With a resistivity of typically 10 megohm-cm or higher

and a reactive silica level (as SiO2) of generally less

than 20 µg/l, high purity water is defined along with

deionized, purified, apyrogenic and ultrapure water in Table 1.

Either electro-deionization (EDI) or mixed bed ion exchange

(MBIX) is required to produce high purity water. Further

processing to reduce total organic carbon (TOC) levels, together

with effective storage and distribution systems, are required to

process still further and produce ultrapure water of consistently

18 megohm-cm resistivity, the highest theoretical level of

purity possible.

High purity water is primarily utilized in the power industry for

feeding super-critical boilers and combined heat and power (CHP)

systems. The removal of silica is critical to prevent deposition and

mechanical failure in the steam raising systems. High purity water

is also utilized in various industrial applications where cleaning of

components is required.

Depending on the nature of the source water, deionized water

is increasingly being produced by RO before undergoing

additional purification. RO is a crossflow membrane separation

process providing a level of filtration down to ionic levels for the

removal of dissolved salts. Permeate is produced from the

membrane with the majority of the dissolved content of the feed

transferred to the waste concentrate stream. RO membranes

typically reject 95-98 % of the total dissolved solids (TDS) in the

feed supply, thereby reducing the ionic loading onto downstream

ion exchange resin.

EDI and MBIX are the two technology types involved in

producing high purity water. EDI technology is a continuous and

compact process for producing high purity water that eliminates the

requirement for regeneration chemicals and waste neutralization. It

is used to further demineralize deionized water by removing CO2,

remaining TDS and TOC. Systems such as USFilter’s CDI-LX™

units utilize ion-selective membranes, ion exchange resins and

electricity to consistently produce high quality water. When fed

with RO permeate of greater than 0.03 megohm-cm resistivity,

CDI-LX systems typically achieve better than 99% salt removal and

produce up to 18 megohm-cm quality water.

MBIX vessels contain a combination of cation exchange and

anion exchange resins. The cation resin has H+ ions attached

that are readily exchanged for cations such as calcium and

magnesium in the feed supply. The anion resin has OH- ions

attached, to be exchanged for anions such as sulphate and

chloride. The resulting H+ and OH- ions released from the resin

combine to form water. MBIX resins are readily available from

suppliers such as Rohm & Haas Co, Purolite International Ltd

and Lewatit. The treated water produced by MBIX is equivalent

to that generated by EDI. As the H+ and OH- ions are removed

the performance of the MBIX system deteriorates and the mixed

resin becomes ‘exhausted’.

The plant is regenerated utilizing hydrochloric acid or

sulphuric acid, sodium hydroxide, treated water and air.

Backwashing with treated water fluidizes the resin and separates

the heavier cation resin from the lighter anion resin. Diluted acid

is passed upwards through the cation resin and diluted sodium

hydroxide is passed downwards through the anion resin. The

combined flow exits at the cation-anion resin interface via a

collection system and is transferred to the waste neutralization

28 September 2004 ISSN 0015-1882/04 © 2004 Elsevier Ltd. All rights reserved

Advances in high purity waterfiltration technologies

In the following article Anthony Bennett, freelance writer forFiltration+Separation, defines high purity water, reviews technologies

available for its production and describes four case studies from the USAand the UK where various process combinations have been used

to generate high purity water of various grades. He also demonstrates how reverse osmosis (RO) is increasingly being used to deionize raw water

for feeding on to either electro-deionization or mixed bed ion exchangetechnologies for further purification.

featurearticle

Definition Resistivity Maximum Maximum Reactive Total organic megohm-cm microorganisms total dissolved silica µg/l carbon (TOC)at 25 °C cfu/ml* solids (TDS) mg/l (as SiO2) mg/l

Deionized water 0.05 not specified 10 500 not specifiedPurified water 0.2 10 1 100 not specified Apyrogenic water 0.2 1 1 100 not specified High purity water 10 1 0.5 20 not specified Ultrapure water 18 1 0.005 2 0.05

*cfu = colony forming units

Table 1: Water quality classification - adapted from British Water (1986).

system. The acid regenerates the cation resin with H+ ions, while

the sodium hydroxide regenerates the anion resin with OH- ions.

Displaced ions are transferred to the waste stream. The resins

are then re-mixed using low-pressure air.

MBIX can be more economical than EDI where regenerant

chemicals are already available on site, large throughputs are

required and electricity has to be bought in at relatively high cost.

On the other hand, EDI can be more effective where compact

systems are required and electricity is readily available at low cost.

A detailed evaluation of each site is required to determine the most

appropriate technology. The following case studies review four

specific situations where high purity water has been required for a

variety of applications.

Solar system manufacturing processIn 2001, BP Solar, a global provider of integrated photovoltaic

systems, installed a new water treatment system to produce high

purity water at its Frederick Maryland, USA plant. BP Solar

followed USFilter’s recommendation to replace the existing MBIX

system with compact EDI systems as a way to meet its water

quality specification, while eliminating acid and caustic

regeneration and reducing maintenance costs. The specification

for a new water system called for a 46 m3/h loop providing 15

megohm-cm quality water with less than 100 µg/l TOC. The new

system was designed and installed by USFilter (www.usfilter.com).

The system comprises multi-media filters, softeners, carbon

filters and two FlowMAX® RO systems feeding two

17 m3/h CDI-LX EDI systems. The EDI systems are shown in

Figure 1. The high purity water produced supplies a 25 m3

storage tank before being pumped through two UV sterilizers,

four 30 FT3 MBIX polishers, four 0.2 micron cartridge filters

and then out to a polypropylene distribution loop. The loop

returns through a back-pressure regulating valve to the tank.

Started up in May 2001, the system has exceeded BP Solar’s

specification for water quality with 18 megohm-cm resistivity and

less than 10 µg/l TOC. The plant has been

much simpler to operate and maintain than

the pre-existing plant. Maintenance

manager Brian Davidson said “With our

old RO/DI system, we required three to four

people to maintain it. Now we use zero

resources to maintain the system.” He

added, “What I like best about the system,

and the CDI-LX in particular, is that it’s

out of sight, out of mind … If I don’t go

into the room where the system is but once

a week, I don’t feel guilty. With the old

system, I had to check on it two or three

times a day.”

General industrialIn 2001, Anglian Water Services (AWS)

installed a high-capacity water treatment

system to feed industrial water users within

the Greatham and Seal Sands area of

Teesside, UK with deionized water of

greater than 0.05 megohm-cm resistivity.

The system comprises a fully

automated and centralised RO system connected via a

12 kilometre ring-main to supply Hartlepool power station and

Huntsman North Tees. At the Huntsman site the supply is

demineralized using MBIX technology in an additional facility

to produce high purity water. The system is linked with the

nearby AWS RO system installed at Huntsman Tioxide

(Figure 2). Both projects were designed and installed by ACWa

Services Ltd (www.acwa.co.uk).

The centralized design was specified by AWS to provide the most

economical solution for current requirements, but also to allow

extra customers to be connected to the RO permeate ring-main

supply in the future. The RO water source is a chlorinated mains

supply which is relatively high in hardness because of the presence of

calcium carbonate, with levels typically 120 mg/l calcium, 60 mg/l

magnesium and an alkalinity of 400 mg/l as HCO3.

The total combined RO system comprises seven streams of

identical equipment each rated to produce 82 m3/h of deionized

water, providing a total potential output of 574 m3/h. The system

utilizes Koch thin film composite (TFC) high rejection RO

membranes. Avista Vitec anti-scalant is used in the system to

allow recovery at 80% by effectively eliminating calcium

carbonate precipitation in the reject stream.

The MBIX plant at Huntsman North Tees comprises three

mixed beds utilizing Purolite resin. Regeneration is undertaken

automatically via a man-machine interface (MMI) (see Figure 3)

one vessel at a time, initiated by volume throughput primarily,

and using hydrochloric acid and sodium hydroxide regenerant

chemicals.

Combined heat and powerBritish Sugar’s CHP plant at Bury St Edmunds, UK, is supplied

with high purity water of >10 megohm-cm resistivity and reactive

silica (as SiO2 ) of less than 10 µg/l by a water treatment system

comprising pre-treatment, RO and MBIX. The system was

designed and installed by ACWa in 1998.

Filtration+Separation September 2004 29

featurearticle

Figure 1: BP Solar EDI system (image courtesy of USFilter).

The plant accepts 15 m3/h of borehole water, again with high

levels of calcium carbonate, at a TDS of typically 1000 mg/l. The

presence of dissolved iron and manganese in this supply means

that aeration and manganese dioxide filtration are required. The

filtered water is dosed with hydrochloric acid to reduce alkalinity

associated with calcium carbonate hardness. This allows RO

operation at 90% recovery by limiting the calcium carbonate

precipitation potential in the reject.

The RO feed supply is treated with GE Betz Hypersperse

MSI310 antiscalant designed to control silica precipitation

primarily and the double array system of Koch TFC high rejection

membranes produces permeate of 10-20 mg/l TDS, which is

subsequently degassed, eliminating CO2 and increasing resistivity

for feeding onto the MBIX plant.

Two 100% MBIX units utilize Purolite mixed bed resins.

Regeneration is again automatic and similar in sequence to the

Anglian Water system described above.

Whenever the demineralized water tanks are full the RO plant

and degasser shut down. To maintain water quality in the tanks,

the MBIX feed pumps continue to operate, constantly circulating

water round the MBIX process and storage system.

Nuclear powerThe Diablo Canyon power plant (DCPP) in

California, USA, utilizes seawater cooling

water for producing potable water and high

purity water for steam generation.

Ionics Inc (www.ionics.com) installed the

complete water treatment system in 1992,

which it now operates, and the entire

system has operated reliably since

installation.

Seawater is used at the DCPP to cool

down the waste heat generated by the twin

nuclear reactors. The intake pumps draw

millions of litres of water per day. The

seawater RO facility generates a maximum

permeate flow of 102 m3/h and utilizes Dow

Filmtec SW30-8040HR membranes

operated at 45% recovery to produce

desalinated water.

A small proportion of this desalinated

water is processed by another RO system that

produces potable quality water for domestic use on the power plant.

The steam-generation makeup system combines

ultrafiltration, EDI, double-pass RO, vacuum degasification and

polishing MBIX to produce high purity water. Demand from

this system varies from 41 m3/h to 91 m3/h. The vacuum

degasifier reduces the dissolved oxygen to less than 10 µg/l. The

degasifier product is pumped to the MBIX system, which reduces

reactive silica to 5 µg/l as SiO2 and gives 50 µg/l TOC and above

10 megohm-cm resistivity.

When installed in 1992 the MBIX system was considered the

most appropriate. However, future upgrade options will most

likely favour EDI in this power generation application because of

the availability of cheap electricity on site.

ConclusionsThe above case studies demonstrate the effectiveness of RO as

pre-treatment for EDI or MBIX in high purity water systems.

The cost of membrane technology is on a downward trend and

EDI is becoming competitive in comparison to MBIX, so it is

likely that the combination of RO and EDI will become more

popular. But with all projects of this kind, a detailed

investigation of the feed water variability, site constraints and

product water requirements is required before the most suitable

process solution can be implemented.

Reference1. British Water (formerly British Effluent and Water Association). 1986.

Water Quality Classification BEWA: DS.02.86.

30 September 2004 www.filtsep.com

featurearticle

Figure 3: MBIX MMI at Huntsman North Tees(image courtesy of ACWa).

Figure 2: Three RO units installed at Huntsman Tioxide (image courtesy of ACWa).

About the author

Anthony Bennett is managing and technical director of Specialist

Technical Solutions Ltd, which is a technical marketing and

media management services company for process technology

and engineering companies in the water and environmental

sector. He can be contacted on anthony@specialist-technical-

solutions.co.uk or visit www.specialist-technical-solutions.co.uk