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Investigation of two ultrafiltration membranes for treatment of olive oil mill wastewater
Desalination 249 (2009) 660–666
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Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
Investigation of two ultrafiltration membranes for treatment of olive oil mill wastewater Ezgi Oktav Akdemir ⁎, Adem Ozer Dokuz Eylul University, Engineering Faculty, Department of Environmental Engineering, Tinaztepe Campus, 35160 Buca-Izmir, Turkey
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Article history: Accepted 30 June 2008 Available online 6 October 2009 Keywords: Olive oil mill wastewater Pretreatment Ultrafiltration Permeate flux Retention coefficient
a b s t r a c t In this study, a promising treatment method is given for the olive oil mill wastewater (OMWW). Although the same steps of this method have been used in different studies before, flow scheme is novel. The membrane filtration of pretreated OMWW was investigated by using two ultrafiltration membranes in this study. Pretreatment steps were pH adjustment (pH = 2) and cartridge filter filtration, and pH adjustment (pH = 6) and cartridge filter filtration. Each step of cartridge filter filtration was batch process and effluent from the filter was recycled back to OMWW tank. Pretreated OMWW was sent to feed vessel of experimental set-up. Recovery of olive oil in the OMWW was realized collecting it from the top of pretreated OMWW. Ultrafiltration membranes used were JW and MW membranes supplied by Osmonics. The effects of main operating parameters (transmembrane pressure, feed flow rate, pH and membrane type) on the permeate flux and membrane fouling were examined. The effectiveness of the different membranes and operating conditions was evaluated using retention coefficients calculated from COD and TOC of experimental studies. The highest permeate flux (25.9 l/m2 h) was obtained using MW membrane under operational conditions of Qf = 200 l/h flow rate and TMP = 4 bar, while the highest removals were obtained at Qf = 100 l/h flow rate and TMP = 1 bar. COD, TOC, SS, oil and grease concentrations of MW membrane effluent were 6400 mg/l, 2592 mg/l, 320 mg/l, and 270 mg/l, respectively. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Olive oil production is an important agricultural and economic activity in Mediterranean countries such as Italy, Spain, Greece and Turkey. Olive oil mill wastewater (OMWW) is a by-product of the three phase centrifugal olive oil extraction from olives. This dark liquid wastewater is composed of the olive fruit vegetation water, added water during extraction, washing water, and a portion of residual olive pulp oil [1]. The composition of OMWW is not constant and shows great variability because it depends on a lot of parameters such as kind of olive and ripeness, oil extraction technology, and duration of aging. Biochemical oxygen demand (BOD5) and chemical oxygen demand (COD) of OMWW may be as high as 100 and 200 g/l, respectively. The organic fraction of OMWW typically includes sugars, tannins, and phenolic compounds including polyphenols, polyalcohols, pectins and lipids. The color of OMWW may be attributed to the phenolics and varies from dark red to black depending on the age and type of olive processed [2]. In Turkey, most of the olive oil production plants are small and decantralized. For this reason, a centralized treatment of the wastewater seems to be not feasible. It means that a solution must be found for the small plants, which makes OMWW treatment simple
and effective [3]. Physical and chemical treatment methods include flocculation, coagulation, lagoon of evaporation and burning systems partial solutions [4–7]. Biological treatment of OMWW is difficult because of phenolic chemicals, compounds and seasonal production of oil [8,9]. Flexible and efficient treatment plants should assure not only a significant reduction of BOD and COD values, but also the possibility of selectively recovering some valuable compounds that could be used in the same production cycle or as raw material for other processes [10]. In this case, membrane processes should be used. There are limited applications of membrane processes for the treatment of OMWW [10–12]. One common problem of membrane filtration of OMWW is the membrane fouling that drastically reduces the efficiency of permeate and also changes its selectivity. Therefore, a pretreatment step is necessary to decrease membrane fouling and to increase filtration efficiency. By considering this fact, chemical and physical pretreatment steps were applied before ultrafiltration in a flat-sheet membrane module, separately. Water flux calculations and characterization of sample before and after ultrafiltration were given in the content of this paper. 2. Materials and methods 2.1. Sample collection
⁎ Corresponding author. Tel.: +90 232 4127135; fax: +90 232 4531143. E-mail addresses: [email protected] (E.O. Akdemir), [email protected] (A. Ozer). 0011-9164/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2008.06.035
Olive oil mill wastewater (OMWW) was taken from a 3-phase continuous olive oil mill plant located in Izmir-Turkey. Samples were
E.O. Akdemir, A. Ozer / Desalination 249 (2009) 660–666
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collected in December from the effluent of the horizontal decanter. Fresh sample was kept in the dark at 4 °C.
valves. During the filtration experiments, weight of permeate in permeate carrier was continuously monitored with 5 minute interval.
2.2. Pretreatment experiments
2.4. Analytical methods
pH adjustment and cartridge filtration were applied to samples as pretreatment step. H2SO4 and Ca(OH)2 were used for pH adjustment. In the first step, pH of OMWW was adjusted to pH = 2 by using 4 ml/ l H2SO4 with 98% purity. Then OMWW was mixed at 225 rpm for 3 min. This sample was recirculated for 60 min in 20 l tank through 20 µm cartridge filter. Then pH of filtered OMWW in the tank was adjusted to pH = 6 with 45 ml/l 10% Ca(OH)2 solution. This sample was also circulated and filtered through the 20 µm cartridge filter. Pretreated effluent in the tank was used in ultrafiltration membrane experiments. 2.3. Experimental system The membrane experiments were carried out in a laboratory-scale cross-flow membrane system. The feed stream was pumped from the feed vessel to the feed inlet. A portion of the solution permeated through the membrane and flowed into the permeate receiver. The concentrate stream flowed back to the 20 l feed vessel. Schematic flow diagram of experimental set-up is given in Fig. 1. A heat exchanger in the feed vessel was used in all filtration experiments to keep the temperature at 22–24 °C. Osmonics Sepa CF II membrane cell, which consists of two elements (cell body and cell holder), was used. Hydraulic pressure was applied to the top of the holder. This pressure causes the piston to extend downward and compress the cell body against the cell holder. A single piece of rectangular membrane was installed in the bottom cell body with a feed spacer. Two polymeric membranes, which were supplied by Osmonics as a flat sheet, were used in this study. JW membrane (polyvinylidine-difluoride) with a molecular weight cut-off of 30,000 Da and MW membrane (Ultrafilic) with a molecular weight cut-off of 100,000 Da were used. Membrane area was 0.0155 m2 for all membrane type. At the beginning of the experiments, pretreated effluent was filled into feed vessel of experimental set-up. Permeate from membrane was collected in the permeate collection vessel. The permeate volume collected during 1 h is between 170 ml and 300 ml. This permeate volume is around 1.5% of filter stock volume. This amount does not increase the concentration in feed vessel more than 6% even if the concentration in permeate is assumed to be zero. With 1 h interval, permeate in the collection vessel was poured into the feed vessel. The pressure and the recycle flow rate were controlled by regulation
COD, TOC, pH, SS, oil and grease measurements were carried out on the influent and effluent samples for the characterization and treatment studies. COD, SS, oil and grease analyses were carried out according to standard methods [13]. DOHRMANN DC-190 High Temperature TOC Analyzer was used for TOC measurements. pH measurement was done by using 890 MD pH meter. 3. Results and discussion 3.1. Characterization of olive oil mill wastewater OMWW sample was taken from 3-phase olive oil production plant. The main physicochemical characteristics of the used OMWW were as follows: COD: 84,000 mg/l; TOC: 35,542 mg/l; suspended solids: 11,200 mg/l; oil and grease: 25,100 mg/l; and pH: 4.8. OMWW has a dark brown color and characteristic smell. 3.2. Pretreatment experiments Pretreatment steps were pH adjustment and cartridge filtration. pH value of 20 l raw OMWW was adjusted to pH = 2 by using 4 ml/ l H2SO4 with 98% purity. This wastewater was recirculated through 20 µm cartridge filter for 60 min. This time is sufficient for 30 passes through the cartridge filter. More or less the maximum removal ratios were achieved after 2 or 3 passes. pH was adjusted to pH = 6 using 45 ml/l of 10% Ca(OH)2 solution. The same cartridge filtration was repeated. Cartridge filter blocking was not occurred. Using more OMWW it is possible to clog cartridge filter. In the experiments filter clogging was encountered during filtration of raw OMWW using 5 µm cartridge filter. The deposits on filter surface were easily washed by water. In real applications, it is useful to choose the cartridge filter pore size after particle size distribution determination of raw OMWW. Moreover usage of coarse to fine cartridge filter (50 µm or 20 µm) in a series increased the cartridge filter back washing periods. Periodic automatically washing of cartridge filters is possible with the filters available on the market. The COD and TOC concentrations of sample at all steps were measured and given in Table 1. As a result of all pretreatment experiments, COD and TOC concentrations of OMWW were 31,000 mg/l and 8172 mg/l, respectively.
Fig. 1. Schematic flow diagram of the experimental set-up.
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Table 1 Pretreatment results of OMWW. Sample
COD (mg/l)
TOC (mg/l)
Raw wastewater pH adjustment to pH = 2 and cartridge filter filtration pH adjustment to pH = 6 and cartridge filter filtration
84,000 35,000 31,000
35,542 9309 8172
COD removal efficiency was 63% by only pH adjustment and cartridge filtration. This result can be evaluated as quite satisfactory compared to 49% COD removal by two step coagulation [14]. The volume of captured particulate matter was too small compared to settled sludge during the two step coagulation. Total volume of settled sludge was approximately 750 ml during the two step coagulation of 1 l OMWW. On the other hand, the volume of total captured particulate matter was 50 ml for the two step cartridge filtration of 1 l OMWW. Adjustment of pH to 6 and cartridge filter filtration is useful to get bigger fluxes during the ultrafiltration than that of without pH = 6 adjustment. Moreover pH = 6 adjustment of effluent is necessary to satisfy discharge standards. In addition to COD removal, olive oil was collected on the pretreated wastewater. Recovery of olive oil is done by second decanter in olive oil production process. For decanter separation, sometimes hot water addition is necessary. However, hot water addition causes an increase in wastewater volume for the same amount of olive oil production without water addition [15]. 3.3. Membrane experiments One of the most important parameters in the membrane filtration is permeate flux (Jp) and it depends on the permeate volume, membrane area and filtration time. Permeate flux is given in Eq. (1). Permeate flux; Jp =
Permeate volumeðLÞ Membrane areaðm2 Þ × TimeðhÞ
ð1Þ
Permeate flux is affected by some operational conditions such as the transmembrane pressure, feed flow rate, pH and nature of the membrane. In order to investigate the effects of these parameters on permeate flux, experimental study was planned and two different ultrafiltration membranes were used in the content of this study. The performance of the membranes used in this study was evaluated by using COD and TOC removal efficiencies. Retention coefficients of COD and TOC were given by Eqs. (2) and (3). RCOD ð%Þ = ðCODF −CODP Þ = CODF × 100
ð2Þ
RTOC ð%Þ = ðTOCF −TOCP Þ = TOCF × 100
ð3Þ
Where, CODF and CODP represent the COD values measured in the feed and permeate streams; TOCF and TOCP represent the TOC values measured in the feed and permeate respectively. The effects of feed flow rate or cross-flow velocity and the transmembrane pressure on retention coefficients were examined in this study. Naturally, the type and characteristics of membrane determine the removal efficiencies. The filtration experiments were done by two different ultrafiltration membranes (MW and JW), and only the ultrafiltration experimental results for MW membrane of which fluxes are bigger than that of JW membrane are given in the following sections. The removal efficiencies, pressure, flow rate, pH relations and fouling characteristics of these two membrane types were not significantly different to each other. 3.3.1. Pressure–permeate flux–removal efficiency relations The variation of the permeate flux for the increasing transmembrane pressures, and different feed flow rates are given in Fig. 2. Almost a
Fig. 2. Influence of transmembrane pressure on the permeate flux for MW membrane, T = 22 °C, Co constant, ◊ = 200 l/h, □ = 150 l/h, and Δ = 100 l/h.
constant value of flux is reached at higher pressures. A constant flux at higher pressures is reported for microfiltration of oily wastewater [16]. Similar flux–pressure curves are given for the microfiltration and ultrafiltration of cork processing wastewaters [17]. Higher pressures cause the cake layer on membrane surface to compress and accelerate membrane fouling [18]. Some pressure conditions cause the formation of irreversible cake layer and accelerate membrane fouling [18]. Pressure at which flux reaches a constant value can be considered the optimum pressure [17]. In real wastewater problems, cake formation is not avoidable but may remain reversible. From Fig. 2, 3 bar seems to be the ideal pressure value because permeate flux must be as high as possible without cake layer formation at ideal pressure value. Using small permeate flux leads to use of huge membrane area for real olive oil production plants. In Fig. 2, highest flux difference (from 20.6 l/m2 h to 25.2 l/m2 h) is observed for 200 l/h recirculation flow rate corresponding to 18% increase in flux or 18% decrease in membrane area for 2 bar pressure increase (from 1 to 3 bar). On the other hand, higher pressure causes more energy consumption for the same amount of water. Selection of pressure is a matter of engineering economy considering the investment and operational cost. Instead of seeking optimum pressure, to choose the value of 1 bar in order to have the safest operating conditions against fouling issues seems more reasonable. The influence of pressure on the COD and TOC retention coefficients is depicted in Fig. 3. The use of lower pressures gives better removal efficiencies. These results are in agreement with the results of ultrafiltration works done by similar wastewater [18–20]. They have obtained decreasing retention coefficient for increasing pressure. The same result was obtained for OMWW ultrafiltration in this study for the repeated experiments. For the ultrafiltration of effluent from aerobic treatment of OMWW, retention coefficient of COD is almost constant with respect to the pressure [21]. A slight increase of RCOD with respect to pressure is also reported for the ultrafiltration process of cork processing wastewaters [17]. As it can be seen from Fig. 3, the retention coefficients decrease less than 5% for 3 bar pressure increase. COD and TOC retentions did not change significantly with pressure. Therefore it is not economically feasible to run the plant at pressures over 1 bar. 3.3.2. Feed flow rate–permeate flux–removal efficiency relations The effect of the feed flow rate on the permeate flux for the MW membrane at three different transmembrane pressures is given in Fig. 4. The feed flow rate parameter determines the tangential crossflow velocity. An increase in the cross-flow velocity increases turbulence. Accumulated particles on the membrane surface are carried into the bulk of the fluid, the concentration polarization effect decreases and the permeate flux increases [18]. In this study, flux increases are significant with increasing flow rate. Therefore operating at maximum flow rate seems to be reasonable.
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Fig. 3. Influence of transmembrane pressure on the apparent rejection coefficients for MW membrane, T = 22 °C, Co constant, ◊ = 200 l/h, □ = 150 l/h, and Δ = 100 l/h. (a) RCOD and (b) RTOC.
The effects of the flow rate on the COD and TOC retention coefficients for MW membrane are depicted in Fig. 5. These coefficients decrease with increasing flow rate. It is because of turbulence due to increasing
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Fig. 5. Influence of the feed flow rate on the rejection coefficients for MW membrane, T =22 °C, Co constant, x = 4 bar, ♦ = 3 bar, ■= 2 bar, and ▲= 1 bar. (a) RCOD and (b) RTOC.
cross-flow velocity or feed flow rate. Increasing turbulence reduces the membrane fouling. Lower fouling increases the permeate flux through the membrane and decreases the retention coefficients. This effect was also reported by other authors [20–22]. 3.3.3. pH–permeate flux relations In order to see the effect of pH on the permeate flux, pretreated effluent pH was adjusted to three different pH values as 2, 6, and 9. Fatty acids are weak acids and they dissolve at alkali solutions by equilibrium reaction given in Eq. (4). HA þ H2 O↔H3 O þ A
−
ð4Þ
Increasing pH drives the equilibrium to the right side. In this case, fatty acid molecules are converted into ions and their accumulation on the membrane surface is decreased so that the permeate flux increased [23]. Effect of pH on the permeate flux for MW membrane is given in Fig. 6. It is observed that as pH increases to pH = 9 from pH = 6, the permeate flux increases considerably. Since retention coefficients are not decreasing significantly with increasing flux rates, use of maximum fluxes for OMWW filtration plants seems reasonable. But maximum discharge standard is pH = 9 for effluents. Therefore pH values less than 9 must be used.
Fig. 4. Influence of the feed flow rate on the permeate flux for the MW membrane, T = 22 °C, Co constant, x = 4 bar, ♦ = 3 bar, ■ = 2 bar, and ▲ = 1 bar.
Fig. 6. Influence of pH on the permeate flux for the MW membrane, T = 22 °C, Co constant, Q = 150 l/h, ◊ = 3 bar, □ = 2 bar, and Δ = 1 bar.
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Fig. 7. Influence of the type of membrane on the permeate flux, T = 22 °C, Co constant, Q = 200 l/h, ● = MW membrane, and ○ = JW membrane.
In fact in OMWW filtration plants, the cost of use of more alkaline to achieve higher pH values and the benefit of use of less filtration modules running with higher permeate values can be compared. In this study, pH = 6 is selected for the rest of the study. 3.3.4. Membrane type–permeate flux relations The effect of the different types of ultrafiltration membranes used in this study with different transmembrane pressures at pH = 6 is shown in Fig. 7. In all cases of the ultrafiltration membranes, flux reached approximately constant value at the pressure between 1 and 2 bar. The permeate flux values at 2 bar pressure and pH = 6 conditions are 24.7 and 6.29 l/m2 h for MW and JW ultrafiltration membranes, respectively. As it is expected, membrane with the same nature, larger pore size and higher molecular weight cut-off gave higher permeate fluxes. So, MW membrane has higher permeate flux values. 3.4. Membrane fouling experiments 3.4.1. Pressure–membrane fouling–removal efficiency relations Fouling experiments were done at four different pressures and the results are given in Fig. 8. Permeate flux as a function of time at different pressures showed the same trend. At all pressures, declining rate of flux is not significant during the whole filtration period. According to the Fig. 8, at each pressure after about 60 min, flux reaches to more or less constant value because, the cake layer reaches to equilibrium and its growth ceases after this time. So, the cake layer resistance and subsequently permeate flux remain constant [19]. In another filtration experiment with the same membrane and wastewater, the flux value remained constant for 12 h. Fig. 9 shows the effects of transmembrane pressure on COD and TOC retention coefficients during 120 min operation time for MW membrane. For all pressures, in the first minutes of the operation, an important increase in the retention coefficients took place, and after 60 min, more or less steady-state conditions were reached. Retention
Fig. 8. Effect of transmembrane pressure on fouling for MW membrane, T = 22 °C, Co constant, Q = 150 l/h, x = 4 bar, ◊ = 3 bar, □ = 2 bar, and Δ = 1 bar.
Fig. 9. Effect of transmembrane pressure on retention coefficients for MW membrane, T = 22 °C, Co constant, Q = 150 l/h, x = 4 bar, ◊ = 3 bar, □ = 2 bar, and Δ = 1 bar. (a) RCOD and (b) RTOC.
coefficients were not changed considerably after 60 min. As it can be seen from Fig. 9a, COD retention coefficient was 80% after 60 min, and 82% after 120 min operation time at 1 bar transmembrane pressure. 3.4.2. Feed flow rate–membrane fouling–removal efficiency relations Effect of feed flow rate on membrane fouling at three different flow rates is given in Fig. 10. As it can be seen from the figure, permeate flux more or less increases linearly with feed flow rate after 60 min filtration. Permeate flux decline rate decreased gradually in the first 60 min. The decrease of permeate flux between 60 and 120 min is less than 1.5% for all pressures. This percentage of decrease seems to get lower after 120 min. Therefore it can be accepted that steady-state conditions were achieved after 60 min filtration. The permeate flux decline patterns for different feed flow rates were similar for OMWW and MW ultrafiltration membranes used in this study. The similarity of decline pattern of permeate fluxes was observed for flux–time curves at 2, 3, and 4 bar pressured and data not presented here. Influence of feed flow rate on retention coefficients during 120 min operation time is given in Fig. 11 for the MW membrane. Increasing flow rates resulted in decreasing retention coefficients. At 3 bar transmembrane pressure, maximum retention coefficients were achieved at
Fig. 10. Effect of feed flow rate on fouling for MW membrane, T = 22 °C, Co constant, TMP = 1 bar, ♦ = 200 l/h, ■ = 150 l/h, and ▲ = 100 l/h.
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Fig. 13. Influence of the type of membrane on the permeate flux, T = 22 °C, Co constant, Q = 200 l/h, TMP = 1 bar, ● = MW membrane, and ο = JW membrane.
Fig. 11. Effect of feed flow rate on retention coefficients for MW membrane, T = 22 °C, Co constant, TMP= 3 bar, ♦ = 200 l/h, ■ = 150 l/h, and ▲ = 100 l/h. (a) RCOD and (b) RTOC.
100 l/h flow rate, while maximum flux was achieved at 200 l/h flow rate. Reasonable values for retention and flux are located at opposite flow rates. There was an increase in retention coefficients up to 30 min. After 60 min, these coefficients did not change significantly. The change between 60 and 120 min was 1.5% and 1.2% for RCOD and RTOC, respectively. So it can be concluded that retention time can be chosen as 60 min.
retention coefficients are very close to each other (Fig. 14), MW is preferred to JW in this study. The decline of Jp between 20 and 120 min is 0.65 l/m2 h and 0.67 l/ m2 h for MW and JW membranes, respectively. This negligible decline of permeate flux confirms the weak fouling effect [17]. In Fig. 2, membrane fouling conditions start after 2 bar pressure. Therefore 1 bar is suitable to avoid membrane fouling. In order to evaluate the effect of operation time on COD and TOC removal, two membranes were examined. It is interesting that similar trends were observed in the rejection coefficients of two membranes. Fig. 14 shows the plot of the rejection coefficients for membranes JW and MW. In all cases, an important increase of the retention coefficients took place in the first minutes of the filtration, and after 60 min, more or less steady-state conditions were achieved. Similar COD removal efficiencies are obtained with two membranes which give considerably different permeate fluxes. There were different olive fruit particles in the OMWW and these particles were captured by both membranes which have different flux values due to their different structural characteristics.
3.4.3. pH–membrane fouling relations Fouling experiments at different pH values are given in Fig. 12. Permeate flux increased with increasing pH. Retention coefficient curves for different pH values showed similar properties with the curves given in Sections 3.4.1 and 3.4.2. The retention coefficients did not change more than 5% when pH was changed from 2 to 6. On the other hand, flux increased 4 times or the same pH range. Therefore high pH values should be preferred for industrial applications. 3.4.4. Membrane type–membrane fouling–removal efficiency relations The effects of the membrane type on the membrane fouling are given in Fig. 13. As expected, MW membrane gave higher flux values, compared to the JW membrane. It is because of the differences in the pore sizes and molecular weight cut-off values. The flux rates of MW membrane are four times higher than that of JW membrane. Since the
Fig. 12. Effect of pH on fouling for MW membrane, T = 22 °C, Co constant, Qf = 100 l/h, TMP = 1 bar, ◊ = pH: 9, □ = pH: 6, and Δ = pH: 2.
Fig. 14. Influence of the operation time on the rejection coefficients, T = 22 °C, Co constant, ● = MW membrane, and ○ = JW membrane. (a) RCOD and (b) RTOC.
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Table 2 Treatment results of pretreatment and ultrafiltration process.
COD (mg/l) TOC (mg/l) SS (mg/l) Oil–grease (mg/l)
Raw water
JW membrane
MW membrane
84,000 35,542 11,200 25,100
11,200 2534 290 370
6400 2592 320 270
3.4.5. Treatment results of the membrane processes Table 2 shows the results of treatment by ultrafiltration process with JW and MW membranes. Considerable reductions of parameters were achieved. However, the effluent quality of membranes did not satisfy Turkish standards of wastewater discharge to the sewer. These standards in Turkey are pH: 6–10, SS: 500 mg/l, oil and grease: 250 mg/l, and COD: 4000 mg/l. Therefore, additional processes should be applied for the treatment of olive oil mill wastewaters. The alternative and better solution is to improve the removal efficiency of pretreatment steps using cartridge filter with finer pore sizes and then to use ultrafiltration membranes with different characteristics. 4. Conclusions In this study, OMWW was used in order to investigate the variation of COD and TOC removal efficiencies together with permeate fluxes for ultrafiltration process. Before the ultrafiltration process, chemical and physical pretreatments were applied. First step of pretreatment is 20-µm cartridge filtration of OMWW after pH adjustment to pH = 2. Then effluent pH adjusted to pH = 6 and filtered through the cartridge filter again. Each step of cartridge filter filtration was done by recycling the cartridge filter effluent back to the OMWW tank. Pretreated OMWW was sent to ultrafiltration membrane. After pretreatment steps, olive oil in the raw OMWW was recovered by collecting it at the top of the tank. In the ultrafiltration experiment, the permeate flux increased with increasing TMP up to 3 bar and remained more or less constant for higher pressures. As an operation pressure, TMP = 1 bar seems to be safest operating condition against fouling issues. The permeate flux of MW is higher than JW and the permeate flux increased with increasing pressure, feed flow rate and pH for both ultrafiltration membranes. The retention coefficients of COD and TOC decreased with increasing permeate fluxes. The effects of increasing permeate flux on retention coefficients are not significant for OMWW and MW membranes in this study. This result gives savings in the design and operation without losing significant removal efficiency. The change of permeate flux with time was also examined in this study. In these experiments, effect of pressure, flow rate, pH and type of membrane were evaluated. Corresponding removal ratios were also
measured. More or less steady-state conditions were achieved after 60 min and remained constant indicating the weak fouling conditions. Two of the ultrafiltration membranes gave close removal efficiencies. Therefore ultrafiltration membrane with bigger molecular weight cut-off is preferred for OMWW in order to use higher flux value losing small removal efficiency. pH value of pretreated value should be as high as possible in accordance with the effluent discharge standards. The cross-flow velocity can be selected as high as possible considering the long term fouling conditions. Although discharge effluent standards are not obtained COD, TOC, SS, oil–grease removal efficiencies were 92.3%, 92.7%, 97.1%, and 98.9%, respectively. Further investigations on pretreatment using cartridge filter with finer pore size and then ultrafiltration process which can be determined by experimental or pilot plant studies should be considered. It is better to make economic evaluation of the process after pilot plant studies Especially the simplicity and reasonable removal efficiency of pretreatment steps are encouraging the pilot plant studies.
References [1] B. Sassi, A. Boularbah, A. Jaouad, G. Walker, A. Boussaid, Proc. Biochem. 41 (2006) 74–81. [2] A. Jaouani, S. Sayadi, M. Vanthournhout, M. Penninckx, Enzyme Microb. Technol. 33 (2003) 802–809. [3] H. Inan, A. Dimoglo, A. Simsek, M. Karpuzcu, Sep. Purif. Technol. 36 (2004) 23–31. [4] E. Aktas, S. Imre, L. Ersoy, Water Res. 9 (2001) 2336–2340. [5] F.J. Rivas, F.J. Beltran, O. Gimeno, J. Frades, J. Agric. Food Chem. 49 (2001) 1873–1880. [6] M. Mitrakas, G. Papageorgşou, A. Docoslid, G. Sakellaropoulos, Eur. Water Pollut. Control 6 (1996) 10–18. [7] M.C. Annesini, F. Gironi, Water Res. 25 (1991) 1157–1160. [8] C.I. Piperidou, C.I. Chaidou, C.D. Stalikas, K. Soulti, G.A. Pilidis, C. Balis, J. Agric. Food Chem. 48 (2000) 1941–1950. [9] P. Blánquez, G. Caminal, M. Sarrà, M.T. Vicent, Biotechnol. Prog. 18 (2002) 660–667. [10] E. Turano, S. Curcio, M. De Paola, V. Calabrò, G. Iorio, J. Membr. Sci. 209 (2002) 519–531. [11] R. Borsani, B. Ferrando, Desalination 108 (1996) 281–286. [12] P. Canepa, N. Marignetti, U. Rognoni, S. Calgari, Water Res. 22 (1988) 1491–1494. [13] A.E. Greenberg, L.S. Clesceri, A.D. Eaton, Standard Methods for the Examination of Water and Wastewater, APHA/AWWA/WEF, 16th ed., 1992 Washington DC. [14] E. Oktav, A. Ozer, Treatability of olive oil ındustry wastewater with physical and chemical treatment methods, ODTU 2nd National Environmental Pollution Control Symposium Proceeding Books, 2003, pp. 349–352, Ankara, Turkey. [15] M. Niaounakis, C.P. Halvadakis, Olive-Mill Waste Management, Literature Review and Patent Survey, Typothito-George Dandanos Publications, Athens, Greece, 2004. [16] A.B. Koltuniewicz, W. Field, Desalination 105 (1996) 79–89. [17] F.J. Benitez, J.L. Acero, A.I. Leal, Sep. Purif. Technol. 50 (2006) 354–364. [18] T. Mohammadi, A. Esmaeelifar, Desalination 166 (2004) 329–337. [19] T. Mohammadi, A. Esmaeelifar, J. Membr. Sci. 254 (2005) 129–137. [20] E.O. Akdemir, A. Özer, Sep. Purif. Technol. 62 (2008) 222–227. [21] M. Stoller, A. Chianese, Desalination 200 (1–3) (2006) 734–736. [22] M. Minhalma, C.R. Dias, M.N. De Pinho, Adv. Environ. Res. 3 (2000) 539–549. [23] J. Brinck, A.S. Jonsson, B. Jonsson, J. Lindau, J. Membr. Sci. 164 (2000) 187–194.
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Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
Investigation of two ultrafiltration membranes for treatment of olive oil mill wastewater Ezgi Oktav Akdemir ⁎, Adem Ozer Dokuz Eylul University, Engineering Faculty, Department of Environmental Engineering, Tinaztepe Campus, 35160 Buca-Izmir, Turkey
a r t i c l e
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Article history: Accepted 30 June 2008 Available online 6 October 2009 Keywords: Olive oil mill wastewater Pretreatment Ultrafiltration Permeate flux Retention coefficient
a b s t r a c t In this study, a promising treatment method is given for the olive oil mill wastewater (OMWW). Although the same steps of this method have been used in different studies before, flow scheme is novel. The membrane filtration of pretreated OMWW was investigated by using two ultrafiltration membranes in this study. Pretreatment steps were pH adjustment (pH = 2) and cartridge filter filtration, and pH adjustment (pH = 6) and cartridge filter filtration. Each step of cartridge filter filtration was batch process and effluent from the filter was recycled back to OMWW tank. Pretreated OMWW was sent to feed vessel of experimental set-up. Recovery of olive oil in the OMWW was realized collecting it from the top of pretreated OMWW. Ultrafiltration membranes used were JW and MW membranes supplied by Osmonics. The effects of main operating parameters (transmembrane pressure, feed flow rate, pH and membrane type) on the permeate flux and membrane fouling were examined. The effectiveness of the different membranes and operating conditions was evaluated using retention coefficients calculated from COD and TOC of experimental studies. The highest permeate flux (25.9 l/m2 h) was obtained using MW membrane under operational conditions of Qf = 200 l/h flow rate and TMP = 4 bar, while the highest removals were obtained at Qf = 100 l/h flow rate and TMP = 1 bar. COD, TOC, SS, oil and grease concentrations of MW membrane effluent were 6400 mg/l, 2592 mg/l, 320 mg/l, and 270 mg/l, respectively. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Olive oil production is an important agricultural and economic activity in Mediterranean countries such as Italy, Spain, Greece and Turkey. Olive oil mill wastewater (OMWW) is a by-product of the three phase centrifugal olive oil extraction from olives. This dark liquid wastewater is composed of the olive fruit vegetation water, added water during extraction, washing water, and a portion of residual olive pulp oil [1]. The composition of OMWW is not constant and shows great variability because it depends on a lot of parameters such as kind of olive and ripeness, oil extraction technology, and duration of aging. Biochemical oxygen demand (BOD5) and chemical oxygen demand (COD) of OMWW may be as high as 100 and 200 g/l, respectively. The organic fraction of OMWW typically includes sugars, tannins, and phenolic compounds including polyphenols, polyalcohols, pectins and lipids. The color of OMWW may be attributed to the phenolics and varies from dark red to black depending on the age and type of olive processed [2]. In Turkey, most of the olive oil production plants are small and decantralized. For this reason, a centralized treatment of the wastewater seems to be not feasible. It means that a solution must be found for the small plants, which makes OMWW treatment simple
and effective [3]. Physical and chemical treatment methods include flocculation, coagulation, lagoon of evaporation and burning systems partial solutions [4–7]. Biological treatment of OMWW is difficult because of phenolic chemicals, compounds and seasonal production of oil [8,9]. Flexible and efficient treatment plants should assure not only a significant reduction of BOD and COD values, but also the possibility of selectively recovering some valuable compounds that could be used in the same production cycle or as raw material for other processes [10]. In this case, membrane processes should be used. There are limited applications of membrane processes for the treatment of OMWW [10–12]. One common problem of membrane filtration of OMWW is the membrane fouling that drastically reduces the efficiency of permeate and also changes its selectivity. Therefore, a pretreatment step is necessary to decrease membrane fouling and to increase filtration efficiency. By considering this fact, chemical and physical pretreatment steps were applied before ultrafiltration in a flat-sheet membrane module, separately. Water flux calculations and characterization of sample before and after ultrafiltration were given in the content of this paper. 2. Materials and methods 2.1. Sample collection
⁎ Corresponding author. Tel.: +90 232 4127135; fax: +90 232 4531143. E-mail addresses: [email protected] (E.O. Akdemir), [email protected] (A. Ozer). 0011-9164/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2008.06.035
Olive oil mill wastewater (OMWW) was taken from a 3-phase continuous olive oil mill plant located in Izmir-Turkey. Samples were
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collected in December from the effluent of the horizontal decanter. Fresh sample was kept in the dark at 4 °C.
valves. During the filtration experiments, weight of permeate in permeate carrier was continuously monitored with 5 minute interval.
2.2. Pretreatment experiments
2.4. Analytical methods
pH adjustment and cartridge filtration were applied to samples as pretreatment step. H2SO4 and Ca(OH)2 were used for pH adjustment. In the first step, pH of OMWW was adjusted to pH = 2 by using 4 ml/ l H2SO4 with 98% purity. Then OMWW was mixed at 225 rpm for 3 min. This sample was recirculated for 60 min in 20 l tank through 20 µm cartridge filter. Then pH of filtered OMWW in the tank was adjusted to pH = 6 with 45 ml/l 10% Ca(OH)2 solution. This sample was also circulated and filtered through the 20 µm cartridge filter. Pretreated effluent in the tank was used in ultrafiltration membrane experiments. 2.3. Experimental system The membrane experiments were carried out in a laboratory-scale cross-flow membrane system. The feed stream was pumped from the feed vessel to the feed inlet. A portion of the solution permeated through the membrane and flowed into the permeate receiver. The concentrate stream flowed back to the 20 l feed vessel. Schematic flow diagram of experimental set-up is given in Fig. 1. A heat exchanger in the feed vessel was used in all filtration experiments to keep the temperature at 22–24 °C. Osmonics Sepa CF II membrane cell, which consists of two elements (cell body and cell holder), was used. Hydraulic pressure was applied to the top of the holder. This pressure causes the piston to extend downward and compress the cell body against the cell holder. A single piece of rectangular membrane was installed in the bottom cell body with a feed spacer. Two polymeric membranes, which were supplied by Osmonics as a flat sheet, were used in this study. JW membrane (polyvinylidine-difluoride) with a molecular weight cut-off of 30,000 Da and MW membrane (Ultrafilic) with a molecular weight cut-off of 100,000 Da were used. Membrane area was 0.0155 m2 for all membrane type. At the beginning of the experiments, pretreated effluent was filled into feed vessel of experimental set-up. Permeate from membrane was collected in the permeate collection vessel. The permeate volume collected during 1 h is between 170 ml and 300 ml. This permeate volume is around 1.5% of filter stock volume. This amount does not increase the concentration in feed vessel more than 6% even if the concentration in permeate is assumed to be zero. With 1 h interval, permeate in the collection vessel was poured into the feed vessel. The pressure and the recycle flow rate were controlled by regulation
COD, TOC, pH, SS, oil and grease measurements were carried out on the influent and effluent samples for the characterization and treatment studies. COD, SS, oil and grease analyses were carried out according to standard methods [13]. DOHRMANN DC-190 High Temperature TOC Analyzer was used for TOC measurements. pH measurement was done by using 890 MD pH meter. 3. Results and discussion 3.1. Characterization of olive oil mill wastewater OMWW sample was taken from 3-phase olive oil production plant. The main physicochemical characteristics of the used OMWW were as follows: COD: 84,000 mg/l; TOC: 35,542 mg/l; suspended solids: 11,200 mg/l; oil and grease: 25,100 mg/l; and pH: 4.8. OMWW has a dark brown color and characteristic smell. 3.2. Pretreatment experiments Pretreatment steps were pH adjustment and cartridge filtration. pH value of 20 l raw OMWW was adjusted to pH = 2 by using 4 ml/ l H2SO4 with 98% purity. This wastewater was recirculated through 20 µm cartridge filter for 60 min. This time is sufficient for 30 passes through the cartridge filter. More or less the maximum removal ratios were achieved after 2 or 3 passes. pH was adjusted to pH = 6 using 45 ml/l of 10% Ca(OH)2 solution. The same cartridge filtration was repeated. Cartridge filter blocking was not occurred. Using more OMWW it is possible to clog cartridge filter. In the experiments filter clogging was encountered during filtration of raw OMWW using 5 µm cartridge filter. The deposits on filter surface were easily washed by water. In real applications, it is useful to choose the cartridge filter pore size after particle size distribution determination of raw OMWW. Moreover usage of coarse to fine cartridge filter (50 µm or 20 µm) in a series increased the cartridge filter back washing periods. Periodic automatically washing of cartridge filters is possible with the filters available on the market. The COD and TOC concentrations of sample at all steps were measured and given in Table 1. As a result of all pretreatment experiments, COD and TOC concentrations of OMWW were 31,000 mg/l and 8172 mg/l, respectively.
Fig. 1. Schematic flow diagram of the experimental set-up.
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Table 1 Pretreatment results of OMWW. Sample
COD (mg/l)
TOC (mg/l)
Raw wastewater pH adjustment to pH = 2 and cartridge filter filtration pH adjustment to pH = 6 and cartridge filter filtration
84,000 35,000 31,000
35,542 9309 8172
COD removal efficiency was 63% by only pH adjustment and cartridge filtration. This result can be evaluated as quite satisfactory compared to 49% COD removal by two step coagulation [14]. The volume of captured particulate matter was too small compared to settled sludge during the two step coagulation. Total volume of settled sludge was approximately 750 ml during the two step coagulation of 1 l OMWW. On the other hand, the volume of total captured particulate matter was 50 ml for the two step cartridge filtration of 1 l OMWW. Adjustment of pH to 6 and cartridge filter filtration is useful to get bigger fluxes during the ultrafiltration than that of without pH = 6 adjustment. Moreover pH = 6 adjustment of effluent is necessary to satisfy discharge standards. In addition to COD removal, olive oil was collected on the pretreated wastewater. Recovery of olive oil is done by second decanter in olive oil production process. For decanter separation, sometimes hot water addition is necessary. However, hot water addition causes an increase in wastewater volume for the same amount of olive oil production without water addition [15]. 3.3. Membrane experiments One of the most important parameters in the membrane filtration is permeate flux (Jp) and it depends on the permeate volume, membrane area and filtration time. Permeate flux is given in Eq. (1). Permeate flux; Jp =
Permeate volumeðLÞ Membrane areaðm2 Þ × TimeðhÞ
ð1Þ
Permeate flux is affected by some operational conditions such as the transmembrane pressure, feed flow rate, pH and nature of the membrane. In order to investigate the effects of these parameters on permeate flux, experimental study was planned and two different ultrafiltration membranes were used in the content of this study. The performance of the membranes used in this study was evaluated by using COD and TOC removal efficiencies. Retention coefficients of COD and TOC were given by Eqs. (2) and (3). RCOD ð%Þ = ðCODF −CODP Þ = CODF × 100
ð2Þ
RTOC ð%Þ = ðTOCF −TOCP Þ = TOCF × 100
ð3Þ
Where, CODF and CODP represent the COD values measured in the feed and permeate streams; TOCF and TOCP represent the TOC values measured in the feed and permeate respectively. The effects of feed flow rate or cross-flow velocity and the transmembrane pressure on retention coefficients were examined in this study. Naturally, the type and characteristics of membrane determine the removal efficiencies. The filtration experiments were done by two different ultrafiltration membranes (MW and JW), and only the ultrafiltration experimental results for MW membrane of which fluxes are bigger than that of JW membrane are given in the following sections. The removal efficiencies, pressure, flow rate, pH relations and fouling characteristics of these two membrane types were not significantly different to each other. 3.3.1. Pressure–permeate flux–removal efficiency relations The variation of the permeate flux for the increasing transmembrane pressures, and different feed flow rates are given in Fig. 2. Almost a
Fig. 2. Influence of transmembrane pressure on the permeate flux for MW membrane, T = 22 °C, Co constant, ◊ = 200 l/h, □ = 150 l/h, and Δ = 100 l/h.
constant value of flux is reached at higher pressures. A constant flux at higher pressures is reported for microfiltration of oily wastewater [16]. Similar flux–pressure curves are given for the microfiltration and ultrafiltration of cork processing wastewaters [17]. Higher pressures cause the cake layer on membrane surface to compress and accelerate membrane fouling [18]. Some pressure conditions cause the formation of irreversible cake layer and accelerate membrane fouling [18]. Pressure at which flux reaches a constant value can be considered the optimum pressure [17]. In real wastewater problems, cake formation is not avoidable but may remain reversible. From Fig. 2, 3 bar seems to be the ideal pressure value because permeate flux must be as high as possible without cake layer formation at ideal pressure value. Using small permeate flux leads to use of huge membrane area for real olive oil production plants. In Fig. 2, highest flux difference (from 20.6 l/m2 h to 25.2 l/m2 h) is observed for 200 l/h recirculation flow rate corresponding to 18% increase in flux or 18% decrease in membrane area for 2 bar pressure increase (from 1 to 3 bar). On the other hand, higher pressure causes more energy consumption for the same amount of water. Selection of pressure is a matter of engineering economy considering the investment and operational cost. Instead of seeking optimum pressure, to choose the value of 1 bar in order to have the safest operating conditions against fouling issues seems more reasonable. The influence of pressure on the COD and TOC retention coefficients is depicted in Fig. 3. The use of lower pressures gives better removal efficiencies. These results are in agreement with the results of ultrafiltration works done by similar wastewater [18–20]. They have obtained decreasing retention coefficient for increasing pressure. The same result was obtained for OMWW ultrafiltration in this study for the repeated experiments. For the ultrafiltration of effluent from aerobic treatment of OMWW, retention coefficient of COD is almost constant with respect to the pressure [21]. A slight increase of RCOD with respect to pressure is also reported for the ultrafiltration process of cork processing wastewaters [17]. As it can be seen from Fig. 3, the retention coefficients decrease less than 5% for 3 bar pressure increase. COD and TOC retentions did not change significantly with pressure. Therefore it is not economically feasible to run the plant at pressures over 1 bar. 3.3.2. Feed flow rate–permeate flux–removal efficiency relations The effect of the feed flow rate on the permeate flux for the MW membrane at three different transmembrane pressures is given in Fig. 4. The feed flow rate parameter determines the tangential crossflow velocity. An increase in the cross-flow velocity increases turbulence. Accumulated particles on the membrane surface are carried into the bulk of the fluid, the concentration polarization effect decreases and the permeate flux increases [18]. In this study, flux increases are significant with increasing flow rate. Therefore operating at maximum flow rate seems to be reasonable.
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Fig. 3. Influence of transmembrane pressure on the apparent rejection coefficients for MW membrane, T = 22 °C, Co constant, ◊ = 200 l/h, □ = 150 l/h, and Δ = 100 l/h. (a) RCOD and (b) RTOC.
The effects of the flow rate on the COD and TOC retention coefficients for MW membrane are depicted in Fig. 5. These coefficients decrease with increasing flow rate. It is because of turbulence due to increasing
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Fig. 5. Influence of the feed flow rate on the rejection coefficients for MW membrane, T =22 °C, Co constant, x = 4 bar, ♦ = 3 bar, ■= 2 bar, and ▲= 1 bar. (a) RCOD and (b) RTOC.
cross-flow velocity or feed flow rate. Increasing turbulence reduces the membrane fouling. Lower fouling increases the permeate flux through the membrane and decreases the retention coefficients. This effect was also reported by other authors [20–22]. 3.3.3. pH–permeate flux relations In order to see the effect of pH on the permeate flux, pretreated effluent pH was adjusted to three different pH values as 2, 6, and 9. Fatty acids are weak acids and they dissolve at alkali solutions by equilibrium reaction given in Eq. (4). HA þ H2 O↔H3 O þ A
−
ð4Þ
Increasing pH drives the equilibrium to the right side. In this case, fatty acid molecules are converted into ions and their accumulation on the membrane surface is decreased so that the permeate flux increased [23]. Effect of pH on the permeate flux for MW membrane is given in Fig. 6. It is observed that as pH increases to pH = 9 from pH = 6, the permeate flux increases considerably. Since retention coefficients are not decreasing significantly with increasing flux rates, use of maximum fluxes for OMWW filtration plants seems reasonable. But maximum discharge standard is pH = 9 for effluents. Therefore pH values less than 9 must be used.
Fig. 4. Influence of the feed flow rate on the permeate flux for the MW membrane, T = 22 °C, Co constant, x = 4 bar, ♦ = 3 bar, ■ = 2 bar, and ▲ = 1 bar.
Fig. 6. Influence of pH on the permeate flux for the MW membrane, T = 22 °C, Co constant, Q = 150 l/h, ◊ = 3 bar, □ = 2 bar, and Δ = 1 bar.
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Fig. 7. Influence of the type of membrane on the permeate flux, T = 22 °C, Co constant, Q = 200 l/h, ● = MW membrane, and ○ = JW membrane.
In fact in OMWW filtration plants, the cost of use of more alkaline to achieve higher pH values and the benefit of use of less filtration modules running with higher permeate values can be compared. In this study, pH = 6 is selected for the rest of the study. 3.3.4. Membrane type–permeate flux relations The effect of the different types of ultrafiltration membranes used in this study with different transmembrane pressures at pH = 6 is shown in Fig. 7. In all cases of the ultrafiltration membranes, flux reached approximately constant value at the pressure between 1 and 2 bar. The permeate flux values at 2 bar pressure and pH = 6 conditions are 24.7 and 6.29 l/m2 h for MW and JW ultrafiltration membranes, respectively. As it is expected, membrane with the same nature, larger pore size and higher molecular weight cut-off gave higher permeate fluxes. So, MW membrane has higher permeate flux values. 3.4. Membrane fouling experiments 3.4.1. Pressure–membrane fouling–removal efficiency relations Fouling experiments were done at four different pressures and the results are given in Fig. 8. Permeate flux as a function of time at different pressures showed the same trend. At all pressures, declining rate of flux is not significant during the whole filtration period. According to the Fig. 8, at each pressure after about 60 min, flux reaches to more or less constant value because, the cake layer reaches to equilibrium and its growth ceases after this time. So, the cake layer resistance and subsequently permeate flux remain constant [19]. In another filtration experiment with the same membrane and wastewater, the flux value remained constant for 12 h. Fig. 9 shows the effects of transmembrane pressure on COD and TOC retention coefficients during 120 min operation time for MW membrane. For all pressures, in the first minutes of the operation, an important increase in the retention coefficients took place, and after 60 min, more or less steady-state conditions were reached. Retention
Fig. 8. Effect of transmembrane pressure on fouling for MW membrane, T = 22 °C, Co constant, Q = 150 l/h, x = 4 bar, ◊ = 3 bar, □ = 2 bar, and Δ = 1 bar.
Fig. 9. Effect of transmembrane pressure on retention coefficients for MW membrane, T = 22 °C, Co constant, Q = 150 l/h, x = 4 bar, ◊ = 3 bar, □ = 2 bar, and Δ = 1 bar. (a) RCOD and (b) RTOC.
coefficients were not changed considerably after 60 min. As it can be seen from Fig. 9a, COD retention coefficient was 80% after 60 min, and 82% after 120 min operation time at 1 bar transmembrane pressure. 3.4.2. Feed flow rate–membrane fouling–removal efficiency relations Effect of feed flow rate on membrane fouling at three different flow rates is given in Fig. 10. As it can be seen from the figure, permeate flux more or less increases linearly with feed flow rate after 60 min filtration. Permeate flux decline rate decreased gradually in the first 60 min. The decrease of permeate flux between 60 and 120 min is less than 1.5% for all pressures. This percentage of decrease seems to get lower after 120 min. Therefore it can be accepted that steady-state conditions were achieved after 60 min filtration. The permeate flux decline patterns for different feed flow rates were similar for OMWW and MW ultrafiltration membranes used in this study. The similarity of decline pattern of permeate fluxes was observed for flux–time curves at 2, 3, and 4 bar pressured and data not presented here. Influence of feed flow rate on retention coefficients during 120 min operation time is given in Fig. 11 for the MW membrane. Increasing flow rates resulted in decreasing retention coefficients. At 3 bar transmembrane pressure, maximum retention coefficients were achieved at
Fig. 10. Effect of feed flow rate on fouling for MW membrane, T = 22 °C, Co constant, TMP = 1 bar, ♦ = 200 l/h, ■ = 150 l/h, and ▲ = 100 l/h.
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Fig. 13. Influence of the type of membrane on the permeate flux, T = 22 °C, Co constant, Q = 200 l/h, TMP = 1 bar, ● = MW membrane, and ο = JW membrane.
Fig. 11. Effect of feed flow rate on retention coefficients for MW membrane, T = 22 °C, Co constant, TMP= 3 bar, ♦ = 200 l/h, ■ = 150 l/h, and ▲ = 100 l/h. (a) RCOD and (b) RTOC.
100 l/h flow rate, while maximum flux was achieved at 200 l/h flow rate. Reasonable values for retention and flux are located at opposite flow rates. There was an increase in retention coefficients up to 30 min. After 60 min, these coefficients did not change significantly. The change between 60 and 120 min was 1.5% and 1.2% for RCOD and RTOC, respectively. So it can be concluded that retention time can be chosen as 60 min.
retention coefficients are very close to each other (Fig. 14), MW is preferred to JW in this study. The decline of Jp between 20 and 120 min is 0.65 l/m2 h and 0.67 l/ m2 h for MW and JW membranes, respectively. This negligible decline of permeate flux confirms the weak fouling effect [17]. In Fig. 2, membrane fouling conditions start after 2 bar pressure. Therefore 1 bar is suitable to avoid membrane fouling. In order to evaluate the effect of operation time on COD and TOC removal, two membranes were examined. It is interesting that similar trends were observed in the rejection coefficients of two membranes. Fig. 14 shows the plot of the rejection coefficients for membranes JW and MW. In all cases, an important increase of the retention coefficients took place in the first minutes of the filtration, and after 60 min, more or less steady-state conditions were achieved. Similar COD removal efficiencies are obtained with two membranes which give considerably different permeate fluxes. There were different olive fruit particles in the OMWW and these particles were captured by both membranes which have different flux values due to their different structural characteristics.
3.4.3. pH–membrane fouling relations Fouling experiments at different pH values are given in Fig. 12. Permeate flux increased with increasing pH. Retention coefficient curves for different pH values showed similar properties with the curves given in Sections 3.4.1 and 3.4.2. The retention coefficients did not change more than 5% when pH was changed from 2 to 6. On the other hand, flux increased 4 times or the same pH range. Therefore high pH values should be preferred for industrial applications. 3.4.4. Membrane type–membrane fouling–removal efficiency relations The effects of the membrane type on the membrane fouling are given in Fig. 13. As expected, MW membrane gave higher flux values, compared to the JW membrane. It is because of the differences in the pore sizes and molecular weight cut-off values. The flux rates of MW membrane are four times higher than that of JW membrane. Since the
Fig. 12. Effect of pH on fouling for MW membrane, T = 22 °C, Co constant, Qf = 100 l/h, TMP = 1 bar, ◊ = pH: 9, □ = pH: 6, and Δ = pH: 2.
Fig. 14. Influence of the operation time on the rejection coefficients, T = 22 °C, Co constant, ● = MW membrane, and ○ = JW membrane. (a) RCOD and (b) RTOC.
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Table 2 Treatment results of pretreatment and ultrafiltration process.
COD (mg/l) TOC (mg/l) SS (mg/l) Oil–grease (mg/l)
Raw water
JW membrane
MW membrane
84,000 35,542 11,200 25,100
11,200 2534 290 370
6400 2592 320 270
3.4.5. Treatment results of the membrane processes Table 2 shows the results of treatment by ultrafiltration process with JW and MW membranes. Considerable reductions of parameters were achieved. However, the effluent quality of membranes did not satisfy Turkish standards of wastewater discharge to the sewer. These standards in Turkey are pH: 6–10, SS: 500 mg/l, oil and grease: 250 mg/l, and COD: 4000 mg/l. Therefore, additional processes should be applied for the treatment of olive oil mill wastewaters. The alternative and better solution is to improve the removal efficiency of pretreatment steps using cartridge filter with finer pore sizes and then to use ultrafiltration membranes with different characteristics. 4. Conclusions In this study, OMWW was used in order to investigate the variation of COD and TOC removal efficiencies together with permeate fluxes for ultrafiltration process. Before the ultrafiltration process, chemical and physical pretreatments were applied. First step of pretreatment is 20-µm cartridge filtration of OMWW after pH adjustment to pH = 2. Then effluent pH adjusted to pH = 6 and filtered through the cartridge filter again. Each step of cartridge filter filtration was done by recycling the cartridge filter effluent back to the OMWW tank. Pretreated OMWW was sent to ultrafiltration membrane. After pretreatment steps, olive oil in the raw OMWW was recovered by collecting it at the top of the tank. In the ultrafiltration experiment, the permeate flux increased with increasing TMP up to 3 bar and remained more or less constant for higher pressures. As an operation pressure, TMP = 1 bar seems to be safest operating condition against fouling issues. The permeate flux of MW is higher than JW and the permeate flux increased with increasing pressure, feed flow rate and pH for both ultrafiltration membranes. The retention coefficients of COD and TOC decreased with increasing permeate fluxes. The effects of increasing permeate flux on retention coefficients are not significant for OMWW and MW membranes in this study. This result gives savings in the design and operation without losing significant removal efficiency. The change of permeate flux with time was also examined in this study. In these experiments, effect of pressure, flow rate, pH and type of membrane were evaluated. Corresponding removal ratios were also
measured. More or less steady-state conditions were achieved after 60 min and remained constant indicating the weak fouling conditions. Two of the ultrafiltration membranes gave close removal efficiencies. Therefore ultrafiltration membrane with bigger molecular weight cut-off is preferred for OMWW in order to use higher flux value losing small removal efficiency. pH value of pretreated value should be as high as possible in accordance with the effluent discharge standards. The cross-flow velocity can be selected as high as possible considering the long term fouling conditions. Although discharge effluent standards are not obtained COD, TOC, SS, oil–grease removal efficiencies were 92.3%, 92.7%, 97.1%, and 98.9%, respectively. Further investigations on pretreatment using cartridge filter with finer pore size and then ultrafiltration process which can be determined by experimental or pilot plant studies should be considered. It is better to make economic evaluation of the process after pilot plant studies Especially the simplicity and reasonable removal efficiency of pretreatment steps are encouraging the pilot plant studies.
References [1] B. Sassi, A. Boularbah, A. Jaouad, G. Walker, A. Boussaid, Proc. Biochem. 41 (2006) 74–81. [2] A. Jaouani, S. Sayadi, M. Vanthournhout, M. Penninckx, Enzyme Microb. Technol. 33 (2003) 802–809. [3] H. Inan, A. Dimoglo, A. Simsek, M. Karpuzcu, Sep. Purif. Technol. 36 (2004) 23–31. [4] E. Aktas, S. Imre, L. Ersoy, Water Res. 9 (2001) 2336–2340. [5] F.J. Rivas, F.J. Beltran, O. Gimeno, J. Frades, J. Agric. Food Chem. 49 (2001) 1873–1880. [6] M. Mitrakas, G. Papageorgşou, A. Docoslid, G. Sakellaropoulos, Eur. Water Pollut. Control 6 (1996) 10–18. [7] M.C. Annesini, F. Gironi, Water Res. 25 (1991) 1157–1160. [8] C.I. Piperidou, C.I. Chaidou, C.D. Stalikas, K. Soulti, G.A. Pilidis, C. Balis, J. Agric. Food Chem. 48 (2000) 1941–1950. [9] P. Blánquez, G. Caminal, M. Sarrà, M.T. Vicent, Biotechnol. Prog. 18 (2002) 660–667. [10] E. Turano, S. Curcio, M. De Paola, V. Calabrò, G. Iorio, J. Membr. Sci. 209 (2002) 519–531. [11] R. Borsani, B. Ferrando, Desalination 108 (1996) 281–286. [12] P. Canepa, N. Marignetti, U. Rognoni, S. Calgari, Water Res. 22 (1988) 1491–1494. [13] A.E. Greenberg, L.S. Clesceri, A.D. Eaton, Standard Methods for the Examination of Water and Wastewater, APHA/AWWA/WEF, 16th ed., 1992 Washington DC. [14] E. Oktav, A. Ozer, Treatability of olive oil ındustry wastewater with physical and chemical treatment methods, ODTU 2nd National Environmental Pollution Control Symposium Proceeding Books, 2003, pp. 349–352, Ankara, Turkey. [15] M. Niaounakis, C.P. Halvadakis, Olive-Mill Waste Management, Literature Review and Patent Survey, Typothito-George Dandanos Publications, Athens, Greece, 2004. [16] A.B. Koltuniewicz, W. Field, Desalination 105 (1996) 79–89. [17] F.J. Benitez, J.L. Acero, A.I. Leal, Sep. Purif. Technol. 50 (2006) 354–364. [18] T. Mohammadi, A. Esmaeelifar, Desalination 166 (2004) 329–337. [19] T. Mohammadi, A. Esmaeelifar, J. Membr. Sci. 254 (2005) 129–137. [20] E.O. Akdemir, A. Özer, Sep. Purif. Technol. 62 (2008) 222–227. [21] M. Stoller, A. Chianese, Desalination 200 (1–3) (2006) 734–736. [22] M. Minhalma, C.R. Dias, M.N. De Pinho, Adv. Environ. Res. 3 (2000) 539–549. [23] J. Brinck, A.S. Jonsson, B. Jonsson, J. Lindau, J. Membr. Sci. 164 (2000) 187–194.