Better RO results lie in design
Simon Gare of Ecolochem International examines potential causes of reduced performance from RO systems, methods of pre-treatment and factors upon which they are dependant
The aim of a reverse osmosis (RO) pre-treatment system is to condition the
feed water for further processing, protect downstream equipment and minimise
maintenance. One advantage of RO systems is that they reduce fluctuations in
feed water quality, producing continuous and dependable product water. However,
incorrectly designed pre-treatment can reduce the expected benefits of the technology.
This normally results in lower than expected RO permeate flow or quality and
In the last 10 to 15 years the take-up of RO technology has grown enormously,
displacing more established processes to become a common and almost familiar
part of any water treatment plant. Increasing numbers of users have proceeded
hand-in-hand with the development of newer and better membranes, along with
research into ever more diverse applications. The advantages for this are:
- consistent product quality,
- high availability,
- water savings,
- chemical savings,
- manpower reduction,
- ease of operation.
However, to reap the rewards offered by RO systems, correct design of pre-treatment
is essential. Membrane systems, and RO specifically, are less forgiving in terms
of incorrect specification and operation than more established technology such
as ion exchange.
The aim of a pre-treatment system is to provide feed water at a quantity and
quality that allows continuous operation of downstream equipment and protects
that equipment from variability in the feed. Therefore, before designing an
RO system an in-depth knowledge of the feed stream in terms of dissolved and
suspended solids, total organic materials (TOC), potential for biological activity
and variations in temperature is required. It is good practise to then obtain
long-term data relating to the feed stream, to pick out unusual events that
the RO system may face, such as peaks in suspended solids or TOC. Figure 1 shows
the steps involved when designing an RO plant.
Identification of the end use and an 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 obviously have
a huge impact on the system design and overall performance, but also produce
the specification for any pre-treatment of the feed water that may be required
to operate the RO. For make-up water treatment systems this may be simple. For
process applications questions of compatibility and performance in non-aqueous
solutions must also be considered.
RO membranes are manufactured from basically two materials, cellulose acetate
(CA) and polyamide (PA). Alterations to their basic chemistries then give rise
to a variety of sub classifications of membrane. Generic membrane types would
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,
be that 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 100gpm) are designed around the familiar
spiral wound membrane type. The dominance of the spiral configuration is due
to its compact nature, because a large membrane area can be packed into a small
volume creating cost savings in terms of pressure vessels, piping and pumps.
This design increases the potential for operating systems at high recovery while
saving energy via low-pressure drops. The industry standard 8″ diameter,
element can pack a vast membrane area into small volume, typically 350-440ft²
of active membrane into a volume of ~1.2ft³. Other configurations are available
including flat sheet and hollow fibre.
The choice of membrane type and configuration are initially chosen based on
characteristics of the feed solution and required permeate quality. By choosing
a particular membrane type, particular feed water characteristics are demanded
for consistent operation. With any commercially available membrane, the manufacturers
detail very specific operating requirements both for system design and feed
water quality. Regardless of membrane type – CA, PA or any of the sub categories
– the guidelines detail the maximum allowable suspended solids in the feed water,
generally stated in terms of a Silt Density Index (SDI). The dense packing of
all spiral wound membranes means that should large or excessive levels of suspended
solids be sent to an RO membrane they are likely to foul. The result is over
pressuring and eventually membrane damage. Once fouling begins, cleaning of
the membranes becomes very difficult. As a general rule, most membrane systems
will not return to their original performance levels once fouling has occured4.
Guidelines for SDIs from membrane manufactures5,6 state
SDI15s of less than 5 for operation but SDI15s of less
than 3.5 for continuous operation. The relevance of these statements will be
dealt with in a later section.
Other feed water contaminants may adversely affect a membrane’s performance
or life-span, these factors can be generalised by looking at the strengths and
weaknesses of particular membrane materials (See Table 1).
PA membranes are generally the membrane of choice in today’s water treatment
market. This is due to its high salt rejection and lower operating pressures
when compared to the older CA membranes. Other advantages are their ability
to withstand the extremes of pH and their higher temperature stability. However
they do have 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, PA membrane systems then become vulnerable
to excessive biological growth. Bio fouling 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 on cationic coagulants and flocculants. If they are introduced into
the feed stream, near permanent fouling of the RO membranes will occur.
RO system design
Bearing in mind the operating requirements set out by the membrane manufacturer,
a pre-treatment system must be designed to provide a suitable quality and quantity
of feed water. However, the design of the membrane system itself can make the
job of pre-treating the feed water potentially more challenging. Key factors
in RO system designs are:
- flux – permeate production per unit area,
- cross-flow velocity – speed of the proportion of the feed stream that does
not pass through the membrane,
- recovery – percentage of the feed stream that is recovered as permeate.
Manipulation of these factors governs the rate of particulate fouling, risk
of bio fouling and scaling potential. Therefore, good RO design will balance
the key factors to minimise these negative results.
How and why these effects come about and how they can be minimised 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 are concentrated in the concentrate and left behind. At the
membrane surface the flow of water is almost static due to friction against
the membrane. As the flow of water is almost static high concentrations of salts
and particulate matter can build up, this forms a gel layer. This phenomenon
is referred to as concentration polarization¹ (See Figure 2). The static water
near the membrane surface is separated from the well-mixed concentrate stream
at the boundary layer. It is due to these flow conditions that the salt concentrations
can build up, as the only way to reduce the concentration in the gel layer is
to diffuse away from the membrane surface, which is very slow.
The higher the flux and recovery of the system, fewer membranes are needed
to produce the required permeate flow and therefore, smaller feed pumps. However
this has the effect of reducing the cross-flow velocity. It is cross-flow that
provides mixing within the concentrate stream and reduces concentration polarization
by reducing the thickness of the boundary layer, i.e. scale-forming ions can
be kept from reaching saturation levels and because there is enough energy in
the concentrate, small suspended solids can be carried through the process without
Without sufficient mixing within the concentrate, the permeate total disolved
solids (TDS) would 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, more salt passes through the membrane
into the permeate.
|Characteristics of two membrane materials|
Frequent (weeks to months)
Infrequent (months to years)
It is clear then, that 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 on poorly treated surface water and higher than expected
cleaning frequencies. This will reduce permeate quality, quantity and result
in reduced membrane life. Guidelines for flux rates on various feed water sources
are given in Table 2.
All major RO membrane manufacturers provide computer projection packages that
can assist in the design of membrane systems and identify limits of a potential
design. However these programs do not always plainly identify the feed water
requirements that may be demanded by a potential design. The model may well
show no errors but in practise the system may foul or scale very quickly if
the user does not understand the effects of high recovery, low cross-flow and
|Suggested minimum and maximum flux rates for different
The filtration spectrum (See Figure 3) 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’s processes are shown. The range plotted reflects their pore size and
therefore, the size of particles they will reject as they act as a sieve.
Almost all water sources require some form of pre-treatment because of suspended
solids, particularly water from rivers and lakes. Even where water is treated
to potable standards, SDI measurements may be below 5 but will not typically
be below 3.5. Where RO is to be used in some form of process application or
effluent recycling system, solids removal can be very difficult, due either
to the nature of the solids and/or the mass of solids. There are many types
of filtration systems but these can be grouped together based on the size of
solids they are normally designed to remove (See Table 3).
For large solids such as gravel, sand and other large pieces of detritus, common
removal processes would 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 the water. The third acts as a sieve. Course
solids do not usually cause fouling or plugging within RO systems because they
are easily removed. It is smaller particles that require either very long settling
times or some form of assistance to be removed, i.e. flocculation and coagulation.
Figure 4 shows the time in seconds for particles of various sizes to fall through
1m 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 would be prohibitive.
From Figure 4 the settling time of silt is ~3 hours, for colloidal particles
~3 years. 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 addition of
inorganic or organic coagulant and/or flocculant. By their addition, smaller
particles floc, or clump, together.
|Particle size removal verses potential unit process|
Settling ponds and lagoons
It is small particles, such as colloids that contribute to RO fouling. The
term colloid can be used to refer to many different particles in water, from
clay particles (<1,000µm diameter) to poorly ionised organic acids. It is for this reason that filtration systems based on settling, do not typically provide water with a suitable SDI for operating RO.
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 down-stream 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 on using a clarifier to pre-treat RO. A potential
solution for the above is media filtration. This often allows clarifiers to
catch floc carry-over and mop-up very fine floc or overfeeds of coagulant.
Media filters operate by sieving out particles of greater size than the spaces
between the media grains. The filters can therefore be single media, where the
media is all approximately the same size and density, or, two or more media
types, of different size and density, can be combined in the same vessel. In
this 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, good SDI15
of 3 or less can be obtained and maintained from these systems.
|A list of contaminants that should be quantified
to correctly design an RO pre-treatment system
Feed water source
Location of sample point
Water ion analysis (mg/l as ion or CaCO3)
Metals: eg. Fe+++, AI+++, Mn++, Zn++, Cu++, Sn++
Other ions: ef. NH+4, Br¨
Gases: (C12, H2S, O2)
Suspended or colloidal matter (Silt Density Index (SDI))
Organic substances (TOC, BOD, COD)
Biological activity (TBC)
It is possible to use multimedia filters alone on many feed water sources where
the feed concentration of suspended solids, including coagulant addition, is
less than 100ppm. Once the loading in the feed water has exceeded this level
the run length, or time between backwashes, will be too short. This would give
rise to excessive pressure drops and potentially poor quality filtered water.
Screens, clarifiers and multimedia filters are established processes which,
when designed correctly, can give excellent results in terms of suspended solids
removal and reduction of colloidal species such as humic acids and colloidal
silica. Clarifiers are effectively large tanks and for industrial flow rates
of over a 1,000gpm become intensive in terms of space and civil engineering
costs. Also, all filtration processes will give rise to a waste stream containing
the filtered solids and any coagulant that was used. Media filters may generate
3-5% wastewater compared to the treated water throughput. If this water cannot
be returned to the feed water source (typically not if a coagulant has been
added), the derived sludge must be dewatered, either by settlement or some mechanical
The last five years have seen the development of alternatives to clarifier
and filter systems, these are based on membranes. The types of materials used
are extensive, including 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 as they have fixed pore sizes. However, flux rate, cross-flow
and cleaning frequency are very important in the design of these systems. Many
of the systems available do foul and have high cleaning frequencies or redundancy
to cope with this. Maintaining permeate flow is, therefore, always a concern.
Pilot testing of these installations is highly recommended before proceeding
with a full-scale design.
Case Study One
Figure 5 shows a simple process flow diagram for the pre-treatment of this system.
The most challenging aspects of the feed water to this particular system were
the potential biological activity and the possibility of ammonia within the
feed water source. The solids in the feed are present due to carry-over from
the clarifier within the activated sludge process that provides the water. Any
pre-treatment system used would require a high dose of biocide to kill the bio
solids to prevent bio fouling within the RO and for removal of the solids by
For this application chlorine was originally chosen as the biocide. However,
due to the presence of ammonia the demand was in excess of 100ppm on occasions.
For this reason chlorine dioxide was used instead. The dosage was significantly
reduced (~20ppm) and the fast acting and extremely aggressive nature of ClO2
gave good quality, sterile water to the RO system. Suspended solids following
the primary filtration are typically 1 nephelometric turbidity unit (NTU), the
filters having a backwash frequency of 2 in 24h. Following the polishing filters
SDI15s of 3.8 to 4 are maintained.
Following the installation of this plant, significant variation in the feed
water quality was noted. This resulted in poor performance of the RO and unscheduled
cleaning. The solution to the problem was installation of turbidity monitors
to give an early warning when solids were likely to cause a problem and the
provision of a secondary low solids water source, which was used to backwash
the filters when the solids loading was high, therefore keeping the filters
Chemical pre-treatment to RO systems is primarily concerned with the prevention
of scaling due to the presence of poorly soluble salts and the minimisation
or control of biological growth. The source of the feed water and the membrane
type dictate the chemical additions necessary. Table 4 is a potential list of
contaminants that should be quantified to correctly design an RO system and
its required pre-treatment.
Chemical additions should always be kept to minimum where possible, over addition
will increase the feed TDS to the RO system and therefore the RO permeate TDS.
Table 5 lists common foulants within RO systems, their potential sources and
possible pre-treatments. From Table 5 it should be noted that over addition
of some chemicals within the RO pre-treatment system might also harm the RO’s
performance. Inorganic coagulants based on iron or aluminium and organic polymers
are good examples of this.
Scaling of RO is closely linked to the system’s recovery and cross-flow. As
a general rule, the higher the recovery of the system, the more likely it is
that scaling will occur. Sufficient control and monitoring of the RO and its
pre-treatment is vital.
In systems that operate at low recoveries (65-70%) with little or no iron present,
the addition of acid may be the only chemical pre-treatment necessary – this
will prevent scaling of the RO system with calcium carbonate. The feed waters
pH will typically be adjusted to ~6.0 at which point the majority of the carbonate
within the water will be found either as bicarbonate or carbon dioxide – these
do not form scales with calcium. Acid addition is a simple system to control
using the pH of the water as feedback to meter the dosage of acid required.
This potentially creates two problems. Firstly, the CO2
created will freely pass into the RO permeate requiring degassing downstream.
Secondly, large doses of acid will increase the feed TDS to the system. If hydrochloric
acid is used, this will increase the permeate TDS as the chloride added is poorly
rejected by the membrane. If sulphuric acid is used, the permeate TDS will not
be as badly affected, as sulphate is well rejected by RO membranes. However,
if the dosage of sulphuric acid is high enough, precipitation of calcium sulphate
becomes a potential risk.
As recovery increases to 75-80%, acid addition alone cannot control all of
the species within the feed that are likely to scale. There are two potential
alternatives to this problem:
- remove the scale forming ions from the feed with some form of softening
- use a scale inhibitor
Ion exchange softening is an excellent pre-treatment to RO systems. By removing
calcium, magnesium, barium and iron II the formation of carbonate, sulphate
and fluoride scales is prevented. Additional benefits from this treatment are
the polishing filtration provided by the ion exchange beds further improving
SDIs. This approach to pre-treatment is particularly suited to systems using
PA membranes. As the membrane is stable over a wide range of pH, no acid addition
is required when compared to CA membrane systems. Therefore, carbonate species
can be rejected by the RO as NaHCO3, reducing the need
for down stream degassing of CO2.
As with all ion exchange processes, the regenerant water from the process can
be difficult to dispose of due to the high salt content. However, by softening
the feed water to the RO the concentrate can occasionally be used as a secondary
make-up water source to cooling systems. This improves the chemical efficiency
of the RO system and allows for greater cycling within the cooling systems,
thereby reducing fresh water needs.
The alternative to removing the divalent cations from the feed water is to
use an inhibitor. Most inhibitors or antiscalents used for water treatment at
this time can be classified as threshold inhibitors (Figure 6a shows normal
scale formation, Figure 6b shows the method operation of threshold inhibitors).
These materials are extremely good at preventing carbonate and sulphate scales.
Using these, it is possible to operate at higher recoveries without significantly
altering the feed water composition, as the dosage of antiscalent when compared
to acid is extremely small. The need to de-gas CO2 from
the permeate is reduced because the antiscalents do not lower the pH and therefore
evolve CO2 from the alkalinity present. It is important
to realise this when designing RO plants, as these materials do not prevent
scaling from occurring but delay the formation of large crystals that form scales.
However, the effectiveness of these chemicals is quite short-lived – potentially
only 30 minutes. This effects the operation of the RO. While 30 minutes should
be more than the hydraulic retention time of the feed water within the RO, when
the RO shuts-down the concentrate may start to scale the membranes. RO systems
that use these forms of scale inhibition should be designed to go through a
flush sequence when shut-down, even under alarm conditions.
|Common foulants and scalents within RO systems
and potential pre-treatments4
Feed water/acid addiction
Feed water / (Fe(ll) / Coagulant addition (Allll)
Oxidation of reduced species
Control of coagulant addition
Free water/free chlorine
Addition of a reducing agent
Feed water / humic acids / polymers
Coagulant and filtration
Control of polymer addition
Internal to RO
On line biocide addition
Off line sanitisation
Coagulation and filtration
Biological fouling of RO membrane systems is the most common cause of reduction
in permeate flow and quality3. 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. 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, on 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 of the oxidising biocide must be removed to prevent severe membrane
damage. Typically, chlorine tolerances for PA membranes are quoted at around
1,000ppm/h6, therefore the membrane’s rejection will drop below that
quoted after 500h operation with a feed concentration of 2ppm. CA membranes
can have less problems with biological fouling due to their tolerance of oxidants,
but this must be measured against their lower salt rejection and pH range requirements.
Where a PA membrane has been chosen the removal of oxidants is therefore required.
This can be done by either the addition of a reducing agent, such as sodium
bisulphite or adsorption of the free oxidant, such as chlorine on to 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 of the reaction to occur. Over addition is quite possible
because the residual of the oxidant may vary and the feed water TDS to the system
will be increased. Most importantly, the system is vulnerable to mechanical
failure of the dosing pumps, to avoid this GAC can be used as 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, particularly town’s water sources. Town’s water is typically supplied
with a residual of a free oxidant, usually ~0.05ppm chlorine in the UK. SDI15s
measured on raw town’s water will typically reach 4-5, therefore polishing filtration
will further increase the water quality to ~3.5. The solids found in drinking
water supplies tend to be rust from the distribution system or fine silt.
Once 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 man3, therefore
in within RO systems biological growth must be controlled. Out-of-control biological
growth is typically referred to as bio fouling due to the presence of thick
films of cells and their secretions. It is identified by increased pressure
drops, reduced permeate flow and decreasing salt rejection. Where an oxidising
biocide can not be added, such as in a PA system, non-oxidising 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 that may be used 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.
Case Study Two
Figure 7 shows simple process flow diagram of the pre-treatment system. The
key factor controlling the design of this system was the silica content of the
water. Due to the local geology the feed water was almost saturated with it.
Silica is highly insoluble, the maximum concentration at pH of 6 to 9 being
~130ppm, therefore, the use of some form of scale inhibition was required. Threshold
inhibitors on the market will guarantee prevention of silica scale up to a concentration
of ~220ppm8. This would limit the recovery
of the plant to ~58% which was considered an uneconomic use of the available
resource. By increasing the pH of the feed water the solubility of silica can
be enhanced, at a pH of 9.8 the solubility of silica is increased to ~240ppm.
This allows the system to operate at a recovery of ~65%.
However at a pH of 9.8 any hardness present in the feed will combine with carbonate
to form calcium carbonate and scale the system. Therefore it was necessary to
soften the feed water to less than 1ppm total hardness as CaCO3.
An advantage of using the softener system up-front of the RO is that colloidal
silica present in the feed is filtered out to give SDI15s of <3.5.
This system has operated well producing high quality feed water to the customer,
cleaning frequencies have been low and no evidence of bio fouling has been detected.
Hardness leakage, or upsets to the softeners operation, are a potential weakness
within this system, for that reason polishing softening was installed.
In exploring approaches to RO pre-treatment systems and some of the factors
upon which they depend, such as water source, water availability, membrane type,
downtime and environmental impacts, we have examined potential causes for reduced
performance from RO systems and how in some circumstances it may be the pre-treatment
system that introduces the foulant.
Two facts are certain, RO systems are still fouling and the business of RO
cleaning chemicals is a growth area. This is due to poor or aggressive RO design
and results in the inability of pre-treatment systems to supply water to the
To improve RO system design, good quality feed water data over an extended
period should always be sorted out 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.
1) Course notes from Membrane Processes, short course by the School of Water
Sciences, Cranfield University, 2000.
2) Betz Handbook of Industrial Water Conditioning, 9th Edition, 1991. USA.
3) Membranes and Micro Organisms, H.C. Flemming, Proceedings of Membrane Technology
in Water and Wastewater Treatment 27th -29th March 2000, Royal Society of Chemistry.
4) Death, Taxes and RO Membrane Fouling, Nancy Mulhern, Osmonics Inc. 11/01/95
5) Koch Membranes Catalogue, 2000. Koch Membrane Systems, The Granary, Stafford.
6) Osmonics Membrane Catalogue, 2000. Osmonics, Inc. Minnetonka, Minnesota.
7) Important considerations in the design and construction of semiconductor
water systems. Steven Gagonon, Feb 2000. Ultrapure Water. Pub Tall Oaks. USA.
8) Permacare International presentation to Ecolochem International, Oct 2000.
9) Reverse Osmosis Pre-Treatment of High Silica Waters, Simon Gare, Ecolochem
International, Proceeding of the International Water Conference Oct 2000, Pittsburgh
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