Polyacrylamide (PAM) acts as a flocculant in the plate and frame filtration process. Its primary purposes are:
Flocculation: To agglomerate fine suspended particles in wastewater into large, dense flocs.
Reducing Specific Resistance: The floc structure facilitates easier solid-liquid separation, significantly lowering the sludge’s filtration resistance and allowing water to pass through the filter cloth more rapidly.
Improving Efficiency: To shorten the filtration cycle time and increase processing capacity per batch.
Enhancing Results: To produce drier, firmer filter cakes with higher solid content, reducing cake volume for cheaper transportation and disposal.
Incorrect product selection is the most common cause of poor filter press performance. The three main types of PAM are: Cationic (CPAM), Anionic (APAM), and Non-ionic (NPAM). Selection must be determined through laboratory jar testing, following the principle of “Lab Test → Pilot Test → Full-scale Test.”
This is the primary basis for selection. Sludge characteristics are determined by its origin.
| Sludge Type | Key Characteristics | Recommended PAM Type | Rationale |
|---|---|---|---|
| Organic Sludge | Negatively charged, highly hydrophilic, difficult to dewater. e.g., Municipal sewage, food processing, slaughterhouse, paper mill sludge. |
Cationic (CPAM) | The positive charge groups neutralize the negative charges on sludge particles (charge neutralization) and bridge them together (polymer bridging), forming strong, compact flocs. This effectively overcomes the hydrophilicity and negative charge of organic sludge. |
| Inorganic Sludge | Often positively charged, higher specific gravity, settles easily. e.g., Steel mill, electroplating, coal washing, kaolin slurry. |
Anionic (APAM) | Anionic polymers primarily function through powerful polymer bridging to connect dispersed inorganic particles into large, strong flocs. Charge neutralization is a secondary mechanism. |
| Neutral or Mixed Sludge | Complex properties, neutral or weak charge. | Non-ionic (NPAM) or Weak CPAM/APAM |
Non-ionic types are less affected by pH in neutral or acidic conditions and rely on their strong adsorption and bridging capabilities. Suitable for mixed sludges where charge is difficult to determine. |
Simple Rule of Thumb:
Municipal Wastewater Treatment Plants: Almost exclusively use Cationic PAM, typically with an Ionicity between 40%-60%.
Industrial Wastewater Sludge: Requires jar testing based on the specific industry and water characteristics.
Mineral Processing, Coal Washing: Mostly use Anionic PAM with very high molecular weight.
Ionicity (For CPAM/APAM):
Refers to the proportion of charged groups on the polymer chain.
Principle: The stronger the negative charge of the sludge (higher organic content, more colloids), the higher the ionicity of the CPAM required.
Common Range: CPAM ionicity typically ranges from 20% to 60%. Municipal sludge commonly uses 40%-50%. Too high an ionicity can re-stabilize particles; too low provides insufficient charge neutralization.
Molecular Weight:
Refers to the length of the polymer chain. Higher molecular weight means longer chains, greater bridging ability, and larger flocs.
Principle: For filter presses, we need flocs that are not only large but also dense and resistant to squeezing.
Common Range: Medium to high molecular weight (8 – 18 million Daltons) is typically selected. Very high MW (>20 million) may produce large but fluffy “cotton candy” flocs that break under pressure and blind the filter cloth.
Improper dissolution will render even the correctly selected PAM ineffective.
Preparation Concentration: Typically 0.1% – 0.3% (i.e., 1-3 kg of PAM powder per ton of water). Concentrations for filter presses may be slightly higher than for centrifuges or DAF.
Dissolution Time: Requires 40-60 minutes of gentle agitation to fully dissolve. The final solution should be transparent, viscous, and free of visible fish-eyes (undissolved gel clusters).
Crucial Note: NEVER add dry powder directly to the sludge! Avoid high-shear, violent mixing during dissolution, as it will shear (break) the polymer chains, destroying their effectiveness. Use dedicated automatic preparation units.
Correctly selected and applied PAM delivers the following significant benefits:
Dramatically Improved Feeding Efficiency:
Unconditioned sludge can blind filter cloth channels immediately, causing feed pressure to rise rapidly and extending feed time.
PAM-formed flocs maintain cloth permeability, allowing for faster pumping and shorter overall cycle times.
Significantly Lower Cake Moisture:
This is the core benefit. Dense floc structures release water more effectively under high pressure instead of blinding the cloth.
For municipal sludge, using the right CPAM can reduce cake moisture from >85% (without chemical) to <60% or lower. This reduces cake volume by over half, drastically cutting disposal costs.
Formation of a Solid, Releasable Cake:
Good flocculation produces a uniform cake structure that doesn’t stick to the cloth, allowing for more complete cake release during the opening cycle and reducing cloth cleaning efforts.
Reduced Cloth Blinding and Extended Life:
Fine particles can penetrate and irreversibly clog the cloth. PAM traps these fines within large flocs that sit on the cloth surface, making them easier to wash off, thereby extending cloth life.
Dosing Point and Mixing Energy:
The PAM solution should be mixed with the sludge upstream of the filter press, ideally in a static mixer or a conditioning tank.
Mixing energy is critical: Too low results in uneven distribution and poor performance; too high shears and breaks apart the formed flocs. Observe floc size (ideally 3-5mm) and adjust conditions.
Dosage:
More is not better. Overdosing can re-stabilize sludge particles (especially with CPAM), making flocs sticky and dispersible, which blinds the filter cloth and hinders dewatering. The optimal dose (e.g., 3-5 kg per ton of dry solids) must be found through testing.
Blending with Other Chemicals (Conditioning):
For particularly difficult sludges (e.g., oily, viscous), a combination of “PAM + inorganic coagulant (e.g., PAC, Ferric Chloride)” is often used.
Sequence: Typically, add the coagulant (PAC) first for charge neutralization and destabilization, followed by PAM for flocculation. This combination can further reduce costs and cake moisture.
Test First: There is no universal PAM. Always conduct laboratory jar tests with samples of different ionicities and molecular weights. Observe floc size, strength, settling speed, and supernatant clarity.
Focus on Floc Quality: Filter presses require “dense and strong” flocs, not “big and fluffy” ones.
System Optimization: PAM performance is intertwined with sludge properties (pH, concentration), PAM preparation, dosage, mixing conditions, and the filter press’s operating pressure and cycle time. It must be treated as a integrated system to find the optimal operating parameters.
