Dosing method for treating high turbidity water with polyaluminum chloride
Release time: 2025-07-28
For the scientific application of polyaluminum chloride (PAC) in high turbidity water bodies, combined with industry practice and experimental data, the following step-by-step operation guidelines and key precautions are as follows:
I. Method of determining the basic dosage
- Formal estimate (theoretical reference)
- PAC dosage (kg) = total water volume (ton) × COD concentration of sewage (mg/L) × PAC dosage concentration (mg/L)/alkalinity ratio
- Alkalinity ratio: generally take 1.2~1.5, according to the proportion of alkaline ions in water quality.
- Application scenario: It is suitable for scenarios with clear water quality parameters and small fluctuations, but it needs to be combined with experimental verification.
- Alkalinity ratio: generally take 1.2~1.5, according to the proportion of alkaline ions in water quality.
- Small trial (precise recommendation)
- Solution preparation: Mix solid PAC with clean water in a weight ratio of 1:5, dissolve it, and then dilute it to a concentration of 2-3% (below 1% is easy to hydrolyze and fail).
- Bottle of coffee simulation:
- Solution preparation: Mix solid PAC with clean water in a weight ratio of 1:5, dissolve it, and then dilute it to a concentration of 2-3% (below 1% is easy to hydrolyze and fail).
- Multiple samples of equal amount of high turbidity water were taken and PAC solution was added at different doses (e.g., 5mg/L, 10mg/L, 15mg/L).
- Stir rapidly for 1 minute to simulate mixing, and stir slowly for 10 minutes to simulate flocculation. Observe the size of alum flowers, sedimentation speed and turbidity of supernatant after standing.
- Best dose: choose the dosage with dense alum flowers, fast sedimentation (within 10 minutes) and transparent supernatant.
II. Reference dosage in different turbidity ranges
Adjust the amount of PAC according to the initial turbidity of water:
- Turbidity 100-500mg/L: 5~10kg PAC per thousand tons of water (i.e. 5~10mg/L).
- Turbidity>500mg/L: 10-20kg PAC per thousand tons of water should be added, and strengthening coagulation measures should be combined.
- Extreme high turbidity (e.g., flood period): can increase to 20-30kg/1000 tons of water, and the precipitation effect is monitored simultaneously.
III. Dynamic adjustment techniques during operation
- Observe the condition of sedimentation tank:
- Insufficient dosage: fine alum flowers, slow precipitation, high residual turbidity → 10%~20% dosage needs to be increased.
- Excessive dosage: large but upside-up flocculation, turbid effluent → reduce dosage and strengthen sludge discharge.
- Insufficient dosage: fine alum flowers, slow precipitation, high residual turbidity → 10%~20% dosage needs to be increased.
- Dealing with water quality fluctuations:
- Low temperature (<4℃): PAC hydrolysis is slowed down, and the dosage should be increased by 20%~30%.
- pH unusual ：
- Acidic water (pH <6): inhibit coagulation, it is necessary to add PAC or pre-adjust pH to neutral.
- Alkaline water (pH>8): enhance coagulation, can be reduced appropriately.
- Acidic water (pH <6): inhibit coagulation, it is necessary to add PAC or pre-adjust pH to neutral.
- Low temperature (<4℃): PAC hydrolysis is slowed down, and the dosage should be increased by 20%~30%.
IV. Key efficiency improvement and pit avoidance suggestions
- Combining drugs improves the effect
- For high turbidity water (such as mine water and coal washing wastewater), PAC is first added to neutralize the charge, and then anionic polyacrylamide (PAM) bridge flocculation is added, which can reduce the amount of PAC by more than 30%.
- Avoid equipment corrosion
- The PAC solution is weakly acidic (pH 4~6), and the dosing equipment should be made of corrosion-resistant materials (such as PVC), and the operators should wear protective equipment.
- Regular optimization and monitoring
- Check the injection system every quarter to prevent pipeline blockage or measurement deviation.
- When the turbidity of raw water changes>20%, adjust the parameters again in small test.
- Check the injection system every quarter to prevent pipeline blockage or measurement deviation.
In high-turbidity water bodies, PAC dosing should follow the principle of "initial dosage determined by pilot tests + fine-tuning during operation": Pilot test phase Select initial dosage based on turbidity → Operation phase Adjust dosage according to observed floc formation → Special scenarios Combine PAM/ dosage adjustment pH. Through scientific dosing, the effluent turbidity from sedimentation tanks can be stabilized at ≤5 NTU, reducing treatment costs per ton of water by 15%-30%. For practical applications, refer to:
- Coal mine wastewater treatment: turbidity 500 → 5mg/L
- River water purification in flood period: 20mg/L standard
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