Ind. Eng. Chem. Res. 2007, 46, 2249-2253
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Influence of the Adopted Pretreatment Process on the Critical Flux Value of Batch Membrane Processes Marco Stoller* and Angelo Chianese Department of Chemical Engineering, UniVersity of Rome “La Sapienza”, Via Eudossiana, 18 00184 Rome, Italy
This work deals with an experimental investigation on the effect of different pretreatments on the critical flux of both an ultrafiltration and a nanofiltration membrane process, applied at pilot scale, on the purification of an olive washing wastewater. Two pretreatments were considered: flocculation and biofiltration. First, the flocculation process was optimized, taking into account two different coagulants and varying the dosage and the mixing procedure. The same wastewater was treated by a biological filter. The obtained pretreated solutions were then processed by using wound spiral membrane modules of ultrafiltration and nanofiltration to determine the critical flux for each membrane process. For both the examined membrane processes the critical flux value was around 40% higher when the flocculation pretreatment was applied. Introduction In the production of olive oil, a preliminary important step is the washing of the collected olives before their milling. The washing process is performed in order to clean the olives by removing earth and all other undesired substances attached to the fruit peel. Up to 500 L of potable water is required per ton of olives. Such a utility is hardly available in Mediterranean regions. The wastewater produced by the washing process is a dark odorous (smelly) liquid with a rather high organic matter load. This polluted water must be treated before its disposal according to present-day European environmental rules, with high costs for the olive oil mill manufacturer. Membrane technology may recover efficiently up to 80% of potable water from this wastewater1 by a batch membrane process consisting of ultrafiltration (UF) and nanofiltration (NF) in sequence. With respect to continuous processes, batch processes are easier to handle and allow a reduced active membrane area in order to reach the separation target. One of the main problems of batch processes is the increase of the pollutant concentration of the feedstock along the operation. This fact often leads to a continuous adjustment of the operating conditions, in particular by increasing pressure. On the contrary, when the batch process plant is performed at constant operating conditions, we may obtain weaker performances or heavy fouling on the membranes. The fouling on the membranes is one of the main challenges of the recent widespread application of membrane technology. Fouling decreases productivity and shortens membrane life. Critical flux determination seems to be the best method to identify process conditions for minimizing fouling. At the critical flux point the drag forces on the particles depositing on the membrane surface are equal to the dispersive forces, leading to a quasi-stationary layer. Under these conditions, only reversible fouling, which can be periodically soft-cleaned, takes place. Field et al.2 introduced the concept of critical flux with respect to microfiltration, stating that it is the maximum permeate flux below which fouling is not observed. It was immediately clear * To whom correspondence should be addressed. E-mail: stoller@ ingchim.ing.uniroma1.it.
that the newly developed concept is an effective criterion for the optimization of a membrane separation operation. Afterward, critical flux was also defined for UF and NF operations.3 On the other hand, Cho and Fane4 pointed out that to operate below the critical flux may not be sufficient in order to avoid long-term fouling. These authors introduced the concept of sustainable flux, at which the desired separation can be operated in a profitable manner, minimizing fouling but without eliminating it. Nowadays the critical flux values cannot be theoretically predicted, but they may be only experimentally evaluated. Critical flux depends, among other factors, on hydrodynamics5 and feed stream nature.6 Similar hydrodynamic conditions may be attained at different scales if the same mean velocity over the membrane surfaces is applied. The feedstock quality is a crucial issue with regard to the critical flux; this is particularly true for batch processes, which are characterized by a change of the pollutant concentration due to batch concentration. The purification of the olive washing wastewater is usually accomplished by a preliminary process to reduce the COD content and separate the suspended particles, and a series of membrane processes to get the purity grade required by sewage disposal. The membrane separation performances are strongly affected by the nature of the pretreatment process. In this work, two different pretreatment processes were performed on a real sample of olive washing wastewater: flocculation and an aerobic treatment by using a biological immobilized bed reactor. The outlet stream from each pretreatment was processed by two membrane operations in sequence, i.e., UF and NF. The effects of the two different pretreatments were then compared on the basis of the critical flux, experimentally evaluated, of both the UF and NF processes. Experimental Section Two different experimental apparatuses were used in this work, each concerning a different pretreatment followed by the UF and NF separation steps. The first pilot plant (“1”), built up at a laboratory scale, was used to investigate the flocculation pretreatment, whereas the effect of the biological pretreatment was examined by using a pilot plant (“2”), at larger scale, located on the site of a mill factory. The main characteristics of the two experimental pilot plants are reported in Table 1.
10.1021/ie060964k CCC: $37.00 © 2007 American Chemical Society Published on Web 02/13/2007
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Table 1. Characteristics of the Membrane Modules of the Two Pilot Plants pretreated feedstock vol [L] pilot plant 1
100
pilot plant 2
780
membrane modules UF: NF: UF: NF:
Desal GM 2540F Desal DK 2540F Desal GM 4040F Desal DK 4040F
Table 2. Chemical Analysis of the Olive Washing Wastewater pH conductivity [µS/cm] BOD5 [mg/L] COD [mg/L] total phenols [mg/L] dry matter [mg/L]
5.3 966.0 1215.0 1840.0 5.6 1650.0
Both pilot plants are equipped with the same spiral wound membrane modules characterized by a 1.27 mm high feed spacer and supplied by Osmonics.7 The feed flow rate to the different scale membrane modules was fixed to attain the same mean velocity over the membrane surfaces. Since for the two pilot plants very close pure water permeability mw values were measured, as shown in Table 1, the same mass transfer resistance was considered for both cases. In Table 2 the chemical analysis of the experimented olive washing wastewater is reported. In the pilot plant, the pretreatment consists of a filtration through a 50 µm filter paper followed by a flocculation undertaken by adding aluminum sulfate (AS) or aluminum hydroxide (AH). The flocculation operation was carried out in batch mode and by using a 20 L reactor. The coagulant was added to the wastewater agitated at a relatively high stirred rate. The fast mixing regime lasted a few minutes; afterward the suspension was mixed slowly for 20 min and, finally, it was left to separate over 24 h. The second pretreatment process was accomplished by means of a biofilter 1 m in diameter, filled with approximately 1 m3 of rounded shaped expanded clay beads, with a diameter ranging from 4 to 8 mm (Filtralite).8 An air stream of 15 N m3/h was fed to the reactor. The start-up of the reactor was accomplished by recycling the wastewater at a reduced flow rate for 5 days. After this period of time, the inlet stream flow rate was set at 250 L/h, corresponding to a residence time in the reactor of 8 h. In this work the efficiencies of the pretreatment processes were compared on the basis of the maximum allowable productivity of the subsequent membrane processes. As is wellknown, the main limitation to the flux through a membrane is fouling. The main cause of fouling is the gel formation at the membrane surface, which can be reduced as well as the stability of the solution can be increased.9 Without the gel layer, the permeability of the solvent through the membrane is higher, leading to greater permeate fluxes accompanied by lower concentration gradients toward the membrane surface. On the contrary, when higher pollutant concentrations are present in the treated solution, the concentration gradient close to the membrane surface increases until crystallization is induced. At this latter condition irreversible fouling occurs. Since the critical flux is a suitable measurement of the fouling behavior of the system, the influence of each adopted pretreatment process of the feedstock on the critical flux was examined. The positive influence of a flocculation pretreatment is wellknown.10 During the flocculation process, a great amount of suspended particles in the form of flocs can be separated, leading to a clarified solution which as a result is more stable and less
MWCO [Da] UF: NF: UF: NF:
8000 300 8000 300
total membrane area [m2] UF: NF: UF: NF:
mw [L/(h m2 bar)]
2.5 2.5 32.0 32.0
UF: NF: UF: NF:
15.71 9.19 15.52 9.30
Table 3. Results Obtained from the Coagulation Experiments coagulant [mg/L]
mixing time [s]
pH
∆COD [%]
125 250 500 250 250 500 250 500
AS 180 180 180 90 180 180 180 180
5 5 5 5 6 6 7 7
35.1 43.6 41.8 47.0 57.6 58.4 64.4 65.8
10 30 50 30 30 50 30 50
AH 180 180 180 90 180 180 180 180
5 5 5 5 6 6 7 7
63.2 66.1 62.3 69.2 38.7 45.1 0 0
prone to induce fouling. A strong reduction of the suspended matter is also allowed by the use of a biofilter, which has the advantage, with respect to flocculation, of not requiring the addition of chemical compounds. Critical flux can be measured by different procedures. In this work the method proposed by Benkhala et al.11 and recently applied by Stoller and Chianese1 to olive washing wastewater was adopted. It consists of cycling the applied pressure up and down, and measuring the corresponding permeate fluxes. The last pressure value at which the same permeate flux is obtained, before and after the pressure cycle, is the critical flux. Results and Discussion In the first pretreatment study, the effects of the coagulant amount, the pH, and the fast mixing time were investigated, for both used coagulants. The solution pH was adjusted by adding a small amount of a weak acid or base. The obtained COD reductions, ∆COD, due to the flocculation step are reported in Table 3 together with the applied operating conditions. The best performances of both coagulants were obtained for a fast mixing time of 90 s. The optimal mass of AS, that is leading to the largest COD reduction, was equal to 250 g/m3 regardless of the solution pH. On the contrary, the performances of the coagulation process by using AH appeared to be strongly affected by the solution pH. In this case the best performances were obtained at pH 5 for a coagulant amount of 30 mg/L. At pH 6 the COD reduction was significantly smaller; finally, at pH 7 or higher no COD reduction occurred since AH did not dissolve. In conclusion, around two-thirds of the organic matter was eliminated by the flocculation process with one or the other of the adopted coagulants. Moreover, with both coagulants, a significant reduction of polyphenols occurred and just 1.2 kg of dry mud per m3 of treated wastewater was produced. At the maximum flocculation efficiency, the cost of the used coagulant per cubic meter of pretreated water, the organic content reduction, and the settling time were more or less the same for both coagulants. However, AS seems to work better
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Figure 1. Critical flux values for the wastewater after flocculation. Table 4. Chemical Analysis of the Wastewater Stream after the Two Pretreatment Processes pretreatment process operating conditions and outlet stream characteristics residence time [h] coagulant amount [g/m3] pH conductivity [µS/cm] BOD5 [mg/L] COD [mg/L] total phenols [mg/L] dry matter [mg/L]
biofilter 8.0 5.9 943.0 668.0 837.0 4.4 1380.0
flocculation with AS 24.3 250.0 6.7 1043.0 702.0 895.0 1.0 430.0
than AH because of its invariant performance at different pH values. In fact, since the addition of AH to the feedstock introduces additional OH- ions, which interact with the protons in the solution, its effect strongly depends upon the value of initial pH. Since a typical value of the pH for the washing olive wastewater is nearer 6 than 5, the addition of AH leads to worse performances with respect to those assured by the use of AS. As a consequence, in this study only the addition of AS was considered for the flocculation process. In Table 4 the results of the biological pretreatment are reported and compared with the best performances of the flocculation process at pH 5.3, attained without any pH adjustment. The two pretreatment processes performed similarly with respect to COD and BOD5 reductions, whereas remarkable reductions of both the residual phenol content and the final dry matter were exhibited by the flocculation process. In each plant, the pretreated wastewater was first submitted to a UF membrane batch process. The UF permeate was then collected in a second reservoir from where it was fed to an NF membrane batch operation. This latter process step provided the separation of the major part of polyphenols and saline compounds, which were retained in the concentrate stream. The NF permeate water, that is, the water recovered by the overall membrane process, was 80% of the initial wastewater volume and reached a quality consistent with agriculture irrigation use. For each plant and for both membrane processes the critical flux, as fouling index, was measured by adopting the method reported in detail elsewhere.1 The critical flux is obviously a function of the impurity content of the feedstock, which greatly increases throughout a batch membrane process. Therefore, in this study the critical flux was measured for different grades of impurities for each membrane process. In a previous work1 the authors showed that the electrical conductivity (EC) is fully representative of the overall impurities, with regard to the critical flux analysis; that is, it may be assumed to be the key impurity parameter. Accordingly, the critical flux values were measured at different EC values of the feedstock.
Figure 2. wastewater.
Critical flux values for the biologically pretreated
The critical flux values measured for the two membrane processes, together with their fitting curves, are reported in Figures 1 and 2 referring to plants 1 and 2, respectively, First of all, we may observe that starting from the same wastewater different critical flux values are obtained, as a consequence of the two different pretreatments. In particular, higher critical fluxes are always obtained when the flocculation pretreatment is applied. As previously suggested by the same authors,1 a semilogarithmic fitting of the experimental values of the critical flux was adopted, that is
Jc(EC) ) A - B ln(EC)
(1)
where A and B are fitting parameters. It is convenient to express the parameter A as a function of an EC reference value ECref. By applying eq 1 to this reference condition, we have
A ) Jc(ECref) + B ln(ECref)
(2)
and substituting eq 2 in eq 1, the following expression is obtained:
Jc(EC) ) Jc(ECref) - B ln(EC/ECref)
(3)
In this work the reference value ECref was assumed equal to 400 µS/cm, that is, the lowest EC experimented value. The fitting equations of the critical flux for the two separation membrane processes in the case of flocculation pretreatment, reported in Figure 1, are
Jc,UF ) 10.2 - 0.90 ln(EC/400)
(4)
Jc,NF ) 6.0 - 0.23 ln(EC/400)
(5)
whereas the fitting equations of the critical flux for the two separation membrane processes with the biological pretreatment, reported in Figure 2, are
Jc,UF ) 8.0 - 1.29 ln(EC/400)
(6)
Jc,NF ) 5.7 - 0.96 ln(EC/400)
(7)
As far as the membrane process rejection is concerned, UF operation exhibits in the two plants an almost constant rejection with respect to the pressure that is equal to 10% ( 1%. This is not the case for the NF rejection, which significantly
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Figure 3. Separation rejection for NF in the case of a feedstock’s EC equal to 600 µS/cm. Table 5. Critical Flux Values for UF and NF at 90% Recoverya optimal critical flux value [L h-1 m-2]
UF NF a
OWW biologically treated
OWW treated by flocculation with AS
6.24 4.05
9.21 5.52
OWW, olive washing wastewater.
increased on increase of the transmembrane pressure TMP (see Figure 3). The fitting equation for both used pretreatments is
RNF(EC) ) 0.67(TMP)(TMP + 1)-1
(8)
where TMP ) P - π and π is the osmotic pressure of the solution. During each membrane batch process, since the impurity concentration continuously increases, the feed flow rate was continuously decreased to avoid the attainment of critical flux conditions. The optimal policy, suggested by the authors in a previous work, seems to be to operate from the operation start point at a permeate flux smaller than the critical flux corresponding to the final feedstock quality. The prediction of this latter value requires the use of a simulation model. In this work this policy was adopted to predict the critical flux for both UF and NF for the two membrane processes by considering one or the other pretreatment process. By using a simulation model (MEMPHYS1) and assuming a 90% recovery factor for each separation step, the critical fluxes corresponding to the final process conditions were predicted, as reported in Table 5. Table 5 shows that the flocculation pretreatment allows an increased critical flux value of 48% for UF and 36% for NF, respectively. The effect of the different pretreatments is stronger in the first membrane separation step, that is for UF, as expected. In fact, the main cause of fouling is the gel-forming materials, which are mainly removed by UF operation. The lower fouling observed by using flocculation as pretreatment is due to a more effective elimination of the smaller particles by this operation. Since UF membrane blocking is affected mainly by particles with a length similar to the pore size,12 which is hundreds of nanometers, a residual content of very small suspended particles entails a higher fouling rate on the membrane. The same concept applies for the nanometer range to NF. The introduced coagulant in the solution is adsorbed by the small suspended particles, reducing the energy required for agglomeration. The so-formed agglomerates can be removed by sedimentation. However, an amount of coagulant still remains in the feedstock, and particles may continue to agglomerate as soon as they get into contact with each other on the membrane surface due to concentration polarization. These agglomerates can then be easily suspended by the feed stream and removed from the membrane. According to Bacchin and Aimar,9 under
these conditions fouling is less. On the contrary, the biofilter retains only very coarse particles, acting like a gravel filter, where smaller particles are not retained. These can successively deposit on the on the membrane surface, leading to irreversible fouling. This interpretation is confirmed by the chemical analysis reported in Tables 2 and 4. The flocculation operation eliminates more solids from the feedstock with respect to the bioreactor one, and the particles surviving in the pretreated stream are fewer. Moreover, the content of polyphenols, which have molecular weights in the range 500-20 000 Da and thus are in the same range as the membrane’s MWCO (see Table 1), are more efficiently removed by flocculation. Thus, polyphenol molecules may block the pores of both UF and NF membranes to a different extent according to the pretreatment separation efficiency. In conclusion, flocculation is the more efficient pretreatment since it eliminates micro- and nanoparticles from the solution by aggregation. This effect is visible during the initial sedimentation of the formed flocs, but continues to stay active during the whole membrane separation process. Confirming the observations reported by Fuchs et al.,10 the coagulant can be considered as an antifouling agent for membrane operations. Conclusions In this work different critical flux values were observed for both UF and NF separation processes as a consequence of different pretreatment processes performed on olive washing wastewater. Flocculation and biofiltration were used as pretreatments. It was found that flocculation performed better than biofiltration as a pretreatment. The effect of each pretreatment on the membrane processes was evaluated by adopting the fouling critical flux as the key parameter. The critical flux values predicted for both examined membrane processes after the flocculation pretreatment was up to 40% greater than ones obtained after biofiltration. This result seems to be strictly related to the amount and size of the residual suspended particles in the feedstock after the pretreatment. Nomenclature A ) fitting parameter B ) fitting parameter EC ) electrical conductivity [µS/cm] Jc ) critical flux [L h-1 m-2] mw ) pure water permeability [L h-1 m-2 bar-1] P ) operating pressure [bar] π ) osmotic pressure [bar] R ) rejection [%] TMP ) transmembrane pressure [bar] Note Added after ASAP Publication. The version of this paper that was published on the Web February 13, 2007 had mathematical errors in eqs 3-7. The correct version of this paper was published February 15, 2007. Literature Cited (1) Stoller, M.; Chianese, A. Optimization of membrane batch processes by means of the critical flux. Desalination 2006, 191, 62-70. (2) Field, R. W.; Wu, D.; Howell, J. A.; Gupta, B. B. Critical flux concept for microfiltration fouling. J. Membr. Sci. 1995, 100, 259272. (3) Ma¨ntta¨ri, M.; Nysto¨rm, M. Critical flux in NF of high molar mass polysaccharides and effluents from the paper industry. J. Membr. Sci. 2000, 170, 257-273.
Ind. Eng. Chem. Res., Vol. 46, No. 8, 2007 2253 (4) Cho, B. D.; Fane, A. G. Fouling transients in nominally sub-critical flux operation of a membrane bioreactor. J. Membr. Sci. 2002, 209, 391403. (5) Vyas, H. K.; Bennett, R. J.; Marshall, A. D. Performance of cross flow MF during constant TMP and constant flux operations. Int. Dairy J. 2002, 12, 473-479. (6) Lipp, P.; Lee, C. H:, Fane, A. G.; Fell, C. J. D. A fundamental study of the UF of oil-water emulsions. J. Membr. Sci. 1988, 36, 161-177. (7) Desal Osmonics, 760 Shadowridge Dr., Vista, CA 92083-7986. (8) Maxit Group, Filtralite Dept, Calle Hermosilla 100, 4b, E-28009 Madrid, Spain. (9) Bacchin, P.; Aimar, P. Critical fouling conditions induced by colloidal surface interaction: from causes to consequences. Desalination 2005, 175, 21-27.
(10) Fuchs, W.; Braun, R.; Theiss, M. Influence of various wastewater parameters on the fouling capacity during membrane filtration. ICOM2005 Proceedings, Seoul, South Korea, 2005; p 128. (11) Benkahala, Y. K.; Ould-Dris, A.; Jaffrin, M. Y.; Si-Hassan, D. Cake growth mechanism in cross-flow MF of mineral suspensions. J. Membr. Sci. 1995, 98, 107-117. (12) Sethi, S.; Wiesner, M. R. Performance and cost modelling of ultrafiltration. J. EnViron. Eng. 1995, 12, 874-882.
ReceiVed for reView July 24, 2006 ReVised manuscript receiVed January 11, 2007 Accepted January 11, 2007 IE060964K
