Industrial wastewater refers to the wastewater, sewage, and waste liquid generated in the industrial production process, which contains industrial production materials, intermediate products and products that are lost with water, as well as pollutants generated in the production process. Six treatment processes for industrial wastewater, including chemical industry wastewater, printing and dyeing industry wastewater, papermaking industry wastewater, dye production wastewater, food industry wastewater, and pesticide wastewater, are summarized.

1. Characteristics and treatment process of desulfurization wastewater from coal-fired power plants

Characteristics of desulfurization wastewater from power plants:

Most desulfurization devices in power plants use the flue gas limestone gypsum wet desulfurization process. This process mainly consists of a limestone slurry preparation system, a gypsum dewatering system, and a desulfurization wastewater treatment system. The water in the slurry of the desulfurization device will accumulate heavy metal elements and Cl - during continuous circulation, which accelerates the corrosion of the desulfurization equipment and affects the quality of gypsum. Therefore, it is necessary to discharge the wastewater in a timely manner.

Power plant desulfurization wastewater treatment process:

The process flow of desulfurization wastewater treatment in power plants: desulfurization wastewater → wastewater tank → wastewater pump → pH neutralization tank → settling tank → flocculation tank → clarifier → outlet tank → outlet pump → standard discharge

The desulfurization wastewater treatment system includes three parts: wastewater treatment, dosing, and sludge treatment. The wastewater treatment system mainly consists of waste water tank, triple box, clarification tank, sludge pump, outlet water tank, clean water pump, fan, dewatering machine and other equipment. In addition to a large amount of Cl - and Mg2+, impurities in desulfurization wastewater also include fluoride, nitrite, etc; Heavy metal ions, such as Cu2+, Hg2+, etc; Insoluble CaSO4 and fine dust. To meet the wastewater discharge standards, corresponding wastewater treatment devices need to be equipped.

2. Chemical industry wastewater

Chemical industry wastewater mainly comes from production wastewater discharged from petrochemical industry, coal chemical industry, acid-base industry, fertilizer industry, plastic industry, pharmaceutical industry, dye industry, rubber industry, etc.

The main measures for the prevention and control of chemical wastewater pollution are: first, reform of production processes and equipment, reduction of pollutants, prevention of wastewater discharge, comprehensive utilization and recovery; The treatment level of wastewater that must be discharged should be selected based on water quality and requirements.

The primary treatment mainly separates suspended solids, colloids, floating oil or heavy oil in water. Methods such as water quality and quantity regulation, natural sedimentation, flotation, and oil separation can be used.

Secondary treatment mainly involves the removal of biodegradable organic dissolved substances and some colloidal substances, reducing the biochemical oxygen demand and some chemical oxygen demand in wastewater, usually using biological methods for treatment. After biological treatment, there is still a considerable amount of COD remaining in the wastewater, sometimes with high color, smell, and taste, or due to high environmental hygiene standards, a three-level treatment method needs to be used for further purification.

The third level treatment mainly aims to remove organic pollutants and dissolved inorganic pollutants that are difficult to biodegrade from wastewater. The commonly used methods include activated carbon adsorption and ozone oxidation, as well as ion exchange and membrane separation technologies. Various chemical industry wastewater can be treated using different methods according to the requirements of different water quality, quantity, and discharged water quality after treatment.

3. Printing and dyeing industry wastewater

The printing and dyeing industry consumes a large amount of water, typically consuming 100-200 tons of water per 1 ton of textile processed. 80% to 90% of them are discharged as printing and dyeing wastewater. The commonly used governance methods include recycling and harmless treatment.

Recycling:

Wastewater can be recycled and reused according to the characteristics of water quality, such as the separation of bleaching and refining wastewater and dyeing and printing wastewater. The former can be washed by convection. One water can be used for multiple purposes to reduce emissions;

Alkali recovery and utilization usually use the evaporation method. If the amount of alkali is large, it can be recovered by three effect evaporation. If the amount of alkali is small, it can be recovered by thin film evaporation;

Dye recycling, such as Shilin dyes, can be acidified to form cryptic acid, which forms colloidal particles and is suspended in the residual solution. After precipitation and filtration, it is recycled and reused.

Harmless treatment can be divided into:

There are physical treatment methods such as precipitation and adsorption. The sedimentation method mainly removes suspended solids from wastewater; The adsorption method is mainly used to remove dissolved pollutants and decolorize wastewater.

Chemical treatment methods include neutralization, coagulation, and oxidation. The neutralization method is to regulate the acidity and alkalinity of wastewater, and can also reduce the chromaticity of wastewater; The coagulation method is used to remove dispersed dyes and colloidal substances from wastewater; The oxidation method involves oxidizing reducing substances in wastewater to precipitate sulfide and reducing dyes.

Biological treatment methods include activated sludge, biological rotary table, biological rotary drum, and biological contact oxidation. In order to improve the effluent quality, meet discharge standards or recovery requirements, it is often necessary to use several methods for joint treatment.

4. Paper industry wastewater

The wastewater from papermaking mainly comes from the two production processes of pulping and papermaking in the papermaking industry. Pulping is the process of separating fibers from plant raw materials, making pulp, and then bleaching it; Paper making is the process of diluting, shaping, pressing, and drying pulp to produce paper. Both processes discharge a large amount of wastewater.

The wastewater generated from pulping is the most severely polluted. The wastewater discharged during pulp washing is black brown in color, known as black water. The concentration of pollutants in black water is high, with BOD reaching 5-40g/L, containing a large amount of fibers, inorganic salts, and pigments. The wastewater discharged from the bleaching process also contains a large amount of acidic and alkaline substances. The wastewater discharged by 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 papermaking industry wastewater should focus on improving the recycling water rate, reducing water consumption and wastewater discharge, and actively exploring various reliable, economical, and fully utilizing useful resources in wastewater treatment methods. For example, flotation can recover fibrous solid substances from white water with a recovery rate of up to 95%, and clarified water can be reused; The combustion method can recover sodium hydroxide, sodium sulfide, sodium sulfate, and other sodium salts combined with organic matter from black water.

Neutralization method to regulate the pH value of wastewater; Coagulation sedimentation or flotation methods can remove suspended solids from wastewater; Chemical precipitation method can decolorize; The biological treatment method can remove BOD and is more effective in treating kraft paper wastewater; The wet oxidation method has been successful in treating sulfite pulp wastewater. In addition, domestic and foreign treatment methods such as reverse osmosis, ultrafiltration, and electrodialysis are also used.

5. Dye production wastewater

Dye production wastewater contains acids, bases, salts, halogens, hydrocarbons, amines, nitro compounds, dyes and their intermediates, as well as pyridine, cyanide, phenols, biphenylamine, and heavy metals such as mercury, cadmium, and chromium. These wastewater components are complex, toxic, and difficult to treat. Therefore, the treatment of dye production wastewater should be based on its characteristics and discharge requirements, and appropriate treatment methods should be selected.

For example, to remove solid impurities and inorganic substances, coagulation and filtration methods can be used; The main methods for removing organic matter and toxic substances include chemical oxidation, biological methods, and reverse osmosis; The decolorization process can generally be achieved through a combination of coagulation and adsorption methods, while the removal of heavy metals can be achieved through ion exchange methods.

6. Food industry wastewater

The food industry has a wide range of raw materials and a wide variety of products, resulting in significant differences in the amount and quality of wastewater discharged.

The main pollutants in wastewater include: solid substances floating in the wastewater, such as vegetable leaves, fruit peels, minced meat, poultry feathers, etc; The substances suspended in wastewater include oils, proteins, starch, colloidal substances, etc; Acids, alkalis, salts, sugars, etc. dissolved in wastewater; Mud, sand, and other organic matter carried by raw materials; Pathogenic bacteria and toxins.

The characteristics of food industry wastewater are high content of organic matter and suspended solids, easy spoilage, and generally no significant toxicity. Its harm mainly lies in eutrophication of water bodies, causing the death of aquatic animals and fish, promoting the odor of organic matter deposited at the bottom of the water, deteriorating water quality, and polluting the environment.

In addition to appropriate pre-treatment according to the characteristics of water quality, biological treatment is generally recommended for the treatment of food industry wastewater. If there is a high requirement for the quality of the effluent or due to the high organic content in the wastewater, a two-stage aeration tank or two-stage biological filter, or a multi-stage biological rotary table can be used. Alternatively, a combination of two biological treatment devices can be used, or an anaerobic aerobic series connection can be used.

Source: Environmental Engineering