1 chemical oxidation method
Chemical oxidation is to use the oxidizability of strong oxidant to react with organic pollutants in the middle water under certain conditions, so as to eliminate the pollution. The common strong oxidants are chlorine, chlorine dioxide, ozone, hydrogen peroxide, perchloric acid and hypochlorite.
Ozone has a good decolorization effect on the middle water because of its high oxidation potential (E0 = 2.07v). When the ozone concentration is 20mg/l, 90% of the color of the middle water can be removed in 90 minutes, and 85% of them are completed within 15 minutes. The chemical oxidation process with a large number of free radicals is called advanced chemical oxidation method. This treatment process can decompose the organic pollutants in wastewater completely, and it is a new technology of water pollution treatment which has been paid attention to in recent years. For example, ozone and ultraviolet (UV), ultrasonic, catalyst and other combined use greatly improve the oxidation decolorization performance. The energy provided by these auxiliary means not only catalyzes the production of oxygen and hydrogen radicals with strong oxidation, but also stimulates the substances in water, makes it into excited state and accelerates the rate of oxidation reaction.
Photocatalytic oxidation is a kind of oxidation decomposition reaction which takes place under special light irradiation conditions, and finally decomposes the organic matter into non-toxic substances. The photocatalysis oxidation method has strong oxidation and reduction ability due to the electron hole, which can oxidize the toxic inorganic substances, degrade most organic substances, and finally generate simple inorganic substances, which will minimize the impact of middle water on the environment. Wushubin studied the application of TiO2 photocatalysis oxidation technology in papermaking wastewater treatment. It was found that the treatment time of room temperature was not more than 1 hour under the action of O2 and ultraviolet light, and the total organic chlorine and chroma in the middle water could be reduced by more than 80%. After biological oxidation, COD, TOC and chroma were almost completely removed.
2 physicochemical method
The physical and chemical methods include adsorption, coagulation and membrane separation.
Adsorption is a method that uses porous solid adsorbent to transfer organic pollutants from wastewater to solid adsorbent by means of material transfer on solid-liquid interface, so as to separate and remove it from wastewater. At present, the adsorbents used in water treatment are mainly activated carbon, diatomite, silica, active alumina, zeolite and ion exchange resin. Activated carbon is the first used decolorization adsorbent. Although it can effectively remove the color of wastewater, it is expensive, difficult to regenerate and has high loss rate. Therefore, it is generally only used for wastewater treatment or deep treatment with low concentration. The main composition of bentonite is aluminate, and the intercalation of the layered structure is exchangeable plasma of calcium, magnesium and sodium. The surface of bentonite particles is often charged, so it has good adsorption. Wang Chunfeng and others used sulfuric acid activation method to make activated fly ash adsorption materials. The adsorption properties of activated fly ash adsorption materials on COD in papermaking wastewater were studied. The results showed that when the pH = 7, the fly ash had obvious removal effect on organic matter at 20 ℃ and pH = 7. The production of the adsorption material and its application in the treatment of industrial wastewater are low cost, and the purpose of comprehensive utilization of waste is achieved.
The principle of coagulation treatment of middle water is the same as that of black liquor. By coagulation, turbidity and chroma of middle water can be reduced, and polymer substances, suspended or colloidal organic pollutants and some heavy metal substances can be removed. The coagulant commonly used in middle water treatment mainly includes aluminum sulfate, magnesium sulfate, 2 or 3 valent iron salt, alumina, calcium oxide, sulfuric acid, phosphoric acid, polyamide organic polymer, etc. Panruting and other people used oxidation coupling flocculation method to treat the middle water of the paper making which is difficult to degrade, and the influence of various factors on the treatment effect was investigated. The results show that the COD removal rate is 85% under the condition of 2:1 of the mass ratio of modified aluminum salt to modified calcium salt, 150mg/l of total dosage, pH = 7-8, and reaction time of 20min. The treatment effect under the best conditions is higher than that of aluminum sulfate, iron trichloride and PAC. The wastewater can be discharged up to standard after treatment. Membrane separation is a new technology of separation, purification and concentration. The membrane separation process is to use selective permeable membrane as the separation medium, and push some impetus on both sides to make the separated material selectively pass through the membrane, so as to achieve the purpose of separation or purification. Membrane separation can be divided into ultrafiltration, electrodialysis, nanofiltration and other technologies. Ultrafiltration is a membrane separation operation that uses differential pressure as driving force to select particles and macromolecules contained in the solution according to particle size; electrodialysis is a membrane separation operation which takes potential difference as driving force and takes the selective permeability of ion exchange membrane to remove or enrich electrolyte from solution; nanofiltration is a membrane separation operation which takes differential pressure as power and is between reverse osmosis and ultrafiltration, and separates substances from solution Process. The technology of membrane separation has been used in the paper industry in the United States, Finland, Norway and Sweden to treat bleaching wastewater, and the production technology has been mature. In 1970s, China has also begun to study membrane separation technology to treat papermaking wastewater, and has made some progress.
3 biological method
Biological method is to use microorganism to degrade and metabolize organic matter as inorganic substance to treat wastewater. The environment suitable for microbial survival and reproduction is created by human beings, which can make it propagate in large quantities to improve the efficiency of oxidation and decomposition of organic matter. According to the types of microorganisms used, it can be divided into aerobic method, anaerobic method and biological enzyme method.
Aerobic method is a method of degrading and treating wastewater by aerobic microorganism under aerobic conditions. The commonly used aerobic treatment methods include activated sludge method, biofilm method, biological contact oxidation method, biological fluidized bed and other methods. Anaerobic method is a method to treat wastewater by anaerobic microorganism degradation and metabolism under the condition of no oxygen. The operating conditions of anaerobic method are more stringent than aerobic method, but it has better economic benefits, so it also has an important position. At present, the developed methods include anaerobic pond method, anaerobic filter bed method, anaerobic flow bed method, anaerobic expansion bed method, anaerobic rotating disk method, anaerobic pool method, upflow anaerobic sludge bed (UASB) method, etc. Generally, in order to achieve better treatment effect, aerobic method and anaerobic method are used together. The results show that the biodegradability of wastewater increases from 0.2 to 0.25 to 0.4-0.5 when HRT is 6h, and the optimal HRT of SBR is 8h, which is operated separately, COD removal is removed The results showed that the SBR treatment effect was improved obviously, COD removal rate was about 80%, and the combined process was good. The removal rate of COD and BOD5 was about 90%, and the impact load resistance was strong.
The mechanism of biological enzyme treatment of organic matter is that the free radicals are formed by enzyme reaction, and then the free radicals are polymerized to form polymer compounds precipitation. Compared with other microbial treatment, enzyme treatment has the advantages of high catalytic efficiency, mild reaction conditions, low requirements for wastewater quality and equipment, fast reaction speed, wide range of adaptability to temperature, concentration and toxic substances, and can be reused. The results showed that the enzyme activity and AOX removal rate of immobilized cells were higher than that of free bacteria, and the range of adaptation to temperature and pH was wide. The continuous treatment of papermaking bleaching wastewater for one month showed that the removal rate of the wastewater was stable from 65% to 81% when the retention time was 2.4h. The results show that the comprehensive treatment effect of organic chloride and COD in wastewater is better when the relative concentration of the middle bleaching water is 50%, pH is 7.0 and the amount of bacteria solution is 2ml.
4 electron beam method
The electron beam method relies on the radiation of high energy electron beam on water to produce active radicals. Through the action of these active radicals and organic matter in water, the purpose of removing organic matter in water can be achieved. High energy electron beam has a good killing effect on bacteria and viruses, and the electron beam does not produce by-products, no secondary pollutants, and its own process is clean, which is a more advanced pollution treatment technology. The experiment of electron beam irradiation was carried out on the wastewater from the middle section of alkali straw pulp by wulihua. The results show that the electron beam irradiation can degrade the harmful chemicals in wastewater which are not biodegraded, and the results are good. It is worth noting that electron beam irradiation can degrade the pollutants which are difficult to be degraded by biochemical treatment. Therefore, if the irradiation technology and biochemical technology are combined, the best conditions can be found out, the economic and rapid use of biochemical technology and irradiation technology can degrade all kinds of pollutants of different properties, and each can achieve ideal treatment effect.
5 electrochemical method
Electrochemical method is to produce new ecological free radicals with strong living property through electrode reaction. The chromogenic organic matter in wastewater will undergo oxidation-reduction reaction under the action of these free radicals, and degrade to colorless small molecular matter or form flocculation to settle down. After treatment, the color and COD of water are reduced. The electrochemical method has been improved. In the electrochemical reactor, aluminum or iron is used as anode, and the hydroxides of aluminum (iron) are hydrolyzed to form aluminum (iron) by hydrolysis of al3+ (fe2+) produced during electrolysis. Compared with the aluminum (iron) inorganic salt put into the coagulation method, it has higher activity and stronger flocculation effect, which makes the organic suspended matter and colloid particles in the middle wastewater agglomerate to form flocs. Hydrogen generated on the cathode is discharged in the form of micro bubbles, and is separated by floating up to the water surface together with the floc. This method is called electroflocculation. Sunjinyong and others used electro flocculation method to treat the waste paper deinking wastewater, and discussed the influence of electrode material, current density, plate spacing, pH of system and electrolysis time on wastewater treatment. The results show that aluminum is used as electrode material and the current density is 1.7a/dm3. The results showed that the removal rate of turbidity and COD of wastewater can reach 95% and 60% respectively under the condition of the electrode spacing of 10 mm, the system ph5-6.5 and the electrolysis time of 20min. Jingfeng et al. Combined electrochemical method and coagulation precipitation method to treat papermaking wastewater, the COD removal rate of papermaking wastewater reached 55% - 70%, and the chroma removal rate was 90% - 95%.
6 physical method
Physical method is to use various screens, screens, inclined screens, grilles and other pretreatment middle water, which mainly block the large waste pulp fibers in the water, and then return to the production of common board paper or felt raw paper. The amount of waste pulp fiber is generally 10% - 15%, and the recovery can get certain economic benefits. In addition, microfiltration and vibrating screen technology, as a simple mechanical filtration method, has been gradually applied to the pretreatment of middle stage sewage. It is suitable for separating the micro suspended substances, organic residue and other suspended solids in the wastewater to the maximum extent, which greatly reduces the post-treatment load, and has a large water treatment capacity, convenient management, and good quality of recycled waste pulp. It is a promising technology in the middle water pretreatment of paper making.
7 comprehensive method
The above mentioned introduces some methods of water treatment in the middle section of paper making. In fact, these methods are mostly used comprehensively. Each method has its own advantages and disadvantages. It is not only cost high to use a single method to treat wastewater, but also difficult to meet the discharge standard. Therefore, they are often used together to find the best matching way for different water quality, so that the process can be simplified. Hanbiao used hydrolysis aerobic process to treat the middle wastewater produced by a pulp and paper mill in Guangxi. After sampling and monitoring on site, the effluent quality was good and COD removal rate was over 98%.
The SBR physical and chemical method is used to treat the middle water of paper making, such as jianjinghua, etc., with less investment, low operating cost, stable and up to standard water quality outside the paper plant, and the treatment cost is within the acceptable range of the manufacturer.