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Early warning of biofouling in spiral wound nanofiltration and reverse osmosis membranes
Desalination 265 (2011) 206–212
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Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
Early warning of biofouling in spiral wound nanofiltration and reverse osmosis membranes J.S. Vrouwenvelder a,b,⁎, M.C.M. van Loosdrecht b, J.C. Kruithof a a b
Wetsus, Centre of Excellence for Sustainable Water Technology, Agora 1, P.O. Box 1113, 8900 CC Leeuwarden, The Netherlands Delft University of Technology, Department of Biotechnology, Environmental Biotechnology Group, Julianalaan 67, 2628 BC Delft, The Netherlands
a r t i c l e
i n f o
Article history: Received 21 May 2010 Received in revised form 19 July 2010 Accepted 20 July 2010 Available online 24 August 2010 Keywords: Biofouling monitor Biofilm Feed spacer channel Linear flow velocity NF RO Costs
a b s t r a c t In spiral wound nanofiltration and reverse osmosis installations several fouling types may occur. Simultaneous screening of all fouling types could be carried out to establish the impact of each individual fouling type on membrane performance. In extensively pre-treated water biofouling is the major fouling type. Membrane manufacturers recommend to take corrective actions based on a 15% pressure drop increase criterion. In general this approach is not successful. For an adequate anti-biofouling strategy early warning monitoring plays an essential role. Early warning of biofouling requires (i) a Membrane Fouling Simulator (MFS) supplied with feed water of the membrane filtration installation, (ii) a sensitive differential pressure drop transmitter over the MFS to monitor the pressure drop increase, and (iii) a higher linear water velocity in the MFS compared to practical conditions to increase the biofilm formation rate and pressure drop, enabling earlier detection. Action based on this early warning monitoring system for biofouling control is more reliable and successful than the approach recommended by membrane manufacturers and the costs are a fraction only of the potential annual savings. © 2010 Elsevier B.V. All rights reserved.
1. Introduction An early warning monitoring system should be an essential part of an adequate anti-biofouling strategy for reverse osmosis (RO) and nanofiltration (NF). Earlier detection of biofouling enables corrective actions at an earlier stage. This strategy is expected to be more effective than control actions after a pressure drop increase of 15% over the total installation, as recommended by membrane manufacturers, when already much biomass is accumulated at the feed side of the membrane installation. The feed water quality plays a major role in membrane biofouling. This suggests the use of a biological parameter to assess the biofouling potential of the feed water. Vrouwenvelder et al. [1] evaluated the use of feed water parameters such as adenosinetriphosphate (ATP) [2] and total direct cell counts [3], representing the concentration of microorganisms, Assimilable Organic Carbon (AOC) [4] as a measure for growth promoting substances, and the Biofilm Formation Rate (BFR) in a glass-ring biofilm monitor [5]. These water quality parameters ATP, total direct cell counts, AOC and the BFR were not sensitive enough to be used for early warning of biofouling [1]. In extensively pre-treated water the pressure drop increase over the membrane modules in the installation is directly related to ⁎ Corresponding author. Wetsus, centre of excellence for sustainable water technology, Agora 1, P.O. box 1113, 8900 CC Leeuwarden, The Netherlands. E-mail address: [email protected] (J.S. Vrouwenvelder). 0011-9164/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2010.07.053
membrane biofouling [6,7]. Standard pressure drop measurements commonly used in practice are not sensitive enough to quantify biofouling timely and therefore cannot be used as an early warning system enabling control actions [1]. More sensitive and accurate differential pressure drop transmitters – than commonly used standard pressure equipment – are required for early warning of biofouling. In addition to sensitive differential pressure transmitter use early warning monitoring of biofouling also requires conditions facilitating more early detection such as a higher linear water velocity causing a higher biofouling accumulation rate. Recently, interest in early warning system for biofouling has been growing [8,9]. Several methods for monitoring of biofouling/biofilm accumulation have been described [10–21]. The ideal tool for biofouling monitoring in spiral wound membrane systems has to meet a large number of requirements [12,13,16], e.g. representativeness and sensitive in-situ non destructive assessment of performance loss and fouling. However, no available monitor was fulfilling the combination of requirements. Therefore, a tool was developed for the validation of membrane fouling: the Membrane Fouling Simulator [22–24]. The major advantages of the MFS are representativeness for spiral wound membranes and the small size requiring small amounts of water and chemicals. Using the MFS, fouling can be monitored by (1) operational parameters like pressure drop, (2) non-destructive (visual, microscopic) observations using the sight glass and (3) analysis of coupons sampled from the membrane and spacer sheet in the MFS.
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The objective of this study was to develop an early warning system for biofouling enabling preventive actions. 2. Effect of permeate production on biofouling In spiral wound membrane modules, two types of pressure drop can be discriminated: the trans membrane pressure drop (TMP) and the feed spacer channel pressure drop (FCP). The TMP is the differential pressure between feed and permeate lines, describing the frictional resistance over the membrane. The TMP is related to the membrane flux (permeation rate). The definition of flux (permeation rate) is the water volume flowing through the membrane per unit area and time (L m−2 h−1). The FCP is the pressure drop between the feed and brine lines [25]. The FCP-increase was the same for membrane elements at the same position in a nanofiltration pilot plant (Fig. 1A, [26]) operated with and without permeate production. Periodically, the permeate side of the membrane elements operated without permeate production was shortly (20 min) opened to determine permeate production and normalized flux. The normalized flux of the nanofiltration installation was not declining during 146 days of operation. With and without permeate production, elements at the same position in the installation had the same normalized flux, which was constant in time (Fig. 1B). For fouling diagnosis, after 146 days of continuous operation autopsies of membrane elements operated with and without flux were performed. In all membrane elements, the total concentration of inorganic compounds was below the detection limit of the analysis performed (≤0.2 mg m−2 membrane surface area), illustrating that the performance of the installation in this study was not hampered by accumulation of inorganic compounds (scaling). In membrane elements operated with and without permeate production at the same position in the installation the same biomass concentration was found (Fig. 1C, lead membrane modules from stage 1). The pressure drop and biofilm development in the feed channel were not affected by the (absence of) flux in membrane elements. The high biomass concentrations in the lead modules were not affecting the normalized flux but resulted in a pressure drop increase over the feed channel of the membrane modules (Fig. 1, [26]). The membrane flux is related to the TMP. When the flux is not affected, the TMP is not affected. The influence of biofouling on flux decline and feed channel pressure drop increase has also been studied in parallel on monitor, test rig and full-scale for extensively pre-treated water [27]. The same concentrations of biomass were observed in the two MFSs (without
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permeate production), in the test rigs and in the full-scale installation (with permeate production), indicating that permeate production was not influencing biofouling. In other words, the feed spacer channel pressure drop and biofilm concentration increased, irrespective of permeate flux applied, supporting the results of the NF pilot plant studies. So it was shown that permeation was not significantly accelerating biofouling. Calculations on mass transfer aspects supported the observations that the flux is not playing a significant role in substrate supply to the fouling layer [26]. Test-rig and full-scale studies with different types of feed water showed that biofouling of membrane modules correlated very well with FCP-increase [1]. Moreover, in systems suffering from biofouling cleaning cycles are governed by the pressure drop over the feed channel. Therefore, biofouling is considered a feed spacer channel problem [26,27]. 3. Localisation of biofouling in NF/RO installations High biomass concentrations were observed in the first stage RO/ NF lead modules during studies in practice [1,7,28,29]. A systematic study on biofouling development in individual membrane modules and stages of a NF pilot plant showed a feed channel pressure drop increase over the lead membrane module higher than the pressure drop increase over the following membrane modules (Fig. 2). Also the lead module gave the highest contribution to the pressure drop over the total installation [7]. In agreement with these observations, most biomass was present in the lead element. Biofouling studies using a flat sheet monitor with the length of a membrane module showed a stronger feed channel pressure drop increase and higher biomass concentrations in the first half of the monitor compared to the second half, indicating that biofouling was located at the inlet side of lead membrane modules [23]. High biomass concentrations on the feed side of NF/RO installations cause most of the total pressure drop increase. Evidently, early detection of biofouling requires measurements in that part of the installation where most of the biofouling develops, being the feed side of the NF/RO installation. 4. Biofouling monitoring In a suitable early warning system, biofouling should be detected much earlier than in the membrane installation.
Fig. 1. Feed channel pressure drop (A) and normalized flux (B) over nanofiltration lead membrane modules in time (pilot plant data). Biomass concentrations (C) in the membrane modules determined after 146 days continuous operation. The elements were operated with and without flux for 146 days. Periodically, the permeate side of the element operated without flux was shortly (about 20 min) opened to determine the flux. The flux was normalized for pressure and temperature. Adapted from [26].
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5. Methods for measuring biofouling
Fig. 2. Pressure drop in time over individual membrane elements in the first pressure vessel of a NF installation (adapted from [7]).The number represents the position of the elements in the pressure vessel from the feed side (1 = lead module and 6 = element on the concentrate side of the pressure vessel).
A small tool with an effective length of 0.20 m named Membrane Fouling Simulator (MFS) was developed. This tool can be used to study membrane biofouling by (i) operational parameters like the pressure drop, (ii) non-destructive (visual, microscopic) observations using the MFS sight window and (iii) analysis of coupons sampled from the membrane sheet in the MFS [22]. Comparison studies of the monitor with test rigs and a full-scale installation showed the same pressure drop development and biomass accumulation [22], indicating that the MFS is a representative tool to monitor biofouling. In general, biomass starts to accumulate at the inlet side of the lead membrane module of the first stage (Fig. 2). Therefore, monitoring of the differential pressure with a sensitive differential pressure transmitter over (i) a MFS supplied with feed water of the membrane filtration installation and/or (ii) lead membrane module of the first stage will enable early detection of biofouling (Fig. 3). For early warning monitoring, the MFS supplied with feed water of the installation is preferred over monitoring a lead membrane module in the installation or a test rig. Biomass predominantly accumulates on the inlet side of lead membrane modules only [1,23], which equals the total length of the membrane and spacer sheets present in the MFS. This enables the MFS to provide more early detection of biofouling (Fig. 3). Using the same feed water as a full-scale installation suffering from biofouling problems, within a month the pressure drop increase over the MFS was 6.6 times higher compared to a (40 in long commercially available) membrane module in a test rig, operated with the same linear velocity [22]. Over the total installation, the pressure drop increase would have been even lower than in the test rig, illustrating that the MFS is most ideally suited to provide early warning monitoring of biofouling.
Characklis [6] stated that biofouling causes energy losses due to increased fluid frictional resistance (e.g. in pipelines and in porous media like spiral wound membranes), indicating that in spiral wound membrane systems biofouling may cause a pressure drop increase. Obviously, since the pressure drop increase is the operational problem, the pressure drop must be measured. In practice, pressure is measured before and after stages of the membrane filtration installation, using pressure transmitters. Then, the pressure drop is derived by subtracting the two measured pressures. The pressure drop is relatively small compared to both pressures, especially for high pressure membrane processes NF and RO. So, since two large numbers for the measured pressures must be subtracted, already a small error in the measured pressure has a large impact on the pressure drop [7]. A factor 325 to 800 times higher accuracy for the pressure drop value can be obtained by using sensitive differential pressure drop transmitters instead of standard, high precision pressure transmitters [7]. Robust sensitive differential pressure drop transmitters have shown to be useful tools in membrane biofouling studies [7,26,27,30]. Biofouling inhibitor dosing to the MFS feed water showed within 17 days that this chemical caused a high pressure drop increase and high biomass concentrations compared to the blank: the biofouling inhibitor was not inhibiting biofouling, but was even contributing to biofouling [31], illustrating the potential of MFS studies combined with a differential pressure drop transmitters. In addition to the pressure drop over the MFS, observations using the sight window and analysis of the accumulated material from the MFS will provide information on the fouling type present [22,27,30]. The biomass parameters adenosinetriphosphate, a measure for active biomass and the total direct cell count are suitable parameters for diagnosis of biofouling [1]. 6. Improving sensitivity of fouling detection An increase of the linear flow velocity has a non-linear effect on the pressure drop (see formula 1). Δp = λ⋅
ρ⋅v2 L ⋅ 2 dh
ð1Þ
where λ is the friction coefficient, ρ the specific liquid density, v the linear velocity, L the length of the membrane or MFS and dh the hydraulic diameter. MFS studies showed that both the pressure drop of a non-fouled system and the pressure drop increase caused by biomass accumulation were affected by the linear flow velocity. In other words, the pressure drop increase caused by a constant biomass concentration is a function of linear flow velocity (Fig. 4). The largest difference between pressure drop and pressure drop increase was observed for the highest linear flow velocity, indicating that early detection of biofouling should be carried out at high linear flow velocities (Fig. 4), improving the accuracy of the pressure drop measurement. Also a temporarily linear flow velocity increase will enable earlier detection of biofouling. 7. Increasing rate of biofouling
Fig. 3. Biofouling is earlier detected when differential pressure monitoring is applied at a Membrane Fouling Simulator (MFS) supplied with feed water of the membrane filtration installation. The major biofouling zone indicates the area where biofouling starts.
A higher linear flow velocity resulted in an initial higher pressure drop (Figs. 2 and 5A; t:0), a faster and stronger pressure drop increase in time (Fig. 5A) and a stronger biomass accumulation after 8 days of operation (Fig. 5B) [32]. In other words, continuous operation at a high linear flow velocity increased (i) the pressure drop range and accuracy of the pressure drop measurement (Fig. 4) and (ii) the biofilm accumulation rate, both enabling earlier biofouling detection. A linear flow velocity of about 0.33 m s−1 in the MFS containing a feed
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Table 1 Experimental conditions for the monitors with the same substrate load, as acetate carbon, and different linear flow velocities and substrate concentrations. Monitor No.
Linear velocity (m s−1)
Feed (L h−1)
Acetate concentration (μg L−1)
Acetate load (μg m−2 s−1)
1 2 3 4 5
0.041 0.082 0.123 0.163 0.245
4 8 12 16 24
400 200 133 100 66
56 56 56 56 56
9. Early warning strategy
Fig. 4. Pressure drop as a function of the linear flow velocity (adapted from [32]), illustrating that the effect of biomass concentration on pressure drop increase is affected by linear flow velocity. The difference between the pressure drop in nonfouling conditions and pressure drop in fouling conditions is the pressure drop increase caused by accumulated fouling (biomass).
spacer (thickness 31 mil, porosity 0.85) from practice results in a feed flow of 32 L h−1 indicating that high linear velocities can be obtained with restricted water use. An inventory of full-scale NF/RO installations showed that linear flow velocities in lead modules ranged between 0.07 and 0.20 m s−1 [32]. 8. Impact of substrate load Comparative studies with MFSs were performed with varying linear flow velocities at a constant substrate load (Table 1). The pressure drop increase increased with linear flow velocity (Fig. 6), but the accumulated amount of biomass in the monitors at the end of the research period was not significantly different (Fig. 6). The same amounts of accumulated biomass in the monitors operated with different linear flow velocities and acetate concentrations but the same acetate load indicate that biomass accumulation was substrate load related. Replicated experiments showed similar results for various biomass parameters (data not shown). In other words, in the monitors supplied with the same substrate load, the amount of accumulated biomass was constant but the pressure drop increase was a function of the linear flow velocity. Biomass accumulation was related to the substrate load (substrate concentration and linear flow velocity), illustrating that increasing the linear flow velocity, thereby increasing the substrate load, resulted in a higher biomass accumulation rate (Fig. 5).
Executing the proposed early warning biofouling strategy requires (i) a MFS supplied with feed water of the membrane filtration installation, (ii) a sensitive differential pressure drop transmitter over the MFS for monitoring the pressure drop increase, and (iii) an increased linear water velocity in the MFS compared to the lead membrane modules of the full-scale installation. A scheme of the MFS system for early warning monitoring of biofouling and the MFS are shown in Fig. 7 and equipment details are provided in the supplementary data. The MFS has multiple possibilities to detect fouling quickly and accurately. Measuring the pressure drop with a sensitive differential pressure transmitter over the MFS may provide a simple early warning system by triggering an alarm when the pressure drop exceeds a set point. The use of the transparent window and the destructive study of membrane coupons from the MFS also facilitate (early) detection of fouling [22,27]. Membrane manufacturers recommend corrective actions when the pressure drop increase over the total installation is 15% of the start-up value determined under “industrial conditions” and restrict guarantees when this pressure drop increase is exceeded. The 15% pressure drop increase criterion over the total installation, used in practice, is not a well defined guideline to take corrective actions. When the pressure drop over the total installation is 15%, in general the pressure drop increase over a lead module will be much higher than 15% while the pressure drop increase over the other elements in the pressure vessel will be much lower than 15% and can be even close to zero [7]. The use of low pressure NF/RO membranes will even cause earlier exceeding the 15% increase criterion while the absolute pressure drop is still relatively low. So monitoring the pressure drop over the total installation or complete stages is not suitable for early biofouling detection. Monitoring the pressure drop over a lead membrane module from the first stage can be a – less attractive – alternative for
Fig. 5. Pressure drop development (A) and biomass concentration in the monitor (B) after 8 day operation with different linear flow velocities and with the same concentration biodegradable compound (200 μg acetate-C L−1) in the feed water (adapted from [32]).
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Fig. 6. Pressure drop increase (A) and amount of accumulated biomass in the membrane fouling simulator (B) and after 11 day operation with the same acetate load and different linear flow velocities. Adapted from [32].
Fig. 7. Scheme of early warning monitoring system for biofouling (A) including connection to feed water of a NF/RO installation, a pressure reducing valve and manometer (I), the MFS and differential pressure transmitter (II), and flow controller (III). The MFS (B) can be used without (IV) and with a sight window (V).
the MFS, because monitoring the lead module requires connections for pressure drop measurements on the pressure vessel, while a (temporary) operation at a high velocity is not always a realistic option. Separating the lead module from the rest of the NF/RO installation can provide the same operational possibilities as with the MFS, but realizing the necessary adaptations in practice is complex. 10. Monitoring all fouling types Other fouling types such as scaling, particulate and organic fouling can reduce membrane performance as well. However, extensive pretreatment such as ultrafiltration will avoid other fouling types than
Table 2 Setup for assessment of the contribution of individual fouling mechanisms using four MFSs in parallel fed with feed water (taken from [22]). MFS
Fouling
Pretreatment
Production
MFS content
Dosage of biocide
Analysis
1 2
biofouling particulate fouling NOM
UF no
no no
spacer spacer
no yes
ΔP ΔP
ILC/UF
yes
yes
MTC
all fouling types
no
yes
spacer + RO membrane spacer + RO membrane
no
ΔP + MTC
3 4
UF = ultrafiltration; ΔP = pressure drop; ILC = in line coagulation; MTC = normalized flux.
biofouling causing a pressure drop increase over the lead module in the first pressure vessel. Practical experience has shown that not foreseen fouling types may contribute to reduced membrane performance [1]. Therefore, insight in all fouling types occurring in NF/RO installations enables actions aimed at specific fouling types. Adaptation of pre-treatment may be required to control fouling [1]. Fouling mechanisms can be studied individually or simultaneously by systematic MFS use. The obtained information can be used for a better pre-treatment selection to overcome (site specific) fouling. Also, the contribution and interaction of individual fouling types related to practical fouling problems can be unravelled. The effect of pre-treatment on biofouling, particulate and organic fouling (also named NOM-fouling) can be determined within one setup, using four MFSs in parallel fed with the feed water (Table 2, Fig. 8). Scaling can be studied by monitoring the concentrate of the installation. Therefore, a monitor has to concentrate the water thereby producing permeate. For this purpose an adapted MFS is under development [31]. 11. Long term strategy MFS use for early warning When early warning of biofouling has been detected, the question arises whether the membrane and spacer sheets in the MFS should be replaced by new sheets or not. Preferred is not to replace the sheets but apply for the MFS the same cleanings as for the full-scale installation. Subsequently, the MFS can be used to determine cleaning efficiency and continue the early warning monitoring of biofouling.
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Fig. 8. Scheme showing locations to monitor biofouling, particulate fouling, organic fouling and scaling in the concentrate stream.
When the MFS sheets need to be replaced, the MFS with new sheets will still be predictive for biofouling, since only the biofouling location is monitored and the biofouling accumulation rate in the MFS is higher, albeit the accumulated biomass in the MFS will temporarily be lower than in the lead elements of the full-scale installation. The effects of MFS membrane and spacer replacement in relation to early warning can be determined using several MFSs (with and without membrane and spacer replacement) in parallel.
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The costs of an early warning monitoring system are a fraction only of the potential annual savings in BWRO and SWRO plants. In addition to biofouling monitoring, the MFS can also be used for selecting and optimizing of pre-treatment of NF/RO installations, for selection of chemicals dosed to the feed water (e.g. acids, dechlorinators, flocculants, antiscalants and biofouling inhibitors) and for optimizing cleaning strategies to reduce biofouling in a more cost effective way. Also an approach to screen all fouling types simultaneously has been proposed since in practice other types of fouling may occur. Acknowledgements This work was performed at Wetsus, Centre of Excellence for Sustainable Water Technology. Wetsus is funded by the Ministry of Economic Affairs. The authors like to thank the participants of the theme ‘Biofouling’ for the fruitful discussions and their financial support. The input of Jacques van Paassen, Simon Bakker, Arie Zwijnenburg, Wim Borgonje, and Harm van der Kooi is fully acknowledged.
12. Cost effective early warning Appendix A. Supplementary data The costs related to biofouling are generally composed of (i) additional energy costs: increase of pressure drop requires an increase of feed pressure, (ii) additional chemical cleaning (and waste disposal) and related manpower as well as increased down time, and (iii) decrease of membrane life, impairing the reliability of the installation. Furthermore, to avoid excessive cleaning, membrane plants suffering from biofouling are generally operated outside the manufacturer's warrantee condition of less than 15% increase of the normalized pressure drop increase over the total installation between cleanings. Early biofouling warning enables to take preventive measures, either by pre-treatment optimizing or by preventive membrane cleaning. Cleaning at an early stage requires less chemicals and less down time and lowers the risk of irreversible fouling and membrane damage. Cleaning was more successful at early stage biofouled membrane systems [33]. The potential savings of an early warning of biofouling are estimated to be 10% of the annual membrane replacement and chemical costs (including labour for cleaning) and between 2 and 5% of energy costs. The costs of membrane filtration are based on average operational data from well designed installations [34,35]. The annual savings are based on an average capacity of 1000 m3 h−1 for brackish water reverse osmosis (BWRO) and 10,000 m3 h−1 for sea water reverse osmosis (SWRO), and an average electricity rate of 0.10 $ kWh−1. The costs of an early warning monitoring system are a fraction only of the potential annual savings (Table 3). 13. Concluding remarks An early warning monitoring system for biofouling is developed. Early warning of biofouling requires (i) a Membrane Fouling Simulator (MFS) supplied with feed water of the membrane filtration installation, (ii) a sensitive differential pressure drop transmitter over the MFS for monitoring the feed channel pressure drop increase, and (iii) a high linear water velocity over the MFS.
Table 3 Potential annual savings by early warning of biofouling.
BWRO SWRO
Energy (kWh m−3)
Membranes ($ m−3)
Chemicals ($ m−3)
Average capacity (m−3 h−1)
Potential annual savings ($)
0.5–1.0 3.5–4.2
0.014–0.028 0.05–0.1
0.03 0.05–0.06
1000 10,000
70,000 2,000,000
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[20] A. Pereira, J. Mendes, L.F. Melo, Using nanovibrations to monitor biofouling, Biotechnol. Bioeng. 99 (2008) 1407–1415. [21] E. Eguía, A. Trueba, B. Río-Calonge, A. Girón, J.J. Amieva, C. Bielva, Combined monitor for direct and indirect measurement of biofouling, Biofouling 24 (2008) 75–86. [22] J.S. Vrouwenvelder, J.A.M. Van Paassen, L.P. Wessels, A.F. Van Dam, S.M. Bakker, The membrane fouling simulator: a practical tool for fouling prediction and control, J. Membr. Sci. 281 (2006) 316–324. [23] J.S. Vrouwenvelder, S.M. Bakker, M. Cauchard, R. Le Grand, M. Apacandie, M. Idrissi, S. Lagrave, L.P. Wessels, J.A.M. Van Paassen, J.C. Kruithof, M.C.M. Van Loosdrecht, The membrane fouling simulator: a suitable tool for prediction and characterization of membrane fouling, Water Sci. Techn. 55 (2007) 197–205. [24] J.S. Vrouwenvelder, S.M. Bakker, L.P. Wessels, J.A.M. Van Paassen, The membrane fouling simulator as a new tool for biofouling control of spiral wound membranes, Desalination 204 (2007) 170–174. [25] J. Mallevialle, P.E. Odendaal, M.R. Wiesner, Water Treatment Membrane Processes, McGraw-Hill, New York, 1996. [26] J.S. Vrouwenvelder, J.A.M. Van Paassen, J.M.C. Van Agtmaal, M.C.M. Van Loosdrecht, J.C. Kruithof, A critical flux to avoid biofouling of spiral wound nanofiltration and reverse osmosis membranes: fact or fiction? J. Membr. Sci. 326 (2009) 36–44. [27] J.S. Vrouwenvelder, D.A. Graf von der Schulenburg, J.C. Kruithof, M.L. Johns, M.C.M. van Loosdrecht, Biofouling of spiral wound nanofiltration and reverse osmosis membranes: a feed spacer problem, Water Res. 43 (2009) 583–594. [28] R.P. Carnahan, L. Bolin, W. Suratt, Biofouling of PVD-1 reverse osmosis elements in the water treatment plant of the city of Dunedin, Florida, Desalination 102 (1995) 235–244.
[29] H.S. Vrouwenvelder, J.A.M. Van Paassen, H.C. Folmer, J.A.M.H. Hofman, M.M. Nederlof, D. Van der Kooij, Biofouling of membranes for drinking water production, Desalination 118 (1998) 157–166. [30] J.S. Vrouwenvelder, J. Buiter, M. Riviere, W.G.J. Van der Meer, M.C.M. Van Loosdrecht, J.C. Kruithof, Impact of flow regime on pressure drop increase and biomass accumulation and morphology in membrane systems, Water Res. 44 (2010) 689–702. [31] J.S. Vrouwenvelder, C. Hinrichs, A.R. Sun, F. Royer, J.A.M. Van Paassen, S.M. Bakker, W.G.J. Van der Meer, J.C. Kruithof, M.C.M. Van Loosdrecht, Monitoring and control of biofouling in nanofiltration and reverse osmosis membranes, Water Sci. Techn.: Water Supply –WSTWS 8.4 (2008) 449–458. [32] J.S. Vrouwenvelder, C. Hinrichs, W.G.J. Van der Meer, M.C.M. Van Loosdrecht, J.C. Kruithof, Pressure drop increase caused by biofilm accumulation in spiral wound membrane systems: role of substrate load, substrate concentration and flow velocity, Biofouling 25 (2009) 543–555. [33] S.A. Creber, J.S. Vrouwenvelder, M.C.M. Van Loosdrecht, M.L. Johns, Chemical cleaning of biofouling in reverse osmosis membranes evaluating using magnetic resonance imaging, J. Membr. Sci. (2010), doi:10.1016/j.memsci.2010.06.052. [34] N.M. Wade, Distillation plant development and cost update, Desalination 136 (2001) 3–12. [35] M. Wilf, Reverse Osmosis, Nanofiltration and MF/UF Systems Process, Design, Applications and Economics”, 4-day intensive course membrane desalination and membrane water filtration, February 20–23, 2006, L'Aquila, Italy, 2006.
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Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
Early warning of biofouling in spiral wound nanofiltration and reverse osmosis membranes J.S. Vrouwenvelder a,b,⁎, M.C.M. van Loosdrecht b, J.C. Kruithof a a b
Wetsus, Centre of Excellence for Sustainable Water Technology, Agora 1, P.O. Box 1113, 8900 CC Leeuwarden, The Netherlands Delft University of Technology, Department of Biotechnology, Environmental Biotechnology Group, Julianalaan 67, 2628 BC Delft, The Netherlands
a r t i c l e
i n f o
Article history: Received 21 May 2010 Received in revised form 19 July 2010 Accepted 20 July 2010 Available online 24 August 2010 Keywords: Biofouling monitor Biofilm Feed spacer channel Linear flow velocity NF RO Costs
a b s t r a c t In spiral wound nanofiltration and reverse osmosis installations several fouling types may occur. Simultaneous screening of all fouling types could be carried out to establish the impact of each individual fouling type on membrane performance. In extensively pre-treated water biofouling is the major fouling type. Membrane manufacturers recommend to take corrective actions based on a 15% pressure drop increase criterion. In general this approach is not successful. For an adequate anti-biofouling strategy early warning monitoring plays an essential role. Early warning of biofouling requires (i) a Membrane Fouling Simulator (MFS) supplied with feed water of the membrane filtration installation, (ii) a sensitive differential pressure drop transmitter over the MFS to monitor the pressure drop increase, and (iii) a higher linear water velocity in the MFS compared to practical conditions to increase the biofilm formation rate and pressure drop, enabling earlier detection. Action based on this early warning monitoring system for biofouling control is more reliable and successful than the approach recommended by membrane manufacturers and the costs are a fraction only of the potential annual savings. © 2010 Elsevier B.V. All rights reserved.
1. Introduction An early warning monitoring system should be an essential part of an adequate anti-biofouling strategy for reverse osmosis (RO) and nanofiltration (NF). Earlier detection of biofouling enables corrective actions at an earlier stage. This strategy is expected to be more effective than control actions after a pressure drop increase of 15% over the total installation, as recommended by membrane manufacturers, when already much biomass is accumulated at the feed side of the membrane installation. The feed water quality plays a major role in membrane biofouling. This suggests the use of a biological parameter to assess the biofouling potential of the feed water. Vrouwenvelder et al. [1] evaluated the use of feed water parameters such as adenosinetriphosphate (ATP) [2] and total direct cell counts [3], representing the concentration of microorganisms, Assimilable Organic Carbon (AOC) [4] as a measure for growth promoting substances, and the Biofilm Formation Rate (BFR) in a glass-ring biofilm monitor [5]. These water quality parameters ATP, total direct cell counts, AOC and the BFR were not sensitive enough to be used for early warning of biofouling [1]. In extensively pre-treated water the pressure drop increase over the membrane modules in the installation is directly related to ⁎ Corresponding author. Wetsus, centre of excellence for sustainable water technology, Agora 1, P.O. box 1113, 8900 CC Leeuwarden, The Netherlands. E-mail address: [email protected] (J.S. Vrouwenvelder). 0011-9164/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2010.07.053
membrane biofouling [6,7]. Standard pressure drop measurements commonly used in practice are not sensitive enough to quantify biofouling timely and therefore cannot be used as an early warning system enabling control actions [1]. More sensitive and accurate differential pressure drop transmitters – than commonly used standard pressure equipment – are required for early warning of biofouling. In addition to sensitive differential pressure transmitter use early warning monitoring of biofouling also requires conditions facilitating more early detection such as a higher linear water velocity causing a higher biofouling accumulation rate. Recently, interest in early warning system for biofouling has been growing [8,9]. Several methods for monitoring of biofouling/biofilm accumulation have been described [10–21]. The ideal tool for biofouling monitoring in spiral wound membrane systems has to meet a large number of requirements [12,13,16], e.g. representativeness and sensitive in-situ non destructive assessment of performance loss and fouling. However, no available monitor was fulfilling the combination of requirements. Therefore, a tool was developed for the validation of membrane fouling: the Membrane Fouling Simulator [22–24]. The major advantages of the MFS are representativeness for spiral wound membranes and the small size requiring small amounts of water and chemicals. Using the MFS, fouling can be monitored by (1) operational parameters like pressure drop, (2) non-destructive (visual, microscopic) observations using the sight glass and (3) analysis of coupons sampled from the membrane and spacer sheet in the MFS.
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The objective of this study was to develop an early warning system for biofouling enabling preventive actions. 2. Effect of permeate production on biofouling In spiral wound membrane modules, two types of pressure drop can be discriminated: the trans membrane pressure drop (TMP) and the feed spacer channel pressure drop (FCP). The TMP is the differential pressure between feed and permeate lines, describing the frictional resistance over the membrane. The TMP is related to the membrane flux (permeation rate). The definition of flux (permeation rate) is the water volume flowing through the membrane per unit area and time (L m−2 h−1). The FCP is the pressure drop between the feed and brine lines [25]. The FCP-increase was the same for membrane elements at the same position in a nanofiltration pilot plant (Fig. 1A, [26]) operated with and without permeate production. Periodically, the permeate side of the membrane elements operated without permeate production was shortly (20 min) opened to determine permeate production and normalized flux. The normalized flux of the nanofiltration installation was not declining during 146 days of operation. With and without permeate production, elements at the same position in the installation had the same normalized flux, which was constant in time (Fig. 1B). For fouling diagnosis, after 146 days of continuous operation autopsies of membrane elements operated with and without flux were performed. In all membrane elements, the total concentration of inorganic compounds was below the detection limit of the analysis performed (≤0.2 mg m−2 membrane surface area), illustrating that the performance of the installation in this study was not hampered by accumulation of inorganic compounds (scaling). In membrane elements operated with and without permeate production at the same position in the installation the same biomass concentration was found (Fig. 1C, lead membrane modules from stage 1). The pressure drop and biofilm development in the feed channel were not affected by the (absence of) flux in membrane elements. The high biomass concentrations in the lead modules were not affecting the normalized flux but resulted in a pressure drop increase over the feed channel of the membrane modules (Fig. 1, [26]). The membrane flux is related to the TMP. When the flux is not affected, the TMP is not affected. The influence of biofouling on flux decline and feed channel pressure drop increase has also been studied in parallel on monitor, test rig and full-scale for extensively pre-treated water [27]. The same concentrations of biomass were observed in the two MFSs (without
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permeate production), in the test rigs and in the full-scale installation (with permeate production), indicating that permeate production was not influencing biofouling. In other words, the feed spacer channel pressure drop and biofilm concentration increased, irrespective of permeate flux applied, supporting the results of the NF pilot plant studies. So it was shown that permeation was not significantly accelerating biofouling. Calculations on mass transfer aspects supported the observations that the flux is not playing a significant role in substrate supply to the fouling layer [26]. Test-rig and full-scale studies with different types of feed water showed that biofouling of membrane modules correlated very well with FCP-increase [1]. Moreover, in systems suffering from biofouling cleaning cycles are governed by the pressure drop over the feed channel. Therefore, biofouling is considered a feed spacer channel problem [26,27]. 3. Localisation of biofouling in NF/RO installations High biomass concentrations were observed in the first stage RO/ NF lead modules during studies in practice [1,7,28,29]. A systematic study on biofouling development in individual membrane modules and stages of a NF pilot plant showed a feed channel pressure drop increase over the lead membrane module higher than the pressure drop increase over the following membrane modules (Fig. 2). Also the lead module gave the highest contribution to the pressure drop over the total installation [7]. In agreement with these observations, most biomass was present in the lead element. Biofouling studies using a flat sheet monitor with the length of a membrane module showed a stronger feed channel pressure drop increase and higher biomass concentrations in the first half of the monitor compared to the second half, indicating that biofouling was located at the inlet side of lead membrane modules [23]. High biomass concentrations on the feed side of NF/RO installations cause most of the total pressure drop increase. Evidently, early detection of biofouling requires measurements in that part of the installation where most of the biofouling develops, being the feed side of the NF/RO installation. 4. Biofouling monitoring In a suitable early warning system, biofouling should be detected much earlier than in the membrane installation.
Fig. 1. Feed channel pressure drop (A) and normalized flux (B) over nanofiltration lead membrane modules in time (pilot plant data). Biomass concentrations (C) in the membrane modules determined after 146 days continuous operation. The elements were operated with and without flux for 146 days. Periodically, the permeate side of the element operated without flux was shortly (about 20 min) opened to determine the flux. The flux was normalized for pressure and temperature. Adapted from [26].
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5. Methods for measuring biofouling
Fig. 2. Pressure drop in time over individual membrane elements in the first pressure vessel of a NF installation (adapted from [7]).The number represents the position of the elements in the pressure vessel from the feed side (1 = lead module and 6 = element on the concentrate side of the pressure vessel).
A small tool with an effective length of 0.20 m named Membrane Fouling Simulator (MFS) was developed. This tool can be used to study membrane biofouling by (i) operational parameters like the pressure drop, (ii) non-destructive (visual, microscopic) observations using the MFS sight window and (iii) analysis of coupons sampled from the membrane sheet in the MFS [22]. Comparison studies of the monitor with test rigs and a full-scale installation showed the same pressure drop development and biomass accumulation [22], indicating that the MFS is a representative tool to monitor biofouling. In general, biomass starts to accumulate at the inlet side of the lead membrane module of the first stage (Fig. 2). Therefore, monitoring of the differential pressure with a sensitive differential pressure transmitter over (i) a MFS supplied with feed water of the membrane filtration installation and/or (ii) lead membrane module of the first stage will enable early detection of biofouling (Fig. 3). For early warning monitoring, the MFS supplied with feed water of the installation is preferred over monitoring a lead membrane module in the installation or a test rig. Biomass predominantly accumulates on the inlet side of lead membrane modules only [1,23], which equals the total length of the membrane and spacer sheets present in the MFS. This enables the MFS to provide more early detection of biofouling (Fig. 3). Using the same feed water as a full-scale installation suffering from biofouling problems, within a month the pressure drop increase over the MFS was 6.6 times higher compared to a (40 in long commercially available) membrane module in a test rig, operated with the same linear velocity [22]. Over the total installation, the pressure drop increase would have been even lower than in the test rig, illustrating that the MFS is most ideally suited to provide early warning monitoring of biofouling.
Characklis [6] stated that biofouling causes energy losses due to increased fluid frictional resistance (e.g. in pipelines and in porous media like spiral wound membranes), indicating that in spiral wound membrane systems biofouling may cause a pressure drop increase. Obviously, since the pressure drop increase is the operational problem, the pressure drop must be measured. In practice, pressure is measured before and after stages of the membrane filtration installation, using pressure transmitters. Then, the pressure drop is derived by subtracting the two measured pressures. The pressure drop is relatively small compared to both pressures, especially for high pressure membrane processes NF and RO. So, since two large numbers for the measured pressures must be subtracted, already a small error in the measured pressure has a large impact on the pressure drop [7]. A factor 325 to 800 times higher accuracy for the pressure drop value can be obtained by using sensitive differential pressure drop transmitters instead of standard, high precision pressure transmitters [7]. Robust sensitive differential pressure drop transmitters have shown to be useful tools in membrane biofouling studies [7,26,27,30]. Biofouling inhibitor dosing to the MFS feed water showed within 17 days that this chemical caused a high pressure drop increase and high biomass concentrations compared to the blank: the biofouling inhibitor was not inhibiting biofouling, but was even contributing to biofouling [31], illustrating the potential of MFS studies combined with a differential pressure drop transmitters. In addition to the pressure drop over the MFS, observations using the sight window and analysis of the accumulated material from the MFS will provide information on the fouling type present [22,27,30]. The biomass parameters adenosinetriphosphate, a measure for active biomass and the total direct cell count are suitable parameters for diagnosis of biofouling [1]. 6. Improving sensitivity of fouling detection An increase of the linear flow velocity has a non-linear effect on the pressure drop (see formula 1). Δp = λ⋅
ρ⋅v2 L ⋅ 2 dh
ð1Þ
where λ is the friction coefficient, ρ the specific liquid density, v the linear velocity, L the length of the membrane or MFS and dh the hydraulic diameter. MFS studies showed that both the pressure drop of a non-fouled system and the pressure drop increase caused by biomass accumulation were affected by the linear flow velocity. In other words, the pressure drop increase caused by a constant biomass concentration is a function of linear flow velocity (Fig. 4). The largest difference between pressure drop and pressure drop increase was observed for the highest linear flow velocity, indicating that early detection of biofouling should be carried out at high linear flow velocities (Fig. 4), improving the accuracy of the pressure drop measurement. Also a temporarily linear flow velocity increase will enable earlier detection of biofouling. 7. Increasing rate of biofouling
Fig. 3. Biofouling is earlier detected when differential pressure monitoring is applied at a Membrane Fouling Simulator (MFS) supplied with feed water of the membrane filtration installation. The major biofouling zone indicates the area where biofouling starts.
A higher linear flow velocity resulted in an initial higher pressure drop (Figs. 2 and 5A; t:0), a faster and stronger pressure drop increase in time (Fig. 5A) and a stronger biomass accumulation after 8 days of operation (Fig. 5B) [32]. In other words, continuous operation at a high linear flow velocity increased (i) the pressure drop range and accuracy of the pressure drop measurement (Fig. 4) and (ii) the biofilm accumulation rate, both enabling earlier biofouling detection. A linear flow velocity of about 0.33 m s−1 in the MFS containing a feed
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Table 1 Experimental conditions for the monitors with the same substrate load, as acetate carbon, and different linear flow velocities and substrate concentrations. Monitor No.
Linear velocity (m s−1)
Feed (L h−1)
Acetate concentration (μg L−1)
Acetate load (μg m−2 s−1)
1 2 3 4 5
0.041 0.082 0.123 0.163 0.245
4 8 12 16 24
400 200 133 100 66
56 56 56 56 56
9. Early warning strategy
Fig. 4. Pressure drop as a function of the linear flow velocity (adapted from [32]), illustrating that the effect of biomass concentration on pressure drop increase is affected by linear flow velocity. The difference between the pressure drop in nonfouling conditions and pressure drop in fouling conditions is the pressure drop increase caused by accumulated fouling (biomass).
spacer (thickness 31 mil, porosity 0.85) from practice results in a feed flow of 32 L h−1 indicating that high linear velocities can be obtained with restricted water use. An inventory of full-scale NF/RO installations showed that linear flow velocities in lead modules ranged between 0.07 and 0.20 m s−1 [32]. 8. Impact of substrate load Comparative studies with MFSs were performed with varying linear flow velocities at a constant substrate load (Table 1). The pressure drop increase increased with linear flow velocity (Fig. 6), but the accumulated amount of biomass in the monitors at the end of the research period was not significantly different (Fig. 6). The same amounts of accumulated biomass in the monitors operated with different linear flow velocities and acetate concentrations but the same acetate load indicate that biomass accumulation was substrate load related. Replicated experiments showed similar results for various biomass parameters (data not shown). In other words, in the monitors supplied with the same substrate load, the amount of accumulated biomass was constant but the pressure drop increase was a function of the linear flow velocity. Biomass accumulation was related to the substrate load (substrate concentration and linear flow velocity), illustrating that increasing the linear flow velocity, thereby increasing the substrate load, resulted in a higher biomass accumulation rate (Fig. 5).
Executing the proposed early warning biofouling strategy requires (i) a MFS supplied with feed water of the membrane filtration installation, (ii) a sensitive differential pressure drop transmitter over the MFS for monitoring the pressure drop increase, and (iii) an increased linear water velocity in the MFS compared to the lead membrane modules of the full-scale installation. A scheme of the MFS system for early warning monitoring of biofouling and the MFS are shown in Fig. 7 and equipment details are provided in the supplementary data. The MFS has multiple possibilities to detect fouling quickly and accurately. Measuring the pressure drop with a sensitive differential pressure transmitter over the MFS may provide a simple early warning system by triggering an alarm when the pressure drop exceeds a set point. The use of the transparent window and the destructive study of membrane coupons from the MFS also facilitate (early) detection of fouling [22,27]. Membrane manufacturers recommend corrective actions when the pressure drop increase over the total installation is 15% of the start-up value determined under “industrial conditions” and restrict guarantees when this pressure drop increase is exceeded. The 15% pressure drop increase criterion over the total installation, used in practice, is not a well defined guideline to take corrective actions. When the pressure drop over the total installation is 15%, in general the pressure drop increase over a lead module will be much higher than 15% while the pressure drop increase over the other elements in the pressure vessel will be much lower than 15% and can be even close to zero [7]. The use of low pressure NF/RO membranes will even cause earlier exceeding the 15% increase criterion while the absolute pressure drop is still relatively low. So monitoring the pressure drop over the total installation or complete stages is not suitable for early biofouling detection. Monitoring the pressure drop over a lead membrane module from the first stage can be a – less attractive – alternative for
Fig. 5. Pressure drop development (A) and biomass concentration in the monitor (B) after 8 day operation with different linear flow velocities and with the same concentration biodegradable compound (200 μg acetate-C L−1) in the feed water (adapted from [32]).
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Fig. 6. Pressure drop increase (A) and amount of accumulated biomass in the membrane fouling simulator (B) and after 11 day operation with the same acetate load and different linear flow velocities. Adapted from [32].
Fig. 7. Scheme of early warning monitoring system for biofouling (A) including connection to feed water of a NF/RO installation, a pressure reducing valve and manometer (I), the MFS and differential pressure transmitter (II), and flow controller (III). The MFS (B) can be used without (IV) and with a sight window (V).
the MFS, because monitoring the lead module requires connections for pressure drop measurements on the pressure vessel, while a (temporary) operation at a high velocity is not always a realistic option. Separating the lead module from the rest of the NF/RO installation can provide the same operational possibilities as with the MFS, but realizing the necessary adaptations in practice is complex. 10. Monitoring all fouling types Other fouling types such as scaling, particulate and organic fouling can reduce membrane performance as well. However, extensive pretreatment such as ultrafiltration will avoid other fouling types than
Table 2 Setup for assessment of the contribution of individual fouling mechanisms using four MFSs in parallel fed with feed water (taken from [22]). MFS
Fouling
Pretreatment
Production
MFS content
Dosage of biocide
Analysis
1 2
biofouling particulate fouling NOM
UF no
no no
spacer spacer
no yes
ΔP ΔP
ILC/UF
yes
yes
MTC
all fouling types
no
yes
spacer + RO membrane spacer + RO membrane
no
ΔP + MTC
3 4
UF = ultrafiltration; ΔP = pressure drop; ILC = in line coagulation; MTC = normalized flux.
biofouling causing a pressure drop increase over the lead module in the first pressure vessel. Practical experience has shown that not foreseen fouling types may contribute to reduced membrane performance [1]. Therefore, insight in all fouling types occurring in NF/RO installations enables actions aimed at specific fouling types. Adaptation of pre-treatment may be required to control fouling [1]. Fouling mechanisms can be studied individually or simultaneously by systematic MFS use. The obtained information can be used for a better pre-treatment selection to overcome (site specific) fouling. Also, the contribution and interaction of individual fouling types related to practical fouling problems can be unravelled. The effect of pre-treatment on biofouling, particulate and organic fouling (also named NOM-fouling) can be determined within one setup, using four MFSs in parallel fed with the feed water (Table 2, Fig. 8). Scaling can be studied by monitoring the concentrate of the installation. Therefore, a monitor has to concentrate the water thereby producing permeate. For this purpose an adapted MFS is under development [31]. 11. Long term strategy MFS use for early warning When early warning of biofouling has been detected, the question arises whether the membrane and spacer sheets in the MFS should be replaced by new sheets or not. Preferred is not to replace the sheets but apply for the MFS the same cleanings as for the full-scale installation. Subsequently, the MFS can be used to determine cleaning efficiency and continue the early warning monitoring of biofouling.
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Fig. 8. Scheme showing locations to monitor biofouling, particulate fouling, organic fouling and scaling in the concentrate stream.
When the MFS sheets need to be replaced, the MFS with new sheets will still be predictive for biofouling, since only the biofouling location is monitored and the biofouling accumulation rate in the MFS is higher, albeit the accumulated biomass in the MFS will temporarily be lower than in the lead elements of the full-scale installation. The effects of MFS membrane and spacer replacement in relation to early warning can be determined using several MFSs (with and without membrane and spacer replacement) in parallel.
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The costs of an early warning monitoring system are a fraction only of the potential annual savings in BWRO and SWRO plants. In addition to biofouling monitoring, the MFS can also be used for selecting and optimizing of pre-treatment of NF/RO installations, for selection of chemicals dosed to the feed water (e.g. acids, dechlorinators, flocculants, antiscalants and biofouling inhibitors) and for optimizing cleaning strategies to reduce biofouling in a more cost effective way. Also an approach to screen all fouling types simultaneously has been proposed since in practice other types of fouling may occur. Acknowledgements This work was performed at Wetsus, Centre of Excellence for Sustainable Water Technology. Wetsus is funded by the Ministry of Economic Affairs. The authors like to thank the participants of the theme ‘Biofouling’ for the fruitful discussions and their financial support. The input of Jacques van Paassen, Simon Bakker, Arie Zwijnenburg, Wim Borgonje, and Harm van der Kooi is fully acknowledged.
12. Cost effective early warning Appendix A. Supplementary data The costs related to biofouling are generally composed of (i) additional energy costs: increase of pressure drop requires an increase of feed pressure, (ii) additional chemical cleaning (and waste disposal) and related manpower as well as increased down time, and (iii) decrease of membrane life, impairing the reliability of the installation. Furthermore, to avoid excessive cleaning, membrane plants suffering from biofouling are generally operated outside the manufacturer's warrantee condition of less than 15% increase of the normalized pressure drop increase over the total installation between cleanings. Early biofouling warning enables to take preventive measures, either by pre-treatment optimizing or by preventive membrane cleaning. Cleaning at an early stage requires less chemicals and less down time and lowers the risk of irreversible fouling and membrane damage. Cleaning was more successful at early stage biofouled membrane systems [33]. The potential savings of an early warning of biofouling are estimated to be 10% of the annual membrane replacement and chemical costs (including labour for cleaning) and between 2 and 5% of energy costs. The costs of membrane filtration are based on average operational data from well designed installations [34,35]. The annual savings are based on an average capacity of 1000 m3 h−1 for brackish water reverse osmosis (BWRO) and 10,000 m3 h−1 for sea water reverse osmosis (SWRO), and an average electricity rate of 0.10 $ kWh−1. The costs of an early warning monitoring system are a fraction only of the potential annual savings (Table 3). 13. Concluding remarks An early warning monitoring system for biofouling is developed. Early warning of biofouling requires (i) a Membrane Fouling Simulator (MFS) supplied with feed water of the membrane filtration installation, (ii) a sensitive differential pressure drop transmitter over the MFS for monitoring the feed channel pressure drop increase, and (iii) a high linear water velocity over the MFS.
Table 3 Potential annual savings by early warning of biofouling.
BWRO SWRO
Energy (kWh m−3)
Membranes ($ m−3)
Chemicals ($ m−3)
Average capacity (m−3 h−1)
Potential annual savings ($)
0.5–1.0 3.5–4.2
0.014–0.028 0.05–0.1
0.03 0.05–0.06
1000 10,000
70,000 2,000,000
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