A sewage treatment plant needs to add various chemicals during the treatment process to ensure that the effluent quality meets the standards. Commonly used chemicals include coagulants, flocculants, carbon sources, etc. This article will introduce the dosage ratios, technical indicators and practical operation techniques of these chemicals. In actual use, adjustments should be made according to the specific water quality and treatment objectives, and the optimal dosage may need to be determined through experiments.
Coagulant dosage ratio
Coagulants are mainly used to remove suspended solids and colloidal particles in sewage. Commonly used coagulants include aluminum sulfate, polyaluminum chloride (PAC), and ferric chloride.
1. Aluminum sulfate
Scope of application: suitable for sewage with high pH and high alkalinity.
Dosage ratio: generally 50-150 mg/L.
Technical index: the content of aluminum sulfate’s active ingredient Al2O3 should be above 15%.
Operating tips: Aluminum sulfate should be fully dissolved before use, and the dissolution time is about 30 minutes. The general dissolution concentration is 5-10%.
Example: A sewage treatment plant with a treatment capacity of 1000 m³/h and a raw water pH of 7.5 needs to add aluminum sulfate. Based on experience, the dosage is 100 mg/L, which means 100 kg of aluminum sulfate needs to be added per hour.
2. Polyaluminum chloride (PAC)
Scope of application: Suitable for sewage with various pH values, especially high turbidity sewage.
Dosing ratio: Generally 10-50 mg/L.
Technical index: The alkalinity of PAC should be between 50-80%.
Operating tips: PAC should be fully dissolved before use, and the dissolution time is about 30 minutes. The general dissolution concentration is 5-10%.
Example: A sewage treatment plant with a capacity of 500 m³/h and high raw water turbidity needs to add PAC. Based on experience, the dosage is 30 mg/L, which means 15 kg of PAC needs to be added per hour.
The actual concentration should be adjusted according to the specific water quality and treatment goals, and the optimal dosage may need to be determined through experimentation.
Typical dosage concentration: The typical dosage concentration of polyaluminium chloride is 10%-16%. This means that 10-16 kg of PAC dry powder is contained in a 100 kg PAC solution. This concentration refers to the concentration of the chemical solution, not directly related to the volume of water.
Dosage
The dosage per ton of sewage: The dosage of polyaluminium chloride is about 0.5-2kg of dry PAC powder per ton of sewage. This dosage is determined based on the characteristics of the sewage and the treatment requirements. The actual use should be adjusted according to the specific water quality and treatment objectives.
Dosage for different water qualities: The dosage of PAC varies under different water quality conditions. Here are some reference values:
Domestic water: 2.5-25kg/1,000 tons of water
Industrial water: 2.5–25 kg/1,000 tons of water
Municipal sewage: 15–50 kg/1,000 tons of water
Electroplating wastewater: 20–100 kg/1,000 tons of water
Metallurgical wastewater: 20–150 kg/1,000 tons of water
Paper-making wastewater: 50–300 kg/1,000 tons of water
Tanning wastewater: 100–300 kg/1,000 tons of water
Food waste water: 50-150 kg/1,000 tons of water
Chemical waste water: 50-100 kg/1,000 tons of water
Emulsification waste water: 50-200 kg/1,000 tons of water
Coal washing waste water: 30-100 kg/1,000 tons of water
These figures provide a rough ratio and dosage of different chemicals per ton of water treated. The actual usage should be adjusted according to the specific water quality and treatment objectives, and the optimal dosage may need to be determined through experimentation.
3. Ferric chloride
Scope of application: suitable for sewage containing heavy metal ions.
Dosage ratio: generally 10-50 mg/L.
Technical index: the content of the active ingredient Fe2O3 in ferric chloride should be above 30%.
Operating tips: Ferric chloride should be fully dissolved before use, with a dissolution time of about 30 minutes and a general dissolution concentration of 5-10%.
Example: An industrial wastewater treatment plant with a treatment capacity of 200 m³/h needs to remove heavy metal ions and needs to add ferric chloride. Based on experience, the dosage is 20 mg/L, which means 4 kg of ferric chloride needs to be added per hour.
Dosage ratio of flocculant
Flocculants are mainly used to enhance the coagulation effect. Commonly used flocculants include polyacrylamide (PAM), etc.
1. Polyacrylamide (PAM)
Scope of application: suitable for flocculation treatment of various sewage.
Dosage ratio: generally 0.1-0.5 mg/L.
Technical index: the molecular weight of PAM should be above 10 million.
Operating technique: PAM should be fully dissolved before use, and the dissolution time is about 60 minutes. The general dissolution concentration is 0.1-0.5%.
Example: A sewage treatment plant with a treatment capacity of 1000 m³/h needs to add PAM. According to experience, the dosage is 0.3 mg/L, so 0.3 kg of PAM needs to be added per hour.
Dosing ratio of carbon source chemicals
Carbon source chemicals are mainly used in the biological nitrogen removal process. Commonly used carbon source chemicals include methanol, ethanol, acetic acid, etc.
1. Methanol
Scope of application: suitable for denitrification and nitrogen removal processes.
Dosing ratio: generally 3 mg/L (for every 1 mg/L of nitrate removed).
Technical index: the purity of methanol should be above 99%.
Operating tips: Methanol should be directly added to the anoxic section and thoroughly mixed with the influent and the mixed liquid by the mixer.
Example: A sewage treatment plant with a treatment capacity of 1000 m³/h needs to remove 20 mg/L of nitrate nitrogen and needs to add methanol. Based on experience, the dosage is 60 mg/L, which requires the addition of 60 kg of methanol per hour.
Practical operating tips
1. Dissolving and storing chemicals
Chemicals should be fully dissolved before use, and the dissolution time and concentration should be determined according to the specific characteristics of the chemical. Chemicals should be stored in a dry, cool, well-ventilated place, away from direct sunlight and high temperatures.
2. Selection of dosing points
The dosing point for chemicals should be selected in a place with strong turbulent water flow as much as possible to ensure that the chemicals can spread quickly and evenly. For multi-stage treatment processes, chemicals should be dosed in stages to improve the treatment effect.
3. Dosing equipment
A metering pump should be selected for the dosing equipment to ensure the accuracy of the dosage. The dosing equipment should be regularly maintained to ensure its normal operation.
4. Online monitoring and control
An online monitoring system should be installed to monitor the influent and effluent water quality in real time and adjust the dosage of chemicals in a timely manner. An automatic control system should be installed to automatically adjust the dosage of chemicals according to changes in water quality to ensure that the effluent water quality meets the standards stably.
By reasonably selecting and dosing chemicals, the sewage treatment plant can effectively remove pollutants in the sewage and ensure that the effluent quality meets the standards. At the same time, by optimizing the dosing process and equipment, the treatment efficiency can be improved and operating costs reduced.
The dosing ratio of various chemicals needs to be determined based on the specific water quality and experimental results to achieve the best treatment results.