DESALINATION Desalination 137 (2001) 123-131
Spring water treatment with ultrafiltration and stripping Corinne Cabassud”*, Caroline Burgaudb, Jean-Michel Espenan’ Yaboratoire
d ‘Ingenierie des ProctWs de I ‘Environnement, Institut National aks Sciences Appliquees, 135 Av. de Rangueil, 31077 Toulouse Ceakx, France Tel. +33 (5) 6155 97 73; Fax +33 (5) 6155 97 60; email: [email protected]
bLaboratoires Soludia, Route de Revel 31450 Fourquevaux, France ‘Polymem, Route de Revel, 31450 Fourquevaux, France
Received 11 August 2000; accepted 28 August 2000 Abstract This study focuses on an hybrid process which couples ultrafiltration and stripping. The aim is to treat spring water to disinfect it and to enhance its pH without adding reactants. The first part of the study focused on stripping in a batch stirred reactor without UF. Air was chosen as the stripping gas, and the influence of the air flow rate on the water quality was studied for the stirred reactor and then for the hybrid process. Benefits of gas sparging have previously been pointed out in terms of fouling removal. This study demonstrates that gas sparging can be used to enhance the pH by removing a dissolved compound by gas stripping. Very good mass transfer coefficients are obtained in comparison with conventional systems like the batch stirred reactor or bubble columns. Keywords: Spring water; Ultrafiltration; Hybrid process; Stripping; Disinfecting
1. Introduction Spring water showed some interesting proper-
ties as it was rich in minerals such as fluorine, lithium and strontium. It also contained some salts and oligo-elements including calcium, magnesium and potassium. Because of these characteristics, this water could be used to make therapeutic dental products. However, it can
be contaminated by bacteria and its pH was too low for this application (7.1) as the required pH for a dental use is nearly 8. The aim of the study was to develop an integrated process to disinfect the water and to raise the pH without adding chemicals and without modifying the composition of the fluorine, lithium and strontium. sometimes
*Corresponding author. Presented at the conference on Desalination Strategies in South Mediterranean Countries, Cooperation between Mediterranean Countries ofEurope and the Southern Rim of the Mediterranean, sponsored by the European Desalination Society and Ecole Nationale d’lngenieurs de Tunis, September 11-13, 2000, Jerba, Tunisia. 00 1 l-91 64/O l/$- See front matter 0 2001 Elsevier Science B.V. All rights resewed PII:SOOll-9164(01)00210-7
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2. Choice of a process The first quality problem for the spring water was the pH, which had to be increased. Chemical treatments using sodium carbonate or hydroxide or lime are commonly used. But the use of chemical products should be avoided for this application. Another way of increasing the pH is to strip the dissolved CO, out from the water using a gas. When stripping is performed, the concentration in dissolved CO, [CO,(liq)] decreases in the liquid. As a consequence, the equilibria in the solution are displaced so as to counterbalance the loss of CO,. The main reactions occurring in water and involving the carbonic system [l] are: CO, (liq) * CO, (gas)
H&O, * CO, (liq)
HCO; + H30+ * H,CO, + H,O
Reactions 2 and 3 will be displaced to increase the CO, concentration. This will lead to a consumption of HCO; and H,O” ions and thus an increase of the pH. These reactions involve three successive steps (cf. Fig. 1) [l]. The first one is a transfer of CO, from the liquid phase to the gas phase. The second one concerns the reactions occurring in the liquid phase. It is never the limiting step as the rate of reaction is very high . The third step is the mass transfer between the liquid and some solid crystals in the solution (like for example calcium carbonate) and which can appear or disappear, depending on the operating conditions. The second water quality problem is bacteria contamination. Chemical methods such as oxidation by chlorine or ozone cannot be used for this application as they will modify the water composition. Membrane processes and ultrafiltration (UF) are more and more used for water disinfecting. UF by hollow-fibre membranes is
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increasing in development for drinking water production. Nowadays, more than 50 UF plants are in operation in the world. As pointed out by numerous recent studies, the main advantage of UF membranes is that they are able to totally remove bacteria, viruses and particles without modifying the content in dissolved compounds of the water . UF membranes can thus be used to disinfect water without removing or modifying the elements which are necessary for human health, as some of those present in spring water. The main limitation of UF lies in the flux decline associated with the formation of a particle deposit on the membrane surface. This fouling tends to limit the ultrafiltered water production. Numerous studies aim to suggest a process to prevent or to limit fouling. Recent developments are focusing on a process which uses air sparging in the concentrate compartment of inside-out UF or microfiltration membranes. In that process, air is added to the liquid feed stream at the inlet of the module throughout the filtering period, and both the feed and gas flow together inside tubular or hollow-fibre membranes (Fig. 2). Some studies have pointed out the interest of gas sparging to enhance the permeate flux for different applications (drinking water production , biological treatment [5,6], macromolecule separation ) and different membrane geometries (hollow fibres [4,7], flat sheet , tubular membranes [S]). Flux enhancement has always been observed. It should be pointed out that the process is less efficient in the case of microfiltration because internal fouling occurs and the two-phase flow has no effect on this fouling . In the field of drinking water production, the potentialities of gas sparging in UF hollow fibres have previously been studied by experiments using clay suspensions, clay + dextran suspensions and natural river water [4,8]. In each case the tangential gas/liquid two-phase flow inside the tibres allows the enhancement of the permeate flux by preventing particle deposition.
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137 (2001) 123431
Fig. 1. Schematicof CO2reactionand transfer.
composition in key elements are: l stripping to rise the pH l ultrafiltration for disinfecting. Membrane Cake deposit
Fig. 2. Principleof gas sparginginside membranes. The tangential gas/liquid two-phase flow inside the fibres allows enhancement of the permeate flux by a factor of 1.2 to 3, depending on the suspension. The interest of gas sparging at the industrial scale is a decrease of both the membrane area to install and the energy consumption in some’operating conditions . To conclude, the best processes to improve the water quality without perturbing its
Moreover, use of gas sparging inside UF hollow fibres can reduce the fouling problems. So a synergy between the two processes could be found by using a hybrid process which couples UF and stripping inside the fibres. The principle of this process is shown in Fig. 3. The aim of the study is then to study the potentialities of this process and to determine the best operating conditions. The first step in the study concerns the stripping conditions. Then the hybrid process and the interactions between stripping and UF will be studied in terms of permeate flux and of mass transfer from the liquid to the gas phase.
3. Experimental study of the stripping To begin, a gas (air or nitrogen) had to be chosen for the stripping, and it was necessary to
C. Cabassud et al. /Desalination I37 (2001) 123-I 31 Concentrate
Bacteria, Viruses Poor in dissolved CO, +Air
D%i!iid Poor in dissolved CO, Porous membrane
Spring water pH=7
Bacteria, viruses Rich in dissolved CO, Fig. 3. Principle and quality aims of the hybrid stripping and ultrafiltration process.
determine which gas could increase the pH without affecting the most interesting elements such as fluoride, lithium and strontium.
, I Fivity
3. I. Experimental set-up (Fig. 4) The spring water was fed into a PTFE batch reactor (OS7L) which was immersed in a thermoregulated water bath at 23 “C. The gas was injected at the bottom of the reactor as bubbles through a porous bubble distributor. Air or nitrogen could be used as a stripping gas. The gas flow rate was fixed to 2 L.min- ‘. pH, temperature and conductivity in the reactor were measured for 2 h. Samples were taken and the following quality parameters were measured: the total alkaline strength (TAC = [HC03-] + [COf-] + [OH-]), alkaline strength: (TA= [CO:-] 12 + [OH-]), and the concentrations of Mg’+, N03-, SO:-, PO:-, total Fe, and Cl-. From the TAC, TA and pH,
Fig. 4. Experimental apparatus for stripping studies.
of HC03- and CO:- were calcu-
3.2. Choice of a stripping gas The time-variation of pH is shown in Fig. 5 for air and nitrogen. Experiments confirmed that
C. Cabassud et al, /Desalination pH ‘.*I
Fig. 5. Time variation of pH for stripping performed with air and nitrogen. Spinr water: volume = 0.57L; gas flow rate = 2 L.min-‘; T = 23“C.
a gas flow was able to increase the pH to the required value: the stabilized pH was nearly 8.2. The same pH variations were obtained with air or with nitrogen. During the experiments, negligible variations of conductivity were observed. Therefore, the conductivity can be considered to be constant (near 1600 @/cm), which means that no significant precipitation occurred. For the mineral compounds which were analyzed, similar results were obtained for the stripping carried out with air and nitrogen. As the results of stripping with air and nitrogen gas are nearly the same, and taking into account economic aspects, air was chosen as the stripping gas.
3.3. Influence of the airflow rate Influence of the air flow rate was studied on the pH variation (Fig. 6) and on the water quality. Whatever the air flow rate, the pH increased with time. But the higher the air flow rate was, the faster the increase of the pH became. To reach a pH of about 8.2, the stripping took 1h when air flow rate (0.07L. using a 0.04L.min’ w hereas it lasted only 10 min with mir-‘.,&-l) an air flow rate of 2 L.min-’ (3.3 L.min-‘.,A-‘). These results demonstrate that the mass transfer
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of CO, between the liquid and the gas phase is the limiting step. Moreover, analyses showed that the air flow had no significant influence on carbonates, bicarbonates or calcium. Therefore, we can consider that no precipitation occurs. This is confirmed by the graph characterizing the carbonic system [ 11. The point representative of the water at the beginning of the experiment is under the equilibrium curve, which means that precipitation had already started before the experiments. We will then focus on the transfer of CO, from the liquid phase to the gas phase, which seems to be the main mechanism in the stripping process for this water. In a closed batch reactor, the mass balance on the CO, in the liquid phase can be written: d(CO,)/dt = k,a [(CO,)* - (CO,)] where k,a is a global mass transfer coefficient, and (CO,)* is the concentration of dissolved CO, in the liquid phase in equilibrium with the CO, in the gas phase. The partial pressure of CO, in the ambient air was considered as constant -pco2 = 3.1 0e4 atm -which means that (CO,)* = 0.01 mmoY1. (CO,) was then calculated using the following relationship, where Kl = 6.445: lo+’ = (HC03-) (H’) / (CO,) The plot of In [(CO,)-(CO,)*] against time is a straight line whatever the air flow rate. The slope of the straight line was obtained by linear regression and determined k,a. The k,a values and the regression coefficients (R2) are introduced in Table 1. The mass transfer coefficient k,a is independent on the air flow rate. The average k,a is equal to 3.7. 1O-4s-l. This value is of the same range of magnitude than the k,a normally obtained in bubble columns.
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lime (min) Fig. 6. Influenceof air flow rate on pH. Springwater:volume= 0.57 L; T= 23°C.
Table 1 Effectof the air flow rateon the globalCO,masstransfer coefficientin the closed stirred reactor Air flow rate, L.min-’ R,a, S-I
2 1 0.06 0.04
72 76 83 86
3.6 x 1O-4 3.8 x 1O-4 3.5 x 1o-4 3.8 x 1O-4
Fig. 7. Pilot plant for studying the stripping + ultrafiltrationprocess.
4.Study of the hybrid process: ultrafiltration and stripping 4.1. Thepilotpht The pilot plant is shown in Fig. 7. The peristaltic volumetric pump (2) allowed the water to be taken from the stirred feed tank (l), and injected into the hollow-fibre module (5), going through a pulse absorber (4). The membrane module was installed vertically. Air was added to the liquid feed stream at the inlet of the module through a porous membrane. Stripping thus occurs inside the fibres. The liquid and gas flow rates were controlled using flow meters (3,6). The liquid velocity, U,, and the gas velocity, U,,
were calculated as if each phase were alone in the fibres. The liquid velocity, U,, was fixed to 0.2m.s-‘. The inlet inner pressure, P,,and the pressure drop, AI’, along the module were measured by pressure gauges. The transmembrane pressure was adjusted manually using the valve (7). The concentrate was recycled into the feed tank and pH was measured in the permeate. The membranes were UF hollow fibres, 15 in a module. The fibre mean pore diameter was 0.01 pm, the inner diameter was 0.93 mm and the length was 1.20m. The filtering area was 0.0526m’.
C. Cabassud et al. /Desalination I3 7 (2001) 123-13 1
1.4 1.6 Ptm (bar)
Fig. 8. Variation of permeate flux with transmembranepressure for ultrafiltratedwater nd spring water without gas sparging. 4.2. Characterization
of the fouling ability of the
To begin with, UF experiments were performed without using air sparging so as to characterize the fouling generated by the spring water. Variations of permeate flux with transmembrane pressure are showed in Fig. 8 for the reference water (ultrafiltrated water) and for a sample of spring water. Darcy’s law is obtained for the pure water. The membrane permeability is nearly 100 l.h-‘.m-2.bar-1 at 20°C. For the spring water, fouling was observed for pressures higher than 0.7 bar, which is thus the critical pressure for this water; 0.6 bar was therefore chosen as the transmembrane pressure for the rest of the experiments on the pilot plant. 4.3. Influence of gas sparging on permeate Jrux and on the water quality An important operating parameter for gas sparging is the air injection ratio E, defined by
and which should be between 0.1 and 0.9 for hollow tibres to obtain a slug flow inside the fibres and the best effects on fouling [4,8].
The liquid velocity was maintained constant, and the gas sparging experiments were performed with different gas velocities between 0.15 and 0.5m.s-’ (E between 0.4 and 0.7) and at the constant transmembrane pressure of 0.6 bar. The permeate quality was very good: as expected with UF membranes, bacteria and viruses were completely removed and the mineral composition was not changed. Nearly the same pH (about 8.1) was reached whatever the air velocity was (Fig. 9). Nevertheless, this pH was obtained faster with a higher air velocity. This confirms the observations done with the batch reactor without UF. For the used filtration conditions, the spring water showed a very low fouling effect with the permeate flux between 85 and 90% of the pure water flux, J,. As a consequence, very low flux improvement can be observed, and the permeate flux appears as independent of the air flow rate (Fig. 10). We have to point out that in a previous study, it was shown that the higher flux enhancement can be achieved for the feed having the higher fouling ability . Higher flux enhancement could probably have been observed if an higher transmembrane pressure had been used.
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80 Time (min)
Fig. 9. Influence of gas sparging on the pH time variation for different gas velocities. Spring water: T = 23°C.
120 Time (min)
Fig. 10. Influence of gas sparing on the permeation ratio for different gas velocities. Spring water: T = 23°C.
4.3 Influence of the UF module on the transfer of
The hybrid UFfstripping system induces a new way to transfer a compound from a liquid phase to a gas phase. It seemed then interesting to characterize this mass transfer. The mass balance on CO, on the UF module on the liquid phase can be written as:
+k,a (CO,-CO; QP(co2~pe,,
This mass balance considers the module as a perfectly stirred reactor under permanent conditions and that transfer and reaction occur in the volume, V, in which the gas and liquid are in
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contact, mainly in the fibres. In the mass balance, the reaction term (r,V) concerns the global reaction (4). CO, +
OH- * CO:- + H,O
For reaction (4), the rate of reaction can be writtenr,=R(CO,).(OH-) withk=5x103 l.mol-‘. s-l. Considering that the concentrations in the reactor are equal to the concentrations in the permeate, the reaction term R=r,V can be computed using our experimental data on the concentrations. The value obtained is R = 6.7x 1O-gmol.s-l. This value is negligible in comparison with the others terms in the mass equation, as for example, the terms Q(C0,) range between lo-’ and 6.5~ lo-’ .This computation confirms our previous hypothesis: precipitation reactions are not so important for this water. Moreover, the mass balance equation was used to estimate the gas/liquid transfer coefficient in the module, and the k,a value found was 0.033 s-l, which is much higher than the k,a obtained in the stripping reactor. The mass transfer from a liquid to a gas phase is, therefore, more efficient in an UF module operated with gas sparging than in bubble columns or in the conventional stirred reactor.
5. Conclusions A hybrid process coupling air sparging and UF obtained the required water quality for spring
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water without adding chemicals to the system. The membrane was used to remove bacteria and viruses. In the same time air sparging strips the CO, with very good transfer coeffkients in comparison with conventional stripping systems (bubble columns or stirred cells) and in reaching a good pH. It could also in the same way prevent fouling, should this be important. An industrial plant is now being built which is based on this innovative process. Long-term data will be available to validate the interest of the process and its impacts on the water quality.
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