Journal of Hazardous Materials 263P (2013) 168–176

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Effective treatment of olive mill effluents from two-phase and three-phase extraction processes by batch membranes in series operation upon threshold conditions J.M. Ochando-Pulido a,∗ , G. Hodaifa b , M.D. Victor-Ortega a , S. Rodriguez-Vives a , A. Martinez-Ferez a a b

Department of Chemical Engineering, University of Granada, 18071 Granada, Spain Molecular Biology and Biochemical Engineering Department, University of Pablo de Olavide, 41013 Seville, Spain

h i g h l i g h t s • • • • •

Effective reclamation of two-phase and three-phase olive oil extraction effluents. Membranes behavior accurately optimized by means of threshold flux theory. Fouling control and optimal hydrodynamics ensuring safe design and steady operation. Economic boost due on recoverable lab-made ferromagnetic-core photocatalyst. Standards for discharging OVW-2 and OVW-3 in sewers upon near-zero membrane fouling.

a r t i c l e

i n f o

Article history: Received 9 February 2013 Received in revised form 13 March 2013 Accepted 16 March 2013 Available online 23 March 2013 Keywords: Reverse osmosis Nanofiltration Ultrafiltration Threshold flux Olive mill wastewater Wastewater reclamation

a b s t r a c t Production of olive oil results in the generation of high amounts of heavy polluted effluents characterized by extremely variable contaminants degree, leading to sensible complexity in treatment. In this work, batch membrane processes in series comprising ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO) are used to purify the effluents exiting both the two-phase and tree-phase extraction processes to a grade compatible to the discharge in municipal sewer systems in Spain and Italy. However, one main problem in applying this technology to wastewater management issues is given by membrane fouling. In the last years, the threshold flux theory was introduced as a key tool to understand fouling problems, and threshold flux measurement can give valuable information regarding optimal membrane process design and operation. In the present manuscript, mathematical approach of threshold flux conditions for membranes operation is addressed, also implementing proper pretreatment processes such as pH-T flocculation and UV/TiO2 photocatalysis with ferromagnetic-core nanoparticles in order to reduce membranes fouling. Both influence the organic matter content as well as the particle size distribution of the solutes surviving in the wastewater stream, leading, when properly applied, to reduced fouling, higher rejection and recovery values, thus enhancing the economic feasibility of the process. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Olive oil industry, actually one of the main agricultural activities of the Mediterranean Basin countries, generates two main wastewater streams, the first one from the washing of the fruit (olives washing wastewater, OWW) and the second one from the extraction of the olive oil (olive mill wastewater, OMW, a mixture of the proper olive-fruit humidity along with process-added water).

∗ Corresponding author. Tel.: +34 958241581; fax: +34 958248992. E-mail address: [email protected] (J.M. Ochando-Pulido). 0304-3894/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jhazmat.2013.03.041

These effluents are commonly referred to as olive mill effluents (OME). In traditional olive oil mills working with the batch press method, no longer in wide use, 0.4–0.6 m3 of OMW were produced per ton of processed olives, whereas nowadays an average-sized modern olive oil factory leads to a daily amount of up to 10–15 m3 of OMW, in sum to 1 m3 /day of OWW (Table 1). Only in Spain, this raises a total volume of more than 9 million m3 of OMW per year. Considering that in the two-phase extraction water injection is only practiced in the final vertical centrifugation step, the volume of liquid effluent derived from the decanting process (OMW-2) is reduced by one third on average if compared to the amount required for the three-phase system (Table 1). Moreover, much of

J.M. Ochando-Pulido et al. / Journal of Hazardous Materials 263P (2013) 168–176 Table 1 Flow rates of the different effluents of continuous extraction processes [11]. Effluent flow rate, L kg−1

3-phase extraction

2-phase extraction

Washing of olives (OWW) Horizontal centrifuge Vertical centrifuge Cleaning Total

0.06 0.90 0.20 0.05 1.21

0.05 0 0.15 0.05 0.25

the organic matter remains in the solid waste, which contains more humidity than the pomace from the three-phase system (60–70% in two-phase systems vs. 30–45% in three-phase ones, OMW-3) and hence OMW-2 exhibits lower pollutants degree, too (Table 2). The disposal of the solid waste stream is not the objective of the present work, which aims only towards the management problem related to the reclamation of the liquid effluents. Some solutions already proposed for the management of the pomace waste are for instance adsorption of heavy metals [1–3], dyes [4] and phenols [5], as well as composting [6] or biogas production [7], among others. The two-phase system appears to be more ecological and thus has been strongly promoted in Spain. Nevertheless, the threephase system is still surviving in other countries where scarcity of financial support did not permit the technological switch. In this work, purification of the effluents from both decanting processes is specifically addressed, a hard task given their highly variable physico-chemical composition, which also depends on edaphoclimatic and cultivation parameters, the type, quality and maturity of the olives [8–10]. OMW-2 and OMW-3 are among the heaviest polluted industrial effluents by organic matter and are characterized by strong odor nuisance, acid pH, intensive violet-dark color and high saline toxicity, exhibiting considerable electroconductivity (EC) values [10–12]. Uncontrolled disposal of these effluents represents an environmental hazard, causing soil contamination, underground leakage and water body pollution. Summarized compositions of the wastewater from the olives cleaning (OWW) and from the olive oil extraction (OMW), including batch press (OMW-P) as well as two-phase (OMW-2) and three-phase (OMW-3) continuous olive oil decanting processes, are reported in Table 2. Major organic pollutants concentration is found in the effluent exiting the centrifuges (OMW-2 and OMW-3), including phenols, organic acids, tannins and organohalogenated contaminants which are mostly phytotoxic and thus recalcitrant to biological degradation. The presence of these substances would be hardly reflected by measurements of the biological oxygen demand (BOD5 ), thus the chemical oxygen demand (COD) appears more appropriate as key parameter together with total phenols (TPh) concentration (Table 2). Due to this presence of high levels of COD as well as refractory compounds, and also fats and lipids, direct disposal of these effluents to the municipal sewage treatment systems is prohibited. Legal limits are established in order to prevent difficulties to the municipal sewer wastewater treatment plants, which rely on biomasses that must be maintained alive. At the moment there is no specific European legislation regarding the regulation of olive mill discharges, and standard procedures are left to individual countries. Only one EU directive exists,

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Directive 2000/60/CE, pointing out the necessity of conferring maximum protection to water and introducing the idea of the use of regenerated wastewater, thus encouraging the establishment of a legal framework capable to achieve this goal. Therefore, direct discharge of OMW on the ground fields and public superficial waters is actually prohibited in Spain; this is not the case of Italy and other EU countries where partial discharge on suitable terrains is still allowed. Up to now, various treatment processes for the management and reclamation of OMW have been proposed [8]. Biological treatment of OMW is a hard task and right now not applied on a large scale due to the resistance of OMW to biological degradation [13–17]. Other treatment practices have been developed, such as lagooning or natural evaporation and thermal concentration [9,18], treatments with lime and clay [19,20], composting [21–23], physico-chemical procedures as coagulation-flocculation [24–26] and electrocoagulation [27,28], advanced oxidation processes including ozonation [29], Fenton’s reagent [30,31] and photocatalysis [32] and also electrochemical [33–35] and hybrid processes [36–39]. Membranes were revealed in the last decade as a very promising technology, but even though it is now more than half a century that the first membranes appeared, this technology is still in continuous development, and much effort is to be further done for better comprehension and improvement of membrane processes. Pressure-driven membrane processes – microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO) – have been applied in the last years for municipal, agricultural and industrial wastewater reclamation [40–46]. Several works have been conducted in the past by means of membrane technology with the target to reduce the organic load of OMW [47–52], but only few focusing on OMW-2 [53–55]. Furthermore, some authors have tried the extraction of added-value compounds contained in this effluent, mainly low-molecular-weight polyphenols and sugars, by concentration with membranes [56–58]. In all these works membrane fouling has been noticed to play a key role during operation, leading to increase in operating and energy costs as well as frequent plant shut-downs for in situ membrane cleaning and also irretrievable membrane life shortening. Hence, control of fouling is key to increase the profitability and competiveness of this technology. Some research groups have observed that the non-adoption of specifically tailored OMW pretreatment processes leads straightly to rapid development of fouling on the membranes [49,50,53,54]. Moreover, other factors exhibiting high influence on membranes performances apart from feedstock composition are hydrodynamic conditions and membrane type, roughness and porosity. Field et al. [59] introduced for the first time the concept of critical flux for MF membranes, defining it as the permeate flux below which fouling is not promptly observed, and afterwards critical flux values were also identified in UF and NF membranes [60–62]. However, later on some authors noted that this behavior is not always strictly observed in the treatment of real wastewater streams by membrane processes, and hence this theory was extended to these cases by introducing the concept of threshold flux [63–65]. Confirmation of the existence of a threshold flux in the case of the treatment of OMW with membranes has been recently reported by Stoller and Ochando [66]. The threshold flux makes reference

Table 2 Characteristics of effluents of batch or continuous olive oil extraction processes. Process

ID

COD, g O2 ·L−1

Olives cleaning Batch press Three phase Two phase

OWW OMW-P OMW-3 OMW-2

0.8–2.2 130–130 30–220 4–18

Tss: total suspended solids.

BOD5 , g O2 ·L−1 0.3–1.5 90–100 5–45 0.8–6.0

Tss, g L−1 8–18 10–12 5–35 2–7

pH 5.5–6.6 4.5–5.0 3.5–5.5 3.5–6.0

EC, mS cm−1 2.5–3.0 2.0–5.0 2.0–7.9 1.5–2.5

TPh, g L−1 0–0.1 1.0–2.4 0.3–7.5 0.1–1.0

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to the maximum permeate flux at which fouling builds up at a very low and constant rate, and above which the rate of fouling increases exponentially. Threshold flux values may be enhanced by properly tailored raw wastewater pretreatment processes and optimized operating conditions. Until the present moment, there is no study addressing a treatment suitable for both the effluents from the two-phase (OMW-2) and three-phase (OMW-3) olive oil extraction processes available. In this work, the treatment of both effluents by a batch membranes process consisting of UF followed by NF and finally RO in sequence will be discussed. Beforehand, both feedstocks were processed by the following pretreatment processes: (i) pH-temperature flocculation, (ii) UV/TiO2 photocatalysis. At the end, compliance with municipal sewers discharge and irrigation standards was checked. 2. Experimental 2.1. Analytical procedure Chemical oxygen demand (COD), total phenols (TPh), total suspended solids (Tss), electroconductivity (EC) and pH measurements were performed following standard methods [67]. COD was measured by means of a LASA 100 photometer with the COD cuvettes LCK014 supplied by Dr. Hach Lange, whereas EC was measured by the 8706R1 portable instrument supplied by Delta Ohm. Last, particle size distribution analysis of suspended and colloidal matter was carried out with a Plus90 nanosizer supplied by Brookhaven. All analytical methods were applied at least in triplicate with analytical-grade reagents. The list of reagents used were 70% (w/w) HNO3 , 98% (w/w) NaOH, 98% (w/w) Na2 SO3 , 30% (w/w) NH4 OH, 37% (w/w) HCl and 30% (w/w) FeCl3 , supplied by Panreac, whereas 70% (w/w) TiO2 P-25 nanopowder was provided by Degussa. 2.2. Raw feedstock pretreatment In first instance, both raw feedstocks (OMW-2 and OMW-3) were subjected to gridding (cut-size equal to 300 ␮m) in order to remove coarse particles. Subsequently, two different pretreatment steps were applied on both feedstocks, the first consisting in pH-T flocculation and the other one in photocatalysis with TiO2 nanopowder under UV light (nominal power 45 W, wavelength 365 nm). The best operating conditions as well as the optimal chemical dosage for both pretreatment steps were taken from a previous work, and checked again at lab scale [68]. The pH-T flocculation process is inexpensive if compared with others for the removal of suspended matter, and consisted in adding 70% w/w HNO3 or 1 N NaOH to whether reduce or increase the pH values of the feedstock (from 2 to 7) at various temperatures ranging from 15 to 50 ◦ C, to promote formation of easily sedimentable flocks. The UV light/TiO2 photocatalysis is an advanced oxidation process (AOP) potentially useful not only for the oxidative degradation but also reductive cleavage of a wide range of organic solutes into non-toxic compounds such as CO2 and H2 O [69]. Moreover, it is rather cost-effective in contrast with other AOPs [27–39,70], since it may be exploited at ambient conditions and also activated under sunlight in case of properly doped TiO2 [71]. The details about the production of the lab-made photocatalyst are described elsewhere [68,72,73]. Briefly explained, the photocatalyst was produced through a sol–gel process by means of a spinning-disk reactor, and consisted of a ferromagnetic core (␥Fe2 O3 , modal particle size of 30 nm) and two subsequent layers of silica and titania (␥-Fe2 O3 /SiO2 /TiO2 ) (Fig. 1). This obtained nanophotocatalyst presented a final mean particle size of 79 nm with pure anatase titania phase and traces of brookite. The magnetic

Fig. 1. Ferromagnetic core catalyst ␥-Fe2 O3 /SiO2 /TiO2 layer-structure.

property of the photocatalyst permits its recovery from the treated wastewater stream and reuse, rending this procedure extremely cost effective. The two pretreatment steps used in this work were scaled up: pH-T flocculation was carried out in a 20 L stirred batch reactor provided with a turbine impeller stirrer, whereas photocatalysis was conducted in an agitated 8 L batch reactor, equipped with an UV lamp on top. Finally, achieved reduction of COD, total phenols and Tss concentration was measured at the end of each pilot-scale pretreatment step. 2.3. Description of membranes, filtration procedures and pilot-scale plant The membranes pilot plant, schematically represented in Fig. 2, is provided with a 100 L feed tank (FT1 ) where the various feedstocks were loaded. Two different pumps – a centrifugal booster (P1 ) and a volumetric piston (P2 ) pumps - served to drive the raw effluents to the spiral-wound (SW) membrane module fitted in housing M1 . The characteristics of the membranes chosen for this research, all polymeric ones supplied by GE Water and Process Technologies, are reported in Table 3. The used membrane modules were model GM for UF, model DK for NF and model SC for RO, all three with an active area equal to 2.5 m2 . These membranes were previously employed in other previous experiments with raw olive mill wastewaters for more than 1000 h operation time, and thus exhibited low pure water permeability values if compared to virgin ones. Although the use of new membrane modules would lead to better

Fig. 2. Membrane filtration pilot plant flow diagram, FT1 : feed tank, P1 : booster pump, P2 : volumetric pump, V1 : bypass regulation valve, V2 : concentrate regulation valve, E1 and E2 : plate heat exchangers, M1 : membrane housing provided with SW membrane.

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171

Table 3 Membranes measured characteristics. Membrane type

Model series

Kw , L h−1 m−2 bar−1

Pore size, nm

MWCO, Da

RNaCl , %

Surface, m2

Max. P, bar

Max. T, ◦ C

UF NF RO

GM DK SC

5.2 2.5 1.9

2 0.5 4000). Otherwise, permeate flux was gauged during operation time through a precision electronic mass balance (AX-120 Cobos, 0.1 mg accuracy). Then, threshold flux estimation was carried out with one of the methods available in the scientific literature for critical flux measurement. Both critical and threshold flux values cannot be theoretically predicted and experimental determination is needed [74]. The chosen method, proposed by Espinasse et al. [75], consists basically in a hysteresis cycle for the pressures range of each corresponding membrane, increasing and decreasing stepwise the net driving pressure up and down, in a way that complete restoration of the permeate flux must be observed for the same pressure level after one cycle to stay within threshold flux conditions. Hence, the highest pressure value at which this condition is ultimately observed divides the low fouling region from the high fouling region. After each measurement, the threshold flux value (Jth ) and its corresponding operating pressure (Pth ) were noted. To maintain the characteristics of the feedstocks constant during threshold flux measurements, both permeate and concentrate streams were cooled down to the feedstock temperature and then mixed and recycled back to the raw wastewater tank (recycling mode). Finally, the permeate flux profiles during batch runs – that is collecting the permeate stream whereas steadily recirculating the concentrate flow back to the feed tank – were examined for all the membranes. At the end of each experiment, rinsing of the membrane with tap water for 30 min was performed. If no longer necessary, the membrane module was stored in fresh tap water, after which chemical cleaning of the circuit with 1 N NaOH solution was performed in closed loop for 30 min.

10

5

3. Results and discussion 3.1. Tailored pretreatment efficiency 3.1.1. pH-T flocculation Physico-chemical composition of both raw feedstocks (OMW-2 and OMW-3) after incipient sieving is given in Table 4, confirming much higher COD and total phenols as well as EC and Tss

0

0

2

4

pH Fig. 3. Lab-scale pH-T flocculation results with regard to OMW-2 (caption A) and OMW-3 (caption B); 䊉 = 50 ◦ C,  = 25 ◦ C,  = 15 ◦ C.

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Table 5 OMW-2 and OMW-3 physicochemical composition after pilot-scale pH-T flocculation. Parameters

pH Tss COD TPh

Table 7 OMW-2 and OMW-3 physicochemical UV/ferromagnetic TiO2 photocatalysis.

composition

after

pilot-scale

Final parametric values, g L−1

Removal efficiencies, %

Parameters

OMW-2

OMW-3

OMW-2

OMW-3

OMW-2

OMW-3

2.5 1.6 14.5 0.172

3.0 7.9 19.1 0.905

– 72.4 12.7 6.5

– 76.1 41.4 1.7

pH Tss, g L−1 COD, g L−1 TPh, mg L−1

2.9 1.15 11.1 139

3.2 5.1 15.2 723

Tss removal in the range of acid pH (2.5–3) and low ambient temperature (15–25 ◦ C). Keeping in mind the economical effectiveness of the process, pH-T flocculation was transferred to a pilot scale at ambient temperature (25 ± 0.5 ◦ C) and pH value equal to 3.0 ± 0.25 (Fig. 3). At acid pH values (pH < 4) much of the organic matter present in OMW-2 and OMW-3, such as aromatic compounds and organic acids, becomes protonated and thus neutrally charged. The lack of net charge leads to absence of electrostatic repulsion among these macromolecules, hence aggregation is enhanced resulting in large flocks which can sediment with ease [68,76]. Moreover, this phenomenon is boosted at lower temperature conditions, since formation of sub-micron and supra-micron aggregates is favored as solubility of organic matter becomes reduced [77]. Results of the pilot-scale pH-T flocculation pretreatment (Table 5) showed up to 72.4% and 76.1% Tss removal together with 12.7% and 41.1% COD abatement for OMW-2 and OMW-3 respectively. This implies that even higher percentage of Tss removed by the pH-T flocculation process correspond to organic substances in case of OMW-3. Furthermore, the resultant sludge rounded 12.5% (v/v) and 10.8% for OMW-2 and OMW-3, which correspond to recovery values of 87.5% (v/v) and 89.2% of the clarified supernatant respectively. These results reveal the pH-T flocculation process as an economic alternative to current coagulation-flocculation processes with commercial flocculants for the removal of Tss load in olive mill effluents. 3.1.2. UV/TiO2 photocatalysis The supernatant of the pH-T flocculation pretreatment process was subsequently either directly carried on the membranes (stream hereafter called FS1 or FS2 for OMW-2 and OMW-3, respectively) or further processed by UV/TiO2 photocatalysis upstream   the membranes-in-series operation (assigned FS1 or FS2 for OMW2 and OMW-3 respectively). Results of lab-scale UV/TiO2 tests are reported in Table 6. Higher reduction of the key depuration parameter (−COD) was found to be achieved with the laboratory-made ferromagneticcore catalyst for FS1 (pre-flocculated OMW-2), proving better performance than the commercial TiO2 nanopowder Degussa P-25 used for comparison purposes. After 4 h of operation a maximum reduction of the COD parameter of 20.7% was achieved by the commercial catalyst upon a dosage of 9 g L−1 , but even better result

(21.7%) could be ensured by using the laboratory-made ferromagnetic catalyst at a sensible lower dosage (1 g L−1 ). The same catalyst was selected to carry out the UV/TiO2 photocatalysis process for FS2 (pre-flocculated OMW-3). For this raw feedstock, a higher dosage of 1.5 g L−1 resulted in providing the highest COD removal after 2 h of operation time (18.3%). The major organic matter abatement ensured by the lab-made ferromagnetic-core catalyst can be justified on the basis of its TiO2 phase consisting in pure anatase with small traces of brookite, whilst commercial Degussa P-25 consists of 70% anatase and 30% rutile. Anatase is been proved to be the most photoactive among the three TiO2 phases (anatase, rutile and brookite). Moreover, catalyst activation also depends on the particle habit and size, and the narrow particle size distribution of the self-made catalyst permits to maximize the photocatalytic performances [72,73]. By using optimized operating conditions and ferromagnetic catalyst dosages for each feedstock, UV/TiO2 photocatalysis pretreatment step was scaled-up. Results at a pilot scale are reported in Table 7, showing values of up to 23.4% COD, 19.2% total phenols and 28.1% Tss removal efficiencies for FS1 whereas higher Tss removal (35.4%) was accomplished for FS2 though slightly lower COD abatement (20.4%) was observed. What is more, the novel developed lab-made titania ferromagnetic-core photocatalyst (␥-Fe2 O3 /SiO2 /TiO2 ) can be whether recovered back from the wastewater stream by a magnetic trap and reused or even fixed to the photocatalysis reactor, giving a solution to the problem of the recovery of the catalyst and thus considerably enhancing the cost-effectiveness of the process. 3.2. Membranes threshold flux analysis The scope of the present paper was to study the impacts of the differently pretreated feedstocks, OMW-2 and OMW-3 solely pretreated by pH-T flocculation (FS1 and FS2 , respectively) or further pretreated by photocatalysis with the lab-made ferromagnetic  core nanocatalyst (FS1 and FS2 , respectively), on the threshold flux values of the different membranes used for the purification of both olive mill effluents. The permeate stream of each membrane step was send as feedstock to the next one. The equations used to describe the flux behavior of the membranes as a function of the operation time in relationship to the threshold flux state as follows [65,75]: Jp (t) = Jp,0 − a · t;

Jp (t) ≤ Jth

(1)

Table 6 COD reduction in OMW-2 and OMW-3 after lab-scale UV/TiO2 photocatalysis. Raw wastewater

Catalyst type

Catalyst dosage, g L−1

−COD2h , %

−COD4h , %

CODfinal , g L−1

OMW-2

Degussa P-25

1 3 9 20 1 3 9 1.5 3

13.4 16.2 18.9 17.6 16.8 12.9 6.9 18.3 14.6

16.5 18.6 20.7 18.6 21.4 13.3 10.3 – –

12.1 11.8 11.5 11.8 11.4 12.5 13.1 15.6 16.3

Ferromagnetic TiO2

OMW-3

Ferromagnetic TiO2

J.M. Ochando-Pulido et al. / Journal of Hazardous Materials 263P (2013) 168–176

(2)

where Jp (t), Jp,0 and Jth are the permeate flux at a given time, the initial permeate flux and the threshold permeate flux respectively (L h−1 m−2 ), whereas a and b are fouling fitting parameters. In turn, Jp,0 can be calculated with the next equation: Jp,0 = K0 · PTM

(3)

in which K0 is the membrane initial permeability (L h−1 m−2 bar−1 ) and PTM (bar) represents the net driving pressure across the membrane. Below threshold flux conditions (Eq. (1)), a certain constant but very low permeate flux drop is observed during operation time, given by the a parameter. On the other hand, permeate flux loss increases noticeably above threshold flux conditions (Eq. (2)) and fouling starts to develop rapidly over the membrane, fact given by the b parameter which indicates additional exponential permeate flux decay. Threshold flux values corresponding to each membrane stage regarding the various feedstocks were estimated with the method proposed by Espinasse et al. [74]. During the pressure cycling experiments, steady-state conditions were apparently reached after 15 min operation and thus these data were taken as the steady-state permeate flux points. Taking into account that even below threshold flux conditions loss of membrane permeability is suffered, according to Eq. (3) the theoretical permeate flux gap (Jp∗ (t)) when the same pressure level (PTM ) is applied after one cycle can be expressed as follows [75]: Jp∗ (t, PTM ) = Jp (t1 , PTM ) − Jp (t1 , PTM ) = K · PTM

(4)

Also, combining Eqs. (1) and (3) and deriving with respect to the time: K = −a · dt

(5)

And thus linking Eq. (5) to Eq. (4) a final expression for Jp∗ is attained: Jp∗ (t, PTM )

= −a · PTM · t

(6)

where the value of the constant fouling fitting parameter a can be calculated from the value of Jp measured at the lowest PTM : a = (K0 · PTM,1 − Jp,1measured )/t

(7)

With these data, graphical representations of theoretical (Jp∗ (t)) and experimental (Jp (t)) permeate flux gaps vs. applied net driving pressure (PTM ) during the pressure cycles were constructed for each feedstock and membrane (Figs. 4–6). The threshold flux (highlighted with circled points in those figures) is the one at the highest PTM value where the measured Jp does not deviate significantly from the theoretical Jp∗ (Eq. (1)) value. When this premise fails to apply, threshold flux boundary conditions are surpassed and Eq. (2) fits the flux trend instead. The values of Jp∗ (t) vs. Jp (t) as function of the applied PTM dur

ing the pressure cycles for the UF membrane with FS1 and FS1 are given in Fig. 4. As it can be observed, for OMW-2 solely pretreated by pH-T flocculation (FS1 ) the experimental permeate flux gaps begin to deviate from the theoretical values at a pressure of 9 bar, thus the value of Pth is estimated to be 8 bar. However, this deviation from the theoretical flux trend is observed at a value of 10 bar for  OMW-2 further pretreated by UV/TiO2 photocatalysis (FS1 ) with the ferromagnetic-core nanopowder. Similarly, higher Pth was registered (8 bar) upon NF of the UF permeate of OMW-2 pretreated by both pH-T flocculation and UV/TiO2 photocatalysis in contrast with that solely pretreated by the former (Pth equal to 7 bar), as shown in Fig. 5. Hence, it appears

A)

1.0

0.8

ΔJp, L/hm2

Jp (t) > Jth

0.6

0.4

0.2

0.0

B)

0

2

4

6

8

10

12

8

10

12

PTM, bar 0.6 0.5

ΔJp, L/hm2

Jp (t) = (Jp,0 − Jth ) · e−b·t + Jth − a · t;

173

0.4 0.3 0.2 0.1 0.0

0

2

4

6

PTM, bar Fig. 4. Theoretical (−−) vs. experimental (о) permeate flux gaps (Jp ) during UF pressure cycling for OMW-2 pretreated by pH-T flocculation (caption A) vs. further pretreated by UV/TiO2 photocatalysis (caption B).

that UV/TiO2 photocatalysis performed by using the lab-made ferromagnetic nanopowder helps both UF and NF membrane stages to work at higher Pth , which increases the threshold flux value accordingly (Jth ). This result may be justified by the additional polyphenols concentration removal (19.2%) yielded by the UV/TiO2 photocatalysis (Table 7), which present molecular weights between 0.5 and 20 kDa and thus in the same range as the membrane’s molecular weight cut-off (MWCO). In light of these results, OMW-3 was directly pretreated by pHT flocculation and UV/TiO2 photocatalysis with the ferromagnetic  nanoparticles (FS2 ). In Fig. 6, the threshold flux conditions for the  treatment of FS2 with the UF and NF membranes sequenced are examined. In this case, a value of Pth of 4 bar was estimated for the UF membrane, owed to the fact that deviation from the theoretical threshold flux linear fit was noticed at a value of 5 bar. Otherwise, Pth was found to be equal to 5 bar in the case of the NF membrane. After the determination of the threshold flux values, both UF and NF operations were performed in batch mode. To close the treatment process loop, a final purification stage consisting in a RO membrane was conducted, for which an operating pressure of 20 bar was selected. Threshold fluxes of RO were not measured since no sensible fouling can be observed for this membrane when put in contact with the almost clear wastewater pretreated by UF and NF. In other words, the RO system relies within threshold flux regimes even at high operating pressures, above operational limit.

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A)

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A)

1.0

0.20

ΔJp, L/hm2

ΔJp, L/hm2

0.8

0.6

0.4

0.2

0.0

0.15

0.10

0.05

0

2

4

6

8

10

12

14

PTM, bar

B)

0.25

0.00

B)

0

2

4

6

8

10

6

8

10

PTM, bar 0.6

1.0

0.5

ΔJp, L/hm2

ΔJp, L/hm2

0.8

0.6

0.4

0.4

0.3 0.2 0.1

0.2

0.0

0.0

0

2

4

6

8

10

12

0

2

4

14

PTM, bar Fig. 5. Theoretical (−−) vs. experimental (о) permeate flux gaps (Jp ) during NF pressure cycling for OMW-2 pretreated by pH-T flocculation (caption A) vs. further pretreated by UV/TiO2 photocatalysis (caption B).

Results are summarized in Table 8, where the threshold flux values (Pth – Jth ) of every membrane step for each feedstock are given, as well as the results referring to the batch membranes-in-series operation including the COD abatement (RCOD ), the recovery rate (Y) and the experimental steady-state permeate flux registered (Jss ) for each membrane stage. It can be observed that steady-state permeate flux values (Jss ) observed during the batch membranes sequence were, upon threshold hydrodynamic conditions (Pth ), in good line with the threshold flux values (Jth ) previously estimated by the pressure cycling method. Furthermore, high recovery rate values were attained for all membrane steps regardless of the feedstock.

PTM, bar Fig. 6. Theoretical (−−) vs. experimental (о) permeate flux gaps (Jp ) during UF (caption A) and NF (caption B) pressure cycling for OMW-3 pretreated by pH-T flocculation and UV/TiO2 photocatalysis.

However, not only higher steady-state/threshold permeate fluxes were provided for the OMW-2 effluent ulteriorly pretreated by UV/TiO2 photocatalysis with the ferromagnetic nanoparticles after pH-T flocculation (9.6, 12.3 and 13.2 L h−1 m−2 vs. 7.6, 10.2 and L h−1 m−2 ) but also major recovery rates (88.1%, 85% and 83.3% for UF, NF and RO respectively vs. 87.4%, 84.2% and 75.8%) and organic matter rejection efficiencies (RCOD ) were ensured for every membrane operation (48.5, 76.6 and 90.5% vs. 39.9, 69.4 and 82.8%). These results confirm that, even though photocatalysis did not lead to very high COD removal efficiencies, similarly as reported by other authors [78], it is quite relevant as a pretreatment step before the membranes-in-series process, leading to improved threshold flux production. This has important economic implications since

Table 8 Results analyses of the membrane-in-series steps. Raw effluent

Membrane

Feedstock

Pth , bar

Jth , L h−1 m−2

Jss , L h−1 m−2

RCOD , %

Y, %

OMW-2

UF

FS1  FS1 FS1 , UF  FS1 , UF FS1, UF+NF  FS1 , UF+NF  FS2  FS2 , UF  FS2 , UF+NF

8 9 7 8 20 20 4 5 20

7.3 9.4 10.3 12.5 – – 0.8 6.9 –

7.6 9.6 10.2 12.3 10.5 13.2 0.6 6.6 22.6

39.9 48.5 69.4 76.6 82.8 90.5 28.2 63.1 89.1

87.4 88.1 84.2 85.0 75.8 83.3 74.5 76.7 80.2

NF RO OMW-3

UF NF RO

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fouling inhibition enables continuous operation of the plant and reduces sensibly the running costs for membrane module substitution. On the other hand, lower steady-state/threshold flux values were confirmed for UF and NF membrane stages in the treatment of OMW-3 pretreated by both pH-T flocculation and photocatalysis in contrast to the values corresponding to OMW-2, as well as lower RCOD (see Table 8), given the higher EC and COD values in the former feedstock. Moreover, slightly minor recovery values were achieved. However, these results are quite satisfactory taking into account the major pollutants load in the raw OMW-3. What is more, quite higher (41.6%) steady-state RO permeate flux was   registered for FS2 , UF+NF if compared to FS1 , UF+NF upon the same operating pressure (20 bar), owed maybe to the fact that the higher  presence of organic matter in FS2 , UF+NF may derive in molecular aggregation leading to bigger particles more easily retained by the RO membrane, confirmed by similar RCOD in spite of its higher organic concentration. In conclusion, final COD values equal to 452 mg O2 L−1  (FS1, UF+NF ) and 121 mg O2 L−1 (FS1 , UF+NF ) were measured in the RO permeate streams after both OMW-2 treatments, whereas 466 mg  O2 L−1 (FS2 , UF+NF ) for OMW-3. This means the achievement of quality standards for irrigation (values below 1000 mg O2 ·L−1 ) in all cases, as well as for discharge not only in Italian, but also in Spanish sewer systems (values below 500 and 125 mg O2 L−1 , respectively) in case of OMW-2. 4. Conclusions The pretreatment process including pH-T flocculation followed by UV/TiO2 photocatalysis with ferromagnetic-core nanoparticles appears to be very promising for efficient pretreatment of olive mill effluents from both two-phase (OMW-2) and threephase (OMW-3) continuous extraction processes before batch membranes-in-series operation consisting of UF followed by NF and finally RO polymeric membrane modules. This pretreatment procedure ensured higher and steady threshold permeate flux values in all membrane separation stages, major COD rejection values and increased recovery rates, enhancing the cost-effectiveness of the management process of both OMW-2 and OMW-3 by the proposed batch membranes sequence. Moreover, the purified wastewater stream can be discharged in Italian and Spanish sewers. The concept of the threshold flux is a key tool for controlling fouling problems common to all large-scale membranes applications, giving valuable information regarding optimal hydrodynamics to ensure safe design and steady operation of the plant. Acknowledgments The membrane pilot plant was constructed in the framework of the European project the research work was performed in the framework of the European project PHOTOMEM (contract no. FP7-SME-2011, grant 262470) and was revamped under the European project ETOILE (contract no. FP7-SME-2007–1, grant 222331). Funding by the EC is gratefully acknowledged. The Spanish Ministry of Science and Innovation is also gratefully acknowledged for having funded the projects CTQ2007-66178 and CTQ2010-21411. References [1] R. Baccar, J. Bouzid, M. Feki, A. Montiel, Preparation of activated carbon from Tunisian olive-waste cakes and its application for adsorption of heavy metal ions, J. Hazard. Mater. 162 (2-3) (2009) 1522–1529. [2] E. Malkoc, Y. Nuhoglu, M. Dundar, Adsorption of chromium (VI) on pomace – An olive oil industry waste: batch and column studies, J. Hazard. Mater. 138 (1) (2006) 142–151.

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