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Technical evaluation of a small-scale reverse osmosis desalination unit for domestic water
Desalination 203 (2007) 319–326
Technical evaluation of a small-scale reverse osmosis desalination unit for domestic water H. Elfila*, A. Hameda, A. Hannachib a
b
Institut National de Recherche Scientifique et Technique, B.P. 2050, Hammam-Lif; Tunisia Tel. þ216 71 430 470; Fax þ216 71 430 934; email: [email protected] National Engineering School of Gabes, ENIG, Omar Ibnelkhattab Street, 6029 Zrig, Gabes, Tunisia Received 9 February 2006; accepted 6 March 2006
Abstract Tunisian standards for drinking water tolerate a maximum Total Dissolved Salts (TDS) of 1.5 g/L. The domestic water presents usually a salinity greater than 0.5 g/L. In the last few years, several small capacity reverse osmosis desalination prototypes have been marketed. They are used to desalinate brackish water with TDS lower than 1.5 g/L. The performances of such type of RO units with respect to the Tunisia tap waters are needed. A technico-economical evaluation of small-scale (100 L/day) reverse osmosis desalination unit has been studied. Water pre-treatment is composed of three filtration operations. Water is pumped through the RO membrane with maximum pressure of 6 bars. Before use, the desalinated water is treated with UV light. The salinity and the temperature of the tested domestic water are located respectively between 0.5 and 1.3 g/L and between 12 and 29 C. The pre-treatment allows eliminating all the suspension matters, as the turbidity and the Solid Density Index are reduced to zero FTU and surrounding one unit respectively. No chemicals are used in the pre-treatment, so membrane scaling can not be avoided if reject water presents a high scaling power. The supersaturation relative to calcium carbonate and gypsum were estimated for reject water. Their values indicate that the tested waters have no risk to scale the RO membrane. The recovery rate of the RO unit was evaluated vs. different operating conditions such as applied pressure, raw water TDS and water temperature. The small capacity unit was able to deliver a treated water of a 100 mg/L TDS with a conversion rate ranging between 25 and 37%. The water treatment cost was evaluated at 0.01 E/L which is roughly the tenth of that of bottled table water. Keywords: Desalination; Tap water; Small scale RO unit; Performances; Treatment cost *Corresponding author. Presented at EuroMed 2006 conference on Desalination Strategies in South Mediterranean Countries: Cooperation between Mediterranean Countries of Europe and the Southern Rim of the Mediterranean. Sponsored by the European Desalination Society and the University of Montpellier II, Montpellier, France, 21-25 May 2006. 0011-9164/07/$– See front matter 2007 Published by Elsevier B.V>. doi:10.1016/j.desal.2006.03.530
320
H. Elfil et al. / Desalination 203 (2007) 319–326
1. Introduction The maximum tolerated Total Dissolved Salts (TDS) of tap water in many south Mediterranean countries like Tunisia is fixed at 1500 ppm [1]. This TDS upper limit is much higher than the Word Health Organisation standard set at 500 ppm. Continuous chemical analyse of domestic water showed that water TDS changes with seasons and regions from 400 to 1400 ppm. To have a good drinking water quality many solutions were embraced. The former one, often practiced in many places in Tunisia, is to rely on rain water stored in private underground concrete tanks. Bottled mineral water is gaining success among households. But it is still beyond the average family income range. In the last few years, the marketing of small scale reverse osmosis unit [2,3] for low TDS brackish water desalination offered an alternative solution to obtain drinking water with TDS lower than 500 ppm. The price of these small units is no longer a problem for ordinary household’s budget as it has drastically decreased in the last three years. Nonetheless, the performances of these limited yields reverse osmosis units are not known especially for varying water quality and temperature. This is the case for many places in Tunisia, where the TDS is twice as much higher in the warmer season than in winter. The aim of this work is to carry out a technico-economical evaluation of the performances of a small-scale reverse osmosis unit fed with tap water. The investigated unit has a 100 L/day production capacity of treated water. The investigation period was 15 months. 2. Experimental investigations The used reverse osmosis unit is composed of:
Three filtration operations (cartridge prefilter, granulate activate carbon filter and
cartridge filter 5 mm) as water pretreatment, a pump with a maximum outlet pressure of 6 bar, a reverse osmosis modulus, a UV and activated carbon post-treatment module, and a 10 L storage container.
This unit is sold at an approximate price of 300 E. Its daily production capacity is a hundred liters of treated water out of tap water of a TDS of around 1 g/L. For all experiments, only tap water was used. The feed water quality and temperature were varying along the seasons. Upper and lower values for the important features of the feed water are given in Table 1. For example, in the year 2005, the water TDS and temperature have ranged between 400 and 1100 mg/L; and 12 and 29 C respectively. Fig. 1 gives the variation of the feed water conductivity along the study period that lasted for fifteen months. The increase of water conductivity is synonymous to a raise of the water salinity. As clearly shown in Fig. 1, the variation of the water parameters can be related to the prevailing climate conditions. Indeed, salinity and temperature were lower for rainy cold periods of the year. On the other hand they were higher in July and August, the hottest summer months where the salinity often exceeded 1 g/L and the temperature ranged between 24 and 29 C. Several operating parameters were monitored, of which the flow rates, the conductivities, the pH, the temperatures and the applied pressure where systematically measured. All water steams, in RO unit, were occasionally subject to a chemical analysis. Intermittently, turbidity as well as the Silt Density Index (SDI) [4] was determined for feed water before and after the pretreatment process. The saturation indexes with respect to calcium carbonate and calcium sulfate
321
H. Elfil et al. / Desalination 203 (2007) 319–326 Table 1 Upper and lower limits for some physical-chemical characteristics of tap water along 2005 Cond. mS/cm
pH
T C
Turb. FTU
LSI
Ca2þ ppm
Mg2þ ppm
HCO3 ppm
SO4 2 ppm
Naþ ppm
Cl ppm
Kþ ppm
400 1100
480 1700
6.9 8.0
12 29
0.2 1.4
2.3 3.4
60 140
15 32
108 160
80 241
60 175
94 255
1 5
dihydrate (gypsum) for tap and rejected water were also monitored to assess the risk of membrane scaling to avoid sealing.
the tap water is thrown away for some time. While the turbidity and the SDI for the feed water ranged between 0.2 and 1.4, and 2.3 and 3.4 respectively; after the pre-treatment process the water turbidity is merely null the SDI was kept between 0.8 and 1.5. These SDI figures are well below the recognized values for reverse osmosis membranes (SDI < 3) [5]. The reverse osmosis unit is designed to work with no chemical (acid or inhibitor) use. That is, all water steams should not present any scaling tendency to prevent membrane obstruction. Water scaling tendency was evaluated mainly for temperatures exceeding 20 C where blinding risk is higher. The scaling water properties were assessed by determining the supersaturations with respect to CaCO3 monohydrate (CCM) and to gypsum (gypsum). It has been shown that spontaneous germination of CaCO3 would occur if CCM>1 [6]. Table 2 gives the CCM for
3. Results and discussion 3.1. Evaluation of the membrane scaling and fouling risk
2000 1600 1200 800 400
06 1/ /0
2/
05 25
05 1/
/1 14
/1
9/
05 02
05
/0
8/
Date
21
/0
6/
05 10
05 5/
/0 29
/0
4/
05 18
05 2/
/0 06
/0
05 1/
2/
/0 12
/1 01
0/ /1 20
Fig. 1. Water conductivity variation along the study period.
04
0 04
Raw water conductivity (µS/cm)
The aim of the pre-treatment operation, consisting of a three step filtration process, is to eliminate any suspended material and chlorine contained in the tap water. The efficiency of such pre-treatment with respect to catching the suspended material was evaluated by measuring the turbidity and the SDI for inlet and outlet streams. In general, as can be seen in Table 1, the tap water has a very low content in suspended material. This is not the case in some situations, particularly when water is run for the first time after a lasting period. That is why, before feeding the unit,
23
Min Max
TDS mg/L
322
H. Elfil et al. / Desalination 203 (2007) 319–326
Table 2 Supersaturation values relative to calcium carbonate monohydrate and gypsum for different reject waters Feed water
T( C)
TDS (mg/L) 1043 977 916 905
25 28.5 26 25 25
1288
25 30
Reject water TDS mg/L
pH
Ca2þ ppm
HCO3 ppm
SO4 2 ppm
CCM
Gypsum
1497 1314 1158 1102 1675
7.90 8.01 8.02 7.86 7.8
179.2 166.0 143.2 148.0 222.6
239.9 228.7 208.6 195.2 255.0
398.8 352.6 328.1 293.7 417.2
0.61 0.45 0.37 0.30 0.9
0.12 0.10 0.08 0.07
1.3 1.1
0.18
8.0 7.8
the rejected flux in cases where feed water salinity and temperature are highest, i.e. for cases of high scaling risk. All CCM values are well below 1, hinting that there is no risk of CaCO3 precipitate occurrence in these conditions. This is consistent with the fact that no significant decline of the reverse osmosis unit performances occurred all along the study period. Indeed, the recovery rate or yield, defined as the ratio of the permeate flow rate to that of the water feed, along with the salinity of the produced water did not experience any considerable variation during the investigation period. In some experiments, the rejected flux was fed to the unit instead of the tap water. These cases are reported in the last three lines of Table 2. In such cases, the working pressure was set at 4.5 bar giving a yield of approximately 28%. In those cases, the rejected flux presents a scaling tendency as CCM>1 increasing the risk of membrane obstruction. 3.2. Evaluation of the RO unit performances As stated earlier, the recovery rate () is defined as the ratio of the permeate flow rate to that of the tap water fed to the unit: ¼ 100:
Qp Qf
ð1Þ
Where Qf and QP are respectively, the feed and permeate flow rates. Both the recovery rate and the salinity of the produced were used to evaluate the reverse osmosis unit performances. These parameters were evaluated all along the investigation period. During the 15 months study period, the recovery rate ranged between 27 and 37% (Fig. 2). The lowest yields were obtained for cold periods in winter. Despite the high feed water salinity, recovery rate is higher in summer time offering an increase of 6–8% with respect to that in winter. Despite the fact that the tap water pressure is highest in winter and its variation could be as high as 0.5 bar, the increase of the recovery rate can only be explained by the temperature effects on the membrane and the water properties and particularly the viscosity. Warmer water lead to an increase of the membrane permeability given in the Eq. 2 relating the permeate flux and the deriving force for the mass transfer through the membrane. Jp ¼ AðP Þ
ð2Þ
Where Jp, P and are respectively, the permeate flux, the differential pressure and the osmotic pressure difference across the membrane. The membrane permeability A at
323
H. Elfil et al. / Desalination 203 (2007) 319–326 39 Recovery rate (%)
35 31 27 23 19
/0 6
/0 5
/0 1 25
/1 2
/0 5 14
/0 5
/1 1
Date (week)
any temperature t is often given by a relationship of an exponential type similar to: At ¼ A0 B0 exp½B1 ð1=T0 1=TÞ
ð3Þ
Where, A0 is the permeability at the reference temperature t0, B0 and B1 are constants that depend mainly on the membrane nature, and T is the absolute temperature. When applying the Van’t Hoff equation for dilute solutions, the osmotic pressure is given by: ¼ CRT
ð4Þ
In this equation, C and R are respectively, the solute concentration and the ideal gas
constant. In summer the salinity is almost twice as much as in winter, the driving force for water transfer through the membrane decreases by approximately 10%. On the other hand, the increase in temperature between 15 and 30 C accounts for a raise in permeability of almost 40%. This is consistent with the increase of permeate flux of almost 7% in the hot months. Several experiments were conducted to investigate the temperature effects on the unit recovery rate for various water salinities. These experiments were conducted at transmembrane pressure of 4.5 bar and two temperatures: 16 and 25 C. The results of this set of experiments are summarized in Fig. 3.
Conversion rate (%)
40
Fig. 3. Variation of the conversion rate versus the feed water total dissolved salts.
02
/0 9
/0 5 21
/0 5
/0 8 10
/0 6
/0 5 29
/0 5
/0 5 18
/0 4
/0 5 06
/0 5
/0 2 23
/0 1
/0 4 /1 2
12
01
Fig. 2. Year long recovery rate of the reverse osmosis desalination unit.
20
/1 0
/0 4
15
T = 16 °C T = 25 °C
36 32 28 24 20 200
P = 4,5 bar
400
600
800 TDS (mg/L)
1000
1200
1400
324
H. Elfil et al. / Desalination 203 (2007) 319–326 40 TDS = 700 mg/L; T = 16 °C TDS = 800 mg/L; T = 25 °C
Recovery rate (%)
36 32 28 24 20 1
2
3
4
5
P (bar)
They clearly show an increase of the conversion rate as the temperature increases and a steady decrease of the yield as the feed water becomes more saline. The pressure effects on the recovery rate have been considered as well. These effects can be seen on Fig. 4 where the pressure was varied in the interval 2 to 5.2 bar for two different water qualities. As expected, the conversion rate steadily increases with cross membrane pressure difference. Beyond a differential pressure of 4.5 bar, the yield apparently reaches a plateau and an increase of the transmembrane pressure has no effect on the recovery rate. This cross membrane pressure difference limit could be considered as an optimal operating value for the reverse osmosis desalination unit.
6
Fig. 4. Variation of the recovery rate versus the cross membrane pressure difference.
It is also important to notice that the tap water is commonly delivered with a pressure of 2–3 bar throughout the year. This implies that the unit would be capable of providing treated water with no pumping needed and hence with no electricity consumption at a recovery rate ranging between 25 and 30%. An important parameter in studying the performances of the reverse osmosis unit is the rate of salt rejection. This parameter is a good indicator for the treated water quality and is given by: SR ¼ 100ðCf CP Þ=Cf
ð5Þ
Where Cf and CP are respectively, the feed and permeate salt concentrations. Table 3 gives a summary of few key elements’
Table 3 Some ion contents in the feed, permeate and rejected fluxes and rejection rates (P ¼ 4,5 bar T ¼ 28.5 C) ions Ca2þ Mg2þ Naþ Kþ HCO3 Cl SO4 2 TDS
Cf (mg/L)
Cp (mg/L)
Cr (mg/L)
Rejection rate (%)
113 32.2 164.2 4.87 158.6 246 241 977.0
2.04 1.3 41.3 0.91 13.4 47.9 0 1356.0
166 47.9 227.9 7.0 228.7 355.3 352 99.0
98.2 96.1 74.8 81.3 91.5 80.5 100 90.0
325
H. Elfil et al. / Desalination 203 (2007) 319–326 Table 4 Some ions’ rejection rates and TDS permeate water (P ¼ 4.5 bar) T ( C) Wf TDS (mg/L)
25 1043
28.5 977
24 960
26 916
25 905
22 613
20 520
Wp TDS (mg/L)
121
99
95
97
92
42
30
95.9 95.2 80.9 86.1 88.0 79.7 100 88.6
98.2 96.1 74.8 81.3 91.5 80.5 100 89.7
98.1 96.8 71.3 84.7 93.1 83.0 100 90.1
98.2 96.1 74.8 81.3 91.5 80.5 100 89.4
98.2 97.1 77.8 85.0 92.8 84.3 100 89.8
99.1 96.7 81.7 88.4 95.0 93.2 100 93.1
98.3 96.6 74.7 87.3 94.1 87.7 100 94.2
2þ
Ca Mg2þ Naþ Kþ HCO3 Cl SO4 2 TDS
concentrations in the feed, permeate and rejected fluxes and their respective rejection rates for a typical summer day, the 29th of July 2005. The rejection rate of divalent ions is between 96 and 100%. For monovalent ions, it is between 75 and 93%. These same rejection rates’ figures were also obtained for different feed water qualities, as shown in Table 4. As expected, the salt rejection rates are better for lower total dissolved salt contents. The water permeate salinity depends on those of feed water and varies from 30 to 121 mg/L. For feed water with around 1 g/L TDS, the permeate salinity is near 100 mg/L, witch corresponds to a salt reject rate of 90%.
3.3. Treated water cost estimation The water treatment cost has been estimated for the current tap water [7] and electricity prices [8] which were supposed not to very all a long the assumed unit replacement period of 5 years. The treatment cost also accounts for an annual replacement of the membrane, filter cartridge and the UV light lamp. As can be seen from Fig. 5, the treatment cost decreases as the production rate approaches the maximum unit production capacity. The minimum treatment cost is approximately 9,5 E/m3 which is relatively a much higher cost than that of large scale water desalination units (0,3–0.9 E/m3) [9,10]. Nonetheless, the treated water cost
Treated Water cost (Euro/L)
0,10
Fig. 5. Treated water cost versus the unit production rate.
0,08 0,06 0,04 0,02 0,00
0
20
40
60
80
100
326
H. Elfil et al. / Desalination 203 (2007) 319–326
(0.01 E/L) is approximately the tenth of that of noble water such as bottled table water priced at roughly 0.1 E/L in Tunisia. 4. Conclusion A technico-economical evaluation of small-scale (100 L/day) reverse osmosis desalination unit has been conducted. The investigation was continued for fifteen months between October 2004 and January 2006. The unit was fed with the tap water whose properties were followed all along the study period. The tap water salinity and temperature varied all the time and were very much related to the prevailing climate conditions. The raw water TDS and temperature have ranged respectively between 400 and 1100 mg/L; and 12 and 29 C. The membrane obstruction risk has been assessed through the evaluation of the reject flux scaling properties. The reverse osmosis desalination unit performances were checked for several operating conditions. The effect on the recovery rate and salt reject rates of the raw water temperature, salinity and the transmembrane pressure have been explored. The recovery rate of the unit ranged between 25 and 37% with a salt reject exceeding 75% for monovalent ions and 95% for divalent ions. The water treatment cost was evaluated at slightly less then 0.01 E/L witch is approximately the tenth of that of bottled table water in Tunisia.
References [1] Kamel Fethi, L’expe´rience tunisienne en matie`re de dessalement des eaux saumaˆtres, Cours: Dessalement des eaux par les membrane d’osmose inverse; MEDRC-SONEDE-INRST, Tunis; 14–17 March 2005. [2] J. Ayoub and R. Alward, Water requirements and remote arid areas: the need for smallscale desalination. Desalination, 107 (1996) 131–147. [3] E. El-Zanati and S. Eissa, Development of a locally designed and manufactured small-scale reverse osmosis desalination system. Desalination, 165 (2004) 133–139. [4] C.K. Teng, M.N.A. Hawlader and A. Malek. An experiment with different pretreatment methods. Desalination, 156 (2003) 51–58 [5] E. Brauns, E. Van Hoof, B. Molenberghs, C. Dotremont, W. Doyen and R. Leysen, New method of measuring and presenting the membrane fouling potential. Desalination, 150 (2002) 31–43. [6] H. Elfil and H. Roques, Role of hydrate phases of calcium carbonate on the scaling phenomenon. Desalination, 137 (2001) 177–186. [7] Available: http://www.sonede.com.tn/fra/IU.html, 5 January 2006. [8] Available: http://www.steg.com.tn/fr/clients_res/ tarif.html, 5 January 2006. [9] K. Fethi and H. Chheibi, Performances de la station de dessalement de Gabe`s (22,500 m3/j) apre`s cinq ans de fonctionnement. Desalination, 136 (2001) 263–272. [10] J.A. Medina, 20 years evolution of desalination cost in spain. International conference on desalination costing, MEDRC, Cyprus, 6–8 December 2004.
Technical evaluation of a small-scale reverse osmosis desalination unit for domestic water H. Elfila*, A. Hameda, A. Hannachib a
b
Institut National de Recherche Scientifique et Technique, B.P. 2050, Hammam-Lif; Tunisia Tel. þ216 71 430 470; Fax þ216 71 430 934; email: [email protected] National Engineering School of Gabes, ENIG, Omar Ibnelkhattab Street, 6029 Zrig, Gabes, Tunisia Received 9 February 2006; accepted 6 March 2006
Abstract Tunisian standards for drinking water tolerate a maximum Total Dissolved Salts (TDS) of 1.5 g/L. The domestic water presents usually a salinity greater than 0.5 g/L. In the last few years, several small capacity reverse osmosis desalination prototypes have been marketed. They are used to desalinate brackish water with TDS lower than 1.5 g/L. The performances of such type of RO units with respect to the Tunisia tap waters are needed. A technico-economical evaluation of small-scale (100 L/day) reverse osmosis desalination unit has been studied. Water pre-treatment is composed of three filtration operations. Water is pumped through the RO membrane with maximum pressure of 6 bars. Before use, the desalinated water is treated with UV light. The salinity and the temperature of the tested domestic water are located respectively between 0.5 and 1.3 g/L and between 12 and 29 C. The pre-treatment allows eliminating all the suspension matters, as the turbidity and the Solid Density Index are reduced to zero FTU and surrounding one unit respectively. No chemicals are used in the pre-treatment, so membrane scaling can not be avoided if reject water presents a high scaling power. The supersaturation relative to calcium carbonate and gypsum were estimated for reject water. Their values indicate that the tested waters have no risk to scale the RO membrane. The recovery rate of the RO unit was evaluated vs. different operating conditions such as applied pressure, raw water TDS and water temperature. The small capacity unit was able to deliver a treated water of a 100 mg/L TDS with a conversion rate ranging between 25 and 37%. The water treatment cost was evaluated at 0.01 E/L which is roughly the tenth of that of bottled table water. Keywords: Desalination; Tap water; Small scale RO unit; Performances; Treatment cost *Corresponding author. Presented at EuroMed 2006 conference on Desalination Strategies in South Mediterranean Countries: Cooperation between Mediterranean Countries of Europe and the Southern Rim of the Mediterranean. Sponsored by the European Desalination Society and the University of Montpellier II, Montpellier, France, 21-25 May 2006. 0011-9164/07/$– See front matter 2007 Published by Elsevier B.V>. doi:10.1016/j.desal.2006.03.530
320
H. Elfil et al. / Desalination 203 (2007) 319–326
1. Introduction The maximum tolerated Total Dissolved Salts (TDS) of tap water in many south Mediterranean countries like Tunisia is fixed at 1500 ppm [1]. This TDS upper limit is much higher than the Word Health Organisation standard set at 500 ppm. Continuous chemical analyse of domestic water showed that water TDS changes with seasons and regions from 400 to 1400 ppm. To have a good drinking water quality many solutions were embraced. The former one, often practiced in many places in Tunisia, is to rely on rain water stored in private underground concrete tanks. Bottled mineral water is gaining success among households. But it is still beyond the average family income range. In the last few years, the marketing of small scale reverse osmosis unit [2,3] for low TDS brackish water desalination offered an alternative solution to obtain drinking water with TDS lower than 500 ppm. The price of these small units is no longer a problem for ordinary household’s budget as it has drastically decreased in the last three years. Nonetheless, the performances of these limited yields reverse osmosis units are not known especially for varying water quality and temperature. This is the case for many places in Tunisia, where the TDS is twice as much higher in the warmer season than in winter. The aim of this work is to carry out a technico-economical evaluation of the performances of a small-scale reverse osmosis unit fed with tap water. The investigated unit has a 100 L/day production capacity of treated water. The investigation period was 15 months. 2. Experimental investigations The used reverse osmosis unit is composed of:
Three filtration operations (cartridge prefilter, granulate activate carbon filter and
cartridge filter 5 mm) as water pretreatment, a pump with a maximum outlet pressure of 6 bar, a reverse osmosis modulus, a UV and activated carbon post-treatment module, and a 10 L storage container.
This unit is sold at an approximate price of 300 E. Its daily production capacity is a hundred liters of treated water out of tap water of a TDS of around 1 g/L. For all experiments, only tap water was used. The feed water quality and temperature were varying along the seasons. Upper and lower values for the important features of the feed water are given in Table 1. For example, in the year 2005, the water TDS and temperature have ranged between 400 and 1100 mg/L; and 12 and 29 C respectively. Fig. 1 gives the variation of the feed water conductivity along the study period that lasted for fifteen months. The increase of water conductivity is synonymous to a raise of the water salinity. As clearly shown in Fig. 1, the variation of the water parameters can be related to the prevailing climate conditions. Indeed, salinity and temperature were lower for rainy cold periods of the year. On the other hand they were higher in July and August, the hottest summer months where the salinity often exceeded 1 g/L and the temperature ranged between 24 and 29 C. Several operating parameters were monitored, of which the flow rates, the conductivities, the pH, the temperatures and the applied pressure where systematically measured. All water steams, in RO unit, were occasionally subject to a chemical analysis. Intermittently, turbidity as well as the Silt Density Index (SDI) [4] was determined for feed water before and after the pretreatment process. The saturation indexes with respect to calcium carbonate and calcium sulfate
321
H. Elfil et al. / Desalination 203 (2007) 319–326 Table 1 Upper and lower limits for some physical-chemical characteristics of tap water along 2005 Cond. mS/cm
pH
T C
Turb. FTU
LSI
Ca2þ ppm
Mg2þ ppm
HCO3 ppm
SO4 2 ppm
Naþ ppm
Cl ppm
Kþ ppm
400 1100
480 1700
6.9 8.0
12 29
0.2 1.4
2.3 3.4
60 140
15 32
108 160
80 241
60 175
94 255
1 5
dihydrate (gypsum) for tap and rejected water were also monitored to assess the risk of membrane scaling to avoid sealing.
the tap water is thrown away for some time. While the turbidity and the SDI for the feed water ranged between 0.2 and 1.4, and 2.3 and 3.4 respectively; after the pre-treatment process the water turbidity is merely null the SDI was kept between 0.8 and 1.5. These SDI figures are well below the recognized values for reverse osmosis membranes (SDI < 3) [5]. The reverse osmosis unit is designed to work with no chemical (acid or inhibitor) use. That is, all water steams should not present any scaling tendency to prevent membrane obstruction. Water scaling tendency was evaluated mainly for temperatures exceeding 20 C where blinding risk is higher. The scaling water properties were assessed by determining the supersaturations with respect to CaCO3 monohydrate (CCM) and to gypsum (gypsum). It has been shown that spontaneous germination of CaCO3 would occur if CCM>1 [6]. Table 2 gives the CCM for
3. Results and discussion 3.1. Evaluation of the membrane scaling and fouling risk
2000 1600 1200 800 400
06 1/ /0
2/
05 25
05 1/
/1 14
/1
9/
05 02
05
/0
8/
Date
21
/0
6/
05 10
05 5/
/0 29
/0
4/
05 18
05 2/
/0 06
/0
05 1/
2/
/0 12
/1 01
0/ /1 20
Fig. 1. Water conductivity variation along the study period.
04
0 04
Raw water conductivity (µS/cm)
The aim of the pre-treatment operation, consisting of a three step filtration process, is to eliminate any suspended material and chlorine contained in the tap water. The efficiency of such pre-treatment with respect to catching the suspended material was evaluated by measuring the turbidity and the SDI for inlet and outlet streams. In general, as can be seen in Table 1, the tap water has a very low content in suspended material. This is not the case in some situations, particularly when water is run for the first time after a lasting period. That is why, before feeding the unit,
23
Min Max
TDS mg/L
322
H. Elfil et al. / Desalination 203 (2007) 319–326
Table 2 Supersaturation values relative to calcium carbonate monohydrate and gypsum for different reject waters Feed water
T( C)
TDS (mg/L) 1043 977 916 905
25 28.5 26 25 25
1288
25 30
Reject water TDS mg/L
pH
Ca2þ ppm
HCO3 ppm
SO4 2 ppm
CCM
Gypsum
1497 1314 1158 1102 1675
7.90 8.01 8.02 7.86 7.8
179.2 166.0 143.2 148.0 222.6
239.9 228.7 208.6 195.2 255.0
398.8 352.6 328.1 293.7 417.2
0.61 0.45 0.37 0.30 0.9
0.12 0.10 0.08 0.07
1.3 1.1
0.18
8.0 7.8
the rejected flux in cases where feed water salinity and temperature are highest, i.e. for cases of high scaling risk. All CCM values are well below 1, hinting that there is no risk of CaCO3 precipitate occurrence in these conditions. This is consistent with the fact that no significant decline of the reverse osmosis unit performances occurred all along the study period. Indeed, the recovery rate or yield, defined as the ratio of the permeate flow rate to that of the water feed, along with the salinity of the produced water did not experience any considerable variation during the investigation period. In some experiments, the rejected flux was fed to the unit instead of the tap water. These cases are reported in the last three lines of Table 2. In such cases, the working pressure was set at 4.5 bar giving a yield of approximately 28%. In those cases, the rejected flux presents a scaling tendency as CCM>1 increasing the risk of membrane obstruction. 3.2. Evaluation of the RO unit performances As stated earlier, the recovery rate () is defined as the ratio of the permeate flow rate to that of the tap water fed to the unit: ¼ 100:
Qp Qf
ð1Þ
Where Qf and QP are respectively, the feed and permeate flow rates. Both the recovery rate and the salinity of the produced were used to evaluate the reverse osmosis unit performances. These parameters were evaluated all along the investigation period. During the 15 months study period, the recovery rate ranged between 27 and 37% (Fig. 2). The lowest yields were obtained for cold periods in winter. Despite the high feed water salinity, recovery rate is higher in summer time offering an increase of 6–8% with respect to that in winter. Despite the fact that the tap water pressure is highest in winter and its variation could be as high as 0.5 bar, the increase of the recovery rate can only be explained by the temperature effects on the membrane and the water properties and particularly the viscosity. Warmer water lead to an increase of the membrane permeability given in the Eq. 2 relating the permeate flux and the deriving force for the mass transfer through the membrane. Jp ¼ AðP Þ
ð2Þ
Where Jp, P and are respectively, the permeate flux, the differential pressure and the osmotic pressure difference across the membrane. The membrane permeability A at
323
H. Elfil et al. / Desalination 203 (2007) 319–326 39 Recovery rate (%)
35 31 27 23 19
/0 6
/0 5
/0 1 25
/1 2
/0 5 14
/0 5
/1 1
Date (week)
any temperature t is often given by a relationship of an exponential type similar to: At ¼ A0 B0 exp½B1 ð1=T0 1=TÞ
ð3Þ
Where, A0 is the permeability at the reference temperature t0, B0 and B1 are constants that depend mainly on the membrane nature, and T is the absolute temperature. When applying the Van’t Hoff equation for dilute solutions, the osmotic pressure is given by: ¼ CRT
ð4Þ
In this equation, C and R are respectively, the solute concentration and the ideal gas
constant. In summer the salinity is almost twice as much as in winter, the driving force for water transfer through the membrane decreases by approximately 10%. On the other hand, the increase in temperature between 15 and 30 C accounts for a raise in permeability of almost 40%. This is consistent with the increase of permeate flux of almost 7% in the hot months. Several experiments were conducted to investigate the temperature effects on the unit recovery rate for various water salinities. These experiments were conducted at transmembrane pressure of 4.5 bar and two temperatures: 16 and 25 C. The results of this set of experiments are summarized in Fig. 3.
Conversion rate (%)
40
Fig. 3. Variation of the conversion rate versus the feed water total dissolved salts.
02
/0 9
/0 5 21
/0 5
/0 8 10
/0 6
/0 5 29
/0 5
/0 5 18
/0 4
/0 5 06
/0 5
/0 2 23
/0 1
/0 4 /1 2
12
01
Fig. 2. Year long recovery rate of the reverse osmosis desalination unit.
20
/1 0
/0 4
15
T = 16 °C T = 25 °C
36 32 28 24 20 200
P = 4,5 bar
400
600
800 TDS (mg/L)
1000
1200
1400
324
H. Elfil et al. / Desalination 203 (2007) 319–326 40 TDS = 700 mg/L; T = 16 °C TDS = 800 mg/L; T = 25 °C
Recovery rate (%)
36 32 28 24 20 1
2
3
4
5
P (bar)
They clearly show an increase of the conversion rate as the temperature increases and a steady decrease of the yield as the feed water becomes more saline. The pressure effects on the recovery rate have been considered as well. These effects can be seen on Fig. 4 where the pressure was varied in the interval 2 to 5.2 bar for two different water qualities. As expected, the conversion rate steadily increases with cross membrane pressure difference. Beyond a differential pressure of 4.5 bar, the yield apparently reaches a plateau and an increase of the transmembrane pressure has no effect on the recovery rate. This cross membrane pressure difference limit could be considered as an optimal operating value for the reverse osmosis desalination unit.
6
Fig. 4. Variation of the recovery rate versus the cross membrane pressure difference.
It is also important to notice that the tap water is commonly delivered with a pressure of 2–3 bar throughout the year. This implies that the unit would be capable of providing treated water with no pumping needed and hence with no electricity consumption at a recovery rate ranging between 25 and 30%. An important parameter in studying the performances of the reverse osmosis unit is the rate of salt rejection. This parameter is a good indicator for the treated water quality and is given by: SR ¼ 100ðCf CP Þ=Cf
ð5Þ
Where Cf and CP are respectively, the feed and permeate salt concentrations. Table 3 gives a summary of few key elements’
Table 3 Some ion contents in the feed, permeate and rejected fluxes and rejection rates (P ¼ 4,5 bar T ¼ 28.5 C) ions Ca2þ Mg2þ Naþ Kþ HCO3 Cl SO4 2 TDS
Cf (mg/L)
Cp (mg/L)
Cr (mg/L)
Rejection rate (%)
113 32.2 164.2 4.87 158.6 246 241 977.0
2.04 1.3 41.3 0.91 13.4 47.9 0 1356.0
166 47.9 227.9 7.0 228.7 355.3 352 99.0
98.2 96.1 74.8 81.3 91.5 80.5 100 90.0
325
H. Elfil et al. / Desalination 203 (2007) 319–326 Table 4 Some ions’ rejection rates and TDS permeate water (P ¼ 4.5 bar) T ( C) Wf TDS (mg/L)
25 1043
28.5 977
24 960
26 916
25 905
22 613
20 520
Wp TDS (mg/L)
121
99
95
97
92
42
30
95.9 95.2 80.9 86.1 88.0 79.7 100 88.6
98.2 96.1 74.8 81.3 91.5 80.5 100 89.7
98.1 96.8 71.3 84.7 93.1 83.0 100 90.1
98.2 96.1 74.8 81.3 91.5 80.5 100 89.4
98.2 97.1 77.8 85.0 92.8 84.3 100 89.8
99.1 96.7 81.7 88.4 95.0 93.2 100 93.1
98.3 96.6 74.7 87.3 94.1 87.7 100 94.2
2þ
Ca Mg2þ Naþ Kþ HCO3 Cl SO4 2 TDS
concentrations in the feed, permeate and rejected fluxes and their respective rejection rates for a typical summer day, the 29th of July 2005. The rejection rate of divalent ions is between 96 and 100%. For monovalent ions, it is between 75 and 93%. These same rejection rates’ figures were also obtained for different feed water qualities, as shown in Table 4. As expected, the salt rejection rates are better for lower total dissolved salt contents. The water permeate salinity depends on those of feed water and varies from 30 to 121 mg/L. For feed water with around 1 g/L TDS, the permeate salinity is near 100 mg/L, witch corresponds to a salt reject rate of 90%.
3.3. Treated water cost estimation The water treatment cost has been estimated for the current tap water [7] and electricity prices [8] which were supposed not to very all a long the assumed unit replacement period of 5 years. The treatment cost also accounts for an annual replacement of the membrane, filter cartridge and the UV light lamp. As can be seen from Fig. 5, the treatment cost decreases as the production rate approaches the maximum unit production capacity. The minimum treatment cost is approximately 9,5 E/m3 which is relatively a much higher cost than that of large scale water desalination units (0,3–0.9 E/m3) [9,10]. Nonetheless, the treated water cost
Treated Water cost (Euro/L)
0,10
Fig. 5. Treated water cost versus the unit production rate.
0,08 0,06 0,04 0,02 0,00
0
20
40
60
80
100
326
H. Elfil et al. / Desalination 203 (2007) 319–326
(0.01 E/L) is approximately the tenth of that of noble water such as bottled table water priced at roughly 0.1 E/L in Tunisia. 4. Conclusion A technico-economical evaluation of small-scale (100 L/day) reverse osmosis desalination unit has been conducted. The investigation was continued for fifteen months between October 2004 and January 2006. The unit was fed with the tap water whose properties were followed all along the study period. The tap water salinity and temperature varied all the time and were very much related to the prevailing climate conditions. The raw water TDS and temperature have ranged respectively between 400 and 1100 mg/L; and 12 and 29 C. The membrane obstruction risk has been assessed through the evaluation of the reject flux scaling properties. The reverse osmosis desalination unit performances were checked for several operating conditions. The effect on the recovery rate and salt reject rates of the raw water temperature, salinity and the transmembrane pressure have been explored. The recovery rate of the unit ranged between 25 and 37% with a salt reject exceeding 75% for monovalent ions and 95% for divalent ions. The water treatment cost was evaluated at slightly less then 0.01 E/L witch is approximately the tenth of that of bottled table water in Tunisia.
References [1] Kamel Fethi, L’expe´rience tunisienne en matie`re de dessalement des eaux saumaˆtres, Cours: Dessalement des eaux par les membrane d’osmose inverse; MEDRC-SONEDE-INRST, Tunis; 14–17 March 2005. [2] J. Ayoub and R. Alward, Water requirements and remote arid areas: the need for smallscale desalination. Desalination, 107 (1996) 131–147. [3] E. El-Zanati and S. Eissa, Development of a locally designed and manufactured small-scale reverse osmosis desalination system. Desalination, 165 (2004) 133–139. [4] C.K. Teng, M.N.A. Hawlader and A. Malek. An experiment with different pretreatment methods. Desalination, 156 (2003) 51–58 [5] E. Brauns, E. Van Hoof, B. Molenberghs, C. Dotremont, W. Doyen and R. Leysen, New method of measuring and presenting the membrane fouling potential. Desalination, 150 (2002) 31–43. [6] H. Elfil and H. Roques, Role of hydrate phases of calcium carbonate on the scaling phenomenon. Desalination, 137 (2001) 177–186. [7] Available: http://www.sonede.com.tn/fra/IU.html, 5 January 2006. [8] Available: http://www.steg.com.tn/fr/clients_res/ tarif.html, 5 January 2006. [9] K. Fethi and H. Chheibi, Performances de la station de dessalement de Gabe`s (22,500 m3/j) apre`s cinq ans de fonctionnement. Desalination, 136 (2001) 263–272. [10] J.A. Medina, 20 years evolution of desalination cost in spain. International conference on desalination costing, MEDRC, Cyprus, 6–8 December 2004.