Current trends and future prospects in the design of seawater reverse osmosis desalination technology
Highlights
► Technological innovations are trying to improve the reverse osmosis process. ► A comprehensive review of them is presented for seawater reverse osmosis. ► Reverse osmosis membrane and rack design improvements are deeply described. ► Special focus is placed on the use of renewable energies for medium/ large capacities.
Introduction
Desalination is an increasingly common solution to supply fresh water in many regions of the world where this resource is scarce. Among all desalination technologies, seawater reverse osmosis (SWRO) is the most internationally widespread technology. It is fully mature and is present in all coastal areas in the world with limited natural hydrological resources. Mainly for these reasons the advances and the R&D on this technology are continuous. Basically, main recent innovations, as well as existing research lines, focus on trying to further reducing the process energy consumption. In addition, researchers focus in minimizing the negative effects of scaling and fouling on membranes and obtaining membranes with higher permeate flux [1].
In the previous context, this paper focuses on the comprehensive description of innovations and reliable future trends in the design of SWRO desalination technology. In the compendium of previous innovations, it cannot remain outside the consideration of using renewable energy for large-scale desalination plants as a highly innovative trend. This should help to balance the progressive dependence on fossil fuels.
SWRO has superseded distillation from entry into the XXI century and will represent the technology with a highest degree of current and future implementation. From 2005 to 2008, annual worldwide contracted capacity of RO technology increased from 2.0 million to 3.5 million m3/d, whereas the annual values of overall distillation processes were between 2.0 million and 0.5 million m3/d [2]. The 61.1% of the worldwide capacity installed, including seawater and brackish water desalination, is attributable to RO. The status of desalination market is thoroughly reported in reference [3].
At the end of the 1970s, SWRO plants consumed up to 20 kWh/m3 [4], but thanks to the development of more efficient membranes, the use of energy recovery devices, new materials with less friction and variable-frequency drive devices, the energy consumption has been reduced drastically. Excellent specific energy consumption (SEC) as low as 1.80 to 2.20 kWh/m3 could be obtained in new SWRO plants [5], [6], as follows: SWRO plants with capacities up to 1000 m3/d have led achievable energy consumption below 2.0 kWh/m3 [7], [8]. This is due to the use of highest-efficiency energy recovery devices based on isobaric chambers and positive displacement pumps that can have yields over 90%. Currently, in medium–large capacity SWRO plants the RO process energy consumption is usually between 2.2 and 2.5 kWh/m3 [9], [10].
An example of installations with SEC lower than 2.00 kWh/m3 is located in Sal Island — Cape Verde. 1000 m3/d SWRO plant (64.90 mS/cm seawater conductivity) equipped with a RO Kinetic® energy recovery device obtains a specific energy consumption of 1.98 kWh/m3 [8].
Besides that, the most relevant desalination experience is the following. Affordable Desalination Collaboration (ADC) is a Californian non-profit organization composed of leading U.S. companies and agencies in the desalination industry. The ADC, since 2004, has agreed to pool its resources and to share its expertise in the mission to achieve the affordable seawater desalination. Using a combination of energy efficient, commercially available RO technologies including axial piston pumps, last generation membranes and ERI® high-efficiency energy recovery equipment, the ADC has demonstrated that SWRO can produce water at a cost and energy consumption rate comparable to other alternative water supplies. With a variable RO capacity of 200–300 m3/d, the lowest RO process energy consumption of 1.58 kWh/m3 was demonstrated using the Filmtec XLE membranes at a water flux of 244 l/(m2 d) — 42.5% of recovery rate [7].
Within this context, in the following sections the main innovations detected are described. These are divided into three categories — agreeing with which part of the technology process affects:
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RO membrane and rack design improvements: These innovations will ensure to obtain the best design of the main process unit, the pressure vessel (PV). They are innovations that will modify membranes in use, their constructive characteristics and dimensions as well as how to locate them inside a PV.
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SWRO desalination plant designs: These enhancements will impact mainly on external elements of the membranes rack or the process in general. With these, lower energy consumption, specific water quality or improvements in the pre-treatment and post-treatment generally used are guaranteed.
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Energy supply — application of renewable energies in the medium–high capacity SWRO desalination plants: This innovation, with a view to the future, will enable an energy contribution in quality and quantity of current energy supply based on fossil fuel.
Section snippets
Last generation membranes
Current research regarding spiral wound membranes is focused on providing greater filtration surface within the same volume. These elements have an increased productivity as well as a higher salts rejection [11], [12], [13]. At the present, membrane manufacturers offer membranes of low-energy and high-productivity up to 47.5 m3/d (12,500 gpd).
Besides that, membrane manufacturers work in developing membranes with high boron rejection due to the requirement to reduce the boron content in the
High efficiency of energy recovery devices
After more than a decade of using pressure exchange systems in desalination plants, the maturity of this technology is enabling the stable reduction of specific energy consumption of the desalination process. Periodically, new systems with better materials come onto the market, achieving greater efficiencies and lengthen the equipment lifetime. The challenge in these devices is to reduce the barrier of the 2.00 kWh/m3 in medium and large capacity plants in the desalination process. To date, this
Energy supply innovations. Application of renewable energies in medium–high capacity seawater reverse osmosis plants
Like water, energy is a scarce resource in many countries of the world and is becoming a source of instability in many regions. About 80% [49] of the energy produced in the world comes from greenhouse gas emitting sources (coal, oil, natural gas…) which contribute to the planet-global warming and air pollution. Since desalination processes exhibit intensive energy consumption, the desalination worldwide capacity – more than 60 million m3/d – results in significant energy consumption. From this
Conclusions
Seawater reverse osmosis (SWRO) technology is fully mature and commonly used at industrial scale at present in all coastal areas around the world that have limited natural hydrological resources. It has superseded distillation processes from the beginning of the XXI century and will continue to be the technology with the highest degree of current and future implementation.
There are many technological advances and innovations which achieve significant improvements in the process. In particular,
Acknowledgments
The authors wish to thank the European Commission for their financial assistance with the POWERSOL project — Mechanical power generation based on solar heat engines (FP6-INCO2004-MPC3-032344) within the International Cooperation Activities Programme.
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