RO/NFUFMBRChemicals

C A S E S T U DY
RO | UF | MBR |Drinking Water
North Dakota, USA

Design Expectations Exceeded with Toray Ultrafiltration Membranes atOliver-Mercer-North Dunn
SYSTEM DESIGNThe Oliver-Mercer-North Dunn (OMND) Drinking
Water Treatment Plant (WTP) operates as partof the Southwest Pipeline Project (SWPP) todeliver a continuous supply of drinking water tocommunities in southwest North Dakota.
OMND uses a two-stage ultrafiltration (UF) systemwith Toray's hollow-fiber UF membrane modules totreat Lake Sakakawea's surface water. The second
stage treats backwash water from the first stage,and the product water from the second stage isreturned to the head of the plant. Table 1 lists theraw water quality of the UF feed.
The UF filtrate is fed to a reverse osmosis (RO)system using Toray's low-pressure membraneelements to reduce hardness, Total Dissolved Solids(TDS), and sulfate to the City's potable standards.
The UF/RO membrane treatment system's final
product water is blended with 60% RO permeate and 40% UF filtrate.

Figure 1: UF system by Wigen Water Technologies (www.wigen.com)

EXPANSION
In 2014, OMND expanded its UF capacity from 4.0 to 7.5 MGD byadding two and one skids to its primary and secondary stages,respectively. The RO capacity was increased from 2.0 to 3.0 MGD
by adding one skid. The total finished water capacity of the UF/
RO membrane plant would become 5.2 MGD. Table 2 outlines
additional information regarding the expansion.
As shown in Figure 2, fourteen months of Toray's UF demonstrated
stable operation with minimal fouling of the UF membranes and atrouble-free operation of the RO system. Toray's proprietary thermallyinduced phase separation (TIPS) spinning method of the hollow-fibers
has produced one of the most durable PVDF (polyvinylidene fluoride)
membranes in the industry, with high chemical tolerance and robust
fiber integrity.

DataParameter

Unit

Total Dissolved Solids (TDS)

mg/L380510Turbidity

NTU

< 1.0

31.0

Total Organic Carbon (TOC)

mg/L

2.0

4.5

Alkalinity (as CaCO3)

mg/L

150

180

Total Iron (Fe)

mg/L

0.05

0.15

Total Manganese (Mn)

mg/L

0.01

0.02

Hardness (as CaCO3)

mg/L

170

260

7.5

8.5°F / °C35 / 268 / 20pHTemperature

Min.

Max.

Table 2 — After Phase 2 Expansion

PROJECT HIGHLIGHT
The UF modules were packaged in a preservative solution toprevent the fibers from drying out but became subject to freezing
as the temperatures dropped under sub-zero Fahrenheit (-20°C) while in transit. If frozen, the fibers could become brittle andsnap with any jarring motion. Remarkably, out of the 150 modules
that shipped, only two modules suffered damage, with one module
restored within the manufacturing Quality Control Release Value(QCRV ). This case demonstrates the mechanical strength andintegrity of Toray's durable PVDF UF membrane fibers.

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Table 1 — 1st stage UF feed water quality

1 of 2UFsystem

1st stagend2 stage

RO system
Productrecovery

Primary UF: 5 skids / 52 modules
Secondary UF: 2 skids / 16 modules
3 skids / 1 MGD per skid (17:9 array, 7M)

UF>99% (95% primary, 90% secondary)

RO

80%

LRV calculation

Based on conservative Darcy pipe flow modelPre-treatment

200 µm screenDesign Expectations Exceeded with Toray Ultrafiltration Membranes atOliver-Mercer-North Dunn

C A S E S T U DY
RO | UF | MBR |Figure 2: Instantaneous Trans-membrane Pressure (TMP)

Figure 3 — RO skid using Toray RO membrane elements

MEMBRANE PERFORMANCERESULTSFirst Stage Operation• The successful operation of phase 1 led to the expansion ofphase 2, and design expectations were exceeded.

• The transmembrane pressure (TMP) did not exceed 10 psi,
one-third of the maximum TMP (29 psi), as shown in Figure 2.
• Clean-in-place (CIP) performed after one year of operation foroperator training purposes only.
• After observing the positive performance of the membranes interms of fouling and permeability, the backwash flux rate wasdecreased from 1.5x to 1.1x filtrate flow.
• The first stage instantaneous flux could be increased 20% to
a new temperature corrected flux of 58 gfd at 20°C, granting
plant operators the flexibility of adjusting the softened bypass
water blend ratio to 50-50 to increase the plant capacity, ifdesired.
Second Stage Operation• With feed turbidities 20 times that of the first stage, the second
stage was cleaned only once within fourteen months, indicating
excellent process resiliency and ability to handle upsets.
• The train was cleaned after the TMP reached approximately
10 psi demonstrating the membranes' ability to handle a highconcentration of solids while maintaining a low fouling rate.
• The TMP rose above 10 psi after ten months of operation. CIP
was performed after one year of operation, which reduced theTMP from 13 to less than 1 psi.

• The first stage UF system flux rate can be increased by 20% if
desired by the end-user.
• The second stage treated feed turbidities of 55 NTU with peaksof up to 620 NTU, where the operation was stable for over one
year with TMP values below the operating limit.
• Daily Log Removal Values (LRV) at 4-log using very conservativeparameters.
Five years after the expansion:• No pinning of fibers was required on any of the UF modules.
• LRV values continue to be stable at greater than 4-log on theprimary UF system.
• CIP performed only once per year on the primary UF systemand only 2–3 times per year on the secondary UF system.
REFERENCE
Guibert, Susan. "Operations and LRV Calculations at North
Dakota's Southwest Pipeline Project Oliver-Mercer-North Dunn(OMND) Drinking Water Treatment Facility." 2014 Membrane
Technology Conference & Exposition. Las Vegas, NV. March 2014.

TORAY INDUSTRIES, INC.
Head Office: Nihonbashi Mitsui Tower 24th Floor, 1-1, Nihonbashi-Muromachi 2 chome, Chuo-ku,Tokyo, 103-8666, JAPANwww.water.toray+81 3 3245-4540

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All information presented herein is believed reliable and in accordance with accepted engineering practices. Toray makes nowarranties as to the completeness of this information. Users are responsible for evaluating individual product suitability forspecific applications. Toray assumes no liability whatsoever for any special, indirect or consequential damages arising from thesale, resale or misuse of its products. © 2021 Toray Industries, Inc. Subject to change without notice.
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