Life of reverse osmosis membrane and identification of pollution types
2021-03-09
Reverse osmosis membrane is the core component of reverse osmosis, which is a kind of artificial semi permeable membrane with characteristics made by simulating biological semi permeable membrane. It is generally made of polymer materials. Such as cellulose acetate membrane, aromatic polyhydrazide membrane, aromatic polyamide membrane. The diameter of the surface micropores is generally between 0.5 nm and 10 nm, and the permeability is related to the chemical structure of the membrane itself. Some polymer materials have good repellency to salt, but the permeability of water is not good. Some polymer materials have more hydrophilic groups in chemical structure, so the water transmission speed is relatively fast. Therefore, a satisfactory reverse osmosis membrane should have an appropriate permeability or desalination rate.
1、 The reverse osmosis membrane should have the following characteristics:
1. It should have high desalination rate at high flow rate;
2. It has high mechanical strength and service life;
3. It can function under low operating pressure;
4. It can tolerate the influence of chemical or biochemical action;
5. It is less affected by pH value, temperature and other factors;
6. The raw material source of the membrane is easy, the processing is simple, and the cost is low.
The structure of reverse osmosis membrane can be divided into asymmetric membrane and homogeneous membrane. Cellulose acetate and aromatic polyamide are the main membrane materials currently used. Its components are hollow fiber type, roll type, plate frame type and tube type. It can be used for separation, concentration, purification and other chemical unit operations, mainly used in pure water preparation and water treatment industry.
2、 Principle of reverse osmosis
Reverse osmosis, also known as reverse osmosis, is a kind of membrane separation operation with pressure difference as driving force to separate solvent from solution. When the pressure exceeds its osmotic pressure, the solvent will reverse the direction of natural permeation. Thus, the permeated solvent is obtained on the low pressure side of the membrane, that is, the permeate; and the concentrated solution is obtained on the high pressure side, that is, the concentrated solution. If seawater is treated by reverse osmosis, fresh water is obtained at the low pressure side of the membrane and brine is obtained at the high pressure side.
In reverse osmosis, the permeation rate of solvent, i.e. liquid flow energy N, is as follows:
N=Kh（Δp－Δπ）
Where KH is the hydraulic permeability coefficient, which increases slightly with the increase of temperature; Δ P is the static pressure difference on both sides of the membrane; Δ π is the osmotic pressure difference of solution on both sides of the membrane.
The osmotic pressure of dilute solution is π
π=iCRT
Where I is the number of ions generated by ionization of solute molecules; C is the molar concentration of solute; R is the molar gas constant; t is the standard temperature.
Asymmetric membrane and composite membrane are usually used in reverse osmosis. The equipment used in reverse osmosis is mainly hollow fiber type or roll type membrane separation equipment.
Reverse osmosis membrane can intercept various inorganic ions, colloidal substances and macromolecular solutes in water, so as to obtain purified water. It can also be used for preconcentration of macromolecular organic solution. Because of its simple process and low energy consumption, reverse osmosis has developed rapidly in recent 20 years. It has been widely used in desalination of seawater and brackish water (see brine), softening of boiler water and wastewater treatment, and combined with ion exchange to produce high purity water. Its application scope is expanding, and it has been used in the concentration of dairy products and fruit juice, as well as the separation and concentration of biochemical and biological agents.
3、 Life of reverse osmosis membrane
In the process of using the equipment, in addition to the normal attenuation of performance, the attenuation of equipment performance caused by pollution is more serious. The common pollution mainly includes chemical scale, organic matter and colloid pollution, microbial pollution and so on. Different pollution shows different symptoms. Different membrane companies have different symptoms of membrane fouling. In the project, we found that the duration of pollution is not the same, the symptoms are not the same.
For example, when the fouling time is one week, the desalting rate decreases rapidly, the pressure difference increases slowly, and the change of water yield is not obvious. The pollution time was one year (a water purifier). The salt flux increased from 2mg / L to 37mg / L (raw water: 140mg / L ~ 160mg / L), and the water yield decreased from 230L / h to 50L / h. After cleaning with citric acid, the salt flux decreased to 7Mg / L, and the water yield increased to 210L / h. Moreover, pollution is often not single, and its symptoms are different, which makes the identification of pollution more difficult.
4、 Identification of fouling types of reverse osmosis membrane
To identify the type of pollution, the raw water quality, design parameters, pollution index, operation records, equipment performance changes and microbial indicators should be considered
1. Colloid pollution
When colloid pollution occurs, it is usually accompanied by the following two characteristics: A. in the pretreatment, the micro filter is blocked very quickly, especially the pressure difference increases very quickly. B. The SDI value is usually above 2.5.
2. Microbial contamination
When microbial contamination occurs, the total number of bacteria in permeated water and concentrated water of RO equipment is relatively high, and maintenance and disinfection are not carried out as required. To prevent the performance damage of ultrafiltration RO membrane. New reverse osmosis membrane elements are usually soaked with 1% NaHSO3 and 18% glycerine solution and stored in sealed plastic bags. If the plastic bag is not broken, it will not affect its life and performance even if it is stored for about one year. When the plastic bag is opened, it should be used as soon as possible to avoid adverse effects on the components due to oxidation of NaHSO3 in the air. Therefore, the membrane should be opened before use. After the trial run of the equipment, we used two methods to protect the film. The equipment has been put into trial operation for two days (15-24h), and then maintained with 2% formaldehyde solution; or after running for 2-6h, 1% NaHSO3 aqueous solution is used for maintenance (the air in the equipment pipeline shall be drained to ensure that the equipment does not leak, and all inlet and outlet valves shall be closed). Both methods can get satisfactory results. The former method is more expensive and can be used when idle time is long, while the second method can be used when idle time is short.
3. Calcium scale
It can be judged according to raw water quality and design parameters. For carbonate type water, if the recovery rate is 75%, the LSI of the concentrated solution should be less than 1 if the scale inhibitor is added in the design; if the scale inhibitor is not added, the LSI of the concentrated solution should be less than 0, so the calcium scale will not be produced.
4. 1 / 4 inch PVC plastic pipe can be inserted into the assembly to test the performance change of different parts of the assembly.
5. Determine the type of pollution according to the change of equipment performance.
6. Acid pickling (such as citric acid and dilute HCl) can be used to judge the calcium scale according to the cleaning effect and the cleaning solution, which is further confirmed by the composition analysis of the cleaning solution.
7. Chemical analysis of cleaning solution
Take raw water, cleaning solution and cleaning solution for analysis. After determining the type of pollution, it can be cleaned according to the method in 1, and then disinfected for use. When the type of pollution cannot be determined, cleaning + disinfection + 0.1% HCl (pH = 3) is usually used.
