Summary Overview
In simple terms, PAC and PAM are a classic “dynamic duo” in wastewater treatment, typically used together in the coagulation and flocculation process.
PAC acts as a Coagulant. Its main role is to neutralize charges and destabilize tiny suspended particles and colloids, causing them to form micro-flocs.
PAM acts as a Flocculant. Its main role is adsorption and bridging, which pulls the micro-flocs created by PAC together into large, dense, and fast-settling flocs.
Their relationship can be analogized as: PAC is the “glue” that binds sand into small clumps, and PAM is the “net” that gathers the clumps into a large, easy-to-remove boulder.
1. Polyaluminum Chloride (PAC)
1.1. Mechanism of Action
PAC is an inorganic polymer coagulant. Its coagulation mechanism is based on three key principles:
Charge Neutralization: Upon dissolving in water, PAC hydrolyzes to produce a multitude of positively charged complex ions (e.g., [Al(OH)₂]⁺, [Al₈(OH)₂₀]⁴⁺). Most colloidal particles in wastewater (e.g., clay, organic colloids) carry a negative surface charge. These like charges cause the particles to repel each other and remain stable in suspension. The positive ions from PAC effectively neutralize these negative charges, compressing the electrical double layer and causing destabilization. This allows the particles to collide and aggregate.
Adsorption and Bridging: The hydrolyzed products of PAC, such as aluminum hydroxide gel [Al(OH)₃], form a vast, net-like structure with a large surface area and strong adsorption capacity. This gel can “bridge” between the destabilized particles and suspended solids, forming initial, small “micro-flocs” or “pin flocs”.
Sweep Flocculation: At an optimal pH range, PAC generates a substantial precipitate of aluminum hydroxide. As this precipitate settles, it enmeshes and sweeps suspended particles down with it, similar to a falling net.
1.2. Usage Method
Solution Preparation: Solid PAC must first be dissolved into a liquid solution. A typical working solution concentration is 5%-10% (i.e., 50-100g of solid PAC per liter of water). It should be added slowly under agitation to prevent the formation of lumps.
Dosing Point: Typically added at the “rapid mix” or “coagulation tank” stage of the treatment process.
Mixing and Reaction: After dosing, rapid and intense mixing (approx. 1-3 minutes) is required to ensure complete and uniform dispersion of PAC in the wastewater, facilitating charge neutralization and initial floc formation.
Dosage Control: The optimal dosage must be determined by jar testing. Overdosing can lead to charge reversal (re-stabilization) and worse performance. Typical range is 50-200 mg/L.
1.3. Treatment Effects
Effective Removal of: Suspended Solids (SS), colloidal particles, some dissolved organic matter, and color.
Reduction in Key Parameters: Turbidity, Chemical Oxygen Demand (COD), and Total Phosphorus (TP, via the formation of aluminum phosphate precipitate).
Forms: Small, light flocs that settle slowly.
2.1. Mechanism of Action
PAM is an organic polymer flocculant. Its core mechanism is the bridging action of its long-chain polymer structure.
Adsorption and Bridging: PAM molecules have extremely long chains (molecular weights in the millions to tens of millions) with numerous active functional groups (e.g., amide groups -CONH₂, or ionic groups). These long chains act like “strings” or “bridges,” adsorbing onto the surface of one micro-floc and extending into the solution to adsorb onto others. This connects multiple flocs into large, dense, and rapid-settling agglomerates.
Charge Effects (Ionic Type):
Cationic PAM: Possesses a positive charge. In addition to bridging, it also provides charge neutralization, making it highly effective for organic, negatively charged sludges (e.g., municipal sewage, food processing wastewater). It is the most common type used for sludge dewatering.
Anionic PAM: Possesses a negative charge. It primarily relies on long-chain bridging and is often used in conjunction with inorganic coagulants like PAC, targeting the weakly positive flocs formed after coagulation.
Non-ionic PAM: Effective in acidic or neutral conditions and offers good salt tolerance.
2.2. Usage Method
Type Selection is Critical:
Inorganic suspensions (e.g., mineral processing, sand wash water): Often use Anionic PAM.
Organic sludge thickening and dewatering (e.g., municipal, industrial): Primarily use Cationic PAM.
Acidic or high-salinity wastewater: Non-ionic PAM can be considered.
Solution Preparation: Typically prepared as a 0.1%-0.3% solution. Dissolution requires more time (40-60 minutes) and gentle agitation to avoid shearing (breaking) the long polymer chains. An aging tank is essential for complete “activation” of the polymer.
Dosing Point: Added after PAC, in the “flocculation tank” or “slow mix zone.” Here, slow stirring (laminar flow conditions) is required to promote floc growth without breaking the formed agglomerates.
Dosage Control: The required dosage is very low, typically 1-5 mg/L, and must be determined by jar testing.
2.3. Treatment Effects
Significantly accelerates the settling velocity of flocs.
Forms large, strong, and compact flocs that are easy to separate.
Greatly improves sludge dewatering efficiency and increases the solid content of the dewatered cake.
Produces a clearer supernatant (treated water).
3. Collaborative Process & Key Considerations
Typical Process Flow:
Raw Water → (pH Adjustment) → PAC Dosing → Rapid Mix → PAM Dosing → Slow Mix → Sedimentation → Clarified Effluent / Sludge Handling
Key Considerations & Precautions:
Dosing Order is Critical: It must be PAC first, then PAM. Adding PAM first will cause it to adsorb onto individual stable particles, potentially shielding them and preventing PAC from effectively neutralizing their charges, leading to treatment failure.
pH Dependence: PAC works best in a pH range of 6.5 to 8.0. Extreme pH values impair its hydrolysis and effectiveness. pH adjustment may be necessary.
Complete Dissolution of PAM: Incompletely dissolved PAM, with “fish eyes” (gel lumps), is a waste of chemicals and severely reduces performance.
Mixing Intensity: High G-value (rapid mixing) for PAC dispersion; Low G-value (slow mixing) for PAM flocculation.
Jar Testing is Essential: Water quality varies significantly. Jar tests are the most cost-effective method to determine the optimal type and dosage of PAC/PAM, pH, and dosing points.
Safety & Environment: The PAM monomer (acrylamide) is a neurotoxin. Always select high-quality products with low residual monomer content. Treated sludge must be disposed of properly.
Summary of Combined Effects
Through the synergistic action of PAC and PAM, a wastewater treatment system can achieve:
Highly efficient removal of suspended and colloidal solids, with turbidity removal rates often exceeding 90%.
Significant reduction in COD and Total Phosphorus, which is crucial for meeting discharge standards.
Greatly enhanced solid-liquid separation, reducing the required footprint and retention time of sedimentation tanks.
Creation of better conditions for subsequent biological or advanced treatment processes.
Substantial improvement in sludge dewaterability.
