1. 1 activated carbon adsorption and ion exchange
Activated carbon is a porous substance, which is easy to control automatically and has strong adaptability to the changes of water quantity, water quality and water temperature. Therefore, activated carbon adsorption is a kind of advanced sewage treatment technology with broad application prospects. Activated carbon has a very obvious removal effect on organic matter with molecular weight of 500 ~ 3 000, and the removal rate is generally 70% ~ 86.7%. Its biggest feature is that there is no phase change in the separation process, and a high separation effect can be obtained only by using a certain pressure as the driving force. It is a very energy-saving separation technology.
Microfiltration can remove bacteria, viruses and parasites, and can also reduce the phosphate content in water. The sewage treatment plant of Tianjin Development Zone uses microfiltration membrane to deeply treat the secondary effluent of SBR to meet the needs of municipal and domestic miscellaneous uses such as landscape, road washing and toilet flushing [8].
Macromolecules were removed by ultrafiltration, and the removal rate of COD and BOD in secondary effluent was more than 50%. Beijing Gaobeidian Sewage Treatment Plant uses ultrafiltration to deeply treat the secondary effluent, and the effluent quality meets the standard of domestic miscellaneous water. Reusing sewage for car washing can save 4 700 m3 of water every year [9].
Reverse osmosis is used to reduce salinity and remove total dissolved solids. The desalination rate of secondary effluent is over 90%, the removal rate of COD and BOD is about 85%, and the removal rate of bacteria is over 90% [10]. Reverse osmosis membrane combined with electric desalting technology is used for boiler make-up water in a power plant in Myanmar. The water treated by reverse osmosis can remove most inorganic salts, organic matters and microorganisms [1 1].
Nanofiltration is between reverse osmosis and ultrafiltration, and the operating pressure is usually 0.5 ~ 1.0 MPa. A remarkable feature of nanofiltration membrane is its ion selectivity. Its removal rate of divalent ions is as high as 95%, while the removal rate of monovalent ions is relatively low, ranging from 40% to 80% [12]. Pan et al. used membrane bioreactor-nanofiltration membrane integrated technology to treat molasses alcohol wastewater and achieved good results. The effluent COD is less than 100 mg/L, and the wastewater reuse rate is more than 80%.
The application of membrane technology in advanced treatment in China is still far from the advanced level in the world. The future research focuses on developing and manufacturing membrane materials with high strength, long service life, anti-pollution and Qualcomm content, focusing on solving key problems such as membrane pollution, concentration polarization and cleaning.
1.3 advanced oxidation method
There are many kinds of high-concentration organic pollutants and toxic and harmful pollutants discharged in industrial production, some of which are difficult to biodegrade and have inhibitory and toxic effects on biochemical reactions. However, the advanced oxidation method produces highly active free radicals (such as? Oh, wait. ), so that refractory organic pollutants can be transformed into easily degradable small molecular substances, and even CO2 and H2O can be directly generated, thus achieving the purpose of harmlessness.
1.3. 1 wet oxidation method
Wet oxidation (WAO) is to oxidize organic or inorganic substances in water at high temperature (150 ~ 350℃) and high pressure (0.5 ~ 20 MPa), using O2 or air as oxidant, so as to remove pollutants. The final products are carbon dioxide and H2O[ 14]. In 2002, WAO process was introduced into Fujian Refinery, which completely solved the problems of subsequent treatment of alkali residue and odor pollution, with low operating cost and high oxidation efficiency [15].
1.3.2 wet catalytic oxidation method
Wet catalytic oxidation (CWAO) is to add suitable catalyst to the traditional wet oxidation treatment process, so that the oxidation reaction can be completed under milder conditions and in a shorter time, thus reducing equipment corrosion and operating costs [16, 17]. At present, the continuous-flow CWAO industrial experimental device built in Kunming has shown good economy [18].
Catalysts for wet catalytic oxidation are generally divided into three categories: metal salts, oxides and composite oxides. At present, considering the economy, the most widely used catalysts are transition metal oxides such as copper, iron, nickel, cobalt and manganese and their salts. The use of solid catalyst can also avoid the loss of catalyst, secondary pollution and waste of funds.
1.3.3 supercritical water oxidation method
Supercritical water oxidation method is to raise the temperature and pressure above the critical point of water, and the water in this state is called supercritical water. In this state, the density, dielectric constant, viscosity, diffusion coefficient, conductivity and solvent chemical properties of water are different from those of ordinary water. Higher reaction temperature (400 ~ 600℃) and pressure also accelerate the reaction speed, and can achieve high destruction efficiency of organic matter in a few seconds.
Harrington, Texas, USA applied supercritical water oxidation to treat sludge for the first time on a large scale, with a daily treatment capacity of 9.8 t. The system operation proved that the COD removal rate was over 99.9%, and all organic components in sludge were converted into harmless substances such as CO2 and H2O, with low operating cost [19].
1.3.4 photocatalytic oxidation method
At present, photochemical catalytic oxidation methods are mainly divided into Fenton reagent method, Fenton-like reagent method and oxidation method with TiO2 as the main body.
Fenton reagent method was discovered by Fenton in the 20th century, and now it has been re-evaluated as a meaningful research method in the field of wastewater treatment. Fenton reagent is generated by H2O2 and Fe2+ salt? Oh, this reactant is a very attractive oxidation system for wastewater treatment, because iron is a rich non-toxic element, and H2O2 is also easy to operate and safe to the environment [20]. Fenton reagent can destroy toxic compounds such as phenol and herbicide in wastewater. At present, there are many studies on Fenton reagent used in printing and dyeing wastewater treatment in China. The results show that Fenton reagent has a good decoloration effect on printing and dyeing wastewater. In addition, studies at home and abroad have also proved that Fenton reagent can effectively treat wastewater containing oil, alcohol, benzene series, nitrobenzene and phenol.
Fenton-like reagent method has the advantages of simple equipment, mild reaction conditions and convenient operation, and has great application potential in treating toxic and harmful biodegradable organic wastewater. The main problem of this method in practical application is that the treatment cost is high, and it is only suitable for the treatment of low concentration and small amount of wastewater. As a pretreatment or advanced treatment method of refractory organic wastewater, it is combined with other treatment methods (such as biological method and coagulation method). ), it can better reduce the cost of wastewater treatment, improve the treatment efficiency and broaden the application scope of this technology.
Photocatalytic method is to induce strong oxidative free radicals by irradiating some semiconductor photocatalysts with energy band structure, such as titanium dioxide, zinc oxide, cadmium sulfide and WO3. Oh, so many difficult chemical reactions can be carried out under normal conditions. Anatase TiO2 _ 2 has the characteristics of high stability, excellent performance and low cost. The latest research in the world is to obtain improved (doped with other components) TiO22 _ 2 with wider absorption line and higher quantum yield.
1.3.5 electrochemical oxidation method
Electrochemical oxidation, also known as electrochemical combustion, is a branch of environmental electrochemistry. Its basic principle is to oxidize organic matter under the action of electrocatalysis on the electrode surface or free radicals generated by electric field. In addition to completely oxidizing organic matter into CO2 and H2O, electrochemical oxidation can also be used as a pretreatment process of biological treatment, which can convert non-biocompatible substances into biocompatible substances after electrochemical transformation. The method has high energy utilization rate and can be carried out at low temperature. The equipment is relatively simple, the operation cost is low, and it is easy to control automatically; No secondary pollution, etc.
1.3.6 ultrasonic radiation degradation method
Ultrasonic radiation degradation method mainly comes from cavitation bubbles produced by liquid under ultrasonic radiation, which absorb acoustic energy and collapse to release energy in a very short time, and produce high temperature of 1 900 ~ 5 200 K and high pressure exceeding 50 MPa in a very small space around it. Water molecules entering cavitation bubbles can undergo decomposition reaction, resulting in high oxidation activity. Oh, induce organic matter degradation; In addition, water molecules on the surface of cavitation bubbles can form supercritical water, which is beneficial to the improvement of chemical reaction speed.
Ultrasonic wave has a remarkable effect on dehalogenation and oxidation of halides. The final degradation products of chlorinated organic compounds such as chlorophenol, chlorobenzene, CH2Cl2, CHCl3 and CCl4 are HCl, H2O, CO and CO2. Ultrasonic degradation is also very effective for denitrification of nitro compounds. Adding O3, H2O2, Fenton reagent and other oxidants will further enhance the ultrasonic degradation effect. The combination of ultrasound and other oxidation methods, such as US/O3, US/H2O2, US/Fenton and US/ photochemical method, is a hot research topic at present. At present, the research on degradation of water pollutants by ultrasonic radiation is still in the experimental exploration stage.
1.3.7 radiation method
Radiation method is a sewage radiation purification method developed by using high-energy rays (γ, χ rays) and electron beams to destroy compounds. It is generally believed that the reaction mechanism of treating organic wastewater by radiation technology is to produce water under the action of high energy radiation. Oh, H2O2? High activity particles such as HO2. And then induced by these highly active particles to degrade harmful substances.
The radiation method has the advantages of high efficiency and simple operation. The main problems of this technology are that the equipment used to generate high-energy particles is expensive and the technical requirements are high, and this method consumes a lot of energy and has low energy utilization rate. In addition, in order to avoid radiation damage to the human body, special protective measures need to be taken. For more information, please visit Easy Water Purification Network. Therefore, to put this method into practice, a lot of research and exploration work is needed.
1.4 ozone method
Ozone is highly oxidizing and reacts with many organic substances or functional groups, thus effectively improving water quality. Ozone can oxidize and decompose the color and smell caused by various impurities in water, and the decoloration effect is better than that of activated carbon. It can also reduce the turbidity of effluent, play a good flocculation role, improve the filtration speed or extend the filtration period. At present, the domestic ozone generation technology and process are relatively backward, and the operating cost is too high, so it is difficult to popularize.