featurearticle ,

The ability of reverse osmosis [RO] to produce a continuous and dependable water product has seen its use within the water treatment

industry grow. However: incorrectly designed pre-treatments to this technology can reduce the expected benefits. Simon Gare, Ecolochem

International Inc, UK, explores both physical and chemical pre-treatment methods routinely used with RO systems, and discusses some of the

factors that are important when choosing the right pre-treatment.

RO Systems: the Importance of Pre-treatment

0 ver the last lo-15 years the take-up of RO technology has

grown enormously, displacing many well established

processes to become a familiar part of any water

treatment plant. The increasing numbers of users have been

accompanied by the development of newer and better

membranes and more research into a diverse range of

applications.

To reap the rewards offered by RO systems, however, the

correct design of RO pre-treatment is essential. Membrane

systems, and RO specifically, are less forgiving in terms of

incorrect specification and operation than more traditional

technologies such as ion exchange. This article attempts to

discuss the options for pre-treatment available to the end user of

RO plants.

The aim of a pre-treatment system is to provide feed water of

a quantity and quality that allows the continuous operation of

downstream equipment and protects this equipment from

variability in the feed. To begin designing an RO system an in-

depth knowledge of the feed stream, in terms of dissolved and

suspended solids, organic materials (TOC), potential for

biological activity and variations in temperature is required. It is

good practise to obtain long-term data relating to the feed

stream, so that unusual events that the RO system may face, such

as peaks in suspended solids or TOC, can be identified. Figure 1

highlights the steps involved when designing an RO plant.

The identification of the end-use and the available water

source lead to the specification of the membrane type and

required permeate quantity and quality These factors then

dictate the operational parameters of the RO system. These

Figure 1: The steps involved in designing an RO system.

obviously have a huge impact on the system’s design and overall

performance, but also produce the specification for any pre-

treatment of the feed water.

I?0 Membrane Types

In general RO membranes are manufactured from two

materials - cellulose acetate (CA) and polyamide (PA).

Alterations to these materials’ basic chemistry give rise to a

variety of membrane sub-classifications. Generic membrane

types may include ‘High Rejection’, ‘Low Energy’, ‘Low Fouling’

and ‘Hot Water’ membranes. The choice of one of these

membrane types to meet the particular needs of an end-user,

i.e. high salt rejection or the ability to hot water sanitise, will

require compromise in terms of membrane choice, system

design and operational requirements.

Most large RO systems (greater than 455 L/min) tend to be

designed around the ‘spiralwound’ membrane type (Figure 2).

The dominance of the spiral configuration is due to its compact

nature; i.e. a large membrane area can be packed into a small

volume, creating cost savings in terms of pressure vessels, piping

and pumps. The spiralwound design increases the opportunity to

operate systems at high recovery, while saving energy via low-

pressure drops.

Several other membrane configurations are available

including flat sheet and hollow fibre. The choice of membrane

type and configuration are initially chosen based on the

characteristics of the feed solution and the required permeate

quality

SPECIFICATIONS RO SYSTEM CHARATERISTICS

REQUIREMENTS

PERMEATE QUALITY END USE MEMBRANE TYPE CROSS-FLOW

AVAILABLE WATER CLEANING FREQUENCY RECOVERY MEMBRANE LIFE

22 January/February 2002 www.filtsep.com

x _)__.s_.a I. featurearticle

For any commercially available membrane, manufacturers

detail specific operating requirements for both system design and

feed water quality. Regardless of membrane type the guidelines

provide the maximum allowable suspended solids in the feed

water; generally stated in terms of a Silt Density Index (SDI).

Figure 2: Schematic of a spiralwound membrane type.

The dense packing of spiralwound membranes mean that if

excessive levels of suspended solids are senr to a RO membrane

the membranes are likely to foul. The result is over-pressuring

and eventually membrane damage. Once fouling begins,

cleaning of the membranes becomes difficult. As a general rule,

most membrane systems will not return to their original

performance levels once fouling has occurred [ 11. Guidelines for

SDIs from membrane manufactures [2, 31 state SDI,,s of less

than 5 for operation, but SDI,,s of less than 3.5 for continuous

operation.

Produ water

roduct water flow

through membrane)

Product water

Other feed water contaminants can adversely affect a

membrane’s performance or life span. These factors can be

generalized by looking at the strengths and weaknesses of

particular membrane materials (Table 1).

Membrane (impermeabl barrier in composite membrane modules)

PA membranes are the membrane of choice in today’s water

treatment market. This is because of their high salt rejection and

lower operating pressures when compared to the older CA

membranes. Other advantages are their ability to withstand

extremes of pH and high temperatures. However they do possess

several disadvantages, primarily their susceptibility to attack

by oxidants. While oxidants such as free chlorine can be easily

removed from water sources, and therefore prevent membrane

damage, the PA membrane system becomes vulnerable to

excessive biological growth. Biofouling of RO systems is the

most frequent cause of poor performance. PA membranes also

exhibit an anionic surface charge. This surface charge limits

the use of cationic coagulants and flocculants. If they are

introduced into the feed stream, near permanent fouling of the

RO membranes occurs.

Brine side spacer screen Single-knit

poyester cloth

Salt Rejection

Driving Pressure

Pre-Treatment

Requlremants

pH Limits

Surface Charge

Cleaning Frequency

Polyamide

>99%

150-250 psi

Very strict

Cellulose Actetate

-95%

200-400 PSI

High

Organics Removal

Biogrowth

Oxidants

l-l 3

AnIonIc

Frequent

[weeks to months]

Effective

Problematic

Intolerant .~____._._______._

4-6

Neutral

Infrequent

[months to years]

Good

No Problem

Tolerant

RO System Design

Bearing in mind the operating requirements set out by

membrane manufacturers, a pre-treatment system must be

designed to provide a suitable quality and quantity of feed

water. However the design of the RO membrane system itself

can make the job of pre-treating the feed water challenging.

Key factors in the design of an RO system are: flux (permeate

production per unit area), crossflow velocity (the speed of the

proportion of the feed stream that does not pass through the

membrane) and recovery (the percentage of the feed stream

that is recovered as permeate).

phenomenon is referred to as concentration polarization [4]

(Figure 3). The static water at the membrane surface is separated

from the well-mixed concentrate stream at the boundary layer. It

is because of these flow conditions that the salt concentration

builds up. The only way to reduce the concentration in the gel

layer is for the salt to diffuse away from the membrane surface,

but this occurs at a very slow rate.

Manipulation of these factors governs the rate of particulate

fouling, the risk of biofouling and the scaling potential. A good

RO design will balance the key factors to minimize these

negative results.

How and why these effects come about and how they can be

minimized must be based on an understanding of what is

occurring at the membrane surface. As water is forced through

the RO membrane, the rejected salts and any solids present in the

feed stream become concentrated in the concentrate. At the

membrane surface the flow of water is almost static because of

friction against the membrane. As the flow of water is almost

static, high concentrations of salts and particulate matter can

build up to form a gel layer at the membrane surface. This

The higher the flux and recovery of a system the fewer

membranes are needed to produce the required permeate flow.

However this has the effect of reducing the crossflow velocity.

Crossflow produces mixing within the concentrate stream, and

limits concentration polarization by reducing the thickness of

the boundary layer, i.e. scale-forming ions are prevented from

reaching saturation levels, and because there is sufficient energy

in the concentrate, small suspended solids can be carried through

the process without fouling it.

In the absence of sufficient mixing within the concentrate, the

permeate TDS will also be higher than expected. This is because

the passage of dissolved salts through the membrane is

concentration driven (the water flux is pressure driven), i.e. the

greater the recovery, the higher the salt concentration at the

membrane surface, and therefore the more salt will pass through

the membrane into the permeate.

Filtration+Separation January/February 2002 23

Turbulent

Figure 3: A schematic illustrating the concentration boundary layer

t Feed / Cross Flow

and / ILMernbrane the resultant gradient in TDS across a membrane.

I Back Diffusion

1 Layer 1

Based on the discussion so far, it can be seen that the flux and

recovery chosen at the design stage must be based on accurate

knowledge of the feed water source and the likely quality

following some form of pre-treatment. Choosing too high a flux

will lead to particulate fouling and higher than expected cleaning

frequencies. These in turn will reduce permeate quality, quantity

and result in a reduced membrane life.

Physical Pre-treatment Methods The filtration spectrum (Figure 4) illustrates some of the solid

materials that are present in naturally occurring feed waters,

particularly those of surface origin, and separates them in terms

of their physical size. Filtration and various membrane processes

are shown, the range plotted reflects their pore size, and there-

fore the size of particles they reject.

Almost all water sources require some form of pre-treatment

because of suspended solids, particularly when the water comes

from rivers or lakes. Where water is treated to potable standards,

Figure 4: Filtration spectrum illustrating the solid materials commonly found in feed water.

Salts

Water

SD1 measurements tend to be below 5,

but they are not typically less than 3.5.

For RO, where it is to be used in some

form of process application or effluent

recycle system, solids removal can be

very difficult due to either the nature of

the solids or the mass of solids. There

are many types of pre-treatment

filtration systems and they can be

grouped together based on the size of

solids they are designed to remove.

For large solids such as gravel or

sand common removal processes include

settlement tanks, lagoons or grit

screens. While all are well established

techniques, the first two simply rely on

the weight of the particles being heavier

than that which the water can suspend

by turbulent mixing, and the third acts as a sieve. Typically,

course solids do not cause fouling or plugging within RO

systems because they are easily removed. It is the smaller

particles that require either very long settling times or some form

of assistance, i.e. flocculation and coagulation, to be removed.

Figure 5 shows the time in seconds for particles of various sizes

to fall through 1 m of static water. It is possible to build settling

ponds and lagoons to remove silt sized particles, but the physical

size and civil engineering required for such a process makes it

prohibitive. The settling time of silt has been found to be

approximately 3 hours, while colloidal particles take close to 3

years.

It is small particles such as colloids that contribute to RO

fouling. The term colloid can be used to refer to a variety of

particles in water, ranging from clay particles (< 1000 urn

diameter) to poorly ionized organic acids. It is for this

reason that filtration systems based on settling do not

typically produce water with a suitable SD1 for operating RO

systems.

.ECTRON MICROSCOF

COLI

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VIRUSES I c

SALTS I

MICROSCOPY 1 VISUAL SPECTRUM

.

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RAIN 4 b

; POLLE

SEWING NEEDLE

I

I MICROFILTRATION FILTRATION

ULTRAFll!TRATlON ’

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24 January/February 2002 www.filtsep.com

featurearticle

To improve the removal of small

particles, clarification can be used. This

process is based on settling rates similar

to settling ponds or lagoons, but the

mass and the settling rate of the solids is

increased by the addition of inorganic

or organic coagulants and/or

flocculants.

Problem Source Pre-Treatment

Coagulation can be added to improve

performance, but continuous operation

will see spikes in feed water quality,

which will lead to poorly clarified water

being fed to downstream processes.

Carry-over from clarifiers is also a

problem either due to incorrect dosing

of coagulant or high flow rates through

the equipment. Control can be a major

limitation to using a clarifier to pre-treat a RO system.

Sulphate Scale Feed water /

Acid additton

Metal Hydroxides Feed water / [Fe[ll] /

Coagulant addition (Allll)

Oxidants Free water / Free

Chlorine

Dissolved Organics Feed water / Humic

Acids / Polymers

Bio Films Internal to RO

Suspended Solids Feed water

--.. I” _ _” . -~_I-~~_~

Antiscalents /

IX Softeners

Oxidation of reduced species

IX Softeners

Control of coagulant add&ion

Adsorptlon (GAC]

Addition of a reducing agent

Coagulant and filtration

Control of polymer addition

On-line biocide addition

Off-line sanitization

Coagulatton and filtratton

--“--

results in terms of suspended solids removal and reduction of

colloidal species such as humic acids and colloidal silica. But

all filtration processes will give rise to a waste stream

containing the filtered solids and any coagulant used.

Media filters can generate a wastewater volume of between 3-

5% of their treated water throughput. If this water cannot be

returned to the feed water source (typically the case if a

coagulant has been added), the derived sludge must be

dewatered either by settlement or by some mechanical

process.

A potential solution for the above is media filtration; this

often allows clarifiers to catch floe carry over and mop up very

fine floe or overfeeds of coagulant. Media filters operate by

sieving out particles of greater size than the spaces between the

media grains. The filters can be single media, where the media is

all of the same approximate size and density. Alternatively, two

or more media types can be combined in the same vessel, where

they are of different sizes and densities. In the latter process the

lighter and coarser material sits on top of the denser, finer

material. In this way the filter will act to remove more than one

size of particle. In all media filters the filtered solids act as an

active filtration layer within the bed, further aiding the removal

process. Typically a SDI,, of 3 or less can be obtained and

maintained from these systems.

It is possible to use multimedia filters alone on feed water

sources where the feed concentration of suspended solids,

including coagulant addition, is less than 100 parts per million

(ppm). However, once the loading in the feed water has exceeded

this level, the run length or time between backwashes will be too

short. This gives rise to excessive pressure drops and potentially

poor quality filtered water.

Screens, clarifiers and multimedia filters are all established

processes which, when designed correctly, can give excellent

1 COLLOID 1

The last five years have seen the development of alternatives

to clarifier plus filter pre-treatment systems. These are based on

membranes. The types of materials used are extensive and

include polypropylene, alumina and cloth. The configurations

and modes of operation in which they are used are also

numerous. Membrane based filter systems can offer absolute

guarantees regarding the filtered water quality because they have

fixed pore sizes. However flux rate, crossflow and cleaning

frequency are very important in the design of these systems.

Many of the systems available do foul, and therefore have high

cleaning frequencies or redundancy to cope with this.

Maintaining permeate flow is always a concern. Pilot testing of

these installations is highly recommended before proceeding with

a full-scale design.

Setting time vs particle size Figure 5: Settling rates of various sized solids in water [2].

1 OOOOOOOOr

- g 10000000 E 1000000 - ~ 0

ii 100000

.E 10000 -

E

I-yyy@q

1000 - ‘S P 100 - [Fizyiq

.- t: 63 10 -

l-

0.1 1 0.0001 0.0010 0.0100 0.1000

Particle size in mm

January/February 2002 25

featurearticle

Figure 6: The normal stages of crystallization [6].

Figure 7: The m addition of an threshold inhibitor slows the normal crystallization process [ 61.

PROTONUCLEI

26 January/February 2002

THRESHOLD INHIBITOR

Chemical Pre-treatment Methods

Chemical pre-treatment to RO systems is primarily concerned

with the prevention of scaling by poorly soluble salts and the

minimization or control of biological growth. The source of the

feed water and the membrane type dictate the chemical additions

necessary

Chemical additions should always be kept to a minimum,

where possible, because over addition can increase the feed TDS

to the RO system, and therefore the RO permeate’s TDS. Table 2

lists common foulants within RO systems, their potential sources

and suitable pre-treatments. It should be noted that over addition

of some chemicals within the RO pre-treatment system could

also harm the RO’s performance. Inorganic coagulants based on

iron or aluminium and organic polymers are good examples of

this.

The scaling of RO membranes is closely linked to the system’s

recovery and crossflow. As a general rule, the higher the recovery

of the system, the more likely scaling will occur. Therefore,

sufficient control and monitoring of the RO and its pre-

treatment are vital.

In systems that operate at low recoveries (6.570%,) with little

or no iron present, the addition of acid may be the only chemical

pre-treatment necessary to prevent scaling of the RO system with

calcium carbonate. The feed water’s pH will typically be

adjusted to approximately 6.0, at which point the majority of the

carbonate within the water can be found either as bicarbonate or

carbon dioxide (CO,) - these do not form scales with calcium.

Acid addition is a simple system, using the pH of the water as

feedback to meter the dosage of acid required. However, this

potentially creates two problems, firstly, the CO, created will

freely pass into the RO permeate, which will then require

degassing downstream. Secondly, large doses of acid will increase

the feed TDS to the system. If hydrochloric acid is used, it will

increase the permeate TDS because the added chloride is poorly

rejected by the membrane. Sulphuric acid, on the other hand,

will not affect the permeate TDS as badly because sulphate is

well rejected by RO membranes. However, if the dosage of the

acid is too high, precipitation of calcium sulphate becomes a

risk.

As recovery increases to 7580%, acid addition alone cannot

control all of the species within the feed that are likely to cause

scaling. There are two alternatives: removal of the scale forming

ions from the feed via some form of softening, or use a scale

inhibitor.

Ion exchange softening is an excellent pre-treatment for RO

systems. By removing calcium, magnesium, barium and iron (II),

the formation of carbonate, sulphate and fluoride scales is

prevented. An additional benefit of this treatment is that the

polishing filtration provided by the ion exchange beds further

improves SDIs. This approach to pre-treatment is particularly

suited to systems using PA membranes. As mentioned earlier PA

membranes are stable over a wide range of pH, so no acid

addition is required. Therefore, carbonate species can be rejected

by the RO as NaHCO,, reducing the need for downstream need

to degas CO,.

As with all ion exchange processes, the regenerant water from

the process can be difficult to dispose of becuase of its high salt

content. However, by softening the feed water to the RO the

concentrate can sometimes be used as a secondary make-up

water source for cooling systems. This improves the chemical

efficiency of the RO system and allows for greater cycling within

the cooling systems, and overall reducing fresh water demands.

The alternative to removing the divalent cations from the feed

water is to use an inhibitor. The majority of inhibitors or

antiscalents used for water treatment today can be classified as

‘threshold inhibitors’. Figure 6a shows normal scale formation,

while Figure 7 shows the method of operation of threshold

www.filtsep.com

featurearticle

inhibitors. Threshold inhibitors are extremely good at preventing

carbonate and sulphate scales. It is possible to operate at high

recoveries without significantly altering the feed water

composition because the dosage of antiscalent used is extremely

small. The need to degas CO, from the permeate is reduced

because the antiscalents do not lower the pH, and therefore

evolve CO, from the alkalinity present. It is important to realise

this when designing RO plants, as these materials do not prevent

scaling from occurring, but rather delay the formation of large

crystals that form scales. However the effectiveness of these

chemicals is relatively short lived - approximately 30 minutes.

This effects the operation of the RO system. While 30 minutes

should be more than the hydraulic retention time of the feed

water within the system, when it shuts down the concentrate can

begin to scale the membranes. RO systems that use this form of

scale inhibition should be designed to go through a flush

sequence when shutdown.

Biological fouling of an RO membrane system is the most

common cause of a reduction in permeate flow and quality [7].

This is because most water sources that RO systems operate on

are not sterile. Solids removal techniques such as clarification and

filtration (media or membrane based) can remove bacteria and

higher organisms from the feed stream by particle exclusion.

However, this can be improved by the addition of a biocide to the

feed stream to kill off any organisms present. At this point

membrane choice is again a critical factor. For surface water

sources with a high potential for biological fouling the continuous

addition of an oxidising biocide may be beneficial. If a PA

membrane has been chosen for this duty any residual oxidising

biocide must be removed to prevent severe membrane damage. CA

membranes have fewer problems with biological fouling due to

their tolerance of oxidants, but this must be measured against a

lower salt rejection and pH range requirements.

Where a PA membrane has been chosen the removal of

oxidants can be done by either the addition of a reducing agent

such as sodium bisulphite or adsorption of the free oxidant onto

granulated activated carbon (GAC), both options have pros and

cons. The addition of a reducing agent relies on the correct dose

being injected into the feed stream and adequate mixing and

contact time for the reaction to occur. Over-addition can occur

because the residual oxidant concentration may vary. The system

is also vulnerable to mechanical failure of the dosing pumps. To

avoid this GAC can be used as both a polishing filter and

absorber for oxidants.

GAC is well suited to pre-treating RO systems fed with low

suspended solids and biological activity, or waters which have

already been treated for solids removal such as town’s water

sources. Town’s water is typically supplied with a residual of a

free oxidant; usually 0.05 ppm chlorine in the UK. SDJSs

measured on raw town’s water will typically reach 4-5, with

polishing filtration further increasing the water quality to 3.5.

Once the oxidants have been removed from the feed water to

a PA system there is generally no other form of continuous

biological control. As a result any organism that passes into the

system is likely to be trapped within it. Microorganisms are

capable of colonising almost every material or surface known

to man [3], and therefore within RO systems biological growth

must be controlled. Out of control biological growth is

typically referred to as biofouling because of the presence of

thick films of cells and their secretions. It is identified by

Filtration+Separation January/February 2002 27

increased pressure drops, reduced permeate flow and decreasing

salt rejection.

Where an oxidizing biocide cannot be added, non-oxidizing

biocides may be used. These are not added while the system is

on-line due to the slow action of these materials; usually the RO

will be placed in a rinse mode for this procedure. Examples of

non-oxidising biocides are gluteraldehyde and isothiazolone. To

control biological growth over extended periods it is important

to vary the biocide used, this prevents the build up of colonies

that are tolerant of the material used.

Conclusions

The aim of this article was to explore approaches to RO pre-

treatment systems and some of the factors upon which they

depend upon, such as water source, membrane type, downtime

and environmental impacts. This paper has also detailed

potential causes of reduced RO systems performance and how in

some circumstances it may be the pre-treatment system that

introduces the foulant.

The article does not contain an exhaustive list of available pre-

treatment processes or detail their suitability for specific applic-

ations, but it does attempt to give some guidelines on the limits of

some processes. Two facts are certain, RO systems are still fouling

and the business of RO cleaning chemicals is a growth area.

To improve RO system design good quality feed water data

over an extended period should always be collected and attention

paid to membrane manufactures guidelines. This will produce

more reliable systems and continue the rise of RO and other

membrane technologies into the 21st century. l

References

1.

2.

3.

4.

5.

6.

7.

Nancy Mulhern. 1995. Death, Taxes and RO Membrane Fouling,

Water Technology, Osmonics Inc, USA (11101195).

Koch Membranes Catalogue. 2000. Koch Membrane Systems, UK.

Osmonics Membrane Catalogue. 2000. Osmonics Inc, USA.

Course notes from Membrane Processes. 2000. A short course given

by the School of Water Sciences, Cranfield University, UK.

Betz Handbook of h&trial Water Conditioning. 1991.9th Edition,

USA.

l’ermacare international presentation to Ecolochem

International. October 2000. Houston, Texas, USA.

H C Flemming. 2000. Membranes and Micro Organisms,

Proceedings of Membrane Technology in Water and

Wastewater Treatment, Royal Society of Chemistry, UK.

Contact: Simon Gare, BSc,

Ecolochem International Inc, Hydrohouse,

Peterborough, PE2 6SE, UK.

Tel: +44 1733 394566; Fax: +44 1733 390179:

E-mail: sales@ecolochem.comi Wabsite: www.ecolochem.com