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Demineralization of brackish surface water by reverse osmosis: The first experience in Morocco
Journal of Environmental Chemical Engineering 7 (2019) 102937
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Journal of Environmental Chemical Engineering journal homepage: www.elsevier.com/locate/jece
Demineralization of brackish surface water by reverse osmosis: The first experience in Morocco
T
Hicham Boulahfaa,⁎, Sakina Belhamidia, Fatima Elhannounia, Mohamed Takya, Abdelhakim El Fadilb, Azzedine Elmidaouia a b
Laboratory of separation processes, Department of chemistry, Faculty of sciences, Ibn Tofail University, P.O. Box 1246, Kenitra, 14000, Morocco Dow Deutschland Anlagengesellschaft, Industriestrasse1, Rheinmunster, Germany
ARTICLE INFO
ABSTRACT
Keywords: Surface water Brackish water Reverse osmosis Desalination Industrial
Desalination technique is among the focused theme of the Moroccan kingdom’s universities. The implementation of reverse osmosis (RO) technology at the industrial level has proved that this RO technology delivers an affordable production of good water quality and quantity. The conception of mid-size to large RO (FILMTEC™ product) desalination plants are performed by means of reverse osmosis system analysis (ROSA) software. This study shed the light on how realistic the predictions are done by means of ROSA software to the real daily performance of the RO (FILMTEC™XLE440) membranes. The results obtained are then compared against the real industrial data collected from Khenifra desalination plant. This RO plant is designed to produce 36.290 m3/d of drinking water at the horizon of the year 2030. The performances collected are the pressure, temperature, permeate flow and permeate conductivity. The brackish water RO desalination plant has operated successfully and showed a steady behavior from the second half of 2013. The data collected within around 400 days showed that the ROSA modeling prediction matches in a conservative way to the real performance data collected from the plant at about 97%.
1. Introduction Water shortage is one of substantial challenges worldwide [1,2], whereby many countries of the Middle East, South East Asia and North Africa are now classified as water-stressed regions [3,4]. The pressure on fresh water availability for human consumption is exacerbated by the climate change and population growth [5]. According to the World Health Organization (WHO), around 2.1 billion people around the world lack safe drinking water, whereas by 2025, half of the world's population are predicted to be living in water-stressed areas [6,7]. In order to solve the problem of the increasing demand for fresh water, strategies like water reuse and seawater desalination have already been adopted [8,9]. More than 15.000 desalination plants around the world are supplying fresh water from saline water through which this number will continue to rise as researchers study to enhance the treatment processes, both in terms of cost effectiveness and energy efficiency [10].
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According to the International Energy Agency (IEA), today, about 58% of the world's total desalination capacity lies in the Middle East and North Africa combined, while globally, about 90 million m3 of water is desalinated each day [11]. During the last century, industrial water treatment has evolved from an optional and voluntary approach into a must-have multistage operation, which is applied greatly enhance the drinking water quality. After many trials and failures, the technology has established a set of obvious treatment processes that are extensively applied to diverse raw waters [12]. To supply high quality water, membrane technology is one of the most promising processes [13,14]. Among the most frequently used technologies, reverse osmosis membrane process is extensively used in seawater desalination, drinking water production, brackish water treatment and wastewater treatment [15–17]. The reverse osmosis (RO) technology has been increasingly considered as one of the promising methods for water desalination
Corresponding author. E-mail address: [email protected] (H. Boulahfa).
https://doi.org/10.1016/j.jece.2019.102937 Received 8 October 2018; Received in revised form 31 January 2019; Accepted 1 February 2019 Available online 06 February 2019 2213-3437/ © 2019 Elsevier Ltd. All rights reserved.
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[18–20]. Also known as hyper-filtration is a membrane separation process which allows the removal of very small particles such as monovalent ions with diameters in the order of 10−3– 10-4mm [21]. The reverse osmosis process is a pressure driven separation, based on the selective permeability of membrane [22] and is a multi-variable complex system that requires an insight analysis of the mutual interaction between the different operating and design parameters [23]. However, the effectiveness of this technology is compromised by fouling of the membranes. Fouling impacts the performance, which results in an increase in the energy demand, cleaning frequency, chemicals consumption, membrane replacements and hence the cost of the water treatment [24,25]. The secret of success of a reverse osmosis system is the pretreatment. The science of pretreatment in desalination is complicated by the natural variation in the water quality associated with the many water sources which can be desalted by membrane processes. These applications generally include: brackish groundwater, brackish surface water, seawater, municipal and industrial wastewater, produced water, flow back water and ultrapure water [26]. Pretreatment has been mainly used in RO systems and it has the advantage of improving the feed water quality greatly to ensure efficient RO operation as well as to extend membrane life. The water composition analysis is the main parameter to select an adequate pretreatment processes [16]. Among the most commonly used pretreatment processes, coagulation is widely used in water treatment plants due to its low cost. Several factors may influence the aggregation of colloids, such as solution pH, turbidity, chemical composition of the water samples, coagulant dosage, water temperature, surface area of colloids, and mixing conditions [27,28]. The kingdom of Morocco is working to alleviate an imperative decline in water availability that is projected to continue falling as a result of changing rainfall patterns. Morocco, located in the far west of northwest Africa in a semi-arid climate zone, receives an average of 346 mm of rainfall per year. This rainfall is roughly half the rainfall experienced in the European Union. Current per capita availability is 760.000 L/year, but that availability is expected to fall to 560.000 L/ year by 2030 [29]. This fall results from a projected reduction in rainfall and an increase in the average temperature of 1.5–2.5 °C, which increases the susceptibility of lakes and dams more susceptible to evaporation. Erosion and pollution also contribute to the loss of available fresh water. Fortunately, the Moroccan government has launched programs of water management to counteract the effects of climate change on water availability. Morocco employs a multi-sectoral approach to combat the effects of climate change, including efforts of government ministries and policy makers, universities, and NGOs to enact management strategies, develop new sources of potable water, and encourage public awareness on the importance of environmental conservation. The kingdom’s universities and research facilities are working to develop more effective wastewater treatment and desalination techniques. Morocco has an extensive system of dams, which mobilize a total of 17.6 billion m3 of water [29]. There are 139 total dams in Morocco, 13 of which are major water sources. Morocco has considerable experience in water treatment; desalination is the commonly used process of exploitation of non-conventional resources in the country [30]. To meet the growing demand of the
population and prevent over-exploitation of conventional resources, the competent authorities have appealed for years to the exploitation of non-conventional resources; it is mainly the desalination of sea water and brackish water. Therefore, brackish and seawater desalination is being progressively utilized, despite the higher costs in comparison with conventional techniques [31]. The first demineralization unit in Morocco was installed in Tarfaya in 1975. This unit was designed for the electrodialysis demineralization of brackish water containing 5 g/L of dissolved salt [30]. In 1983, the National Office of Water and Electricity (ONEE) received a new demineralization installation based on the reverse osmosis process to reinforce the electrodialysis demineralization unit of Tarfaya. The same year, the ONEE undertook the implementation of a project elaborated to enhance drinking water quality, thanks to the assistance of the program of United Nations for Development (PNUD) and the collaboration of KIWA Institute in Netherlands, specialized brackish water demineralization [30]. The expertise thus gained through the use of these diverse techniques strengthened by ONEE collaborations with the countries skilled in desalination technologies (the Middle East, the Canary Islands, Malta) led to the adoption of reverse osmosis desalination at Laayoune and Boujdour cities. Thus, in 1995 two reverse osmosis desalination plants using DuPont B-10 hollow fiber membranes with production capacities of 7000 m3/d and 800 m3/d were built in Laayoune and Boujdour, respectively [32]. The main aims of this paper are to share the first industrial experience on brackish surface water reverse osmosis treatment in Morocco, analyze the RO performances during one year of the plant operation and shed the light on how realistic the predictions are done by means of ROSA software to the real daily industrial performance of the RO membranes. 2. The reverse osmosis plant set up and methods 2.1. General description of the Khenifra region Khenifra, a city of 117.510 inhabitants, is located in the Middle Atlas of Morocco (Fig. 1), with a global surface of 41.033 km2 (5.77% of Morocco surface). It occupies a strategic position to the crossroads of two important collars which connect the high pond of Oum Rbiâa with the high ponds of Melouiya and Beht / Sebou. So, it is situated in the center of Morocco in the intersection of the richest agricultural regions and between two strongly robust regional city networks (Fes, Meknes and Tadla). The Khenifra region is a domain varied by hills, mountains and basins, which experience a big difference in climate, with rainy winters and relatively dry summers. The annual average pluviometry is approximately 600 mm at the level of the Khenifra station. More than 95% of rains fall between October and May. The rainiest months are in November, December and March. The hydrological network of Khenifra region is so constituted by the Oum Rbiâa river and its effluents. Oum Rbiâa is the second largest Moroccan river of 550 km with a flow of 117 m3/s. It takes its source at 1800 m altitude in the Middle Atlas, 40 km from Khenifra city and flows into the Atlantic Ocean .
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Fig. 1. Location of the Khenifra city.
2.2. Plant description
have been used most widely, and they can change surface charge properties to promote agglomeration and/or enmeshment of smaller particles into larger flocs [27,33]. 2 Acidification (H2SO4): eventually required for an adequate coagulation for the suspended matter 3 Flocculation using anionic polymer: the role of flocculation is to promote contact between the destabilized particles by means of a slow mixture. These particles agglomerate to form a floc that can easily be removed by settling. 4 Lamellar sedimentation: is a technique for separating suspended matter and colloids collected in flocs during the coagulation flocculation step. 2.2.2.1.3. Dual media filtration. There are seven parallel dual media filters of 36 m² containing 0.1 m gravel, 0.8 m very fine sand and 0.8 m pumice for the purpose of trapping fine particles, get a filtered water turbidity of less than 0.5 NTU and thus improve the silt density index (SDI). All cells are backwashed in sequence with filtered water and air.
2.2.1. Brackish water supply system The plant raw water supply is provided from the Tanfnit El Borj dam, located at 15 km from the treatment plant. 2.2.2. Pretreatment process 2.2.2.1. Water system 2.2.2.1.1. Scrubbers. There are two independent clarifiers, designed to handle a level of suspended solids up to 20 g/L by unit at the nominal flow rate of 1250 m3/h. Their function is needed when the suspended solids concentration exceeds 2 g/L, if not they serve as contact tank for pre-chlorination. 2.2.2.1.2. Lamellar clarifiers. There are two independent clarifiers (Fig. 2); their role is to eliminate suspended solids upstream of the dual media filters. They comprise the following steps: 1 Coagulation using aluminum sulfate: Aluminum-based coagulants
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Fig. 2. Block flow diagram for the pretreatment system. FWT: Filtered water tank.
2.2.2.2. Sludge system. At various stages Khenifra water processing plant produce different wastes:
top surface. Each layer is tailored to specific requirements. A schematic diagram of the membrane is shown in Fig. 3
- Sludge of scrubbers - Sludge of lamellar clarifiers - Clarifier of dirty water coming from sand filters This sludge cannot be dismissed directly to the natural environment. In addition, in order to minimize the loss of water from the plant, a part of the dirty water washing will be recycled. The sludges extracted from the clarifiers, lamellar clarifiers as well as of the decanter of dirty water filter backwash are sent according to their concentration either toward the tarp of sludge, or directly toward the natural environment. The tarp of sludge feed the thickener to concentrate the sludge and send them by following to the drying beds to reduce their volume by drying and avoid returning them to the natural environment.
Fig. 3. Membrane composition: a: polyamide 0.2 micro-m, b: micro-porous polysulfone 40 micro-m, c: polyester support web, ultra-thin barrier layer 120 micro-m.
The table below shows the characteristics of the reverse osmosis trains installed in the Khenifra plant (Table 1). Table 1 Characteristics of the Khenifra brackish surface water reverse osmosis trains. Number of trains Number of stages by train Number of membranes by the pressure vessel Number of pressure vessels on the first stage Number of pressure vessels on the second stage Feed flow rate by train Permeate flow rate by train Concentrate flow rate by train Recovery rate
2.2.3. Reverse osmosis process 2.2.3.1. Low pressure pumps and microfiltration. A group of three-micron cartridge filters (MCF) with 5 μm mesh installed on the common manifold discharge of low pressure pumps, each one contains 224 cartridges (Reference PALL Claris 50). After the low-pressure pumping, filtered water receives three additional treatments: ○ Acidification: Adjusting pH by injection of H2SO4 98%. ○ Sequestering injection reagent upstream of microfiltration to prevent the precipitation of calcium carbonate (CaCO3) and calcium sulfate (CaSO4) at the membranes level. ○ Injection of sodium bisulfate to neutralize the remaining residual chlorine contained in the feed water.
3
2 7 52 23 506 m³/h 410 m³/h 95 m³/h 81%
2.2.3.3. High pressure pumps. The high pressure pumps and RO section of the plant is divided into three trains, each one with a production capacity of 9.850 m3/d. One stand-by high pressure pump (the fourth), is installed and connected to the three trains. Each reverse osmosis train contain 2 stages of pressure vessels installed in "series-rejection", with 52 pressure vessels on the first stage and 23 on the second stage. Each pressure vessel contains 7 membranes. There is no pumping or energy recovery between the two stages. The pretreated water is pumped up to a pressure of 12 bars. The maximum design pressure is 14.5 bars (Fig. 4).
2.2.3.2. FilmTec™reverse osmosis for brackish water. The membrane used in this plant named XLE440. It belongs to the FILMTEC™ FT30 membrane family. It is composed of thin film composite membrane consisting of three layers: a polyester support web, a micro-porous polysulfone interlayer, and an ultra-thin polyamide barrier layer on the
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Fig. 4. Block flow diagram for the reverse osmosis process. FWT: filtered water tank; CF: cartridge filter; LPP: low pressure pump; HP: high pressure pump; T: tank; SBS: sodium bisulfate.
2.3. Cleaning in place (CIP)
production systems starting from RO plants to avoid the aforementioned challenges [34]. Before the water distribution in Khenifra plant, the remineralization is done by blending of one third of filtered water coming from dual media filters and two thirds of permeate. The remineralization installation is on stand-by to produce lime water. Lime water is produced by the saturator. It is a cylindro-conical device [34] which receives, firstly, a quantity of lime in the form of lime milk (50 g/L), and secondly a quantity of spraying water which arrives under pressure at the base of the device. The lime is then dissolved to saturation in order to produce a water of lime having 10 NTU in turbidity and approximately 200°F in complete alkalimetric title (CAT), which is collected in the upper portion of the saturator (Fig. 5).
Clean-in-place (CIP) is a method of cleaning the interior surfaces of pipes, vessels, process equipment, filters and associated fittings, without disassembly. The desalination plant is equipped with a chemical cleaning system for membranes cleaning. Two pumps are allocated to allow cleaning and recirculating the cleaning solution on the membranes then returning them in the tank. 2.4. Remineralization unit The permeate water coming out of a desalination plant cannot be directly used because it is unpalatable, corrosive and unhealthy. Hence, a remineralization step is an essential part of the drinking water
Fig. 5. The remineralization process. (a) saturator, (b) lime milk preparation tank, (c) milk injection pumps, (d) purge, (e) injection pumps for permeated water, (f) permeated water tank, (g) lime water tank, (h) distribution of remineralized water.
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2.5. Product delivery to the city
3.4. Methods
• The reverse osmosis membranes projection was carried out using the ROSA software of the Dow Chemical Company, version 7.2.1. • The dosing rate of the Genesys LF antiscalant was carried out by using the software GENESYS MM4 version: v1. 22.1 (DLL v1.8.7). • Sodium bisulfate (SBS) dosing rate was defined using stoichiometric
Table 2: The quality of drinking water waited at the outlet of the plant. Table 2 The quality of drinking water waited at the outlet of the plant. Parameters
Unit
Value
pH Ca Complete alkalimetric title (CAT) Total dissolved solids (TDS) Turbidity Aluminum Iron Manganese
Unit pH °F °F mg/L NTU mg/L mg/L mg/L
7 to 8.5 8 8 TDS < 800 < 0,5 < 0.2 < 0.3 < 0.1
• •
calculations. To neutralize 1 mg/L of free chlorine it is necessary to inject 1.34 mg/L of sodium bisulfate. The permeate conductivity was measured using WTW laboratory conductivity meter inoLab Cond 7110. The silt density index (SDI) was measured using a kit that allows the filtration of water through a membrane of porosity 0.45 μm and 47 mm in diameter at a pressure of 2.1 bars.
4. Results and discussion 4.1. Silt density index (SDI) assessment
3. Materials and methods
The silt density index (SDI) is an indicator of the colloids and suspended particulate matter present in water [35–37]. Fig. 6 presents the evolution of the silt density index (SDI) of filtered water coming from the dual media filters and micro-filtered water after the 5 μm cartridge filters; the graph shows the effectiveness of the pretreatment process; it illustrates that the SDI values are always lower than those of the design (<3) after microfiltration. The obtained results prove that the pretreatment process inlet reverse osmosis section operates successfully, and the microfiltration enhances the filtered water quality in terms of SDI (Fig. 6).
3.1. Feed water characteristics Table 3 gives the mean values of the raw water quality, with an indication of World Health Organization (WHO) recommendations and Moroccan drinking water standards. Table 3 The water quality used as a reference for the demineralization unit design. Chemical composition
Brackish surface water
Moroccan standards
WHO standards
pH Alkalinity (°F) Calcium (mg/L) Magnesium (mg/L) Sodium (mg/L) Potassium (mg/L) Manganese total (mg/L) Aluminum (mg/L) Ammonium (mg/L) Iron (mg/L) Strontium (mg/L) Barium (mg/L) Bicarbonate (mg/L) Chloride (mg/L) Sulfate (mg/L) Nitrate (mg/L) Fluoride (mg/L) Silica (mg/L) Total dissolved solids TDS (mg/L)
6.8 17.6 95 32.5 466 2.9 0 < 0.2 0 < 0.03 0 0 214 866 117 6.49 0 15 1800
6.5-8.5 – <500 100 <200 – 0.5 0.2 0.5 0.3 – 0.7 – 750 200 <50 1.5 – 1000-2000
6.5-8.5 – <270 <50 <200 – – 0.2 – 0.2 – 0.7 – <250 <200 <50 1.5 – <1000
4.2. Performances of reverse osmosis plant
Fig. 6. SDI values at the inlet and outlet of the microfiltration.
4.2.1. Seasonal conductivity variation Fig. 7 shows that the raw water supplying the demineralization plant undergoes significant seasonal variations in terms of conductivity. It increases during the summer reaching 2512.3μS/cm; due to the evaporation of the water causing the salts concentration high and decreases during the winter reaching 1333.8 μS/cm, due to the rainfall dilution process. This increase will noticeably impact the membrane performance in short and long term. Short term the quality of the permeate will decline gradually and the productivity as well. Long term if the recovery and the antiscalant’s dosing are not adjusted, then the membranes would experience a heavy scaling or even irreversible fouling which will lead to a drop in the membrane plant performance.
3.2. RO membrane element characteristics The reverse osmosis membrane used in this study was an 8-inch spiral wound membrane element (FILMTEC™XLE440, USA) having 28 mil feed spacers. From the manufacturer, the permeate flow and salt rejection of the membrane elements under the standard conditions were 36.8 L/min and 99.5%, respectively (standard conditions: 2000 ppm NaCl, 125 psi (8.6 bar), 77 °F (25 °C), pH 8, and 15% recovery). 3.3. Microfiltration cartridges The cartridges installed were PALL Claris with 5 μm mesh. The material of construction is polypropylene.
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The decline in permeate performance influenced directly the other operating parameters. Effectively, the feed pressure increased on both stages (Fig. 10). In fact, the membrane fouling causes the decrease of the membrane active surface area and consequently the increase of the applied pressure. The increase in pressure also depends on the decrease in temperature during the winter period and on the increase in conductivity during the summer period (Figs. 7 and 8). The recovery rate increased in the first stage and decreased in the second stage (Fig. 13) due to the automatic regulation which attempts to reach the set point flow rate of the RO unit. The pressure drop increased slightly on the first stage because of the flow increase in this stage; moreover, it decreased on the second stage, because of the flow diminution through the membranes due to the membrane fouling (Fig. 11). The conductivity remained stable in the first stage (Fig. 12); furthermore, it increased in the second stage due to the augmentation of the salt passage, generated by the salts deposition on the membrane surface, especially, NaCl, CaCO3 and CaSO4 contained in the feed water. The gradual increase of permeate salinity indicates a mineral scaling. However, the decrease of permeate flow indicates organic fouling [39]. To restore the original RO performance and overcome the fouling problems [40]. The Dow FilmTec™ team suggested to perform a classic membrane cleaning, using alkaline (1% 4Na-EDTA + 0.1% NaOH) and acid (0.2% HCl) cleaning agents [41]. Alkaline solutions such as sodium hydroxide (NaOH) remove organic fouling and biofouling by hydrolysis and solubilization; whereas acidic solutions such as hydrochloric acid (HCl) dissolve scaling, disrupt the bacterial cell wall structure and precipitate proteins [16,40,42]. Cleaning protocols for spiral wound RO membranes consist of several phases of high flow recirculation and soaking, lasting anywhere between 6.5–24 hours in duration at a typically applied temperature of 35 °C [40]. The acid solution was used to dissolve the salts deposited on the membranes, in particular, NaCl, CaCO3 and CaSO4 contained in the feed water. Alkaline solution is used to eliminate organic fouling including biological matter, which can be caused by a possible bacterial growth or by the organic matter contained in the brackish surface water supplying the plant. After the first chemical cleaning, which has been much effective, the initial flow rate of the first stage, second stage and global system were restored. Since this intervention, the RO unit has operated successfully during nearly eight months (from October 2013 to June 2014) with stable performances. Another membrane fouling was recorded after eight months of operation. In fact, the RO unit showed the same behavior that occurred during the first three months of operation after the plant commissioning. Indeed, the permeate flow passes from 299 to 370 m3/h for the first stage; form 110 to 16 m3/h for the second stage and from 411 to 347 m3/ h for the global system (Fig. 9). The other operating parameters were evolved in the same way recorded during the first membranes fouling. A second classic chemical cleaning was carried out to return to the normal operation.
Fig. 7. Feed water conductivity variations.
4.2.2. Seasonal temperature variation Fig. 8 shows the feed water temperature changes. It undergoes important seasonal variations. It declines during winter reaching 10.4 °C and rises during summer reaching 22 °C. The performance of the RO system will be influenced by the feed water temperature changes. For example, a feed temperature increase of 4 °C will cause a permeate flow increase of about 10%. This, however, is a normal phenomenon [38].
Fig. 8. Feed water temperature variations.
4.2.3. Reverse osmosis performance assessment The data represented on the graphs corresponding to the operating parameters evolution was followed every day. Five graphs of the plant evolution through the first year of operation are shown to indicate operating time versus the performances. Figs. 9–13 give the performances of the reverse osmosis plant especially the variation during one year of permeate flow, feed pressure, pressure drop, permeate conductivity and recovery rate. In the beginning of the RO system operation, the 1st stage permeate flow decreased from 295 to 252 m3/h. This decrease is due to the system settings during the commissioning period. The permeate flow increased during the first days and stabilized after nearly two weeks. This membrane flux stabilization is known as the so called wet out of the membranes [38]. The wet out process ended after nearly after 13 days of operation. After nearly three months of the plant commissioning (from July 2013 to October 2013), the permeate flow performance has changed significantly. It passes from 295 to 362 m3/h for the first stage and form 114 to 34 m3/h for the second stage (Fig. 9). The augmentation of the first stage permeate flow is explained by the automatic regulation PID (proportional, integral, and derivative), linked to the speed controller of the high pressure pump. This regulation attempts to reach the set point flow rate (410 m3/h) of the RO unit. It compensates the flow decline in the second stage by increasing the first stage flow rate. This performance decline was essentially due to an operating error which led to an overdose of the antiscalant at the feed of the 1st stage.
Fig. 9. 1st stage, 2nd stage and RO unit permeate flow evolution. 7
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performance of RO systems in conditions outside of the standard 2000 ppm NaCl at which FILMTEC™ products are warranted. This software is developed based on empirical data that is generated by testing FILMTEC™ products at different conditions to generate the ratio between A-value and B-value. The raw water quality used as reference for the reverse osmosis unit design is represented in Table 3. Tables 4 and 5 show the operating results versus the expected design values in the same conditions of temperature and feed water TDS after nearly one year of operation. The ROSA projection and actual performances were compared after the membranes chemical cleanings and reprise of the normal operating parameters recorded during the plant start up. Throughout the whole period of operation, the feed flow, the permeate flow; the rejection flow and the recovery rate of the train were the same as the expected design values. For the feed pressure, it was slightly above the expected design value for the first stage of the RO train. However, it was the same in the second stage .
Fig. 10. 1st stage, 2nd stage and RO unit feed pressure evolution.
Table 4 Comparison of operating results with ROSA projection after one year of operation. Parameters
ROSA
3
Feed flow (m /h) Feed pressure (bar) Permeate flow (m3/h) Rejection flow (m3/h) Recovery rate (%)
Fig. 11. 1st stage, 2nd stage and RO unit pressure drop evolution.
RO TRAIN
1st stage
2nd stage
1st stage
2nd stage
506 10.4 297.24 208.76 58.7
208.76 8.99 112.89 95.87 54.1
507.96 11.13 299.64 208.32 59
208.32 8.9 108.72 99.6 52.2
Table 5 compares the ROSA prediction values with the experimental results collected in the RO plant in the same conditions of temperature and feed water TDS. The results obtained prove that the real permeate TDS values are close to the ROSA predictions and the reverse osmosis membranes exhibit high and stable performances . Table 5 Comparison of the permeate TDS results with the ROSA projection. Concentration
TDS (mg/L)
Fig. 12. 1st stage and 2nd stage permeate conductivity evolution.
ROSA
RO TRAIN
1st stage
2nd stage
1st stage
2nd stage
101.58
258.52
99.2
252.6
5. Conclusion In comparing the real industrial data versus ROSA simulation, the following key learning points were generated:
• RO membrane at large industrial scale needs rigorous technical care • • •
Fig. 13. 1st stage, 2nd stage and RO unit recovery rate evolution.
4.2.4. Rosa projection versus RO trains performances Reverse osmosis system analysis (ROSA) software is developed by Dow Chemical to help their customer’s model and predict the
at the start up and a keen technical follow up to reach a steady stabilized performance. ROSA predicted the performance of FILMTEC™XLE440 at about 97% close to the real industrial data. The weather change from season to another tends to affect the dam water temperature as well as its quality which noticeably influence the RO membrane performance. The change in water quality requires a rethinking and sometime also reselection of the adequate antiscalant molecule and dosage.
The feed water quality piped to the Khenifra reverse osmosis (RO) desalination plant is changing. This change is due to rainfall deviation 8
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and seasonal temperature fluctuation which leads to the evaporation of the water from the dam. A scientific approach combined with RO technical expertise is vital to operate the plant and to maintain a longterm reliable membrane performance. A further study would be interesting to understand the impacts of this changes and how to overcome them in the RO desalination plant. Morocco has a significant experience in water desalination but this one is still the first in terms of brackish surface water treatment by reverse osmosis, making it a great challenge for the kingdom, and will be the occasion for many important investigations.
[21] [22] [23] [24] [25]
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Croue, How different is the composition of the fouling layer of wastewater reuse and seawater desalination RO membranes? Water Res. 59 (2014) 271–282, https://doi.org/10. 1016/j.watres.2014.04.020. L. Henthorne, B. Boysen, State-of-the-art of reverse osmosis desalination pretreatment, Desalination 356 (2015) 129–139, https://doi.org/10.1016/j.desal.2014.10. 039. H. Xu, F. Xiao, D. Wang, Effects of Al2O3 and TiO2 on the coagulation process by Al2(SO4)3 (AS) and poly-aluminum chloride (PACl) in kaolin suspension, Sep. Purif. Technol. 124 (2014) 9–17, https://doi.org/10.1016/j.seppur.2014.01.010. R. Jiao, H. Xu, W. Xu, X. Yang, D. Wang, Influence of coagulation mechanisms on the residual aluminum – the roles of coagulant species and MW of organic matter, J. Hazard. Mater. 290 (2015) 16–25, https://doi.org/10.1016/j.jhazmat.2015.02. 041. J. Kurtze, M. Morais, E. Platko, H. Thompson, S. Mccauley, A. Sakulich Water, R. Al Fath, Advancing Water Management Strategies in Morocco, J. Kurtze, M. Morais, E. Platko, H. Thompson, S. Mccauley, A. Sakulich Water, R. Al Fath, Advancing Water Management Strategies in Morocco, (2015) 2015https://web.wpi.edu/Pubs/ E-project/Available/E-project-101615-040211/unrestricted/IQP-Sponsor_Edition. pdf (Accessed 04, November 2018) https://web.wpi.edu/Pubs/E-project/ Available/E-project-1. H. Zidouri, Desalination of Morocco and presentation of design and operation of the Laayoune seawater reverse osmosis plant, Desalination 131 (2000) 137–145, https://doi.org/10.1016/S0011-9164(00)90014-6. K. Tahri, The prospects of fresh water supply for Tan Tan City from non-conventional water resources, Desalination 135 (2001) 43–50, https://doi.org/10.1016/ S0011-9164(01)00137-0. M. Hafsi, Analysis of Boujdour desalination plant performance, Desalination 134 (2001) 93–104, https://doi.org/10.1016/S0011-9164(01)00119-9. Z. Yang, B. Gao, Q. 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Pilipovik, Prediction of the behaviour of the Silt Density Index (SDI) in the Caribbean Seawater and its impact on RO desalination plants, Desalination 268 (2011) 262–265, https://doi.org/10.1016/j.desal.2010.09.049. M. Habib, U. Habib, A.R. Memon, U. Amin, Z. Karim, A.U. Khan, S. Naveed, S. Ali, Predicting colloidal fouling of tap water by silt density index (SDI): pore blocking in a membrane process, J. Environ. Chem. Eng. 1 (2013) 33–37, https://doi.org/10. 1016/j.jece.2013.03.005. A. Gorenflo, M. Brusilovsky, M. Faigon, B. Liberman, High pH operation in seawater reverse osmosis permeate: first results from the world’s largest SWRO plant in Ashkelon, Desalination 203 (2007) 82–90, https://doi.org/10.1016/j.desal.2006. 05.004. S. Farhat, M. Bali, F. Kamel, Membrane autopsy to provide solutions to operational problems of Jerba brackish water desalination plant, Desalination 445 (2018) 225–235, https://doi.org/10.1016/j.desal.2018.08.013. F. Beyer, J. Laurinonyte, A. Zwijnenburg, A.J.M. Stams, C.M. Plugge, Membrane Fouling and Chemical Cleaning in Three Full-Scale Reverse Osmosis Plants Producing Demineralized Water, J. Eng. 2017 (2017) 1–14, https://doi.org/10. 1155/2017/6356751. D.N. Nada, Large SWRO PROJECT FOR DRINKING WATER IN SHUQAIQ, Int. Desalin. Assoc. (2011). Hydranautics, Technical Service Bulletin Foulants and Cleaning Procedures for Composite Polyamide RO Membrane Elements (ESPA, ESNA, CPA, LFC, NANO and SWC), (2014) (Accessed 01, January 2019), http://membranes.com/docs/tsb/ TSB107.pdf.
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Demineralization of brackish surface water by reverse osmosis: The first experience in Morocco
T
Hicham Boulahfaa,⁎, Sakina Belhamidia, Fatima Elhannounia, Mohamed Takya, Abdelhakim El Fadilb, Azzedine Elmidaouia a b
Laboratory of separation processes, Department of chemistry, Faculty of sciences, Ibn Tofail University, P.O. Box 1246, Kenitra, 14000, Morocco Dow Deutschland Anlagengesellschaft, Industriestrasse1, Rheinmunster, Germany
ARTICLE INFO
ABSTRACT
Keywords: Surface water Brackish water Reverse osmosis Desalination Industrial
Desalination technique is among the focused theme of the Moroccan kingdom’s universities. The implementation of reverse osmosis (RO) technology at the industrial level has proved that this RO technology delivers an affordable production of good water quality and quantity. The conception of mid-size to large RO (FILMTEC™ product) desalination plants are performed by means of reverse osmosis system analysis (ROSA) software. This study shed the light on how realistic the predictions are done by means of ROSA software to the real daily performance of the RO (FILMTEC™XLE440) membranes. The results obtained are then compared against the real industrial data collected from Khenifra desalination plant. This RO plant is designed to produce 36.290 m3/d of drinking water at the horizon of the year 2030. The performances collected are the pressure, temperature, permeate flow and permeate conductivity. The brackish water RO desalination plant has operated successfully and showed a steady behavior from the second half of 2013. The data collected within around 400 days showed that the ROSA modeling prediction matches in a conservative way to the real performance data collected from the plant at about 97%.
1. Introduction Water shortage is one of substantial challenges worldwide [1,2], whereby many countries of the Middle East, South East Asia and North Africa are now classified as water-stressed regions [3,4]. The pressure on fresh water availability for human consumption is exacerbated by the climate change and population growth [5]. According to the World Health Organization (WHO), around 2.1 billion people around the world lack safe drinking water, whereas by 2025, half of the world's population are predicted to be living in water-stressed areas [6,7]. In order to solve the problem of the increasing demand for fresh water, strategies like water reuse and seawater desalination have already been adopted [8,9]. More than 15.000 desalination plants around the world are supplying fresh water from saline water through which this number will continue to rise as researchers study to enhance the treatment processes, both in terms of cost effectiveness and energy efficiency [10].
⁎
According to the International Energy Agency (IEA), today, about 58% of the world's total desalination capacity lies in the Middle East and North Africa combined, while globally, about 90 million m3 of water is desalinated each day [11]. During the last century, industrial water treatment has evolved from an optional and voluntary approach into a must-have multistage operation, which is applied greatly enhance the drinking water quality. After many trials and failures, the technology has established a set of obvious treatment processes that are extensively applied to diverse raw waters [12]. To supply high quality water, membrane technology is one of the most promising processes [13,14]. Among the most frequently used technologies, reverse osmosis membrane process is extensively used in seawater desalination, drinking water production, brackish water treatment and wastewater treatment [15–17]. The reverse osmosis (RO) technology has been increasingly considered as one of the promising methods for water desalination
Corresponding author. E-mail address: [email protected] (H. Boulahfa).
https://doi.org/10.1016/j.jece.2019.102937 Received 8 October 2018; Received in revised form 31 January 2019; Accepted 1 February 2019 Available online 06 February 2019 2213-3437/ © 2019 Elsevier Ltd. All rights reserved.
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[18–20]. Also known as hyper-filtration is a membrane separation process which allows the removal of very small particles such as monovalent ions with diameters in the order of 10−3– 10-4mm [21]. The reverse osmosis process is a pressure driven separation, based on the selective permeability of membrane [22] and is a multi-variable complex system that requires an insight analysis of the mutual interaction between the different operating and design parameters [23]. However, the effectiveness of this technology is compromised by fouling of the membranes. Fouling impacts the performance, which results in an increase in the energy demand, cleaning frequency, chemicals consumption, membrane replacements and hence the cost of the water treatment [24,25]. The secret of success of a reverse osmosis system is the pretreatment. The science of pretreatment in desalination is complicated by the natural variation in the water quality associated with the many water sources which can be desalted by membrane processes. These applications generally include: brackish groundwater, brackish surface water, seawater, municipal and industrial wastewater, produced water, flow back water and ultrapure water [26]. Pretreatment has been mainly used in RO systems and it has the advantage of improving the feed water quality greatly to ensure efficient RO operation as well as to extend membrane life. The water composition analysis is the main parameter to select an adequate pretreatment processes [16]. Among the most commonly used pretreatment processes, coagulation is widely used in water treatment plants due to its low cost. Several factors may influence the aggregation of colloids, such as solution pH, turbidity, chemical composition of the water samples, coagulant dosage, water temperature, surface area of colloids, and mixing conditions [27,28]. The kingdom of Morocco is working to alleviate an imperative decline in water availability that is projected to continue falling as a result of changing rainfall patterns. Morocco, located in the far west of northwest Africa in a semi-arid climate zone, receives an average of 346 mm of rainfall per year. This rainfall is roughly half the rainfall experienced in the European Union. Current per capita availability is 760.000 L/year, but that availability is expected to fall to 560.000 L/ year by 2030 [29]. This fall results from a projected reduction in rainfall and an increase in the average temperature of 1.5–2.5 °C, which increases the susceptibility of lakes and dams more susceptible to evaporation. Erosion and pollution also contribute to the loss of available fresh water. Fortunately, the Moroccan government has launched programs of water management to counteract the effects of climate change on water availability. Morocco employs a multi-sectoral approach to combat the effects of climate change, including efforts of government ministries and policy makers, universities, and NGOs to enact management strategies, develop new sources of potable water, and encourage public awareness on the importance of environmental conservation. The kingdom’s universities and research facilities are working to develop more effective wastewater treatment and desalination techniques. Morocco has an extensive system of dams, which mobilize a total of 17.6 billion m3 of water [29]. There are 139 total dams in Morocco, 13 of which are major water sources. Morocco has considerable experience in water treatment; desalination is the commonly used process of exploitation of non-conventional resources in the country [30]. To meet the growing demand of the
population and prevent over-exploitation of conventional resources, the competent authorities have appealed for years to the exploitation of non-conventional resources; it is mainly the desalination of sea water and brackish water. Therefore, brackish and seawater desalination is being progressively utilized, despite the higher costs in comparison with conventional techniques [31]. The first demineralization unit in Morocco was installed in Tarfaya in 1975. This unit was designed for the electrodialysis demineralization of brackish water containing 5 g/L of dissolved salt [30]. In 1983, the National Office of Water and Electricity (ONEE) received a new demineralization installation based on the reverse osmosis process to reinforce the electrodialysis demineralization unit of Tarfaya. The same year, the ONEE undertook the implementation of a project elaborated to enhance drinking water quality, thanks to the assistance of the program of United Nations for Development (PNUD) and the collaboration of KIWA Institute in Netherlands, specialized brackish water demineralization [30]. The expertise thus gained through the use of these diverse techniques strengthened by ONEE collaborations with the countries skilled in desalination technologies (the Middle East, the Canary Islands, Malta) led to the adoption of reverse osmosis desalination at Laayoune and Boujdour cities. Thus, in 1995 two reverse osmosis desalination plants using DuPont B-10 hollow fiber membranes with production capacities of 7000 m3/d and 800 m3/d were built in Laayoune and Boujdour, respectively [32]. The main aims of this paper are to share the first industrial experience on brackish surface water reverse osmosis treatment in Morocco, analyze the RO performances during one year of the plant operation and shed the light on how realistic the predictions are done by means of ROSA software to the real daily industrial performance of the RO membranes. 2. The reverse osmosis plant set up and methods 2.1. General description of the Khenifra region Khenifra, a city of 117.510 inhabitants, is located in the Middle Atlas of Morocco (Fig. 1), with a global surface of 41.033 km2 (5.77% of Morocco surface). It occupies a strategic position to the crossroads of two important collars which connect the high pond of Oum Rbiâa with the high ponds of Melouiya and Beht / Sebou. So, it is situated in the center of Morocco in the intersection of the richest agricultural regions and between two strongly robust regional city networks (Fes, Meknes and Tadla). The Khenifra region is a domain varied by hills, mountains and basins, which experience a big difference in climate, with rainy winters and relatively dry summers. The annual average pluviometry is approximately 600 mm at the level of the Khenifra station. More than 95% of rains fall between October and May. The rainiest months are in November, December and March. The hydrological network of Khenifra region is so constituted by the Oum Rbiâa river and its effluents. Oum Rbiâa is the second largest Moroccan river of 550 km with a flow of 117 m3/s. It takes its source at 1800 m altitude in the Middle Atlas, 40 km from Khenifra city and flows into the Atlantic Ocean .
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Fig. 1. Location of the Khenifra city.
2.2. Plant description
have been used most widely, and they can change surface charge properties to promote agglomeration and/or enmeshment of smaller particles into larger flocs [27,33]. 2 Acidification (H2SO4): eventually required for an adequate coagulation for the suspended matter 3 Flocculation using anionic polymer: the role of flocculation is to promote contact between the destabilized particles by means of a slow mixture. These particles agglomerate to form a floc that can easily be removed by settling. 4 Lamellar sedimentation: is a technique for separating suspended matter and colloids collected in flocs during the coagulation flocculation step. 2.2.2.1.3. Dual media filtration. There are seven parallel dual media filters of 36 m² containing 0.1 m gravel, 0.8 m very fine sand and 0.8 m pumice for the purpose of trapping fine particles, get a filtered water turbidity of less than 0.5 NTU and thus improve the silt density index (SDI). All cells are backwashed in sequence with filtered water and air.
2.2.1. Brackish water supply system The plant raw water supply is provided from the Tanfnit El Borj dam, located at 15 km from the treatment plant. 2.2.2. Pretreatment process 2.2.2.1. Water system 2.2.2.1.1. Scrubbers. There are two independent clarifiers, designed to handle a level of suspended solids up to 20 g/L by unit at the nominal flow rate of 1250 m3/h. Their function is needed when the suspended solids concentration exceeds 2 g/L, if not they serve as contact tank for pre-chlorination. 2.2.2.1.2. Lamellar clarifiers. There are two independent clarifiers (Fig. 2); their role is to eliminate suspended solids upstream of the dual media filters. They comprise the following steps: 1 Coagulation using aluminum sulfate: Aluminum-based coagulants
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Fig. 2. Block flow diagram for the pretreatment system. FWT: Filtered water tank.
2.2.2.2. Sludge system. At various stages Khenifra water processing plant produce different wastes:
top surface. Each layer is tailored to specific requirements. A schematic diagram of the membrane is shown in Fig. 3
- Sludge of scrubbers - Sludge of lamellar clarifiers - Clarifier of dirty water coming from sand filters This sludge cannot be dismissed directly to the natural environment. In addition, in order to minimize the loss of water from the plant, a part of the dirty water washing will be recycled. The sludges extracted from the clarifiers, lamellar clarifiers as well as of the decanter of dirty water filter backwash are sent according to their concentration either toward the tarp of sludge, or directly toward the natural environment. The tarp of sludge feed the thickener to concentrate the sludge and send them by following to the drying beds to reduce their volume by drying and avoid returning them to the natural environment.
Fig. 3. Membrane composition: a: polyamide 0.2 micro-m, b: micro-porous polysulfone 40 micro-m, c: polyester support web, ultra-thin barrier layer 120 micro-m.
The table below shows the characteristics of the reverse osmosis trains installed in the Khenifra plant (Table 1). Table 1 Characteristics of the Khenifra brackish surface water reverse osmosis trains. Number of trains Number of stages by train Number of membranes by the pressure vessel Number of pressure vessels on the first stage Number of pressure vessels on the second stage Feed flow rate by train Permeate flow rate by train Concentrate flow rate by train Recovery rate
2.2.3. Reverse osmosis process 2.2.3.1. Low pressure pumps and microfiltration. A group of three-micron cartridge filters (MCF) with 5 μm mesh installed on the common manifold discharge of low pressure pumps, each one contains 224 cartridges (Reference PALL Claris 50). After the low-pressure pumping, filtered water receives three additional treatments: ○ Acidification: Adjusting pH by injection of H2SO4 98%. ○ Sequestering injection reagent upstream of microfiltration to prevent the precipitation of calcium carbonate (CaCO3) and calcium sulfate (CaSO4) at the membranes level. ○ Injection of sodium bisulfate to neutralize the remaining residual chlorine contained in the feed water.
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2 7 52 23 506 m³/h 410 m³/h 95 m³/h 81%
2.2.3.3. High pressure pumps. The high pressure pumps and RO section of the plant is divided into three trains, each one with a production capacity of 9.850 m3/d. One stand-by high pressure pump (the fourth), is installed and connected to the three trains. Each reverse osmosis train contain 2 stages of pressure vessels installed in "series-rejection", with 52 pressure vessels on the first stage and 23 on the second stage. Each pressure vessel contains 7 membranes. There is no pumping or energy recovery between the two stages. The pretreated water is pumped up to a pressure of 12 bars. The maximum design pressure is 14.5 bars (Fig. 4).
2.2.3.2. FilmTec™reverse osmosis for brackish water. The membrane used in this plant named XLE440. It belongs to the FILMTEC™ FT30 membrane family. It is composed of thin film composite membrane consisting of three layers: a polyester support web, a micro-porous polysulfone interlayer, and an ultra-thin polyamide barrier layer on the
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Fig. 4. Block flow diagram for the reverse osmosis process. FWT: filtered water tank; CF: cartridge filter; LPP: low pressure pump; HP: high pressure pump; T: tank; SBS: sodium bisulfate.
2.3. Cleaning in place (CIP)
production systems starting from RO plants to avoid the aforementioned challenges [34]. Before the water distribution in Khenifra plant, the remineralization is done by blending of one third of filtered water coming from dual media filters and two thirds of permeate. The remineralization installation is on stand-by to produce lime water. Lime water is produced by the saturator. It is a cylindro-conical device [34] which receives, firstly, a quantity of lime in the form of lime milk (50 g/L), and secondly a quantity of spraying water which arrives under pressure at the base of the device. The lime is then dissolved to saturation in order to produce a water of lime having 10 NTU in turbidity and approximately 200°F in complete alkalimetric title (CAT), which is collected in the upper portion of the saturator (Fig. 5).
Clean-in-place (CIP) is a method of cleaning the interior surfaces of pipes, vessels, process equipment, filters and associated fittings, without disassembly. The desalination plant is equipped with a chemical cleaning system for membranes cleaning. Two pumps are allocated to allow cleaning and recirculating the cleaning solution on the membranes then returning them in the tank. 2.4. Remineralization unit The permeate water coming out of a desalination plant cannot be directly used because it is unpalatable, corrosive and unhealthy. Hence, a remineralization step is an essential part of the drinking water
Fig. 5. The remineralization process. (a) saturator, (b) lime milk preparation tank, (c) milk injection pumps, (d) purge, (e) injection pumps for permeated water, (f) permeated water tank, (g) lime water tank, (h) distribution of remineralized water.
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2.5. Product delivery to the city
3.4. Methods
• The reverse osmosis membranes projection was carried out using the ROSA software of the Dow Chemical Company, version 7.2.1. • The dosing rate of the Genesys LF antiscalant was carried out by using the software GENESYS MM4 version: v1. 22.1 (DLL v1.8.7). • Sodium bisulfate (SBS) dosing rate was defined using stoichiometric
Table 2: The quality of drinking water waited at the outlet of the plant. Table 2 The quality of drinking water waited at the outlet of the plant. Parameters
Unit
Value
pH Ca Complete alkalimetric title (CAT) Total dissolved solids (TDS) Turbidity Aluminum Iron Manganese
Unit pH °F °F mg/L NTU mg/L mg/L mg/L
7 to 8.5 8 8 TDS < 800 < 0,5 < 0.2 < 0.3 < 0.1
• •
calculations. To neutralize 1 mg/L of free chlorine it is necessary to inject 1.34 mg/L of sodium bisulfate. The permeate conductivity was measured using WTW laboratory conductivity meter inoLab Cond 7110. The silt density index (SDI) was measured using a kit that allows the filtration of water through a membrane of porosity 0.45 μm and 47 mm in diameter at a pressure of 2.1 bars.
4. Results and discussion 4.1. Silt density index (SDI) assessment
3. Materials and methods
The silt density index (SDI) is an indicator of the colloids and suspended particulate matter present in water [35–37]. Fig. 6 presents the evolution of the silt density index (SDI) of filtered water coming from the dual media filters and micro-filtered water after the 5 μm cartridge filters; the graph shows the effectiveness of the pretreatment process; it illustrates that the SDI values are always lower than those of the design (<3) after microfiltration. The obtained results prove that the pretreatment process inlet reverse osmosis section operates successfully, and the microfiltration enhances the filtered water quality in terms of SDI (Fig. 6).
3.1. Feed water characteristics Table 3 gives the mean values of the raw water quality, with an indication of World Health Organization (WHO) recommendations and Moroccan drinking water standards. Table 3 The water quality used as a reference for the demineralization unit design. Chemical composition
Brackish surface water
Moroccan standards
WHO standards
pH Alkalinity (°F) Calcium (mg/L) Magnesium (mg/L) Sodium (mg/L) Potassium (mg/L) Manganese total (mg/L) Aluminum (mg/L) Ammonium (mg/L) Iron (mg/L) Strontium (mg/L) Barium (mg/L) Bicarbonate (mg/L) Chloride (mg/L) Sulfate (mg/L) Nitrate (mg/L) Fluoride (mg/L) Silica (mg/L) Total dissolved solids TDS (mg/L)
6.8 17.6 95 32.5 466 2.9 0 < 0.2 0 < 0.03 0 0 214 866 117 6.49 0 15 1800
6.5-8.5 – <500 100 <200 – 0.5 0.2 0.5 0.3 – 0.7 – 750 200 <50 1.5 – 1000-2000
6.5-8.5 – <270 <50 <200 – – 0.2 – 0.2 – 0.7 – <250 <200 <50 1.5 – <1000
4.2. Performances of reverse osmosis plant
Fig. 6. SDI values at the inlet and outlet of the microfiltration.
4.2.1. Seasonal conductivity variation Fig. 7 shows that the raw water supplying the demineralization plant undergoes significant seasonal variations in terms of conductivity. It increases during the summer reaching 2512.3μS/cm; due to the evaporation of the water causing the salts concentration high and decreases during the winter reaching 1333.8 μS/cm, due to the rainfall dilution process. This increase will noticeably impact the membrane performance in short and long term. Short term the quality of the permeate will decline gradually and the productivity as well. Long term if the recovery and the antiscalant’s dosing are not adjusted, then the membranes would experience a heavy scaling or even irreversible fouling which will lead to a drop in the membrane plant performance.
3.2. RO membrane element characteristics The reverse osmosis membrane used in this study was an 8-inch spiral wound membrane element (FILMTEC™XLE440, USA) having 28 mil feed spacers. From the manufacturer, the permeate flow and salt rejection of the membrane elements under the standard conditions were 36.8 L/min and 99.5%, respectively (standard conditions: 2000 ppm NaCl, 125 psi (8.6 bar), 77 °F (25 °C), pH 8, and 15% recovery). 3.3. Microfiltration cartridges The cartridges installed were PALL Claris with 5 μm mesh. The material of construction is polypropylene.
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The decline in permeate performance influenced directly the other operating parameters. Effectively, the feed pressure increased on both stages (Fig. 10). In fact, the membrane fouling causes the decrease of the membrane active surface area and consequently the increase of the applied pressure. The increase in pressure also depends on the decrease in temperature during the winter period and on the increase in conductivity during the summer period (Figs. 7 and 8). The recovery rate increased in the first stage and decreased in the second stage (Fig. 13) due to the automatic regulation which attempts to reach the set point flow rate of the RO unit. The pressure drop increased slightly on the first stage because of the flow increase in this stage; moreover, it decreased on the second stage, because of the flow diminution through the membranes due to the membrane fouling (Fig. 11). The conductivity remained stable in the first stage (Fig. 12); furthermore, it increased in the second stage due to the augmentation of the salt passage, generated by the salts deposition on the membrane surface, especially, NaCl, CaCO3 and CaSO4 contained in the feed water. The gradual increase of permeate salinity indicates a mineral scaling. However, the decrease of permeate flow indicates organic fouling [39]. To restore the original RO performance and overcome the fouling problems [40]. The Dow FilmTec™ team suggested to perform a classic membrane cleaning, using alkaline (1% 4Na-EDTA + 0.1% NaOH) and acid (0.2% HCl) cleaning agents [41]. Alkaline solutions such as sodium hydroxide (NaOH) remove organic fouling and biofouling by hydrolysis and solubilization; whereas acidic solutions such as hydrochloric acid (HCl) dissolve scaling, disrupt the bacterial cell wall structure and precipitate proteins [16,40,42]. Cleaning protocols for spiral wound RO membranes consist of several phases of high flow recirculation and soaking, lasting anywhere between 6.5–24 hours in duration at a typically applied temperature of 35 °C [40]. The acid solution was used to dissolve the salts deposited on the membranes, in particular, NaCl, CaCO3 and CaSO4 contained in the feed water. Alkaline solution is used to eliminate organic fouling including biological matter, which can be caused by a possible bacterial growth or by the organic matter contained in the brackish surface water supplying the plant. After the first chemical cleaning, which has been much effective, the initial flow rate of the first stage, second stage and global system were restored. Since this intervention, the RO unit has operated successfully during nearly eight months (from October 2013 to June 2014) with stable performances. Another membrane fouling was recorded after eight months of operation. In fact, the RO unit showed the same behavior that occurred during the first three months of operation after the plant commissioning. Indeed, the permeate flow passes from 299 to 370 m3/h for the first stage; form 110 to 16 m3/h for the second stage and from 411 to 347 m3/ h for the global system (Fig. 9). The other operating parameters were evolved in the same way recorded during the first membranes fouling. A second classic chemical cleaning was carried out to return to the normal operation.
Fig. 7. Feed water conductivity variations.
4.2.2. Seasonal temperature variation Fig. 8 shows the feed water temperature changes. It undergoes important seasonal variations. It declines during winter reaching 10.4 °C and rises during summer reaching 22 °C. The performance of the RO system will be influenced by the feed water temperature changes. For example, a feed temperature increase of 4 °C will cause a permeate flow increase of about 10%. This, however, is a normal phenomenon [38].
Fig. 8. Feed water temperature variations.
4.2.3. Reverse osmosis performance assessment The data represented on the graphs corresponding to the operating parameters evolution was followed every day. Five graphs of the plant evolution through the first year of operation are shown to indicate operating time versus the performances. Figs. 9–13 give the performances of the reverse osmosis plant especially the variation during one year of permeate flow, feed pressure, pressure drop, permeate conductivity and recovery rate. In the beginning of the RO system operation, the 1st stage permeate flow decreased from 295 to 252 m3/h. This decrease is due to the system settings during the commissioning period. The permeate flow increased during the first days and stabilized after nearly two weeks. This membrane flux stabilization is known as the so called wet out of the membranes [38]. The wet out process ended after nearly after 13 days of operation. After nearly three months of the plant commissioning (from July 2013 to October 2013), the permeate flow performance has changed significantly. It passes from 295 to 362 m3/h for the first stage and form 114 to 34 m3/h for the second stage (Fig. 9). The augmentation of the first stage permeate flow is explained by the automatic regulation PID (proportional, integral, and derivative), linked to the speed controller of the high pressure pump. This regulation attempts to reach the set point flow rate (410 m3/h) of the RO unit. It compensates the flow decline in the second stage by increasing the first stage flow rate. This performance decline was essentially due to an operating error which led to an overdose of the antiscalant at the feed of the 1st stage.
Fig. 9. 1st stage, 2nd stage and RO unit permeate flow evolution. 7
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performance of RO systems in conditions outside of the standard 2000 ppm NaCl at which FILMTEC™ products are warranted. This software is developed based on empirical data that is generated by testing FILMTEC™ products at different conditions to generate the ratio between A-value and B-value. The raw water quality used as reference for the reverse osmosis unit design is represented in Table 3. Tables 4 and 5 show the operating results versus the expected design values in the same conditions of temperature and feed water TDS after nearly one year of operation. The ROSA projection and actual performances were compared after the membranes chemical cleanings and reprise of the normal operating parameters recorded during the plant start up. Throughout the whole period of operation, the feed flow, the permeate flow; the rejection flow and the recovery rate of the train were the same as the expected design values. For the feed pressure, it was slightly above the expected design value for the first stage of the RO train. However, it was the same in the second stage .
Fig. 10. 1st stage, 2nd stage and RO unit feed pressure evolution.
Table 4 Comparison of operating results with ROSA projection after one year of operation. Parameters
ROSA
3
Feed flow (m /h) Feed pressure (bar) Permeate flow (m3/h) Rejection flow (m3/h) Recovery rate (%)
Fig. 11. 1st stage, 2nd stage and RO unit pressure drop evolution.
RO TRAIN
1st stage
2nd stage
1st stage
2nd stage
506 10.4 297.24 208.76 58.7
208.76 8.99 112.89 95.87 54.1
507.96 11.13 299.64 208.32 59
208.32 8.9 108.72 99.6 52.2
Table 5 compares the ROSA prediction values with the experimental results collected in the RO plant in the same conditions of temperature and feed water TDS. The results obtained prove that the real permeate TDS values are close to the ROSA predictions and the reverse osmosis membranes exhibit high and stable performances . Table 5 Comparison of the permeate TDS results with the ROSA projection. Concentration
TDS (mg/L)
Fig. 12. 1st stage and 2nd stage permeate conductivity evolution.
ROSA
RO TRAIN
1st stage
2nd stage
1st stage
2nd stage
101.58
258.52
99.2
252.6
5. Conclusion In comparing the real industrial data versus ROSA simulation, the following key learning points were generated:
• RO membrane at large industrial scale needs rigorous technical care • • •
Fig. 13. 1st stage, 2nd stage and RO unit recovery rate evolution.
4.2.4. Rosa projection versus RO trains performances Reverse osmosis system analysis (ROSA) software is developed by Dow Chemical to help their customer’s model and predict the
at the start up and a keen technical follow up to reach a steady stabilized performance. ROSA predicted the performance of FILMTEC™XLE440 at about 97% close to the real industrial data. The weather change from season to another tends to affect the dam water temperature as well as its quality which noticeably influence the RO membrane performance. The change in water quality requires a rethinking and sometime also reselection of the adequate antiscalant molecule and dosage.
The feed water quality piped to the Khenifra reverse osmosis (RO) desalination plant is changing. This change is due to rainfall deviation 8
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and seasonal temperature fluctuation which leads to the evaporation of the water from the dam. A scientific approach combined with RO technical expertise is vital to operate the plant and to maintain a longterm reliable membrane performance. A further study would be interesting to understand the impacts of this changes and how to overcome them in the RO desalination plant. Morocco has a significant experience in water desalination but this one is still the first in terms of brackish surface water treatment by reverse osmosis, making it a great challenge for the kingdom, and will be the occasion for many important investigations.
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