1.chemical industrial wastewater
Chemical industry wastewater mainly comes from the production wastewater discharged by industries such as petroleum chemistry, coal chemistry, acid-base industry, fertilizer industry, plastic industry, pharmaceutical industry, dye industry, and rubber industry.
The main measures for the prevention and control of chemical wastewater pollution are as follows: Firstly, the production process and equipment should be reformed to reduce pollutants and prevent wastewater from being discharged externally. Comprehensive utilization and recycling should also be carried out; for the wastewater that must be discharged externally, the treatment degree should be selected based on the water quality and requirements.
Primary treatment mainly separates suspended solids, colloids, floating oil or heavy oil, etc. from the water. Methods such as water quality and quantity adjustment, natural sedimentation, floating, and oil separation can be employed.
The secondary treatment mainly aims to remove the biodegradable organic dissolved substances and some colloidal substances, reducing the biochemical oxygen demand and part of the chemical oxygen demand in the wastewater. It is usually carried out using biological methods. After the wastewater has undergone biological treatment, there is still a considerable amount of COD remaining. Sometimes, it has high levels of color, odor, and taste. Or due to the high environmental hygiene standards, a tertiary treatment method is required for further purification.
The tertiary treatment mainly aims to remove the organic pollutants that are difficult to be biodegraded and the soluble inorganic pollutants from the wastewater. Common methods include activated carbon adsorption and ozone oxidation. Other techniques such as ion exchange and membrane separation can also be employed. For various chemical industrial wastewater, different treatment methods can be selected based on different water quality, water volume and the requirements of the treated effluent water quality.
2. Textile dyeing and printing wastewater
The dyeing and printing industry consumes a large amount of water. Usually, for every 1 ton of textiles processed, 100 to 200 tons of water are consumed. Among this, 80% to 90% is discharged as dyeing wastewater. The common treatment methods include recycling and harmless treatment.
Recycling:
Wastewater can be recycled and reused according to its specific water quality characteristics. For example, the separate collection of bleaching and refining wastewater and dyeing and printing wastewater. The former can be treated through direct flow washing and reused, thereby reducing the discharge volume.
The recovery and reuse of alkaline solutions is usually achieved through evaporation. If the amount of alkaline solution is large, three-effect evaporation can be used for recovery; if the amount is small, thin-film evaporation can be employed for recovery.
Dye recovery. For example, stilbene dyes can be acidified to form phthalic acid, which exists as colloidal particles. These particles are suspended in the residual liquid and can be recovered and reused after sedimentation and filtration.
Harmless treatment can be divided into:
The physical treatment methods include precipitation and adsorption methods, etc. The precipitation method mainly removes suspended solids from wastewater; the adsorption method mainly removes dissolved pollutants and decolorizes the wastewater.
Chemical treatment methods include neutralization method, coagulation method and oxidation method, etc. The neutralization method aims to adjust the pH value of the wastewater and also reduce the color of the wastewater; the coagulation method is used to remove dispersed dyes and colloidal substances in the wastewater; the oxidation method is to oxidize the reducing substances in the wastewater, causing the sulfur dyes and reducing dyes to precipitate.
Biological treatment methods include activated sludge, biological rotating discs, biological rotating drums and biological contact oxidation. To improve the effluent quality and meet the discharge standards or recovery requirements, it is often necessary to adopt a combination of several methods for treatment.
3. Wastewater from the paper industry
Papermaking wastewater mainly comes from the two production processes in the papermaking industry: pulping and papermaking. Pulping involves separating the fibers from plant raw materials to make pulp, which is then bleached; papermaking involves diluting the pulp, shaping it, pressing it, and drying it to produce paper. Both of these processes discharge a large amount of wastewater.
The wastewater generated during pulping is the most severely polluted. The wastewater discharged during the bleaching process is dark brown and is called "black water". The concentration of pollutants in this water is very high, with BOD ranging from 5 to 40 g/L. It contains a large amount of fibers, inorganic salts and pigments. The wastewater discharged during the bleaching process also contains a large amount of acidic and alkaline substances. The wastewater discharged from the paper machine is called "white water", which contains a large amount of fibers and fillers and adhesives added during the production process.
The treatment of wastewater from the paper industry should focus on increasing the rate of recycled water usage, reducing water consumption and wastewater discharge. At the same time, various reliable, economical and methods that can fully utilize the useful resources in the wastewater should also be explored. For example, the flotation method can recover the fibrous solid substances in the white water with a recovery rate of up to 95%, and the clarified water can be reused; the incineration method can recover sodium hydroxide, sodium sulfide, sodium sulfate and other sodium salts combined with organic substances in the black water.
The neutralization method is used to adjust the pH value of the wastewater; coagulation sedimentation or flotation methods can remove the suspended solids in the wastewater; chemical precipitation methods can decolorize; biological treatment methods can remove BOD, and are more effective for kraft paper wastewater; wet oxidation method is relatively successful in treating sulfite pulp wastewater. In addition, reverse osmosis, ultrafiltration, and electrodialysis treatment methods have also been adopted both domestically and internationally.
4. Dye production wastewater
The wastewater from dye production contains substances such as acids, bases, salts, halogens, hydrocarbons, amines, nitro compounds, dyes and their intermediates. Some of it also contains pyridine, cyanide, phenol, biphenylamine, as well as heavy metals like mercury, cadmium and chromium. The composition of this wastewater is complex and it is toxic, making it difficult to treat. Therefore, the treatment of dye production wastewater should be based on the characteristics of the wastewater and the requirements for its discharge, and the appropriate treatment methods should be selected.
For example: To remove solid impurities and inorganic substances, methods such as coagulation and filtration can be used; to remove organic substances and toxic substances, mainly chemical oxidation methods, biological methods and reverse osmosis methods are adopted; for decolorization, a process composed of coagulation and adsorption methods can be used; for removing heavy metals, ion exchange methods can be employed, etc.
5. Wastewater from the food industry
The characteristics of wastewater from the food industry are high content of organic substances and suspended solids, which is prone to spoilage and generally has no significant toxicity. The main harm is that it causes eutrophication of water bodies, leading to the death of aquatic animals and fish, promoting the formation of odors from organic substances deposited at the bottom of the water, deteriorating water quality and polluting the environment.
For the treatment of wastewater from the food industry, in addition to appropriate pre-treatment based on the characteristics of the water quality, biological treatment is generally preferred. If the effluent quality is highly demanding or the organic content in the wastewater is very high, two-stage aeration tanks or two-stage biological filters, or multi-stage biological disks, or the combined use of two biological treatment devices, or anaerobic-aerobic series can be adopted.
