In conversation with Desalination Expert - Nikolay Voutchkov

The Water Network team had the pleasure to interview Nikolay Voutchkov - president of the Water Globe Consulting.

Nikolay Voutchkov has more than 25 years of experience in the field of desalination and water reuse as an independent technical advisor to public utilities implementing large desalination projects in Australia, USA, and the Middle East; and to private companies and investors involved in the development of advanced membrane technologies.

In this interview, we talk with Nikolay about different desalination technologies, differences between them and their impact on the environment. He also gave us incredible insight on the future of the desalination industry and its role in providing the world with sustainable water treatment options. Bellow are the excerpts from the interview.

Q1. How does the use of desalination technology compare with other drinking water generation methods like waste water reuse?

Desalination is complimentary to water reuse and conventional water supply sources such as surface or groundwater.

Q2. What are the different types of desalination technologies commonly implemented? What are the criteria for the selection of appropriate technology for different applications?

Sea and brackish water are typically desalinated using two general types of water treatment technologies – thermal evaporation (distillation) and membrane separation. In thermal distillation fresh water is separated from the saline source by evaporation.

In RO desalination fresh water is produced from saline source water by its pressure-driven transport through semi-permeable membranes. The main driving force in RO desalination is pressure which is needed to overcome the naturally occurring osmotic pressure, which in turn is proportional to source water salinity.

Besides thermal and RO membrane separation two other mainstream desalination technologies widely applied at present are electrodialysis (ED) and ion exchange (IX). Electrodialysis is electrically driven desalination where salt ions are removed out of the source water by exposure to direct electric current. The main driving force for ED separation is electric current, which is proportional to the salinity of the source water.

Ion exchange is a selective removal of salt ions from water by adsorption on ion-selective resin media. The driving force in this desalination process is the ion charge of the IX resin, which can selectively attract and retain ions of opposite charge contained in the saline source water.

Table 1 provides a general indication of the range of source water salinity for which distillation, RO separation, ED and IX can be applied cost effectively for desalination. For processes with overlapping salinity ranges, a life-cycle cost analysis for the site-specific conditions of a given desalination project is typically applied to determine the most suitable desalination technology for this project.

Table 1

Desalination Process Applicability

Currently, approximately 60 % of the world’s desalination systems are reverse osmosis (RO) membrane separation plants and 34 % are thermal desalination facilities (GWI/IDA, 2012). The percentage of RO desalination installations has been increasing steadily over the past 10 years due to the remarkable advances in membrane separation and energy recovery technologies, and associated reduction of the overall water production costs. At present, ED and IX-based technologies, contribute less than 6 percent of the total installed desalination plant capacity worldwide.

Q3. The two major thermal desalination processes are Multi-Effect Distillation, or MED, and Multi-Stage Flash, or MSF. What are the differences in these technologies?

All thermal desalination technologies apply distillation (i.e. are based on heating of the source water) to produce water vapor, which is then condensed into a low-salinity water. Since the energy for water evaporation is practically not dependent on the source water salinity concentration, thermal evaporation is very suitable for desalination of high salinity waters and brine. This is one of the reasons why thermal desalination has been widely adopted by Middle Eastern countries such as Saudi Arabia, Oman, Qatar, United Arab Emirates, Bahrain and Kuwait, which use some of the most saline water bodies on the planet for water supply (i.e., the Red Sea, Persian Gulf, Gulf of Oman and the Indian Ocean). At present, approximately 75 % of the total world’s thermal desalination plants are located in the Arabian Peninsula, have of which are in Saudi Arabia.

All thermal desalination plants have five key streams: source water (seawater, brackish water or brine) used for desalination; steam needed for evaporation of the source water; cooling water to condense the fresh water vapor by generated from source water evaporation; and concentrate (brine) which contains the salts and other impurities separated from the source water (see Figure 1).

Figure 1– General Schematic of Thermal Evaporation Technologies

The three most commonly used types of thermal desalination technologies are: multistage flash distillation (MSF); multi-effect distillation (MED); and vapor compression (VC). Each class of these technologies has evolved over the past 40 to 60 years towards improvements in efficiency and productivity. For example, “MSF-BR” (see Figure 1-1) is a multistage flash distillation process with brine recycle, which reduces the source water volume and steam needed for evaporation. Similarly – “MED-TC” stands for multi-effect distillation with thermal compression – a state-of-the art MED technology; and “MVC” is an acronym for mechanical vapor compression – a VC technology which can run without the need of outside source of steam.

The three types of thermal technologies mainly differ by the temperature and pressure at which the source water is boiled to generate fresh water vapor. The oldest thermal evaporation process – MSF - boils water at near atmospheric pressure and temperature close to 100 0C (212 0C). This type of process requires large quantity of high temperature steam.

MED and VC are newer thermal desalination technologies which improved efficiency stems from the fact that water can be boiled at lower temperature if the boiling process occurs at pressure lower than the atmospheric pressure. Boiling water at lower temperature allows using less and lower quality steam for production of the same volume of water.

As shown of Figure 1, in MED desalination vessels the boiling process typically occurs at lower temperatures and pressures these of MSF distillation systems. VC thermal desalination systems operate at lower pressure than MSF and MED, which allows these systems to evaporate water at even lower temperatures and to self-generate steam rather than to depend on outside steam sources.

The ratio of the mass of produced low salinity water (distillate) to the mass of heating steam used to produce this water is commonly referred to as a gained output ratio (GOR) or performance ratio. Depending on the thermal desalination technology used, the site specific conditions and source water quality, the GOR typically varies between 4 and 40 – i.e., thermal desalination technologies produce 4 to 40 kilograms of fresh water using one kilogram of steam. The higher the technology GOP the more efficient it is because it produces more fresh water from the same amount of steam.

As seen on Figure 1, all thermal desalination technologies generate very low salinity water (TDS in a range of 10 to 25 mg/L). This fresh water also has very low content of pathogens and other contaminants of concern such as boron, bromides and organics.

Thermal desalination is most popular in the Middle East, where seawater desalination is typically combined with power generation that provides low-cost steam for the distillation process. Thermal desalination requires large quantity of steam.

Most power plants outside the Middle Eastern region are not designed to yield significant amounts of waste steam as a side product of power generation. This is one of the key reasons why thermal desalination has not found wider application outside of this region.

Multistage Flash Distillation (MSF)

In the MSF evaporator vessels (also referred to as “flash stages” or “effects”) the high-salinity source water is heated to a temperature of 90 to 115 0C (194 to 239 0F) in a vessel (Figure 2 – “Heating Section”) to create water vapor. The pressure in the first stage is maintained slightly below the saturation vapor pressure of the water. So, when the high pressure vapor created in the heating section enters into the first stage, its pressure is reduced to a level at which the vapor “flashes” into steam.

Figure 2 – Schematic of MSF Distillation System

Steam (waste heat) for the heating section is provided by the power plant collocated with the desalination plant. Each flash stage (effect) has a condenser to turn the steam into distillate. The condenser is equipped with heat exchanger tubes, which are cooled by the source water fed to the condensers.

Entrainment separators (mist eliminators/demister pads) remove the high-salinity mist from the low-salinity rising steam. This steam condenses into pure water (distillate) on the heat exchanger tubes and is collected in distillate trays from where it is conveyed to a product water tank. Distillate flows from stage to stage and it is collected at the last stage.

The concentrate (brine) generated in each stage and after collection at the last stage, some of it typically is recycled to the source water stream in order to reduce the total volume of source water that would need to be collected by the intake for desalination. The recirculated brine flowing through the interior of the condenser tubes also removes the latent heat of condensation. As a result, the recirculated brine is also pre-heated close to maximum operating temperature, thereby recovering the energy of the condensing vapor and reducing the overall source water heating needs. This “brine recycle” (BR) feature is adopted in practically all most recent MSF facility designs and allows to improve significantly the overall cost-competitiveness of MSF installations.

Each flash stage typically produces approximately 1 % of the total volume of the desalination plant condensate. Since typical MSF unit has 19 to 28 effects, the total MSF plant recovery (i.e., volume of distillate expressed as percentage of the total volume of processed source water) is typically 19 to 28 %. For comparison seawater desalination plants have recovery of 40 to 45 %. The latest MSF technology has 45-stage units i.e., can operate at 45 % recovery. This feature allows it to compete with RO systems in terms of recovery.

Historically, MSF is the first commercially available thermal desalination technology applied for production of potable water in a large-scale, which explains its popularity. Over 80 percent of the thermally desalinated water today is produced in MSF plants. The GOR for the MSF systems is typically between 2 and 8. Latest MSF technology has GOR of 7 to 9. Pumping power required for the operation of the MSF systems is 2.0 to 3.5 kWh per cubic meter of product water.

Multiple Effect Distillation (MED)

In multiple-effect distillation systems (MED) s aline source water is typically not heated – cold source water is sprayed via nozzles or perforated plates over a heat exchanger tube bundles. This feed water sprayed on the tube bundles boils and the generated vapor passes through mist eliminators, which collect brine droplets from the vapor. The feed water that turned into vapor in the first stage (effect) is introduced into the heat exchanger tubes of the next effect. Because the next effect is maintained in slightly lower pressure, although the vapor is slightly cooler, the lower pressure allows it to condense into fresh water at this lower temperature. This process of reducing the ambient pressure in each successive stage allows the feed water to undergo multiple successive boilings without introducing new heat. Steam flowing through the exchanger tubes is condensed into pure water (see Figure 3) and collected from each effect. Heating steam (or vapor) introduced in the heat exchanger tubes of the first effect is provided by a steam ejector from an outside source.

Figure 3 – Schematic of MED System

The MED system shown on Figure 3 is also equipped with brine recycle system, which allows to introduce warmer than ambient water in the first effects of the MED system thereby (1) reducing the volume of feed water that will need to be collected by the plant intake system and the overall energy needs of the system.

The main difference between the MED and MSF processes is that while in MSF system vapor is created through flashing, evaporation of feed water in MED is achieved through heat transfer from the steam in the condenser tubes into the source water sprayed on these tubes. This heat transfer at the same time results in condensation of the vapor to fresh water.

MED desalination systems typically operate at lower temperatures than MSF plants (maximum brine concentrate temperature of 62 to 75 0C vs. 115 0C) and yield higher GORs. Newest MED technologies which include vertically positioned effects (vertical tube evaporators or VTEs) may yield GOR of up to 24 kilograms of potable water per kilogram of steam. Pumping power required for the operation of MED systems is also lower than that typically needed for MSF plants (0.8 to 1.4 kWh per cubic meter of product water). Therefore, MED is now increasingly gaining ground over MSF desalination, especially in the Middle East where thermal desalination is still predominant method for potable water production from seawater.

Vacuum Compression (VC)

The heat source for VC systems is compressed vapor produced by a mechanical compressor or a steam jet ejector rather than a direct exchange of heat from steam (see Figure 4).

Figure 4 – Schematic of VC System

In VC systems the source water is evaporated and the vapor is conveyed to a compressor. The vapor is than compressed to increase its temperature to a point adequate to evaporate source water sprayed over a tube bundles through which the vapor is conveyed. As the compressed vapor exchanges its heat with the new source water, which is being sprayed on the evaporation tubes, it is condensed into pure water. Feed water pre-heater (plate type heat exchanger) is used to start the process and reach evaporation temperature.

VC and MED work based on similar principles. However, while in MED the steam produced by source water evaporation is introduced and condensed in a separate condenser located in the downstream effect, in VC the steam generated from evaporation of new source water sprayed on the outside surface of the pressure exchanger tubes is recirculated by the vapor compressor and introduced into the inner side of the of the same pressure exchanger tubes in which it condenses to form distillate.

VC desalination has found applications mostly for small municipal and resort water supply systems, and industrial applications. The total amount of power required for the operation of mechanical VC systems is typically between 8 to 12 kWh/m3 (30 to 45 kWh/1,000 gallons) of product water.

Q4. Solar membrane distillation systems are being piloted, and Egypt is testing a low temperature distillation (LTD) system to reduce the need for heat and thus energy. Is this a viable option to save energy in desalination plants?

Yes, on a small scale for desalination plants that operate intermittently because of the natural constraints of availability of sunlight for only 6 to 8 hours per day and lack of consistently intense sunlight at al times. In a longer term the technology may develop into a viable municipal water supply option – but this will not happen for the next 5 to 10 years.

Q5. Environmental experts are concerned about the large scale (50 million gallon per day) desalination plants like the one in Carlsbad (that is under construction). Is this environment friendly, in terms of the amount of brine released into sea?

Desalination plants, large and small, are environmentally safe if they are designed and operated appropriately. Plants of the size of Carlsbad have been in operation in many other parts of the world and such plants have not caused any measurable environmental impact. What opponents of desalination often call “brine” is in fact concentrated seawater, which contains substances of natural origin – salts. Desalination is not any different than natural evaporation in terms of its impact on the environment. Both natural evaporation due to sunlight irradiation and desalination cause concentration in certain regions of the ocean without changing the chemical composition of the ocean water – so what opponents of desalination call “brine” is in fact nothing more than concentrated seawater. In fact, based on the environmental review of the Carlsbad project, we have to build around 100 desalination plants along the California coast in order to match the effect of natural evaporation of this coast.

Q6. Usage of renewable energies, like solar, photovoltaic, thermal, wind, geothermal, can be a good alternative to improve the energy efficiency of desalination plants. While usage of solar energy is getting more popular by the day. How feasible are the other alternative sources of energy?

Solar power is the least feasible alternative in terms of cost of energy production – usually the cost of energy produced by solar power desalination plants exceeds US$40 cents/kWh. For comparison, power from conventional water resources such as clean natural gas can per produced at 4 to 6 cents/kWh. Wind power is potential alternative for power supply to desalination plants but I consider it the most unreliable source of long term power supply. Wind direction and speed along the coast, which define the feasibility of near-coast wind power generation project, are determined by the direction and velocity of near-shore ocean currents. While in the past ocean current direction has remained approximately the same over long periods of time (over 50 years), recently as a result of the impacts of global warming, often near-shore ocean currents and therefore the winds they impacts are changing much more frequently – every 5 to 15 years – this means that the fact that there was a wind in a certain location does not guarantee that such wind will be available in the long term. Wind power generation is very costly as well – 30 to 40 cents/kWh. In my opinion, the most reliable future alternative source of seawater desalination plants will be wave-generated energy. Tidal wave movement direction and speed are not impacted by global warming and are the most cost-effective future source of alternative power along the coast.

Q7. Having seen the development of desalination technology over the last 30 years, what have been the major milestones according to you?

Advances in reverse osmosis membrane productivity, rejection and durability have been the main milestones for reducing the cost of desalinated water with approximately 2 times for the last 20 years. The other key milestone is the development of pressure-exchanger technology for energy recovery.

Q8. What is the future of desalination technology?

Non-reverse osmosis based desalination technologies, which apply enzyme-driven water transport through the membranes rather than osmotic pressure. We still have a lot to learn from nature in terms of low-energy separation of salts from water.

Q9. What has been the most exciting case study that you have worked in the last years?

The 200,000 m3/day Carlsbad SWRO desalination plant in California – it is a great illustration of the fact that “common sense is not that common” when it comes to desalination – the project took over 10 years to develop and implement in an environment of political hostility driven by misguided environmental groups mainly representing their own interest rather than the citizens of California as a whole. It is great to see that this project has become a reality. This is the first desalination project in the world with zero-carbon footprint.

Q10. What is your take home message to desalination practitioners around the world?

Seawater desalination is the future backbone of reliable and drought-proof water supply worldwide – the life on this planet has originated in the ocean and it is only natural to consider it our main nurturing source of water. But we still have a lot to learn from nature – for example, dolphins can produce fresh milk in highly saline environment without using large amount of energy and intake of fresh water – if they can - we should be able to do it one day too – hopefully in not so distant future.

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Taxonomy

• Desalination
• Thermal Desalination
• Solar Desalination
• Desalination Plant