An optimization strategy for a forward osmosis-reverse osmosis hybrid process for wastewater reuse and seawater desalination: A modeling study
Graphical abstract
Introduction
The demand for clean water production technologies has been consistently emphasized as a way of coping with water security issues. The water security issues can be induced by an increasing population, climate change, and geographical accessibility [[1], [2], [3]]. To provide sufficient fresh water, alternative water resources, such as seawater and wastewater, have been investigated. Currently, the seawater desalination process can provide additional fresh water successfully using the reverse osmosis (RO) process [3,4]. However, a further reduction in energy consumption in the seawater reverse osmosis (SWRO) process has been continuously demanded. To achieve this, a combination of forward osmosis (FO) and SWRO processes has been considered as a possible solution.
FO is an osmotically-driven process that recovers water from dilute feed solution to concentrated draw solution through a semipermeable membrane [5]. In the FO-RO hybrid process for seawater desalination, wastewater and seawater can be utilized as the feed and draw solutions, respectively. The FO part of the FO-RO hybrid process functions as a SWRO pretreatment process, diluting seawater to reduce energy consumption and enhancing the recovery rate of the SWRO process. Moreover, by using the FO-RO hybrid process, the wastewater can be reused as fresh water [[6], [7], [8], [9]]. Furthermore, the fouling reversibility of the FO process provides additional applicability [[10], [11], [12]].
To investigate the feasibility of the FO-RO hybrid process using wastewater and seawater, various modeling and pilot studies have been conducted using wastewater and seawater [[13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25]]. Studies of the FO-RO hybrid process have shown that the contaminants in the FO feed solutions can be removed effectively through the dual barrier process [[13], [14], [15]]. Subsequently, direct potable reuse of FO-RO treated water has been discussed [13,18]. In addition, en economic analysis of the FO-RO hybrid process was conducted [23,25]. Recently, Choi et al. reported the long-term stable operation of the FO-RO hybrid process using wastewater and seawater that shows better performance than stand-alone RO process [20]. However, the quantitative analyses of parameters affecting RO energy consumption in the FO-RO hybrid process in terms of the membrane, membrane element, and the process have yet to be conducted for the FO-RO hybrid process design.
The main purpose of this study is to derive major factors for controlling and suggesting an optimization strategy for the FO-RO hybrid process in terms of RO energy consumption. In this study, the impacts of water quality, intrinsic membrane parameters, and control parameters for operation were systematically and quantitatively analyzed with regard to FO-RO hybrid RO energy consumption using a modeling approach. Wastewater and seawater were considered in the FO-RO hybrid process. In the FO and RO model, water flux, solute flux, internal and external concentration polarization, and flow rate change inside of the membrane element were considered. In addition, the efficiency of the pump and the energy recovery device were accounted for in the calculation of RO energy consumption. To assess the feasibility of the FO-RO hybrid process, a performance comparison with the stand-alone RO process was conducted. Subsequently, major factors affecting RO energy consumption in the FO-RO hybrid process were derived and ranked using sensitivity analysis. In addition, an optimization strategy of the FO-RO hybrid process to minimize RO energy consumption was discussed. In conclusion, this study suggests an optimization strategy for the FO-RO hybrid process that considers the intrinsic membrane parameters, membrane element design, and the control parameters for operation.
Section snippets
FO, RO, and FO-RO hybrid process model development
Fig. 1(a) shows a schematic diagram of the FO-RO hybrid process. The FO-RO hybrid process model is composed of two unit process models, the FO model and the RO model. Fig. 1(b) illustrates a flow chart of the FO-RO hybrid process model. In the FO process, seawater and wastewater were considered as FO draw and FO feed solutions, respectively. To evaluate the performance of the FO process, the flow velocity and concentration of the FO feed and draw solutions, the FO water flux, and the FO solute
FO and RO performance in the FO-RO hybrid process
The water flux and concentration distributions in the FO-RO hybrid process are shown in Fig. 2. The concentration of the FO feed solution (i.e., wastewater) and draw solution (i.e., seawater) were 1000 and 35,000 mg/L, respectively. The flow velocity of the FO feed and draw solution was 0.05 m/s. The temperature of the FO feed, FO draw, and RO feed solutions was 300 K. The applied pressure on the RO process was 30 bar (see Table 1). Fig. 2(a) describes the water flux in the FO process with
Conclusion
The feasibility of the FO process to the RO-based hybrid process has been investigated in various studies. This study investigated the FO-RO hybrid process for desalination using numerical modeling. Beginning with the development of the FO-RO hybrid process model, performance comparison between the FO-RO hybrid process and the stand-alone RO process, sensitivity analysis of the FO-RO hybrid process, and an optimal design strategy for the FO-RO hybrid process were discussed. The main conclusions
Acknowledgement
This Research has been performed as a project No KK1923-10 (Development of key membranes for high efficiency seawater desalination) and supported by the Korea Research Institute of Chemical Technology (KRICT).
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