Brackish Water Reverse Osmosis System - wseas

Computational Methods in Science and Engineering

Comparison of One Stage and Two Stage- Brackish Water Reverse Osmosis System: A Simulation study MUSTAQIMAH², M.A. ALGHOUL 1*, P. POOVANAESVARAN¹, ASSIM FADHIL¹, F.ANNISA ACEK¹, K.SOPIAN¹ 1

Solar Energy Research Institute, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia 2 Department of Agricultural Engineering, Faculty of Agricultural, Syiah Kuala University, Darussalam 23111, Banda Aceh, Indonesia *Corresponding author: [email protected]

Abstract: - A 3 kW brackish water reverse osmosis (BWRO) desalination system is simulated using Reverse Osmosis System Analysis (ROSA) under different feed flow and number of membrane elements. Two different designs were simulated, one stage and two stage configuration to provide at least 35 m3/day of drinking water for 110 households in Mersing, Johor, Malaysia. Permeate flow will increase with the increment of feed flow, feed pressure and number of membrane elements. However, by increasing feed pressure, the permeate TDS will decrease and the permeate flow increases making the system to conserve more power. Single stage and two stage portraits the same behavior in terms of relationship between the parameters, the difference is two stage produces more fresh water and the permeate TDS is higher due to higher TDS passes through the second stage. Key-Words: - desalination, Brackish water, 3 kW reverse osmosis load, one stage/two stage design configuration, ROSA software

1 Introduction separation. The mineral content of the water is usually measured by the water quality parameter named total dissolved solids (TDS), concentration of which is expressed in milligrams per liter (mg/L), or parts per million (ppm). The World Health Organization (WHO) has established a maximum TDS concentration of 500 mg/L as a potable water standard. This TDS level can be used as a classification limit to define potable (fresh) water [7]. RO systems consist of the following basic components: Feed water supply unit, pretreatment system, high pressure pumping unit, membrane element assembly unit, instrumentation and control system, permeate treatment and storage unit, and cleaning unit [8, 9]. In this paper, basic designs of one stage and two stage configuration are discussed, Figs 1 -2. In each pressure vessel numbers of elements are

About 1.76 billion people live in areas facing a high degree of water stress [1]. ‘‘Water stress’’ is at the top of the international agenda of critical problems, at least as firmly as climate change [2]. As a result, the need for desalination is increasing, even in regions where water supply is currently adequate. Increasing demand for water is a big problem[3]. Reducing the mineral content in brackish water and sea water to turn them in drinkable supply, a process called “desalination”. Brackish water is water that has more salinity than fresh water, but not as much as seawater [4, 5]. Reverse osmosis (RO) technology is ranked among the most desirable in desalination systems [6]. Reverse osmosis (RO) desalination is a treatment process for production of fresh, low salinity potable water from saline water source (sea or brackish water) via membrane

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assumed 6-8. The total numbers of elements are 12-16 as shown in Figs 1 -2. The design is for small scale BWRO system. Permeate flow, TDS, specific energy and energy recovery of each design are going to be compared and evaluated.

2 Materials and Method Fig.2 Schematic diagram of two stages 3 kW BWRO

The source of brackish water is from Mersing, Johor, Malaysia. Most of residents in Mersing are near about 4 km to 7 km from the sea. The TDS of the water is 13,400 mg/L. This sample is categorized as brackish water. According to National Research Council (2004), TDS content in brackish water is between 1,000 mg/L to 30,000 mg/L[21]. Acceptable TDS of drinking water is under 500 mg/L. The water demand is assumed for a residential complex with about 110 households. According to Marie [11], the average freshwater demand is about 0.32m3/household/day. By using ROSA 7.2, it can be determined RO designs capable of producing more than 35 m3/day of freshwater. The type of membrane that used in this research is BW30-4040. Reverse osmosis system analysis (ROSA) is a sophisticated RO design program that predicts the performance of membranes and energy requirements for desalination in user-specified systems. ROSA has been used for designing desalination plants in different parts of the world [10]. Energy recovery devices can also be included in the design. Greenlee, et al, (2009) have studied that the energy recovery can save 75 % for brackish water.

3. Design assumptions 1. One stage RO design and is limited with one pressure vessel. 2. Two stage RO design and is limited with one pressure vessel for each stage 3. The load of the RO unit is 3 kW only. 4. The type of membrane that used in this research is BW30-4040. 5. The permeate TDS is less than 500ppm.

4 Results and Discussion 4.1 effect of feed flow and number of membrane elements on permeate flow Figures 3-4 show the effect of feed flow and number of elements on permeate flow at single and two stage designs. On single stage, the highest permeate flow is 38.25 m3/d at feed flow 60 m3/d, feed pressure 39bar and using 8 membrane elements.

Fig.1 Schematic diagram of one stage 3 kW BWRO

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Fig.3 Effect of feed flow and number of membrane element on permeates flow (1 stage).

minimum Permeate TDS at single stage design is 230ppm while it is 63.7ppm at two stage design. Also, increasing feed pressure will increase the permeate TDS as well. If number of membrane element increases, it will increase the permeate TDS significantly.

The permeate flow can be described by [19]

where is the total membrane surface area, is the hydraulic permeability of the membrane (depends on the type and product of the membrane), is the average transmembrane pressure, is the reflection coefficient of membrane, is the average osmotic pressure difference between feed and permeate . is average transmembrane net driving pressure, . At two stage, the permeate flow increases to 43.74 m3/d at feed flow of 69 m3/d, feed pressure 35bar and using 8 membrane elements. As seen from these two figures, number of elements significantly effects the permeate flow at both designs. Also it can be seen that two stage showed higher permeate flow at lower pressure.

Fig.5 Effect of feed flow and number of membrane elements on permeate TDS (1 stage).

Fig.6 Effect of feed flow and number of membrane elements on permeate TDS (2 stages).

4.3 Effect of feed flow and number of membrane elements on specific energy Fig.4 Effect of feed flow and number of membrane element on permeate flow (2 stages).

Specific energy consumption (SEC) of membrane desalination system can be expressed in terms of pressure difference, , pump efficiency, , and recovery ratio, Y [20]

4.2 Effect of feed flow and number of membrane elements on permeate TDS Figures 5-6 show the effect of feed flow and number of elements on permeate TDS at single and two stage designs. It is seen that when feed flow increases the permeate TDS reduces in both designs. The

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Figures 7-8 show the effect of feed flow and number of membrane elements on specific energy at single and two stage designs respectively. Specific energy is less in two stage compare to single stage design due to higher production of fresh water. The number of membrane elements affects the specific energy. Increasing the number of elements reduce significantly the specific energy

When feed flow increases, the recovery will decrease. Increasing number of element, the recovery will increase. The highest recovery occurs at lowest feed flow, in highest feed pressure and maximum number of membrane elements. In two stage configuration, recovery is higher than in single stage.

Fig.9 Effect of feed flow and number of membrane elements on recovery (1 stage).

Fig.7 Effect of feed flow and number of membrane elements on specific energy (1 stage).

Fig.10 Effect of feed flow and number of membrane elements on recovery (2 stages)

5 Conclusions

Fig.8 Effect of feed flow and number of membrane elements on specific energy (2 stages).

Permeate flow will increase with the increment of feed flow, feed pressure and number of membrane elements. However, by increasing feed pressure, the permeate flow increase and permeate TDS decrease. Increasing feed flow and pressure, increases power consumption. Single stage and two stage portraits the same behavior in terms of relationship between the parameters, the difference is two stage produces more fresh water and the TDS of the fresh water

4.4 Effect of feed flow and number of membrane elements on system recovery Figures 9-10 show the effect of feed flow and number of elements on recovery at single and two stage designs.

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is higher due to higher TDS passes through the second stage. [11] Marie A, Pederson J, Urban households and urban economy in Eritrea. Analytical report from the urban Eritrean household income and expenditure survey 1996/1997. Statistic and evaluation office, Asmara, Eritrea, 2001. [12] Widiasa, N., Paramita, V & Kusumayanti, H., BWRO Desalination for potable water supply enhancement in coastal regions. Coastal development,Vol. 12, No.2, 2009,pp.81-88.

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