BWRO Silica 65 Ppm Case Study Results

7/30/2019 BWRO Silica 65 Ppm Case Study Results

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Operational Experience With BWRO Membrane Fouling With High Silica

Groundwater

N.W. Swain*#

Arup Water,

Level 2, Optus Centre, 431-439 King William Street, Adelaide, SA 5000, Australia

Tel. +61 (8) 8212-5580; Fax +61 (8) 8212-5590; email: [email protected]

Abstract

A brackish water reverse osmosis plant (BWRO) treating groundwater with silica levels up to 65 mg/L was

installed in October 2000 to supply 1,300 m per day of potable water to the township of Yulara in the Northern

Territory of Australia. This paper presents data from 827 days of operation for this high silica BWRO

application, demonstrating that designers of such applications should allow contingencies for reduced recovery,

increased cleaning and reduced membrane life. The BWRO was operated at an average recovery of 73% using

a silica specific antiscalant compared to the design target of 76.8%. At this recovery fouling of the membranes

was experienced but was managed by cleaning with an ammonium bifluoride solution every 87 days. Cleaning

was not completely effective in removing the fouling and the membranes had to be replaced after 2 years,

compared to the target 3 years. The reduced recovery, specialised cleaning and reduced membrane life due to

the silica scaling was estimated to add 4.6 per m3

to the cost of producing the water, around 4% of the full cost

of treatment.

1. Introduction

In October 2000, a brackish water reverse osmosis plant (BWRO) was installed to supply

1,300 m per day of potable water to the township of Yulara in the Northern Territory of

Australia. Yulara is the gateway to one of Australias premier tourist attractions, the Uluru

and Kata Tjuta National Park and serves more than 350,000 international and local visitors

each year. Water infrastructure at Yulara is owned and operated by Power and Water

Corporation, a Northern Territory government corporation that engaged Arup Water to

undertake concept design, tender documentation preparation and project management for the

provision of the BWRO. The plant was supplied by QED Occtech under a design and

construct contract and replaced an electro dialysis reversal (EDR) plant that had been inoperation since 1983. BWRO was selected in preference to EDR during the tender stage due

to lower overall costs.

The feed water for the plant is sourced from the Dune Plains aquifer and contains up to

2,200 mg/L of total dissolved solids (TDS) and 65 mg/L of silica. The aquifer is recharged on

an irregular and unpredictable basis. The unreliable nature of the recharge necessitates strict

groundwater resource conservation in this arid environment. To this end it was vital to

maintain as high a recovery as possible from the BWRO but it was understood from the

outset of the project that this goal would be limited by the potential for silica scaling.

This paper presents data from 2 years of operation for this high silica BWRO application

demonstrating that designers of such applications should allow contingencies for reducedrecovery, increased cleaning and reduced membrane life.


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2. Water Treatment Plant Description

Raw water to the Yulara Water Treatment Plant is a blend of five individual bores and has atypical analysis as presented in Table 1. Upstream of a reverse osmosis stage (RO), the water

LVSXPSHGWKURXJKD PVFUHHQD89GLVLQIHFWLRQXQLWDQGILQDOO\D PFDUWULGJHILOWHUThe water is dosed continuously with a silica specific antiscalant and fortnightly with a non-

oxidising biocide. The RO was configured with Hydranautics 8 ESPA1 elements in twostages with seven pressure vessels and 42 elements in the first stage and three pressure

vessels and 18 elements in the second stage. The design capacity was 1,300 m3

per day at a

recovery of 76.8% giving a flux of 24.2 L/m2/h. Following RO, the water is blended with a

side stream of raw water, passed through a degassing tower and finally dosed with caustic

soda and chlorine to achieve the desired water quality.

Table 1. Typical Blended Bore Water Quality (all mg/L unless stated or pH)

Parameter Value Parameter Value Parameter Value

pH 7.2 Na 339 Al


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On the basis of successful reference sites for the antiscalant proposed and the fact that aside

from hardness there appeared to be no obvious additional risk factors, the tendered design

was accepted.

4. Operating Data

Operating data for the 827-day period from start up on October 31, 2001 to February 5, 2003

was examined. The plant operates according to the levels in the treated water storage tanks

and on average operated for 18.6 hours per day, meeting an average demand of 900 m per

day. Table 2 presents a comparison between the predictions at the design stage with the

average values for this period.

Table 2 Comparison of Design Predictions with Actual Operation

Parameter Design Actual

Permeate Production Rate 1,300 m per day 1,160 m per day

Recovery 76.8% 73%

Flux 24.4 L/m/h 21.7 L/m/h

Feed Pressure-New Membranes 10.0 bar 10.6 bar

Feed Pressure-Old Membranes 11.1 bar

(@ 3 yrs)

12.0 bar

(@ 2.5 yrs)

Table 2 shows that the design targets have not been met. This is due to fouling being more of

a problem than anticipated. A recovery of around 73% has been adopted and while this

appears to be achievable, it has not been rigorously optimised.

Figure 1 and Figure 2 present trends of membrane permeability over the period examined

showing a gradual reduction in the overall membrane permeability from a starting point of

40 L/h/m2/kPa to 26.6 L/h/m

2/kPa at the end of the period examined, a reduction of 34%.

Both figures reveal that the silica fouling was managed via regular cleaning, however it can

be seen that cleaning would not fully restore permeability. At many sites a reduction inpermeability of 34% would be considered unacceptable but it was tolerated as there was

sufficient spare capacity available.

From Figure 1 and Figure 2 it should be noted that:

A faulty flowmeter causing a high (but unknown) recovery resulted in catastrophic

fouling on plant start up.

An apparent increase in permeability of stage 1 in May 2002 was due to failure of

seals and leakage of raw water into the permeate tubes.

The permeability of Stage 1 has also decreased although not to the same extent as

Stage 2. The reason for this has not been investigated.

Figure 3 presents the trend of normalised permeate TDS over time. Many of the increases in

TDS including the excursions during the May 2001 to March 2003 were caused by

problems with leaking permeate tube seals. However the general trend shows a gradual

increase in permeate TDS consistent with fouling. Cleaning had the expected effect of

reducing the TDS of the permeate.


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5. Evaluation of Silica Scaling Management Strategy

Silica scaling has been primarily managed by the use of a silica specific antiscalant and

regular cleaning. In general, the best silica antiscalant is selected by test work or field

trials [5] and in this case two proprietary silica specific antiscalant chemicals have been used.

Permatreat 510 supplied by PermaCare was used from plant start up until October 2002 after

which it was changed to MT1300i supplied by QED Occtech in an effort to see if the rate offouling could be reduced. It is unclear from the data presented in Figure 1 and Figure 2

whether this change has had any significant impact. However the plant operators are

continuing to use the MT1300i.

Periodic cleaning of the membranes is undertaken using an ammonium bifluoride based

chemical solution and cleaning procedure developed by the plant supplier QED Occtech. A

key element of the procedure is to circulate the cleaning solution for no longer than 3 hours

otherwise a reduction in membrane permeability may be experienced for two to three

days afterwards [5]. The supplier claims that this cleaning removes the order of 97% of the

silica. Although the removal rate cannot be practically quantified, the fact that membrane

permeability could not be restored suggests that cleaning was indeed less than 100%. The

membranes were cleaned on 10 occasions during the period examined, i.e., an average ofevery 83 days. The cleaning operation was effective in restoring membrane permeability in

the short term as illustrated in Figure 2. However over the longer term the membrane

permeability has deteriorated at relatively high rates, most likely due to a combination of

incomplete removal of scale and damage to the membranes by the cleaning operation. The

membranes were replaced on 20 May, 2003 as the plant could not meet an increased demand.

The resultant membrane life was around 2 years compared to the 3 years that was originally

expected. It is planned to conduct autopsies on the membranes removed to provide further

insights on the fouling problems.

A key question arising from the evaluation of the silica management strategy is what is the

impact on the cost of operation of not meeting the original design expectations? The author

has estimated that the cost of these items amounted to a total of 4.6 /m3, made up of

contributions from reduced recovery (1.2 ), increased cleaning (1.3 ), higher operating

pressures (0.5 ) and reduced membrane life (1.6 ). The additional cost is in the order of 4%

of the full cost of treating the water.

6. Conclusions

A review of the operation of the Yulara BWRO with 65 mg/L of silica in the feed feedwater

has identified that:

An achievable recovery was 73% using a silica specific antiscalant and this corresponds

to a nominal silica level in the brine of 240 mg/L.

Cleaning of the membranes was required every 80 days with an ammonium bifluoride

solution.

A membrane life of 2 years was achieved.

The additional costs due to the silica fouling were in the order of 4.6 per m3, around 4%

of the full cost of treatment.


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Acknowledgements

The author would like to thank Power and Water Corporation for giving permission to

publish this paper and to Neil Poulton, Andrew Mills and Mark Skinner of Power and Water

for their assistance in preparing the paper.

References

[1] Koo T, Lee Y.J., Sheikholeslami R, Silica Fouling and Cleaning of Reverse Osmosis

Membranes, Desalination 139(2001), 43-56.

[2] Darton E.G., RO Plant Experience with High Silica Waters in the Canary Islands, EDS

Conference, Desalination and the Environment, Las Palmas, November 1999.

[3] Sheikholeslami R, Tan S, Effects of Water Quality on Silica Fouling of Desalination

Plants, Desalination 126(1999), 267-280.

[4] Sahachaiyunta P, Koo T, Sheikholeslami R, Effect of Several Inorganic Species on Silica

Fouling in RO Membranes, Desalination 144(2002), 373-378.

[5] Wende N, QED Occtech, personal communication, 27/7/03.

Figure 1 Trend In Overall Membrane Permeability

15

20

25

30

35

40

45

10/11/2000 1/19/2001 4/29/2001 8/7/2001 11/15/2001 2/23/2002 6/3/2002 9/11/2002 12/20/2002 3/30/2003

Permea

bility

(x10^3L/h/m

2/kPa

)


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Figure 2 Trend In Stage 1 and Stage 2 Membrane Permeability

(arrows show cleaning events)

Figure 3 Trend In Normalised Permeate Total Dissolved Solids

0.00

10.00

20.00

30.00

40.00

50.00

60.00

10/11/2000 1/19/2001 4/29/2001 8/7/2001 11/15/2001 2/23/2002 6/3/2002 9/11/2002 12/20/2002 3/30/2003

Permea

bility

(x10^3L/h/m2/k

Pa

)

Stage 1

Stage 2

0

50

100

150

200

250

300

350

10/21/2000 1/29/2001 5/9/2001 8/17/2001 11/25/2001 3/5/2002 6/13/2002 9/21/2002 12/30/2002 4/9/2003

Date

NormalisedTDS(mg/L)

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BWRO Silica 65 Ppm Case Study Results