Abstract: This thesis is to introduce the main technological principles and applications of Reverse Osmosis (RO) and analyze the problems of membrane pollution caused during the operation of RO equipments and major purpose and trends of RO membrane. Therefore, it’s concluded that the application of RO technology will become the development tendency for treatment of boiler make-up water in power plant.
Keywords: osmosis, reverse osmosis (RO for short), sea water desalinization, membrane separation
0. Introduction
In 1953, the conception of sea water desalinization through RO technology was first put forward by Reid, et al. of Florida University in US. Later on, the famous initiator of RO membrane theory, Dr. Sourirajan, in Canadian National Institute, succeeded in innovating the first piece of unsymmetrical RO membrane with high desalinization rate and high water flux with the collaboration of his colleague Loeb. Since then, the gate towards RO technology research and application was open. In the early 1980s, the first piece of composite polyamide membrane was developed in the American governmental laboratory. Compared with cellulose membrane, it possesses a higher water flux and a higher rejection rate of salt, which accelerated the RO application greatly. RO technology is applied in a vast area in US and Japan. And in China, RO technology research could be traced back to 1966. In recent years, it’s been applied widely, from desalting sea water at the beginning, and then extended to the brackish water desalting, and food processing, medicine and health, drinking purifying and ultrapure water producing etc.. The application of RO technology brings profound economic benefits.
1. RO Separation Principles
If the concentrated solution (e.g. brine water) and dilute solution (e.g. pure water) are separated by using of a piece of semi-permeable membrane, the dilute solution will permeate into the concentrated side, keeping a corresponding osmotic pressure. This phenomenon is so called the osmosis. But if a bigger pressure is put on the concentrated solution than the dilute side, the concentrated solution will infiltrate towards the dilute one. This situation is called reverse osmosis. The size of micro-pore on RO membrane is 10 angstrom (i.e., 10×10⁻10m) commonly, with operation pressure: 1.0~10.0MPa and the divided molecular weight<500. This kind of RO membrane could entrap salt or small mol. Wt and decrease the ion content in water by 96% ~ 99%. The RO removal performance has the following regular laws:
1.1 The removal capacity of high-valence ion is bigger than that of low-valence ion.
Al3+>Fe3+>Mg2+>Ca2+>Li+
1.2 The organics removing performance of membrane is affected by its affinity and molecular structure.
Mol. Wt: high mol. Wt > low mol. Wt
Affinity: aldehydes >acids> amine
Side chain structure: the 3rd stage> mal-position >the 2nd stage>the 1st stage
1.3 For the electrolyte and the non-electrolyte with mol. Wt > 300 could be removed efficiently. For the mol. Wt: 100~300, its removal capacity is above 90%.
2. Membrane Pollution
In recent years, RO technology has been in widespread use due to its easy operation, high reliability and free of second pollution etc.. However, after a long time of operation, some colloid, metallic oxide, bacteria, organics and scale etc. will attach to the concentrate side of membrane, which will lead into the membrane pollution and cause the decrease of desalting rate and water output, and the increase of pressure difference etc. So, once the RO performance declines to a certain degree, the membrane should be cleaned timely and effectively to avoid membrane becoming too polluted to be restored. As for the necessity of system cleaning, it depends on the water output, desalting rate and change of pressure difference. If the water output drops by 10%~20% comparing with the original output or the water output after cleaning last time, or the desalting rate goes down above 10%, or the pressure difference of the equipment rises more than 15%, the RO system must be cleaned. For different pollutants, different detergents should be adopted. Cleaning the membrane is an effective measure to protect it. The following points should be cared while cleaning: the water temperature should be preferably no more than 35oC; the detergent concentration and PH value (2.0~12.0) should be well-controlled. Make sure that there is no free chlorine ion or other oxides/oxidizing matters. And the cleaning direction should be same as that of operation. Reverse cleaning is forbidden. Or else, the membrane elements will be damaged.
3. The Application of RO Membrane
3.1 For sea water and brackish water desalinization
RO membrane separation technology has been used in sea water desalinization vastly. By adopting RO membrane to desalt sea water, its maximum working pressure is 5.6~7MPa, and its energy consumption is only 1/3~1/2 of the multi-stage flash evaporation with an good water quality (in that the RO membrane stopping rate to monovalent metallic ion is more than 99.4%; for bivalent metallic ion is 99.9%; and total stopping rate for dissolved solids>99.6%). In the world, 30% of sea water desalting units is adopted RO technology. And RO membrane could entrap more than 99% of salt ion in the sea water.
3.2 For heavy-metal wastewater treatment
A wide study on heavy metal wastewater treatment has been carried both at home and abroad. Sine 1970s, RO technology has been used in treating electroplating wastewater. And for some specific projects, RO technology has also been applied in disposing the rinsing water of galvanization, nickel-plating, chrome-plating and treating the mixed heavy metal wastewater. At present, there are about 100 sets of RO equipment in China for electroplating wastewater. And its assembly mostly adopts the internal pressure tube-type or roll-type.
3.3 For pure water and ultrapure water production
For a long time, to produce the pure water is at the cost of a huge amount of acid and alkali. But the acid and alkali will bring the environmental pollution inevitably during production, transportation, storage and usage. The acid and alkali will also corrode the equipments and may harm the human body. The maintenance cost remains high all the time. The application of RO greatly reduces the consumption of acid and alkali and RO equipment is easy to operate with a steady output.
4. The conclusion
As a new branch in the field of separation science, RO membrane separation technology, developing from last 60s, has a broad and promising prospect. Based on its outstanding separation performance, RO technology will be employed in more and more areas.
Hereafter, the main developmental direction of RO technique will be:①. To continue in exploiting the new style of anti-oxidative and anti-acid / alkali membrane but with high water permeability. ②. To exploit the new RO assembly with some special characteristic, such as low energy consumption, anti-pollution, high-temperature resistance, and specific separation etc. ③. The combination and application of RO assembly and the other membrane assembly of ultra-filtration, micro-filtration, nano-filtration and EDI etc..
Bibliography
The Application and Research of Ultra-Filtration In Boiler Make-Up Water Treatment [J] Hebei Electric Power Technology, 2005(5)
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