Negative effects of activated carbon and ultrafiltration membrane technology
Negative effects of activated carbon and ultrafiltration membrane technology
Microbial growth. Activated carbon technology and ultrafiltration membrane technology are essentially physical filtration, so it will become a hotbed of microorganisms, and the health system experts will spend 10 days or so when using a water purifier without a bacteriostatic (sterilization) device. The index will be greatly increased. Regardless of whether the ultrafiltration membrane is used for positive or recoil cleaning, the effect is not satisfactory. Besides, 7 days, the consumer is not annoying?
The nitrite concentration is high. Activated carbon and ultrafiltration membrane technology are the best solutions for dealing with organic pollution and residual chlorine. However, everything has two sides. In areas with serious organic pollution and obvious turbidity, this technology will lead to an increase in microbial indicators, resulting in nitrite concentration. Increase, its most direct performance is that the water purifier takes a period of time, the water quality will become salty! Not only affects the taste, but also seriously threatens human health.
Let me talk about the application of activated carbon in water purifiers.
Since foreign countries have restricted the development of activated carbon, some domestically produced activated carbons have been developed in the same way as the international brands of activated carbon after so many years of development.
There are many types of activated carbon, and the use of domestic water treatment technology is generally compressed activated carbon, granular activated carbon, and powdered activated carbon (the latter two are also called loose carbon).
From the material point of view: the material is divided into coal, coconut shell, fruit shell and so on.
From the adsorption effect: powdered activated carbon is larger than granular activated carbon than compressed activated carbon (can also play the role of pp cotton)
How ultrafiltration works:
Under the action of a certain pressure, the water and small molecule solute permeate through the membrane to become a permeate, while the macromolecular solute membrane is retained as a concentrate. There are three main situations in the ultrafiltration process:
1 is adsorbed on the surface of the filter membrane and in the pores (substantially adsorbed);
2 is retained in the hole or discharged from there (blocked):
3 mechanically trapped on the surface of the filter membrane (screening).
Features of ultrafiltration: First, its working range is very wide. It can separate bacteria, E. coli, heat source, virus, cavity particles, macromolecular organic substances, etc. in water treatment, and can also be used for the separation of special solutions. Filtration can be carried out at room temperature, so separation, concentration, refining, etc. of heat sensitive substances such as pharmaceuticals, protein preparations, fruit juices, enzyme products, etc., will not affect product quality; third, no phase change occurs in the ultrafiltration process, so The consumption is low; the fourth is that the ultrafiltration process is the driving force of the pressure, so the device has a simple structure, convenient operation and easy maintenance. Therefore, ultrafiltration has developed rapidly. In the past 10 years, the average annual growth rate of ultrafiltration membranes worldwide has been around 12%.
Process of ultrafiltration membrane in water treatment application
1 pre-treatment
Ultrafiltration can be used as a pretreatment of the process or as an advanced treatment of the process in water treatment and other industrial purification, concentration and separation processes. In the widely used water treatment process, it is often used as a means of deep purification. According to the characteristics of hollow fiber ultrafiltration membranes, there are certain requirements for pre-water supply treatment. Because suspended solids, colloids, microorganisms, and other impurities in the water adhere to the surface of the membrane, the membrane is contaminated. Due to the relatively large water flux of the ultrafiltration membrane, the concentration of trapped impurities on the membrane surface rapidly increases to produce a so-called concentration polarization phenomenon. More seriously, some very fine particles enter the membrane pores and block the water passage. In addition, viscous substances formed by microorganisms and their metabolites in water also adhere to the surface of the membrane. These factors can lead to a decrease in the permeability of the ultrafiltration membrane and a change in the separation performance. At the same time, there are certain limits on the ultrafiltration water supply temperature, PH value and concentration. Therefore, the ultrafiltration water supply must be properly pretreated and adjusted to meet the water supply requirements to extend the service life of the ultrafiltration membrane and reduce the cost of water treatment.
A, the killing of microorganisms (bacteria, algae):
When the water contains microorganisms, some of the trapped microorganisms may adhere to the pretreatment system, such as the media surface of the multi-media filter, after entering the pre-treatment system. When it adheres to the surface of the ultrafiltration membrane, it grows and propagates, which may completely block the micropores and even completely block the hollow fiber lumen. The presence of microorganisms is extremely harmful to hollow fiber ultrafiltration membranes. Microorganisms such as bacteria and algae in raw water must be taken seriously. In the water treatment project, an oxidizing agent such as NaClO or O 3 is usually added, and the concentration is generally 1 to 5 mg/l. In addition, UV sterilization can also be used. The hollow fiber ultrafiltration membrane module is sterilized in the laboratory and can be circulated for 30 to 60 minutes with hydrogen peroxide (H 2 O 2 ) or potassium permanganate aqueous solution. Killing microbes can only kill microorganisms, but it does not remove microorganisms from the water, only preventing the growth of microorganisms. Process of ultrafiltration membrane in water treatment application - Shanghai seawater treatment equipment - Zhengzhou water treatment equipment
B, reduce the influent turbidity:
When the water contains suspended solids, colloids, microorganisms and other impurities, the water will have a certain degree of turbidity. The turbidity will hinder the transmission of light. This optical effect is related to the size, size and shape of the impurities. The turbidity of water is generally expressed in terms of corrosion degree, and the turbidity produced by 1 mg/l SiO 2 is 1 degree. The larger the degree, the more impurities are contained. There are different requirements for water supply turbidity in different fields. For example, for general domestic water, the turbidity should not exceed 5 degrees. Since the measurement of turbidity is to measure the amount, color, and opacity of light reflected by the particles in the water through the raw water, the size, quantity, and shape of the particles affect the measurement, and the relationship between the turbidity and the suspended solids is random. For particles smaller than a few microns, turbidity is not reflected.
In the membrane treatment, the precise microstructure, the retention of molecular or even ionic particles, the use of turbidity to reflect the water quality is obviously inaccurate. In order to predict the tendency of raw water pollution, an SDI value test was developed.
The SDI value is mainly used to detect the amount of particles such as colloids and suspended solids in water, and is an important indicator for characterizing the influent water quality of the system. The method for determining the SDI value is generally to use a microporous membrane with a pore size of 0.45 μm under a constant water flow pressure of 0.21 MPa, first recording the time t0 required to filter the water through 500 ml of water, and then continue to pass the water under the same conditions. At 15 min, the time t15 required to filter the 500 ml water sample was recorded again, and then calculated according to the following formula: SDI = ( 1-t0/t15 ) × 100/15
The value of SDI in water roughly reflects the degree of colloidal contamination. Well water SDI < 3, surface water SDI above 5, SDI limit value is 6.66 ..., that is, pretreatment is required.
Ultrafiltration technology is most effective in reducing the SDI value. SDI=0 is treated by hollow fiber ultrafiltration membrane. However, when SDI is too large, especially large particles have serious pollution to hollow fiber ultrafiltration membrane. In the process, pretreatment must be carried out, that is, using quartz sand, activated carbon or a filter equipped with various filter materials. There is no fixed mode for the treatment process, because the water supply source is different, so the pretreatment method is also Different.
For example, for tap water or groundwater with lower turbidity, 5 to 10 μm precision filters (such as honeycomb, melt-blown and PE sintered tubes) can be reduced to about 5. Before the precision filter, it is necessary to add flocculant and filter with double or multi-layer media filter. Under normal circumstances, the filtration speed should not exceed 10m/h, 7-8m/h, and the slower the water filtration rate. The better the quality of filtered water.
Removal of C, suspended solids and colloidal substances:
For impurities with a particle size of 5 μm or more, a filter with a filtration accuracy of 5 μm can be used, but fine particles and colloids between 0.3 and 5 μm are difficult to remove by the conventional filtration technique described above. Although ultrafiltration has an absolute removal of these particles and colloids, the hazard to hollow fiber ultrafiltration membranes is extremely serious. In particular, colloidal particles carry a charge, which is a polymer of matter molecules and ions. The colloid can be stably present in water, mainly because the colloidal particles of the same charge are mutually exclusive. A charged substance (flocculant) opposite to the colloidal particles is added to the raw water to break the stability of the colloidal particles, and the charged colloidal particles are neutralized to be electrically neutral to cause the dispersed colloidal particles to aggregate into large agglomerates. It can then be removed relatively easily by filtration or sedimentation. Commonly used flocculants are inorganic electrolytes such as aluminum sulfate, polyaluminum chloride, ferrous sulfate and ferric chloride. Organic flocculants such as polyacrylamide, sodium polyacrylate, polyethyleneimine, and the like. Since the organic flocculant polymer can neutralize the surface charge of the colloidal particles, form hydrogen bonds and “bridgeâ€, and the coagulation sedimentation is completed in a short time, so that the water quality is greatly improved, so in recent years, the polymer flocculant has been replaced. The trend of inorganic flocculants.
At the same time as the flocculant is added, a coagulant such as a pH regulator lime, sodium carbonate, an oxidizing agent chlorine and a bleaching powder, a reinforcing agent underwater shifting agent and a sorbent polyacrylamide may be added to improve the coagulation effect.
The flocculant is often formulated as an aqueous solution, which is added by means of a metering pump or directly into the water treatment system using an ejector mounted on the water supply line.
D, removal of soluble organic matter:
Soluble organic matter cannot be completely removed by flocculation, multi-media filtration and ultrafiltration. Oxidation or suction methods are currently used.
(1) Oxidation method using chlorine or sodium hypochlorite (NaSO) for oxidation is better for removing soluble organic matter. In addition, ozone (O 3 ) and potassium permanganate (KMnO 4 ) are also good oxidants, but the cost is slightly higher.
(2) Adsorption method Activated carbon or macroporous adsorption resin can effectively remove soluble organic matter. However, it is still necessary to treat the alcohol, phenol, etc. which are difficult to adsorb.
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