By J. S. Baker and L. Y. Dudley, PermaCare®
ABSTRACT
The use of microbicides, particularly chlorine may be
advantageous to operation but can also exacerbate
biofouling problems. Microorganisms subjected to low
levels of biocides often exude large amounts of Extracellular Polysaccharides (EPS) as a protection; it is
this EPS material that forms the biofilm.
This paper examines the causes and effects of obstinate biofilms in membrane elements. In these cases,
problems of increased differential pressure have
proven difficult to correct during routine cleaning
cycles. Consequently, regrowth rates, as indicated by
this differential pressure for such biofilms, have been
rapid.
Experimental data has been taken from more than a
hundred membrane autopsies around the world. These
autopsies have been undertaken when such problematic biofilms are encountered. They confirm that
biofilms formed in process and membrane systems are
comparable except that membrane biofilms contain
greater numbers of fungi.
Some operating systems have reported the cessation
of chlorination with a significant reduction of
biofouling. This paper considers possible causes for
this. It also considers the increasing use of proprietary
non-oxidising microbicides.
Conclusions are that biofouling is endemic within
membrane systems, yet many systems operate satisfactorily even with a biofilm. Foulant layers can be
‘conditioned’ or ‘hardened’ by the repeated use of cleaning programmes. There is a strong case for alternating cleaners and biocides as used by the cooling water
treatment industry.
INTRODUCTION
Biofouling and its control remains a major operating
problem for many reverse osmosis (RO) plants, particularly those in tropical and sub-tropical regions.
Presented at Membranes in Drinking and Industrial Water Production,
Amsterdam, September 1998
Figures 1 and 2 — Biofouled membrane (top) and
cartridge filter (bottom)
The sequence of biofouling and its effect on membrane
systems is well documented and has been the subject
of some informative reviews.1 Biofouling occurs despite
the use of pre-treatment systems and the addition of
disinfectants such as chlorine (Figures 1 and 2).
Biofilms occurring in membrane systems may cause
severe loss of performance and the use of costly cleaning procedures to maintain output and quality. The
Reprint R-790
Biofouling in Membrane Systems – A Review
fouling is often so severe that acceptable operation
cannot be maintained and membrane replacement is
needed.
Bacteria are capable of colonising almost any surface
and have been found at extreme conditions such as
temperatures from –12°C to 110°C and pH values
between 0.5 and 13.2,3 Bacteria embedded in a biofilm
are more resistant to biocides than the same bacteria in a dispersed state. This applies to many toxic
substances commonly used in membrane processes.4,5
The important influences on the rate of biofilm development are the carbon:nitrogen:phosphorus ratio,
temperature, redox potential and pH.6 Extracellular
polysaccharides (EPS), substance responsible for the
slimy nature of biofilms and a product of the microorganisms themselves, are produced to a greater or
lesser extent by a variety of genera such as Pseudomonas.
Despite a great quantity of published information,
some of the questions posed by Characklis in 19817
whilst discussing biofilms in cooling water treatment
applications remain unanswered. Some of these are
particularly appropriate to membrane applications:
Figure 3 — Biofilm blockage of feed channel spacer
In a spirally wound element, there is the possibility
for some sections of the flow channel to become
blocked such that the water flow will be concentrated
in other parts of the feed channel. Channelling problems also arise in hollow fibre bundles when the individual fibres become bound together by foulant.
These conditions have often been observed during
autopsies of severely fouled membranes and are difficult to clean as the cleaning solution fails to reach
the fouled membrane leaves or fibres. For this reason, it is essential that biofouling is detected and dealt
with during its early stages as biofilm removal is more
difficult when it occurs to this extent.
• How do biofilm properties influence energy losses?
• How do biofilm properties change when biocides
are applied continuously?
• How does the inorganic content of the water influence biofilm properties?
If these questions could be answered with any degree of accuracy, it would be a good start in the formulation of anti-biofouling strategies for membranebased systems. Such an understanding would be a
clear and positive advance in the understanding of
biofilm control.
Channelling causes rapid salt concentration in the
affected areas. This leads to the precipitation of sparingly soluble salts such as calcium carbonate and calcium sulphate, the latter being a specific problem with
sulphuric acid dosed feedwaters. For example, a
mixed foulant recovered from a blocked sea water
membrane feed channel was found to be 83% calcium
sulphate. Calcium sulphate would not ordinarily be
a scale problem in a low recovery sea water plant,
but this can occur as a result of biofilm blockages.
Biofouling of the feed channels and spacers in spiral
wound elements (Figure 3) often causes a significant
increase in differential pressure (∆P), which is often
detected before normalised product flow or quality is
affected. Such fouling phenomena in any channel are
known to influence frictional energy losses. With
membrane biofilms, increased hydraulic resistance
(i.e., ∆P) has often been attributed to the visco-elastic
properties of the biofilm itself. Clearly these properties are dependent upon the biofilm composition,
which is itself dependent upon environmental factors.
The physical structure of biofilms found in membrane
systems can be compact and ‘gel’ like or ‘slimy and
adhesive’ with some consisting of a large ratio of
polysaccharide slime to viable microorganisms. Other
biofilms contain higher proportions of microorganisms. Between 106 and 108 colony forming units (cfu)
of bacteria per cm2 of membrane are common.
PermaCare have performed many destructive autopsies
from RO plants around the world. In many of these,
the plant operators have experienced difficulty in recovering plant performance by routine cleaning so
these autopsies were undertaken to characterise the
foulant and evaluate alternative cleaning procedures.
The objective of this paper is to document the main
chemical, physical and microbiological characteristics found during these autopsies and to discuss the
available options for biological control.
2
LABORATORY CHARACTERISATION OF
MEMBRANE BIOFILMS
• >5% Fe as iron oxide when treating brackish water
• high microbiological counts (>106 cfu/cm2) including bacteria, fungi and sometimes yeasts
BIOFILM COMPOSITION
Table 1 summarises analytical results from a number of biofouled systems producing potable quality
water for municipal or process use.
MICROBIOLOGICAL ENUMERATION AND
IDENTIFICATION
In summary, these laboratory studies and others
performed within the last year have shown that a
typical biofilm has the following characteristics:
The dilution pour plate method is used to enumerate
the numbers of viable bacteria, fungi and yeasts
present as sessile organisms on the membrane surface, plastic spacer material and permeate carrier.
The viable counts are expressed as cfu/cm2 for membrane, plastic spacer and permeate carrier samples.
Table 2 gives the ranges of viable bacteria enumerated from autopsied membranes.
• >90% moisture content
• of dried deposit, >50% total organic matter
• up to 40% humic substances as % of total organic
matter in high coloured waters
• low inorganic content
Table 1 — Summary results of recent autopsies
Plant Location
Size
(m3/hr)
Water Source
Major Foulants
Netherlands
18
brackish
44% organics
30% Fe
10% SiO2
89%
Canary Islands
63
sea water
63% organics
4.7% MgCO3
1.7% CaSO4
92%
Spain
12
brackish
66% organics
14% alumina
3.4% SiO2
94%
Italy
36
brackish
26% organics
36% Fe
13% SiO2
85%
Argentina
160
brackish
44% organics
37% SiO2
5% Fe
93%
Spain
20
brackish
90% organics
4% phosphate
1.9% Fe
0.5% SiO2
94%
Germany
22
brackish
76% organics
7.1% SiO2
5.1% CaPO4
85%
Spain
1000
brackish
67% organics
13% SiO2
4.5% alumina
90%
Egypt
200
brackish
50% organics
39% Fe
2.9% CaSO4
92%
USA
125
brackish
63% organics
10% alumina
85%
3
Foulant Moisture
Content
Table 2 — Typical microbiological activity in biofouled spiral wound elements
Microbiological identification found only two species
of Pseudomonas present on the membrane surface
and spacer. This is unusual in biofouled membrane
systems where more diverse microbial assemblages
are normally encountered.
Range of viable
Range of
bacteria counts fungal counts
cfu/cm2
cfu/cm2
Fouled membrane
1 x 102 - 1 x 108
2
Plastic spacer material* 4 x 10 - 5 x 10
Permeate carrier
6
< 102 - 1 x 106
Bacteria: Pseudomonas vesicularis, Pseudomonas
fluorescens
0 - 1 x 103
3
0 - 1 x 10
None
Microbiological Counts
*viable bacteria enumerated per cm2 of the spacer mesh
Identification of microorganisms in RO biofilm have
been carried out. Several species of bacteria were
present in the majority of biofilms investigated. Table 3
details genera commonly encountered.
Corynebacterium, Pseudomonas, Bacillus,
Artrobacter, Flavobacterium, Aeromonas
Fungi
Penicillium, Trichoderma, Mucor, Fusarium,
Aspergillus
Yeasts
Occasionally identified in significant numbers
2.9 x 106 cfu/cm2 bacteria
Plastic spacer
1.81 x 105 cfu/cm2 bacteria
Permeate carrier
<102 cfu/cm2 bacteria
This particular biofilm, when viewed with SEM (Figures 4 and 5), after careful preparation, was found to
contain relatively few bacteria, in contrast to large
amounts of apparently EPS material. It may be significant to note that the feed to this membrane was
undergoing chlorination and dechlorination in the
pre-treatment train, which may have been responsible for the large quantities of EPS present in this
system.
Table 3 — Common microorganisms identified in
membrane biofilm
Bacteria
Membrane surface
Although good kill rates were obtainable with
oxidising (100%) and non-oxidising biocides (99.9%),
laboratory cleaning tests indicated that this biofilm
was particularly difficult to remove using standard
cleaning procedures. The only success in removing
the foulant was found with a surfactant at pH 13.
This high pH exceeded that normally advised by the
membrane manufacturer but approved for occasional
intensive cleaning.
The most commonly occurring are the bacterium
Pseudomonas and Corynebacterium and the fungal
genera Penicillium and Trichoderma. The autopsy
procedure investigates the efficacy of biocides on
viable organisms isolated from the autopsied membrane. This allows an indication of the potential
effectiveness of a biocide agent in specific systems.
In practice, this cleaning programme, used in conjunction with regular maintenance cleaning and careful monitoring, limited the increase in ∆P caused by
the biofilm accumulation and growth. This has allowed the site to minimise the detrimental effects of
increases in ∆P to within tolerable levels.
CASE STUDIES
MUNICIPAL DRINKING WATER PLANT,
SOUTHERN EUROPE, BRACKISH
FEEDWATER
This plant produces 1,000 m3/h operating between
70 and 80% recovery. There is an extensive pre-treatment system comprising sand filtration, chlorination/
dechlorination, acid and phosphonate-based antiscalant dosing. High ∆P was evident for this plant,
which was proving difficult to control. An autopsy of
an 8" spiral wound membrane produced the following data:
POTABLE WATER PLANT, NORTHERN
EUROPE
The SEM shown in Figures 6 and 7 reveal the fouling layer taken from a potable plant treating a brackish surface water where biofouling had caused an
overall 30% decrease in membrane performance.
Foulant composition
Moisture content: 86% water
Organic content:
73%, of which 34% was humic
substances
Inorganic content: iron as iron oxide (17 %)
silica (3%)
calcium as calcium sulphate (2%)
alumina (0.6%)
Foulant Composition
Moisture content: 96% water
Organic content:
76%, of which 44% was humic
substances
Inorganic content: silica (7%)
calcium as calcium sulphate (3%)
iron as iron oxide (3%)
4
Figures 4 and 5 — SEM views of the biofouled membrane (municipal drinking water plant)
Figures 6 and 7 — SEM views of the biofouled membrane (potable water plant)
lowed by an off-line non-oxidising biocide to control
biogrowth.
Significant aspects of the biofilm included high
organics, iron and humic acid content. Microbiological analysis of the fouled membrane showed 7.3 x
105 cfu/cm2 of bacteria present on the separating surface and 4.3 x 105 cfu/cm2 on the spacer material. In
addition, high numbers of fungi and yeasts were enumerated on both the membrane (1.51 x 103 cfu/cm2)
and plastic spacer (7.44 x 103 cfu/cm2). The numbers
of fungi and yeast observed were significant.
Factors such as CIP tank size, the ability to clean
stages separately and the provision of temperature
control affect cleaning performance and the rate of
microbiological regrowth. Practical experience of biofouled systems has shown that a three-stage cleaning procedure such as detailed below gives the best
cleaning efficiency.
The following types of microorganism were identified
on the membrane surface:
Cleaning recommendation
Bacteria: Rod-shaped bacteria, Corynebacterium
and Arthrobacter
Fungi:
Penicillium
Yeasts:
Candida
Stage 1: Alkaline surfactant and chelating agent to
condition and break down the organic fouling.
Cleaning conditions: pH 10.5, 30°C, 4 hour recirculation and soaking.
It was recognised that the major influence on fouling
was due to the presence of iron bound with organics
and EPS. For this reason, the most suitable cleaning
product recommended was an alkaline surfactant fol-
Stage 2: Broad-spectrum non-oxidising biocide to
eliminate microbiological growth.
Cleaning conditions: 30°C, 30 minute recirculation.
5
Stage 3: Alkaline and chelating surfactant, to remove
microorganisms and organic debris.
reported after chlorine use was discontinued in a
Mediterranean sea water plant.8 In this case, cleaning frequency was reduced from biweekly to yearly.
Cleaning conditions: pH 10.5, 30°C, 4 hour recirculation and soaking.
The reasons for this improvement are not completely
clear and it is probable there are several contributory factors:
An optional acidic clean may be performed to follow
step 3 to remove remaining traces of inorganic scale.
The sequence of applying cleaning chemicals is important. For example, certain humic acids can become
difficult to remove if subjected to acid conditions. In
such cases where the exact nature of the foulant is
not known, it is always advisable to commence cleaning with an alkaline product.
• It is inevitable that some bacteria will survive disinfection protected by EPS or as part of a planktonic community. The latter could be sloughed
biofilm that has become detached from pre-treatment equipment. Bacteria arriving in the membranes in this compromised condition may produce
EPS as a defence mechanism which, in turn, will
make the bacteria resistant to the biocide and more
difficult to eliminate. The emergence of a oxidant
resistant biofilm can cause problems since this may
be composed of a high proportion of EPS, which
can result in significant energy losses.
Membrane compatibility is essential for all cleaners
and biocides used in the cleaning process and desirable characteristics are:
•
•
•
•
•
•
non-oxidising
stability at pH 2-12
nonionic or anionic
nonfilming
compatible with other cleaning products
stable at 20 – 35°C
• It is probable that nonviable microorganisms in
the water supply following chlorination can act as
a nutrient source.
• Oxidising biocides may break down humic acids
into smaller components, which may become available as a nutrient to the microbiological flora.
CHEMICAL BIOCIDES
OXIDISING BIOCIDES
Many oxidising biocides are available for use in industrial water treatment processes:
•
•
•
•
•
•
•
SODIUM BISULPHITE SHOCK DOSING
The efficacy of NaHSO3 as a biocide for sea water is
dependent on the use concentration, the exposure
time and the type of microorganisms present.9 With
an exposure time of 30 minutes at a concentration of
500 ppm, kill rates up to 99% have been reported for
sea water microflora. Whilst these values appear
impressive, other data presented for aerobic marine
bacteria indicated a greater resistance to sodium
bisulphite with only a 75% kill obtained after 4 hours
of contact at 500 ppm. It is probable that certain
microorganisms such as sulphate-reducing bacteria
would adapt to these anaerobic conditions.
chlorine
bromine
chloramine
chlorine dioxide
hydrogen peroxide
peroxyacetic acid
ozone
The use of oxidising biocides requires caution since
they are incompatible for long-term contact with
polyamide-based membranes. Where compounds like
chlorine are used in the pre-treatment system, it is
essential to remove any residual chlorine with sodium
bisulphite well in advance of the membrane array.
When chlorine or ozone are dosed to sea water, hypobromous acid is formed, which will eliminate bacteria but also cause membrane damage.
NON-OXIDISING BIOCIDES
Oxidising biocides such as peroxyacetic acid/ hydrogen peroxide and chloramine are used in composite
membranes, provided the pH, temperature and
contact time are strictly controlled. However, the
advantages of non-oxidising biocides are evident.
CHLORINATION—FRIEND OR FOE?
It has long been standard practice to control biological growth in the feedwater by using chlorine. Current theory and practical experience indicates that
this is not always successful in controlling biofouling.
In fact, it has been found on occasion to worsen the
biofouling potential. Significant improvements in
DuPont hollow fibre RO plant performance have been
Many non-oxidising biocides such as formaldehyde,
glutaraldehyde and quaternary ammonium compounds
etc. are used by the water treatment industry,
although non-oxidising biocides are not approved for
potable applications.
6
Several proprietary biocides have been developed for
use in membrane systems. These are membrane compatible, suitable for discharge and biologically effective. Many of these compounds are used intermittently at low dose rates, and are a cost-effective means
of maintaining a clean membrane surface. Low
molecular weight compounds can pass through the
membrane to treat the product water side.
Experience with many of these biocides in process
applications has shown that their long-term use often causes microbial resistance. Continuous low dose
rates should be avoided and it is advised they be ‘shock
dosed’ to the system water. For instance, one such
proprietary non-oxidising biocide is recommended at
200-300 ppm for 1/2 – 1 hour. This treatment may be
necessary daily, weekly or monthly depending upon
rates of regrowth and cleaning efficiency. In process
applications, it is common practice to employ a secondary non-oxidising biocide to prevent the emergence of resistant strains. This is a technique we suggest be used in membrane applications.
CONCLUSIONS
Many autopsies have shown that some biofilms are
difficult to eliminate by routine cleaning procedures.
The biofilm becomes ‘conditioned’ and ‘resistant’ to
chemical cleaning procedures due to the establishment of resistant microorganisms or the massive production of protective slimes. In the latter case, high
energy losses will usually be evident.
Resistant fouling necessitates cleaning outside recommended temperature and pH limits, which is
inadvisable. Very often the only option is to try and
maintain conditions at a tolerable level before an
irrecoverable position is reached and the membrane
must be replaced. Consideration should then be given
to improved control procedures and more effective
membrane biocides and cleaners.
The following points are important:
• Membrane biofouling is endemic.
• Although most biofilms have common characteristics, their structure varies greatly.
• The impact of biofilm upon plant performance depends on the composition and structure of the
biofilm. Many plants with biofilm work satisfactorily.
• The resistance of the biofilm to chemical cleaning
and biocides will increase if inefficient control
measures are employed.
• A biofilm formed under natural conditions may
prove easier to control in the long term than one
formed where biocides are in use.
• Non-oxidising biocides should always be applied
as a ‘shock’ dose. Consideration should be given to
the use of two non-oxidising biocides in a complementary biogrowth control programme.
REFERENCES
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membrane biofouling and prevention. In Reverse
Osmosis: Membrane Technology, Water Chemistry,
and Industrial Applications. Ed. Amjad Z. Publ.
Van Nostrand Reinhold, New York.
2. Lessel T., Motsch H. and Hennig E. (1975). Experience with a pilot plant for the irradiation of sewage sludge. In Radiation for a Clean Environment.
Publ. International Atomic Energy Agency, Vienna.
3. Characklis W. G. and Marshall K. C. Eds (1990).
Biofilms. Publ. John Wiley, New York.
4. Nichols P. (1989). Susceptibility of biofilms to toxic
compounds. In Structure and Function of Biofilms.
Eds. Characklis W. G. and Wilderer P. A. Publ. John
Wiley, New York.
5. LeChevalier M.W., Cawthon C. C. and Lee R. G.
(1988). Inactivation of biofilm bacteria. Applied and
Environmental Microbiology 54 2492-2499.
6. Cullimore D. R. (1992). Practical Manual of Groundwater Microbiology. Publ. Lewis Michigan.
7. Characklis W. G. (1981). Fouling biofilm development: A process analysis. Biotechnology and
Bioengineering. 23 1923-1960.
8. Hamida A. B. and Moch I. (1996). Controlling
biological fouling in open sea intake RO plants
without continuous chlorination. Desalination and
Water Reuse. 6 (3) 40-45.
9. Applegate L. E. and Erkenbrecher C. W. (1987).
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