Physicochemical analysis and adequation of olive oil mill wastewater after advanced oxidation process for reclamation by pressure-driven membrane technology

Science of the Total Environment 503–504 (2015) 113–121

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Physicochemical analysis and adequation of olive oil mill wastewater after advanced oxidation process for reclamation by pressure-driven membrane technology Javier Miguel Ochando-Pulido a,⁎, Maria Dolores Victor-Ortega a,1, Gassan Hodaifa b, Antonio Martinez-Ferez a,1 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 • • • • •

Complete physicochemical characterization of OMW exiting secondary treatment (ST). Particle size distribution reveals 75.1% particle concentration below 2 μm. 31.7% of organic pollutants with size below 3 kDa very relevant to membrane fouling Saturation index warns to work below 90% feedstock recovery factor to avoid scaling. Quality standards improved to reuse the treated effluent for irrigation.

a r t i c l e

i n f o

Article history: Received 15 April 2014 Received in revised form 17 June 2014 Accepted 24 June 2014 Available online 10 July 2014 Keywords: Advanced oxidation processes Membrane technology Olive oil mill wastewaters Particle size distribution Reverse osmosis Wastewater reclamation

a b s t r a c t Physicochemical characterization of olive mill wastewaters (OMW) was studied after a primary and secondary treatment was implemented in an olive oil factory in Jaén (Spain), comprising natural precipitation, Fentonlike reaction, flocculation–sedimentation and olive stone filtration in series. The application of membrane technology in improving the quality of the secondary-treated OMW (OMW/ST) was examined, to reduce the hazardous electroconductivity (EC) values (2–3 mS cm−1). Particle size distribution on OMW/ST shows supramicron colloids and suspended solids as well as sub-micron particles with a mean size below 1.5 μm remaining in considerable concentration. The high organic pollutants percentage (31.7%) registered with an average diameter below 3 kDa is sensibly relevant for membrane fouling. Mesophilic aerobic bacteria growth warns of possible membrane biofouling formation. The saturation index indicates to work upon recovery factor below 90%. Finally, operating at a pressure equal to 15 bar ensured low fouling and high flux production on the selected NF membrane (69.9 L h−1 m−2) and significant rejection efficiencies (55.5% and 88.5% for EC and COD). This permits obtaining an effluent with good quality according to the recommendations of the Food and Agricultural Association (FAO) with the goal of reusing the regenerated water for irrigation. © 2014 Elsevier B.V. All rights reserved.

1. Introduction In the past decades, extraction of olive oil by means of the traditional batch press method was substituted by more efficient continuous centrifugation processes, capable of coping with the growing demand of olive oil worldwide. Two main wastewater streams are generated during the olive oil production process based on the two-phase continuous centrifugation technology: the first one from the washing of the fruit (olive washing wastewater, OWW) and the second one from the olive ⁎ Corresponding author. Tel.: +34 958241581; fax: +34 958248992. E-mail address: [email protected] (J.M. Ochando-Pulido). 1 Tel.: +34 958241581; fax: +34 958248992.

http://dx.doi.org/10.1016/j.scitotenv.2014.06.109 0048-9697/© 2014 Elsevier B.V. All rights reserved.

oil washing during the vertical centrifugation (olive oil washing wastewater, OOW). These effluents are commonly referred to as olive mill wastewater (OMW) (Niaounakis and Halvadakis, 2006; Paraskeva and Diamadopoulos, 2006). The highest organic pollutant concentration remains in the effluent exiting the centrifuge (OOW), including phenolic compounds, organic acids, tannins and organohalogenated contaminants, which are phytotoxic and refractory, thus resistant to biological degradation. Consequently, biological oxygen demand measurements (BOD5) hardly reflect the presence of these substances and therefore COD and total phenolic compound (TPh) concentration are more appropriate. On the other hand, the effluent from the olive washing procedure (OWW) is a slightly polluted wastewater stream.

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Nomenclature OMW ST OMW/ST OWW OOW MF UF NF RO COD BOD5 EC TPh TSS PSD MPSD NPSD IC SI BGA MWCO TSA FAO dP DP

olive mill wastewater secondary treatment olive mill wastewater after secondary treatment olives washing wastewater olive oil washing wastewater microfiltration ultrafiltration nanofiltration reverse osmosis chemical oxygen demand (mg L−1) biological oxygen demand (mg L−1) electroconductivity (mS cm−1) total phenolic compounds (mg L−1) total suspended solids (mg L−1) particle size distribution micrometric particle size distribution nanometric particle size distribution ionic chromatograms saturation index bacterial growth analysis molecular weight cut off tryptone soy agar Food and Agricultural Association mean particle diameter membrane mean pore diameter

Olive oil production is currently one of the main agricultural activities in the Mediterranean Basin countries, but it is also an emergent agro-food industry in China and many other countries such as the USA, Australia and the Middle Earth. Hence, the treatment of the olive mill effluents is becoming a task of global concern. Small size and geographical dispersions of olive oil mills make the management of OMW sensibly difficult. Furthermore, the pollutant load of these effluents is extremely variable depending not only on the extraction process, but also on the edaphoclimatic and cultivation parameters, as well as on the type, quality and maturity of the olives (Niaounakis and Halvadakis, 2006; Paraskeva and Diamadopoulos, 2006). Direct disposal of these effluents to surface waters, although still practiced, is both hazardous and illegal, and results in severe pollution consequences. Neither can OMW be disposed directly for irrigation purposes (Niaounakis and Halvadakis, 2006; Paraskeva and Diamadopoulos, 2006). OMW discharge leads to odor nuisance, soil contamination, plant growth inhibition, underground leaks, water body pollution and hindrance of self-purification processes, as well as dramatic impacts to the aquatic fauna and to the ecological status (Asfi et al., 2012; Danellakis et al., 2011; Karaouzas et al., 2011; Ntougias et al., 2013). In 1981 the Spanish Government prohibited the direct discharge of these effluents into rivers, since high pollution levels were detected in the Guadalquivir River Basin. In countries like Spain the general solution applied has been the construction of artificial lagoons for natural evaporation. Over the years, this rule has resulted to be inefficient as a consequence of the low evaporation potential of these ponds, the odor release to the surroundings and the hazardous underground leakages derived from the typical deficiencies in their construction. Chemical remediation strategies are thereafter required for the depuration of these bio-refractory wastewaters (De Caprariis et al., 2012; Martínez Nieto et al., 2011a; Niaounakis and Halvadakis, 2006; Paraskeva and Diamadopoulos, 2006; Sacco et al., 2012). In former

works, a treatment process based on Fenton's reaction was optimized for the reclamation of OMW in an olive oil mill located in Jaén (Spain) working with the modern two-phase olive oil extraction procedure. The treatment process comprises natural precipitation to remove coarse particles (primary treatment, PT) followed by Fenton-like advanced oxidation, flocculation–sedimentation and filtration through olive stones in series (secondary treatment, ST) (Martínez Nieto et al., 2010, 2011a, b; Hodaifa et al., 2013a,b). However, the measured electroconductivity (EC) values in the effluent at the outlet of the ST (OMW/ST) are well above the range 2–3 mS cm−1, thus presenting hazardous salinity levels according to the standards established by the Food and Agricultural Association (FAO) with the goal of reusing the regenerated water for irrigation purposes (Sancho Cierva, 2000). Membranes appear in the last decades as a very promising technology, offering a wide range of advantages if compared to classic separation processes (Iaquinta et al., 2009; Stoller and Chianese, 2006a,b). However, even though it is now more than half a century that the first membranes appeared, this technology is still in development, and much effort is necessary to further improve the competitiveness of membrane processes on large-scale industrial applications. The main drawback concerning membrane technologies is membrane fouling, and this is especially relevant in the case of wastewater purification processes (Ochando-Pulido et al., 2012a; Stoller and Chianese, 2007; Stoller et al., 2013a,b; Stoller, 2009, 2011). As a consequence of the fouling issues, flux drop is promptly observed and membrane performance becomes sensibly reduced. The decay in productivity increases the operating and energy costs and leads to frequent plant shut-downs for in-situ membrane cleaning, which are not capable to overcome the progressive deterioration of the membrane module. Available studies on OMW reclamation by membrane technology often report significant concentration polarization and fouling buildup on the used membranes, leading to sensible flux decline and permeate quality loss that boost the energy costs and make the process economically unfeasible. This is a consequence of the underestimation of the fouling issues, due not only to improper operating conditions but also to wrong membrane-type choice. In this research work, the physicochemical characterization of OMW after secondary treatment (ST) comprising Fenton-like reaction, flocculation and filtration in series (OMW/ST) was examined to assess the appropriate membrane election, and avoid fouling on this ulterior pressure-driven membrane operation for final purification of this effluent. The OMW/ST characterization included bacterial growth analysis (BGA), saturation index (SI), particle size distribution (PSD) and organic matter molecular weight cut off (MWCO) distribution with the goal of gathering information about biological, organic and inorganic matter that cause fouling on membranes. Finally, one membrane was selected and the operating conditions were examined. The goal was to reuse the purified effluent for irrigation and thus reducing the environmental impact of the olive oil production process. 2. Experimental 2.1. Sample preparation Effluent samples were collected from the olive washing machines (OWW) as well as at the outlet of the centrifuges (OOW) from several olive oil mills in the Andalusian provinces of Jaén and Granada (Spain), operating with the two-phase centrifugation system, during winter months and rapidly analyzed in the lab, whereas refrigerated for further research when necessary. Currently, an average-sized modern olive oil factory leads to a daily amount of up to 10–15 m3 of OOW, in sum to 1 m3 of OWW per ton of processed olives. Therefore, OWW and OOW were mixed in 1:1 (v/v) proportion to stabilize the average organic matter concentration of the effluent stream (OMW) entering the treatment system and thus avoid sensible fluctuations in the COD parameter.

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After this, OMW was conducted to a secondary treatment on a pilot scale comprising the following process stages: Fenton-like advanced oxidation, flocculation–sedimentation and olive stone filtration (Martínez Nieto et al., 2010, 2011a,b; Hodaifa et al., 2013a,b).

membrane system, which is very useful to elaborate corrosion control programs to prevent from scaling on the membranes (APHA, AWWA, WPCF, 1992; ASTM International, 2001). The SI can be determined by means of the following expression (ASTM International, 2001):

2.2. Effluent physicochemical characterization

SI ¼ pH‐pHs

2.2.1. Bacterial growth Bacterial growth was examined in OMW/ST in order to prevent biofouling on the selected membrane. Mesophilic bacterial growth was studied by performing a filtration protocol following Greenberg et al. (1992). The method consisted in preparing a tryptone soy agar (TSA) culture medium of salinity in the range of that of OMW/ST (UNE-EN ISO 9308-1 and 6222). Thereafter, the growth of mesophilic aerobic bacteria in OMW/ST was studied under controlled atmosphere in an incubation stove at 22 ± 2 °C and observed after 24–48 h. The ingredients and dosages for the preparation of the TSA culture medium are reported in Table 1. For the preparation of the TSA culture medium, the ingredients were dissolved under heat and stirring. If necessary, pH was adjusted to a value of 7.2 ± 0.1 at 25 °C. The TSA culture medium was poured in tubes (250 mL) for sterilization in an autoclave at 121 ± 1 °C for 20 min, after which the tubes were taken out and allowed to cool down to a temperature of 50 ± 5 °C. Finally, TSA was extended (5 mm thickness) in double–layer plaques. Proliferation of mesophilic aerobic bacteria was studied on 10 mL samples of OMW/ST and 1:10 (v/v) dilutions of the latter with MilliQ® water. OMW/ST samples were taken from water treated in batch process and compared with samples taken directly during the continuous operation of the process at the pilot plant for comparison purposes.

where pH is that in OMW/ST, whereas pHs is the solubility pH of OMW/ST, which can be calculated as follows (Fariñas Iglesias, 1998):

2.2.2. Particle size distribution The study of the particle size distribution (PSD) was carried out by conducting a micrometric particle size distribution analysis (MPSDA) and a nanometric particle size distribution analysis (NPSDA) on OMW/ST prior to the membrane stage. The MPSDA was conducted by means of a liquid sampler model LS-200 (Beckman Coulter), whereas NPSDA was performed with a dynamic light scattering device (Plus90 nanosizer) supplied by Brookhaven. In both cases, 1 mL samples of OMW/ST were taken for their examination. Furthermore, molecular weight distribution or molecular weight cut off (MWCO) of the organic matter remaining in OMW/ST was also studied. The procedure performed consisted of filtering 100 mL samples of OMW/ST through 0.45 μm nitrate cellulose membrane filters (supplied by Sartorius). After this, 15 mL of the filtered samples were poured into Falcon tubes provided with membranes of different mean pore diameters (MWCO ranging from 3 to 100 kDa). Finally, the Falcon tubes were subsequently centrifuged at 4000 rpm for 3 min, after which the COD of the centrifuged-filtered samples was analyzed. 2.2.3. Saturation index The saturation index (SI) provides information about the tendency of the feedstream to whether dissolve or form precipitates on the

Table 1 Composition of TSA culture medium used for bacterial growth analysis. Ingredient, g Caseine triptyque digestate Soy peptone Sodium chloride Agar–agar (in powder) Distilled water

15 5 5–43 15–25 Up to 1000 mL

ð1Þ

h i 2þ − þ p½HCO3  þ 5 p f m pHs ¼ pK2 ‐ pKs þ p Ca

ð2Þ

where K2 is the second dissociation constant for the carbonic acid at the feedstream temperature, Ks is the product solubility constant for CaCO3 at the feedstream temperature, [Ca2+] and [HCO3−] are the calcium and bicarbonate ion concentration (mol kg − 1 ) and f m is the activity coefficient for the monovalent species at the feedstream temperature. The p index indicates—Log10. The expression used for the calculation of the activity coefficient (fm) is equal to (Fariñas Iglesias, 1998): pffiffi I pffiffi−0; 3  I pf m ¼ A 1þ I

ð3Þ

where A is the activity which is equal to A = 1.82 · 106 · (E · T)−1,5, E is the dielectric constant (60,954 / (T + 116) − 68.937) and I the ionic strength (Fariñas Iglesias, 1998): I¼

n 1X 2 ½x zi 2 i¼1 i

ð4Þ

where [xi] is the concentration (mol kg−1) and zi is the net charge of the i component. Therefore, for a target recovery factor of the feedstream fixed (Y), the concentration of a component i in the concentrate stream will be (ASTM International, 2001): ½xir ¼

½xi  .f Y 1−



ð5Þ

100

where: [xi]r = concentration (mol kg-1) of the i component in the concentrate stream. [xi]f = concentration (mol kg−1) of the i component in the feedstream. Y = feed recovery factor (%).

Table 2 Nominal characteristics of the selected NF membrane. Parameters

Parametric value

Model Supplier Surface, m2 Permeability (Kw), L h–1 m−2 bar−1 Configuration Dimensions, cm Chemical structure Chemical composition Surface nature MWCO, Da Average pore size, nm Maximum pressure, bar Maximum temperature, °C pH range

DK GE Water & Process Tech. 0.02 6.5 ± 0.5 Flat sheet 3.9 × 33.5 × 14.2 Thin film composite (TFC) Polyamide/polysulfone Hydrophilic 300 0.5 40 50 1−11

MWCO: molecular weight cut off.

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2.3. Filtration unit, selected membrane and operating mode Once the OMW/ST was characterized, a nanofiltration (NF) membrane was chosen to conduct well-controlled crossflow filtration experiments. The characteristics of the NF virgin membrane are reported in Table 2. The NF membrane was provided by GE Water and Process Technologies (USA) and presented an active area of 200 cm2. The membrane filtration tests were carried out in a lab-scale crossflow membrane unit designed by Prozesstechnik GmbH (Basel, Switzerland) equipped with a plate-and-frame housing, a nonstirred double-walled tank (5 L capacity) and a diaphragm pump (Hydra-Cell model D-03). The main process parameters (operating pressure, temperature, tangential velocity) were measured and displayed. Operating pressure was finely adjusted with a spring loaded pressure-regulating valve on the concentrate outlet and monitored by a digital pressure gauge, allowing independent control of operating pressure and flow rate. Operating temperature was regulated automatically via an electronic temperature controller (Ochando-Pulido et al., 2012b,c). Bench-scale tangential-flow NF experiments were run in semicontinuous operation mode (diafiltration), which consisted in recycling continuously the concentrate stream back to the feedwater tank whereas steadily collecting the permeate stream, replacing the permeate outlet volume by fresh OMW/ST. Fluctuations in the feedstock composition made the evaluation of the membrane performance more difficult, thus experiments were replicated. Both the operating pressure and temperature were maintained constant (PTM set point ± 0.01 bar, Tset point ± 0.1 °C), whereas the feed flow rate was controlled at a fixed value equal to turbulent tangential velocity over the membrane (5.09 m s−1, that is, a crossflow velocity to promote a Reynolds number NRe = 2.6 · 104). 2.4. Analytical methods Chemical oxygen demand (COD), total suspended solids (TSS), total phenolic compounds (TPh), total iron, electroconductivity (EC) and pH were measured in the raw effluent samples and at the end of each depuration step according to standard methods, as well as carbonates/ − bicarbonates (CO2− 3 /HCO3 ) in OMW/ST (Greenberg et al., 1992). EC and pH measurements were carried out with a Crison GLP31 conductivity-meter and a Crison GLP21 pH-meter, whereas a Helios Gamma UV–visible spectrophotometer (Thermo Fisher Scientific) served for COD, TPh and total iron measurements (Standard German methods ISO 8466-1 and German DIN 38402 A51) (Greenberg et al., 1992). Ionic concentrations were analyzed in the effluent at the end of each depuration step with a Dionex DX-120 ion chromatograph, as described in previous works (Ochando-Pulido et al., 2012b,c). All chemicals used for the analytical proceedings presented analytical grade with purity over 99%, and the analyses were performed in triplicate. 3. Results and discussion 3.1. Characterization of OMW before and after secondary treatment Summarized physicochemical characterization of the raw olive mill effluents (OOW and OWW) is presented in Table 3. It is worth pointing that the pollutant load of OWW stands normally below the limits for discharge on superficial suitable terrains (Guadalquivir Hydrographical Confederation in Spain, 2006: TSS below 500 mg L−1 and COD below 1000 mg L− 1, whereas in Italy the COD limit is more restrictive, 500 mg L−1). For OWW, significant organic matter load and phenolic compounds are only present in case of rupture of the olives during the recollection process, and concentration values may exceed the established standards only in case the water used in the washing machines during the olive cleaning is renewed with low frequency.

Table 3 Physicochemical characterization of the raw OMW used. Parameters

OOW

OWW

1:1 (v/v) OMW

pH Moisture, % Total solids, % Organic substances, % Ashes, % BOD5, mg O2 L−1 COD, mg O2 L−1 Total phenolic compounds, mg L−1 EC, mS cm−1

4.9 99.3 0.6 0.49 0.11 790.1 7400.5 157.0 1.3

6.3 99.7 0.27 0.10 0.17 500.0 800.2 4.0 0.9

5.7 99.5 0.44 0.29 0.14 645.1 4100.3 80.5 1.1

Otherwise, more stringent limit values are established for discharge in public waterways (TSS below 35 mg L−1 and COD below 125 mg L−1). On the contrary, major organic load was registered in OOW (Table 3). The organic matter is formed by phenolic compounds, organic and long-chain fatty acids, tannins and organohalogenated contaminants, which are phytotoxic compounds recalcitrant to biological degradation (Martínez Nieto et al., 2011a; Niaounakis and Halvadakis, 2006; Paraskeva and Diamadopoulos, 2006). In contrast to the three-phase olive oil extraction process (OOW-3), in the two-phase system water is only injected in the final vertical centrifugation step, thus the volume of liquid effluent by-produced from the decanting process (OOW-2) is reduced by one third on average and, furthermore, most of 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, OOW-3) and hence OOW-2 exhibits lower pollutant degree, too (Ochando-Pulido et al., 2013). The two-phase system thus seems more ecological and has been strongly promoted in Spain, but the three-phase system is still surviving in those countries where scarcity of financial support has not permitted the technological change yet. In Table 4, results of the complete physicochemical composition of OMW/ST are reported. Operating at ambient temperature, without pH control, agitation speed 60 rpm., oxidizing agent concentration [H2O2] = 5% w/v and [FeCl3]/[H2O2] ratio = 0.01–0.04 w/w, as withdrawn from previous works on lab scale (Martínez Nieto et al., 2011a; Hodaifa et al., 2013a), yielded up to 92.3% COD and 98.8% total phenolic compound removal at the outlet of the Fenton-like reactor (Table 4). Spontaneous acidification of the effluent exiting the Fenton-like oxidation tank took place (pH around 3.0, at which the reaction optimally occurred) as a result of the addition of the catalyst as well as to the cleavage of complex organic solutes into organic acids (Martínez Nieto et al., 2011a). Hence, a neutralizing agent (NaOH, 1N) was added to the second tank, to adjust the pH to neutrality. Subsequently, the

Table 4 OMW characterization determined at the outlet of the different operation units that formed the secondary treatment. Parameters

pH EC, mS cm−1 TSS, mg L−1 COD, mg O2 L−1 Total phenolic compounds, mg L−1 Total iron, mg L−1 Cl−, mg L−1 Na+, mg L−1 a

Secondary treatment of OMWa Fenton oxidation

Flocculation– sedimentation

Gravel and olive stone filtration

3.2 3.81 141.2 315.9 0.9 50.8 1096.6 725.5

7.9 3.62 80.5 295.2 0.8 3.69 1095.1 724.9

7.7 3.43 13.1 150.8 0.4 0.028 990.9 718.6

OMW is a 1:1 (v/v) mixture of OWW and OOW.

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a

117

b -

Na

Cl

+

Fig. 1. Ionic chromatograms of anionic (panel a) and cationic (panel b) for species concentration present in water treated at the outlet of OMW/ST. Panel a: 1, fluoride; 2, chloride; 3, bromide; 4, nitrate and 5, sulfate. Panel b: 1, sodium; 2, potassium; 3, lithium; 4, calcium and 5, magnesium.

flocculation process (6 mg L− 1 oil-based Nalco-77171 flocculant) ensured 12.3% v/v final sludge separation and 87.7% v/v final clarified water (Table 4) (Martínez Nieto et al., 2011b). The iron-rich sludge was ulteriorly recirculated to the oxidation tank to minimize the catalyst consumption. At the exit of the secondary treatment, COD and total phenolic compounds concentration removal efficiencies reached values of up to 92.7% and 99.5% respectively (Martínez Nieto et al., 2010; Hodaifa et al., 2013b). The proposed secondary treatment process is among the most efficient and economically feasible ones, ensuring major mineralization of the recalcitrant organic pollutants present in the raw effluent. Estimated operating costs in the range of 2.4–4.0 € kg−1 equivalent O2 for conductive-diamond electrochemical oxidation, 8.5–10.0 € kg− 1 equivalent O2 for ozonation and 0.7–3.0 € kg− 1 equivalent O2 for Fenton-like process highlight the latter as the most cost-effective one (Cañizares et al., 2009). The cost of the flocculation–sedimentation operation is negligible considering that the concentration of flocculant used is very low (6 mg L−1) and its price is about 30 € per kg, whereas filtration through olive stones is cost free, as it is a by-product of olive oil industries. However, the secondary treatment was not able to abate the high concentration of dissolved monovalent and divalent ions, which cannot be removed by conventional physicochemical treatments. Furthermore, even higher EC values were noticed in OMW/ST if compared to the raw OMW (Martínez Nieto et al., 2010, 2011a,b; Hodaifa et al., 2013a,b). The high concentration of dissolved monovalent and divalent ions in OMW treated by ST was verified in the ionic chromatograms (IC) reported in Fig. 1, where striking peaks of chloride (Fig. 1a) and sodium ions (Fig. 1b) can be observed. IC analysis (corresponding to 1:10 v/v diluted OMW/ST) confirmed concentrations for both ions (Cl− and Na+) in the range 875.8–1045.1 mg L−1 and 534.0–728.7 mg L− 1, respectively (Table 5). These high concentrations are owed to the addition of the

catalyst (FeCl3) and neutralizing agents (NaOH) in the Fenton reaction tank (Martínez Nieto et al., 2010, 2011a,b; Hodaifa et al., 2013a,b). OMW/ST presented EC values in the range of 3.2–3.6 mS cm−1. These salinity levels are potentially hazardous according to the standards established by the FAO for reusing the regenerated water for irrigation purposes (Table 6) (Sancho Cierva, 2000). Otherwise, the concentration of other ionic species in OMW/ST were within the typical range in natural water, such as of calcium, magnesium and bicarbonate ions, equal to 84.1–103.8 mg L−1, 21.4–32.6 mg L−1 and 129.3– 132.9 mg L− 1, respectively (Table 5). Otherwise, the organic matter load of OMW/ST is below the limits for discharge on superficial suitable terrains but does not comply with the limits for discharge into public waterways (Table 7).

3.2. Bacterial growth on the secondary treated olive mill wastewater (OMW/ST) The build-up of microbiological fouling on the membranes depends mainly on the characteristics of the feedstream. The significant presence of organic pollutants in OMW/ST acts as a potential culture medium which can lead to rapid proliferation of mesophilic aerobic bacteria on the membrane surface during filtration if not well controlled. This is an important task that needs prevention, since emerge of biofouling results in severe performance loss and can cause irretrievable service-life shortage of the selected membrane module. Biofouling propensity was examined by analyzing the growth of mesophilic aerobic bacteria in the secondary treated olive mill wastewater (OMW/ST). For this purpose, proliferation of mesophilic aerobic bacteria in undiluted OMW/ST samples was compared with the growth in 1:10 (v/v) diluted OMW/ST samples. Results of the bacterial growth analysis are reported in Table 8. Incubation of the samples of OMW/ST at the same conditions to those expected during the secondary treatment in the industrial plant

Table 5 Concentrations of ionic species determined by IC at the outlet of the secondary treatment of OMW. Anionic species, mg L−1

Cationic species, mg L−1

[F−] [Cl−] [Br−] [NO− 3 ] [SO2− 4 ] [PO3− 4 ]

[Na+] [K+] [Li+] [Ca2+] [Mg2+] [HCO+ 3 ]

0.2–1.9 875.8–1045.1 1.8–2.8 7.4–8.6 127.3–139.3 0.5–3.2

534.0–728.7 41.5–65.5 0.1–0.2 84.1–103.8 21.4–32.6 129.3–132.9

Table 6 Standard water quality values (electroconductivity) established for irrigation purposes according to Food and Agriculture Organization (FAO). EC, S cm−1

Water quality

Hazard due to salinity

0–1 1–3 N3

Excellent–good Good–poor Poor–non acceptable

Low–medium High Very high

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Table 7 Standard water quality values established for irrigation purposes according to the Guadalquivir Hydrographical Confederation in Spain, 2006. Regenerated water use

Parameter

Standard limit, mg L−1

Discharge on superficial suitable terrains

COD TSS COD TSS

1000 500 125 35

3.5 3.0 2.5

Lognp

Discharge into public waterways

a

2.0 1.5 1.0 0.5 0.0

0

0.5

1

1.5

2

2.5

mean particle diameter (dp), µm

b

4.5 4.0 3.5 3.0

Lognp

(ambient temperature and saline concentration) warns from quick proliferation of mesophilic aerobic bacteria: i) in batch system between 2 and 3 CFU were registered after 24 h and uncountable after the first 48 h in OMW/ST samples without dilution, whereas 4–9 CFU were observed in the 1:10 (v/v) diluted OMW/ST samples after 48 h; ii) however, no mesophilic aerobic bacterial growth was detected when operating continuously during the secondary treatment process (Table 8). It is worth highlighting that in previous works Hodaifa et al. (2013a) have pointed the existence of a percentage of residual hydrogen peroxide remaining unreacted at the outlet of the oxidation reactor and at the exit of the secondary treatment plant depending on the [catalyst]/[H2O2] ratio used. The results obtained by the authors showed that for 3 h of operating space–time (τ), the residual oxidant concentration at the outlet of the secondary treatment, was about of 1134 mg L−1, thus exhibiting biocide potential against proliferation of mesophilic aerobic bacteria in OMW/ST during the continuous operation of the plant, which is consistent with the mesophilic aerobic bacterial growth results obtained (Table 8). Moreover, note that the detection of residual hydrogen peroxide at the end of the process is due to the fact that the initial hydrogen peroxide dosage used was estimated for complete organic load oxidation. This oxidation needs higher oxidation time in the reactor (more than 3 h). Therefore, the water quality parameters in the treated water accumulation raft are usually lower than those registered at the output of the plant, in other words, the continuous oxidation reaction after the plant treatment.

2.5 2.0 1.5 1.0 0.5 0.0

0

10

20

30

40

50

mean particle diameter (dp), µm Fig. 2. Micrometric particle size distribution in OMW/ST. Particle range: 0.2–2.5 μm (panel a), and 2.5–50 μm (panel b). np: particle number, dp: mean particle diameter.

3.3. Particle size distribution With the aim to determine the effect of the particles present in the effluent at the outlet of secondary treatment (OMW/ST) on the membranes prior to their usage in the filtration stage, particles size distributions study was performed. Semilogarithmic representation of the micrometric particle size distribution (MPSD) in OMW/ST is given in Fig. 2. In this analysis, the particle size was divided in two different particle ranges: 0.2–2 μm (sub-micron particles, Fig. 2a) and 2–50 μm (supra-micron colloids and suspended solids, Fig. 2b). Higher concentration values (75.1%, Table 9) of sub-micron particles (0.2–2 μm) presenting a mean diameter between 1 and 1.5 μm were registered (Fig. 2a). These dissolved organic pollutants and small colloids are very prone to cause severe membrane fouling (Gwon et al., 2003; Winfield, 1979). Small particles may lead to blocking and

plugging of the pores of the membrane, given the fact that pore blocking and plugging may statistically be significant when the size of the particles (dP) is in the near range of the membrane mean pore diameter (DP), which as a rule of thumb is 0.1 b dP/DP b 10 (Gwon et al., 2003; Stoller, 2009; Winfield, 1979). On the other hand, a semilogarithmic pattern was observed for the particles with a size (diameter, dp) above 2 μm (Fig. 2b). These particles are supra-micron colloids and suspended solids, which remain in significant concentration (24.9%) in OMW/ST (Table 9), as measured by the TSS parameter (Table 4). Winfield (1979) indicated that, for reverse osmosis (RO) and nanofiltration (NF) units, solid particles greater in size than 5 μm play a minor role in determining the rate of membrane fouling than sub-micron particles. The nanometric particle size distribution analysis (NPSDA) of OMW/ST, performed at different temperatures (10–30 °C), demonstrates a decrease in the dp with temperature, from a value of 0.6 nm at 10 °C to a value of about 0.25 nm at 30 °C (Fig. 3). OMW is considered to be closely related to humic compounds because it is dark colored,

Table 8 Mesophilic bacterial growth analysis in OMW/ST on batch and continuous systems. Dilution

System used for OMW/ST Batch

Incubation time, h CFU/10 mL CFU/1 mL a

0 1/10

Saline concentration = 5 g L−1.

24 2–3 0

Continuous 48 Uncountable 4–9

24 0 0

48 0 0

Table 9 Total particle number (np) and percentage in each size range. np total 0.2 μm b np b 2 μm 2 μm b np b 50 μm

23,349 17,519 5830

% 75.1 24.9

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25

It is relevant to notice not only the existence of a significant concentration of suspended organic solids (dp N 0.45 μm), up to 35.8 mg L−1 corresponding to 19% of the total organic matter load, but also a very important presence of low-sized colloidal organic particles (dp between 3 and 30 kDa, up to 63.3%). Of these latter, those organic particles with a mean dp below 3 kDa (over 59.7 mg L−1, that is more than 31.7% of the total organic pollutant load in OMW/ST) are especially relevant for the ulterior membrane process, as they can foul the membranes by getting trapped in the nodules of the spacers and in the membrane support, as well as in the valleys of the ridge-and-valley structured active layer and in the membrane surface voids and defects. Moreover, colloidal particles can cause deleterious abrasive fouling on the active surface of the membranes (Gwon et al., 2003; Winfield, 1979).

np,%

20 15 10 5 0 0

0.5

1

3.4. Scaling formation on the membranes

mean particle diameter (dp), nm Fig. 3. NPSD in OMW/ST in the range of 0.1–10 nm measured at several temperature values: 10 º C (- · -), 15 º C (- - -), 22 °C ( ) and 30 °C ( ).

contains phenolic compounds and shares some of the properties of humic substances. Jin et al. (2009) studies on particle size distribution (NPSD) of humic acid by dynamic light scattering confirmed higher hydrodynamic diameters upon lower temperature values, predominantly in the form of sub-micron aggregates and a small fraction of supramicron aggregates. This can be attributed to the minor solubility of the organic matter, which enhances molecular aggregation. Similar trends with regard to natural organic matter (NOM) macromolecules were observed by Hong and Elimelech (1997). This fact is quite relevant since OMW is by-produced during the olive oil production campaign, which lasts four months on average during the autumn–winter period, when low temperatures are usually expected. The decrease in temperature may have important implications in the relationship between the average dp of the particles present in OMW/ST and the mean Dp of the selected membranes. In addition to the PSD analysis, molecular weight cut off (MWCO) of the organic solutes remaining in OMW/ST was conducted to obtain complementary information besides the PSD of the particles present in OMW/ST prior to the final membrane process. Results are reported in Fig. 4, where the concentration of the remaining organic particles (measured by the COD parameter) versus MWCO range are reported.

MWCO < 3 kDa

10 kDa < MWCO < 30 kDa

MWCO

119

30 kDa < MWCO < 50 kDa

50 kDa < MWCO < 100 kDa

dp > 0.45 µm

0

20

40

60

80

COD, mg L-1 Fig. 4. Organic pollutants (measured as COD) and molecular weight cut off (MWCO) distribution.

The values of the saturation index calculated as a function of the feedstream pH and the target recovery factor (Y, %) are reported in Table 10. Results show the maximum feed recovery factor that should be fixed to avoid scaling on the selected membranes. A positive SI warns of the risk of scaling formation due to precipitation of CaCO3 on the membrane surface, whereas a negative SI indicates that the stream is under-saturated; nil SI indicates equilibrium (ASTM International, 2001). As it can be seen in Table 10, choosing major feed recovery factor results in higher saturation indexes as the concentrate stream will be more concentrated, thus there is risk of scaling formation on the membrane surface owed to the recirculation of the concentrate stream back the membrane system. The results obtained highlight that for a maximum feed recovery value equal to 90%, the saturation index of the concentrate stream (SIr) is well above a value of 0.5, thus is a serious warning of scaling formation. Scaling is a very severe form of membrane fouling that causes irreversible permeate flux decay and separation performance loss (Gwon et al., 2003; Hong and Elimelech, 1997; Winfield, 1979). Therefore, two strategies can be adopted, that is whether adjusting the pH of the feedstream (OMW/ST) to a value equal to pH = 6 or setting the value of the feed recovery factor below 90%, that is the upper boundary to avoid scaling problems (Table 10). 3.5. NF performance Once the OMW/ST was characterized, a NF membrane (characteristics reported in Table 2) was chosen to conduct well-controlled cross-flow filtration experiments. Based on the previous analyses, the feed recovery factor (Y) was fixed at a value of 90% by adopting a set point in the pH controller equal to 6 ± 0.5. In Fig. 5, the values of the EC and COD rejection efficiencies measured on the NF membrane as function of the net driving pressure (ΔP − Δπ) are reported. Furthermore, the summarized results of initial (Jp0) and steady-state permeate flux values (Jpss) of each experimental NF run at the different operating pressure values (PTM) studied are reported in Table 11. The measured EC rejection values of the selected NF membrane ranged from 32.5% upon a net driving pressure equal to 5 bar, 55.5% at 15 bar to 62.3% at 25 bar, whereas the COD rejection efficiency ranged from 71.9% at 5 bar, 88.4% at 15 bar up to 92.9% at 25 bar (Fig. 5). Moreover, the initial permeate flux (Jp0) was found to be increased linearly with the net driving pressure (ΔP − Δπ) applied on the NF membrane (Fig. 6). However, fouling on the NF membrane leads always to certain permeate flux loss during the operation time, and the extent of the fouling build-up on the membrane strongly depends on the applied net driving pressure (PTM). Similar patterns were observed for the permeate flux variation during operation time in all experiments, consisting in initial sharp flux decay for a few minutes due to concentration polarization caused

120

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100

COD rejection RCOD,%

EC rejection REC,%

70

60

50

40

30

90

80

70

60

20

50 0

5

10

15

20

25

30

0

5

10

15

20

25

30

Fig. 5. EC (left panel) and COD rejection (right panel) as function of the net driving pressure (ΔP − Δ) applied on the NF membrane.

by the fast accumulation of solutes within the boundary region of the NF membrane, followed by gradual decrease as a result of fouling and concentration polarization, and then finally, upon the appropriate operating conditions, steady state (Jpss) due to the equilibrium of foulant attachment and detachment between the membrane surface and the feed solution (OMW/ST). The values of the permeate flux loss (− ΔJp) at the end of the NF run are given in Table 11. The permeate flux decay was found to be much higher for the experiment at the highest PTM performed, that is, 15.5% at 25 bar, and no steady-state permeate flux (Jpss) was reached. However, negligible difference on the −ΔJp parameter was found for the NF runs at PTM of 5 and 15 bar (2.3% vs. 2.6%), which ensured low fouling on the NF membrane and permitted high steady-state permeate flux values. The high permeate flux, as a result of the low fouling, and rejection values measured are explained on the basis of the molecular weight cut off (MWCO) of the selected NF membrane (300 Da), according to the pore blocking model (0.1 b dP/DP b 10). Otherwise, the compulsive transport of pollutants towards the membrane surface by the convective force at very high PTM values (25 bar) overcomes the membrane– colloid repulsion, resulting in particle deposition and consequently in severe membrane fouling. Operating at a PMT equal to 15 bar led to a high permeate flux value, 69.9 L h−1 m−2, major feed recovery, 90%, and rejection efficiencies of 55.5% and 88.5% for EC and COD, that is, final EC and COD values in the purified effluent equal to 1.5 S cm−1 and 17.3 mg L−1, respectively. This permits obtaining an effluent with good quality standards according to the recommendations of the Food and Agricultural Association (FAO) with the goal of reusing the regenerated water for irrigation purposes (Table 6) and also complies with the water quality standard values established by the Guadalquivir Hydrographical Confederation (Spain) for discharge into public waterways.

4. Conclusions Physicochemical analysis of secondary-treated olive mill wastewaters (Fenton-like reaction, flocculation–sedimentation and filtration by filters packed with sand and olive stones, OMW/ST) coming from a two-phase olive oil extraction process is necessary to identify the appropriate membrane election and avoid fouling during the tertiary treatment of these effluents by membrane technology. Sub-micron particles (mean diameter below 1.5 μm) and organic pollutants with a mean size below 3 kDa present in OMW/ST (31.7% of the total organic matter concentration) are sensibly relevant to membrane fouling, whereas mesophilic aerobic bacterial growth analysis warns of biofouling formation on the membrane. Finally, very low membrane fouling, thus high permeate flux production, as well as major feed recovery and rejection values were yielded by the selected NF membrane upon the proper operating framework. Operating at a pressure equal to 15 bar led to a high steady-state flux value of the purified effluent, 69.9 L h−1 m−2, major feed recovery, 90% (upper boundary fixed by the saturation index) and rejection efficiencies of 55.5% and 88.5% for EC and COD, that is, final EC and COD values in the purified effluent are equal to 1.5 S cm−1 and 17.3 mg L−1, respectively. This permits obtaining an effluent with good quality standards according to the recommendations of the Food and Agricultural Association (FAO) with the goal of reusing the regenerated water for irrigation purposes and also complies with the water quality standard values established by the Guadalquivir Hydrographical Confederation (Spain) for discharge into public waterways, and thus reduce the environmental impact of the industrial olive oil production process.

Conflict of interest The Authors declare no conflict of interests.

Table 10 Saturation index (SI) as function of feed recovery (Y) and pH. Y, %

pHOMW/ST

pHr

pHs, r

SIr

90

6.5 6 6.5 6 6.5 6

7.16 6.70 7.00 6.56 6.86 6.40

6.26 6.26 6.60 6.60 6.82 6.82

0.90 0.44 0.40 −0.04 0.04 −0.42

80 70

pHr: concentrate pH; pHs, r: concentrate saturation pH; SIr: concentrate saturation index.

Table 11 Results of initial (Jp0) and steady-state permeate flux values (Jpss) of each experimental NF run at the different operating pressure values (PTM) studied. PTM, bar

Jp0, L h−1 m−2

Jpss, L h−1 m−2

−ΔJp, %

Y, %

REC, %

RCOD, %

5 15 25

25.4 71.8 123.9

24.8 69.9 104.2

2.3 2.6 15.5

90 90 90

32.5 55.5 62.3

71.9 88.4 92.9

vt: tangential velocity; NRe: Reynolds number; J0: initial permeate flux; Jss: steady-state permeate flux; −ΔJp: permeate flux loss at the end of the NF run; Y: volume recovery.

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160 140 120 100 80 60 40 20 0 0

5

10

15

20

25

30

Fig. 6. Permeate flux as function of the net driving pressure (ΔP − Δπ) applied on the NF membrane.

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