36

Application of ZeeWeed® Membranes for Water Reuse and Pre-Treatment for Brackish and Sea Water Reverse Osmosis Desalination

Roy Arviv, Diana Mourato and John Coburn

ZENON Environmental Inc.

3239 Dundas Street West, Oakville, Ontario, L6M 4B2 Canada ABSTRACT

In many parts of the world, availability of water has become a barrier to land development and growth. In arid regions, this has been compensated primarily by desalination of seawater. The reuse of wastewater is less expensive than seawater desalination and, when possible, thus constitutes an attractive alternative to desalination. Water reuse projects are currently being evaluated in many water short countries.

Ultrafiltration (UF) membranes can remove particulate and colloidal materials from water. For desalination, the primary reason to use UF is indeed to protect sensitive spiral wound or hollow fiber RO elements from particulate and biological fouling. But equally important is the fact that a membrane is a physical barrier that provides constant quality. Conventional pre-treatment systems may produce acceptable feed water when properly tuned, but upsets result in changes in water that are detrimental to the operation of the RO.

For water reuse, UF has successfully been used in combination with biological processes to treat wastewater. There are two basic configurations for combining a biological process with UF. In the first configuration, the two processes are integrated into a single step membrane bioreactor. This configuration, which offers real synergies between the two processes is described elsewhere (Adham et al., 2001; Thompson et al., 2001; Phagoo et al., 2000).

The second configuration, tertiary filtration, involves using ultrafiltration downstream of a biological process. This configuration may be more practical when reusing water from an existing biological plant. Biological treatment and UF provide adequate water quality in many water reuse applications. They also constitute the best pre-treatment for reverse osmosis if the removal of dissolved contaminants or salts are required.

One of the most attractive and rising applications is in the industrial use where the motivation for reuse is double due to the cost of sewage treatment combined to the cost of buying and treating process water.

This paper introduces the use of the ZeeWeed� ultrafiltration immersed membranes in water reuse and desalination applications. In water reuse, pilot, demonstration and full-scale data are presented from test sites in the United States and a full-scale site in Poland. In desalination, piloting results from 3 studies conducted in the United States, United Arab Emirates and Europe are presented.

37

INTRODUCTION

Ultrafiltration (UF) membranes completely remove particulate and colloidal materials from water. In comparison to conventional granular filtration the UF technology is much more efficient in terms of footprint, bacterial and virus log removal etc. To monitor the efficiency of the UF systems, qualitative methods such as turbidity or Silt Density Index – are being used.

Ultrafiltration became much more accepted and affordable in recent years, as membranes

could be produced cheaper, systems are nowadays controlled easier and operators seem to accept new technology. The technology today is basically PLC-supported and controlled which enables such systems to perform better and have almost zero upsets.

Today we can see UF applications in many disciplines related to different types of water. Sometimes we see it as a leading and ultimate solution that meet the sufficient requirements of the user and sometimes just as an intermediate technology, which incorporate in the overall treatment scheme for producing clean water even from dissolved salts.

The increased use of UF membranes resulted not only of technological improvement, but

also because of the economic aspect shown by the overall project economics: - Integrated membrane systems such as UF and RO can provide useful solutions for

controlling or minimizing waste discharge streams as well as reclaiming water for supplementing plant usage requirements.

- Feasibility of combining membranes in the process, which later enable to reuse the water instead of buying new water continuously and paying more for disposal as shown on Figure 1. purchase treatment use sewage reduce disposal water contamination for disparage cost cost cost cost buying less use sewage reduce Water contamination for reuse ½ cost cost Figure 1: Motivation for reuse

UF membranes are ultimate barriers for particles removal. For desalination, the primary reason to use UF is indeed to protect sensitive spiral wound or hollow fibre RO elements from particulate and biological fouling. But equally important is the fact that a membrane is a

38

physical barrier that provides constant quality. Conventional pre-treatment systems may produce acceptable feed water when properly tuned, but upsets result in changes in water that are detrimental to the operation of the RO.

For water reuse, UF membranes have successfully been used in combination with

biological processes to treat wastewater. There are two basic configurations for combining a biological process with UF. In the first configuration, the two processes are integrated into a single step membrane bioreactor. This configuration, which offers real synergies between the two processes is described elsewhere (Adham et al., 2001; Thompson et al., 2001; Phagoo et al., 2000).

The second configuration, tertiary filtration, is described in this paper and involves using

ultrafiltration downstream of a biological process. This configuration may be more cost effective when reusing water from an existing biological plant. Biological treatment and UF provide adequate water quality in many water reuse applications. They also constitute the best pre-treatment for reverse osmosis if the removal of dissolved contaminants or salts are required.

Many other UF membrane applications for water reuse can be found today in the municipal

and industrial areas where the regulation for sewage disposal and water reuse are strictly enforced by governmental authorities and the continuous shortage of water.

In industry, for example, there is always an opportunity to recycle water because there are many areas within the plant where water can be reused (cooling towers, equipment wash/clean, boiler feed water) that require different levels of treatment quality.

The following lecture will attempt to demonstrate the case studies and pilots for Municipal

and Industrial reuse, as well as RO pretreatment.

39

THE ZEEWEED���� ULTRAFILTRATION MEMBRANE

Key features of the ZeeWeed� ultrafiltration immersed membrane are illustrated in Figure 2. The membrane is a hollow fibre with filtration from the outside-in under gentle suction. The module is shell-less and immersed directly in the water to be filtered. Air is used to scour the membrane surface and de-concentrate the hollow fibre bundles. Feed and purge operations are done at the tank level.

ZeeWeed� are asymmetric ultrafiltration membranes that reject all suspended and colloidal

solids, including viruses. They are made from a chlorine-tolerant, hydrophilic proprietary polymer. The same membrane is used in the two configurations of modules described below, the ZeeWeed� 500 series and the ZeeWeed� 1000 series.

Figure 2. The ZeeWeed���� immersed membrane principles of operations

The ZeeWeed� 500 Series The ZeeWeed� 500 series are built with a reinforced, large diameter hollow fibre. The 1.9

mm outside diameter hollow fibres are flexible and have a very high tensile strength, two properties that allow vigorous air scouring in difficult applications.

Modules are rectangular frames containing thin bundles of hollow fibres. The hollow fibres

are mounted vertically between headers with some slack to allow movement, air penetration and water renewal within the bundle.

Modules are assembled side by side into cassettes, leaving space for water circulation and

air scouring. Cassettes have integrated headers for permeate collection and air distribution. Cassettes are the building blocks that are immersed into the filtration tank and connected to permeate and air headers. A ZeeWeed� 500 cassette is shown in Figure 2a.

Four generations of ZeeWeed� 500 series cassettes have been introduced over the past 7

years. Using the same membrane and overall cassette size to ensure backward compatibility, improvements have been made to the module, the cassette and the aeration system. The results of these efforts are illustrated in Figure 3 showing the evolution for flow per cassette

� Outside-in hollow fibre

� Shell-less module

� Open tank

� Gentle suction

� Air scouring

� Feed & purge at tank level

Air

Feed

Purge

40

(Figure 3a) and energy consumption per unit volume of water treated (Figure 3b). For both parameters, the improvements represent a factor of more than 9 over a 6-year period.

Figure 3. ZeeWeed���� immersed membrane cassettes

Figure 4. Evolution of the ZeeWeed���� 500 series membrane

The ZeeWeed� 1000 Series ZeeWeed� 1000 is the fifth generation of the ZeeWeed® immersed membrane targeted at low

suspended solids applications (Côté et al., 2001). It is based on the proven chlorine-tolerant membrane chemistry used in the ZeeWeed� 500 series.

a) ZeeWeed� 500 cassette

b) ZeeWeed� 1000 cassette

a) Flow per cassette

b) Energy consumption

0

100

200

300

400

500

600

Flow

per

cas

sett

e (m

3 /d)

ZW-150(1994)

ZW-500a(1997)

ZW-500b(1999)

ZW-500c(2000)

0

100

200

300

400

500

600

Flow

per

cas

sett

e (m

3 /d)

ZW-150(1994)

ZW-500a(1997)

ZW-500b(1999)

ZW-500c(2000)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Ene

rgy

(kW

h/m

3 )

ZW-150(1994)

ZW-500a(1997)

ZW-500b(1999)

ZW-500c(2000)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Ene

rgy

(kW

h/m

3 )

ZW-150(1994)

ZW-500a(1997)

ZW-500b(1999)

ZW-500c(2000)

41

The building block of a ZeeWeed� 1000 filtration system is a parallelepiped membrane element with hollow fibres mounted between two vertical headers. Fine hollow fibres provide high module packing density. Shrouds enclose the fibres, leaving only the bottom and top open to create a vertical flow channel through the fibre bundles.

Cassettes are built by assembling elements in the vertical and horizontal dimensions. In the vertical dimension, a standard stack has three elements, but stacks of one and two elements can be easily assembled to fit in shallow tanks. The cassette illustrated in Figure 2a contains 48 elements in three rows. An element can be inserted into, or removed from the cassette by sliding it like a book into a bookcase. Each stack of elements is connected to a permeate manifold that runs horizontally above the cassette.

When compared to the ZeeWeed� 500 series, the ZeeWeed� 1000 represents a significant

reduction in footprint and energy consumption. Furthermore, it was designed to be easily retrofitted into tanks of various dimensions to upgrade existing conventional treatment plants.

Application Guidelines

The ZeeWeed� 500 and ZeeWeed� 1000 membrane are complementary. The ZeeWeed� 500 membrane is targeted at high solids applications such as:

1. the ZenoGem® process (membrane bioreactor) where the membranes are immersed in

8,000 to 12,000 mg/L of biomass; 2. the enhanced coagulation process where coagulated solids are separated directly by the

membrane; 3. the thickening of conventional water treatment plant residuals; 4. second stage membrane filtration when a very high recovery (>98%) is required.

The ZeeWeed� 1000 membrane is targeted at low solids applications including:

1. filtration of surface water 2. polishing of settled water 3. pre-treatment of sea water for reverse osmosis

Both membranes can be used for tertiary filtration of wastewater. However, the ZeeWeed�

500 is preferred for small plants when the flow is variable and upsets resulting in variable effluent quality are expected. In addition, it is well adapted when biological (e.g. polishing removal of BOD/COD) or chemical (e.g. phosphorous precipitation) complementary treatment is required.

CASE STUDIES FOR WATER REUSE Demonstration Study The Orange County Water District (OCWD) has been investigating MF/UF technology for

treatment of municipal wastewater at its Water Factory 21 plant. A principal objective of the research is to investigate MF/UF suitability to provide direct influent to the reverse osmosis

42

process. Ultimately, the treated water will be percolated through the ground into Orange County’s underground aquifers where it will be used to maintain an underground freshwater barrier against seawater infiltration.

Many membrane technologies have been evaluated at Water Factory 21 by OCWD and

their consultants. As part of their ongoing research a 300 m3/d demonstration unit equipped with ZeeWeed� 500 membranes was evaluated for a period of eight months from April to November 2000. The feed to the demonstration unit was secondary effluent from the full-scale wastewater treatment plant, pre-chlorinated with 4-6 mg/L sodium hypochlorite.

Throughout all phases of testing, analytical results demonstrated a high quality treated

water, well suited for reverse osmosis feed. ZeeWeed® filtration completely removed suspended solids and turbidity and produced a filtrate with an average silt density index (SDI) of 2.0. E. Coli counts were beyond detection for the duration of the demonstration, without any downstream disinfection. Analytical results are presented in Table 1.

Table 1. Average analytical results from demonstration plant and pilot plant

Parameter Units Number of Analyses

Secondary effluent

ZeeWeed® filtrate

Suspended Solids mg/L 23 4.7 < 1

Turbidity NTU 20 0.9 < 0.1

Silt Density Index (SDI) - 12 - 2.0

E. Coli MPN/100 ml 23 1.5 x 105 < 1

Coliphage* PFU/100 ml 9 2.9 x 104 < 6.8

* Coliphage results from pilot plant testing conducted in 1998; all other results from demonstration plant

43

Industrial Case Study - petrochemical Integrated membrane systems like Ultrafiltration (UF) and Reverse Osmosis (RO) provide

useful solutions for controlling or minimizing waste discharge streams as well as reclaiming water for supplementing plant usage requirements.

With this in mind, PEMEX, a Mexican state-owned oil company invited several membrane manufacturers to conduct pilot studies in order to choose the full-scale system.

Four skids mounted pilot plants were tested simultaneously in a refinery site, receiving an identical feed coming out from a biological secondary treatment Analytical results are presented in Table 2.

Table 2. Average analytical results from the pilot test.

COD BOD TSS 105 Colour SDI Oil&gre TPH Feed 76 9.2 21.1 145 Infinite 9.7 3.4 Pilot no1 32 2.9 1.8/<1.0 18/12 1.2/0.8 3.4 <1 Pilot no2 31 3.0 2.0/<1.0 21/13.5 3.0/1.9 2.4 <1 ZENON 31 2.2 1.7/<1.0 17/11 3.0/2.2 1.7 <1 Pilot no4 29 3.3 2.2/<1.0 19/11 1.7/1.3 2.3 <1 Requested <100 <20 0.0 <30 3.0 0.0 0.0

PEMEX finally chose ZENON to provide a tertiary filtration with ZeeWeed® membranes

as a RO pre-treatment. The system is rated for a peak flow capacity of 300 l/s. A full scale plant will be commissioned soon and will save the Pemex refinery about 65% from their fresh water needs. A process flow diagram of Zenon Ultrafiltration as pretreatment for RO presented in Figure 5. Figure 5. Zenon Ultrafiltration as Pretreatment for RO in Phemex Mexico (II)

Pemex Mexico (II)

Air Compressor

...Reject

Permeat

Luft

...Reject

Permeat

Luft

...Reject

Permeat

Luft

...Reject

Permeat

Luft

...

Feed

To ReverseOsmosis

Reject

44

Industrial case study - Power Plant In Poland, a ZeeWeed® 500 filtration system is used to treat the secondary sewage from a local wastewater treatment plant and recycle it as make-up water for the cooling circuit of a power plant (Figure 6). This solution was retained at the most cost-effective and reliable option when compared to treating coalmine drainage or using the local potable water. The plant has a nominal capacity of 5,400 m3/d and was designed as 3 parallel tanks containing 6 cassettes of ZeeWeed® 500 modules each. The secondary effluent is passed through a 1 mm backwashable screen. The plant was commissioned in the Fall 1999. The results from the first year of operation are presented in Table 3. ZeeWeed® filtration completely removed suspended solids and turbidity and produced a filtrate with a silt density index (SDI) of between 0.9-2.4, suitable for reverse osmosis feed. Of particular interest is the excellent removal of BOD, which ranged between 2-32 mg/L in the secondary effluent and was reduced to less than 2 mg/L consistently through removal of suspended solids and complementary biodegradation.

Table 3. Analytical results from Polish plant (year 2000)

Parameter Units Number of analyses

Secondary effluent

ZeeWeed® filtrate

Suspended Solids mg/L 75 6-32 < 1 Turbidity NTU 10 10-50 < 1 Silt Density Index (SDI) - 31 - 0.9-2.4 Biochemical Oxygen Demand (BOD5)

mg/L 70 2-32 < 2

Fecal Coliforms cfu/100 ml 18 3-600 < 4 Total Kjeldahl Nitrogen (TKN) mg/L 15 0.9-2.8 0.3-1.2 Chemical Oxygen Demand (COD)

mg/L 7 35-51 27-34

Iron (total) mg/L 18 0.07-1.1 0.02-0.2 Hydraulic results for the first year of operation are presented in Figure 7. For the first 9 months, the plant was run at a net flux of 34 L/m2/h and shut itself off when no make-up water was needed. Starting in October, the operation strategy was modified so that each train would run 20 hour/day. This resulted in a flux reduction to 27 L/m2/h. The filtration trans-membrane pressure ranged between 15 to 25 kPa, for water temperature between 7 and 20ºC, except for a fouling event through September and October. The normal maintenance cleaning procedure involved backpulsing the membrane for 30 seconds every 30 minutes with break-point chlorinated permeate. Every 2-3 weeks, a backpulse was performed with a warm, 300 mg/L chlorine solution over a 10-minute period.

45

Figure 6. A ZeeWeed® 500 filtration system In Poland

Figure 7. Hydraulic results from Polish plant (year 2000)

The fouling event was due to a seasonal increase in inorganic constituents in the secondary effluents. During this period, an acid backpulse was necessary twice per week to maintain productivity, until water characteristics came back to normal.

0

5

10

15

20

25

30

35

40

45

50

Jan-00 Feb-00 Mar-00 Apr-00 May-00 Jun-00 Jul-00 Aug-00 Sep-00 Oct-00 Nov-00 Dec-00

Tra

nsm

embr

ane

Pres

sure

(kPa

)Fl

ux (L

/m2/

h)T

empe

ratu

re (C

)

TemperatureFluxTMP

Annual power plant shutdown

46

During the first year, operators were present at the plant the equivalent of one hour/shift/day.

CASE STUDIES FOR DESALINATION Pilot Study Both, ZeeWeed� 500 and ZeeWeed� 1000 system have been piloted at more than 20

locations over the past two years. Three pilot studies, one on a brackish water in California and two on seawater in Europe and United Arab, are presented below.

Brackish Water A ZeeWeed® 500 pilot was evaluated over 3 months on a surface water under the influence

of tide, causing wide and rapid variations in salinity, temperature and turbidity. Feed turbidity over the duration of the study averaged 21.7 NTU and ranged between 1.1 and 100 based on an on-line turbidity meter calibrated on a 0-100 scale (Figure 8); laboratory analyses on grab samples showed spikes as high as 140. Other analytical results are presented in Table 4 below. As would be expected from an ultrafiltration membrane, turbidity and suspended solids were not detected in the permeate. The SDI ranged between 1.4 and 2.9 and averaged 2.2.

Table 4. Analytical results for a pilot study on a California brackish water Feed Permeate Units Average Range (#) Average Range (#)

Colour

Co Units 20 (5) 10-50 2 (5) 0-5

Silica

mg/L SiO2 14.8 (5) 12-19 14.1 (5) 9.6-18

Silt Density Index (SDI)

SDI N.A. — 2.2 (7) 1.4-2.9

Suspended solids mg/L 28 <6.0-61 <1.0 — Temperature

ºC 22 (on-line)

12.5-30.5 Same

Total dissolved solids

mg/L 3,700 (7) 1,600-5,000 Same

Total organic carbon (TOC)

mg/L 2.8 (6) 2.0-4.0 2.6 (6) 1.8-4.0

Turbidity

NTU 21.7 (on-line)

1.1-100 <0.1

—

Note: number of analyses in bracket

47

0

5

10

15

20

25

30

35

40

45

50

0 20 40 60 80 100

Elapsed Time (days)

Tra

nsm

embr

ane

Pres

sure

(kPa

)Fl

ux (l

/m2 /h

)

0

20

40

60

80

100

120

140

160

180

200

Feed

Tur

bidi

ty (N

TU

)

TMP Flux Turbidity

Figure 8. Piloting results for the California brackish water study

Hydraulic results are presented in Figure 6, showing net flux and trans-membrane pressure. In the first phase of the study, the flux was increased in steps from 25 to 40 L/m2/h in order to find the maximum sustainable flux; when TMP did not stabilize at 40 L/m2/h, the flux was decreased to 35 L/m2/h to demonstrate stable operation during the second phase. Hydraulic recovery was 96% during the second phase.

Sea Water Pilot #1 Figure 9 shows the early results of a pilot study underway on a seawater in Europe. The

feed is taken from a surface water intake and has a low turbidity ranging between 1 to 5 NTU. Due to the rather low turbidities a ZW-1000 pilot system was chosen here. During the first month of the study, the temperature varied between 6.8 and 10�C. The graph clearly shows the influence of flux on cleaning requirements. During the first phase of the study, the flux was 25 L/m2/h and TMP was perfectly stable at 25 kPa. At a flux of 30 L/m2/h, a slow increase in TMP can be observed, which would lead to a cleaning frequency of 4-6 times per year.

48

0

5

10

15

20

25

30

35

40

45

50

0 5 10 15 20 25 30 35

Elapsed Time (days)

Tra

nsm

embr

ane

Pres

sure

(kPa

)Fl

ux (l

/m2 /h

)

0

2

4

6

8

10

12

14

16

18

20

Feed

Tur

bidi

ty (N

TU

)

TMP Flux Turbidity

Figure 9. Piloting results for the European seawater study Sea Water Pilot #2 Another pilot study with a ZW-1000 membrane is currently underway on a seawater in United Arab Emirates. The results available to date are shown in Figure 10. The pilot was started at a flux of 20 L/m²/h and 90% recovery with raw water turbidities in the range of 1 to 4 NTUs. Because of stable membrane performance, the flux was subsequently increased to 25 and then 30 L/m²/h. Based on the data available, the projected cleaning intervals are in excess of 3 months at 20 and 25 L/m²/h and less than 4 weeks at 30 L/m2/h flux. The lower cleaning intervals at 30 L/m²/h was likely due to the precipitation of calcium carbonate caused by sodium hypochlorite addition in order to disinfect the backpulse tank. Since then, the sodium hypochlorite addition has been stopped and membrane performance has improved. The SDI values have been consistently less than 1.5 throughout this study, which indicates that pre-treatment using ZW-1000 membranes will result in a significant improvement in the performance of downstream RO unit.

49

Figure 10: Piloting Results for UAE Sea Water Study CONCLUSIONS

The ZeeWeed® ultrafiltration membrane has been demonstrated successfully in many

different applications for water reuse and desalination. In water reuse, the ZeeWeed® 500 can be used in an integrated membrane bioreactor or as

a separate tertiary filtration process. As a tertiary filter, it offers the following benefits: 1) tolerance to variable concentrations of suspended solids that may result from upsets of

the upstream biological process; 2) ability to provide complementary biodegradation or chemical treatment, and 3) utrafiltration quality treated water with low SDI when used as pre-treatment to reverse

osmosis. Results from pilot, demonstration and full-scale plants were presented to illustrate design

and operation features of the technology and the water quality obtained in this application. The ZeeWeed� 1000 is an immersed membrane targeted at low suspended solids

applications. It is based on a fine outside-in, very low-pressure ultrafiltration hollow fibre membrane capable of removing all suspended solids and colloids without chemicals. It offers the smallest footprint and lowest energy consumption when compared to competing products. ZeeWeed� 1000 is ideally suited for the construction of new plants of any size and especially for increasing the capacity and water quality of existing plants.

50

Piloting results were presented on brackish water and seawater, demonstrating the ability of the ZeeWeed� 1000 to produce a high quality reverse osmosis feed water under low pressure and stable operating conditions.

REFERENCES Adham, S., Gagliardo, P. and Trussel, R., (2001), “Water reclamation with membrane bioreactors”, IDA World Congress on Desalination and Water Reuse, Manama, Bahrain, October 26-31, 2001. Côté, P., Simon, R. and Mourato, D., (2001), “New developments in immersed membranes”, AWWA Membrane Technology Conference & Exhibition, San Antonio, TX, March 4-7, 2001. Phagoo, D. and Côté, P., (2000), “The use of ZenoGem® for recycling wastewater in commercial buildings”, International Meeting on Technologies for Urban Water Recycling (TUWR), Cranfield University, Bedforshire (UK), January 19, 2000. Thompson, D., Schneider, C., Chomic, C. and Gassett, R., (2001), “Application of a Membrane Bioreactor for Municipal Wastewater Re-Use in the Florida Keys”, Florida Water Resources Conference, Jacksonville, Florida, April 9-11, 2001.