In the last post, we looked at the tremendous growth of the desalination industry in the last few decades, as well as the benefits and drawbacks of desalination plants. This post will look at the technology of desalination, especially the major desalination methods in use globally today.
1) Intake — getting the water from its source to the processing facility;
2) Pretreatment — removing suspended solids to prepare the water for further processing;
3) Desalination — removing dissolved solids, primarily salts and other inorganic matter, from a water source;
4) Post-treatment — adding chemicals to the desalinated water to prevent corrosion of downstream infrastructure pipes; and
5) Concentrate management and freshwater storage — handling and disposing or reusing the waste from the desalination, and storing freshwater before it’s provided to consumers.
We will mainly focus on the third stage — desalination — where the majority of advancements in technology have happend. There are three categories of desalination methods: membrane, thermal or distillation, and ion exchange. The thermal and membrane methods are the two most widely-used today. The ion exchange process won’t be discussed here since ion exchangers are only economical in removing small amounts of salt. The process of ion exchange is very effective at producing ultrapure water, but is limited at desalting on a large scale.
Before 1998, most desalination plants were thermal. However, in recent years, technological improvements in reverse osmosis (RO) desalination, a membrane filtration method, has made the number of plants using membrane technology surpass that of thermal. As of 2008, membrane processes accounted for 56 percent of desalination capacity worldwide while thermal processes accounted for 43 percent. Small-scale ion exchangers and hybrid processes accounted for the remaining one percent.
Below, I’ll provide an overview of the membrane and thermal methods because currently, they are the two primary categories of desalination used at the utility scale. Although a number of different desalination processes fall under each of these categories, for each category, I will focus more on the specific process that is most prevalently used in desalination plants worldwide. For the membrane process, this is RO filtration; for the thermal process, this is multistage flash distillation (MSF).
Membranes can be designed to selectively allow or prevent the passage of certain ions, including salts. Membranes play an important role in the separation of salts in natural processes (such as osmosis and dialysis), and this principle has been adapted for commercial use. Commercially-available membrane processes include RO, nanofiltration (NF), electrodialysis (ED), and electrodialysis reversal (EDR).
Membrane technologies can be used not only for desalting brackish water and seawater sources, but also for treating wastewater because of their ability to remove contaminants other than salts (e.g., organic contaminants, bacteria, and viruses). Typically, 35 to 60 percent of the seawater fed into a membrane process is recovered as product water. For brackish water desalination, water recovery can range from 50 to 90 percent.
Reverse Osmosis Filtration
In the natural process of osmosis, solvents (such as water) diffuse or pass through a semipermeable membrane (think cheese cloth) that blocks the passage of solutes (such as salts). More specifically, when solvents of different concentrations of solutes are separated by a membrane, the solvent wants to move from the low to the high concentration of solutes to achieve equilibrium. At this point, osmotic pressure across the membrane becomes equal (usually 350 pounds per square inch).
RO, as the name implies, is the opposite of what happens in osmosis. A pressure greater than the osmotic pressure is applied to saline water to cause freshwater to flow through the membrane while holding back the solutes, or salts. The water that comes out of this process is so pure that they have to add back salts and minerals to make it taste like drinking water.
As mentioned in the section above, new membrane desalination capacity has surpassed new thermal capacity mostly due to significant advances in RO technology. RO desalination is popular because of its sustainability, cost effectiveness, and simplicity. RO plants typically use less energy than thermal distillation, which has led to a reduction in overall desalination costs over the past decade.
The largest RO plant in the world, located near Ashkelon, Israel, produces 320,000 cubic meters of water daily (m3/day) — about 6% of the country’s total water needs. The cost of producing one cubic meter of water is a bit more than $0.50 USD, one of the world’s lowest prices for desalinated water.
The basic principle of the thermal processes is to apply heat to create water vapor, which then condenses into pure water, separating it from most of the salts and impurities. Thermal processes include multistage flash (MSF), multiple effect distillation (MED), and mechanical vapor compression (MVC). Thermal processes are configured to use and reuse the energy required to evaporate water.
Thermal distillation was the earliest method used to desalinate seawater on a commercial basis. Thermal processes are used across the Middle East and will continue to be a logical choice for the region for several reasons.
First, the regional seas are very saline, hot, and periodically have high concentrations of organics, which are challenging conditions for RO desalination technology. Second, RO plants have only recently approached the large production capacities required in this region, so much of the existing desalination capacity is thermal-based. Third, dual-purpose cogeneration facilities built there combine water production with electric power generation to take advantage of shared intake and discharge structures, as well as to improve energy efficiencies (usually by 10-15 percent). These cogeneration facilities allow the thermal desalination processes to use low-temperature waste steam from the power generation turbines. These reasons, combined with the artificially-low cost of energy in the region, make thermal processes the dominant desalination technology in the Middle East.
Multistage Flash Distillation
MSF is the most robust of all desalination technologies and is capable of very large production capacities. The number of stages used in the MSF process is directly related to how efficiently the system will use and reuse the heat that it is provided.
The MSF process consists of a series of stages, or chambers, maintained at decreasing pressures from the first stage (hot) to the last stage (cold). In this illustration of the process, seawater flows in on the right side through tubes in the upper part of the chambers where it is warmed by the water vapor produced in each stage. Its temperature increases from sea temperature to the temperature of the heater on the left as it travels in that direction. The seawater then flows through the heater (the squiggly line through the cloud, which represents steam) where it receives the necessary heat for the process.
At the outlet of the heater, when entering the bottom of the left-most chamber (the first stage), the seawater is overheated compared to the temperature and pressure of that stage. It will immediately release heat (known as “flashing“), and thus vapor, to reach equilibrium with the conditions in that chamber. The vapor is then condensed into freshwater on the tubes at the top of the chamber. The process takes place again in the next stage, and so on until the last and coldest stage (the chamber on the right end). The freshwater builds up and is extracted from the coldest stage (the blue-colored distillate flow). Seawater slightly concentrates from stage to stage and builds up the brine flow at the bottom, which is also extracted from the last stage.
The state of desalination technology today: comparisons and areas for improvements
No single method of desalination is the “best” choice. Globally, both thermal and membrane technologies are used widely for seawater desalination. Both processes require energy for the separation of salts, and various energy sources can be used. Brackish water is typically desalinated using membrane processes (such as RO, NF, or ED).
The combined energy requirements of thermal technologies are greater than those of membrane technologies, but it is not so simple to compare the total energy use of these very different processes. Thermal processing such as MSF and MED are capable of using waste, or low-grade, heat (such as in cogeneration facilities mentioned above), which can significantly improve the economics of thermal desalination. For example, many of the largest modern cruise ships use the MED desalination process to make freshwater at sea; MED requires 20 to 33 percent of the energy required for RO and the ships’ propulsion engines can provide the required heat.
The major desalination technologies in use today are generally efficient and reliable, but the cost and energy requirements are still high. Ongoing research efforts are aimed at either reducing cost (by powering plants with less-expensive energy sources, such as low-grade heat) or overcoming operational limits of a process (by increasing energy efficiency).
Improvements will be incremental since the current technologies are relatively mature. Ultimately, no desalination process can overcome the thermodynamic limit of desalination, and we’re pretty close to approaching that limit.