High-Recovery Desalination and Water Treatment
KEVIN L. ALEXANDER
Hazen and Sawyer
There are many water supply challenges facing government, municipal, commercial, and industrial industries throughout the United States and around the world. As demand for water continues to increase and conventional water supplies are depleted, alternative water supplies are being developed. The processes to treat the recovered waters must create a water supply that lasts well into the future without a legacy of environmental challenges.
Alternative water supplies being considered include low quality surface water, irrigation runoff, brackish groundwater, municipal and industrial wastewater, and seawater. These water sources can be low quality and substantially more expensive to treat than conventional water sources, but when the cost of treating the alternative source drops below that of the currently available source, opportunity is created.
The production of high-quality water often requires implementation of desalination technologies, which use energy in the form of pressure, heat, or electricity to remove salt from water, resulting in high energy consumption and production of a high-salinity waste stream. Solutions to these challenges may be found in new technologies and combinations of new and conventional technologies.
Thermal (i.e., distillation) processes have been used for desalination since the 1960s and remain prevalent in areas that have cheap energy sources or waste heat. Although there have been advances in thermal technologies, including multistage distillation and multistage flash distillation, thermal processes are still more expensive than reverse osmosis (RO) membrane technology.
RO desalination technology, which was incubated at universities with funding from the US government, is used in many industries that require high-quality and low-salinity water. The first commercial RO desalination project in 1971 was to produce less than a few thousand gallons per day for Texas Instruments. Today, projects requiring well over 100 million gallons per day (MGD) (378.5 megaliters per day, MLD) are being implemented and others over 1 billion gallons per day (3.785 billion liters per day) are being considered.
Advances in the field of desalination include improvements in fundamental materials, manufacturing techniques, packaging techniques, and mechanical energy recovery techniques. In most cases, the objective has been to improve operational aspects of the desalination technology to achieve maximum water recoveries at the lowest energies possible while minimizing high-salinity waste flows. Improvements have reduced temperatures, operating pressures, and electrical parameters to near theoretical levels for applications in many types of water supplies. Other advances include treatment chemicals that allow for additional water recovery by extending the saturation limits of salts in solution.
Ideally, the salt and dissolved constituents remaining after the water is separated from the source water are highly concentrated or in solid form. Achieving high water recovery minimizes the high-salinity waste stream that must be discharged. With an increasingly stringent regulatory environment, many areas of the United States will not allow discharge of these streams even to wastewater treatment facilities.
The technologies being developed to take advantage of the opportunity costs that are available with the rising cost of water (due to limited supplies) vary in their approach to minimize both brine streams and energy requirements. They include controlled-scaling RO, closed-circuit desalination, forward osmosis, electrodialysis metathesis, membrane distillation, capacitive deionization, and brine-bulb technologies. Some of these technologies result from incremental improvements to existing technologies while others represent entirely new concepts.
This paper reviews these technologies and applications. It defines the opportunities and opportunity costs to show the major drivers in the market that are attracting investors and inventors alike to the field of desalination technologies.
HOOVER DAM: A WATER SUPPLY SOLUTION EXAMPLE FOR THE NEXT GENERATION
In the 1920s and 1930s, during the Great Depression, the US government saw an opportunity to get the economy moving again by building the Hoover Dam and other major infrastructure projects along the Colorado River. The government foresaw the need for more water supply in the major urban and agricultural areas developing from Colorado through California and determined that taming the Colorado River would provide such a supply. Construction of the Hoover
Dam enabled the region to grow exponentially and contributed significantly to its economy and security.
The Hoover Dam project provides a stable and sustainable source of fresh water within and beyond the contributing watershed. Although it has brought cross-state and cross-border supply challenges, the Hoover Dam and others on the Colorado River are great examples of managed water supply solutions. The project also ensures lower total dissolved solids (TDS) water for the region, a further benefit compared to brackish sources used throughout the lower Colorado River states. Although the system of dams has been criticized for its environmental impact on the Colorado River, the advantages from power supplied, water storage, and social/recreational and economic benefits have been significant. However, in recent years the project’s storage and drought mitigation capacity are being tested as demand for water increases and it becomes clear in the drought-stricken region that there is not as much water available as was originally predicted.
Today’s challenges require leaders, planners, and researchers to look to the next 100 years and assess the opportunities and risks of water supply solutions. Desalination technologies, when looked at in this context, may be the Hoover Dam of this and future generations. They provide access to potential water supplies such as brackish groundwater, wastewater, and seawater, sources not typically considered because cheaper and more abundant sources were available. And they allow for immediate access to water in aquifers and the ocean and to wastewater, all of which are drought-proof sources. Water supply solutions of this generation must be able to defend against drought and unpredictable weather. One of the most important considerations when looking at long-term solutions that use lower-quality water sources is the ability to remove constituents and contaminants to levels that make the water quality commensurate with the proposed uses.
Desalination technology has challenges that affect the environment, such as higher energy consumption, which translates into greenhouse gas emissions. The technology also creates a residual brine or concentrate waste stream that can have an adverse impact on the environment if not managed properly.
A BRIEF HISTORY OF DESALINATION
The desalination technology of today was conceived in the 1960s, when President Lyndon Johnson supported Israel in the development of desalination through the US Department of the Interior’s Office of Saline Water. The Israelis invested effort in technology using freezing in a vacuum, but the technology was not commercialized and did not receive widespread acceptance. However, from the research and development efforts two technologies were developed: multistage flash distillation (MSF) and multieffect distillation (MED) technology. They are still in use in desalination projects around the world.
While Israel was developing desalination technology, the US government was supporting the development of reverse osmosis. Sidney Loeb at UCLA had
developed a semipermeable cellulose acetate RO membrane that was capable of removing salt from water. The challenge was in commercializing the technology. The US government saw an opportunity and hired General Atomic to develop a commercial product using RO membranes. In 1966 ROGA, a division of General Atomic, hired Richard G. Sudak to develop the technology for commercial applications. ROGA developed the first commercial applications and in 1971 sold the first RO system to Texas Instruments. Since then, both thermal and RO desalination technologies have seen widespread application and improvements.
DESALINATION TECHNOLOGIES TODAY
Desalination technology has been used in many fields to achieve specific water quality. For example, in the power industry distillation technology is used to generate service water and potable water. In the oil refining industry, RO membranes are used to generate 2,000-pound boiler feed water, which requires very low hardness and silica. In the field of microchip manufacturing, RO membranes are used to generate 18 megohm water for specialized washing. And for municipal water, RO membranes are used for treating wastewater to meet drinking water standards for applications in indirect and (soon) direct potable water supply.
The most significant advance in RO technology has been improvements in the membrane material. In the early 1990s the industry moved away from cellulose acetate to a polyamide composite, which reduced energy requirements from 300–400 psi for brackish water applications to near theoretical osmotic pressures of 100–200 psi. The main disadvantage of the latest membrane material is that it is not oxidant tolerant and is therefore more susceptible to fouling during operation.
RO membrane manufacturing changed to automated processes starting in early 2000. Prior to that, membranes were manufactured by hand gluing and rolling. Hand rolling was challenged by quality control and a loss in membrane area. Automated rolling and manufacturing have increased the membrane area by greater than 10 percent within the systems. Automated rolling also allowed for packaging changes from 8" to 16" and 18" diameter membrane elements.
The major advances for the MSF and MED technologies have been in materials improvements to address corrosion and heat transfer. A wide range of materials can now resist corrosion and are more economical, reducing the energy consumption of the technology. In addition, because the technologies work with vacuum, they have addressed operational challenges with the aid of better sealing technology.
Energy recovery devices and approaches to energy optimization in desalination have provided significant opportunity. In recent years, there have been more energy recovery devices introduced into the market, including high-efficiency pressure exchange devices that use corrosion-resistant ceramic materials. The energy consumption for seawater desalination using RO with energy recovery has declined from 11–14 kWh per 1,000 gallons (kgal) to 8–11 kWh/kgal. In
the MSF and MED technology, the energy consumption associated with process improvements and materials has decreased from 15 kWh/kgal to 13 kWh/kgal.
Challenges remain in efforts to achieve the objective of desalination technology: to maximize water recovery while minimizing high-salinity waste stream and at the lowest possible energy consumption.
Challenge 1. The amount and type of salt in the water are directly proportional to how much water can be recovered. As a salt solution becomes concentrated by the removal of water, it approaches saturation, with the liquid salt crystalizing and becoming a solid. The crystal formation can form a scale on the equipment and damage the RO membranes or the equipment. The difficulty in predicting when crystal formation and scaling will occur is compounded when there are significant numbers of different types of salts in a solution and each has a different saturation level. Controlling the scaling and crystal formation becomes a major treatment challenge, with hours spent analyzing the water quality and treatment technology to determine whether and where scaling could occur.
Challenge 2. The higher the salinity, the more pressure or electrical energy required to remove the salt from the solution. For seawater at around 35,000 mg/L of salinity, the theoretical pressure required to overcome the osmotic pressure and reverse the flow of water through the membrane from the higher saline side to the clean or fresh water side of the membrane is approximately 700 psi. For sewage in a normal wastewater plant, the pressure required to overcome osmotic pressure is approximately 20–30 psi.
Challenge 3. The remaining high-salinity waste flow must be handled as part of the overall treatment process, but there are not many practical places to discharge the waste stream. Historically, the most likely discharge locations have been back to the ocean for seawater desalination plants on the coast, into streams and lakes where there are higher volumes of fresh water to dilute the flow in inland areas, into sewers for eventual treatment in sewage treatment plants, and in some locations such as Florida there is the possibility of injecting the waste flow back into the ground through injection wells. In the right environments, such as the arid regions of the country, there are opportunities for evaporation and enhanced evaporation. Unfortunately, with most of the options, the remaining water in the high-salinity waste stream cannot be recovered as a potential water source.
Limited locations and the inability to discharge flows have made desalination a challenge. However, with the ever increasing cost of water where supplies are limited, there are opportunities. For example, current and projected cost of importing and treating surface water in San Diego are projected to be $1,926 per acre foot ($6.13/kgal) by 2021. The cost of desalinating seawater is currently between
$1,500 and $3,000 per acre foot ($4.60 and $9.20/kgal) depending on location and project specific considerations along the California coast.
The cost of seawater is increasing as well due to environmental, permitting, and other concerns. However, treating sewage to drinking water is much lower, at less than $1,000 per acre foot ($3.06/kgal), and treating groundwater in the Riverside area is around $625 per acre foot ($1.90/kgal).
NEW DESALINATION TECHNOLOGIES
Because the cost of water is rising rapidly, there is opportunity for desalination technology development to allow for the production of water at a lower cost than the projected cost of importing or treating seawater in areas such as southern California. In the desalination industry, companies such as GE and venture capitalists are exploring the market and investing to capitalize on the opportunity.
Investment in research on new and improved technologies for recovering water from impaired water sources has led to many different approaches to treating the water. Some technologies that show promise and are gaining acceptance and experience are described below:
Controlled scaling RO (CSRO) allows a third or fourth stage of RO to treat the concentrate from a primary RO system. The technology operates in the final stage of the RO system beyond the theoretical saturation and uses cleaning chemicals to remove scale from the membranes to restore performance. The system is operated beyond saturation for some constituents such as calcium carbonate, calcium sulfate, silica, and potentially others. CSRO is operated in a forward and reversing operation to control scaling and membrane life. The system is cleaned on a frequent basis with various cleaners, acids, and bases to keep the membranes operating. This type of system is used at Water Replenishment District of Southern California.
Desalitech™ uses closed circuit desalination or batch desalination. The high-salinity waste stream is recirculated through the RO unit, allowing for recoveries on a batch basis as high as 97 percent. This technology uses conventional RO equipment operated in a different configuration. At the end of a batch the high-salinity waste stream is discharged and fresh water filled into the feed tank and recirculated. The system operates on low-TDS water conditions that are typical for inland desalination and wastewater desalination projects. The technology has some limitations in comparison to straight RO, such as varying water quality from the beginning to the end of each batch.
Forward osmosis uses a high-salinity stream as a draw solution to pull water from lower-salinity feed water. The feed water can be from a number of sources such as wastewater effluent, high-salinity waste streams from desalination systems, brackish well water, or other high-salinity sources as long as the salinity of the feed is much lower than the draw solution. The difference in salinity between
the feed and draw solutions provides the energy to move the water across the membrane. The draw solution in some cases is a special solution that can be separated from the water. Alternatively, this technology could be used in a seawater application, with ocean water drawing fresh water from a brackish water source, followed by dilution of the seawater to reduce salinity, reducing the feed pressure and energy required to desalinate the ocean water. This could be a viable way to treat seawater using wastewater as a source of pure water while improving the economics and environmental impact.
Brine bulb technology uses AC current across a brine stream to generate heat for evaporation under a vacuum condition, using various technologies to improve the efficiency of the water recovery in a batch process. The technology combines electrocoagulation and vapor removal to separate the salt from the solution. The benefit of the system is that it allows for further recovery of the water.
Dewvaporation, developed at Arizona State University, is a specific process of humidification-dehumidification desalination (patented as AltelaRain®) that uses air as a carrier-gas to evaporate water from saline feeds and form pure condensate at constant atmospheric pressure. The heat needed for evaporation is supplied by the heat released by dew condensation on opposite sides of a heat transfer wall. Because external heat is needed to establish a temperature difference across the wall, and because the temperature of the external heat is variable, the external heat source can be from waste heat, such as solar collectors or fuel combustion. The unit is constructed of thin wettable plastics and operated at atmospheric pressure. The technology is currently sold in the oil field but could have applications in concentrate treatment.
Other desalination technologies on the cutting edge include capacitive deionization and membrane distillation. Companies such as GE are investing in their Aquasel desalination technology, high-efficiency RO(HERO), and electrodialysis metathesis. All of these promising technologies are being tested in various applications.
SOLUTIONS FOR FUTURE GENERATIONS
The Hoover Dam solved many water supply issues for future generations. It provided a water source and energy supply source of immense volume that enabled unfettered growth in the western United States. At the time it also improved the regional economy with all of the jobs associated with the project, and that economic boost continues with jobs, tourism, energy, and water storage. The dam and the reservoir it created, Lake Mead, help to ensure a water quality balance for the region. The effects of saltwater tributaries to the Colorado River are mitigated by stored water, reducing the salinity for lower Colorado River users.
When the dam and lake were created, they offered a drought-proof water
supply. Today the limits of the water supply have been reached and the Lake is being impacted by climate change and extended drought conditions in the region.
In today’s ever changing water landscape, leaders in the water industry need to consider the next generations and what solutions today will solve water supply and water quality challenges long into the future. Desalination coupled with high-recovery technologies are the tools that will be used to solve these challenges. They provide access to the next available water supplies and to water supplies once considered impossible to utilize, ensuring safe and reliable water supplies. They enable access to drought-proof water supplies such as wastewater effluent and irrigation drainage flows, and secure the region against the effects of climate change and extended drought. They require educated, trained, and skilled labor and thus yield economic and social benefits for communities. Last, the technology can offer a stable solution within the cost boundaries of the ever increasing cost of local supplies.
Desalination technologies, not unlike the Hoover Dam, are capable of solving water supply challenges. High-recovery applications are being investigated and will likely provide a path toward successful implementation of large-scale desalination in arid and inland regions with limited ability to discharge concentrate streams.
This is an exciting time in the industry to look forward and at the same time consider the past risks and rewards of visionary thinking.