In modern water treatment, ultrafiltration membrane (UF) and reverse osmosis membrane (RO) have become the main processes of advanced treatment. However, when the content of organic, inorganic, microbial and colloidal substances in raw water is too high, it will cause membrane surface and membrane. The accumulation of holes will cause blockage of the membrane surface. It will affect the operating life of the membrane. The use of activated carbon as a pretreatment of UF can remove most of the organic matter in the sewage, reduce COD, thereby alleviating the problem of UF membrane and RO membrane blockage and membrane fouling, and prolong the membrane use time. The removal rate of organic matter with relative molecular mass of 500~3000 is generally 70%~86.7%. However, the organic matter with relative molecular mass >3000 can not effectively remove it. However, UF has good removal of organic matter of macromolecules. rate. Therefore, the combination of activated carbon and UF has a good effect on removing organic matter.
1. Activated carbon has been used in water treatment processes for more than 70 years. In the 1960s, activated carbon adsorption was widely used in the advanced treatment of urban water, urban sewage and industrial wastewater. It is a traditional mature water treatment technology.
2. According to the raw water quality and treatment process, the carbon type and the dosing point are selected. The reasonable process is adopted in the process of adding and adding, which is of great significance for improving the removal rate of organic matter.
3. Activated carbon adsorption technology has the advantages of low equipment investment, low price, fast adsorption speed, and strong adaptability to short-term and sudden water pollution.
4, powdered activated carbon and ultrafiltration membrane combined with organic sewage treatment • Can complement each other, better effect on the removal of organic matter in sewage.
[Features of powdered activated carbon]
The main component of activated carbon is carbon, which contains about 90% of carbon. It is usually made from various plant scraps (wood chips, coconut shells and bagasse) or appropriate coal or charcoal. There are many very fine pores inside the activated carbon. The diameter of the pores is small, so the total surface area is large and has a strong adsorption capacity. Due to the different raw materials and manufacturing methods, there are quite a variety of activated carbons, which are suitable for different uses. The micropores of the powdered activated carbon can adsorb low molecular weight gas and small molecules in the solution, but the molecules with higher molecular weight cannot enter the micropores; the mesopores provide channels into the micropores, which in turn can adsorb substances with higher molecular weight; Large pores provide both channel and adsorption. If activated carbon is used to adsorb small molecules, such as certain gases (toxic gases) and low molecular weight organics, products with more micropores can be used; however, if larger molecules are to be adsorbed, products with more mesopores should be used. The state of the pores of the activated carbon depends on the raw materials used, the components thereof, the manufacturing methods and conditions, and the like.
The shape of powdered activated carbon products mainly includes powder and granular. Powdered activated carbon is a very fine powder, most of which can pass through a 200 mesh screen, most of which can pass through a 325 mesh screen. The size of the powder is between 1 and 150 μm (average about 40 μm); usually, the finer the carbon powder It has a higher adsorption rate for impurities. Therefore, the activated carbon product is often highly crushed and screened to obtain a fine powder. The disadvantage of powdered charcoal is that it is difficult to regenerate, and it is usually not used for regeneration, so the consumption is large (in recent years, research has also been carried out to regenerate it). Granular activated carbon is usually recycled and consumes less. It has two kinds of amorphous particles and columnar particles, and the particle size is between 0.5 and 4 mm. The former is obtained by appropriate crushing and screening, and the latter is by pressing the raw material through a granulator.
Powdered activated carbon is usually produced from wood chips and chemically activated. Most of the granular activated carbon is made of coal or charcoal. After crushing, it is pressed and formed with a binder (such as coal tar, lignin sulfonic acid, etc.). It is carbonized by dry distillation and then activated to be processed (or broken into appropriate before activation). Granularity).
[Quality and performance of powdered activated carbon]
The quality of powdered activated carbon has a number of physical and chemical indicators, such as: moisture, ash, acid solubles, various metals and acid radicals, and its adsorption properties. For different purposes of activated carbon, different substances and methods are often used to test its adsorption properties, such as methylene blue adsorption value, iodine adsorption value, caramel adsorption value, quinine sulfate adsorption value. Among them, the methylene blue adsorption value is the most commonly used. Methylene blue is a dark blue dye, and its adsorption amount reflects the ability of activated carbon to adsorb small molecular substances; activated carbon with a large number of micropores, this value is higher. The caramel adsorption value (or caramel decolorization rate, or molasses adsorption rate) is an activated carbon that reflects the adsorption performance of activated carbon on a colored substance having a relatively high molecular weight, and has a good performance, and the value reaches 100 to 110.
Activated carbon manufactured at home and abroad has a kind of product called "activated carbon for sugar", which can be used in sugar factories, and can also be used in other similar industries, such as glucose solution and refined decolorization of MSG solution. Its main feature is that it has more mesopores and is therefore suitable for treating solutions containing more macromolecular organics. This activated carbon has a relatively high caramel adsorption value. China's "carbon powder for decolorization of sugar liquid" national standard (GB/T13803.3-1999) stipulates that activated carbon products are divided into three categories: superior grade, first grade and second grade. The water content is less than 10%; the decolorization rate of caramel is higher than 100, 90 and 80 respectively, and the ash content is lower than 3%, 4% and 5% respectively (the activated carbon produced by the phosphoric acid method can be 7% to 9%, regardless of whether Grade), acid solubles below 1%, 1.5% and 2%, respectively, as well as iron and chlorine content. Their pH is between 3 and 5.
[Technical Specifications and Main Characteristics of Activated Carbon]
Activated carbon is a very fine carbon particle with a large surface area, and there are finer pores in the carbon particles - the capillary. This capillary has a strong adsorption capacity and is sufficiently in contact with gas (impurities) because of its large surface area. When these gases (impurities) hit the capillary, they are adsorbed and purify. The surface area of ​​activated carbon is very important. The specific surface area detection data of activated carbon can only be verified by the BET method. There are many instruments in China that can only be directly compared, and now it has been eliminated in China. At present, the specific surface area test at home and abroad adopts the multi-point BET method. The specific surface area measurement standards developed at home and abroad are based on the BET test method. Please refer to the national standard of China (GB/T 19587-2004) - Gas adsorption BET principle A method of determining the specific surface area of ​​a solid material. The specific surface area detection is actually a relatively time-consuming task. Due to the different adsorption capacity of the sample, some samples may take a whole day to test. If the test process is not fully automated, the tester can not leave at all times. High concentration, observing the dashboard, and manipulating the knobs, a little inattention will lead to the failure of the test process, which will waste a lot of valuable time for the testers. The F-Sorb 2400 specific surface area tester is an instrument that can truly realize the BET detection function (both direct contrast method). The more important F-Sorb 2400 specific surface area tester is the only fully automated and intelligent specific surface area inspection equipment in China. The test results are highly consistent with international standards, and the stability is also good, while reducing human error and improving the accuracy of test results.
Adsorption capacity of activated carbon for each gas (unit: ml/cm3):
H2, O2, N2, Cl2, CO2
4.5, 35, 11, 494, 97
[Mechanical characteristics]
(1) Particle size: A set of standard sieve sieving method was used to determine the weight of activated carbon remaining in and passing through each sieve to indicate the particle size distribution.
(2) Static density or bulk density: the weight of the activated carbon per unit volume of the dietary pore volume and the interparticle void volume.
(3) Bulk density and particle density: the weight of the activated carbon per unit volume of the dietary pore volume without the intergranular void volume.
(4) Strength: the crushing resistance of activated carbon.
(5) Abrasion resistance: that is, wear resistance or anti-friction performance.
These mechanical properties directly affect the application of activated carbon, for example: density affects the size of the container; the thickness of the powder affects the filtration; the particle size distribution of the carbon affects the fluid resistance and pressure drop; the fracture affects the service life of the activated carbon and the regeneration of the waste carbon.
[chemical characteristics]
In addition to physical adsorption, adsorption of activated carbon also involves chemical adsorption. The adsorption of activated carbon depends on both the pore structure and the chemical composition.
Activated carbon not only contains carbon, but also contains a small amount of chemically bonded, functional group-starting oxygen and hydrogen, such as carbonyl, carboxyl, phenols, lactones, terpenes, ethers. Some of the oxides and complexes contained on these surfaces are derived from derivatives of raw materials, some of which are formed by the action of air or water vapor upon activation and activation. Surface sulfides and chlorides are sometimes formed. In the activation, the minerals contained in the raw materials are concentrated into activated carbon to become ash, and the main components of the ash are salts of alkali metals and alkaline earth metals, such as carbonates and phosphates.
[Safety issues in activated carbon applications]
It is generally believed that there is no safety problem with the application of activated carbon, but there is no absolute safety. The safety in the application of activated carbon cannot be taken lightly, and the nature of the activated carbon and the possibility of unsafeness must be known.
A. About the fire
1) Activated carbon is not classified as dangerous, but flammable. There is no flaming burning after the fire, only smoldering.
2) Activated carbon does not spontaneously ignite, may ignite in the air, and mix with gasoline, diesel, etc., can cause burning.
3) If the ventilation of the activated carbon is insufficient, toxic carbon monoxide will be formed.
B. About storage
1) Activated carbon must be stored in a building that is as fire resistant as possible.
2) Activated carbon cannot be mixed with oxidant
3) Open fire, sparks and smoking are prohibited in the storage area
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