Chemical oxidation method is to use the strong oxidation of chemical oxidant to completely oxidize inorganic and organic substances in wastewater into nontoxic small molecular substances or gases, so as to achieve the purpose of treatment. Chemical oxidation can remove most organic pollutants and some inorganic substances in wastewater. Common chemical oxidants are O3, H2O2, ClO2, KMnO4 and K2FeO4. These oxidants are usually strong oxidants, which can oxidize various organic pollutants in acidic and alkaline solutions. In particular, Fenton reagent, which is composed of soluble Fe2+ and H2O2 in a certain proportion, can oxidize many organic substances, and the operation does not require high temperature and high pressure. The treatment effect is good, but there are some insurmountable weaknesses. At present, the cost of chemical oxidation is still high, and it is only used for drinking water treatment, special industrial water treatment, toxic and harmful high-concentration organic wastewater treatment and advanced wastewater treatment and reuse.

Chemical catalytic oxidation method is to add suitable catalyst to the traditional wet oxidation treatment process to reduce the temperature and pressure required for the reaction, improve the oxidative decomposition ability, shorten the reaction time, prevent equipment corrosion and reduce the cost.

Chemical catalytic oxidation is mainly used for the treatment of wastewater from petroleum refining and chemical industry. It has successful examples in dealing with gas pollutants, liquid pollutants and solid pollutants. This method has been used to treat gas pollutants, the catalytic conversion of sulfur dioxide and nitrogen oxides, and the treatment of organic wastewater.

Wet oxidation technology is an effective water treatment method to treat toxic, harmful and high-concentration organic wastewater developed in 1950s.

The main principle of supercritical water oxidation method is to use supercritical water as a medium to oxidize and decompose organic matter [6]. The oxidation process of organic pollutants in supercritical water is very rapid and complete. Organic carbon is converted into CO2, hydrogen into H2O, halogen atoms into halogen ions, sulfur and phosphorus into SO42- and PO43- respectively, and nitrogen into N2 or NO3-and NO2-. At the same time, a lot of heat is released in the process of supercritical water oxidation, and once the reaction starts, it can be maintained by itself without external energy [7]. In order to accelerate the reaction rate, reduce the reaction time, reduce the reaction temperature, optimize the reaction procedure, and make the supercritical water oxidation method give full play to its own advantages, many scholars have introduced catalysts into the supercritical water oxidation technology and developed the supercritical wet oxidation technology, which has become an important research direction.

Photocatalytic oxidation degradation of organic pollutants in water has outstanding advantages such as low energy consumption, simple operation, mild reaction conditions and reduction of secondary pollution. At the same time, it has strong purification ability for high concentration organic industrial wastewater. In addition, it can make full use of solar energy, which is of great significance for saving energy, protecting the environment, maintaining ecological balance and realizing sustainable development. In the treatment of dye wastewater, surfactant, pesticide wastewater, oily wastewater, cyanide pharmaceutical wastewater, organophosphorus compounds, polycyclic aromatic hydrocarbons and other wastewater, photocatalytic reaction can be effectively carried out to transform them into inorganic small molecules, thus achieving the goal of complete inorganic. Similarly, photocatalytic reaction has broad application prospects for removing many inorganic substances, such as CN-, Au (CN) 4-, I-, s CN-, Cr2O3-, Hg(CH3)2, Hg2+ and so on [10]. Many foreign scholars have studied the degradation of 4-CP, nitrophenol, phenol, anisole, methyl parathion and other typical organic pollutants by using light-assisted Fenton reagent, and also studied the degradation of landfill leachate. Wang Yili, a domestic scholar, used a suspension reactor to study the photodegradation of eight kinds of dye wastewater, such as reactive brilliant red, reactive yellow and cationic pink. The results show that when the dosage of TiO2 _ 2 is 1 g/L and the illumination time is 4 h, the degradation rate of various dye wastewater can reach above 90%. Zhou Zufei and others studied the photodegradation of NAA. Under the conditions of TiO2 _ 2 dosage of 0. 10 g/L, 254 nm ultraviolet irradiation and aeration, the initial mass concentration of NAA is 50 mg /L, and it drops below 6 mg/L after 3 hours of irradiation. Lei Lecheng and others studied PVA desizing wastewater by using photo-assisted Fenton reagent. The results showed that the removal rate of DOC in PVA wastewater oxidized by photo-assisted Fenton reagent was over 90%.

Electrochemical oxidation is a process in which pollutants undergo a direct electrochemical reaction on the electrode, or pollutants undergo a redox reaction by using strongly oxidizing active species generated on the surface of the electrode to generate harmless substances. The former is called direct electrochemical reaction and the latter is called indirect electrochemical reaction. Direct electrochemical reaction can convert organic pollutants and some inorganic pollutants into harmless substances through anodic oxidation, and cathode can also remove heavy metal ions from water in principle. These two processes are accompanied by the release of H2 and O2, which reduces the current efficiency, but can be prevented by selecting electrode materials and potential control. Indirect electrochemical reaction is to convert pollutants into harmless substances by using redox agents generated by electrochemical reaction, and the redox agents generated at this time are the media for exchanging electrons between pollutants and electrodes. This intermediate can be a catalyst or a short-lived intermediate produced by electrochemistry. In addition, in recent years, O2 was reduced to H2O2 at the cathode, and then (o H) was generated, which can be used to treat phenol, aniline, aldehydes and cyanide.