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This paper discusses a method to operate polyvinylidene difluoride (PVDF) fibers based on outside-in pressurized ultrafiltration (pUF) membranes at high efficiency used as a pretreatment in seawater desalination. Backwash sequence was initially identified as the key contributor to the process efficiency yield. Backwash duration is reduced from 170...
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... use of pressurized ultra ltration as a pretreatment for the reverse osmosis membranes in seawater desalination has experimented an impressive increase as a result of the continuous search for cost-e ff ective technologies that enable a sustainable production of water. 1 Key bene fi ts associated to the ultra fi ltration technology versus conventional pretreatment are a low footprint, the ability to remove virus and bacteria and to signi fi cantly reduce colloids, suspended particles, turbidity, and some total organic carbon. Even more importantly, the ability to reliably provide good quality fi ltrate water to the downstream reverse osmosis are the most remarkable bene fi ts associated with this technology. 2 1.1. Ultra fi ltration Cleanings. The ultra fi ltration process is characterized, unlike reverse osmosis, by having relatively short fi ltration cycles given the need for higher cleaning frequency. The duration of the fi ltration cycle strongly depends on the type of raw water leading to a fi ltration cycle between 10 to 80 min. 3 Between two fi ltration cycles, a backwash (BW) will occur to enable the cleaning of the fi bers and, consequently, a reduction in the trans-membrane pressure (TMP) accumulated during the fi ltration. A second type of cleaning, which takes place with a lower frequency compared to the backwash is the chemically enhanced backwash (CEB). Often, the CEB occurs once or twice per day and is characterized by a longer duration compared to the backwash and also by the use of chemicals. 3 − 12 The last type of cleanings, the cleaning in place (CIP) occurs once every couple of months and is characterized by its longer duration (few hours typically) and higher chemical concentrations used compared to a CEB. 3 1.2. Advanced Cleaning Research. In the past, and in the seawater desalination space, DOW Ultra fi ltration membranes were used in Qingdao 2009 with an e ffi ciency of 80% as some other commercially available ultra fi ltration systems show nowadays. 3 After the fi rst improvement phase done in Barcelona, the e ciency of DOW Ultra ltration was increased to 88%. 3 Previous investigations have focused in reducing the number of backwash steps, so that the steps that contribute the less can be omitted. This reduction from fi ve steps (air scour, draining, backwash top with air scour, backwash bottom, and forward fl ush) to two steps (backwash top with air scour and forward fl ush) at a constant backwash frequency of 30 min increased the e ffi ciency to 95%. 4 Simultaneously, previous investigations focused on reducing the backwash frequency in order to raise the ultra fi ltration e ffi ciency to 95%. The experiments were done keeping the 5 main backwash steps but reducing the backwash frequency from 30 to 90 min. Therefore, it was possible to operate the ultra fi ltration system doing fewer backwashes per day. 13 The aim of this work is to integrate the di ff erent pressurized ultra fi ltration advance cleaning researches described in refs 4 and 13, integrating in a same operation protocol the reduction in the number of backwash steps from fi ve steps to two steps and reducing the backwash frequency from 30 to 90 min. So, combining both approaches, the e ffi ciency representing the total ultra fi ltration process yield can be increased to a very high level. Therefore, the hypothesis of this investigation is that the ultra fi ltration can be operated in a stable way and sustainably by operating with a backwash frequency of 90 min and only using the two main backwash steps previously identi fi ed as being the most e ff ective in cleaning the ultra fi ltration membrane. Doing this, the e ffi ciency can be increased even higher, which ultimately can be translated into cost savings as more water is produced with the same amount of time. Backwash consumes more time per day when compared to CEB and CIP, as it is repeated more often. Therefore, it is identi fi ed as the fi rst cleaning process to be improved in order to increase the e ffi ciency of the ultra fi ltration process. 2.1. Unit Description. This research is done in an experimental containerized seawater desalination plant. This unit represents one of the twenty experimental units that Dow Water & Process Solutions has in its Global Water Technology Development Center in Tarragona, Spain. Figure 1 shows the scheme of the installation, which consists of two independent and parallel lines, both containing ultra fi ltration membranes pretreatment to the reverse osmosis train. The pretreatment before the ultra fi ltration unit includes an Amiad Arkal disk fi lter of 250 μ m. The ultra fi ltration modules used are DOW Ultra fi ltration SFP-2660 modules, and the reverse osmosis used are DOW FILMTEC SW30XLE-4040 membranes. 2.2. Ultra fi ltration Membranes. These two ultra fi ltration modules used are characterized by having a diameter of 165 mm (6.5 in.) and a length of 1500 mm (59.1 in.). The fi bers are made of hydrophilic polyvinylidene di fl uoride (PVDF) polymer, which is mechanically and chemically resistant. With this technology, it is possible to increase the fi ber permeability and make it more fouling resistant. These fi bers have a nominal pore size of 30 nm with a 0.7 mm inner diameter and an outside diameter of 1.3 mm. The module has a total active area of 33 m 2 (355 ft 2 ). DOW Ultra fi ltration modules operate following an outside-in con fi guration. This enables operating the membranes using air scour, which helps clean the fi bers. 2.3. Seawater Characterization. Seawater from the Mediterranean Sea taken from Tarragona Harbor is used for this research. Water inorganic composition is analyzed as a reference at the beginning of the experiment, having a Total Dissolved Solids (TDS) salt content of 39 252 mg/L (ion chromatograph). Table 1 depicts the total ionic seawater characterization. Water organics are analyzed every 4 days during the whole experimental period. Total organic carbon (TOC) has an average value of 1.15 ± 0.19 mg/L (UNE-EN 1484:1998), total suspended solids (TSS) has an average value of 8.29 ± 5.07 mg/L (UN-EN 872:2005), and turbidity (TB) has an average value of 3.11 ± 2.63 Nephelometric Turbidity Unit (NTU) (ISO 7027). This analysis is done in the Water Analytical Laboratory that Dow Water & Process Solutions has in its Global Water Technology Development Center located in Tarragona, Spain. 2.4. Normalization Equations. The normalized (TMP ) is calculated multiplying the measured TMP by the temperature correction factor (TCF) as described by eq 1. The purpose of the temperature correction factor is to take into consideration the e ff ect of the temperature ( T ) in Celsius degrees and its in fl uence on the viscosity of water, as described by eq 2. 14 Therefore, di ff erent TMP values obtained at di ff erent temperatures can be compared and transported to the same reference temperature of 25 ° C. 2.5. E ciency Equations. E ciency is de ned as the net yield of the ultra fi ltration process. It is obtained multiplying the product water recovery yield by the availability yield. E ffi ciency is used to make a fair comparison between these two parameters, making sure both time and water produced are taken into consideration to calculate the overall process yield. This yield is calculated using eq 3. Availability measures the time the ultra ltration module is producing water. Therefore, the time when the unit is not fi ltrating is discounted. This yield is calculated using eq 4. Water product recovery measures net water produced. Filtrated water consumed during backwashes and CEBs is discounted. This yield is calculated using eq 5. 2.6. Cleaning Methods. A typical backwash consists of ve small steps. 15 The fi rst step is the air scour, where air is blown inside an ultra fi ltration membrane at 12 N m 3 /h. Air makes the fi ber shake. This is achieved by opening the air feed valve, which allows air enter the module, and the concentrate valve, which allows air to exit the membrane. The second step, the draining step, empties the module. Draining helps removing all the fouling that have detach from the hollow fi bers and are placed in the water. This is achieved by emptying the concentrate valve, so that air can enter the module, and opening the feed valve, where wastewater exits the module. The third step is the backwash top with air scour. Backwash top with air scour combines the pore unblocking e ff ect of a backwash with the aeration shacking e ff ect. This is achieved by opening the fi ltrate valve while using the backwash pump and opening the concentrate valve so that waste can exit. Meanwhile, the air valve is open so that air can be blown inside the element while the backwash takes place. The fourth step is the backwash bottom, which also unblocks fi bers pores. This is achieved by opening the fi ltrate valve while using the backwash pump and opening the feed valve so that waste can exit. The fi fth step is the forward fl ush, where feedwater is pumped inside the module, having the fi ltrate valve closed and the concentrate valve open. This creates a shear e ff ect above the membrane surface, which helps eliminating fouling. A typically chemical enhanced backwash (CEB) consists of the same steps than the backwash sequence but with a NaClO injection during the ...
Citations
... Feed TDS has a major effect on the quality of water produced [28]. Permeate TDS tends to systematically increase as feed TDS increases [20]. ...
Water scarcity is slowly becoming a threat to the normal livelihoods of people as the human population continues to increase. Affordable options of sourcing potable water are needed, especially in Africa where purification of water is generally expensive. Reverse osmosis (RO) desalination has proven to be one of the most preferred and applied techniques in the desalination of water due to its production of good permeate water quality at low cost compared to other desalination techniques. Research and development for the design and optimization of this process to come up with optimum design parameters has been ongoing. This article summarizes the data extracted from experiments done on one of the major desalination plants in Cape Town, South Africa. The data was collected and statistically analyzed using Microsoft Excel and different relationships of parameters like feed and permeate total dissolved salts (TDS), temperature, pressure, energy and pH were plotted.
... Feed TDS has a major effect on the quality of water produced [28]. Permeate TDS tends to systematically increase as feed TDS increases [20]. ...
Reverse Osmosis (RO) has proven to be the most commonly applied technique in desalination technologies owing to its relatively low energy consumption. Ongoing research on improvement of this technology through the analysis of statistical data extracted from experiments and simulation software is being undertaken so as to maximize permeate water while at the same time using minimum energy. Analysis of experimental and theoretical data extracted from experiments and software helps to describe the performance of the RO unit and come up with satisfactory correlation models for the systems. In this article, statistical analysis of different parameters was carried out and several graphs of correlations were plotted against each other. Important RO parameters such as specific energy consumption, temperature, feed pressure and permeate total dissolved salts (TDS) were plotted against each other to come up with their graphical correlations.
... Calcium carbonate (calcite) at a feed pH of 7, RCaCO3: (10) Calcium sulphate (gypsum), RCaSO4: (11) Calcium fluoride, RCaF2: (12) Total mass balance: (13) Transmembrane pressure difference, ∆P: (14) Normalised specific energy, SEC*: (15) Actual permeate hourly flow rate, Qh, [14]: (16) Total membrane area, Amem: (17) Feed pressure at a given temperature, Pf: ...
... (26) Different TMP values obtained at different temperatures can be compared and transported to the same reference temperature of 25 °C [17]. ...
Reverse osmosis (RO) has proven to be the most effective and efficient desalination method in recent years. Modelling and optimization of RO desalination plants is ongoing in order to come up with sustainable and efficient RO plants, leading to several techniques being employed in relation to mathematical models of mass and heat transfer, salt rejection and membrane solute permeability. Membrane designs and specifications are factors that affect the efficiency of the RO desalination system. Membrane design tools and software such as ROSA and IMSDesign, which are provided by the membrane manufacturing companies, help in the selection and authentication of low energy consumption and high salt rejection membranes for the design of desalination units.
... The purpose of using CIP in DI water was to evaluate the oxidizing of the inorganic foulants present in the membrane, attached with HA during the filtration, which is responsible of scaling during cleaning. However, chemical enhanced backwashing (CEB) solution prepared with permeate were used for regular interval to recover the flux of fouled membrane (Xu et al., 2008;Gilabert Oriol et al., 2013). The CIP solution prepared in the permeate (S4) was mainly applied to compensate the intensity of the chemicals used in the CEB. ...
The goal of this study was to identify the scaling from the chemical cleaning of a polyvinylidene fluoride (PVDF) membrane, fouled by treating a solution containing inorganic foulants (Al, Fe, and Mn) in the presence of kaolin and humic acid as a natural organic matter at Ca+2 strength of 0.5 mMole. Chemical cleaning of the membrane was conducted using solutions prepared in deionized water and permeate water (PW), and the accumulation of insoluble salts on the membrane during cleaning were evaluated. Energy dispersive spectroscopy analysis was used to verify the presence inorganic foulants, and field emission scanning electron microscopy confirmed the changes in membrane symmetry from the accumulation of the foulants. A Fourier-transformed infrared spectroscopy analysis indicated the presence of new functional groups, i.e., C˗Cl and C˗O with bond vibrations at 542 cm −1 and 1,026 cm−1, respectively, on the membrane surface. The adsorbed mass of HA in the presence of inorganic foulants decreased from 3.54 ± 0.045 mg to 2.24 ± 0.095 mg and 1.71 ± 0.075 mg, and the flux recoveries decreased from 93.2% to 85.69% and 81.92%, for the pristine to chemically DI and PW cleaned membrane, respectively. However, the membrane characterization results confirmed that Al was the major contributor to the accumulation of inorganic salts on the membrane during chemical cleaning and its role was more severe in the presence of Mn. The fitting results of Hermia's fouling models and a specific fouling analysis confirmed the contribution of complete blocking model with increase in irreversible fouling was observed after chemical cleaning.
... A more detailed description of the pilot-scale setup and previously performed experiments can be found in Gilabert-Oriol et al. [4], Gilabert-Oriol et al. [6] and Gilabert-Oriol et al. [5]. ...
... Another method used recently for the pretreatment of feed water is membrane filtration, containing microfiltration (MF), ultrafiltration (UF), and nanofiltration (NF) before directed to SWRO modules, which use fewer chemicals than conventional chemical pretreatment and have high removal efficiency of fouling matters, such as turbidity, iron, silica, algae, and microbial contamination (Oriol et al. 2013;Kumar et al. 2006). Removal rating depends on the molecular mass cut-off of the pretreatment membrane to clarify feed water under different operating conditions. ...
Today, seawater reverse osmosis (SWRO) desalination is one of the leading technologies for producing fresh water from the vast oceans which can augment water supply beyond what is available from the hydrological cycle. Quality of SWRO product water, whether used for drinking, domestic purposes, food production, or recreational purposes has an important impact on health. This review identifies 60 priority concerns for the benefit of drinking-water supply system under the framework of an integrated SWRO stages from source to tap, covering the development of up-to-date research trends and full-scale SWRO project experiences.
... Using a Taguchi experimental design method, Akhondi et al. [19] demonstrated that, the roles of backwash duration and backwash strength in affecting membrane fouling rate were much smaller than those of the permeate flux and feed concentration. Gilabert Oriol et al. [140] determined that the optimized backwash duration was reduced from 170 to 100 s by eliminating cleaning steps from numbering five to two, and the optimized backwash interval was increased from 30 to 90 min, which resulted in the efficiency increasing from 88% to 97%. Compared to the backwash procedure of the UF membrane system recommended by a vendor (i.e., backwashing for 5 min and forward flushing for 5 min every 4 h), the optimized backwash parameters by Chen et al. [17] were as follows: backwashing for 1 min every 30 min (adding 1 min of forward flushing), which increased the flux recovery from 86.8% to 96.6%. ...
Conventional drinking water treatment processes have faced several obstacles that are severely affected by water pollution and shortage, which makes it difficult to produce potable water effectively. Low-pressure membrane filtration, which includes ultrafiltration (UF) and microfiltration (MF), is one of the most promising treatment technologies for improving water quality. Periodic hydraulic backwashing is a necessity for the routine operation of UF/MF membranes, but only sparse data are available regarding the optimization of backwashing procedures, while more attention has been directed toward membrane fouling and cleaning. In the current work, we critically review the backwashing parameters of UF/MF membranes used in municipal water supplies. These parameters include pressure, flux, permeability (or resistance), mass balance, and membrane characterization techniques. The factors affecting backwash performance, which include membrane properties, feed water properties and operating conditions, are discussed in detail. The pretreatments of feed water, such as peroxidation, adsorption, coagulation and filtration influence the performance of UF/MF membranes to varying extents. The impacts of the backwash interval, backwash duration, backwash strength, air-assisted backwashing, chemically enhanced backwashing, and backwash water quality on backwash performance are summarized to provide more comprehensive data, which can improve backwash performance in full-scale drinking water and water reuse treatment plants.
... To recover the membrane permeability, a CEB with a solution of 350 mg L À1 of sodium hypochlorite, concentration usually applied in desalination plant (Gilabert Oriol et al., 2013), was performed after the 10th cycle. In the presence of SKC, the efficacy of the chemical cleaning was limited since only 67% and 50% of the initial permeability was recovered for the EP and DP, respectively. ...
Reducing membrane fouling caused by seawater algal bloom is a challenge for regions of the world where most of their freshwater is produced by seawater desalination. This study aims to compare ultrafiltration (UF) fouling potential of three ubiquitous marine algal species cultures (i.e., Skeletonena costatum-SKC, Tetraselmis sp.-TET, and Hymenomonas sp.-HYM) sampled at different phases of growth. Results showed that flux reduction and irreversible fouling were more severe during the decline phase as compared to the exponential phase, for all species. SKC and TET were responsible for substantial irreversible fouling but their impact was significantly lower than HYM. The development of a transparent gel layer surrounding the cell during the HYM growth and accumulating in water is certainly responsible for the more severe observed fouling. Chemical backwash with a standard chlorine solution did not recover any membrane permeability. For TET and HYM, the Hydraulically Irreversible Fouling Index (HIFI) was correlated to their biopolymer content but this correlation is specific for each species. Solution pre-filtration through a 1.2 μm membrane proved that cells and particulate algal organic matter (p-AOM) considerably contribute to fouling, especially for HYM for which the HIFI was reduced by a factor of 82.3.
... The next investigation focused on keeping a low backwash frequency of 90 min, while reducing the number of backwash steps to two. This leaded to an efficiency increase of 97% [14]. ...
... Baseline operation reflects previous operation using filtrated water during backwashes and optimized cleaning steps. The other line uses reverse osmosis brine during backwashes and also uses the two most relevant cleaning steps identified in [4] and [14]. Both lines use a backwash frequency of 90 min as identified in [13]. ...
... Phase 3 focuses on decreasing the backwash frequency to 90 min but keeping the five backwash steps as described in [13]. Phase 4 focuses on decreasing the backwash frequency to 90 min and reducing the backwash steps to two as described in [14]. Phase 5 uses the same conditions as phase 3 but using reverse osmosis brine for backwashing and depicts the experimental part of this work. ...
This paper discusses the feasibility of using reverse osmosis concentrate to backwash ultrafiltration membranes in the seawater reverse osmosis desalination space. Brine is produced through DOW FILMTEC™ reverse osmosis elements and it backwashed every 90 min to DOW™ ultrafiltration membranes. A side-by-side validation is done for 15 d using two parallel ultrafiltration and reverse osmosis integrated systems. One line uses brine for backwashing, while the other uses conventional filtrated water. The optimization is proven to have the same cleaning efficiency than the conventional backwashing methods and no precipitation is observed in the fibers. An additional validation period that uses reverse osmosis brine during backwashes and only two backwash steps is also carried out successfully. These steps are the previously identified backwash top with air scour and forward flush. Fibers also show an excellent integrity after the whole experimental period. A model is built in order to analyze the backwash efficiency of the optimized conditions and the transmembrane pressure increases during the filtration cycle. The results show the same fouling tendency for the line operating with brine and the line operating with filtrated water. The efficiency of the ultrafiltration process is improved from 88 to 98% thanks to this optimization together with the previous researches. This represents filtrating 96 min extra per day and a reduction of 100% in the filtrated water used during backwashes. The chemical equivalent concentration is also optimized from 0.28 to 0.06 mg/L NaClO thanks to the adjustment of the chemically enhanced backwash frequency. This accounts for a 7.1% savings in the ultrafiltration step and for a 1.2% savings in the whole desalination process.
