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Biofouling in membrane systems — A review
DESALINATION ELSEVIER
Desalination 118 (1998) 81-90
B i o f o u l i n g in membrane systems - A review J.S. Baker, L.Y. Dudley* PermaCare International, Houseman Ltd., Windsor, UK
Received 26 July 1998; accepted 30 July 1998
Abstract The use of microbiocides, particularly chlorine may be advantageous to operation but can also exacerbate biofouling problems. Micro-organisms subjected to low levels of biocides often exude large amounts of extracellular polysaccharides (EPS) as a protection, and 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 from around the world. These autopsies have been undertaken when such problematic biofilms are encountered. These 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. The paper considers possible causes for this. It also considers the increasing use of proprietary non-oxidising microbiocides. 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 and there is a strong case for alternating cleaners and biocides as used by the cooling water treatment industry. Keywords:
Membranes; Biofouling
1. Introduction Biofouling and its control remains a major operating p r o b l e m for m a n y reverse osmosis (RO) plants, particularly those in tropical and sub-tropical regions. The sequence of b i o f o u l i n g and its e f f e c t on m e m b r a n e systems is well d o c u m e n t e d and has been the
subject of some informative reviews [1]. B i o f o u l i n g o c c u r s despite the use o f pret r e a t m e n t s y s t e m s and the a d d i t i o n o f d i s i n f e c t a n t s such as c h l o r i n e . B i o f i l m s occurring in m e m b r a n e systems m a y cause severe loss o f p e r f o r m a n c e and the use o f costly cleaning procedures to maintain output and quality. T h e fouling is often so severe
*Corresponding author Presented at the Conference on Membranes in Drinking and Industrial Water Production, Amsterdam, September 21-24, 1998, International Water Services Association, European Desalination Society and American Water Works Association 0011-9164/98/$ - See front matter © 1998 Elsevier Science B.V. All fights reserved. Pll S001 I-9164(98)00091-5
82
J.S. Baker, L.Y. Dudley/Desalination 118 (1998) 81-90
Figs. 1 and 2. Biofouledmembraneand cartridgefilter. that acceptable operation can not 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 ll0°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), the substance responsible for the slimy nature of biofilms and a product of the micro-organisms themselves, are produced to a greater or lesser extent by a variety o f genera such as Pseudomonas. Despite a great quantity of published information some of the questions posed by Characklis in 1981 [7] whilst discussing biofilms in cooling water treatment applications remain unanswered. Some of these are particularly appropriate to membrane applications: - H o w do biofilm properties influence energy losses?
- How do biofilm properties change when biocides are applied on a continuous basis? 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 membrane based systems. Such an understanding would be a clear and positive advance in the understanding of biofilm control. Biofouling of the feed channels and spacers in spiral wound elements (Fig. 3) often causes a significant increase in differential pressure (AP), 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. AP) has often been attributed to the viscoelastic properties of the biofilm itself. Clearly these properties are dependant upon the biofilm composition, which is itself dependant 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
83
J.S. Baker, L.Y. Dudley/Desalination 118 (1998) 81-90 Table 1 Summary results of recent autopsies Plant location Netherlands Canary Islands Spain Italy Argentina Spain Germany Spain Egypt USA
Size (m3/h) 18 63 12 36 t 60 20 22 1,000 200 125
Water source Brackish Sea water Brackish Brackish Brackish Brackish Brackish Brackish Brackish Brackish
Major foulants 44% 63% 66% 26% 44% 90% 76% 67% 50% 63%
Foulant moisture content
organics, 30% Fe, 10% SiO2 orgamcs, 4.7% MgCO3, 1.7% CaSO4 orgamcs, 14% alumina, 3.4% SiO2 organics, 36% Fe, 13% SiO2 orgamcs, 37% SIO2, 5% Fe orgamcs, 4% phosphate, 1.9% Fe, 0.5% SiO2 orgamcs, 7.1% SiO2, 5.1% CaPO4 orgamcs, 13% SiO2, 4.5% alumina orgamcs, 39% Fe, 2.9% CaSO4 orgamcs, 10% alumina
Fig. 3. Biofilm blockage of feed channel spacer.
slime to viable micro-organisms. Other biofilms contain higher proportions of micro-organisms. Between 106 and 108 colony forming units (cfu) of bacteria per cm 2 of membrane are common. 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
89% 92% 94% 85% 93% 94% 85% 90% 92% 85%
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. 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 feed waters. 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. PermaCare International have performed many destructive autopsies from RO plant 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.
J.S. Baker, L.E Dudley/Desalination 118 (1998) 81-90
84
Table 2 Typical microbiological activity in biofouled spiral wound elements Range of viable Range of bacteria counts fungal counts (cfu/cm2) (cfu/cm2) Fouled membrane l×102-1x108 Plastic spacer material* 4x102-5x106 Permeate carrier <102-1×106
0-1xl03 0-1×103 None
*Viable bacteria enumerated per cm2 of the spacer mesh Table 3 Common micro-organisms identified in membrane biofilm Bacteria Fungi Yeasts
2.
Corynebacterium, Pseudomonas, Bacillus, Arthrobacter, Flavobacterium, Aeromonas Penicillium, Trichoderma, Mucor, Fusarium, Aspergillus Occasionally identified in significant numbers
Laboratory
brane
characterisation
of
2.2. Microbiological identification
enumeration
and
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 c f u / c m 2 for membrane, plastic spacer and permeate carrier samples. Table 2 gives the ranges of viable bacteria enumerated from autopsied m e m b r a n e s . I d e n t i f i c a t i o n 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. The most commonly occurring are the bacterium P s e ud omo na s 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.
mem-
biofilms
2.1. Biofilm composition Table 1 summarises analytical results from a number of biofouled systems producing potable quality water for municipal or process use. In summary, these laboratory studies and others performed within the last year have shown that a typical biofilm has the following characteristics: - >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 - > 5 % Fe as iron oxide when treating brackish water - high m i c r o b i o l o g i c a l counts (>106 c f u / c m 2) including bacteria, fungi and sometimes yeasts.
3.
Case studies
3.1. Municipal drinking water plant, Southern Europe, brackish feed-water 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 AP was evident for this plant which was proving difficult to control. An autopsy of an 8" spiral wound membrane produced the following data: Foulant composition Moisture content: 96% water Organic content: 76% of which 44% was humic substances Inorganic content: Silica (7%) calcium as calcium sulphate (3%) and, iron as iron oxide (3%).
J.S. Baker, L.Y. Dudley/Desalination 118 (1998)81-90
85
Figs. 4 and 5. SEM views of the biofouledmembrane.
Figs. 6 and 7. SEM views of the biofouled membrane.
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. Bacteria: Pseudomonas vesicularis, Pseudomonas fluorescens.
Microbiological counts Membrane surface 2.9x10 6 cfu/cm2 bacteria Plastic spacer 1.8 1 × 1 0 5 cfu/cm 2 bacteria Permeate carrier <10 2 cfu/cm2 bacteria. This particular biofilm, when viewed with SEM (Figs. 4 and 5) after careful preparation
86
J.S. Baker, L.Y. Dudley / Desalination 118 (1998) 81-90
was found to contain relatively few bacteria, in contrast to large amount of apparently EPS material. It may be significant to note that the feed to this membrane was undergoing chlorination and dechlorination in the pretreatment train which may have been responsible for the large quantities of EPS present in this system. 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. In practice this cleaning programme, used in conjunction with regular maintenance cleaning and careful monitoring, limited the increase in AP caused by the biofilm accumulation and growth. This has allowed the site to minimise the detrimental effects of increases in AP to within tolerable levels.
were enumerated on both the membrane ( 1 . 5 1 × 1 0 3 cfu/cm 2) and plastic spacer (7.44x103 cfu/cm2). The numbers of fungi and yeast observed were significant. The following types of micro-organism were identified on the membrane surface; Bacteria: Rod shaped bacteria, C o r y n e bacterium and Arthrobacter Fungi: Penicillium Yeasts: Candida. 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 followed by an off-line non oxidising biocide to control biogrowth. Factors such as CIP tank size, the ability to clean stages separately and the provision of t e m p e r a t u r e c o n t r o l affect c l e a n i n g performance and the rate of microbiological re-growth. Practical experience of biofouled systems has shown that a three stage cleaning procedure such as detailed below gives the best cleaning efficiency.
3.2. Potable water plant, Northern Europe The SEM shown in Figs. 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%). Significant aspects of the biofilm included high organics, iron and humic acid content. Microbiological analysis of the fouled membrane showed 7.3x105 cfu/cm 2 of bacteria present on the separating surface and 4.3×105 cfu/cm 2 on the spacer material. In addition, high numbers of fungi and yeasts
Cleaning recommendation: 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; Stage 2 Broad spectrum non-oxidising biocide to eliminate microbiological growth, cleaning conditions: 30°C, 30 minute recirculation; Stage 3 Alkaline and chelating surfactant, to remove micro-organisms and organic debris, cleaning conditions: pH 10.5, 30°C, 4 hour recirculation and soaking. 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
J.S. Baker, L.Y. Dudley/Desalination 118 (1998) 81-90
to acid conditions. In such cases where the exact nature of the foulant is not known it is always adviseable to commence cleaning with an alkaline product. 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 - Non ionic or anionic - Non filming - Compatible with other cleaning products - Stable at 20--35°C.
4.
Chemical
biocides
4. I. Oxidising biocides Many oxidising biocides are available for use in industrial water treatment processes: chlorine, bromine, chloramine, chlorine dioxide, hydrogen peroxide, peroxyacetic acid, and ozone. The use of oxidising biocides requires caution since they are incompatible for long term c o n t a c t with p o l y a m i d e b a s e d 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.
4.2. Chlorination - Friend or foe ? It has long been standard practice to control biological growth in the feed water by the use of 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 reported after
87
c h l o r i n e use was d i s c o n t i n u e d in a Mediterranean sea water plant [8]. In this case, cleaning frequency was reduced from biweekly to yearly. The reasons for this improvement are not completely clear and it is probable there are several contributory factors: -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 which has b e c o m e d e t a c h e d from pre-treatment e q u i p m e n t . B a c t e r i a 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 c o m p o s e d of a high proportion of EPS which can result in significant energy losses. - I t is probable that non viable 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 b e c o m e available as a nutrient to the microbiological flora.
4.3. Sodium bisulphite shock dosing The efficacy of NaHSO3 as a biocide for sea w a t e r is d e p e n d a n t on the use concentration, the exposure time and the type of micro-organisms present [9]. With an e x p o s u r e time of 30 m i n u t e s 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 micro-organisms such as sulphate reducing bacteria would adapt to these anaerobic conditions.
88
J.S. Baker, L.Y. Dudley/Desalination 118 (1998) 81-90
4.4. 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 nonoxidising biocides are evident. Many non-oxidising biocides such as formaldehyde, glutaraldehyde, quaternary ammonium compounds etc. are used by the water treatment industry, although nonoxidising biocides are not approved for potable applications. 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 s y s t e m water. For instance one such p r o p r i e t a r y non o x i d i s i n g b i o c i d e is r e c o m m e n d e d at 200-300 ppm for 1/2-1 hour. This treatment may be necessary daily, weekly or monthly depending upon rates of re-growth and cleaning efficiency. In process applications it is common practice to employ a secondary non oxidising biocide to prevent the emergence of resistant strains and this is a technique we suggest be used in membrane applications.
5.
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 micro-organisms 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 m e m b r a n e biocides and cleaners. The following points are important: Membrane biofouling is endemic. - A l t h o u g h most biofilms have c o m m o n 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 nonoxidising biocides in a complementary biogrowth control programme.
References
[1] H.-C. Flemming, Mechanistic aspects of RO membrane biofouling and prevention. In: Reverse Osmosis: Membrane Technology, Water Chemistry, and Industrial Applications, Z. Amjad, ed., Van Nostrand Reinhold, New York, 1993. [2] T. Lessel, H. Motsch, and E. Hennig, Experience with a pilot plant for the irradiation of sewage sludge. In: Radiation for a Clean Environment, International Atomic Energy Agency, Vienna, 1975. [3] W.G. Characklis and K.C. Marshall K.C., eds., Biofilms, John Wiley, New York, 1990.
J.S. Baker, L.Y. Dudley/Desalination 118 (1998) 81-90 [4] P. Nichols, Susceptibility of biofilms to toxic compounds. In: Structure and Function of Biofilms, W.G. Characklis and P.A. Wilderer, eds., John Wiley, New York, 1989. [5] M.W. LeChevalier, C.C. Cawthon, and R.G. Lee, Inactivation of biofilm bacteria, Applied and Environmental Microbiol., 54 (1988) 24922499. [6] D.R. Cullimore, Practical Manual of Groundwater Microbiology, Lewis Michigan, 1992.
89
[7] W.G. Characklis, Fouling biofilm development: A process analysis, Biotechnology and Bioengineering, 23 (1981) 1923-1960. [8] A.B. Hamida and I. Moch, Controlling biological fouling in open sea intake RO plants without continuous chlorination, Desalination and Water Reuse, 6 (1996) 40-45. [9] L.E. Applegate and C.W. Erkenbrecher, Monitoring and control of biological activity in Permasep seawater RO plants, Desalination, 65 (1987) 331-359.
Desalination 118 (1998) 81-90
B i o f o u l i n g in membrane systems - A review J.S. Baker, L.Y. Dudley* PermaCare International, Houseman Ltd., Windsor, UK
Received 26 July 1998; accepted 30 July 1998
Abstract The use of microbiocides, particularly chlorine may be advantageous to operation but can also exacerbate biofouling problems. Micro-organisms subjected to low levels of biocides often exude large amounts of extracellular polysaccharides (EPS) as a protection, and 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 from around the world. These autopsies have been undertaken when such problematic biofilms are encountered. These 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. The paper considers possible causes for this. It also considers the increasing use of proprietary non-oxidising microbiocides. 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 and there is a strong case for alternating cleaners and biocides as used by the cooling water treatment industry. Keywords:
Membranes; Biofouling
1. Introduction Biofouling and its control remains a major operating p r o b l e m for m a n y reverse osmosis (RO) plants, particularly those in tropical and sub-tropical regions. The sequence of b i o f o u l i n g and its e f f e c t on m e m b r a n e systems is well d o c u m e n t e d and has been the
subject of some informative reviews [1]. B i o f o u l i n g o c c u r s despite the use o f pret r e a t m e n t s y s t e m s and the a d d i t i o n o f d i s i n f e c t a n t s such as c h l o r i n e . B i o f i l m s occurring in m e m b r a n e systems m a y cause severe loss o f p e r f o r m a n c e and the use o f costly cleaning procedures to maintain output and quality. T h e fouling is often so severe
*Corresponding author Presented at the Conference on Membranes in Drinking and Industrial Water Production, Amsterdam, September 21-24, 1998, International Water Services Association, European Desalination Society and American Water Works Association 0011-9164/98/$ - See front matter © 1998 Elsevier Science B.V. All fights reserved. Pll S001 I-9164(98)00091-5
82
J.S. Baker, L.Y. Dudley/Desalination 118 (1998) 81-90
Figs. 1 and 2. Biofouledmembraneand cartridgefilter. that acceptable operation can not 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 ll0°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), the substance responsible for the slimy nature of biofilms and a product of the micro-organisms themselves, are produced to a greater or lesser extent by a variety o f genera such as Pseudomonas. Despite a great quantity of published information some of the questions posed by Characklis in 1981 [7] whilst discussing biofilms in cooling water treatment applications remain unanswered. Some of these are particularly appropriate to membrane applications: - H o w do biofilm properties influence energy losses?
- How do biofilm properties change when biocides are applied on a continuous basis? 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 membrane based systems. Such an understanding would be a clear and positive advance in the understanding of biofilm control. Biofouling of the feed channels and spacers in spiral wound elements (Fig. 3) often causes a significant increase in differential pressure (AP), 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. AP) has often been attributed to the viscoelastic properties of the biofilm itself. Clearly these properties are dependant upon the biofilm composition, which is itself dependant 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
83
J.S. Baker, L.Y. Dudley/Desalination 118 (1998) 81-90 Table 1 Summary results of recent autopsies Plant location Netherlands Canary Islands Spain Italy Argentina Spain Germany Spain Egypt USA
Size (m3/h) 18 63 12 36 t 60 20 22 1,000 200 125
Water source Brackish Sea water Brackish Brackish Brackish Brackish Brackish Brackish Brackish Brackish
Major foulants 44% 63% 66% 26% 44% 90% 76% 67% 50% 63%
Foulant moisture content
organics, 30% Fe, 10% SiO2 orgamcs, 4.7% MgCO3, 1.7% CaSO4 orgamcs, 14% alumina, 3.4% SiO2 organics, 36% Fe, 13% SiO2 orgamcs, 37% SIO2, 5% Fe orgamcs, 4% phosphate, 1.9% Fe, 0.5% SiO2 orgamcs, 7.1% SiO2, 5.1% CaPO4 orgamcs, 13% SiO2, 4.5% alumina orgamcs, 39% Fe, 2.9% CaSO4 orgamcs, 10% alumina
Fig. 3. Biofilm blockage of feed channel spacer.
slime to viable micro-organisms. Other biofilms contain higher proportions of micro-organisms. Between 106 and 108 colony forming units (cfu) of bacteria per cm 2 of membrane are common. 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
89% 92% 94% 85% 93% 94% 85% 90% 92% 85%
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. 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 feed waters. 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. PermaCare International have performed many destructive autopsies from RO plant 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.
J.S. Baker, L.E Dudley/Desalination 118 (1998) 81-90
84
Table 2 Typical microbiological activity in biofouled spiral wound elements Range of viable Range of bacteria counts fungal counts (cfu/cm2) (cfu/cm2) Fouled membrane l×102-1x108 Plastic spacer material* 4x102-5x106 Permeate carrier <102-1×106
0-1xl03 0-1×103 None
*Viable bacteria enumerated per cm2 of the spacer mesh Table 3 Common micro-organisms identified in membrane biofilm Bacteria Fungi Yeasts
2.
Corynebacterium, Pseudomonas, Bacillus, Arthrobacter, Flavobacterium, Aeromonas Penicillium, Trichoderma, Mucor, Fusarium, Aspergillus Occasionally identified in significant numbers
Laboratory
brane
characterisation
of
2.2. Microbiological identification
enumeration
and
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 c f u / c m 2 for membrane, plastic spacer and permeate carrier samples. Table 2 gives the ranges of viable bacteria enumerated from autopsied m e m b r a n e s . I d e n t i f i c a t i o n 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. The most commonly occurring are the bacterium P s e ud omo na s 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.
mem-
biofilms
2.1. Biofilm composition Table 1 summarises analytical results from a number of biofouled systems producing potable quality water for municipal or process use. In summary, these laboratory studies and others performed within the last year have shown that a typical biofilm has the following characteristics: - >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 - > 5 % Fe as iron oxide when treating brackish water - high m i c r o b i o l o g i c a l counts (>106 c f u / c m 2) including bacteria, fungi and sometimes yeasts.
3.
Case studies
3.1. Municipal drinking water plant, Southern Europe, brackish feed-water 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 AP was evident for this plant which was proving difficult to control. An autopsy of an 8" spiral wound membrane produced the following data: Foulant composition Moisture content: 96% water Organic content: 76% of which 44% was humic substances Inorganic content: Silica (7%) calcium as calcium sulphate (3%) and, iron as iron oxide (3%).
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Figs. 4 and 5. SEM views of the biofouledmembrane.
Figs. 6 and 7. SEM views of the biofouled membrane.
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. Bacteria: Pseudomonas vesicularis, Pseudomonas fluorescens.
Microbiological counts Membrane surface 2.9x10 6 cfu/cm2 bacteria Plastic spacer 1.8 1 × 1 0 5 cfu/cm 2 bacteria Permeate carrier <10 2 cfu/cm2 bacteria. This particular biofilm, when viewed with SEM (Figs. 4 and 5) after careful preparation
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was found to contain relatively few bacteria, in contrast to large amount of apparently EPS material. It may be significant to note that the feed to this membrane was undergoing chlorination and dechlorination in the pretreatment train which may have been responsible for the large quantities of EPS present in this system. 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. In practice this cleaning programme, used in conjunction with regular maintenance cleaning and careful monitoring, limited the increase in AP caused by the biofilm accumulation and growth. This has allowed the site to minimise the detrimental effects of increases in AP to within tolerable levels.
were enumerated on both the membrane ( 1 . 5 1 × 1 0 3 cfu/cm 2) and plastic spacer (7.44x103 cfu/cm2). The numbers of fungi and yeast observed were significant. The following types of micro-organism were identified on the membrane surface; Bacteria: Rod shaped bacteria, C o r y n e bacterium and Arthrobacter Fungi: Penicillium Yeasts: Candida. 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 followed by an off-line non oxidising biocide to control biogrowth. Factors such as CIP tank size, the ability to clean stages separately and the provision of t e m p e r a t u r e c o n t r o l affect c l e a n i n g performance and the rate of microbiological re-growth. Practical experience of biofouled systems has shown that a three stage cleaning procedure such as detailed below gives the best cleaning efficiency.
3.2. Potable water plant, Northern Europe The SEM shown in Figs. 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%). Significant aspects of the biofilm included high organics, iron and humic acid content. Microbiological analysis of the fouled membrane showed 7.3x105 cfu/cm 2 of bacteria present on the separating surface and 4.3×105 cfu/cm 2 on the spacer material. In addition, high numbers of fungi and yeasts
Cleaning recommendation: 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; Stage 2 Broad spectrum non-oxidising biocide to eliminate microbiological growth, cleaning conditions: 30°C, 30 minute recirculation; Stage 3 Alkaline and chelating surfactant, to remove micro-organisms and organic debris, cleaning conditions: pH 10.5, 30°C, 4 hour recirculation and soaking. 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
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to acid conditions. In such cases where the exact nature of the foulant is not known it is always adviseable to commence cleaning with an alkaline product. 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 - Non ionic or anionic - Non filming - Compatible with other cleaning products - Stable at 20--35°C.
4.
Chemical
biocides
4. I. Oxidising biocides Many oxidising biocides are available for use in industrial water treatment processes: chlorine, bromine, chloramine, chlorine dioxide, hydrogen peroxide, peroxyacetic acid, and ozone. The use of oxidising biocides requires caution since they are incompatible for long term c o n t a c t with p o l y a m i d e b a s e d 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.
4.2. Chlorination - Friend or foe ? It has long been standard practice to control biological growth in the feed water by the use of 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 reported after
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c h l o r i n e use was d i s c o n t i n u e d in a Mediterranean sea water plant [8]. In this case, cleaning frequency was reduced from biweekly to yearly. The reasons for this improvement are not completely clear and it is probable there are several contributory factors: -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 which has b e c o m e d e t a c h e d from pre-treatment e q u i p m e n t . B a c t e r i a 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 c o m p o s e d of a high proportion of EPS which can result in significant energy losses. - I t is probable that non viable 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 b e c o m e available as a nutrient to the microbiological flora.
4.3. Sodium bisulphite shock dosing The efficacy of NaHSO3 as a biocide for sea w a t e r is d e p e n d a n t on the use concentration, the exposure time and the type of micro-organisms present [9]. With an e x p o s u r e time of 30 m i n u t e s 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 micro-organisms such as sulphate reducing bacteria would adapt to these anaerobic conditions.
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4.4. 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 nonoxidising biocides are evident. Many non-oxidising biocides such as formaldehyde, glutaraldehyde, quaternary ammonium compounds etc. are used by the water treatment industry, although nonoxidising biocides are not approved for potable applications. 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 s y s t e m water. For instance one such p r o p r i e t a r y non o x i d i s i n g b i o c i d e is r e c o m m e n d e d at 200-300 ppm for 1/2-1 hour. This treatment may be necessary daily, weekly or monthly depending upon rates of re-growth and cleaning efficiency. In process applications it is common practice to employ a secondary non oxidising biocide to prevent the emergence of resistant strains and this is a technique we suggest be used in membrane applications.
5.
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 micro-organisms 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 m e m b r a n e biocides and cleaners. The following points are important: Membrane biofouling is endemic. - A l t h o u g h most biofilms have c o m m o n 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 nonoxidising biocides in a complementary biogrowth control programme.
References
[1] H.-C. Flemming, Mechanistic aspects of RO membrane biofouling and prevention. In: Reverse Osmosis: Membrane Technology, Water Chemistry, and Industrial Applications, Z. Amjad, ed., Van Nostrand Reinhold, New York, 1993. [2] T. Lessel, H. Motsch, and E. Hennig, Experience with a pilot plant for the irradiation of sewage sludge. In: Radiation for a Clean Environment, International Atomic Energy Agency, Vienna, 1975. [3] W.G. Characklis and K.C. Marshall K.C., eds., Biofilms, John Wiley, New York, 1990.
J.S. Baker, L.Y. Dudley/Desalination 118 (1998) 81-90 [4] P. Nichols, Susceptibility of biofilms to toxic compounds. In: Structure and Function of Biofilms, W.G. Characklis and P.A. Wilderer, eds., John Wiley, New York, 1989. [5] M.W. LeChevalier, C.C. Cawthon, and R.G. Lee, Inactivation of biofilm bacteria, Applied and Environmental Microbiol., 54 (1988) 24922499. [6] D.R. Cullimore, Practical Manual of Groundwater Microbiology, Lewis Michigan, 1992.
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[7] W.G. Characklis, Fouling biofilm development: A process analysis, Biotechnology and Bioengineering, 23 (1981) 1923-1960. [8] A.B. Hamida and I. Moch, Controlling biological fouling in open sea intake RO plants without continuous chlorination, Desalination and Water Reuse, 6 (1996) 40-45. [9] L.E. Applegate and C.W. Erkenbrecher, Monitoring and control of biological activity in Permasep seawater RO plants, Desalination, 65 (1987) 331-359.