

450

500

550

600

4,0 5,0 6,0 7,0 8,0 9,0 10,0

Operating pressure [bar]

Elementflux[

l/h]

HL-ML1.2 ESNA-ML1.1Polynomisch (HL-ML1.2) Polynomisch (ESNA-ML1.1)

Fig. 7: Element flux of NF-membranes ESNA and HL versus operating pressure

The AG, the NF90 and the XLE were to be evaluated for this purpose. The AG has itsstrength in the high pressure applications, the NF90 and XLE are low pressuremembranes differing in the retention behaviour. In Fig. 8 these diference in retentioncan be seen and it indicates a similar separation performances of these membranes.

Fig. 8: Comparison of retention between NF90 and XLE

The NF90 is known as a dense NF membrane and one can see that the divalent ionspoint out a comparable retention like it with the RO membrane. The HCO3 retention issimilar, the nitrate retention is lower of the NF90. As from the RO part a higher nitrateretention is wanted and the decision for the XLE was drawn based on this relationand on the higher specific flux of the XLE type membrane.

To compare finally yet the AG with the XLE the respective retention diagrams areshown in Fig. 9. These two membranes are quite comparable in all there monvalentions retention coefficients.


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This coincidence is remarkable, because these membranes are manufactured bydifferent companies and the major difference is an almost 5 fold flux.

Fig. 9: Comparison of AG and XLE regarding the respective retention

Another remarkable fact can be shown on the diagram of the XLE membrane, if wecomparing the Na- with the NO3-retention. For the simulationof the treatment processthe experimental data was used to describe the flux versus retention for the ions ofinterest. For that purpose the Push model was used and in Fig. 10 some empiricaldata are shown and the model curve for Na- and nitrate retention. The XLE exhibits ahigher selectivity for nitrate as for sodium, which increases the preference of that

membrane for the given requirement.

XLE 4040 ML2.2

92

93

94

95

96

97

98

99

100

0 100 200 300 400 500

Permeate Flux [l/h,Element]

Rejectionin%

R-NO3

Pusch Model

R-Na

Pusch-Na-Model

Fig. 10: Na- and NO3-retention of the XLE type membrane


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3.3 Energy demands

Based on the the hydraulic data, which was collected during the field tests, theenergy demand in that pilot scale was calculated. For the efficiency of the pumps avalue of 78 % was assumed and power consumption of the feed and circulation

pumps were calculated from the respective volume flows of feed and permeate lines.

In Fig. 11 those result are plotted versus the applied feed pressure based on aconcept with an ESNA in the first stage and a XLE on the 2

ndof the retentate side. In

that concept two third of the water is produced in the NF-stage and 1 third in the ROstage.

For nitrate 50 % reduction was achieved and the hardness were brought to a rangebetween 10 to 12 dH depending on the feed situation.

0,20

0,25

0,30

0,35

0,40

0,45

0,50

4,0 5,0 6,0 7,0 8,0 9,0 10,0

Pressure [bar]

Energydemand[kW/m3]

3. Conclusions and discussion

The aim of the field test investigation was to evaluate the conceptional designof a ground water treatment process, which can be operated on a low energybasis without a need of antiscalants and frequent acidic flushing. The groundwater source for this study was characterized by a high degree of hardnessand nitrate content close to the upper regulation limit. The requirementtherefore is a strictly controlled degree of the hardness on the retentate sidewhich should not exceed a maximum limit of 80 dH.

The combination of NF and RO membrane types has proven advantageous inthe field tests concerning the given targets. The NF membrane contributes inthe softening and in a major flow yield parallel to a small nitrate reduction. The

RO membrane again takes care among others of nitrate reduction and ofcourse to a complete hardness reduction too. The combination of bothmembrane types shows an appropriate way to produce high quality water.


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References

[1] M. A. Shannon, P.W. Bohn, M. Elimelech, J.G. Georgiadis, B.J. Marinas, A.M.

Mayes, Science and technology for water purification in the coming decades,Nature 452 (7185) (2008) 301310

[2] Lauren F. Greenlee et. al.; Reverse osmosis desalination: Water sources,technology, and todays challenges: Water research 43 (2009) 2317-2348

[3] Van der Bruggen, B. et. al.; Application of nanofiltration for removal of pesticides,nitrate and hardness from ground water: rejection properties and economicevaluation: Journal of Membrane Science 193 (2001) 239248

[4] Asmerom M. Gilau, Mitchell J. Small; Designing cost-effective seawater reverseosmosis system under optimal energy options: Renewable Energy33 (2008) 617630

[5] Tansel Berrin, New Technologies for Water and Wastewater Treatment: A Surveyof Recent Patents: Recent Patents on Chemical Engineering, 2008, 1, 17-26

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Membrane Technology in Water Treatment Applications