A chemical substance is added into the sewage to make it exchange with the soluble substances in the sewage, so as to generate a precipitate which is insoluble in water, so as to reduce the soluble substances in the sewage. This treatment method is often used for the treatment of industrial production sewage containing heavy metals, cyanide and so on. According to the difference of precipitants used, Chemical precipitation methods can be divided into lime method (also known as hydroxide precipitation method), sulfide method and barium salt method.
2. Coagulation methods: aluminum sulfate, basic aluminum chloride, iron salt (mainly ferrous sulfate, ferric chloride and ferric sulfate), etc.
Adding coagulant to water can make colloidal particles in sewage lose stability. Coagulation can be used to remove finely dispersed solid particles, emulsified oil and colloidal substances in sewage. This method can be used to reduce the turbidity and chromaticity of sewage, remove a variety of high molecular substances, organic substances, some heavy metal poisons (mercury, cadmium, lead) and radioactive substances, and also remove soluble inorganic substances such as phosphorus that can lead to eutrophication. In addition, it can improve the dewatering performance of sludge. Therefore, coagulation is widely used in industrial sewage treatment, which can be used as an independent treatment process or in combination with other treatment methods as pretreatment, intermediate treatment or final treatment. At present, commonly used coagulants include aluminum sulfate, basic aluminum chloride, iron salt (mainly ferrous sulfate, ferric chloride and ferric sulfate), etc.
When coagulant alone cannot achieve the desired water purification effect, In order to strengthen the coagulation process and save coagulant dosage, coagulant AIDS can often be added at the same time. < P > 3. Neutralization method: lime, sodium hydroxide, limestone, dilute sulfuric acid, CO2, etc. < P > It is used to treat acid wastewater and alkaline wastewater. Alkaline substances such as lime, sodium hydroxide, limestone, etc. are added to the acid wastewater to make the wastewater neutral. The alkaline wastewater can be blown into flue gas containing CO2 for neutralization. Other acidic substances can also be used for neutralization.
4. Redox method: liquid chlorine, ozone, potassium permanganate, reducing agent, etc.
The harmful substances in wastewater are oxidized and decomposed into harmless substances by strong oxidants such as liquid chlorine, ozone, potassium permanganate or anode reaction during electrolysis; Using reducing agent or cathode reaction in electrolysis, the harmful substances in wastewater are reduced to harmless substances. The above methods are collectively called oxidation-reduction method.
The application examples of oxidation-reduction method in sewage treatment are: air oxidation method to treat sulfur-containing sewage; Treatment of cyanide-containing wastewater by alkaline chlorination; Ozone oxidation method has remarkable effects in deodorizing, decolorizing, sterilizing, removing phenol, cyanide, iron and manganese, and reducing BOD and COD of sewage. Reduction method is mainly used to treat chromium-containing sewage at present.