How Does PAC Influence Sludge Settling?

In wastewater treatment, sludge settling is a critical solid-liquid separation process that directly affects effluent quality and treatment efficiency. As a high-performance coagulant, Polyaluminum Chloride (PAC) can significantly improve sludge settling.

But how exactly does PAC affect sedimentation? What factors influence its efficiency? 

Three Key Mechanisms of PAC in Sludge Settling

(1) Charge Neutralization

-PAC hydrolyzes to form highly positive polymeric ions

-Neutralizes negative charges on sludge colloids

-Reduces electrostatic repulsion between particles

(2) Adsorption Bridging

-Active groups on PAC molecules adsorb multiple sludge particles

-Forms “particle-PAC-particle” network structure

-Significantly increases floc size and density

(3) Sweep Flocculation

-PAC forms aluminum hydroxide precipitate networks

-Traps fine suspended solids during settling

-Particularly effective for micron-sized particles

5 Critical Factors Affecting PAC Settling Efficiency

Practical Tips to Optimize PAC Performance

(1) Determine Optimal Dosage

-Conduct jar tests to establish dose-response curve

-Observe floc formation speed and settling ratio

-Avoid “more is better” misconception

(2) Select Appropriate PAC Type

-High basicity PAC(70-80%): For organic sludge

-Low basicity PAC(40-50%): For inorganic sludge

-Custom PAC: For special water quality

(3) Optimize Dosing Points

-Best dosing point is after primary sedimentation

-Multi-point dosing improves efficiency

-Ensure rapid mixing followed by slow stirring

(4) Combine with Other Chemicals

-PAC + Cationic PAM: Enhances dewatering

-PAC + Lime: Adjusts pH and improves settling

-PAC + Activated Carbon: Removes refractory organics

Troubleshooting Common PAC Settling Issues

Issue1: Small, loose flocs

-Causes: Insufficient PAC or improper pH

-Solutions: Increase dosage or adjust pH to 6.5-7.5

Issue2: Turbid supernatant

-Causes: Over-mixing or insufficient settling time

-Solutions: Optimize mixing program, extend settling time

Issue3: Sludge bulking

-Causes: Microbial imbalance or wrong PAC type

-Solutions: Switch to high-basicity PAC or add oxidant

How to Choose Quality PAC Products?

1.Check Basicity: 65-75% is generally recommended

2.Test Settling Speed: Good PAC should settle in 10-15 minutes

3.Measure Residual Aluminum: <0.2mg/L for drinking water

4.Verify Solubility: Quality PAC dissolves completely

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