There are several types of membrane technologies that are used today in the market, and the most predominant one is RO; the separation process using RO depends on the process of pressurizing feed fluid against semipermeable membrane, and such pressure causes only water molecules to cross (pass, migrate) through membrane, where salts and unwanted constituents such as minerals are rejected in a form of brine or concentrate. In RO process there is no heat addition during desalination process, hence no feed phase change occurred during separation process; besides RO technology, there are other membrane desalination systems used at small and limited scale such as nanofiltration (NF), ultrafiltration (UF), and microfiltration (MF).
The RO desalination system is characterized with low amount of energy consumption compared with conventional thermal types and with high water recovery, but membrane lifetime is short due to fouling and scaling; unlike thermal system RO can only treat feed salinity from 50 to 46 000 ppm, and the value of SEC varies from 1 to 6 kWh/m3. As per electrical demand an example of 50 mG/d capacity RO plant requires 20–35 MW in case of using feed seawater (sea water reverse osmosis [SWRO] system) and 8–20 MW in case of using brackish feedwater (brackish water reverse osmosis [BWRO] system).
The following are advantages and disadvantages of RO system:
Advantages of RO:Low energy consumption compared with thermal desalination system.Small in size and compact.Low value in investment.
Disadvantages of RO:Membrane has certain thermal stability limit.Life of a membrane is limited and short.Low feedwater can treat only salinity compared with thermal system.Costly in pretreatment.Use electrical power to drive system.Costly in maintenance.
Example 1.4 Reverse Osmosis
1 A simple one‐stage RO desalination system operates under steady flow; brackish water with feed capacity of 200 m3/h and 3000 ppm salinity needs to be desalted to 150 ppm as a potable water.Find:Concentrate flowrateConcentrate salinityRecovery ratioSolutionMass balance: Assume constant density (incompressible)Then Salinity balance: Recovery ratio: = 70%Extra activity:Student can perform the following:Derive RR relation as a function of three stream salinities.Select one practical operational parameter and examine its effect on RR. Plot and explain.Calculate salt rejection (SR), where SR is defined asSR = 100 − SPSP is salt passage across membrane SP = xp/xf, then perform a proper parametric study. Plot and explain.Perform parametric study to investigate the effect of feed salinity on concentrate salinity. Plot and explain.
1.5 Which Desalination System Is the Best?
Recall that feed type, characteristics, and quality play a major role in selecting desalination system. In addition potable water purity, type of energy consumption, water production cost, and emissions are also factors that play a role during desalination technology selecting; engineers must evaluate all the mentioned parameters to optimize his or her selection process.
For example, if we take a case of 45 000 ppm feed seawater, thermal system can be used, while in other cases such as brackish water with 8000 ppm salinity, membrane system is recommended. In certain applications using two different combined desalination system, thermal and membrane, which is called hybrid (Chapter 10), is another option of selection.
Table 1.1 shows the range operational energy requirement for main desalination systems.
Table 1.1 Energy requirement for main desalination systems.
Source: Alkaisi et al. 2017 [4]. Reproduced with permission of Elsevier.
| MEE | MSF | MEE‐TVC | MVC | RO | ED | |
| Typical unit size (m3/d) | 5000–15 000 | 50 000–70 000 | 10 000–35 000 | 100–2500 | 24 000 | −145 000 |
| Steam pressure (atm) | 0.2–0.4 | 2.5–3.5 | 0.2–0.4 | — | — | — |
| Electrical energy consumption (kWh/m3) | 1.5–2.5 | 4–6 | 1.5–2.5 | 7–12 | 3–7 | 2.6–5.5 |
| Thermal energy consumption (kJ/kg) | 230–390 | 190–390 | 145–390 | None | None | None |
| Electrical equivalent for thermal energy (kWh/m3) | 5–8.5 | 9.5–19.5 | 9.5–25.5 | None | None | None |
| Total equivalent energy consumption (kWh/m3) | 6.5–11 | 13.5–25.5 | 11–28 | 7–12 | 3–7 | 26–5.5 |
1.6 Thermo‐Physical Properties of Water
1.6.1 Potable Water
Potable water (freshwater) is characterized with salinity less than 500 ppm.
Table 1.2 shows water classification based on salinity, and the standards of WHO for freshwater limit is 500 ppm (0.05 mg/kg).
Table 1.2 Water classification based on salinity content.
| Type | Total dissolved salts (TDS) |
| Freshwater | <1500 |
| Brackish water | 1500–10 000 |
| Salt water | >10 000 |
| Seawater | 10 000–45 000 |
| Standard seawater | 35 000 |
The chemical concentration in fresh water with 500 ppm are shown in Table 1.3.
