In this work, the performances of novel nano-filtration (NF) and low-pressure reverse osmosis (RO) polymeric membranes were examined with the aim of recovering the iron used as catalyst in former secondary treatment based on the Fenton-like advanced oxidation of olive mill wastewater (OMW). Results highlight that both membranes exhibit a good performance towards the rejection of iron (99.1% for the NF membrane
One of the key tasks of catalytic processes, from the point of view of cost-efficiency, is the recovery and re-use of the catalyst. This is especially relevant in the case of catalytic treatments aimed for the reclamation of wastewater streams, in which the low added-value of the treated effluent (purified water) makes it imperative to save as much expense as possible. In the case of homogeneous catalytic processes, this is even more difficult to achieve. In the case of heterogeneous catalytic processes, one of the most common solutions can be the fixation of the catalyst to a solid phase. However, this can make the catalyst lose some of its effectiveness, given that a less perfect mix may be achieved. Moreover, in case of dark media like wastewater streams, another problem is added to the former, which is the hindrance of the penetration of light, in the case of photo-catalyzed processes.
In a previous work by the Authors (Ochando-Pulido
In this research paper, an alternative is proposed to our previous work, in this case for two-phase olive mill wastewater (OMW) previously subjected to a secondary treatment based on a homogeneous Fenton-like reaction (Hodaifa
Much effort has been invested to attain novel membranes capable of offering higher technical and economical performances since the development and commercialization of the first cellulose acetate asymmetric membranes. The availability of new membrane materials, designs, module configurations and know-how has succeeded in the promotion of credibility among investors (Akdemir and Ozer,
In the last decades, the effluents generated by olive oil industries (OMW) have significantly increased as a result of the boost of the olive oil agro-industrial sector, also due to the technological conversion into continuous operation centrifugation-based processes. Currently, average-sized modern olive oil mills operating with the two-phase centrifugation technology by-produce daily between 10 and 15 m3 of wastewater derived from the vertical centrifugation process, called olive oil washing wastewater (OOW), together with 1 m3 of olive washing wastewater (OWW) per ton of processed olives (Hodaifa
A wide variety of stand-alone and integrated processes for the treatment of OMW have already been proposed and developed but have not yet led to completely satisfactory results (Borja
The disposal of the solid waste stream is not the objective of the present work, which aims only at the management problem related to the reclamation of liquid effluents. Some solutions already proposed for the management of the pomace waste are for instance adsorption of heavy metals (Baccar
Olive oil industries in their current status, typically small, dispersed mills, cannot afford such high treatment costs. In addition, conventional physicochemical treatments are not effective for the removal of the significant salinity of OMW, reflected in high electro-conductivity (EC), which presents hazardous salinity levels according to the guidelines established by the Food and Agricultural Organization (F.A.O.) for irrigation uses.
Several works have been conducted in the past by means of membrane technology with the aim at reducing the organic load of OMW (Akdemir
In this work, two different membranes, one nano-filtration (NF) and a low-pressure reverse osmosis (RO), are examined with a double aim: the main one is the recovery of the iron used as catalyst in the former Fenton-like secondary treatment of OMW, but at the same time a secondary goal was the removal of the high EC and remaining organic matter in this secondary-treated OMW stream.
For this purpose, the adequate operating pressure for both membranes was studied, with an insight into the impacts on both the productivity and rejection efficiency towards the target species. The fouling issues occurring on both membranes, which deeply influence the performance and cost-effectiveness of the membrane process, were also analyzed and taken into account for the membrane plant dimension. Control of fouling is a key parameter in order to increase the profitability of membrane processes during operation and avoid excessive overdesign of the membrane plants. High fouling rates on the membranes would rapidly lead to zero flux conditions in an irreversible way in case of iron (Yiantsios and Karabelas,
Finally, the suitability for reusing the final effluent (permeate stream) in the olive oil production process and therefore closing the loop was also checked.
Samples of OWW and OOW effluents were collected from several two-phase centrifugation-based olive oil mills in the Andalusian provinces of Jaén and Granada (Spain) during winter months and rapidly analyzed in the lab and refrigerated for further research when necessary.
OWW and OOW were mixed in a 1:1 (v/v) proportion to stabilize the average organic matter concentration of the effluent stream (OMW) entering the treatment system and thus avoiding sensible fluctuations in the COD parameter. After this, OMW was conducted to a secondary treatment on a pilot scale based on Fenton-like advanced oxidation process. The secondary treatment is described in detail in former works by the authors (Martínez-Nieto
The membrane plant used for the experiments was a bench-scale one supplied by Prozesstechnik GmbH (Basel, Switzerland), provided with a plate-and-frame module (
Flow diagram of the bench-scale RO unit. V1, V2: emptying valves (pump inlet and outlet respectively); V3, V4: pressure regulating valves for module 2 and 1 respectively; V5: venting valve for module M1; V6: three-way valve to select desired working membrane module; V7: magnetic valve for cooling jacket inlet; M1: flat-sheet membrane module; M2: spiral-wounded module; P: feedstock pump; FT: feedstock tank; PISH01, PISH02: pressure gauges; TICSH01: temperature gauge.
Nominal characteristics of the selected membranes.
Parameters | Parametric value | |
---|---|---|
|
NF | RO |
|
GE (USA) | GE (USA) |
|
DK | AK |
|
8.2 ± 0.3 | 6.2 ± 0.2 |
|
Flat-sheet | Flat-sheet |
|
|
Asymmetric |
|
0.5 | - |
|
50 - 300 | - |
|
32 | 8.7 |
|
90 | 50 |
PA: polyamide;
PS: polysulfone;
TFC: thin-film composite;
MWCO: molecular weight cut-off.
The membranes plant consists of a non-stirred double walled tank (5 L) and a diaphragm pump (Hydra-Cell model D-03) to drive the effluent stream to a plate-and-frame membrane module M1 (dimensions 3.9 cm width x 33.5 cm length). The plant is also provided with another different membrane module (M2), which can be either a spiral-wound or tubular one, and can be selected with a three-way valve (V6).
The main processing parameters (operating pressure, temperature and feed flow rate) were measured and displayed. The operating pressure could be adjusted finely with a spring-loaded pressure-regulating valve (SS-R4512MM-SP, Swagelok) on the concentrate outlet and monitored by a digital pressure gauge (Endress+Hauser, model Ceraphant T PTC31), allowing independent control of the operating pressure (PTM set point ± 0.01 bar) and the flow rate; the feed flow rate was regulated by means of a feed flow rate valve (Fset point ± 0.1 L/h) to fix the tangential velocity over the membrane (Mott and Untener,
Prior to each NF or RO experiment, the corresponding membrane was stabilized by filtering MilliQ® water at a fixed pressure and temperature until a constant and stable flux was observed. After this, the hydraulic permeabilities (m0) of each of the selected membranes were determined by measuring the pure water flux over the admissible applied pressures range of each one, at ambient temperature and turbulent cross-flow velocity.
Subsequently, 2 L of secondary-treated OMW were poured into the feed-water tank to proceed with the experimental OMW membranes purification. Bench-scale NF and RO experiments were run in a semi-batch mode, conducted in tangential-flow at ambient temperature (22 ± 0.1 °C) and turbulent regime over the membrane. The operating pressures were fixed at 5, 7 and 9 bars for the experimental runs with the NF (DK series) membrane, in order to work in a low-pressure energy-saving range, whereas 3, 5 and 8 (maximum operating pressure 8.7 bar) for the experiments with the RO membrane (AK series), respectively.
All the membrane experiments were run with the highest feed volume recovery possible (Y,%), which is approximately 80 - 90%. The operating procedure consisted of continuously recycling the concentrate stream back into the feed-water tank where it steadily collected the permeate stream, which was replaced by the same volume of fresh pretreated OMW. Periodically, samples of the permeate stream were collected in a cumulative vessel and analyzed in order to evaluate the membrane separation effectiveness with respect to the iron recovery, as well as COD and conductivity rejection. The membrane productivity was assayed by measuring the permeate flux during operation time by weighing the mass of collected permeate on a precision electronic mass balance (AX -120 Cobos, 0.1 mg accuracy).
After each semi-batch run, the membrane was recovered for the following experiment by cleaning it in situ with 0.1-0.15% w/v NaOH and 0.1-0.15% w/v sodium dodecyl sulfate (SDS) solutions (provided by Panreac S.A.).
The membrane performances were measured in terms of permeate flux and solute rejection. The observed iron rejection, as well as COD and conductivity, were calculated as follows: Ri (%) = (1 - (cp,i/cf,i)) x 100 (1)
where
The saturation index (SI) was also calculated following ASTM International (
The SI can be determined by means of the following expression (ASTM International, SI = pH - pHs (2)
where
For a target feed volume recovery of the feed-stream fixed (Y), the concentration of a component [xi]r = [xi]f - (1 - (Y/100)) (3)
where:
[xi]r = concentration (mol/kg) of the
[xi]f = concentration (mol/kg) of the
Y = feed volume recovery factor (%)
All the analytical methods were carried out in triplicate with analytical-grade reagents. Chemical oxygen demand (COD), total phenols (TPh), total suspended solids (TSS), electro-conductivity (EC), pH and particle size distribution (Plus90 nano-sizer, Brookhaven) were measured following standard methods (Greenberg
For the measurement of the total iron concentration, all iron ions were reduced to iron ions (II) in a thioglycolate medium with a derivative of triazine, forming a reddish-purple complex that was determined photometrically at 565 nm (Standard German methods ISO 8466-1 and German DIN 38402 A51) (Greenberg
The physicochemical composition of the pretreated OMW is given in
Physicochemical composition of secondary-treated OMW.
Parameters | Parametric value |
---|---|
|
7.8 - 8.2 |
|
3.5 - 5.5 |
|
14 - 16 |
|
120.5 - 226.6 |
|
390 - 980 |
|
400 - 1000 |
In first place, the virgin NF and RO membranes’ pure water permeability (m0) was calculated (
Next, semi-batch runs were performed with each membrane following the procedure described in
The mean permeate fluxes yielded with each membrane were enhanced linearly upon increasing the operating pressure within the respective pressure range of each of the selected membranes (
Permeate flux values yielded by the selected membranes: AK series (RO) (■ 3, ■ 5 and ■ 8 bar) and DK series (NF) (■ 5, ■ 7 and ■ 9 bar).
The higher permeate flux obtained with the DK membrane is owed to the nano-porous structure nature of NF membranes, in which convective transport occurs, where, as in RO membranes, which are widely accepted to be homogeneous surfaces, though exhibiting imperfections related to their fabrication process, e.g. interfacial polymerization and phase inversion, the solution diffusion takes place.
The selected NF membrane is a highly productive one, capable of offering very high fluxes at low operating pressures, thereby it seemed a priori an optimum membrane from the point of view of the optimization of operating costs. The selection of the proper operating pressure is the key to all membrane processes, in terms of capital and operating expenses, commonly referred in engineering as “capex” and “opex”. Operating at higher pressure leads to major permeate production, involving smaller membrane area and shortened working periods, while the other side of the balance implies more energy consumption for the same amount of influent.
Moreover, another key parameter when projecting membrane treatment plants is the feed volume recovery factor (Y,%). This is very relevant and and is also connected to membrane fouling. An excessive feed volume recovery would lead to fouling issues in a shorter period of time, especially in case of batch or semi-batch systems where the bulk becomes increasingly concentrated, and more irretrievable or irreversible if scaling is a potential type of fouling in the specific system like the one in this research work (due to the presence of colloidal iron).
Inorganic fouling, in particular that generated by colloidal iron precipitates, has paramount importance as evidenced from the manufacturers’ recommendations on iron concentrations in feed waters and from the problems frequently encountered in membrane facilities. Previous studies have warned that a clearly detectable decline in the permeation rate, linear in time, is attained in the iron concentration range of a few ppm. Given that the solubility of ferric ions at pH = 7 is estimated to be 5.9·10−10 mol/L, even at a total concentration of 1 ppb almost all iron will be in precipitated form (Yiantsios and Karabelas,
In addition, calcium leads to the formation of scaling on the fouled membranes, mainly in the form of calcium carbonate, chlorides and sulfates. In this regard, it is important to highlight the role of certain ionic species such as calcium ions, which promote the aggregation of the organic matter by intra and intermolecular bridge formation mechanisms (Madaeni and Samieirad,
The SI calculated following ASTM International (
In
Iron rejection efficiencies and measured values in permeate streams.
Membrane | Op. P, (bar) | Iron rejection (%) | Permeate iron (µg/L) |
---|---|---|---|
|
5 | 95.1 | 19.6 - 49 |
7 | 97.5 | 10 - 25 | |
9 | 99.1 | 3.6 - 9 | |
|
3 | 100 | n.o. |
5 | 100 | n.o. | |
8 | 100 | n.o. |
Required membrane area and overdimension of each membrane operation.
Membrane | Fouling index α, (L/h2m2bar) | OD (%) | Am required, (m2) | Am implemented, (m2) | Nmodules |
---|---|---|---|---|---|
|
0.0072 | 0.8 | 23.8 | 32 | 2 |
|
0.065 | 10.1 | 32.1 | 64 | 4 |
Am required: required membrane area; Am implemented: implemented membrane area; OD: membrane area overdimension; Nmodules: number of membrane modules necessary.
The rejection behavior of the membrane was further studied and modelized by means of a leaky solution-diffusion model (Jain and Gupta, 2006): Ri = PTM ∙ σi / (PTM + βi) (4)
where the rejection of the solute
Results from the fitting of the iron rejection values (
These results indicate that both membranes exhibit a good performance for the rejection of the iron (99.1% for the DK series NF membrane
Finally, the required membrane area was calculated on the basis of a daily amount of 10 m3 of OMW treatment need and considering 10 h operation a day. The number of the necessary membrane modules (Nmodules, 32 m2 each) was also estimated, as well as the overdesign (OD) of the membrane area. For this purpose, the estimated RO membrane area (Am) needed for the treatment of the secondary-treated OMW was calculated with the following equation, derived from the boundary flux theory previously validated by the Stoller and Ochando-Pulido (Ochando and Stoller, Am = Vf ∙ (Y/100) ∙ (1+(OD/100))/Jb (5)
where
The membrane overdesign was estimated with the following expression successfully used in previous work by the same Authors (Stoller and Ochando, OD = 100 ∙ (1–(Jb – α ∙ PTM∙tw))/Jb (6)
where
An
In this work, two different membranes, one nano-filtration (NF) and a low-pressure reverse osmosis (RO), are examined with a double aim: the main one is the recovery of the iron used as catalyst in the former Fenton-like secondary treatment of OMW.
The results indicate that both membranes exhibit a good performance towards the rejection of iron (99.1% for the DK series NF membrane
However, the productivity of the selected NF membrane increases upon lowing operating pressures, about 30.9 L/hm2 under at 8 bar with the RO membrane while 38.2 - 47.4 L/hm2 upon 8- 9 bar with the NF membrane. Moreover, a sensibly lower fouling index was measured on the NF membrane (0.0072 in contrast with 0.065), which ensures major steady-state performance on this membrane and longer service lifetime. Furthermore, this also results in a lower required membrane area, which is 4 modules in the case of RO in contrast with 2 modules for NF.
The Spanish Ministry of Science and Innovation is gratefully acknowledged for having funded the projects CTQ2007-66178 and CTQ2010-21411, as well as the University of Granada.