*5.1.2 Multi-effect distillation (MED)*

One of the oldest industrial desalination processes that are used today is Multieffect distillation (MED) [87, 88]. The MED evaporator consists of cells, called effects, decreasing pressure and temperature from first to last, with temperature typically between 65 and 90°C [89]. Each effect consists of evaporator tube bundles on which seawater is sprayed. Heating steam or hot water through the tubes is supplied in the first effect and it transfers energy to the seawater in each effect, causing partial evaporation [90]. In each effect, the low pressure and temperatures affect the boiling point of water and with decreases of its, water becomes evaporate [88]. By using a heat exchanger and condensing the steam, clean distillate water is produced. This product water is pumped into a storage tank while the brine is pumped back into the sea.

For the production of water in a MED plant with a capacity range between 5000 and 50,000 m<sup>3</sup> /day, we require thermal energy between 145 and 230 MJ/m<sup>3</sup> , which will be equal to 12.2–19.1 kWh/m<sup>3</sup> of electrical energy. Furthermore, for pumps consumption will have been needed 2–2.5 kWh/m<sup>3</sup> of additional electrical energy [91]. Vapor flow and feed configurations are two major parameters that can effect on energy consumption in the MED process.

#### *5.1.3 Electrodialysis (ED)*

Electrodialysis (ED) is an electro membrane process in which with use of an electric field ionic and non-ionic components are removed [29]. In these kinds of processes, Anions and cations migrate towards the positive and negative electrode, respectively, and so the separation process happens. As can be show in the **Figure 10**, an ED system consists of alternately arranged anion exchange membranes (AEM) and cation exchange membranes (CEM).

The energy consumption in ED strongly depends to the salt concentration in feed solution. The rate of salt removal is proportional to the electric current [80, 92]. In order to efficient separation of ions from feed solution with high concentration, would require a high potential difference, thus, the use of ED process for seawater desalination, due to high concentration of ions in seawater and the need for high energy consumption, it is not affordable. This process is suitable for solutions with low-concentration of TDS (<5000 mg/L) such as brackish water [93]. Other parts that consume energy is the pumping unit and electrodes. On the basis of recent study, about 1–3% of the total energy consumption is related to these sections [92, 94].

Theoretically, in ED, for producing water with TDS about 800 mg/L the requirement of energy is 3.3 kWh/m3 and 26 kWh/m<sup>3</sup> for desalination of brackish water and seawater, respectively [95]. On average, 0.7 kWh for each 1000 mg/L

**Figure 10.** *Schematic of electrodialysis desalination.*

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

TDS removed, 0.5–1.1 kWh/m3 for pumping, and roughly 5% accounts for energy losses in a brackish water ED desalination system [96]. In a study that was reviewed by Sajtar and Bagley, they found in order to removal of TDS up to 2000 mg/L in feed stream, the energy consumption is ranges from 0.1 to 1 kWh/m<sup>3</sup> [92, 94]. Although ED is typically applied as a room temperature process, introducing a temperature gradient or increasing the temperature of the system can cause energy reductions [94]. Benneker et al. [97] found that the energy required for ED can be reduced by 9% if the temperature of one of the feed streams is increased by 20°C. Increasing the temperature increases ion mobility, reduces electrical resistance of the solution and decreases solution viscosity.

On the basis of the water salinity, the consumption of the electrical energy by an ED system can be about 0.5–10 kWh/m<sup>3</sup> [98]. For example, to lower TDS from 1500 ppm to 500 ppm, an ED unit would consume 1.5 kWh/m<sup>3</sup> . Due to high energy consumption in ED systems, in order to management and reduced the energy consumption, Recently, multi-stage electrodialysis systems have been investigated. Chehayeb et al. [99] found that by using a two-stage system for brackish water desalination the energy consumption can be reduced up to 29%, that, this can reduce the fixed costs. The application of ED remains limited by the high cost of ion exchange membranes and electrodes, and the electrically-driven degradation of polymeric membranes [100].

### *5.1.4 Membrane distillation (MD)*

Membrane distillation is one kind of separation process which in it, a porous membrane with hydrophobic properties is in contact with aqueous heated feed solution on one side. In MD process, the membranes that was use it works like this, that inhibit from the passage of the liquid water, but on the contrary allowing permeability for free water molecules and thus, for water vapor. These membranes are made of hydrophobic synthetic material (e.g. PTFE, PVDF or PP) and offer pores with a standard diameter between 0.1 and 0.5 <sup>μ</sup>m (3.9 <sup>10</sup><sup>6</sup> and 1.97 <sup>10</sup><sup>5</sup> in) [80, 101].

Due to the high amount of energy consumption and as a result the high cost of water production, MD has not still achieved widespread commercial implementation in desalination. There are four basic MD configurations included [102, 103];


In several studies it has been reported that both AGMD and VMD have greater thermal energy efficiency compared to other configurations, which makes them more popular choices for companies seeking to commercialize MD processes. In **Table 7**, the SEC values for several selected MD systems have been reported [102, 116–118].

## *5.1.5 Forward osmosis*

One kind of osmotic process is called forward osmosis (FO) that, in this process, like RO, in order to the separation of water from dissolved solutes, uses a semipermeable membrane. This process for creating the driving force for separation uses the osmotic pressure gradient, such that a "draw" solution of high


#### **Table 7.**

*Specific energy consumption (SEC) of selected MD systems [80].*

concentration is used to induce a net flow of water through the membrane into the draw solution, thus effectively separating the feed water from its solutes [80, 119]. As a result, separation in FO requires little or no hydraulic pressure as a concentrated draw solution (DS) with a greater osmotic pressure draws in water molecules from the feed solution through a membrane [120].

FO is widely promoted as a low-energy desalination technique. For the determination of the energy consumption in these kinds of plants, a DS recovery step is

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

used. During the osmosis step, in order to overcome dropping the pressure in the feed channel, at 50% water recovery, a low-pressure pump is needed, and the energy consumed is equal to 0.10–0.11 kWh/m<sup>3</sup> [25, 121]. For the osmosis step the values of 0.2–0.55 kWh/m<sup>3</sup> have also been reported [122]. Moon and Lee suggest, in a FO desalination plant, for solute regeneration, the energy consumption range is from 3 to 8 kWh/m<sup>3</sup> [123].
