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Biofouling of membranes for drinking water production
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DESALINATION
ELSEVIER
Desalination 118 (1998) 157-166
Biofouling of membranes for drinking water production H. S. Vrouwenvelder ~*, J. A.M. van Paassen b, H. C. Folmer ~, Jan A.M.H. Hofman d, M. M. Nederlof a, D. van der Kooij a Kiwa N. V., Research and Consultancy, Groningenhaven 7, P.O. Box 1072, 3430 BB Nieuwegein, The Netherlands Tel. +31 30 6069680. Fax +31 30 60 61 165. E-mail: [email protected] bWater Supply Company of Overijssel N. V, cN. V. PWN Water Supply Company of North Holland, ~Amsterdam WaterSupply. Received 27 June 1998 Abstract
In three pilot plants in the Netherlands the performance of either nanofiltration or reverse osmosis in water treatment was studied. Operational problems observed in these systems, viz. increased normalized pressure drop (NPD) and/or declined normalized flux (MTC) values, were attributed to biofouling. To elucidate the role of biofouling, data were collected about biomass accumulation in the membrane elements applying destructive autopsies of the membrane units. Biomass parameters included: total direct cell counts (TDC), adenosinetriphosphate (ATP) analysis and heterotropic plate counts (HPC). Maximum values of biomass parameters at the feed side of the membranes were: 2.1 x 108 cells/cm 2, 1.5 x 104 pg ATP/cm 2 and 1.8 × 10 7 CFU/cm 2, respectively. Microscopic observations of biomass obtained from the membrane systems indicated that the observed organisms were metabolically active in most cases. AOC levels in the feed water were relatively low. In one case, an increase had been observed following the addition of acid to prevent CaCO 3 scaling and also a high biofilm formation rate was observed in relation with a strong NPD increase. Cleaning agents tested under laboratory conditions and in practice did reduce ATP concentrations, but TDC values revealed that biomass removal from the membrane was very limited. These observations show that operational problems caused by biofouling are difficult to solve by cleaning. Therefore, biomass accumulation in membrane elements should be limited by: (i), achieving a far going removal of growth-promoting compounds and micro-organisms from the feedwater, (ii), securing the purity of the chemicals dosed, and (iii), applying effective cleaning procedures for biomass removal. Keywords: Biofouling; Biofilm; Reverse osmosis; Nanofiltration; Membranes; Drinking water; Autopsy; Cleaning agents. 1. Introduction
M e m b r a n e filtration (MF) techniques are very promising in water treatment because of their potential to remove particles, including
microorganisms (disinfection), organic pollutants (pesticides, taste, odor), inorganic compounds (softening, salt) and to achieve biologically-stable water to limit microbial regrowth in the distributionsystem [1, 2]. Additionally, nanofiltration
*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 Seicaee B.V. All rights reserved.
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(NF), reverse osmosis (RO) and even ultrafiltration (UF) can be used to remove viruses [3]. However, fouling of membranes does cause operational problems. Fouling mechanisms include: inorganic scaling (BaSO4, CaCO3), organic scaling (humic acids), colloidal scaling (suspended particles) and biofouling [4]. Different types of fouling can occur simultaneously, influencing each other, e.g. biofouling might enhance concentration polarization which stimulates inorganic scaling [4]. Biofouling - defined as accumulation of microorganisms, i.e. biofilms on a surface by growth and/or deposition at such a level that it is causing operational problems - is difficult to identify. It may cause problems such as flux reduction and/or increased pressure drops during NF or RO leading to early replacement of the membranes. Biofouling phenomena have been described extensively [5,6]. The diagnosis "biofouling" is only justified when a relation is found between the encountered operational problems and biofilm accumulation as described with an adequate parameter. A suite of biomass parameters is available [7], but the question remains which parameter(s) at which level is conclusive for identifying biofouling as the cause of the operational problems and at what level biofilm densities are acceptable. Controlling biological fouling is considered to be a major challenge in operating membrane filtration installations [8,9]. Operational problems attributed to biofouling were observed in several pilot plants in the Netherlands. This study was conducted to: (i), collect information about the amount and the position of biomass in either NF or RO membrane elements, and (ii), to elucidate the relationship between the biomass parameters and operational problems as increased NPD and/or declined MTC. 2. Methods 2.1. Biomass parameters
Biomass parameters which were used to determine the concentration of biomass in the water or on the membranes included: adenosine
triphosphate (ATP), total direct cell counts (TDC) and heterotropic plate counts (HPC). ATP (pg ATP/ml or pg ATP/cm 2) gives an indication of the total amount of active biomass [10]. ATP concentrations are measured by applying an enzymatic reaction using luciferase. The amount of light produced is determined and the ATP concentration is derived from the linear relationship between light production and reference ATP concentrations. TDC's (cells/cm 2) were determined with epifluorescence microscopy using acridine orange as fluorochrome - applying a slightly adapted method to eliminate fading [11,12]. All fluorescing cells are counted but TDC does not discriminate between active and inactive cells. Microscopic observations also give information about the variety and appearance of microorganisms present. HPC's values (CFU/cm 2) are determined by spreading 0.05 ml volumes of water on nutrient poor R2A medium [13]. Colonies were counted after 10 d of incubation at 25 °C. All biomass parameters have been determined in volumes of water collected either directly or from decimal dilutions of water, the suspension either obtained after ultrasonic treatment of the envelope components or prepared with the material collected from the membrane surface. 2.2. Autopsy of membrane elements
Elements were taken from the membrane filtration to determine the amount of accumulated biomass in case of operational problems or at the end of a test period. Directly after collection, the elements were transported and stored at low temperature previous to autopsy, which was usually performed within 24 hours after collection. Following visual inspection of the feed and product side the element was opened mechanically. After visual inspection of unfolded membrane envelopes pieces of membrane and spacer were collected from different locations over the length of the element, using sterile scissors and forceps. The samples were transferred to autoclaved tap
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water and sonicated. Subsequently, the obtained suspension was analyzed to assess the total amount of biomass (ATP, TDC and HPC) present on the membrane, the feed spacer, and the product spacer collected either combined or individually. Swabbing was used to collect biomass either from the feed or the product side of the membrane. Iron and manganese concentrations were determined in the suspensions obtained after acid treatment of the membrane coupons with boiling aqua regia. 2.3. Feed water analysis
In addition to physicochemical water quality parameters (including TOC, pH, temperature), the concentration of biomass (TDC, ATP and HPC) and the biological stability were assessed. For the latter purpose easily assimilable organic carbon (AOC) measurements for determining the heterotropic growth potential of the water [14] and the biofilm formation rate (BFR, pg ATP/cm2.d) were determined. For assessment of the BFR value of the water a biofilm monitor is used in which the accumulation of active biomass (ATP) is determined as a function of time [10,15,16].
3. Pilot plants Three different pilot plants for testing the performance of membrane filtration in surface water treatment were included in this study. Pretreatment in these pilot plants prior to either
NF of RO is characterized as: river bank filtration, ultrafiltration and slow sand filtration, respectively. Table 1 shows feed water quality parameters at the three locations. 3.1. Plant 1
The application of NF for the reduction of hardness, color and pesticides from river-bank filtrate was investigated in a pilot plant at Vechterweerd (Water Supply C o m p a n y of Overijssel). Water from the river Vecht was collected as bank filtrate (residence time of approximately two months). Treatment includes: plate aeration, dual media filtration, aeration, rapid sand filtration and NF. The installation is constructed as three staged (5-3-2), with 5, 3 and 2 pressure vessels in the first, second and third stage, respectively. Each stage contained four 4x40" spiral wound membrane elements (type Hydranautics PVD-1). Addition of hydrochloric acid to the feed water of the NF installation - to prevent scaling of C a C O 3 - made it possible to operate the plant at 80% recovery. 3.2. Plant 2
At Andijk, (PWN Water Supply Company of North Holland) water from Lake IJsselmeer is collected after 34 day storage in an impoundment storage reservoir. Treatment includes: micro straining, coagulation, sedimentation (sludge blanket filtration), rapid sand filtration, UF followed by
Table 1 Water quality parameters (average and range) of the feed water at the three locations after addition of hydrochloric acid Parameter Temperature, °C ATP(ng/I) AOC (~tg C/l) DOC (mg/I) Iron (mg/l) Manganese (mg/1)
Plant 1 10.5-12 8 ( 4 - 14) 17 (8 - 27) 6 (5.6 - 6.8) 0.025 (0.020 - 0.030) < 0.01
Plant 2 2-22 3 (1 - 10) 8 3 < 0.04 < 0.01
Plant 3 12-25 5 ( 3 - 8) 6 ( 3 - 10) 1.4 (1.2 - 2.2) < 0.01 < 0.01
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RO. RO is studied for the removal of organic micropollutants (pesticides), hardness, salt and disinfection [17]. The relatively high salt concentrations in the Lake IJssel are a problem; Dutch guideline values (sodium 120 mg/1 and chloride 150 mg/l) are exceeded in dry years. The two staged (4-2) RO installation with a capacity of 240 m3/d was loaded with a total of 42 Hydranautics 4040-UHA-ESPA spiral wound elements (7 in each pressure vessel). A recovery of 80% was achieved by adding hydrochloric acid. The combined concentrate from the first stage was fed to the second stage. Data on water quality (as achieved after pretreatment and after addition of hydrochloric acid) are presented in Table 1 (Plant 2). 3.3. Plant 3
RO performance was also studied in a pilot plant at Leiduin (Amsterdam Water Supply). The objective of this study was to evaluate direct treatment including RO as an alternative option for dune passage to increase the production capacity. RO is necessary for desalination, softening, removal of pesticides and other organic pollutants and deisinfection. River Rhine water is pretreated with coagulation and rapid sand filtration. After transportation over 55 km the following treatment was performed: ozonation, biological activated carbon filtration and slow sand filtration. Subsequently RO is tested. The RO installation is a three staged tree; in sequence four, two parallel and 1 installed pressure vessels are passed by the water (4-2-1). Each pressure vessel is loaded with 6 (4 x 40") membrane elements from Fluid Systems (TFC--4821 ULP). With hydrochloric acid dosage it is possible to obtain a recovery of 85%. Feed water quality parameters are given in Table 1 (Plant 3). 4. Results
4.1. Plant 1: Nanofiltration of bank filtrate
After a period of about 200 days the NPD
(actual pressure drop normalized for flow and temperature, i.e. viscosity) increased exponentially from about 200 to 400 kPa within a period of 14 days (Fig. 1). Simultaneously the normalizedflux (mass transfer coefficient, MTC, rn/s*kPa) over the tree stages declined. Also the BFR as observed in the biofilm monitor which was continuously fed with acidified feed water increased to about 300 pg ATP/cm2.d (Fig. 1). Cleaning (arrows with C in Fig. 1) had no or only a temporary effect on the NPD value. The first and last (fourth) placed membrane elements from the first stage were removed - before cleaning - 244 days after starting up the installation and autopsies were performed to determine the factors causing the operational problems. The last cleaning action was performed 14 days before removal, within these two weeks again a strong increase of the NPD - from 260 to 400 kPa - was observed. Visual inspection of unfolded membrane envelopes indicated that a dark brown mucous layer was present on the feed side of membranes. This slimy layer was easily removed from the unfolded membrane surface. The print from the feed spacer was still visible in the slime layer after removal of the feed spacer. Clearly less brown deposition was observed on the edges (width of about 10 cm) of the envelopes, were the membranes are glued onto each other. Visual inspection revealed no differences between the two membrane elements in the amount of deposition (intensity of color, amount of slime) on the two membrane elements. Biological analysis of NF elements showed that high biofilm densities were present on the feed side of the membrane (Table 2). All biomass (bacteria, yeasts, fungi, and protozoans) was localized on the feed side and the amount of biomass was equally divided over the membrane and the feed spacer. Biofilm densities on the product side of the membrane and on the product spacer were below the detection limit of the ATP analysis (< 8 pg ATP/cm 2) with a TDC value of about 5 x 103 cells/cm 2 and much lower than the densities o b s e r v e d on the feed side (cf. Table 2).
S. Vrouwenvelderet al./ Desalination 118 (1998) 157-166
500
)tant 1
25000
C C A,C
20000 ~IE O
400
15000 C~ ck Z
.~
300
200
<
......
q ....
~ / L~,- " ,8 0,'
r 5000
i~
,,.-, : i O
100 100
150
200
250
days
Fig. 1. Development of the normalized pressure drop (NPD) over the first stage of the nanofiltrationinstallation and biofilm formation in the biofilm monitor supplied with the acidified feed water. Arrows indicate chemical cleanings (C) and the moment of collecting elements (A) for autopsy. Microscopic investigation of the collected biomass revealed the presence of strongly fluorescing rod shaped cells on the feed side of the membrane indicating that these microorganisms were in good condition as the result of a sufficient nutrient supply. Also clusters (each containing several hundreds of cells) of microorganisms were observed. These clustered cells were embedded in a slightly-fluorescing layer suggesting the presence of extracellular slime. Cells present at the product side were round and small. Table 2 Density and composition of the biofilm on the membranes (feed spacer, membrane and product spacer) collected at elevated NPD (plant 1). Stage 1 Parameter
Element 1
Element 4
ATP (pg ATP/cm2)
15000
2100
TDC (cells/cm2)
2.1 x l0 s
1.1 x 108
HPC (CFU/cm2)
1.8 x 10v
1.9 X
10 6
Biomass densities were highest in the first element. Over the length of this element a decrease of the ATP concentration was observed, but not TDC and HPC values. Flux measurements perfor-
161
med with individual elements at elevated NPD demonstrated that the first placed membrane elements from the first stages were mainly responsible for the strongly increased NPD. The NPD value over the first placed elements was about four times higher than the value directly after installation. In the other elements a 1.4 increase was observed. Elevated AOC values (up to 27 I.tg C/I) showed that a batch of HC1 used to acidify the NF feed had caused the increased biofilm formation in the elements and in the biofilm monitor. Subsequent analysis revealed that the acid had a relatively high TOC concentration. Also considerable iron densities were found in the elements (100 - 200 mg Fe/m2). Enhanced iron accumulation was also observed in the biofilm monitor in the period of strong biofilm formation. 4.2. Plant 2: Reverse osmosis of ultrafiltrate Within an operational period of about 500 days, periodic increases of the NPD ranging from 220 to values over 400 kPa were observed over the RO installation. Simultaneously MTC values declined gradually. Cleaning had no or only temporary effects on these changes. After 500 days, 60 days after a cleaning action, elements were collected from two stages of the installation for autopsy; viz. elements 1, 3 and 7 from a first stage and elements 1 and 7 from the last stage. The end cap of the last placed element (second stage) showed cracks, indicating telescoping. On the feed side of the membrane envelopes a slime layer with a yellowish color was observed. This color was darker with increasing distance from the feed side. The slime layer seemed rather thin, stickier and more difficult to remove from the membrane than the layer found at Plant 1. A gradual increase of active biomass (ATP) was observed over the first stage of the installation, and a plateau level was found in the second stage (Table 3). However, TDC values were fairly constant in both stages and ranged from 2.9 x 107 to 7.9 x 107 cells/cm2. TDC values of the feedwa-
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Table 3 Biofilm composition (average values) on the membranes (Plant 2). Stage 1 Parameter ATP (pg ATP/cm 2) TDC (cells/cm 2) HPC (CFU/cm 2)
Element 1 240 4.2 x 107 2.8 x 105
Stage 2 Element 3 880 7.9 x 107 8.1 x 105
ter were low (4.9 ×10 s cells/ml; detection limit) indicating an effective (> 3.5. logs) removal by the preceding UF. Microscopic investigation of biomass obtained from the membrane showed that > 99% of the microorganisms present on the membrane were brightly fluorescing, filamentous bacteria which adhered to each other. Some iron deposition (1-2 mg/m 2) was observed. Deposition of BaSO4 was found in the last placed elements of the second stage. Biofilm monitor measurements - conducted in a period of about 75 days - and water quality parameters showed that the feed water quality fluctuated considerably in time. Average BFR values up to approximately 10 pg ATP/cm 2 of the feed water were found. The effect of four different cleaning agents on biofouling was tested (pH range 2 - 13). Equallysized coupons (including feed spacer, product spacer and membrane, stacked) were cut from one slip taken from an envelope (from Stage 1 Element 7, Table 3). Cleaning solutions were prepared according to supplier instructions. Several coupons were placed in the cleaning solutions during half an hour (at 35 and 20 °C). After washing with autoclaved tap water with a low
Element 7 4400 7.6 X 10 7 5.6 x 106
Element 1 2100 2.9 x 107 4.9 x 106
Element 7 3000 4.4 X 10 7 5.4 x l06
ATP concentration coupons were subjected to ultrasonic treatment to completely remove biomass for analysis (see methods). Cleaning agents effectively reduced the ATP concentration (up to 99%) but the removal of biomass (TDC) was much less effective (up to 69%). The cleaning acid B was much less effective in the inactivation of biomass. Two step cleaning procedures gave similar performance as single cleanings under the applied conditions. Table 4 Cleaning efficiency of 4 different agents tested on a membrane envelope (Plant 2, Element 7 from Stage 1) under laboratory conditions. Parameter PH ATP removal (%) TDC removal (%)
A 11 99.3 < 0.03
B 2 52 0
C 12 99.7 62
D 12 98.1 69
One of the most effective cleaning agents (C) was tested on a membrane element.Two RO elements were taken from an installation which had been in operation for two months. Within this period an NPD increase was
Table 5 Average biofilm composition on the membranes before and after treatment with an industrial cleaning agent (C, plant 2). Before cleaning After cleaning Reduction % ATP (pg ATP/cm 2) 450 _+ 100 28 + 7 94 TDC (cells/cm 2) 1,8 x 107 _+1.0 × 1 0 7 1.4 × 107 ----. 5.6 × 106 22 (ns)* HPC (CFU/cm 2) 6.7 × 105 _+2.3 × 105 1.3 × 103 _+2.1 x 102 99.8 * ns: not significant
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observed of about 50 kPa per stage, despite four preventive cleanings - alternating acid (pH 1.8) and caustic soda (pH 11.2). The last cleaning had been performed 13 days before autopsy of the ES elements (Element 1 and 2, Stage 1). After assessing the NPD and MTC of the individual elements a cleaning was performed on one of these elements and following this cleaning again NPD and MTC were determined to assess the effect of cleaning. The effect was less than the variations in the m e a s u r e m e n t s of the NPD and MTC values. ATP and HPC values (Table 5) of the biofilm before cleaning were similar to the values observed with the earlier autopsy (Table 3). The frequent and more recent performed cleanings and/or the shorter run time of the element and/or variations in the water quality did not cause an effect on the biomass parameters. However, cleaning gave a clear reduction of ATP (94%) and HPC (99.8%) values but did not significantly reduce the TDC values. This implies nearly all cells were killed but not removed from the element. The ATP content in the washing solution had increased eightfold despite its instability at the high pH value. The TDC increment was limited and mainly single cells were observed. Also, the iron and manganese densities on the membrane before and after the cleaning did not differ from each other, demonstrating that cleaning had not removed these compounds. 4.3. Plant 3: Reverse osmosis of slow sand filtrate
The installation was operated without cleaning during a period of about 350 days. NPD and
MTC values changed little in time over the three stages but stayed within the criteria for cleaning. These criteria were: (i) MTC drop of 15% compared to the initial MTC-value after loading of the installation, (ii) increase of the NPD over a pressure vessel by 400 kPa. The observed slight fouling is not likely caused by precipitation of BaSO4 the mass balances of all stages fitted well. The modified fouling index (MFI) value of the water was below 1 s/12 suggesting that colloidal fouling did not occur [ 18]. The BFR value of the acidified feed water was below 1 pg ATP/cm2.d. This low value and the low AOC concentrations are indicative for a high degree of biological stability which is typical for slow sand filtrate. TDC values of the feed water revealed relatively high cell numbers (3 - 5 _.+.l0 s cells/ml). Most of the cells were small ( 0 < 0.3 Ixm). Autopsy of four elements was performed after 350 days. From a pressure vessel of the first stage the first and last placed elements, and from a second and third stage the last placed elements were collected. On the feed side of the membranes a colorless slimy layer was found. Bacterial analysis revealed that biofilm densities (ATP, TDC and HPC values) on all the investigated elements were similar (Table 6). Large clusters of cells were present in the sonicate resulting in a relatively large standard deviation of the TDC values. Also here the cells were relatively small. HPC values ranged between 1.2 x 10 6 and 2.2 x 10 6 C F U / c m 2 representing a small percentage of the TDC values. Some iron deposition was found (range 5 - 24 mg/m 2) on the membranes in spite -
Table 6 Average biofilm composition on the membranes (Plant 3)
Parameter ATP (pg ATP/cm 2) TDC (cells/cm 2) HPC (CFU/cm 2)
Stage 1 Element 1 2000 6.6 × 107 1.8 × 10 6
Element 6 2000 1.5 × 108 2.1 x 10 6
Stage 2 Element 6 3200 1.7 × 108 2.2 × 10 6
Stage 3 Element 6 2000 7.9 × l0 T 1.2 x 10 6
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of the fact that iron concentrations in the acidified feed water were below the detection limit (< 0.01 mg/l). Highest densities were found in elements from the first stage. The amount of deposited iron declined gradually over the following stages (results not shown).
5. D i s c u s s i o n
and conclusions
In all cases biomass has been observed on the feed side of the membranes (including spacer). Biomass concentrations at the product side were much lower indicating an efficient removal of both cells and growth substrates. Microscopic examination of the bacteria present at the feed side indicated that these bacteria were active, formed clusters of cells and produced extracellular polymer substances. Hence sufficient nutrients were supplied. The microorganisms at the product side were smaller than those at the feed side and no indications for extracellular substances were obtained. Biofouling clearly was the main cause of the operational problems observed at Plant 1. Within a short period of time the NPD value increased simultaneously with the BFR value in the biofilm monitor. Also the AOC levels in the feed water increased from about 12 to 27 I.tg C/1. This fast increase was found to be due to the use of a batch of impure hydrochloric acid. The highest NPD and the highest biofilm densities were found in the first placed membrane elements. The loose structure of the biomass present on the unfolded membrane suggests that it had been produced rapidly and recently. From these observations it can be derived that biomass density values as observed in element 1 (Table 2) are causing severe clogging of the element. Moreover such clogging occurs when the water has a BFR value of about 300 pg ATP/cm~.d. These experiences show that chemicals dosed to water feeding membrane installations should not contain easily degradable components, supporting earlier reports about
biofouling caused by the use of impure acid and/or anti scalants. In one case the time between cleanings could be prolonged from one month to longer then 10 months after stopping the anti-scalant dosage [19, 20]. However, even well defined - chemical pure grade - acid needs close attention and monitoring, since handling and transport may introduce nutrients contributing to biofouling. Also organic anti scalants can cause biofouling of membranes in lead elements in the first stage of an RO-installation [21]. Flocon 100 was used as anti-scalant which contributed to the TOC level of the feed water. Once biofouling has occurred as a result of the use of one improper batch it is possible that irreversible operational problems lead to replacement of elements. In other situations no clear relations between the densities of biomass (ATP, TDC, HPC) on the membranes and operational problems such as increased NPD and declined MTC were observed. ATP levels of about 1000 to 2000 pg ATP/cm 2 and TDC values of about 1-2 x 10 s cells/cm 2 may give rise to increasing MTC values. However in these situations other fouling processes such as organic fouling or scaling may Occur.
The spacial distribution of the biomass concentration in the three different membrane systems shows striking differences (Fig. 2). At Plant 1 the highest biomass densities were found in the first placed element. As explained above, high concentrations of easily degradable nutrients present in the feed water will lead to severe growth of microorganisms followed by clogging of the first placed elements. In Plant 2 a gradual increase of biomass (ATP, HPC) over the first stage were observed. In the following stages the biomass density remained at the same level. The relatively low biomass (ATP and HPC) concentration observed in the first element might have been due to the presence of trace concentration of a cleaning agent originating from the preceding UF unit. Another possibility is that an increasing concentration of biomass and biodegradable com-
S. Vrouwenvelder et al. / Desalination 118 (1998) 157-166
2-10 4
...... 0 n
10 3
•
i.-
/
i/ 10 2 stage
1
stage 2
stage 3
Fig. 2. Distribution of the amount of active biomass (ATP, TDC and HPC) in the elements over the stages of the membrane installations. pounds as caused by the membrane filtration process results in increasing biomass accumulation in the elements. However this tendency did not occur in the second stage and was not confirmed with the TDC values. Such an increase of the amount of biological foulant in elements over a three staged RO-production plant has been reported [22]. Plant 3 shows a constant biofilm density over all stages suggesting that an increasing concentration of cells and growth substrates did not cause increased biomass accumulation on the membranes in this situation. Consequently, predicting the spatial distribution and the concentration of biomass in elements remains difficult on the basis of the obtained data. Most likely different processes occur happen simultaneously affecting water quality at the feed side of the membrane. Microbial activity is causing a decrease of the AOC concentration and the concentration of bacteria and AOC in the water may increase simultaneously during passage through the various stages. Finally, also the flow rate in the elements at the feed side is decreasing over the stages. Cleaning resultsed in a clear - but only temporary - decrease of ATP and HPC values. However, the observations presented above show that chemical cleaning does not always effectively remove cells from the element and operational problems can remain. Probably the structure and the composition of the biofilm, i.e. the pre-
165
sence of extra cellular components (slime) and the shape of the bacterial cells affect biofouling and the effect of cleaning. Removal of biofilms using enzymes capable of degrading polysaccarides has been reported [23]. Experience will show to what extent such enzymes are capable of removing biofouling because the nature of biomass differs from location to location. Laboratory studies demonstrated that biofilm streamers in turbulent water can influence the pressure drop [24]. This is in agreement with (i) findings of (i) branched filamentous biofilm structures on the feed spacer and on the feed side of the membrane, and (ii) the finding that increased pressure drops are not necessarily related to MTC declines. Further research should include analysis of the extracellular polymers. The lowest BFR value (< 1 pg ATP/cm2.d) was found in water feeding an RO installation without direct operational problems (Plant 3) and which did not reach the cleaning criteria within 350 days of operation. The relatively high biomass concentrations and biomass activity on the membranes may have been due to the relatively high total cell counts (3-5 × 105 cell/ml). The BFR found at Plant 2 (10 pg ATP/cm2.d) is difficult to relate to the observed problems because: (i) the BFR fluctuated in time indicating that the water quality varied, (ii) the BFR was determined during a relative short period compared to the run time of the membranes, (iii) highest NPD values were found before the biofilm monitor measurements were conducted and (iv) the water quality was affected by seasonal changes. The presented data demonstrated that biofilm formation occurred in all NF and RO systems. A biomass density of 15,000 pg ATP/cm ~ is causing a strong NPD increase, and values > 2000 pg ATP/cm 2 may be resonsible for MTC increases. Controlling biofouling can be achieved by (i) a far going removal of degradable components from the feedwater (ii) securing the purity of the chemicals dosed, (iii) performing effective cleaning procedures. Also cleaning procedures
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applied when fouling is not a problem might delay biofilm formation. Acknowledgements
The investigations described above have been performed as part o f the Integrated Membrane Systems project (IMS) conducted by a number of water supply companies in the Netherlands and in the US: KIWA, the University of Central Florida ( U C F ) and the A m e r i c a n W a t e r W o r k s Association Research Foundation (AWWA-RF). The IMS project is part o f the Joint Research P r o g r a m o f the N e t h e r l a n d s W a t e r W o r k s Association (VEVIN) and the Research Program o f the A m e r i c a n W a t e r W o r k s A s s o c i a t i o n Research Foundation (AWWA-RF). The authors acknowledge the significant contributions o f S i m o n in 't Veld (Water Supply C o m p a n y o f O v e r i j s s e l N.V. W M O ) , H a r r y Scheerman (N.V. PWN Water Supply Company of N o a h Holland) and Paul Bonn6 (Amsterdam Water Supply) to the study.
References
[1] Kruithof, J.C., J.C. Schippers and J.C van Dijk. J. Water SRT-Aqua, vol. 42, 2 (1994) 47 - 57. [2] Schippers, J.C., J.C. Kruithof. H20, 30 (1997) 179 - 182. [3] Jacangelo, J.G., J-M., LaSn6, K.E. Carns, E.W. Cummings and J. Mallevialle., J. AWWA, 83, 9 (1991) 97 - 106. [4] Amjad, Z. (Eds.), Reverse Osmosis. Membrane Technology, Water Chemistry and Industrial Applications. Van Nostrand Reinhold, New York (1992). [5] Ridgway, H.E and H.C. Hemming. Membrane fouling. Chapter 6 in Water Treatment Membrane Processes (F_xls.J. Mallevialle, P.E. Odendaal, M.R. Wiesner). McGraw-Hill, New York. (1996). [6] Flemming, H-C. Biofouling bei membraanprozessen. Springer Verlag Berlin Heidelberg (1995).
[7] Flemming, H-C., G. Schaule, T. Griebe, Wat. Sci. Tech., 34, 5 (1996) 517 - 542. [8] Flemming, H-C., T. Griebe, G. Schaule. J. Schmitt and A. Tamachkiarowa. Desalination, 113 (1997) 215 - 225. [9] Paul, D. Ultrapure water (May-June 1996) 64 - 67. [10] Van der Kooij, D., H.R. Veenendaal, C. BaarsLorist, D.W. van der Klift and Y.C. Drost. Wat. Res., 29, 7 (1995) 1655 - 1662. [11] Hobbie, J.E., R.J. Daley, S. Jasper. Applied and Environmental Microbiology, 33, 51 (May 1997) 225 - 1228. [12] ASTM Standards on material and environmental microbiology. 2'~ ed. (1993) 31-36. [13] Reasoner D.J. and E.E. Geldreich. E.E. Appl. Envir. Microbiol., 49 (1985) 1-7. [14] Van der Kooij, D. J. Am. Water Works Assoc., 84 (1992) 57-65. [15] Van der Kooij, D., H.S. Vrouwenvelder, H.R. Veenendaal. Wat. Sci. Tech., 32, 8, (1995) 61 -65. [16] Van der Kooij, D., J.S. Vrouwenvelder, H.R. Veenendaal. H 2 O, 25 (1997) 767 - 771. [17] Kamp. P. IWSA/AIDE international workshop membranes in drinking water production, Paris (Mar. 1995). [18] Schippers, J.C. PhD Thesis. Kiwa N.V. Research and ConsultancyAdvice N.V.. The Netherlands (1989). [19] Hiemstra, P., J van Paassen, B. Rietman, M. Nederlof, J. Verdouw. Membrane Technology Conference Proceedings, New Orleans, LA. February 23-26 (1997) 857 - 881. [20] Hoek, J.P., P.A.C. Bonn6, E.A.M. van Soest, A. Graveland, Proc. IWSA/AIDE international workshop membranes in drinking water production, Pads (Mar. 1995) 107- 111. [21] Carnahan, R.P., L. Bolin and W. Suratt. Desalination, 102 (1995) 235-244. [22] Ridgway, H.E, A. Kelly, C. Justice and B.H. Olson. Applied and Environmental Microbiology, (Mar 1983) 1066-1084. [23] Patent number WO 9631610. Membrane Technology, 83 (1997). [24] Stoodley P., Z. Lewandowski, J.D. Boyle, H.M. Lappin-Scott. Biotechnology and Bioengeneering, 57, 5 (Mar. 5, 1998) 536 - 544.
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,
t.,
DESALINATION
ELSEVIER
Desalination 118 (1998) 157-166
Biofouling of membranes for drinking water production H. S. Vrouwenvelder ~*, J. A.M. van Paassen b, H. C. Folmer ~, Jan A.M.H. Hofman d, M. M. Nederlof a, D. van der Kooij a Kiwa N. V., Research and Consultancy, Groningenhaven 7, P.O. Box 1072, 3430 BB Nieuwegein, The Netherlands Tel. +31 30 6069680. Fax +31 30 60 61 165. E-mail: [email protected] bWater Supply Company of Overijssel N. V, cN. V. PWN Water Supply Company of North Holland, ~Amsterdam WaterSupply. Received 27 June 1998 Abstract
In three pilot plants in the Netherlands the performance of either nanofiltration or reverse osmosis in water treatment was studied. Operational problems observed in these systems, viz. increased normalized pressure drop (NPD) and/or declined normalized flux (MTC) values, were attributed to biofouling. To elucidate the role of biofouling, data were collected about biomass accumulation in the membrane elements applying destructive autopsies of the membrane units. Biomass parameters included: total direct cell counts (TDC), adenosinetriphosphate (ATP) analysis and heterotropic plate counts (HPC). Maximum values of biomass parameters at the feed side of the membranes were: 2.1 x 108 cells/cm 2, 1.5 x 104 pg ATP/cm 2 and 1.8 × 10 7 CFU/cm 2, respectively. Microscopic observations of biomass obtained from the membrane systems indicated that the observed organisms were metabolically active in most cases. AOC levels in the feed water were relatively low. In one case, an increase had been observed following the addition of acid to prevent CaCO 3 scaling and also a high biofilm formation rate was observed in relation with a strong NPD increase. Cleaning agents tested under laboratory conditions and in practice did reduce ATP concentrations, but TDC values revealed that biomass removal from the membrane was very limited. These observations show that operational problems caused by biofouling are difficult to solve by cleaning. Therefore, biomass accumulation in membrane elements should be limited by: (i), achieving a far going removal of growth-promoting compounds and micro-organisms from the feedwater, (ii), securing the purity of the chemicals dosed, and (iii), applying effective cleaning procedures for biomass removal. Keywords: Biofouling; Biofilm; Reverse osmosis; Nanofiltration; Membranes; Drinking water; Autopsy; Cleaning agents. 1. Introduction
M e m b r a n e filtration (MF) techniques are very promising in water treatment because of their potential to remove particles, including
microorganisms (disinfection), organic pollutants (pesticides, taste, odor), inorganic compounds (softening, salt) and to achieve biologically-stable water to limit microbial regrowth in the distributionsystem [1, 2]. Additionally, nanofiltration
*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 Seicaee B.V. All rights reserved.
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(NF), reverse osmosis (RO) and even ultrafiltration (UF) can be used to remove viruses [3]. However, fouling of membranes does cause operational problems. Fouling mechanisms include: inorganic scaling (BaSO4, CaCO3), organic scaling (humic acids), colloidal scaling (suspended particles) and biofouling [4]. Different types of fouling can occur simultaneously, influencing each other, e.g. biofouling might enhance concentration polarization which stimulates inorganic scaling [4]. Biofouling - defined as accumulation of microorganisms, i.e. biofilms on a surface by growth and/or deposition at such a level that it is causing operational problems - is difficult to identify. It may cause problems such as flux reduction and/or increased pressure drops during NF or RO leading to early replacement of the membranes. Biofouling phenomena have been described extensively [5,6]. The diagnosis "biofouling" is only justified when a relation is found between the encountered operational problems and biofilm accumulation as described with an adequate parameter. A suite of biomass parameters is available [7], but the question remains which parameter(s) at which level is conclusive for identifying biofouling as the cause of the operational problems and at what level biofilm densities are acceptable. Controlling biological fouling is considered to be a major challenge in operating membrane filtration installations [8,9]. Operational problems attributed to biofouling were observed in several pilot plants in the Netherlands. This study was conducted to: (i), collect information about the amount and the position of biomass in either NF or RO membrane elements, and (ii), to elucidate the relationship between the biomass parameters and operational problems as increased NPD and/or declined MTC. 2. Methods 2.1. Biomass parameters
Biomass parameters which were used to determine the concentration of biomass in the water or on the membranes included: adenosine
triphosphate (ATP), total direct cell counts (TDC) and heterotropic plate counts (HPC). ATP (pg ATP/ml or pg ATP/cm 2) gives an indication of the total amount of active biomass [10]. ATP concentrations are measured by applying an enzymatic reaction using luciferase. The amount of light produced is determined and the ATP concentration is derived from the linear relationship between light production and reference ATP concentrations. TDC's (cells/cm 2) were determined with epifluorescence microscopy using acridine orange as fluorochrome - applying a slightly adapted method to eliminate fading [11,12]. All fluorescing cells are counted but TDC does not discriminate between active and inactive cells. Microscopic observations also give information about the variety and appearance of microorganisms present. HPC's values (CFU/cm 2) are determined by spreading 0.05 ml volumes of water on nutrient poor R2A medium [13]. Colonies were counted after 10 d of incubation at 25 °C. All biomass parameters have been determined in volumes of water collected either directly or from decimal dilutions of water, the suspension either obtained after ultrasonic treatment of the envelope components or prepared with the material collected from the membrane surface. 2.2. Autopsy of membrane elements
Elements were taken from the membrane filtration to determine the amount of accumulated biomass in case of operational problems or at the end of a test period. Directly after collection, the elements were transported and stored at low temperature previous to autopsy, which was usually performed within 24 hours after collection. Following visual inspection of the feed and product side the element was opened mechanically. After visual inspection of unfolded membrane envelopes pieces of membrane and spacer were collected from different locations over the length of the element, using sterile scissors and forceps. The samples were transferred to autoclaved tap
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water and sonicated. Subsequently, the obtained suspension was analyzed to assess the total amount of biomass (ATP, TDC and HPC) present on the membrane, the feed spacer, and the product spacer collected either combined or individually. Swabbing was used to collect biomass either from the feed or the product side of the membrane. Iron and manganese concentrations were determined in the suspensions obtained after acid treatment of the membrane coupons with boiling aqua regia. 2.3. Feed water analysis
In addition to physicochemical water quality parameters (including TOC, pH, temperature), the concentration of biomass (TDC, ATP and HPC) and the biological stability were assessed. For the latter purpose easily assimilable organic carbon (AOC) measurements for determining the heterotropic growth potential of the water [14] and the biofilm formation rate (BFR, pg ATP/cm2.d) were determined. For assessment of the BFR value of the water a biofilm monitor is used in which the accumulation of active biomass (ATP) is determined as a function of time [10,15,16].
3. Pilot plants Three different pilot plants for testing the performance of membrane filtration in surface water treatment were included in this study. Pretreatment in these pilot plants prior to either
NF of RO is characterized as: river bank filtration, ultrafiltration and slow sand filtration, respectively. Table 1 shows feed water quality parameters at the three locations. 3.1. Plant 1
The application of NF for the reduction of hardness, color and pesticides from river-bank filtrate was investigated in a pilot plant at Vechterweerd (Water Supply C o m p a n y of Overijssel). Water from the river Vecht was collected as bank filtrate (residence time of approximately two months). Treatment includes: plate aeration, dual media filtration, aeration, rapid sand filtration and NF. The installation is constructed as three staged (5-3-2), with 5, 3 and 2 pressure vessels in the first, second and third stage, respectively. Each stage contained four 4x40" spiral wound membrane elements (type Hydranautics PVD-1). Addition of hydrochloric acid to the feed water of the NF installation - to prevent scaling of C a C O 3 - made it possible to operate the plant at 80% recovery. 3.2. Plant 2
At Andijk, (PWN Water Supply Company of North Holland) water from Lake IJsselmeer is collected after 34 day storage in an impoundment storage reservoir. Treatment includes: micro straining, coagulation, sedimentation (sludge blanket filtration), rapid sand filtration, UF followed by
Table 1 Water quality parameters (average and range) of the feed water at the three locations after addition of hydrochloric acid Parameter Temperature, °C ATP(ng/I) AOC (~tg C/l) DOC (mg/I) Iron (mg/l) Manganese (mg/1)
Plant 1 10.5-12 8 ( 4 - 14) 17 (8 - 27) 6 (5.6 - 6.8) 0.025 (0.020 - 0.030) < 0.01
Plant 2 2-22 3 (1 - 10) 8 3 < 0.04 < 0.01
Plant 3 12-25 5 ( 3 - 8) 6 ( 3 - 10) 1.4 (1.2 - 2.2) < 0.01 < 0.01
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RO. RO is studied for the removal of organic micropollutants (pesticides), hardness, salt and disinfection [17]. The relatively high salt concentrations in the Lake IJssel are a problem; Dutch guideline values (sodium 120 mg/1 and chloride 150 mg/l) are exceeded in dry years. The two staged (4-2) RO installation with a capacity of 240 m3/d was loaded with a total of 42 Hydranautics 4040-UHA-ESPA spiral wound elements (7 in each pressure vessel). A recovery of 80% was achieved by adding hydrochloric acid. The combined concentrate from the first stage was fed to the second stage. Data on water quality (as achieved after pretreatment and after addition of hydrochloric acid) are presented in Table 1 (Plant 2). 3.3. Plant 3
RO performance was also studied in a pilot plant at Leiduin (Amsterdam Water Supply). The objective of this study was to evaluate direct treatment including RO as an alternative option for dune passage to increase the production capacity. RO is necessary for desalination, softening, removal of pesticides and other organic pollutants and deisinfection. River Rhine water is pretreated with coagulation and rapid sand filtration. After transportation over 55 km the following treatment was performed: ozonation, biological activated carbon filtration and slow sand filtration. Subsequently RO is tested. The RO installation is a three staged tree; in sequence four, two parallel and 1 installed pressure vessels are passed by the water (4-2-1). Each pressure vessel is loaded with 6 (4 x 40") membrane elements from Fluid Systems (TFC--4821 ULP). With hydrochloric acid dosage it is possible to obtain a recovery of 85%. Feed water quality parameters are given in Table 1 (Plant 3). 4. Results
4.1. Plant 1: Nanofiltration of bank filtrate
After a period of about 200 days the NPD
(actual pressure drop normalized for flow and temperature, i.e. viscosity) increased exponentially from about 200 to 400 kPa within a period of 14 days (Fig. 1). Simultaneously the normalizedflux (mass transfer coefficient, MTC, rn/s*kPa) over the tree stages declined. Also the BFR as observed in the biofilm monitor which was continuously fed with acidified feed water increased to about 300 pg ATP/cm2.d (Fig. 1). Cleaning (arrows with C in Fig. 1) had no or only a temporary effect on the NPD value. The first and last (fourth) placed membrane elements from the first stage were removed - before cleaning - 244 days after starting up the installation and autopsies were performed to determine the factors causing the operational problems. The last cleaning action was performed 14 days before removal, within these two weeks again a strong increase of the NPD - from 260 to 400 kPa - was observed. Visual inspection of unfolded membrane envelopes indicated that a dark brown mucous layer was present on the feed side of membranes. This slimy layer was easily removed from the unfolded membrane surface. The print from the feed spacer was still visible in the slime layer after removal of the feed spacer. Clearly less brown deposition was observed on the edges (width of about 10 cm) of the envelopes, were the membranes are glued onto each other. Visual inspection revealed no differences between the two membrane elements in the amount of deposition (intensity of color, amount of slime) on the two membrane elements. Biological analysis of NF elements showed that high biofilm densities were present on the feed side of the membrane (Table 2). All biomass (bacteria, yeasts, fungi, and protozoans) was localized on the feed side and the amount of biomass was equally divided over the membrane and the feed spacer. Biofilm densities on the product side of the membrane and on the product spacer were below the detection limit of the ATP analysis (< 8 pg ATP/cm 2) with a TDC value of about 5 x 103 cells/cm 2 and much lower than the densities o b s e r v e d on the feed side (cf. Table 2).
S. Vrouwenvelderet al./ Desalination 118 (1998) 157-166
500
)tant 1
25000
C C A,C
20000 ~IE O
400
15000 C~ ck Z
.~
300
200
<
......
q ....
~ / L~,- " ,8 0,'
r 5000
i~
,,.-, : i O
100 100
150
200
250
days
Fig. 1. Development of the normalized pressure drop (NPD) over the first stage of the nanofiltrationinstallation and biofilm formation in the biofilm monitor supplied with the acidified feed water. Arrows indicate chemical cleanings (C) and the moment of collecting elements (A) for autopsy. Microscopic investigation of the collected biomass revealed the presence of strongly fluorescing rod shaped cells on the feed side of the membrane indicating that these microorganisms were in good condition as the result of a sufficient nutrient supply. Also clusters (each containing several hundreds of cells) of microorganisms were observed. These clustered cells were embedded in a slightly-fluorescing layer suggesting the presence of extracellular slime. Cells present at the product side were round and small. Table 2 Density and composition of the biofilm on the membranes (feed spacer, membrane and product spacer) collected at elevated NPD (plant 1). Stage 1 Parameter
Element 1
Element 4
ATP (pg ATP/cm2)
15000
2100
TDC (cells/cm2)
2.1 x l0 s
1.1 x 108
HPC (CFU/cm2)
1.8 x 10v
1.9 X
10 6
Biomass densities were highest in the first element. Over the length of this element a decrease of the ATP concentration was observed, but not TDC and HPC values. Flux measurements perfor-
161
med with individual elements at elevated NPD demonstrated that the first placed membrane elements from the first stages were mainly responsible for the strongly increased NPD. The NPD value over the first placed elements was about four times higher than the value directly after installation. In the other elements a 1.4 increase was observed. Elevated AOC values (up to 27 I.tg C/I) showed that a batch of HC1 used to acidify the NF feed had caused the increased biofilm formation in the elements and in the biofilm monitor. Subsequent analysis revealed that the acid had a relatively high TOC concentration. Also considerable iron densities were found in the elements (100 - 200 mg Fe/m2). Enhanced iron accumulation was also observed in the biofilm monitor in the period of strong biofilm formation. 4.2. Plant 2: Reverse osmosis of ultrafiltrate Within an operational period of about 500 days, periodic increases of the NPD ranging from 220 to values over 400 kPa were observed over the RO installation. Simultaneously MTC values declined gradually. Cleaning had no or only temporary effects on these changes. After 500 days, 60 days after a cleaning action, elements were collected from two stages of the installation for autopsy; viz. elements 1, 3 and 7 from a first stage and elements 1 and 7 from the last stage. The end cap of the last placed element (second stage) showed cracks, indicating telescoping. On the feed side of the membrane envelopes a slime layer with a yellowish color was observed. This color was darker with increasing distance from the feed side. The slime layer seemed rather thin, stickier and more difficult to remove from the membrane than the layer found at Plant 1. A gradual increase of active biomass (ATP) was observed over the first stage of the installation, and a plateau level was found in the second stage (Table 3). However, TDC values were fairly constant in both stages and ranged from 2.9 x 107 to 7.9 x 107 cells/cm2. TDC values of the feedwa-
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Table 3 Biofilm composition (average values) on the membranes (Plant 2). Stage 1 Parameter ATP (pg ATP/cm 2) TDC (cells/cm 2) HPC (CFU/cm 2)
Element 1 240 4.2 x 107 2.8 x 105
Stage 2 Element 3 880 7.9 x 107 8.1 x 105
ter were low (4.9 ×10 s cells/ml; detection limit) indicating an effective (> 3.5. logs) removal by the preceding UF. Microscopic investigation of biomass obtained from the membrane showed that > 99% of the microorganisms present on the membrane were brightly fluorescing, filamentous bacteria which adhered to each other. Some iron deposition (1-2 mg/m 2) was observed. Deposition of BaSO4 was found in the last placed elements of the second stage. Biofilm monitor measurements - conducted in a period of about 75 days - and water quality parameters showed that the feed water quality fluctuated considerably in time. Average BFR values up to approximately 10 pg ATP/cm 2 of the feed water were found. The effect of four different cleaning agents on biofouling was tested (pH range 2 - 13). Equallysized coupons (including feed spacer, product spacer and membrane, stacked) were cut from one slip taken from an envelope (from Stage 1 Element 7, Table 3). Cleaning solutions were prepared according to supplier instructions. Several coupons were placed in the cleaning solutions during half an hour (at 35 and 20 °C). After washing with autoclaved tap water with a low
Element 7 4400 7.6 X 10 7 5.6 x 106
Element 1 2100 2.9 x 107 4.9 x 106
Element 7 3000 4.4 X 10 7 5.4 x l06
ATP concentration coupons were subjected to ultrasonic treatment to completely remove biomass for analysis (see methods). Cleaning agents effectively reduced the ATP concentration (up to 99%) but the removal of biomass (TDC) was much less effective (up to 69%). The cleaning acid B was much less effective in the inactivation of biomass. Two step cleaning procedures gave similar performance as single cleanings under the applied conditions. Table 4 Cleaning efficiency of 4 different agents tested on a membrane envelope (Plant 2, Element 7 from Stage 1) under laboratory conditions. Parameter PH ATP removal (%) TDC removal (%)
A 11 99.3 < 0.03
B 2 52 0
C 12 99.7 62
D 12 98.1 69
One of the most effective cleaning agents (C) was tested on a membrane element.Two RO elements were taken from an installation which had been in operation for two months. Within this period an NPD increase was
Table 5 Average biofilm composition on the membranes before and after treatment with an industrial cleaning agent (C, plant 2). Before cleaning After cleaning Reduction % ATP (pg ATP/cm 2) 450 _+ 100 28 + 7 94 TDC (cells/cm 2) 1,8 x 107 _+1.0 × 1 0 7 1.4 × 107 ----. 5.6 × 106 22 (ns)* HPC (CFU/cm 2) 6.7 × 105 _+2.3 × 105 1.3 × 103 _+2.1 x 102 99.8 * ns: not significant
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observed of about 50 kPa per stage, despite four preventive cleanings - alternating acid (pH 1.8) and caustic soda (pH 11.2). The last cleaning had been performed 13 days before autopsy of the ES elements (Element 1 and 2, Stage 1). After assessing the NPD and MTC of the individual elements a cleaning was performed on one of these elements and following this cleaning again NPD and MTC were determined to assess the effect of cleaning. The effect was less than the variations in the m e a s u r e m e n t s of the NPD and MTC values. ATP and HPC values (Table 5) of the biofilm before cleaning were similar to the values observed with the earlier autopsy (Table 3). The frequent and more recent performed cleanings and/or the shorter run time of the element and/or variations in the water quality did not cause an effect on the biomass parameters. However, cleaning gave a clear reduction of ATP (94%) and HPC (99.8%) values but did not significantly reduce the TDC values. This implies nearly all cells were killed but not removed from the element. The ATP content in the washing solution had increased eightfold despite its instability at the high pH value. The TDC increment was limited and mainly single cells were observed. Also, the iron and manganese densities on the membrane before and after the cleaning did not differ from each other, demonstrating that cleaning had not removed these compounds. 4.3. Plant 3: Reverse osmosis of slow sand filtrate
The installation was operated without cleaning during a period of about 350 days. NPD and
MTC values changed little in time over the three stages but stayed within the criteria for cleaning. These criteria were: (i) MTC drop of 15% compared to the initial MTC-value after loading of the installation, (ii) increase of the NPD over a pressure vessel by 400 kPa. The observed slight fouling is not likely caused by precipitation of BaSO4 the mass balances of all stages fitted well. The modified fouling index (MFI) value of the water was below 1 s/12 suggesting that colloidal fouling did not occur [ 18]. The BFR value of the acidified feed water was below 1 pg ATP/cm2.d. This low value and the low AOC concentrations are indicative for a high degree of biological stability which is typical for slow sand filtrate. TDC values of the feed water revealed relatively high cell numbers (3 - 5 _.+.l0 s cells/ml). Most of the cells were small ( 0 < 0.3 Ixm). Autopsy of four elements was performed after 350 days. From a pressure vessel of the first stage the first and last placed elements, and from a second and third stage the last placed elements were collected. On the feed side of the membranes a colorless slimy layer was found. Bacterial analysis revealed that biofilm densities (ATP, TDC and HPC values) on all the investigated elements were similar (Table 6). Large clusters of cells were present in the sonicate resulting in a relatively large standard deviation of the TDC values. Also here the cells were relatively small. HPC values ranged between 1.2 x 10 6 and 2.2 x 10 6 C F U / c m 2 representing a small percentage of the TDC values. Some iron deposition was found (range 5 - 24 mg/m 2) on the membranes in spite -
Table 6 Average biofilm composition on the membranes (Plant 3)
Parameter ATP (pg ATP/cm 2) TDC (cells/cm 2) HPC (CFU/cm 2)
Stage 1 Element 1 2000 6.6 × 107 1.8 × 10 6
Element 6 2000 1.5 × 108 2.1 x 10 6
Stage 2 Element 6 3200 1.7 × 108 2.2 × 10 6
Stage 3 Element 6 2000 7.9 × l0 T 1.2 x 10 6
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of the fact that iron concentrations in the acidified feed water were below the detection limit (< 0.01 mg/l). Highest densities were found in elements from the first stage. The amount of deposited iron declined gradually over the following stages (results not shown).
5. D i s c u s s i o n
and conclusions
In all cases biomass has been observed on the feed side of the membranes (including spacer). Biomass concentrations at the product side were much lower indicating an efficient removal of both cells and growth substrates. Microscopic examination of the bacteria present at the feed side indicated that these bacteria were active, formed clusters of cells and produced extracellular polymer substances. Hence sufficient nutrients were supplied. The microorganisms at the product side were smaller than those at the feed side and no indications for extracellular substances were obtained. Biofouling clearly was the main cause of the operational problems observed at Plant 1. Within a short period of time the NPD value increased simultaneously with the BFR value in the biofilm monitor. Also the AOC levels in the feed water increased from about 12 to 27 I.tg C/1. This fast increase was found to be due to the use of a batch of impure hydrochloric acid. The highest NPD and the highest biofilm densities were found in the first placed membrane elements. The loose structure of the biomass present on the unfolded membrane suggests that it had been produced rapidly and recently. From these observations it can be derived that biomass density values as observed in element 1 (Table 2) are causing severe clogging of the element. Moreover such clogging occurs when the water has a BFR value of about 300 pg ATP/cm~.d. These experiences show that chemicals dosed to water feeding membrane installations should not contain easily degradable components, supporting earlier reports about
biofouling caused by the use of impure acid and/or anti scalants. In one case the time between cleanings could be prolonged from one month to longer then 10 months after stopping the anti-scalant dosage [19, 20]. However, even well defined - chemical pure grade - acid needs close attention and monitoring, since handling and transport may introduce nutrients contributing to biofouling. Also organic anti scalants can cause biofouling of membranes in lead elements in the first stage of an RO-installation [21]. Flocon 100 was used as anti-scalant which contributed to the TOC level of the feed water. Once biofouling has occurred as a result of the use of one improper batch it is possible that irreversible operational problems lead to replacement of elements. In other situations no clear relations between the densities of biomass (ATP, TDC, HPC) on the membranes and operational problems such as increased NPD and declined MTC were observed. ATP levels of about 1000 to 2000 pg ATP/cm 2 and TDC values of about 1-2 x 10 s cells/cm 2 may give rise to increasing MTC values. However in these situations other fouling processes such as organic fouling or scaling may Occur.
The spacial distribution of the biomass concentration in the three different membrane systems shows striking differences (Fig. 2). At Plant 1 the highest biomass densities were found in the first placed element. As explained above, high concentrations of easily degradable nutrients present in the feed water will lead to severe growth of microorganisms followed by clogging of the first placed elements. In Plant 2 a gradual increase of biomass (ATP, HPC) over the first stage were observed. In the following stages the biomass density remained at the same level. The relatively low biomass (ATP and HPC) concentration observed in the first element might have been due to the presence of trace concentration of a cleaning agent originating from the preceding UF unit. Another possibility is that an increasing concentration of biomass and biodegradable com-
S. Vrouwenvelder et al. / Desalination 118 (1998) 157-166
2-10 4
...... 0 n
10 3
•
i.-
/
i/ 10 2 stage
1
stage 2
stage 3
Fig. 2. Distribution of the amount of active biomass (ATP, TDC and HPC) in the elements over the stages of the membrane installations. pounds as caused by the membrane filtration process results in increasing biomass accumulation in the elements. However this tendency did not occur in the second stage and was not confirmed with the TDC values. Such an increase of the amount of biological foulant in elements over a three staged RO-production plant has been reported [22]. Plant 3 shows a constant biofilm density over all stages suggesting that an increasing concentration of cells and growth substrates did not cause increased biomass accumulation on the membranes in this situation. Consequently, predicting the spatial distribution and the concentration of biomass in elements remains difficult on the basis of the obtained data. Most likely different processes occur happen simultaneously affecting water quality at the feed side of the membrane. Microbial activity is causing a decrease of the AOC concentration and the concentration of bacteria and AOC in the water may increase simultaneously during passage through the various stages. Finally, also the flow rate in the elements at the feed side is decreasing over the stages. Cleaning resultsed in a clear - but only temporary - decrease of ATP and HPC values. However, the observations presented above show that chemical cleaning does not always effectively remove cells from the element and operational problems can remain. Probably the structure and the composition of the biofilm, i.e. the pre-
165
sence of extra cellular components (slime) and the shape of the bacterial cells affect biofouling and the effect of cleaning. Removal of biofilms using enzymes capable of degrading polysaccarides has been reported [23]. Experience will show to what extent such enzymes are capable of removing biofouling because the nature of biomass differs from location to location. Laboratory studies demonstrated that biofilm streamers in turbulent water can influence the pressure drop [24]. This is in agreement with (i) findings of (i) branched filamentous biofilm structures on the feed spacer and on the feed side of the membrane, and (ii) the finding that increased pressure drops are not necessarily related to MTC declines. Further research should include analysis of the extracellular polymers. The lowest BFR value (< 1 pg ATP/cm2.d) was found in water feeding an RO installation without direct operational problems (Plant 3) and which did not reach the cleaning criteria within 350 days of operation. The relatively high biomass concentrations and biomass activity on the membranes may have been due to the relatively high total cell counts (3-5 × 105 cell/ml). The BFR found at Plant 2 (10 pg ATP/cm2.d) is difficult to relate to the observed problems because: (i) the BFR fluctuated in time indicating that the water quality varied, (ii) the BFR was determined during a relative short period compared to the run time of the membranes, (iii) highest NPD values were found before the biofilm monitor measurements were conducted and (iv) the water quality was affected by seasonal changes. The presented data demonstrated that biofilm formation occurred in all NF and RO systems. A biomass density of 15,000 pg ATP/cm ~ is causing a strong NPD increase, and values > 2000 pg ATP/cm 2 may be resonsible for MTC increases. Controlling biofouling can be achieved by (i) a far going removal of degradable components from the feedwater (ii) securing the purity of the chemicals dosed, (iii) performing effective cleaning procedures. Also cleaning procedures
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applied when fouling is not a problem might delay biofilm formation. Acknowledgements
The investigations described above have been performed as part o f the Integrated Membrane Systems project (IMS) conducted by a number of water supply companies in the Netherlands and in the US: KIWA, the University of Central Florida ( U C F ) and the A m e r i c a n W a t e r W o r k s Association Research Foundation (AWWA-RF). The IMS project is part o f the Joint Research P r o g r a m o f the N e t h e r l a n d s W a t e r W o r k s Association (VEVIN) and the Research Program o f the A m e r i c a n W a t e r W o r k s A s s o c i a t i o n Research Foundation (AWWA-RF). The authors acknowledge the significant contributions o f S i m o n in 't Veld (Water Supply C o m p a n y o f O v e r i j s s e l N.V. W M O ) , H a r r y Scheerman (N.V. PWN Water Supply Company of N o a h Holland) and Paul Bonn6 (Amsterdam Water Supply) to the study.
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