Forward osmosis (FO)-reverse osmosis (RO) hybrid process incorporated with hollow fiber FO

HFFO performance evaluation

Figure 1 shows the water flux and reverse salt flux (RSF) values of the HFFO element tested at varying (i) operating modes (a and b: FO; c and d: PAO), (ii) flow rates of the FS and DS, and (iii) DS concentrations. The results showed that the flow rate of each side influenced the performance of the element-scale HFFO (Fig. 1a, b). When the DS flow rate was increased from 0.20 to 0.35 L/min with different FS flow rates (0.7, 1.0, and 1.5 L/min), the overall water flux increased (maximum: 35,000 mg/L–1.05 to 1.24 liter per square meter per hour (LMH), minimum: 0.95–1.08 LMH at high DS concentration condition) (maximum: 35,000 mg/L–0.83 to 1.24 LMH at high DS concentration condition, minimum: 0.46–0.60 LMH at low DS concentration condition), although the effect of the FS flow rate was not dominantly than DS flow rate during the HFFO operation. This indicates that the DS flow rate affected the water flux more strongly than the FS flow rate may be due to the flow path diameter and the retention time in the HFFO element. In the HFFO element, the DS flow path was 85 μm (based on inner diameter), and this narrow flow path could significantly enhance the dilution in the channel per unit area (reducing the water flux) (referring Supplementary Tables 1 and 2). However, the FS flow path in the HFFO element did not exist (like a submerged type), and the membranes were packed in a PVC cell with a diameter of 90 mm and a length of 280 mm. Hence, when the DS and FS flow rates were 0.35 and 1.50 L/min, respectively, and a DS concentration of 35,000 mg/L was used, the highest water flux (1.24 L/m²h, LMH) was observed, which was approximately double the flux when the DS concentration was 10,000 mg/L. Interestingly, the overall RSF tendency was more affected by DS flow rates than FS flow rate (e.g., FS 0.7/DS 0.2: 0.0139 gram per square meter per hour (GMH) to FS 0.7/DS 0.35: 0.0266 GMH at 25,000 mg/L DS concentration). The RSF increased when the DS flow rate was increased, like the water flux pattern (refer to Supplementary Tables 1 and 2). However, the RSF tendency does not increase proportionally as well as the water flux tendency, and the fluctuation is relatively high^26,32. It is a relatively small amount of salt mass transport phenomenon, which requires clear identification through future lab-scale experiments. In contrast, when the DS flow rate was increased from 0.20 to 0.35 L/min, the RSF value increased, whereas the RSF value decreased as the FS flow rate was increased over the entire range of the DS concentrations. The RSF showed a decreasing pattern with an increase in DS concentrations (from 10,000 to 35,000 mg/L). It should be noted that the HFFO element showed a relatively low water flux and RSF compared with the different types of FO elements. In previous studies, water fluxes of spiral-wound FO (SWFO) and plate-frame FO (PFFO) elements were 26.5 and 17.7 LMH, respectively. In addition, the RSF values were observed as 12.4 and 8.4 g/m²h (GMH), respectively, at a DS concentration of 35,000 mg/L^26,28,33,36. However, at 35,000 mg/L, the HFFO showed 0.7–1.3 LMH of water flux (around 20 times less than that of SWFO and PFFO) and 0.005–0.030 GMH of RSF, which is much less than the other elements. Therefore, in the case of the HFFO element, the influence of process operating conditions is not serious, which indirectly shows that RSF consideration is not required for HFFO–RO–sHFFO process operation.

A lower water flux can be overcome slightly by operating the FO in the PAO mode. As indicated in Fig. 1c, d showing the HFFO operation results in the PAO mode, when the FS and DS flow rates were increased from 0.7 to 1.5 and 0.2 to 0.35 L/min, respectively, with an applied pressure of 3 bar, the water flux was approximately double (from 1.39 to 2.33 LMH) that of the FO mode (without any applied pressure) under the same conditions (referring the black dot circle). With the addition of artificial pressure, the DS dilution rate was observed to be a maximum of 408% (35,000 mg/L, FS 1.5, DS 0.35 L/min, 2 bar) and a minimum of 131% (15,000 mg/L, FS 0.7, DS 0.35 L/min, 3 bar). Interestingly, when the DS concentration was similar to the seawater level (35,000 mg/L), the specific RSF (SRSF = RSF/water flux (g/L)) in the PAO mode was much lower than that in the FO mode (PAO = 0.008 g/L and FO = 0.018 g/L) under the same conditions (FS and DS flow rates = 1.50 and 0.35 L/min, respectively). This indicates that the HFFO operation in the PAO mode can be beneficial for stable water reuse with the pretreatment option for seawater desalination.

Detailed water flux, RSF, SRFS values, DS dilution rate, and diluted DS conc. in the FO and PAO modes can be found in Supplementary Tables 1 and 2, respectively.

Feasibility of sHFFO

For the characteristics (concept) of FO–RO–sHFFO desalination process, the sHFFO process was simulated under the HFFO operation in the PAO mode depending on the sea level (from the surface of the sea); various natural water pressures can be applied to the membrane by gravity, water density, and depth, and the sHFFO faced an inevitable difference in concentrations between the FS (seawater) and DS (RO brine). Therefore, during this experiment, the FS concentration was changed from 10,000 to 25,000 mg/L, the DS concentration was changed from 35,000 to 80,000 mg/L, and pressures ranging from 2 to 4 bar were applied to the FS side.

Figure 2a, b shows the water flux and RSF values, respectively, depending on the concentration differences between the FS and DS (DS–FS) and the applied pressure to the FS. The water flux values increased continuously with increasing FS flow rates, applied pressures, and concentration differences. With the pressure of 4 bar, the highest water flux values obtained were 3.92, 1.04, and 1.21 LMH at the FS flow rates of 1.5, 1.0, and 0.7 L/min, respectively (DS flow rate = 0.35 L/min and concentration difference between FS and DS = 70,000 mg/L). However, the RSF values were relatively stable compared with those in the FO mode. This may be due to the applied pressure of the FS hindering the salt passage from the DS to the FS (RSF) during the HFFO operation. In addition, the applied pressure provided a positive effect on the performance, and as expected, when there was a variation in the FS and DS concentrations, the FS flow rate and applied pressure to the FS positively influenced the FO performance (i.e., water flux and RSF)³⁷.

Dilution effect of HFFO (seawater intake and brine management)

Figure 3a, b presents the DS dilution rates and diluted DS concentrations according to the DS and FS flow rates and operation modes (FO and PAO) at the DS concentration of 35,000 mg/L. For the HFFO mode (Fig. 3a), the DS dilution rates were over 150 and 200% when the DS flow rates were 0.20 and 0.35 L/min, respectively. This difference occurred by changing the DS volume and permeation ratio (water flux) as the DS flow rate was changed (Supplementary Tables 1 and 2). Accordingly, the final diluted DS concentrations ranged from 16,000 to 23,000 mg/L, depending on the flow rate. However, when the pressure was applied to the FS side at a constant DS flow rate of 0.35 L/min and varied FS flow rates (0.7 to 1.5 L/min), the diluted DS concentrations decreased further to 11,000 and 9,600 mg/L (at operating pressures of 2 and 3 bar, respectively).

Figure 3c shows the dilution rate and diluted DS concentration depending on the differences between the FS and DS concentrations ranging from 50,000 to 70,000 mg/L, the FS flow rate, and the applied pressure. When the difference between the FS and DS concentrations was 50,000 mg/L with the operating conditions of FS flow rate = 0.70, DS flow rate = 0.35 L/min, and applied pressure = 2 bar, the diluted DS concentration and dilution rate were observed to be 34,000 mg/L and 146%, respectively. If the HFFO element is operated under the suggested conditions (i.e., sHFFO), the DS concentration can be equalized to that of the seawater. Therefore, this condition can be used to optimize (Case 7 in Table 1) the HFFO-based infinity seawater desalination process (FO–RO–sHFFO). With a difference in the concentrations across the membrane and the application of pressure to the FS (in PAO mode), various dilution rates and diluted DS concentrations were observed (Fig. 3c) as to the experiment of the condition where the concentration difference exists (Fig. 3b). This occurs because the external concentration polarization has a significant effect on the FO performance when differential concentrations are presented, and more significant internal concentration polarization occurs with a difference in concentration. With no difference between the FS and DS concentrations, when the FS and DS flow rates were 1.5 and 0.35 L/min, respectively, and a pressure of 3 bar was applied to the FS, a dilution rate of more than 400% dilution rate and a diluted DS concentration of approximately 8500 mg/L could be achieved (Figs. 2 and 3). However, when the difference between the FS and DS concentrations was 70,000 mg/L, approximately 350% of the dilution rate was enabled and the process could dilute the DS concentration to 22,580 mg/L (detailed water flux, RSF, and SRFS values can be found in Supplementary Tables 3). In addition, the expected operating pressures and permeate concentrations with the SWRO process after the HFFO process were simulated under various operating conditions in the cross-flow HFFO process (nine cases including a two-stage SWRO) and two different recovery rates in the RO process (50 and 60%) (Table 1). A total of nine cases, including a control (two-stage RO), were selected based on the HFFO element performance evaluation results under various operating conditions (Sections 1 and 2): four conditions in the FO mode (Cases 1–4) and four conditions in the PAO mode (Cases 5–8). The same operating conditions were applied to the HFFO and sHFFO elements in the HFFO-based infinity desalination process. Depending on the cases, the required pressure and final permeate concentration of the downstream SWRO process were predicted.

However, in the FO–RO–sHFFO desalination process, when the downstream two-stage SWRO process is operated at a recovery rate of 60%, the brine concentration is lower than that of the seawater, making the operation of the sHFFO process impossible. Therefore, for the two-stage SWRO process operated at a higher recovery rate (80%), at which the brine concentration discharged is approximately 60,000 mg/L, the operation pressure, permeate concentration, and specific energy consumption (SEC) value were recalculated, as shown in Table 2. In the two-stage SWRO, for Cases 1 and 2, the operating pressures of the SWRO calculated under such conditions were unacceptable. However, in Case 5, it was still possible to operate under lower pressure (37.9 bar) than with the two-stage SWRO process.

The detailed SEC values, operation pressures of the SWRO process, and the permeate concentrations at various recovery rates can be found in Supplementary Tables 4, 5, and 6.

In the following section, an economic evaluation is described in terms of energy, comparing i) two-stage RO versus FO–RO-sHFFO and ii) SWRO with ZLD versus FO–RO-sHFFO.

Energy evaluation (two-stage SWRO vs FO–RO–-sHFFO)

To evaluate the economic benefits of the FO–RO–sHFFO process, the SEC of both the FO and RO processes were calculated, as shown in Fig. 4a, b. During the calculation, the plant capacity was assumed to be 100,000 m³/day. The pump efficiency and energy consumption were 90% and 0.1 kWh/m³, respectively. Owing to the structural characteristics of the element-scale HFFO process, the energy requirement of the FS pump is higher than that of the DS pump (Fig. 4a). Depending on the HFFO operating conditions (Table 1), the operating energy on the FO side also fluctuates, and the calculated SEC values of the RO process were different (Fig. 4b). Surprisingly, regarding the total SEC values when considering the energy requirement of both the FO and RO sections (Fig. 4c), the lowest energy requirement (1.49 kWh/m³) was observed in Case 5 (FS flow rate = 1.5 L/min, DS flow rate = 0.35 L/min, and applied pressure = 3 bar), and approximately 62% of energy was conserved compared with the two-stage RO process. Consequently, the energy costs based on the SEC value of the FO and RO were calculated (Fig. 4d). The operation period of the desalination plant was assumed to be 20 years. The cost results are similar to those of the SEC, and the FO–RO–HFFO can save approximately 66% of the cost compared with the two-stage RO process (two-stage RO = 280 million USD and FO–RO-HFFO process (Case 5) = 96 million USD). Furthermore, when the recovery rate was increased from 60 to 80%, the SEC value of the two-stage SWRO was increased to 6.02 kWh/m³. However, approximately 170 million USD is saved over the lifetime of the plant compared with the two-stage SWRO at a recovery of 60% (Fig. 4c and Supplementary Fig. S7).

The amortized CAPEX of the FO–RO hybrid process was calculated based on Case 5 considering the installation/service, legal/professional, intake/outfall, pretreatment, piping/high alloy, civil engineering, pumps, pressure vessels, membranes, equipment/materials, and the design/professional costs. In the case of the HFFO, the incorporated desalination process, the costs of pretreatment, and the intake/outfall were excluded. This exclusion also results in significant CAPEX savings: approximately 15.8% (20 million USD) of the amortized total CAPEX_RO and 1.2% (43 million USD) of the amortized total CAPEX_FO in the HFFO-incorporated desalination process including the intake/outfall and pretreatment. Consequently, comparing the total cost of the HFFO-incorporated desalination process with the conventional FO–RO hybrid process based on the conditions and performance of Case 5, the FO–RO–sHFFO desalination process can save as much as 63 million USD during a 20-year period. Detailed data on the economic evaluation are presented in Supplementary Fig. 1.

Economic and environmental impact evaluation (ZLD vs. brine circulation—no brine discharge)

Conventional seawater desalination plants produce clean water, although high-salinity brine is also produced^21,24. Depending on the recovery rate, the quality and quantity of the brine vary. In this section, an evaluation of the energy cost was conducted by comparing the HFFO-based infinity seawater desalination process with a two-stage SWRO combined with the ZLD process. The ZLD process can be defined to remove all liquid waste from the desalination process, reduce any harmful environmental effects, and meet the required regulations²⁰. However, the HFFO-based infinity desalination process does not discharge the brine because the brine is recirculated (or diluted) through the HFFOs and then re-fed into the first HFFO process. Therefore, the HFFO-based infinity desalination process presents environmental cost benefits. As shown in Fig. 5, the energy cost of the two-stage SWRO with a brine concentrator and crystallizer was 1191 million USD. The resulting costs were calculated based on a 100,000 m³/day plant capacity and 60% recovery rate. In addition, the brine capacity (brine concentrator feed water) was 400,000 m³/day from the two-stage SWRO process, and the recovery rate of the brine concentrator was 80%. The inlet flow rate of the downstream crystallizer was 8000 m³/day and the recovery rate was assumed to be 100%. The driving force of the brine concentrator and crystallizer is heat energy, and the high energy consumption is required for thermal-based desalination methods (i.e., MED and MSF)). However, as mentioned in the previous section, the HFFO-based infinity desalination process does not require a circulation pump for the FS and DS to recover the brine to the seawater concentrations. Therefore, the HFFO-based infinity desalination process can save more than 1 billion USD in energy costs over a 20-year period.

If the recovery rate is fixed, the concentration and volume of the brine in the FO–RO–sHFFO process differ from those during the production of 100,000 m³/day for the stand-alone two-stage SWRO process. If the recovery rate is 60% in the stand-alone two-stage SWRO process, the concentration and flow rate of the brine can reach 87,500 mg/L and 66,667 m³/day, respectively. For the FO–RO–-sHFFO process, to achieve a final product volume of 100,000 m³/day, the SWRO can be operated at low pressures (25 bar) and a low inlet flow rate (46,519 m³/day) because the DS, which is diluted by the wastewater during the first HFFO process, can be fed into the SWRO process. However, for the second HFFO process (sHFFO) used in the FO–RO-sHFFO process (infinite circulation for zero brine discharge), the concentration of brine from the SWRO must be higher than that of the seawater for a sustainable operation. This means that the recovery rate of the SWRO process must be >60%. Therefore, an additional economic evaluation was conducted with a fixed capacity of the SWRO process, and it was found that reasonable conditions for the SWRO are as follows: recovery rate = 45%, influent = 222,222 m³/day, final product = 100,000 m³/day, operation pressure = 59.2 bar, and brine concentration = 63,636 mg/L. Considering a brine concentration suitable for the sHFFO process, a recovery rate of approximately 85% was recommended to achieve an optimal operation. In this case, the operating pressure is 37.9 bar, and the brine concentration and flow rate are 65,127 mg/L and 33,333 m³/day, respectively. Under modified conditions, the water production of the FO–RO–sHFFO process is approximately twice that of the stand-alone two-stage SWRO process. Detailed economic evaluation results can be found in Supplementary Fig. 1.

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