Techno-economic assessment of electrodialysis and reverse osmosis desalination plants
Introduction
Electrodialysis desalination (ED) is a robust direct current (DC) driven technology of salt ions separation. It is cost-effective for desalinating brackish water compared to other desalination technologies [1], [2]. Pressure energy provided by a pump drives reverse osmosis (RO). It is reported to be economically feasible for relatively high concentrations when compared with electrodialysis [3]. Systematic economic analysis is very important for the decision-making process. The choice depends on many factors, but the most important are source salinity and water cost. The purpose of this study is to conduct an economic analysis of commercial ED and RO plants. Another critical aspect of the present study is the inclusion of detailed cost sources that were not considered in earlier works for ED and RO plants (see Fig. 1).
Design models can carry out the economic analysis of ED plants. Lee et al. [1] developed a lumped design model that used empirical correlations and typical values to investigate the ED desalination while neglecting the effect of boundary layer and Donnan potential besides the membranes and other aspects such as electro-osmosis. Tsiakis and Papageorgiou [4] updated the basic model [1] by exclusively applying the ED stack model. Qureshi and Zubair [2] examined the electrical conductivity impact on the outcomes of the design model of Lee et al. [1]. Qasem et al. [5] had gone one step ahead by including Donnan's potential in the elementary model of Lee et al. [1]. Recently, they [6] further extended the design model and included boundary layer, water transport, spacer effect, and counter-ion transport. The modified design model [6] can handle salinity up to 200 ppt. Thus, this model [6] is considered a good starting point to economic analysis since its output includes membrane area, number of stacks, plant production, and energy consumption.
As RO is a pressure-driven technology, high-pressure pumps are used to raise the pressure of saline water. The pressure energy is used to overcome the osmotic pressure between the product water and saline water. However, the pressure should be below the operating limit of the membranes. It is important to note that the osmotic pressure is a direct feed salinity function [7]. Thus, RO can be used up to a feed salinity of 50 ppt with a maximum feed pressure of 70–80 bar [8], [9]. However, these days, the maximum pressure handled could reach 120 bar [10]. RO brine pressure energy could be recovered using an energy recovery device such as a pressure exchanger (PX). It is an excellent device reported to have an efficiency of up to 98% [11].
Cost analysis of various desalination technologies is available in the literature. In this regard, Lopez et al. [3] compared specific energy consumption (SEC) for RO and ED while ignoring some potential costs, such as the plant's capital and operation costs. Nayar et al. [12] conducted a cost analysis for a hybrid RO and ED system. Also, Zhang et al. [13] conducted an economic study for RO concentrate salinity reduction using a pilot-scale ED plant. While Choi et al. [14] conducted a cost comparison of pressure retarded hybrid osmosis system. That is RO and membrane distillation. They concluded that the hybrid system performance was better than the standalone RO system. Hamid et al. [15] investigated microfiltration and forward osmosis for wastewater treatment under various operating conditions. At the same time, Choi et al. [16] investigated the economics of ion concentration polarization (ICP) desalination for salinity reduction from 70 ppt. Atab et al. [17] studied the RO desalination plant with 24,000 m3/day production capacity. It was revealed that portable water with a salinity of 400 ppm could be obtained with a feed salinity of 15,000 ppm at 0.14 $/m3. Strathmann [18] investigated ED desalination within a feed salinity range of 1–10 ppt and reported that the ED plant could be used in the capacity range of 100–20,000 m3/day.
Estimating unit product cost depends on the plant's design, site conditions, and plant capacity irrespective of the type of technology [7]. Pump, pre-and post-treatment equipment, and ED membrane area are dependent on the plant capacity. Site characteristics also have a substantial effect on the choice of pre-and post-treatment plants [7]. It is important to note that the operation cost constitutes electricity cost, labor cost, membrane replacement expenditures, chemicals (chlorine, sulfuric acid, caustic soda, antiscalant, etc.) for pre-and post-treatment processing, insurance, and fixed price. The fixed cost can be calculated using an amortization concept linked to the capital cost, which could be broken down into direct capital cost and indirect capital cost [7]. The direct cost comprises land and infrastructure cost, process and auxiliary equipment, well digging and brine disposal cost, and membrane and stack cost. Similarly, indirect cost splits into freight charges, legal and consultancy fees, contingency cost, and construction overhead charges [7]. Thus, the product cost is calculated from the annual operation charges, as shown in the detailed flowchart of Fig. 1.
Land cost depends on the location. For example, it may have no charges at all if the plant is state-owned. Similarly, the well-digging cost depends on the site characteristics and depth required. The ion exchange membrane cost is usually around 100 to 150 $/m2 [1], [7], it is also reported to be about 25 $/m2 [19]. Electricity cost is also not the same and varies with location. The current price in the USA is between 0.10 and 0.12 $/kWh [1]. The power cost and fixed cost are also a strong function of design parameters of ED like salinity of feed stream, linear flow velocity, recovery ratio, cell thickness, and spacer shadow effect, etc.
The same is the case with the interest rate. It is influenced by government policies, time, and location. However, an appropriate number is between 3 and 8% [20]. The chemical expenditures vary with the site conditions and quality of the extracted water from a wellbore. The typical dosage rates of chlorine, caustic soda, and antiscalant are shown in Table 1 [21]. The labor cost is also not identical and differs from location to location. Thus, the unit cost is dependent on plant location.
Other costs such as maintenance cost, membrane replacement cost, and operation cost are also reliant on the propensity of membrane fouling and scale formation [22], [23], [24], [25]. Fouling and scale formation depends on water source location and the quality of pretreatment and suitable foulant and antiscalant [26]. However, ion exchange membranes are less susceptible to fouling and scale formation when compared with the RO membrane [27]. Moreover, the life expectancy of ED membranes is higher due to less feed pressure requirements when compared with RO [28]. These variables' impact is also depicted in the variation of bottled water price per liter of various companies worldwide [29], as shown in Fig. 2.
The literature discussed above compared the energy consumption analysis of ED and RO with a small salinity range <20 ppt [3] using straightforward ED models. Moreover, the detailed cost sources were not considered. Also, RO's cost analysis with and without a pressure energy (PX) was not compared. Thus, the main objective of the present study is (a) to estimate the water production cost of ED and RO technologies (with and without PX), (b) to compare the water cost and SEC of ED and RO, (c) to conduct a detailed sensitivity analysis of ED and RO to determine the most influencing parameters on the product water cost, (d) to investigate the cost of highly saline water desalination (up to 150 ppt) using ED plants, and (e) to evaluate the cost of pumping saline water and brine disposal from far away sources.
Section snippets
Systems description
The ED plant consists of a number of stacks in which each stack has many cell pairs. The cell pair is the fundamental building block of an ED plant that consists of ion exchange membranes, concentrate and diluate compartments, two compartments for rinsing, and electrodes for the application of electric potential. Ion exchange membranes (IEM) are a vital element of the ED plant. IEM organizes cation and anion exchange membranes that are put alternatively to allow solutions both cation and anion
Mathematical modeling
In this section, the modeling approach for ED and RO desalination is presented. The models integrated with design models for cost analysis are also described. The normalized sensitivity analysis is one of the pioneer tools to judge the relative contribution of various variables involved in the product water cost. Therefore, an approach for normalized sensitivity analysis is also briefly described.
Results and discussion
In this section, both ED and RO models are first validated against experimentally reported data available in the literature, as explained in Appendix C.
Concluding remarks
Techno-economic analysis of reverse osmosis (RO) and electrodialysis (ED) plants is conducted. For comparasion purposes, up to 50 ppt salinity range is considered for both RO and ED plants. Also, the ED system is investigated to desalinate highly saline water (~150 ppt). Two configurations of RO are considered, i.e., with and without the pressure exchanger (PX).
Various cost sources, including power cost, fixed cost, and other costs (labor, membrane replacement, maintenance, chemical, and
CRediT authorship contribution statement
Muhammad M. Generous: Methodology, Software, Data curation, Validation, Writing - original draft. Naef A.A. Qasem: Methodology, Data curation, Validation. Usman A. Akbar: Data curation. Syed M. Zubair: Supervision, Project administration, Conceptualization.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors acknowledge the support provided by King Fahd University of Petroleum & Minerals (KFUPM) through the project IN171048. Syed Zubair would also like to acknowledge the financial support provided by the King Abdullah City for Atomic and Renewable Energy (K.A.CARE).
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