**1. Introduction**

Sustainable energy is the key solution for addressing major concerns about the future such as climate change, environmental protection, and balanced growth of the economy and society. In many nations at past two decades have witnessed advancement in economic development. However, industrial advancement, deterioration of the environment, energy shortage, the rapid economic growth and increasing demands of growing populations pose a huge threat for future generations [1–3]. For many years, economic development has been the key focus of many policy makers in sustainable development until the inception of the Kyoto protocol agreement in 1997, which includes environmental quality as a crucial variable for sustainable development [3]. According to global energy consumption, expected that electricity demands to be double in the next twenty-five years, so, major opportunities for innovation in energy production, storage, transmission and use of it have begun to open up. In particular, in order to improving the efficiency of the

processes and reducing the global carbon footprint, there is a huge interest in sustainable energy technologies [3, 4].

Development of an approach to sustainable energy that addresses greenhouse gas emission, environmental concerns, availability of resources, social impact and cost is an immense challenge. The key focus for obtaining energy sustainability is the generation of energy with renewable energy sources and replace them slowly with power fossil fuels [5]. There is much research that has worked for developing the membrane sector, which emphasizes the use of renewable energy in membrane technology. Although the efficiency of the process is still a high priority. Recently, membrane technologies, especially, in the water and energy sector, have begun to play a basic role in developing the infrastructure for sustainable energy. Some of the membrane-based approaches that are currently adapted at an industrial scale include desalination by RO, membrane-based bioreactors (MBR) for pure water generation, lithium-ion batteries, and membrane-based fuel cells and CO2 capture [6–8]. Many advantages of membrane technologies like flexibility, feasibility and adaptability have been able to decrease many concerns related to water scarcity and energy demands in recent years. However, with achievement to advancements in membrane-based technologies. we still need to improve affordability and costs.

### **2. Membrane technology and sustainable water generation**

In the past decades, following the increase in freshwater demand, various techniques including multiple-effect distillation (MED), vacuum distillation, multi-stage flash distillation (MSF), and other membrane-based technologies, such as reverse membrane distillation (MD), osmosis (RO) and etc., in order to sea water desalination, have been developed. Among these technologies, some of the membrane-based techniques such as RO, MD and forward osmosis (FO), because of some advantages like as lower maintenance and operating costs, lower capital requirements and low energy consumption, are considered as suitable alternatives [3, 9].

#### **2.1 Desalination**

Desalination is a process which use for producing freshwater from either sea or brackish water, by removing the salt content either by membrane technologies or by a thermal distillation process.

As can be seen from **Table 1**. the membrane technologies, specifically the RO, mainly, because of lower energy requirements, are preferred over the other technologies. In different technologies, the specific energy consumption (SEC) varies widely and depending on the operation and process control as well as the quality of the produced water, this value might have further differed significantly for a particular technology.

#### **2.2 Reverse osmosis (RO)**

To date, for desalination and stress reduction due to depletion of available water resources, reverse osmosis (RO) is the key technology [1]. In desalination plant such as RO, membrane played a key role which is largely determine the separation performance of the overall plant (**Figure 1**). In several recent studied suggests that in ultra-permeable membranes (UPMs) by increasing the water permeability up to three times than normal could reduce the energy consumption pressure vessels for seawater desalination about 15% and 44%, respectively.

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


*ED = electrodialysis; EDR = electrodialysis reversal; BWRO = brackish water reverse osmosis; SWRO = seawater reverse osmosis; MVC = mechanical vapor compression; MD = membrane distillation; MSF = multi-stage flash; MED = multiple effect distillation; MEB = multi-effect boiling; FO = forward osmosis.*

#### **Table 1.**

*Specific energy consumption (SEC) by different desalination techniques.*

**Figure 1.**

*The RO process diagram with (a) and without (b) pressure recovery for SWRO and BWRO respectively.*

In the context of wastewater reclamation, even greater savings (e.g., 45% less energy input and 63% fewer pressure vessels [10]) can be achieved. Moreover, increasing the properties of the membrane selectivity can cause improvement the quality of the product [11].

Recent studies introduce the promise of developing new membrane materials. These materials can desalinate water while showing far greater permeability than traditional reverse osmosis (RO) membranes. But the question remains whether higher permeability means significant reductions in the cost of desalinated water. Research evaluates the potential of ultra-permeable membranes (UPM) to improve the performance and cost of RO.

#### *2.2.1 Ultra-permeable membranes (UPM)*

By modeling the mass transport inside a reverse osmosis pressure vessel (PV), the study assesses how much tripling water permeability lowers energy consumption. And also lowers the number of required pressure vessels for a particular desalination plant. The findings were very interesting, it proved that a tripling (3) in permeability permits 44% fewer pressure vessels and 15% less energy for a seawater reverse osmosis plant (SWRO) [10, 12]. This is done at a both given capacity and recovery ratio. Moreover, tripling permeability results in 63% fewer pressure vessels or 46% less energy for brackish water reverse osmosis (BWRO). However, it also shows that the energy savings of ultra-permeable membranes (UPM) exhibits a law of diminishing returns due to thermodynamics and concentration polarization at the membrane surface [10].

In terms of reducing energy consumption, the benefits of ultra-permeable membranes (UPM) are limited to approximately 15% in the case of SWRO. It also shows that membranes with 3 higher permeability reduces number of pressure vessels by 44% for seawater reverse osmosis RO plants SWRO. And 63% in brackish water RO plants BWRO. This does not affect the energy consumption or permeate recovery [13].

In order to calculation of systems-level quantities the typical RO process diagram that shown in **Figure 2**, is used. In SWRO systems, for pressurizing the feed using mechanical energy Regenerated force from isobaric brine, pressure recovery devices (PRDs) are used (**Figure 2a**), while at BWRO typically this is not done (**Figure 2b**).

In case of energy consumption, ultra-permeable membranes proved to lower energy consumption of seawater reverse osmosis systems—SWRO—by %15. While on the other hand lowered energy consumption of brackish water reverse osmosis systems—BWRO—by 46%. The research was made at the same permeate flow per pressure vessel as what is typical nowadays. As can be shown in **Figure 2a** by reducing the inlet pressure, lower energy consumption (membrane area, feed flowrate and for a given recovery ratio) would be obtained. In SWRO (the line with purple dye in the figure), the pressure of inlet feed reduces to the outlet of the brine osmotic pressure. This limitation in the membrane, that corresponds to the osmotic pressure of the brine, is independent from membrane performance. As can be seen in the **Figure 2a**, with increasing Am up to triple from 1 to 3 L (m<sup>2</sup> h bar), we can reduce the inlet pressure about 1% and reach from 70 bar to 63 bar. For every 1% reduction in the inlet pressure, the SEC could be reduced up to 1.5%. However, as can be seen in this figure, any further improvements in membrane permeability beyond 3 L (m<sup>2</sup> h bar)<sup>1</sup> , since 63 bar is already within 1% of the osmotic limit for SWRO at the chosen recovery ratio, would have essentially no effect on energy consumption.

As can be shown in **Figure 2a**, in order to achieve 65% recovery in BWRO and with increasing *Am*, inlet pressure rapidly drops. Due to the limitation of the osmotic

#### **Figure 2.**

*Investigation of key performance criteria and their effect in membrane permeability for BWRO at 2000 ppm NaCl (orange) and SWRO at 42,000 ppm NaCl (purple). (a) Energy consumption (dashed) and minimum required inlet pressure (solid lines) at fixed feed flowrate and recovery. In BWRO, energy consumption and pressure are linearly related. (b) Number of pressure vessels required for a total capacity of 100,000 m<sup>3</sup> day<sup>1</sup> at fixed recovery ratio and pressure. Membrane width is held fixed in both subplots.*

for BWRO that is only a fraction of that of SWRO, with the increase in membrane permeability up to triple, it is causing a much greater reduction in inlet pressure, namely down to 6.4 bar from 12 bar in the case of thin-film composite (TFC) membranes (a 46% reduction in pressure and energy consumption). In the membranes with more permeability, with increasing the membrane's water permeability (*Am*) (L (m2 h bar)<sup>1</sup> ) to over 5 L (m2 h bar)<sup>1</sup> , the pressure essentially reaches the asymptotic limit. So, for the RO plant in the stage of brackish water, the UPMs could reduce the energy consumption to half. In the BWRO, the number of pressure vessels is lower than the SWRO. On the basis of **Figure 2b**, with a tripling *Am*, we can reduce the pressure vessels up to 63%, for a given plant capacity, by increasing the feed flowrate per vessel from 139 m3 day<sup>1</sup> to 378 m3 day<sup>1</sup> . Furthermore, increases in feed flowrate have no effect on the energy, since, viscous losses in a BWRO system represent a negligible component of the overall energy consumption [3].

Commercial RO membranes are dominated by TFC polyamide and its derivatives **Figure 3**. These membranes are facing critical challenges such as low selectivity, relatively low water permeability and high fouling tendency [2]. For example, in RO membranes, TFC has a typical water permeability range from 1– 2Lm<sup>2</sup> <sup>h</sup><sup>1</sup> bar<sup>1</sup> for SWRO membranes and <sup>2</sup>–8Lm<sup>2</sup> <sup>h</sup><sup>1</sup> bar<sup>1</sup> for BWRO [10, 14]. So, in synthesizing novel RO membranes, focused on the improvement of separation properties and better antifouling performance that is a key research focus in the field of desalination.

When it comes to capital costs, on the basis of our analysis, we can propose certain qualitative trends. According to Global Water Intelligence, in a typical SWRO plant with capacity of 150,000 m3 day<sup>1</sup> , the levelized capital cost today is about 0.20 \$ per m3 (excluding land) that 20% of this cost is due to piping,, pressure vessels and membranes [15, 16]. So, with using of UPMs membrane, in a surface area similar to conventional membranes but with triple permeability, membranes can be reduced by up to 44%, in this situation the membranes would save on the order of 0.02 \$ per m3 in capital costs. The benefits are more significant for BWRO. in BWRO systems with UPMs membrane, saw that reduction of the energy consumption could be up to 46% [8]. Following increase of membrane permeability mass transfer coefficients and also typical cross-flow velocities decrease. With enhancement of membrane permeability, permeate water flux increases routinely [10].

The consequences of producing a product with less working pressure or more permeability can be estimated with confidence. As described above, the energy savings in SWRO with UPMs membrane could be limited to about 15%. At SWRO plants, because of the high salinity of seawater, operation has been optimized in such a way that these plants work with minimum pressure (60–70 bar) in order to extract permeate water from seawater [8, 10]. The difference between pre- and

**Figure 3.** *Ultra-permeable membranes UPM thin film composite TFC for BWRO and SWRO.*

post-treatment is about 1 kWh m<sup>3</sup> , in RO stage, a 15% reduction in the energy consumption could only reduce 10% of the overall cost of the energy in SWRO plants. With the reducing of the total energy consumption in SWRO plants from 3.8 kWh to 3.5 kWh, If the price of electricity is assumed to be 0.10 \$ per kWh, could be saved the cost about 0.03 \$ per m<sup>3</sup> [17, 18].

Wilf [19] evaluated with replacing the RO elements with membranes which have 80% higher permeability, in situation which recovery ratio and feed salinity was 85% and 1500 ppm, respectively, the SEC of BWRO decrease. He found that in two different averages flux (25.5 LMH and 34 LMH) the SEC was decreased (from 0.52 to 0.40 kWh/m<sup>3</sup> and from 0.72 to 0.49 kWh/m<sup>3</sup> , respectively).

Franks et al. [20] evaluated, in BWRO plants, when a membrane element with 34.1 m<sup>3</sup> /d of permeate flow replace with another elements that has 45.4 m<sup>3</sup> /d of permeate flow, the SEC decrease. In this study, with decreasing the feed pump pressure 9.8–8.3 bar, the specific energy consumption decreased from 0.41 to 0.35 kWh/m<sup>3</sup> (the pump efficiency was 83%, the recovery ratio was 85% and the feed salinity was 1167 ppm (for wastewater). The simulation conditions were shown in **Tables 2** and **3**.

For a BWRO plant, Werber et al. [24] assumed a 85% recovery rate and feed with NaCl concentration about 5844 ppm. They observed, in a single-stage process, with increasing the water permeability in membrane from 4 to 10 LMH/bar, the SEC can be reduced up to 2.2%. On the other hand, in this study observed that in a two-stage RO with membrane permeability of about 4 LMH/bar, the required energy was 22% lower (0.11 kWh/m<sup>3</sup> ) than the single-stage RO, also the SEC decreased by increasing the membrane permeability from 4 to 10 LMH/bar by 12% (0.05 kWh/m3 ) that compare to a single-stage BWRO was slightly larger. In this study, in SWRO with single stage process and membrane permeability about 2 LMH/bar, the hydraulic pressure was only 7.6% above the brine osmotic pressure (**Figures 4** and **5**). The results of their findings of the relationship between membrane water permeability and the SEC have shown.

Busch et al. [29] assessed the CAPEX and OPEX reductions with higher permeable SWRO elements. They compared the energy use, power cost, water cost by replacing SW30HR-380 with 28.4 m<sup>3</sup> /d of permeate flow rate and 99.75% of NaCl rejection rate by SW30HR LE-400 with 34.1 m<sup>3</sup> /d of permeate flow rate and 99.70% of NaCl rejection rate using the test results for each element. Test conditions and calculation assumptions were 32,000 mg/L NaCl of feed concentration, 8% of recovery rate, 55 bar of feed pump pressure, 5 years of operating time, 20% of RO membrane elements replacement rate per year, 90% of pump efficiency, and 0.08 US\$/kWh of power cost. The pretreatment, chemical cleaning, and other costs were not considered. They indicated that decreasing membrane area by using higher water permeability RO elements can decrease the water cost by 4.7% from 0.190 to 0.181 US \$/m3 with the same energy cost.

For SWRO, the energy cost contributes 40–50% of the total water production cost; therefore, the ratio of the specific membrane cost to the total water production cost is about 1.2–6%. Hence, doubling the membrane water permeability halves the specific membrane cost so that the total water production cost is reduced to 0.6–3%. When the cost of pressure vessels is taken into consideration, the decrease of total water production cost is 0.7–3.5% [12]. But, with increasing the membrane permeability, the feed velocity and the pressure loss increase, as a result, more energy is needed, these could increase the SEC up to 6%.

As can be shown in **Figure 6**, Cohen-Tanugi et al. [10] calculated the total number of pressure vessels needed in a single-stage SWRO and BWRO with 100,000 m3 /d permeate and 42,000 ppm and 2000 ppm salinity concentration, respectively.

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


**Table 2.**

 *Simulation conditions of each reference that includes the relationship between membrane water permeability and SEC for SWRO [12].*

