Basic Wastewater Treatment Terms: What Is Seawater Desalination (A System That Produces Freshwater by Desalinating Seawater)
More than 400 years ago, the British royal family offered a reward for an economical and practical seawater desalination method.
Meanwhile, the related supporting processes and equipment have also been continuously optimized and upgraded.
In 1984, based on its seawater desalination research office, the State Oceanic Administration established the Hangzhou Water Treatment Technology Research and Development Center, and China began to attach importance to membrane technology. By then, however, American composite membranes and spiral-wound elements for seawater desalination had already been commercialized on a large scale and put into national and civilian use.
In 1992, to catch up with the world in membrane technology, the State Science and Technology Commission, relying on the Hangzhou Water Treatment Technology Research and Development Center, established the National Engineering Technology Research Center for Liquid Separation Membranes and quietly began developing domestic reverse osmosis membranes.
Before the 21st century, reverse osmosis membrane technology was monopolized by foreign countries, and China did not master reverse osmosis membrane production technology until the late 1990s. This history dates back to the early years of the People's Republic, when national leaders had already recognized the prospects of seawater desalination and its future role in society.
The cumulative seawater desalination capacity of projects built and under construction in China is about 600,000 tons/day. According to policy planning, the market capacity of this industry will grow more than fivefold within ten years, with fairly optimistic prospects. The cost of desalinated seawater has dropped to 4-5 CNY/ton, greatly improving economic feasibility. Considering future cost reductions brought by technological progress and policy support, the seawater desalination industry is expected to see explosive growth.
There are more than 20 seawater desalination technologies worldwide, including reverse osmosis, low-temperature multi-effect distillation, multi-stage flash distillation, electrodialysis, vapor compression distillation, dew point evaporation, combined water and power production, integrated thermal-membrane systems, and nuclear/solar/wind/tidal-energy desalination, as well as microfiltration, ultrafiltration, nanofiltration and other pre- and post-treatment processes.
Broadly, desalination falls into two categories: distillation (thermal) and membrane methods, with low-temperature multi-effect distillation (LT-MED), multi-stage flash (MSF) and reverse osmosis (RO) being the mainstream technologies worldwide. Generally, LT-MED offers energy savings, low feedwater pretreatment requirements and high product water quality; RO features low investment and low energy consumption but requires stringent seawater pretreatment; MSF is mature, reliable and large in output but relatively energy-intensive. LT-MED and RO are generally considered the future direction. During the 12th Five-Year Plan period, China's seawater desalination capacity was expected to reach 1.5-2 million tons/day, three to four times the existing capacity, with an investment scale of about 20 billion CNY.
Principle of freeze desalination: the triple point of seawater is the special point where the vapor, liquid and solid phases of seawater coexist in equilibrium. If pressure or temperature deviates from this triple point, the equilibrium breaks down and the phases automatically tend toward one or two phases. Vacuum freeze desalination exploits this triple-point principle, using water itself as the refrigerant to make seawater evaporate and freeze simultaneously; the ice crystals are then separated and washed to obtain desalinated water — a low-cost method. Compared with distillation and membrane desalination, freeze desalination consumes less energy, causes less corrosion and scaling, requires simple pretreatment and small equipment investment, and can handle high-salinity seawater, making it a fairly ideal desalination method.
In addition, many factors must be considered in actual engineering applications.
Reverse osmosis seawater desalination is developing rapidly, with project costs and operating expenses continuing to fall. The main development trends are lowering RO operating pressure, increasing system recovery, developing cheap and efficient pretreatment, and enhancing system fouling resistance.
Notably, technologies and standards in this field are also constantly developing and improving.
The key to this method is the development of new ion-exchange membranes. An ion-exchange membrane is a functional membrane sheet 0.5-1.0 mm thick; by selective permeability it is classified as a cation-exchange membrane (positive) or anion-exchange membrane (negative). In electrodialysis, cation and anion membranes with selective permeability are arranged alternately to form multiple independent compartments: seawater in alternating compartments is desalinated while that in adjacent compartments is concentrated, so freshwater and concentrate are separated. Electrodialysis can not only desalinate seawater but also serve as a water treatment method contributing to wastewater reuse. It is also increasingly used for concentration, separation and purification in the chemical, pharmaceutical and food industries.
Distillation heats seawater to boiling and condenses the vapor into freshwater. It was the first desalination technology put into industrial application; it is applicable even in heavily polluted, highly bioactive seawater environments and produces very pure water. Compared with membrane desalination, distillation can utilize low-grade heat from power plants and other factories, has low feedwater quality requirements and large unit capacity, making it one of today's mainstream desalination technologies.
Dew point evaporation desalination is a new method for brackish water and seawater. It is based on humidifying and dehumidifying a carrier gas while recovering the heat of condensation; its heat transfer efficiency is controlled by heat transfer on the gas-mixture side. Using air as the carrier, freshwater is produced by humidifying and dehumidifying with seawater or brackish water, and the dehumidification and humidification processes are coupled through heat transfer, so the latent heat of condensation is passed directly to the evaporation chamber to provide latent heat of vaporization for the brine, raising the thermal efficiency of the process. Two humidification/dehumidification-coupled dew point evaporation desalination units with effective heat transfer areas of 9.6 m2 and 2.75 m2 were built, along with corresponding experimental apparatus and computer data acquisition systems, and the basic dew point evaporation process, parameter correlation experiments and enhanced heat/mass transfer desalination experiments were successfully completed.
Moreover, from an industry development perspective, market demand is also driving technological progress.
Flash evaporation refers to the phenomenon in which part of seawater at a given temperature evaporates rapidly when the pressure suddenly drops. In MSF desalination, heated seawater evaporates successively in multiple flash chambers at progressively lower pressures, and the vapor is condensed to obtain freshwater. MSF still accounts for the largest output among global desalination plants, with the most mature technology, high operational safety and flexibility; it is mainly built in conjunction with thermal power stations and suits large and super-large desalination plants, mainly adopted in Gulf countries. MSF is technologically mature and reliable; its development trends are increasing single-unit water output, reducing unit power consumption and improving heat transfer efficiency.
Notably, technologies and standards in this field are also constantly developing and improving.
Seawater desalination has half a century of development and industrial application, during which MSF, reverse osmosis and multi-effect evaporation have become the representative industrial technologies. Experts generally agree that these three technologies will remain dominant in industrial applications for the next 30-40 years, but the share of reverse osmosis will grow. Regionally, Middle East Gulf countries will continue to prefer MSF for its large and super-large scale (single units already produce 40,000-50,000 tons of freshwater per day), its suitability for heavily polluted Gulf water and its low pretreatment costs; outside the Middle East, reverse osmosis or membrane methods will be preferred because of their energy and cost advantages. As North America's development has shown, membrane methods will dominate desalination and water treatment.
Global desalinated water output is about 35 million cubic meters per day, 80% of which is used for drinking water, solving the water supply problem for more than 100 million people — one in fifty of the world's population relies on desalination for drinking water. There are more than 13,000 seawater desalination plants worldwide. As a substitute and incremental technology for freshwater resources, desalination is receiving increasing attention from many coastal countries; about 600 billion cubic meters of seawater is used directly as industrial cooling water each year, replacing large amounts of precious freshwater; some 50 million tons of salt, over 2.6 million tons of magnesium and magnesium oxide, and 200,000 tons of bromine are extracted from the ocean annually. Desalination requires large amounts of energy, so its economic benefits are limited in less affluent countries.
As a technology that opens up new freshwater resources, seawater desalination has become an important way to address the global water crisis. By 2006, more than 120 countries and regions were applying seawater desalination; global output was about 37.75 million tons/day, 80% for drinking water, solving the water supply problem for more than 100 million people.
Although China's seawater desalination basically meets the conditions for industrial development, it still lags considerably behind foreign countries in research level and innovation capability, equipment development and manufacturing capability, and system design and integration. The urgent priority is to quickly form a complete industrial chain for China's seawater desalination equipment market. Focusing on the key issues constraining the reduction of desalination costs, China should develop core technologies such as membranes and membrane materials and key equipment, develop new desalination technologies, processes, equipment and products with independent intellectual property rights, raise the localization rate of key materials and equipment, and strengthen the capability to build large seawater desalination projects independently.
The international seawater desalination market will offer nearly US$70 billion in business opportunities over the next 20 years, and China should secure a full share. According to the national seawater utilization special plan, China's desalination capacity was to reach 800,000-1,000,000 tons/day by 2010 and 2.5-3 million tons/day by 2020. The state actively supports the desalination industry: from January 1, 2008, income from corporate seawater desalination projects has been exempted from income tax. The prospects for China's seawater desalination industry are broad.
Meanwhile, the related supporting processes and equipment have also been continuously optimized and upgraded.
The company claims that the new technology is far more efficient than existing reverse osmosis and similar processes. Tests show it can treat water with salinity as high as 16%, well beyond the concentration of ordinary seawater. On average, it costs 16-27 US cents to produce 1,000 liters of freshwater. Scientists say the device can also treat wastewater. RSE technology achieves a recovery efficiency of up to 95%, versus only 35% for conventional technologies; the investment is only one quarter of that for distillation and reverse osmosis, and operation and maintenance costs are greatly reduced.
China's seawater desalination technology developed under government support and key national research programs, with considerable progress in the research and development of electrodialysis, reverse osmosis and distillation (multi-stage flash, vapor compression and low-temperature multi-effect). Research on electrodialysis desalination first began in 1958; in 1967-1969 the State Science and Technology Commission and the State Oceanic Administration jointly organized a national seawater desalination campaign, whose main force established the country's first seawater desalination research office in Hangzhou (the Seawater Desalination Research Office of the Second Institute of Oceanography, SOA — the predecessor of the Hangzhou Water Treatment Technology Research and Development Center), while research on electrodialysis, reverse osmosis, distillation and other technologies was carried out, laying the foundation for the development of the desalination cause.
In 1965, the Chemistry Department of Shandong College of Oceanology was the first in China to study CA asymmetric reverse osmosis membranes; in the 1970s it studied hollow-fiber and spiral-wound RO membranes and elements and initially industrialized them. Since the 7th Five-Year Plan, the development of RO seawater desalination technology has been listed as a national key research project: medium- and low-salinity RO membranes and modules were developed and demonstration projects built during the 7th Plan period; great progress was made in medium-salinity RO membranes during the 8th; the 9th Plan period saw successful pilot-scale scale-up of new polyamide composite membranes, which, combined with the introduction of key technologies and equipment, are now in production.
In 2000, supported by the Ministry of Science and Technology's major research program, the Hangzhou Water Treatment Center built 1,000-ton/day RO seawater desalination demonstration projects on Changdao Island, Shandong and Shengsi, Zhejiang at the same time, pioneering the application of internationally advanced work-exchange energy recovery devices that reduced water production energy consumption to 4 kWh/ton, reaching the internationally advanced level. The completion of these demonstration projects laid the foundation for the rapid development of China's RO seawater desalination technology, and the project won the Second Prize of the Marine Science and Technology Innovation Award of the State Oceanic Administration in 2001.
In 2002, supported by an NDRC industrialization project, the center built a 10,000-ton/day RO seawater desalination demonstration project in Rongcheng, Shandong. Through optimized design, equipment investment dropped substantially and project economics improved further, forming proprietary technology with independent intellectual property rights, achieving significant economic and social benefits, and driving the rapid development of China's membrane separation technology and especially the RO seawater desalination industry — thus creating an effective technical route for water use by coastal enterprises and institutions and for residential water supply. In 2004, the project was awarded the First Prize of the First 'Science and Technology Award of the China Membrane Industry Association'.
In the 1960s, the Shanghai 704 Research Institute of the former Ministry of Shipbuilding Industry developed 5 m3/d vapor compression distillation desalination units and flash desalination units using the waste heat of diesel engine jacket water for shipboard use. In the 1970s and early 1980s, the Tianjin Science and Technology Commission supported pilot-scale research on 100-ton/day multi-stage flash desalination, yielding certain design parameters and experience. After the 1980s, the Institute of Seawater Desalination and Multipurpose Utilization, State Oceanic Administration (Tianjin) carried out development of 30 m3/d vapor compression distillation units, including a 30 m3/d vertical-tube atmospheric pressure VC unit, a 30 m3/d horizontal-tube negative-pressure VC unit (operating temperature 72 C) and a 30 m3/d OTE/VC unit. The achievements and problems encountered in the above work accumulated rich experience for later research and played an important role in advancing China's distillation desalination technology.
On this basis, industry experts have also carried out a great deal of research and improvement.
The '12th Five-Year Plan for the Seawater Desalination Industry' set out five main tasks. First, strengthen technological innovation, rely on technological progress, enhance innovation capability, and accelerate the R&D of core desalination technologies and key components to quickly improve core competitiveness; second, strengthen equipment manufacturing, increase design and R&D efforts, optimize the design and manufacturing technology of single units and complete plants, and improve the manufacturing capability of key equipment and complete plants; third, emphasize engineering demonstration, improve process design and engineering construction capability, and rely on key projects to promote the industrialization demonstration of core and key technologies; fourth, accelerate the application of desalinated water, expand its use, improve utilization efficiency and benefits, and give play to its safeguarding role; fifth, establish standards and norms to promote the healthy development of the seawater desalination industry.
Moreover, from an industry development perspective, market demand is also driving technological progress.
The Ministry of Water Resources is considering whether to incorporate seawater desalination into its overall planning and related plans such as local water resource allocation. To this end, a major research project of the Ministry in 2014 was 'The Status, Problems and Development Countermeasures of Seawater Utilization in China'.
The Chinese government attaches great importance to seawater desalination and has adopted a series of measures to promote the development of the industry. During the 11th Five-Year Plan period, China's desalination developed rapidly: by the end of 2010, more than 70 desalination plants had been built with a designed capacity of 600,000 cubic meters/day and an average annual growth rate of over 60%; technologies with independent intellectual property rights achieved breakthrough progress, with clear advances in RO seawater membranes, high-pressure pumps and energy recovery devices, and salt rejection improved from 99.2% to over 99.7%; the conditions for industrial development were in place and the desalination market had basically taken shape; governments at all levels attached great importance to desalination, and all sectors of society paid close attention to the industry. At the same time, problems remain, such as insufficient understanding of the strategic significance of desalination, weak independent innovation capability, a low level of industrial development and inadequate supporting policies. Therefore, coordinated planning and accelerated development of the desalination industry is an essential and urgent task. In accordance with the spirit of the Opinions and on the basis of thorough research and extensive consultation, the National Development and Reform Commission organized the compilation of the '12th Five-Year Plan for the Seawater Desalination Industry' (FGHuanZi [2012] No. 3867).
In addition, the technology also has wide applications and practice in related fields.
The keys to seawater desalination are desalting efficiency and the relationship with the environment and energy — the evaluation of any method cannot be separated from environmental impact and energy consumption. Water recovery is 35-55%; the desalination process is also a seawater concentration process, so increasing the freshwater yield raises the enrichment of valuable elements in the concentrated brine, providing good conditions for chemical resource recovery.
