Scale-up Strategies for Membrane-Based Desalination Processes

Membranes based technologies have been applied for water desalination processes which evidenced by the large capacity RO water desalination plants that constructed all over the world in recent years.

The scarcity of fresh water due rapid growth of population and industries. Saline water either brackish water or seawater through the desalination process has received an increasing attention in recent years. More than 17,000 water desalination plants have been operated worldwide with an average production rate of 66.5 million m3/d.

Two categories for desalination technology:

1-     Thermal Based Desalination

2-     Membranes Based Desalination 

Specific energy consumption, water cost, OPEX (operation expenditure) and environmental impacts are the set of parameters that are used to determine the most desirable process to be applied.

High permeate water quality, simple operation and design, plant flexibility, small foot print, lower energy consumption, environmentally friendly and cost effectiveness are the prominent advantages that forced membranes to become the major separation techniques for water desalination process compared with the conventional technologies.

A continuous improvement in membrane materials of construction for higher salts rejection and operating at higher recoveries is a challenge.

It has been reported that 60% of the global desalination capacity is dominated by RO plants, In spite of membrane based desalination has a several advantages but membrane based processes are challenged by limitations. The major obstruction is fouling and scaling phenomena.

For larger capacity of membranes based desalination plant, space requirement, number of components of membranes elements, pressure vessels, piping and instrumentation being used and large quantities of waste disposal (Concentrate brine water) where the brine discharge management becomes a critical concern due to its impact on the environment. These aforementioned factors should be taken in consideration during design and scale up steps of membranes based desalination process.

Membranes based desalination is categorized as :

1-     Electrically based desalination

2-     Pressure based desalination

3-     Thermally based desalination  

Membranes based desalination process that have been commercialized since 1987 up to present time, involve pressure driven membranes, electrically driven membranes and thermally driven membranes.

Seawater contains almost 3.5 g/L of salts components, which means that almost 97% of seawater content is water. Actually, the desalination process seems to be more economically and efficient for treating high salty water through the membranes. However, Researchers are focused on the improvement of membrane characteristic due to fouling potential where RO membranes are prone to fouling (Accumulation of foreign materials on the surface of RO membranes that impact their performance)

Improvements of membranes surface (Hydrophilic characteristic of membranes becomes a challenge to minimize the fouling potential to reverse osmosis membranes which offers superior water permeability than other components in sweater desalination. It is considered more efficient, high permeability and less energy consumption compared with the early RO technology.

1- Reverse Osmosis (RO)

RO is a pressure driven membrane process where the membrane acts as the selective barrier for a particular components while partially or completely blocking others. Feed solution which contains dissolved solids is divided into two streams where in the salt components are rejected into the concentrate stream while the almost pure water is produced in the permeate side. Improvement in RO technology including advances in membranes material, module and process design, pre-treatment, and energy recovery has led to cost reduction which in turn gains interest for its commercial applications. In addition, RO is considered as simple to design and operate compared with other desalination processes. Since the end of the 1970’s, energy consumption of SWRO has been reduced significantly due to process improvement. The optimization of RO membrane configuration (single, two-pass or multiple stages) is required in large-scale design since the specific membrane cost is higher than the specific energy consumption. The two pass of the RO unit are used when the target solute cannot be accomplished in a single pass. However, from an energy consumption point of view, a single pass of RO unit results in a lower specific energy cost than the two-pass process. Up to the present time, there is a continuing investigation on process development of RO technology to achieve a better separation at lower energy demand.

In 1999, approximately 78% of the world’s seawater desalination capacity was made up of multistage flash (MSF) plants while RO represented 10%. Nowadays, most desalination plants use membrane-based process, representing 60% of the total number of worldwide plants. A typical example of the largest SWRO desalination plant was commissioned in 2013 with a production capacity of 624,000 m3/day potable water. The system incorporates a 16″ RO element which is arranged vertically and uses 100,000 m2 land area. The energy consumption is minimized and expected to produce water with a maximum energy consumption of 4 kWh/m3 and contains 0.3 ppm of boron. Every element of the plant was customized to minimize investment costs and environmental impacts. The lower investment costs are achieved by several strategies, such as decreasing the number of pressure vessels, piping headers, control and instrumentation equipment and reducing its footprint.

2-Nano Filtration

 NF is a pressure driven membrane separation process that employs membrane as a selective barrier, which has characteristics between RO and the ultrafiltration (UF) membrane. NF provides higher fluxes than RO and better rejection for small molecules than the UF membrane. NF is able to remove turbidity, microorganisms, hardness, and a fraction of dissolved salts. The membrane used in NF is typically asymmetric with an active top layer. It combines the sieving effect and Donnan’s effect for removing uncharged and charged components respectively. The NF membrane has been widely applied in several areas such as desalination and concentration. Separation and purification drinking water and wastewater treatment. Since NF exhibits high rejection for multivalent salts, it is then introduced as pretreatment for SWRO desalination that improves RO permeate flux. By integrating NF as SWRO pretreatment in a pilot scale of the desalination plant, a higher recovery ratio of RO could be achieved, this is contributed to the reduction of fresh water cost up to 27%. In the other case, the elimination of the scale forming constituent by NF may possibly increase the high top temperature brine (TTB) in the MSF desalination process and prevent the scale formation on desalination equipment, particularly on the heating surface. The scale-up of this NF-SWRO integration plant was constructed at Umm Lujj, Saudi Arabia that increased the SWRO unit water recovery from 26% to 56%. As SWRO pretreatment, NF was found to be successful in removal of turbidity, significant removal of hardness and lowering of the seawater TDS that could improve SWRO performance. In addition, the application of NF can reduce chemical consumption used in conventional pretreatment. However, NF membranes are also prone to fouling that can decrease its performance. 

Fouling occurs due to precipitation of inorganic components such as CaCO3 or CaSO4, accumulation of suspended matters, organic substances, micro-organism or bacteria. High operating pressure is also the disadvantage of using NF as pretreatment. The high pressure leads to high energy consumption that affects the overall operation cost. Another problem for NF and for pressure-driven membrane filtration in general, is the required further treatment of the concentrate stream. Another typical application of NF is water softening which shows advantages such as lime softening. The results of the study show that operation and maintenance costs of lime softening are lower than NF softening. However, the cost difference is decreased with larger capacity. The advantages of NF softening over lime softening are a superior product quality which has additional removal of color and turbidity, process flexibility, and no sludge formation. NF has been applied in desalination of effluent with high concentration of salts. NF was used to treat industrial effluent with a high concentration of salt namely crude iron dextran solution, iminodiacetic acid mother liquor and raw soy sauce. Results of the study indicate that NF is a viable and promising process for removing monovalent salt from the effluent (e.g. food and chemicals) because NF could effectively reject organic solutes while monovalent inorganic salt passed through easily.

3-Electro-dialysis (ED)

ED is an electrically driven membrane process that utilizes the ion exchange membrane as the selective separator for ionic substances and electrochemical potential as the driving force. ED has been used for brackish water desalination for over 50 years and the basic process has been significantly modified into several related processes. The ED related processes are conventional ED, electro-deionization, electro-metathesis, electro-dialysis with bipolar membranes. ED and related processes are comprised of components including direct current supply, electrode, ion exchange membranes and solvents and electrolytes. An ED module (stack) contains the cation exchanger membrane (CM) and anion exchanger membrane (AM) which are employed as active separators.

Both of the membranes are packed in alternating arrangement between electrodes (anode and cathode) while spacer is inserted in between them to form an individual compartment. A stack generally consists of several pairs of dilute and concentrate compartments and a pair of electrode compartments. When an electrolyte solution is transferred through those compartments and an electrical potential is supplied from the electrodes, the cations and anions migrate towards the cathode and anode, respectively. The cations pass through the CM and are excluded by AM. Otherwise, the anions pass through the AM and are excluded by the CM. The ion concentration of solution is depleted in the dilute compartment while it is concentrated in the concentrate compartment. ED has been considered as a reliable process in water desalination for more than half a century. Moreover, ED exhibits several advantages compared to its competing processes such as RO, distillation and conventional ion exchange that includes:

? Very little pre-treatment is required (compared to RO) membrane fouling and scaling is reduced to minimum due to periodic reverse polarity

? More brine concentration can be achieved (compared to RO) there is no osmotic pressure limitation

? The membrane has long durability

? Lower energy and investment cost (compared to distillation)

? No chemical regeneration is required (compared to conventional ion exchange)

However, ED also has several limitations, which correspond to technical and economic factors. The disadvantages of the ED process are

? Uncharged species are not eliminated

? Relatively high energy consumption is required when processing solution with high salt concentration

? The investment cost is relatively high for producing dilute with very low concentration.

? ED is cost effective for a certain range of feed water salt concentration (Low TDS feed water)

4-Electrode-ionization (EDI)

EDI is a modified ED, EDI has continued to be an attractive deionization process with significant advantages over the conventional ion exchange deionization in the production of ultrapure water from a technological and economic standpoint. The main reason for its commercial success is that EDI eliminates the chemical regeneration process and its associated hazardous chemicals. Therefore, this process is increasingly becoming the dominant choice for producing ultrapure water.

EDI is mainly applied for water treatment, but it has also shown potentials to be applied in a number of different applications such as in wastewater treatment, biotechnology and biopharmaceutical, and other potential fields. An EDI module (stack) has similar components as those used in ED wherein ion exchange membranes are employed as active separators. In the EDI system, the dilute compartments are filled with electrically active media, such as ion exchange resins. In some cases, the concentrate and electrode compartments can also be filled, depending on the product quality requirement. By employing ion exchange media inside the cells, EDI is expected to be an effective deionization process to treat a solution with higher electrical resistance compared with ED. The ion transfer from the diluting compartment to the membrane surface is almost entirely mediated by the ion exchangers. The EDI performances for ionic separation have been compared with the ED process, e.g. elimination of nitrate, water demineralization and removal of hardness ions as well demineralization of brackish water. EDI has showed better performances than ED. Recently, the commercial applications of EDI are still focused on high purity water production, since EDI can remove weakly ionized components efficiently through extensive water dissociation. As mentioned previously, the EDI process has been commercialized in over twenty years and has gained widespread acceptance in the production of ultrapure water. The ultrapure water applications include pharmaceutical manufacturing, steam generation or power plants, microelectronics or semiconductor manufacturing and academic and clinical laboratories. Several manufacturing industries that use ultrapure water in their process are semiconductor manufacturer, microelectronic device manufacturer, solar panels or flat-panel displays. The standard design to obtain ultrapure water uses a combination of RO and EDI. With this design, the system can produce water which has specification concentration near or below detection limits. One of the features is a unique “electro-regeneration” for regenerating the resins. The resins are never fully exhausted; therefore the consumption of a chemical regenerants could be eliminated. It means that the cost for the chemical regeneration process which is required in a conventional ion exchange (IX) system including labor and chemical could be eliminated and replaced by a low cost of electricity. In addition, EDI systems can be operated in a continuous process and able to produce pure water with high resistivity.

More than 90% salt rejection could be achieved, which cannot be reached by a conventional ED.

Limitations of EDI

Nevertheless high investment cost and difficulty in repairing the module due to the complex equipment configuration are the issues that render EDI less popular than the conventional ion exchange in many large scale applications.

Scaling is also the problem faced by EDI especially in the concentrate compartment wherein the concentration of bivalent ions such as Mg2+ and Ca2+ are increased. Those components can be precipitated on the membrane surface due to local pH shift caused by concentration polarization. The resin inside the compartment also has potential problems on increasing pressure drop. The worst condition occurs when the resins are agglomerated in the compartment outlet that provides additional hydraulic resistance.

5-Membrane distillation (MD)

MD is a thermally driven membrane based separation that utilizes a hydrophobic microfiltration membrane as a selective barrier for separating the vapor phase from feed stream. The hydrophobic nature of the membrane prevents the liquid phase from entering its pores. In addition, vapor pressure difference which is generated by temperature difference across the membrane, acts as the driving force. MD process has several potential applications, e.g. production of high-purity water, concentration of ionic, concentration of colloids and other non-volatile aqueous solution, and removal of trace volatile organic compounds. The advantages of MD compared to its competing desalination processes are:

? Lower operating temperature

? Lower operating pressure than RO

? High solute rejection

? Not limited by high osmotic pressure

? Work with high solute concentration

? Modular and smaller foot print

The total cost for drinking water with membrane distillation could be less than the RO treatment depending on the source of thermal energy. A pilot scale of the solar driven membrane distillation system has been developed in Aqaba, Jordan, which was designed for a remote area. Chemical pretreatment could be eliminated and will significantly reduce the operational cost of the desalination plant. However, the solar powered membrane distillation technology is still expensive compared to other desalination processes.

Despite their advantages, MD technologies have not been commercialized yet in an industrial scale due to several barriers, such as:

? Relatively low permeate flux.

? Flux declining due to concentration and temperature polarization effects, membrane fouling and total or partial pore wetting.

? Membrane and module design for MD.

? High thermal energy consumption: uncertain energy and economic costs for each MD configuration and applications.

6- Forward osmosis (FO)

Forward osmosis is one of the emerging technologies that produce fresh water by utilizing an osmotic pressure difference across the membrane as a driving force.

FO has been investigated in a wide range of applications, including saline water desalination, clean energy generation, waste-water treatment and food processing which are attributed to a range of benefits. For example, benefiting from the low pressure required to perform FO operation, FO holds the promise of achieving low energy consumption and the operating cost as well.

On the other hand, FO also generates clean energy that is induced by the salinity gradient of fresh and saline water. Thus a combination of both functions makes FO a potential technology to face the global water problem and energy crisis. In addition, since FO requires low operating pressure and temperature, FO has potential applications in liquid food and pharmaceutical processing while maintaining the physical properties and quality of products Nevertheless, the commercialization of the FO membrane has been limited by several challenges such as severe concentration polarization, low flux membrane and the availability of appropriate draw solutions (cost effective and nontoxic) Therefore, there should be further developments in membrane materials, draw solutes and membrane fabrication to bring FO into commercialization.

7-Microbial desalination Cell (MDC)

MDC is a relatively new method that holds the promise of reduction or elimination of electricity power for the desalination process. MDC is a modified process from the microbial fuel cell. Generally MDC consists of three compartments namely anode, cathode, and center (salt) compartment. Compartments are separated by cation and anion exchange membranes. In MDC, exoelectrogenic microorganisms generate electrical potential from degradation of organic substances which are used to drive ions in saline water though the ion exchange membrane.

The exoelectrogenic bacteria oxidize substrates in water and release electrons which are transferred from the anode to cathode. The reactions occur in both anode and cathode chambers and then create a potential gradient to induce ion migration from the center of the compartment into the anode and cathode compartment. It has been reported that 90% of salt in the solution that contains 30-35 ppm NaCl concentration could be removed by MDC. Furthermore, higher salt rejection up to 99.99% could be achieved when it is applied to desalinate brackish water that contains 10 g/L of NaCl (99 %).

As aforementioned, MDC demonstrates its ability to combine energy production, desalination and waste water treatment in a simultaneous process which can be applied in wastewater treatment and the desalination field. However, the desalination efficiency of the MDC is limited by fluctuating voltages generated during anode and cathode reactions.