synergistic effect of pac and pam in water treatment

2025-08-16

In sewage treatment, the combined use of PAC (Polyaluminum Chloride) and PAM (polyacrylamide) is a classic combination in the industry. The fundamental reason lies in the complementary mechanism and synergistic effect of the two, and the best treatment effect cannot be achieved by using them separately. The following is a point-by-point analysis of the core reasons:

First, different mechanisms of action, each with its own functions

● PAC (coagulant) —— Destabilizes

● Charge neutralization: After PAC hydrolysis, a positively charged polymeric hydroxyl complex (such as Al₁₃) is generated to neutralize the surface charge of negatively charged colloidal particles in water and eliminate the electrostatic repulsion.

● Compression of double electric layer: reduce the repulsive force between colloidal particles, promote the destabilization of small particles and coagulate into fine flocs (alum flowers).

● Limitations: The formed flocs are small and loose, slow to settle, poor in dehydration, and high dosage is required for high turbidity wastewater.

● PAM (flocculant) —— Build large flocculation

● Adsorption bridge: The long molecular chain of PAM adsorbs multiple destabilized particles and connects them into a network structure through the "bridge" effect.

● Netting and sweeping: The large flocculent body catches the free particles like a fishing net and accelerates the sedimentation.

● Limitations: It is ineffective for the un-decayed charged colloid, and can not break the emulsion or remove the dissolved organic matter alone.

2. Synergistic effect to break through the bottleneck of a single agent

The dimension of comparison	The defect of PAC is independent	PAM has a defect when used alone	Joint use advantage
Mass of floc	Fine and loose, slow settling	Stable colloids cannot be effectively combined	Form large and dense flocs, the sedimentation rate increased by more than 10 times
Dosage and administration	High doses are required to achieve this	It depends on the premise of detuning	Reduce PAC consumption by 30% -50% to reduce costs and residual aluminum risk
scope of application	Difficult to treat high turbidity/high COD wastewater	Unable to break the milk or remove phosphorus	Efficient treatment of oil, colloid, heavy metal and other complex wastewater
Sludge dewatering properties	Clay cakes have high moisture content	No significant improvement	The fiber is strong and tough, the sludge resistivity is reduced, and the dehydration efficiency is improved

3. The injection order should not be reversed

●PAC before PAM:

● PAC takes 1-3 minutes to complete hydrolysis and destabilization, forming microflocs;

● PAM was added and the microflocs were bridged into large particles by slow stirring (10-30 minutes).

● Consequences of wrong order:

● If PAM is added first, the charged colloid is not neutralized, and the bridging effect is invalid;

● If added at the same time, the positive charge of PAC will make the PAM molecular chain curl up and inactivate.

4. Typical application scenarios verify the necessity of collaboration

● Waterworks/sewage plant: PAC removes phosphorus and decolorizes → PAM accelerates the separation of secondary sedimentation tank;

● Industrial Wastewater (dyeing, paper making): PAC demulsification, charge neutralization → PAM flocculation of heavy metals and organic matter;

● Sludge dewatering: PAC destroys the colloid structure and PAM forms hard flocs that can be filtered.

sum up

PAC and PAM are like the "golden partners" in sewage treatment:

● PAC is a "game-breaker" —— it breaks down the stability of pollutants;

● PAM is the "builder" —— that restructures the fragments into easily separable flocs.

The two work together to achieve the effect of 1+1> 2: achieving water purification and solid-liquid separation goals at lower cost and faster speed. In practical application, PAM types (anionic, cationic, non-ion) should be selected according to water quality characteristics (e.g., pH, turbidity) and the addition parameters should be optimized.