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Desalination for cost-effective water production

In the last four decades there has been a steady increase in the use of desalination for low salinity, high quality water production from high salinity or low quality source water. This increase has been due to improvements in the desalination technologies used, a steady decrease over time in the cost of producing desalinated water compared to treating fresh water from diminishing quality surface or ground water sources, and stricter government regulations for wastewater discharges, making wastewater reclamation an increasingly viable alternative to treating fresh water.

MISHRA DHANANJAY

by Dhananjay Mishra

Director - Water Treatment, Houston

04 August 2017
Desal plant

Seawater desalination represents the largest segment of global desalination capacity at roughly 60%1, but the use of other water sources, such as brackish groundwater, river water, and both domestic and industrial wastewater has been steadily increasing. Similarly, although about 60% of installed desalination capacity is for potable use, a significant portion of new desalination capacity is for industrial uses in areas like power plants, oil fields, and refineries, and a growing segment of industrial desalination involves wastewater treatment and reuse.

The majority of desalinated water is made using one of two general approaches: thermal evaporation or pressure-driven membrane separation. The specific technologies within each of these areas are typically chosen based on the quality of the available source water and the cost of energy at the project site.

Multi-Stage Flash (MSF) Distillation

MSF distillation plants produce about 60% of all desalinated water in the world, and the MSF process is considered to be the most cost-effective process for the production of fresh water from seawater in places where the cost of energy is relatively low, such as in the Arabian Gulf.

In an MSF distillation plant, cold feed water is pumped through successively staged preheaters until it enters the hottest stage where low pressure heating steam (typically < 50 psig) is used to heat the feed water to above 100ºC under pressure, starting the flash process. This hot feed water flows back through the process to the previous lower pressure stage, where a portion of it flashes into vapor. The vapor generated condenses on the preheater in that stage and is collected as distillate. The remaining brine continues to flow back through the stages, with pressure and temperature decreasing from one stage to the next. A small fraction of the brine flashes to steam and drops in temperature in each stage. Typically, an MSF plant contains between 15 and 25 stages. The total desalinated water product rate from this process is typically under 25% of the incoming feed water, depending on the operating temperatures and source water.

Because of the low recovery and therefore large amount of feed water required in an MSF plant, a portion (50% to 75%) of the brine from the last stage is sometimes mixed with the incoming feedwater, recirculated through the heat recovery sections of the brine heater, and flashed again through all of the stages. This mode of operation increases recovery, reduces the amount of water treatment chemicals that must be added, and can significantly lower operating costs. However, the increase in Total Disolved Solids (TDS) of the evaporating brine raises its boiling point temperature and can increase corrosion and scaling, so in order to properly manage brine properties in this system, a significant portion of the concentrated brine must always be discharged.

MSF is considered an energy-intensive desalination process, and requires both thermal and mechanical energy, but higher temperature waste heat can sometimes be used as the primary heat source. MSF systems are also very adaptable to using solar energy as the primary heat source. For this reason, it should be considered where energy is relatively inexpensive and/or solar energy is abundant.

 desal figure 1

  Figure 1 – Diagram of typical MSF process for seawater desalination2

 

Advantages and disadvantages of MSF

  • MSF is considered an energy intensive desalination process, and requires both thermal and mechanical energy, but higher temperature waste heat can sometimes be used as the primary heat source.
  • The quality of the desalinated water made is very good and contains as little as 10 ppm TDS.
  • The quality of the feed water is not as important as when using Reverse Osmosis (RO) technology; the water does not require significant pretreatment beyond filtration to remove solids.
  • Higher operating temperatures (over 115°C) will improve efficiency, but can cause scaling problems on the tube surfaces, and may create thermal, mechanical, and corrosion problems.

Multi-Effect Distillation (MED)

The MED process is the oldest desalination process, and was first used in the 1800s. Beginning about 1960, MSF distillation began to dominate the desalination field, but recent developments in MED technology have allowed MED to once again compete technically and economically with MSF. MED systems are also gaining market share due to better compatibility with indirect solar desalination and a greater suitability for lower capacity applications compared to MSF. An MED evaporator consists of multiple interconnected evaporation stages or effects that are maintained at decreasing levels of pressure and temperature, from the first (hot) to the last (cold) effect. Each effect contains a tube bundle heated by the next hottest effect. The top of the bundle is sprayed with high TDS feed water that flows over and around the outside of the tubes, producing the hot water vapor. The hot water vapor generated from the hottest effect flows into the tubes of the next coolest effect, where it heats and evaporates more feed water and condenses to become distillate. Each effect thus reuses the latent heat energy from the previous effect. Because each effect must evaporate feed water at a lower temperature than the preceding effect, the pressure of each effect must also be lower than the preceding effect. The hot distillate produced from the condensed vapors of each effect’s tube bundles is collected and combined to become the low TDS desalinated water product. The feed water that does not evaporate becomes the high TDS waste stream leaving the process. The primary heat source for the hottest effect is typically low pressure steam or waste heat from a power plant or other industrial process. The hottest temperature at which the feed water is evaporated is typically below 70ºC.

Desal fig 2

Figure 2 – Diagram of a typical MED process for seawater desalination1

Advantages and disadvantages of MED 

  • The MED process operates at lower temperatures compared to MSF (~70°C vs. >100°C), which minimizes tube corrosion and the potential of scale formation on the tube surfaces. However, operation below 100°C requires that a vacuum is maintained on the evaporation system.
  • The power consumption of MED is lower than that of MSF, due to operation at lower temperatures.
  • The quality of the feed water is not as important as with RO technology, and pretreatment, and sometimes operational costs of MED, are relatively low as a result. 
  • The performance efficiency (lb. steam required per lb. distillate produced) in MED plants is higher than that in MSF plants so the MED process can have a lower desalinated water production cost compared to MSF.

Mechanical Vapor Compression (MVC) Desalination

The MVC evaporation process uses electrical/mechanical energy to compress and elevate the temperature of the low-pressure steam evaporating from the feed water flowing into the process. This higher temperature steam is used to heat the boiling brine. Nearly 100% of the latent heat of the water vapor made is thus returned to the brine and little or no external heating steam is required, depending on water recovery and other process factors. That is, the compressor provides all or nearly all the energy to evaporate the water.

MVC desalination is particularly suitable to very high TDS and/or highly scaling wastewaters, such as oil field brines, power plant Flue Gas Desulfurization (FGD) wastewater, and RO system reject. It operates at or slightly above atmospheric pressure, so no vacuum needs to be maintained. Very high distillate recoveries above 95% of the incoming feed water may be possible, limited by the TDS and boiling temperature of the brine. MVC desalination can be used for non-scaling brines that mainly contain Sodium Chloride (NaCl), but it is also suited to brines saturated in minerals such as Calcium Sulfate (CaSO4) and/or Silica, which precipitate in the process. In the case of a CaSO4, the concentration of the precipitating solids in the evaporating slurry is purposely elevated by internally recycling the solids to increase their concentration. In that way, precipitation occurs on the solid “seed” rather than on the heat transfer surfaces of the evaporator.

Because precipitated solids can be handled in the MVC process, the solute limits are based on “total solids,” or the sum of TDS plus TSS (Total Suspended Solids) in the evaporating brine. At a total solids level below 300,000mg/l, a vertical thin-film evaporator can be used, and a forced circulation crystallizer can be used for total solids levels up to about 600,000 mg/l. In both cases, the atmospheric pressure boiling point of the brine is typically limited to 116ºC (240 ºF).

 Figure 3 – Typical MVC evaporator schematic3

Advantages and disadvantages of MVC Evaporation

  • Energy costs are typically lower than MSF or MED, and the equipment footprint is smaller.
  • Much higher distillate recoveries and brine concentrations can be reached, compared to MSF and MED (85% - 97% vs. 15% - 25%).
  • MVC evaporation can be used to desalinate high TDS feed water (higher TDS than seawater). However, high TDS brines and high recoveries typically require the use of corrosion resistant metallurgy, such as duplex stainless steels, titanium, or high nickel alloys.
  • MVC evaporation is suited to feed water with relatively low TDS but high hardness (calcium and magnesium), and requires minimal pretreatment compared to the use of RO for this type of feed water.  

Reverse Osmosis (RO) Desalination

RO desalination is a membrane separation process in which a high TDS feedwater stream is pressurized to overcome natural osmotic pressure and pumped past a polymeric membrane that lets water pass through, but rejects charged ionic species and compounds with molecular weights greater than about 100. A low TDS product water stream (permeate) is produced on the low-pressure side of the membrane, leaving a more concentrated brine stream on the high-pressure side of the membrane. The process is conducted at ambient temperature, and the only energy required for desalination is pumping energy.

Because RO systems operate at ambient temperatures and are relatively energy efficient compared to thermal evaporators, they are increasingly employed in the desalination of seawater to make potable water. RO systems are also used in other large-scale industrial separation applications, such as in the treatment of wastewater, reclamation of minerals, and the concentration of whey and other food products. RO is used for the desalination of process water ahead of an ion exchange demineralization process for the production of boiler feedwater for high-pressure industrial boilers. RO is used to treat domestic and industrial wastewaters to remove salts and other contaminants in order to reuse the wastewater or meet strict discharge limits for certain contaminants. RO has also been used to reclaim wastewater for indirect reuse as potable water.

Thermal evaporation technology was the dominant desalination technology applied worldwide up until the evolution and improvements in the performance of RO membrane desalination technology during the 1980s, which resulted in the adoption of RO desalination in preference to thermal evaporation from that time forward. RO is now the predominant technology applied globally for brackish water desalination and for large seawater desalination facilities.

A typical RO plant consists of a pretreatment system to remove fine TSS, organic or microbial contaminants, and sometimes hardness ions such as Ca+2 and Mg+2; chemical treatment to control feed water pH and mitigate scaling by sparingly soluble compounds, high-pressure pumping, RO membranes; and a post-treatment system, if required, to demineralize the water, stabilize it by removing dissolved gases not rejected by the membranes, or to purify it for potable use. The introduction of and significant performance improvements in ultrafiltration (UF) technology over the past decade have led to the adoption of UF as the preferred pre-treatment for RO systems. The UF process provides an RO feedwater with virtually no particulate matter compared with media filtration, which has resulted in an increase in typical RO membrane life from five years to up to 10 years.

 Figure 4 – Typical seawater to demineralized water process4

Advantages and disadvantages of RO Desalination

  • Membrane operating pressures and membrane costs have dropped significantly over the past decade and membrane durability has increased, which has lowered RO process costs relative to MSF, MED, and MVC evaporation. Depending on the feed water pretreatment required and energy costs, RO system capital and operating costs can be lower for a given capacity than for thermal processes. RO desalination is typically more cost effective than thermal desalination for small capacity plants. Energy recovery devices can be used to recover pressure energy from the RO concentrate stream, decreasing overall energy costs further.
  • Membrane scaling caused by the precipitation of salts is a common problem. Membranes are also susceptible to fouling by fine TSS, oil, and microbes. Pretreatment to mitigate scaling and fouling can add significant cost to the overall process and must be carefully evaluated for each case. This can be a significant issue when using wastewater as feed. Lower TDS, high-hardness feedwaters can be desalinated using RO if they are pretreated using chemical or ion exchange softening to remove hardness, and in this case, recoveries of up to 90% may be feasible.
  • Feed TDS is typically limited to that of seawater (i.e., up to 45,000 mg/l) due to the operating pressure limitations of the technology. Feed pressures are typically limited to 800 psig, and SWRO recoveries typically limited to 45%. However, recently, higher pressure RO membranes have been introduced that operate at up to 1400 psig, potentially increasing RO recovery on seawater beyond 45%.
  • Material corrosion problems are significantly less compared with MSF and MED due to operation at ambient temperature. Polymeric materials can be utilized in place of metal alloys in parts of the RO system.        
  • Single-pass RO permeate quality is typically not as good as the distillate quality from thermal processes. If very low TDS product water is required, multi-pass RO systems and/or demineralization post-treatment can be added, but this adds cost and complexity to the overall RO process.

Desalination Technology Summary

The application of desalination technology to produce low salinity, high quality water from high salinity or low quality source water continues to increase due to improvements in desalination technologies and a steady decrease in the cost of producing desalinated water compared to treating fresh water from low quality sources, as well as stricter government wastewater discharge regulations that make wastewater reclamation a viable alternative to using fresh water. Although seawater desalination represents the largest segment of global desalination capacity, the use of brackish water, river water, and wastewater sources are steadily increasing. Desalination for potable water production is well established, but a significant portion of new desalination capacity is for industrial use, and much of this involves wastewater treatment and reuse.

The majority of desalinated water is made using thermal evaporation technology (MSF, MED, MVC) or membrane separation technology (RO). MSF distillation is particularly suited to the production of fresh water from seawater where the cost of energy is relatively low. The MED process can compete technically and economically with MSF in certain cases. It can use lower grade heat sources than MSF, has better compatibility with indirect solar desalination, and is more suitable for lower capacity applications. MVC desalination is particularly suitable to very high TDS and/or scaling wastewaters, such as oil field brines, power plant wastewater, and RO system reject, and can achieve very high distillate recoveries compared to MSF or MED.

RO desalination requires less energy than any of the thermal processes, and because RO systems operate at ambient temperature and are energy efficient compared to thermal evaporation systems, RO has been increasingly employed in the desalination of seawater to make potable water. Although feed water pretreatment can be extensive and relatively costly for certain low quality source waters, RO membrane operating pressures and pumping costs have dropped significantly during the last several decades, and membrane durability has increased. This continues to lower RO process costs relative to MSF, MED, and MVC evaporation for many types of desalination projects.

References

  1. Matteo, Gazzani. (2015). “Sea Water Desalination: Thermal Desalination vs. Membrane”. Separation Processes Laboratory, ETH Zurich.
  2. Henzell, S. (2010). “Desalination Presentation to JVCEC March 2010”. WorleyParsons.
  3. Heins, W. and Peterson, D. (2006). “Use of Evaporation for Heavy Oil Produced Water Treatment”. GE Water & Process Technologies.
  4. www.aquatech.com (2016). “Project Profile #26 Largest Power Plant in Philippines Uses Membrane Desalination for Purified Water”. Aquatech.

For more information contact: Dhananjay Mishra | dhananjay.mishra@advisian.com  

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