Why Does Water Become Turbid After Adding PAC?

Polyaluminum Chloride (PAC) is a widely used coagulant in industrial wastewater treatment, municipal sewage, and drinking water purification. However, some users find that after dosing PAC, the water becomes more turbid instead of clearer. What causes this issue?

Possible Reasons

Improper Dosage
If PAC is overdosed, excess unreacted PAC remains in the water, increasing turbidity. If underdosed, suspended particles cannot fully coagulate, leading to cloudy water.

Unsuitable Mixing Conditions
Coagulation requires proper mixing. If mixing is too strong, flocs break apart; if too weak, chemicals cannot react effectively, resulting in turbidity.

Unfavorable pH Range
PAC works best in the pH range of 6.5–7.5. Outside this range, its hydrolysis products are less effective, and water may remain turbid.

Complex Raw Water Quality
When wastewater contains high levels of organic matter, oil, dye, or colloids, PAC alone may not be sufficient. Additional chemicals such as PAM (Polyacrylamide) or activated carbon may be required.

Solutions

      √ Jar Test First: Determine the optimal PAC dosage through lab testing before full-scale use.

      √ Adjust pH: Use lime, caustic soda, or acid to adjust water pH to the optimal range before dosing.

      √ Optimize Mixing: Apply fast mixing (30–60 seconds) for dispersion, then slow mixing (10–15 minutes) for floc growth.

      √ Use Combination Treatment:PAC combined with PAM or activated carbon significantly improves results.

FAQ

Q1: Why does water turn cloudy after adding PAC?
A1: Mainly due to incorrect dosage, improper pH, poor mixing, or complex water composition.

Q2: What is the typical PAC dosage?
A2: Usually 10–50 mg/L, but exact dosage must be determined by jar testing.

Q3: Should PAC be used with PAM?
A3: Yes. PAC provides initial coagulation, while PAM enhances flocculation and sedimentation.

Q4: Does PAC require specific pH conditions?
A4: Yes, it works best at pH 6.5–7.5.

Q5: How can turbidity after PAC dosing be avoided?
A5: By optimizing dosage, adjusting pH, improving mixing, and using coagulants in combination.