*5.2.1 Electrodialysis: reverse osmosis hybrid systems (ED-RO)*

Increasing recovery in RO systems requires multiple stages and thus significantly increased capital and operation costs [124]. In the electrical desalination systems such as ED compare to the RO membranes, we cannot achieve to high salt rejection alone [125], and this is very important in energy consumption, however, one of the advantages of ED systems, is operation at higher recovery rate, but and low SEC, by scale formation this process eventually limited [80]. The concept of (ED-RO) hybrid system at first in 1981 by Schmoldt et al. was studied [126]. They proposed the use of ED as a second stage to control permeate quality. However, one of the disadvantages of this system was high energy consumption up to 7.94 kWh/ m<sup>3</sup> for SWRO system with a concentration of 45,000 ppm, that was due to some problems such as lack of high-flux and high-selectivity membranes in this process [80, 126]. But, in their studies they showed in a desalination plant with capacity of 1000 m<sup>3</sup> /day and feed concentration of lower than 4000 mg/L, the investment cost for ED can be lower than RO. They noted that with the development of the high flux membranes and with high salt-rejection, not only the cost of the RO system could be reduced, hence, with incoming feed with lower TDS concentration, the energy consumption of the ED unit also reduce [126].

In a another study, by Turek et al. [127], in order to assessment SEC and recovery rates, in four different configurations (single-stage standalone RO, NF-SWRO, hybrid ED-RO and NF-SWRO-ED system) for seawater desalination plants were been compared. As can be seen in **Table 8**, the highest recovery (81.1%) was achieved for SWRO-ED, but, at this recovery rate, the SEC was 7.77 kWh/m3 , after that the NFSWRO-ED system had more recovery rate (69.0%) at lower SEC (6.90 kWh/m3 ). Although SEC in the SWRO system was much less (2.76 kWh/m3 ), but on the other hand, this single-stage RO system operated at a recovery rate of only 43% [80].

In **Table 9**, a comparison of selected ED-RO studies has been presented.

#### *5.2.2 Reverse osmosis: membrane distillation hybrid systems*

Several advantages of MD system like as operation at high recovery, high separation efficiency and Low capital cost, has made it alternative candidate for hybrid separation technologies [132, 133]. Over the last few years, a few studies on the hybridization of MD and RO in order to treatment of the concentrate stream from the RO process have been done. For example, in a study by Choi et al., economic feasibility of a RO-MD system for desalination of seawater was assessed. In this study, they found that a RO-MD hybrid system or a MD stand-alone system only when the flux and recovery are greater than that for RO, and or the thermal energy


**Table 8.**

*The SEC and water recovery for SWRO-ED, SWRO, NF-SWRO and NF-SWRO-ED systems [80].*


#### **Table 9.**

*Key parameters from selected ED-RO hybridization studies [80].*

that has been supplied for MD, had relatively low cost, can compete with RO system [134]. Although, MD is able to achieve a high water recovery rate of 85%, However, the Energy consumption for RO-MD hybrid systems is still unclear and should be further investigated [80].

#### *5.2.3 Forward osmosis (FO)-RO*

**Table 10** shows the summary of hybrid FO-RO system for seawater desalination [135].

#### *5.2.4 Nanofiltration (NF)-RO*

Using of MF, UF membrane although can be effective for the pretreatment of a SWRO system, but some important parameter such as NOMs, organic matters and dissolved organic matters cannot be fully removed. Since in MF and UF divalent metal ions do not remove, so, the potential of the Scaling cannot be reduced. As we know, in SWRO desalination facility, about 44% of water production costs are related to energy consumption, which is closely related to the salinity of seawater. Hence, in order to pretreatment and effectively reduction of overall salinity (reduce *Energy Recovery in Membrane Process DOI: http://dx.doi.org/10.5772/intechopen.101778*


#### **Table 10.**

*Summary of hybrid FO-RO system for seawater desalination [135].*

divalent cations) in SWRO system, nanofiltration (NF) can be used [140–142]. In **Table 11**, the summary of the NF-RO hybrid systems is shown. From the view point of the energy consumption, addition of NF pretreatment will increase the energy consumption due to the added pumping energy. However, due to the reduction of salinity in the influent feed solution of RO, the energy consumption decrease [135].

#### *5.2.5 Pressure-retarded osmosis (PRO)-RO*

Pressure-retarded osmosis (PRO) is a device to generate power using osmosis. There are two advantages of coupling SWRO and PRO; (1) enhancement of the power generation in PRO due to the higher osmotic pressure of concentrated brine than seawater, (2) dilution of the concentrated brine before discharging to the ocean In order to combination of RO and PRO there are many different ways, but they can be classified in two groups. First one is transferring the high pressure of DS to the RO feed by using of pressure exchanger and other is generation of electricity with high-pressure DS that spins the turbine. So, with these changes, the specific energy required for water production is reduced (**Figures 11**–**13**) [159].

There are a number of simulation studies for the RO-PRO hybrid system but only few experimental works have been done using either a small lab-scale equipment or a large demonstration plant, as summarized in **Table 12**, which was made based on the work of Kim et al. [135, 159].


#### **Table 11.**

*Summary of NF-RO hybrid system [135].*

**Figure 11.** *RO-PRO system in Japan [153].*

*Energy Recovery in Membrane Process DOI: http://dx.doi.org/10.5772/intechopen.101778*

**Figure 12.**

*PRO-MD-RO system in Korea [154].*

**Figure 13.** *RO-PRO system of Achilli et al. [80].*
