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The relation between the RO fouling membrane and the feed water quality and the pretreatment in Djerba Island plant
Desalination 286 (2012) 412–416
Contents lists available at SciVerse ScienceDirect
Desalination journal homepage: www.elsevier.com/locate/desal
The relation between the RO fouling membrane and the feed water quality and the pretreatment in Djerba Island plant Soumaya Farhat a, Fethi Kamel b, Younes Jedoui a, Monem Kallel c,⁎ a b c
Unité de Recherche “Hydrosciences Appliquées, Institut Supérieur des Sciences et Techniques des Eaux de Gabès, Université de Gabès, Zrig, 6072, Gabès, Tunisia Société Nationale d'Exploitation et de Distribution des Eaux,Direction Production Sud Est, BP 525, Médenine 4100, Tunisia Laboratoire de recherche: Eau, Energie et Environnement; Ecole Nationale des Ingénieures de Sfax, Université de Sfax, BP 1173-3038, Sfax, Tunisia
a r t i c l e
i n f o
Article history: Received 27 August 2011 Received in revised form 26 November 2011 Accepted 28 November 2011 Available online 5 January 2012 Keywords: Reverse osmosis Feed water quality Sulfuric water Membrane fouling Autopsy
a b s t r a c t The reverse osmosis system is exposed to many problems that affect its efficiency. The major problem is fouling membrane which is frequently related to the interaction between the quality of feed water and the pretreatment process. This paper treats two problems of RO plant in Djerba Island, the sulfuric water and the fouling membrane. For the first problem, the anaerobic process showed better performance than the aerobic process. Furthermore, the aerobic process of non-sulfuric water was affected after 6 years of operation with a fouling phenomenon at R.O membrane. This problem was correlated with the increase of the operating pressure, pressure drop and permeates conductivity. To identify causes of these problems, different investigations are carried out. Membrane autopsy, chemical analyses of feed water and follow-up of the operation parameters are performed. The membrane autopsy was achieved by SEM/EDS. The results show that the film is composed mostly with organic matter, silica, metallic matter and aluminum phosphate. This result is correlated with the phosphonate based antiscalant composition. Therefore, antiscalant selection and dosage appear to be critical for membrane fouling development. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Many investigations have clearly substantiated the fact that operations of RO units are very sensitive to feed water quality and as such pretreatment is an essential part of membrane desalination process. Pretreatment system for RO plant is designed to produce feed water with a reduced fouling potential by removing particulates, micropollutants and microorganisms as well as preventing the formation of inorganic scales [1–7]. Many problems affect the operating conditions such as permeate flux, recovery factor and the quality of feed. These problems can be caused by a variety of fouling types. The blocking mechanisms of RO membranes include scaling, fouling and biofouling. Scaling of the membrane is caused by super-saturation of salt ions in the feed side of the membrane. Membrane fouling is caused by convective and diffusive transport of suspended or colloidal matter. Therefore, biofouling is caused by proliferation of microorganisms at the membrane surface [8]. Several methods for monitoring of biofouling membrane have been described [9,10]. When fouling is complex and poorly understood, membrane autopsy is a powerful diagnostic tool which can help enhance system performance. The best membrane autopsy approach today is to develop more in situ tools which engage non-destructive analysis of the membrane materials [11]. For an
⁎ Corresponding author. Tel./fax: + 216 74675909. E-mail address: [email protected] (M. Kallel). 0011-9164/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2011.11.058
adequate anti-biofouling strategy, an early warning monitoring with membrane fouling simulator was developed [12,13]. Generally, scaling of the membrane is commanded by the pretreatment process and it has a detrimental effect on both permeate flux and quality. In practice, the deposition of scales at the RO membrane surface is prevented by adding antiscalant to feed water [14–16]. The composition of treated feed water and the reject brine is used to determine the scaling index of different precipitations particularly CaCO3 (calcite), SrSO4 (scelestine), CaSO4,2H2O (gypsum) and SiO2 (silica). Once the salt concentration exceeds its saturation limit, it promotes fouling that is the accumulation of undesired solid materials at the phase interfaces. Most of the recommended pretreatment methods given by the membrane manufacturers are normally based on a single water type. El-Manharawy and Hafez (2001) [3] show the importance differentiation of natural waters in a systematic chemical classification that may provide assistance in RO system design, selection of membrane and proper treatment as well. This classification is based on the real molar concentration of the dissolved ion associations of the investigated water samples. It is possible to identify four water classes (b10, 150, 400, and >600 mM chloride ion), including 10 sub-classes of different water types (b0.1, 0.2, 0.5, 1.0, 2.0, 5.0, 10, 15, 20, >20 (SO4/HCO3) molar ratio. Theses classes and types cover the spectrum from salinity of 200 mg/L to 60,000 mg/L. Sheikholeslami [17] indicates that the thermodynamics of single salt crystallization are not applicable to practical conditions of feed water when mixtures of salts are present. It is shown in various
S. Farhat et al. / Desalination 286 (2012) 412–416
systems that the interactive effects of coexisting salts and the presence of common ion cannot be ignored. Other contaminant considered as a common nuisance, sulfide is the only compound non-regulated parameter that exists in dissolved and gaseous forms. It is a gas formed by the fermentation of organic matter and it is most commonly found in groundwater characterized by relatively low concentrations of dissolved oxygen. Hydrogen concentration gives water a “rotten egg” odor and makes water very corrosive. Most methods for treating sulfur water reply on the oxidation of hydrogen sulfide gas into elemental sulfur that contributes to the formation of turbidity and color. This oxidation can be accomplished with oxygen, chlorine, hydrogen peroxide, ozone, potassium permanganate and dipotassium ferrate [18]. Aeration is most effective when hydrogen sulfide concentrations are lower than 2 mg/L. At a higher concentration, this method may not remove the entire offensive odor unless the air is used to oxidize hydrogen sulfide chemically into solid sulfur and then removing by filtration [19]. The aim of this paper is to study the impact of feed water quality and the chemical products used to control scaling of the membranes. The determination of the scale's chemical characteristics was made in order to investigate a possible relationship between this scale and the concentrations of the chemical products used in the plant. Therefore a systematic approach based on the application of an autopsy of a membrane element and an analysis of the fouling layer was performed. 2. Case study 2.1. The Djerba Island RO desalination plant Tunisia, located in a semi-arid zone, is classified among countries with the least water resources in the Mediterranean basin. Water resources in Tunisia are characterized by scarcity and a pronounced irregularity. The shortage of good water quality particularly in the south regions required a brackish water desalination to supply these regions with potable water. Djerba Island localized at the southern East of Tunisia constitutes one of the most significant touristic poles of the country. It is characterized by seasonal population fluctuation ranging from the simple in winter to the quadruple in summer. The establishment of Djerba RO desalination plant marked the new era in the use of groundwater desalination for domestic purposes in Tunisia [20,21]. The Djerba plant is built in 1999 with a designed capacity of 15,000 m³/d and a conversion rate of 75%. The pretreatment is designed with sand filtration and the injection of sulfuric acid to adjust pH and phosphonate based antiscalant to prevent scale formation. The reverse osmosis comportment is composed by three lines; each one contains 252 polyamide elements. The high pressure pumps operate at 15 bar. Post treatment compartment ensures the gas elimination and the pH permeate adjustment. 2.2. Feed water characterization The analysis of the Mio-Pliocene aquifer water used as feed to the RO Djerba plant is given in Table 1. Feed water presents a salinity ranging from 5300 to 6050 mg/L, a high hardness level typical to groundwater in the south of Tunisia, a variable concentration of H2S and iron reaching respectively 48 mg/L and 1.25 mg/L. The brackish water with high concentration of hydrogen sulfide is present particularly in the center of Djerba Island. Therefore, the high concentration of iron is found in the North-East of the Island. Comparing to the systematic chemical classification of natural water [3], wells water of Djerba Island presented a low alkalinity, medium sulfate and chloride concentration (Table 2). Chloride concentration is between 40 and 55 mM, sulfate concentration ranges between 15 and 20 mM and CaSO4/HCO3 molar ratio ranges between 4 and 13. Water molar classification will provide a useful tool to determine the possible scale type and scaling potential of a concentrated
413
Table 1 Brackish water analysis in Djerba Island. Elements
Concentration
T, °C pH TDS mg/L CE μS/cm TOC mg/L HCO3− mg/L Cl− mg/L SO42 − mg/L NO3 − mg/L NO2− mg/L Hardness °F Ca2 + mg/L Mg2 + mg/L Na+ mg/L K+ mg/L Fe mg/L F mg/L H2S mg/L SiO2 mg/L Sr mg/L Ba mg/L
28–30 7–8 5300–6050 7160–8200 0–6 94–224 1500–2200 1300–1900 0–1 ND 90–185 190–340 95–240 920–1800 19–28 0–1.25 0.1–1.5 0–48 8 4 0.001
ND: none detected.
Table 2 Molar concentrations (mM) and (SO4/HCO3) molar ratio of some selected water samples of different wells in Djerba Island. Well
HCO3
SO4
Cl
SO4/HCO3
Ca
Mg
Na
K
TDS mg/L
G1 G2 G3 G4 G6 G7 G8 G9 G 10 G 11 G12
2.40 2.57 2.36 2.38 3.11 2.13 2.39 2.56 2.05 2.00 1.54
16.25 16.26 17.18 15.88 15.10 16.04 16.03 15.67 17.57 20.15 19.27
48.82 47.63 46.56 54.17 48.68 63.42 59.97 65.89 45.79 44.48 42.93
6.77 6.32 7.28 6.68 4.85 7.53 6.70 6.13 8.57 10.07 12.51
7.50 6.53 7.48 5.95 5.20 7.05 6.48 7.05 9.25 8.48 8.63
5.13 5.17 5.71 4.54 4.17 5.15 4.96 5.25 5.67 9.96 8.92
56.57 56.57 54.00 66.57 62.17 73.17 76.09 74.74 58.04 48.70 46.26
0.73 0.74 0.69 0.67 0.64 0.79 0.77 0.82 0.78 0.72 0.72
5414 5345 5316 5607 5282 5959 5918 6515 5350 5260 5000
solution that comes in contact with RO membrane surface. The relationship between chloride (mM) and the molar ratio CaSO4/HCO3 [3] indicates that the natural water of Djerba Island presents a medium-high carbonate scale potential and a medium sulfate scale potential. This classification covers the sulfate and carbonate scales of Ca and Mg. However, the regular study of scale formed and generated from water types proves that Sr and Ba are always associated with calcium scales over the full range of proposed water types. About amorphous silica scaling temperature, pH and ionic strength factors have substantial influence on the form and on the solubility of the amorphous silica [22,23]. It is observed that silica is usually detected in all of the investigated scales that formed inside the RO elements, in spite of its concentration in the feed water which is much lower than its theoretical saturation level (120 mg/L at 25 °C) [3]. However, higher temperatures and lower cation concentration levels may inhibit deposition of silica form retentate solution in desalination plants [22]. Bonnelye et al. (2004) [6] reported general consideration concerning the comparison of different pretreatment processes in RO desalination operation. They detailed that pretreatment process must be adapted to water quality to be treated. In Djerba RO plant the addition of sulfuric acid is about 14 mg/L to control CaCO3 scaling and a phosphonate based antiscalant (flocon 260) is used to control the other salts particularly sulfates and fluorides of calcium. Fig. 1 shows percent saturation decrease of CaSO4 and CaF2 after antiscalant injection. Indeed the CaSO4 saturation is about 180% in the absence of antiscalant. This rate decreases to 40% after the addition of antiscalant.
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3. The sulfuric water treatment Hydrogen sulfide in contact with oxygen in the oxidation basin takes a milk aspect showing the presence of sulfides in colloidal form. This is very difficult to flocculate and retain on the sand filters or on the cartridge filters. When the sulfides come at the membranes surface an irreversible fouling occurs. The treatment based on an elimination of colloidal sulfide is not efficient [19]. In Djerba RO plant, many experiments and tests are carried out to overcome this problem. Some experiments are developed in aerobic process. It consists of oxidizing the H2S which will be trapped in the vase produced with lime and copper sulfate. This treatment was tested for a short time and was eliminated for environment considerations. The second process used in this plant particularly for the sulfuric water is the anaerobic treatment. This process is applied particularly to first line at three periods: Jun to April 2000, July to September 2002 and Jun to September 2003 (Fig. 2a). The pH of feed water is
Fig. 1. Effect of the injection of antiscalant on the saturation of salts.
a
A1
A2
A3
b
M An
An
An M Stage1
Stage2
c
d A1
A2
A3
M
Stage1
Stage2
e
f A1
A2
A3 M
Stage1
Stage2
Fig. 2. Evolution of the drop pressure and the permeate conductivity at the three lines of the RO plant a,b: line1; c,d:line2; e,f: line3. An: Anaerobic process. A1: Flocon 260 antiscalant; A2: Ameroyal antiscalant; A3: Vitec 5100 antiscalant; M: Change of membranes.
S. Farhat et al. / Desalination 286 (2012) 412–416
Standardless, Elements PEI User Sat : 2 Element K Ratio
Weight %
Atomic %
C K N K O K Na k Mg K Al K Si K P K S K Cl K K K CaK Cr K Fe K Total
3.401 2.592 35.617 2.138 1.134 4.150 5.508 5.384 3.783 1.149 0.973 4.868 4.929 24.373 100.000
6.765 4.421 53.185 2.222 1.114 3.675 4.685 4.153 2.819 0.774 0.594 2.902 2.265 10.426 100.000
0.0340 0.0259 0.3562 0.0214 0.0113 0.0415 0.0551 0.0538 0.0378 0.0115 0.0097 0.0097 0.0487 0.0493
415
Fig. 3. SEM micrograph and associated EDS of deposit sample.
adjusted to 5.5 to maintain all sulfides in the form of gas H2S which penetrates into the membranes easily. Sulfured water moves directly towards the reverse osmosis membranes in anaerobic conditions with an adjusted pH. The sulfured derivatives present in permeate are eliminated by stripping. In order to save the environment from any negative impact, gas H2S is neutralized by concentrated soda. The follow-up of the operation parameters and the experiments applied show that the anaerobic process was adapted to the treatment of sulfured water. However, this application requires preservation of the anaerobic conditions as well as the use of intense sulfuric acid consumption and a great soda quantity to neutralize H2S. That is why this process is used only for the peak periods. In spite of the problems caused by the presence of sulfides in feed water, the treatment by anaerobic process applied to the plant presents acceptable solution but with increasing fouling membrane phenomenon and a decreasing of water quality. This is in relation to the process that treats feed water directly without pretreatment. After operating with the anaerobic processes, the first line showed increasing of pressure drop and salinity (Fig. 2a, b). Until the year 2000, the value of the pressure drop remained 1.6 bar, and then it increased to 2.5 bar at the end of the year 2003. The salinity showed at the same period a brutal increase from 100 μS/cm to 1750 μS/cm (conductivity) at the end of the year 2003. This could be explained by the development of the membrane fouling phenomenon. After changing most of membranes of this line by new ones, operation condition is improved. The pressure drop decreases to 1.6 bar (Fig. 2a) and conductivity decreases to 250 μS/cm (Fig. 2b). Currently the establishment of new wells is carried out in the southwest of Djerba Island zones characterized by low sulfide contents.
4. Membrane autopsy After six years, the exploitation of aerobic process of non-sulfuric water showed in the three lines of RO membrane an increase of the pressure drop and a decrease of the quality of permeate (Fig. 2). From 1.5 bar at the year 2000 the pressure drop reached 3 bar at the year 2008 (Fig. 2c and e). The quality of permeate also decreased rapidly, the conductivity varied from 100 μS/cm to 450 μS/cm at the same period (Fig. 2d and f). This decrease of performance requires an autopsy in order to know the reasons and to find the adequate solutions. To identify the causes of these difficulties analytical technique such as the Scanning Electron Microscope - Energy Dispersive X-ray Spectrometry (SEM/EDS) is used to determine the nature of membrane foulant present on the membrane surface. The key steps of membrane autopsy procedure are selection of representative element, dissection, analysis, identification and remediation [2]. The membrane element is removed from the first stage which presents the majority of problems. The visual inspection of the membrane surface of the element removed shows that it is covered by a dark maroon deposit with gelatinous aspect. The EDS analyses indicate a comparable result of Boubakri et al. [24] that the particulate matter had relatively high levels of: C, O, N, Fe, Si, P and Al. Quite low levels of Ca, S, Cr, Na, Cl, Mg, and K are also present (Fig. 3). The C and O peaks are likely due in part to organic compound present in feed water and in the organic antiscalant [14,15]. J.P van der Hoek et al. [14] indicate that the use of an antiscalant (flocon 100) in combination with H2SO4 was successful to control scaling, but this operation mode resulted in severe biofouling as the antiscalant acted as a nutrient for microbial regrowth in the
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S. Farhat et al. / Desalination 286 (2012) 412–416
membrane elements. The levels of Al and Si in the particulate matter suggest that it is mainly aluminum silicates which are common foulants in RO operation [17]. Given that cartridge filtration with 5 and 1 μm pore size filters is used to pre-treat water, the RO feed is likely to be free from larger size of silt/clay particles. However, finer particles might remain in the feed and subsequently form part of the fouling layer [25]. The three peaks of Fe show that iron represents the major constituent of the deposit. The use of phosphonate based antiscalant in the present case can also contribute to the relatively high levels of P observed. In Djerba plant, the addition of antiscalant is about 2.2 mg/L which is higher than the calculated dose 2.06 mg/L (Fig. 1). Since organic phosphonates are much more resistant to hydrolysis or conversed to orthophosphate (PO43 −), it is possible that the P on the membrane resulted from the reaction of the inhibitor with Ca present in the feed water. In this case, results show that the chemical composition of antiscalant is not adapted to the quality of feed water that presents a moderate salinity with organic matter and iron. Vrouwenvelder et al. [26] indicate that antiscalant dosage could increase both phosphate and substrate concentrations of water. In this way Djerba plant changed the first antiscalant (flocon 260) with ameroyal 363 with 2.2 mg/L in 2006 and with vitec 5100 with 2.4 mg/L in 2009. The performance of the station showed stability after this change which confirms the negative effect of the first antiscalant flocon 260. Fig. 2 shows more stability in pressure drop and permeate conductivity respectively until 2006 after changing the first antiscalant by the ameroyal. This performance is clearly improved after changing ameroyal by vitec 5100 until 2009. Therefore the vitec 5100 is considered to be more suitable to Djerba RO plant feed water. This indicates that antiscalant selection appears critical for fouling development. This result is in agreement with recent research by Vrouwenvelder et al. [26]. 5. Conclusions This paper presents the approach based upon the interpretation of the declining performances of Djerba desalination plant after ten years of operation. In spite of the problems caused by the presence of sulfides in feed water, the treatment by anaerobic process applied to the plant at unit 1 for about three years presents an acceptable solution but with an increasing of membrane fouling phenomenon and an alteration of the quality of water permeate. This is in relation to the process that treats feed water directly without pretreatment. The exploitation of non-sulfuric water in aerobic process shows an increase of both the pressure drop and the salinity of permeate after six years. Membrane autopsy by SED/EDS shows the presence of iron (24.4%), silica (5.5%), phosphate (5.4%), chromium (4.9%), calcium (4.8%), aluminum (4.1%), and sulfur (3.8%). The proportion of organic matter in fouling layer (40%) is identified by the presence of carbon and oxygen. The use of chemical products in the course of the treatment in particularly the antiscalant upstream in the cartridge filters is the principal cause of the presence of phosphate. Therefore, the choice of adequate antiscalant and dosage shows more performance of this plant. According to these results and to overcome these problems, the specific pretreatment must be applied especially for the elimination of iron and the organic matter.
References [1] C. Jucker, M.M. Clark, Adsorption of aquatic humic substances on hydrophobic ultrafiltration membranes, J. Membr. Sci. 97 (1994) 37–52. [2] L.Y. Dudley, E.G. Darton, Pretreatment procedures to control biogrowth and scale formation in membrane systems, Desalination 110 (1997) 11–20. [3] S. El-Manharawy, A. Hafeth, Water type and guidelines for RO system design, Desalination 139 (2001) 97–113. [4] S. Bouguecha, M. Dhahbi, Membrane technologies' role in supplying drinkable and industrial water in Tunisia: conventional process and new tendency, Desalination 151 (2002) 75–86. [5] J.S. Vrouwenvelder, J.W.N.M. Kappelhof, S.G.J. Heijman, J.C. Schippers, D. Kooij, Tools for fouling diagnosis of NF and RO membranes and assessment of the fouling potential of feed water, Desalination 157 (1–3) (2003) 361–365. [6] V. Bonneleye, M.A. Sanz, J.P. Durand, L. PLasse, F. Gueguen, P. Mazounie, Reverse osmosis on open intake seawater, Desalination 167 (2004) 181–200. [7] R. Sheikholeslami, Strategies for future research and development in desalination — challenges ahead, Desalination 248 (2009) 218–224. [8] H. Ivnitskya, I. Katza, D. Minzc, E. Shimonid, Y. Chene, J. Trachitzkye, R. Semiatb, C.G. Dosoretza, Characterization of membrane biofouling in nanofiltration processes of wastewater treatment, Desalination 185 (2005) 255–268. [9] H.C. Flemming, Role and levels of real-time monitoring for successful anti-fouling strategies — an overview, Water Sci. Technol. 47 (2003) 1–8. [10] E. Eguia, A. Trueba, B. Rio-Colonge, A. Giron, J.J. Amieva, C. Bielva, Combined monitor for direct and indirect measurement of biofouling, Biofouling 24 (2008) 75–86. [11] M. Ponte, S. Rapenne, A. Thekkedah, J. Duchesne, V. Jacquemet, J. Leparc, H. Suty, Tools for membrane autopsies and antifouling strategies in seawater feeds: a review, Desalination 181 (2005) 75–90. [12] J.S. Vrouwenvelder, S.M. Bakker, M. Cauchard, R. Le Grand, M. Apacandie, M. Idrissi, S. Lagrave, L.P. Wessels, J.A.M. Van Passen, J.C. Kruithof, M.C.M. Van Loosdrecht, The membrane fouling simulator: a suitable tool for prediction and characterization of membrane fouling, Water Sci. Technol. 55 (2007) 197–205. [13] J.S. Vrouwenvelder, M.C.M. Van Loosdrecht, J.C. Kruitof, Early warning of biofouling in spiral wound nanofiltration and reverse osmosis membranes, Desalination 265 (2011) 206–212. [14] J.P. van der Hoek, J.A.M.H. Hofman, P.A.C. Bonné, M.M. Nederlof, H.S. Vrouwenvelder, RO treatment: selection of a pretreatment scheme based on fouling characteristics and operating conditions based on environmental impact, Desalination 127 (2000) 89–101. [15] J.S. Vrouwenvelder, S.A. Manalarakis, H.R. Veenendaal, D. van der Kooij, Biofouling potential of chemicals used for scale control in RO and NF membranes, Desalination 132 (2000) 1–10. [16] E.E.A. Ghaffour, Enhancing RO system performance utilizing antiscalants, Desalination 153 (2002) 149–153. [17] R. Sheikholeslami, Mixed salts — scaling limits and propensity, Desalination 154 (2003) 117–127. [18] S.J. Duranceau, V.M. Trupiano, M. Lowenstine, S. Whidden, J. Hopp, Innovative hydrogen sulfide treatment methods: moving beyond packed tower aeration, Fla. Water Resour. J. (2010) 4–14. [19] M.A. Thompson, U.G. Kelkar, J.C. Vickers, The treatment of groundwater containing hydrogen sulfide using microfiltration, Desalination 102 (1995) 287–291. [20] F. Kamel, Optimisation de la consommation énergetique dans la station de dessalement de Zarzis (12,000 m3/j), Desalination 137 (2001) 225–231. [21] M. Karime, et al., RO membrane autopsy of Zarzis brackish water desalination plant, Desalination 220 (2008) 258–266. [22] B. Hamrouni, M. Dhahbi, Analytical aspects of silica in saline water — application to desalination of brackish water, Desalination 136 (2001) 225–286. [23] R. Sheikholeslami, I.S. Al-Mutaz, T. Koo, A. Young, Pretreatment and the effect of cations and anions on prevention of silica fouling, Desalination 139 (2001) 83–95. [24] A. Boubakri, S. Bouguecha, Diagnostic and membrane autopsy of Djerba Island desalination station, Desalination 220 (2008) 403–411. [25] T. Tran, B. Bolto, S. Gray, M. Hoang, E. Ostarcevic, An autopsy study of a fouled reverse osmosis membrane element used in a brackish water treatment plant, Water Res. 41 (2007) 3915–3923. [26] . J.S. Vrouwenvelder, F. Beyer, K. Dahmani, N. Hasan, G. Galjaard, J.C. Kruithof, M.C.M. Van Loosdrecht, Phosphate limitation to control biofouling, Water Res. 44 (2010) 3454–3466.
Contents lists available at SciVerse ScienceDirect
Desalination journal homepage: www.elsevier.com/locate/desal
The relation between the RO fouling membrane and the feed water quality and the pretreatment in Djerba Island plant Soumaya Farhat a, Fethi Kamel b, Younes Jedoui a, Monem Kallel c,⁎ a b c
Unité de Recherche “Hydrosciences Appliquées, Institut Supérieur des Sciences et Techniques des Eaux de Gabès, Université de Gabès, Zrig, 6072, Gabès, Tunisia Société Nationale d'Exploitation et de Distribution des Eaux,Direction Production Sud Est, BP 525, Médenine 4100, Tunisia Laboratoire de recherche: Eau, Energie et Environnement; Ecole Nationale des Ingénieures de Sfax, Université de Sfax, BP 1173-3038, Sfax, Tunisia
a r t i c l e
i n f o
Article history: Received 27 August 2011 Received in revised form 26 November 2011 Accepted 28 November 2011 Available online 5 January 2012 Keywords: Reverse osmosis Feed water quality Sulfuric water Membrane fouling Autopsy
a b s t r a c t The reverse osmosis system is exposed to many problems that affect its efficiency. The major problem is fouling membrane which is frequently related to the interaction between the quality of feed water and the pretreatment process. This paper treats two problems of RO plant in Djerba Island, the sulfuric water and the fouling membrane. For the first problem, the anaerobic process showed better performance than the aerobic process. Furthermore, the aerobic process of non-sulfuric water was affected after 6 years of operation with a fouling phenomenon at R.O membrane. This problem was correlated with the increase of the operating pressure, pressure drop and permeates conductivity. To identify causes of these problems, different investigations are carried out. Membrane autopsy, chemical analyses of feed water and follow-up of the operation parameters are performed. The membrane autopsy was achieved by SEM/EDS. The results show that the film is composed mostly with organic matter, silica, metallic matter and aluminum phosphate. This result is correlated with the phosphonate based antiscalant composition. Therefore, antiscalant selection and dosage appear to be critical for membrane fouling development. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Many investigations have clearly substantiated the fact that operations of RO units are very sensitive to feed water quality and as such pretreatment is an essential part of membrane desalination process. Pretreatment system for RO plant is designed to produce feed water with a reduced fouling potential by removing particulates, micropollutants and microorganisms as well as preventing the formation of inorganic scales [1–7]. Many problems affect the operating conditions such as permeate flux, recovery factor and the quality of feed. These problems can be caused by a variety of fouling types. The blocking mechanisms of RO membranes include scaling, fouling and biofouling. Scaling of the membrane is caused by super-saturation of salt ions in the feed side of the membrane. Membrane fouling is caused by convective and diffusive transport of suspended or colloidal matter. Therefore, biofouling is caused by proliferation of microorganisms at the membrane surface [8]. Several methods for monitoring of biofouling membrane have been described [9,10]. When fouling is complex and poorly understood, membrane autopsy is a powerful diagnostic tool which can help enhance system performance. The best membrane autopsy approach today is to develop more in situ tools which engage non-destructive analysis of the membrane materials [11]. For an
⁎ Corresponding author. Tel./fax: + 216 74675909. E-mail address: [email protected] (M. Kallel). 0011-9164/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2011.11.058
adequate anti-biofouling strategy, an early warning monitoring with membrane fouling simulator was developed [12,13]. Generally, scaling of the membrane is commanded by the pretreatment process and it has a detrimental effect on both permeate flux and quality. In practice, the deposition of scales at the RO membrane surface is prevented by adding antiscalant to feed water [14–16]. The composition of treated feed water and the reject brine is used to determine the scaling index of different precipitations particularly CaCO3 (calcite), SrSO4 (scelestine), CaSO4,2H2O (gypsum) and SiO2 (silica). Once the salt concentration exceeds its saturation limit, it promotes fouling that is the accumulation of undesired solid materials at the phase interfaces. Most of the recommended pretreatment methods given by the membrane manufacturers are normally based on a single water type. El-Manharawy and Hafez (2001) [3] show the importance differentiation of natural waters in a systematic chemical classification that may provide assistance in RO system design, selection of membrane and proper treatment as well. This classification is based on the real molar concentration of the dissolved ion associations of the investigated water samples. It is possible to identify four water classes (b10, 150, 400, and >600 mM chloride ion), including 10 sub-classes of different water types (b0.1, 0.2, 0.5, 1.0, 2.0, 5.0, 10, 15, 20, >20 (SO4/HCO3) molar ratio. Theses classes and types cover the spectrum from salinity of 200 mg/L to 60,000 mg/L. Sheikholeslami [17] indicates that the thermodynamics of single salt crystallization are not applicable to practical conditions of feed water when mixtures of salts are present. It is shown in various
S. Farhat et al. / Desalination 286 (2012) 412–416
systems that the interactive effects of coexisting salts and the presence of common ion cannot be ignored. Other contaminant considered as a common nuisance, sulfide is the only compound non-regulated parameter that exists in dissolved and gaseous forms. It is a gas formed by the fermentation of organic matter and it is most commonly found in groundwater characterized by relatively low concentrations of dissolved oxygen. Hydrogen concentration gives water a “rotten egg” odor and makes water very corrosive. Most methods for treating sulfur water reply on the oxidation of hydrogen sulfide gas into elemental sulfur that contributes to the formation of turbidity and color. This oxidation can be accomplished with oxygen, chlorine, hydrogen peroxide, ozone, potassium permanganate and dipotassium ferrate [18]. Aeration is most effective when hydrogen sulfide concentrations are lower than 2 mg/L. At a higher concentration, this method may not remove the entire offensive odor unless the air is used to oxidize hydrogen sulfide chemically into solid sulfur and then removing by filtration [19]. The aim of this paper is to study the impact of feed water quality and the chemical products used to control scaling of the membranes. The determination of the scale's chemical characteristics was made in order to investigate a possible relationship between this scale and the concentrations of the chemical products used in the plant. Therefore a systematic approach based on the application of an autopsy of a membrane element and an analysis of the fouling layer was performed. 2. Case study 2.1. The Djerba Island RO desalination plant Tunisia, located in a semi-arid zone, is classified among countries with the least water resources in the Mediterranean basin. Water resources in Tunisia are characterized by scarcity and a pronounced irregularity. The shortage of good water quality particularly in the south regions required a brackish water desalination to supply these regions with potable water. Djerba Island localized at the southern East of Tunisia constitutes one of the most significant touristic poles of the country. It is characterized by seasonal population fluctuation ranging from the simple in winter to the quadruple in summer. The establishment of Djerba RO desalination plant marked the new era in the use of groundwater desalination for domestic purposes in Tunisia [20,21]. The Djerba plant is built in 1999 with a designed capacity of 15,000 m³/d and a conversion rate of 75%. The pretreatment is designed with sand filtration and the injection of sulfuric acid to adjust pH and phosphonate based antiscalant to prevent scale formation. The reverse osmosis comportment is composed by three lines; each one contains 252 polyamide elements. The high pressure pumps operate at 15 bar. Post treatment compartment ensures the gas elimination and the pH permeate adjustment. 2.2. Feed water characterization The analysis of the Mio-Pliocene aquifer water used as feed to the RO Djerba plant is given in Table 1. Feed water presents a salinity ranging from 5300 to 6050 mg/L, a high hardness level typical to groundwater in the south of Tunisia, a variable concentration of H2S and iron reaching respectively 48 mg/L and 1.25 mg/L. The brackish water with high concentration of hydrogen sulfide is present particularly in the center of Djerba Island. Therefore, the high concentration of iron is found in the North-East of the Island. Comparing to the systematic chemical classification of natural water [3], wells water of Djerba Island presented a low alkalinity, medium sulfate and chloride concentration (Table 2). Chloride concentration is between 40 and 55 mM, sulfate concentration ranges between 15 and 20 mM and CaSO4/HCO3 molar ratio ranges between 4 and 13. Water molar classification will provide a useful tool to determine the possible scale type and scaling potential of a concentrated
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Table 1 Brackish water analysis in Djerba Island. Elements
Concentration
T, °C pH TDS mg/L CE μS/cm TOC mg/L HCO3− mg/L Cl− mg/L SO42 − mg/L NO3 − mg/L NO2− mg/L Hardness °F Ca2 + mg/L Mg2 + mg/L Na+ mg/L K+ mg/L Fe mg/L F mg/L H2S mg/L SiO2 mg/L Sr mg/L Ba mg/L
28–30 7–8 5300–6050 7160–8200 0–6 94–224 1500–2200 1300–1900 0–1 ND 90–185 190–340 95–240 920–1800 19–28 0–1.25 0.1–1.5 0–48 8 4 0.001
ND: none detected.
Table 2 Molar concentrations (mM) and (SO4/HCO3) molar ratio of some selected water samples of different wells in Djerba Island. Well
HCO3
SO4
Cl
SO4/HCO3
Ca
Mg
Na
K
TDS mg/L
G1 G2 G3 G4 G6 G7 G8 G9 G 10 G 11 G12
2.40 2.57 2.36 2.38 3.11 2.13 2.39 2.56 2.05 2.00 1.54
16.25 16.26 17.18 15.88 15.10 16.04 16.03 15.67 17.57 20.15 19.27
48.82 47.63 46.56 54.17 48.68 63.42 59.97 65.89 45.79 44.48 42.93
6.77 6.32 7.28 6.68 4.85 7.53 6.70 6.13 8.57 10.07 12.51
7.50 6.53 7.48 5.95 5.20 7.05 6.48 7.05 9.25 8.48 8.63
5.13 5.17 5.71 4.54 4.17 5.15 4.96 5.25 5.67 9.96 8.92
56.57 56.57 54.00 66.57 62.17 73.17 76.09 74.74 58.04 48.70 46.26
0.73 0.74 0.69 0.67 0.64 0.79 0.77 0.82 0.78 0.72 0.72
5414 5345 5316 5607 5282 5959 5918 6515 5350 5260 5000
solution that comes in contact with RO membrane surface. The relationship between chloride (mM) and the molar ratio CaSO4/HCO3 [3] indicates that the natural water of Djerba Island presents a medium-high carbonate scale potential and a medium sulfate scale potential. This classification covers the sulfate and carbonate scales of Ca and Mg. However, the regular study of scale formed and generated from water types proves that Sr and Ba are always associated with calcium scales over the full range of proposed water types. About amorphous silica scaling temperature, pH and ionic strength factors have substantial influence on the form and on the solubility of the amorphous silica [22,23]. It is observed that silica is usually detected in all of the investigated scales that formed inside the RO elements, in spite of its concentration in the feed water which is much lower than its theoretical saturation level (120 mg/L at 25 °C) [3]. However, higher temperatures and lower cation concentration levels may inhibit deposition of silica form retentate solution in desalination plants [22]. Bonnelye et al. (2004) [6] reported general consideration concerning the comparison of different pretreatment processes in RO desalination operation. They detailed that pretreatment process must be adapted to water quality to be treated. In Djerba RO plant the addition of sulfuric acid is about 14 mg/L to control CaCO3 scaling and a phosphonate based antiscalant (flocon 260) is used to control the other salts particularly sulfates and fluorides of calcium. Fig. 1 shows percent saturation decrease of CaSO4 and CaF2 after antiscalant injection. Indeed the CaSO4 saturation is about 180% in the absence of antiscalant. This rate decreases to 40% after the addition of antiscalant.
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3. The sulfuric water treatment Hydrogen sulfide in contact with oxygen in the oxidation basin takes a milk aspect showing the presence of sulfides in colloidal form. This is very difficult to flocculate and retain on the sand filters or on the cartridge filters. When the sulfides come at the membranes surface an irreversible fouling occurs. The treatment based on an elimination of colloidal sulfide is not efficient [19]. In Djerba RO plant, many experiments and tests are carried out to overcome this problem. Some experiments are developed in aerobic process. It consists of oxidizing the H2S which will be trapped in the vase produced with lime and copper sulfate. This treatment was tested for a short time and was eliminated for environment considerations. The second process used in this plant particularly for the sulfuric water is the anaerobic treatment. This process is applied particularly to first line at three periods: Jun to April 2000, July to September 2002 and Jun to September 2003 (Fig. 2a). The pH of feed water is
Fig. 1. Effect of the injection of antiscalant on the saturation of salts.
a
A1
A2
A3
b
M An
An
An M Stage1
Stage2
c
d A1
A2
A3
M
Stage1
Stage2
e
f A1
A2
A3 M
Stage1
Stage2
Fig. 2. Evolution of the drop pressure and the permeate conductivity at the three lines of the RO plant a,b: line1; c,d:line2; e,f: line3. An: Anaerobic process. A1: Flocon 260 antiscalant; A2: Ameroyal antiscalant; A3: Vitec 5100 antiscalant; M: Change of membranes.
S. Farhat et al. / Desalination 286 (2012) 412–416
Standardless, Elements PEI User Sat : 2 Element K Ratio
Weight %
Atomic %
C K N K O K Na k Mg K Al K Si K P K S K Cl K K K CaK Cr K Fe K Total
3.401 2.592 35.617 2.138 1.134 4.150 5.508 5.384 3.783 1.149 0.973 4.868 4.929 24.373 100.000
6.765 4.421 53.185 2.222 1.114 3.675 4.685 4.153 2.819 0.774 0.594 2.902 2.265 10.426 100.000
0.0340 0.0259 0.3562 0.0214 0.0113 0.0415 0.0551 0.0538 0.0378 0.0115 0.0097 0.0097 0.0487 0.0493
415
Fig. 3. SEM micrograph and associated EDS of deposit sample.
adjusted to 5.5 to maintain all sulfides in the form of gas H2S which penetrates into the membranes easily. Sulfured water moves directly towards the reverse osmosis membranes in anaerobic conditions with an adjusted pH. The sulfured derivatives present in permeate are eliminated by stripping. In order to save the environment from any negative impact, gas H2S is neutralized by concentrated soda. The follow-up of the operation parameters and the experiments applied show that the anaerobic process was adapted to the treatment of sulfured water. However, this application requires preservation of the anaerobic conditions as well as the use of intense sulfuric acid consumption and a great soda quantity to neutralize H2S. That is why this process is used only for the peak periods. In spite of the problems caused by the presence of sulfides in feed water, the treatment by anaerobic process applied to the plant presents acceptable solution but with increasing fouling membrane phenomenon and a decreasing of water quality. This is in relation to the process that treats feed water directly without pretreatment. After operating with the anaerobic processes, the first line showed increasing of pressure drop and salinity (Fig. 2a, b). Until the year 2000, the value of the pressure drop remained 1.6 bar, and then it increased to 2.5 bar at the end of the year 2003. The salinity showed at the same period a brutal increase from 100 μS/cm to 1750 μS/cm (conductivity) at the end of the year 2003. This could be explained by the development of the membrane fouling phenomenon. After changing most of membranes of this line by new ones, operation condition is improved. The pressure drop decreases to 1.6 bar (Fig. 2a) and conductivity decreases to 250 μS/cm (Fig. 2b). Currently the establishment of new wells is carried out in the southwest of Djerba Island zones characterized by low sulfide contents.
4. Membrane autopsy After six years, the exploitation of aerobic process of non-sulfuric water showed in the three lines of RO membrane an increase of the pressure drop and a decrease of the quality of permeate (Fig. 2). From 1.5 bar at the year 2000 the pressure drop reached 3 bar at the year 2008 (Fig. 2c and e). The quality of permeate also decreased rapidly, the conductivity varied from 100 μS/cm to 450 μS/cm at the same period (Fig. 2d and f). This decrease of performance requires an autopsy in order to know the reasons and to find the adequate solutions. To identify the causes of these difficulties analytical technique such as the Scanning Electron Microscope - Energy Dispersive X-ray Spectrometry (SEM/EDS) is used to determine the nature of membrane foulant present on the membrane surface. The key steps of membrane autopsy procedure are selection of representative element, dissection, analysis, identification and remediation [2]. The membrane element is removed from the first stage which presents the majority of problems. The visual inspection of the membrane surface of the element removed shows that it is covered by a dark maroon deposit with gelatinous aspect. The EDS analyses indicate a comparable result of Boubakri et al. [24] that the particulate matter had relatively high levels of: C, O, N, Fe, Si, P and Al. Quite low levels of Ca, S, Cr, Na, Cl, Mg, and K are also present (Fig. 3). The C and O peaks are likely due in part to organic compound present in feed water and in the organic antiscalant [14,15]. J.P van der Hoek et al. [14] indicate that the use of an antiscalant (flocon 100) in combination with H2SO4 was successful to control scaling, but this operation mode resulted in severe biofouling as the antiscalant acted as a nutrient for microbial regrowth in the
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membrane elements. The levels of Al and Si in the particulate matter suggest that it is mainly aluminum silicates which are common foulants in RO operation [17]. Given that cartridge filtration with 5 and 1 μm pore size filters is used to pre-treat water, the RO feed is likely to be free from larger size of silt/clay particles. However, finer particles might remain in the feed and subsequently form part of the fouling layer [25]. The three peaks of Fe show that iron represents the major constituent of the deposit. The use of phosphonate based antiscalant in the present case can also contribute to the relatively high levels of P observed. In Djerba plant, the addition of antiscalant is about 2.2 mg/L which is higher than the calculated dose 2.06 mg/L (Fig. 1). Since organic phosphonates are much more resistant to hydrolysis or conversed to orthophosphate (PO43 −), it is possible that the P on the membrane resulted from the reaction of the inhibitor with Ca present in the feed water. In this case, results show that the chemical composition of antiscalant is not adapted to the quality of feed water that presents a moderate salinity with organic matter and iron. Vrouwenvelder et al. [26] indicate that antiscalant dosage could increase both phosphate and substrate concentrations of water. In this way Djerba plant changed the first antiscalant (flocon 260) with ameroyal 363 with 2.2 mg/L in 2006 and with vitec 5100 with 2.4 mg/L in 2009. The performance of the station showed stability after this change which confirms the negative effect of the first antiscalant flocon 260. Fig. 2 shows more stability in pressure drop and permeate conductivity respectively until 2006 after changing the first antiscalant by the ameroyal. This performance is clearly improved after changing ameroyal by vitec 5100 until 2009. Therefore the vitec 5100 is considered to be more suitable to Djerba RO plant feed water. This indicates that antiscalant selection appears critical for fouling development. This result is in agreement with recent research by Vrouwenvelder et al. [26]. 5. Conclusions This paper presents the approach based upon the interpretation of the declining performances of Djerba desalination plant after ten years of operation. In spite of the problems caused by the presence of sulfides in feed water, the treatment by anaerobic process applied to the plant at unit 1 for about three years presents an acceptable solution but with an increasing of membrane fouling phenomenon and an alteration of the quality of water permeate. This is in relation to the process that treats feed water directly without pretreatment. The exploitation of non-sulfuric water in aerobic process shows an increase of both the pressure drop and the salinity of permeate after six years. Membrane autopsy by SED/EDS shows the presence of iron (24.4%), silica (5.5%), phosphate (5.4%), chromium (4.9%), calcium (4.8%), aluminum (4.1%), and sulfur (3.8%). The proportion of organic matter in fouling layer (40%) is identified by the presence of carbon and oxygen. The use of chemical products in the course of the treatment in particularly the antiscalant upstream in the cartridge filters is the principal cause of the presence of phosphate. Therefore, the choice of adequate antiscalant and dosage shows more performance of this plant. According to these results and to overcome these problems, the specific pretreatment must be applied especially for the elimination of iron and the organic matter.
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