The reverse osmosis method can effectively remove inorganic substances, organic substances, bacteria, pyrogens and other particles dissolved in water. To understand the "reverse osmosis" principle, we must first explain the concept of "osmosis." The so-called osmosis refers to the use of a semi-permeable membrane to separate two different concentrations of the solution, where the solute cannot penetrate the semi-permeable membrane, the lower concentration One water molecule will pass through the semi-permeable membrane to reach the other one with higher concentration, until the concentration on both sides is equal. Before the equilibrium is reached, pressure can be gradually applied to the one with higher concentration, and the aforementioned state of water molecule movement will Temporarily stop, the required pressure at this time is called "osmotic pressure". If the applied force is greater than the osmotic pressure, the movement of water will go in the opposite direction, that is, from the high concentration side to the low side. On the one side of the concentration, this phenomenon is called "reverse osmosis." The purification effect of reverse osmosis can reach the level of ions, and the removal rate of monovalent ions can reach 90% -98%, while divalent ions can be Up to 95% -99% (can prevent the passage of substances with molecular weight greater than 200 Dalton).
Semi-permeable membrane materials commonly used for reverse osmosis water treatment include fibrous membranes, aromatic polyamines, polyimide or polyfuranes, etc. As for its structural shapes, there are spiral wound, hollow fiber, and tubular types. (Tubular) and so on. As for the cellulose membrane of these materials, the advantage is high chlorine resistance, but under alkaline conditions (pH ≥ 8.0) or in the presence of bacteria, the service life will be shortened. The disadvantage is poor tolerance to chlorine and chlorammonia.
If the pre-treatment is not done before the reverse osmosis, there is a tendency for dirt to accumulate on the osmosis membrane, such as calcium, magnesium, iron and other ions, causing the reduction of the reverse osmosis function; some membranes are easily damaged by chlorine and chlorammonia. The membrane should be pretreated with activated carbon and softener.
The debugging of reverse osmosis system is particularly important. We can learn from the following aspects:
Operating conditions Preparation for test run before operation Separation process There are several common processes in reverse osmosis membrane separation process design:
① One-stage one-stage method This method is that after the feed liquid enters the membrane module, the concentrated liquid and the produced water are continuously led out. In this way, the water recovery rate is not high and there are fewer industrial applications. The other form is a one-stage one-cycle process, which returns part of the concentrated water to the feed tank, so that the concentration of the concentrated solution is continuously increased, so the water production is large, but the quality of the produced water is reduced.
② One-stage multi-segment method When reverse osmosis is used as the concentration process, when a single concentration fails to meet the requirements, this multi-step method can be used. In this way, the volume of concentrated liquid can be reduced, the concentration can be increased, and the water production can be increased accordingly.
③ Two-stage and one-stage method When the desalination rate of seawater requires NaCl to be reduced from 35000 mg / L to 500mg / L, the desalination rate is required to be as high as 98.6%. That is, the first step is to remove 90% of NaCl, and the second step is to remove 89% of NaCl from the effluent of the first step, which can meet the requirements. If the membrane has a low salt removal rate and high water permeability, it is more economical to use a two-step method. At the same time, it can increase the membrane's service life when it is operated at low pressure and low concentration.
④Multi-stage reverse osmosis process In this process, the first-stage concentrated liquid is used as the second-stage feed liquid, and the second-stage concentrated liquid is used as the next-stage feed liquid. Both are discharged directly to the body, so as the number of stages increases, the recovery of water increases, and the volume of concentrated liquid decreases, and the concentration increases. In order to ensure a certain flow rate of the liquid and control the concentration polarization at the same time, the number of membrane modules should be gradually reduced.