PolyDADMAC is a commonly used cationic flocculant/conditioner. Its field performance is determined by multiple factors. This article focuses on the mechanisms and engineering countermeasures for three major categories of factors: pH, temperature and water quality, to assist formulators and engineers in product selection and process optimisation.

1. pH: Decisive Influence on Charge Neutralisation and Adsorption-Bridging

Recommended operating range: pH ≈ 4–9 (for most systems).

Excessive acidity: pH < ~4 significantly reduces PolyDADMAC efficacy.

Mechanism: Abundant H⁺ in water neutralises or reverses the negative charge on colloidal surfaces (rendering particles neutral or positively charged), preventing cationic polymers from functioning via ‘charge neutralisation’. Concurrently, excess H⁺ may alter the conformation of dissolved organic matter/surfactants, inhibiting bridging formation.

Excessive alkalinity: pH > ~9 reduces flocculation stability.

Mechanism: High OH⁻ concentrations form hydroxide complexes (e.g., Al(OH)₄⁻) with colloidal surfaces and certain metal ions. These complexes may carry negative charge and stabilise particles. Furthermore, OH⁻ interactions with specific functional groups alter surface adsorption sites, impairing PolyDADMAC's adsorption/bridging efficacy.

Note: Specific ‘thresholds’ may shift slightly depending on water composition (e.g., high dissolved organic matter or saline water bodies), thus requiring laboratory confirmation.

pH Adjustment Chemical Selection (Engineering Recommendations)

Preferred: NaOH (sodium hydroxide) — Pure, rapid, and does not significantly increase total hardness.

Alternative: Sodium carbonate (Na₂CO₃) — Suitable for systems where strict low hardness is not required and alkaline buffering is beneficial.

Use with caution: Lime (Ca(OH)₂) — Introduces Ca²⁺, increasing hardness, and may cause calcium carbonate precipitation (impeding process) in CO₂-rich or high-carbonate environments. Suitable for scenarios requiring simultaneous alkaline addition and precipitation of metals/phosphorus.

Operational Key Points: pH adjustments should be made incrementally → Following each adjustment, allow brief contact/agitation and observe floc formation and effluent turbidity before deciding whether to continue adjustment.

2. Temperature: Physical effects outweigh chemical degradation

Applicability Range: 20–35°C yields good results under common operating conditions; low temperatures (<10°C) significantly reduce flocculation kinetics; High temperatures (>45°C) require vigilance regarding changes in physical stability.

Mechanism

Low temperatures (↓): Water viscosity ↑, molecular diffusion and collision frequency ↓ → Reduced contact/adsorption between polymer chains and colloidal particles → Slower floc formation with poor strength. At low temperatures, it is often necessary to moderately increase dosage or extend contact/reaction time, or combine with inorganic coagulants to enhance collision efficiency.

High Temperature (↑): Enhanced thermal motion, reduced viscosity, and shear forces (under pumping/agitation) readily disrupt established adsorption-bridging structures. Additionally, elevated temperatures may alter solubility and surfactant conformation, thereby affecting polymer adsorption sites and bridging efficiency. Consequently, flocs may appear large yet remain ‘loose and fragile’, disintegrating during sedimentation or filtration and causing turbidity.

Engineering countermeasures: For high-temperature effluent, reduce intense shear, optimise mixing energy, and extend slow mixing phases; for low-temperature conditions, increase contact time or incorporate high-efficiency coagulant aids/temperature-raising pretreatment (where feasible).

3. Water Quality Conditions: Systematic Impacts from Ion Composition to Organics

3.1 Turbidity / Suspended Solids (SS)

Low turbidity (colloidal-dominant): PolyDADMAC primarily neutralises charges; low molecular weight grades react rapidly with minimal residue.

High turbidity (coarse/dense particles): Requires stronger bridging (medium/high molecular weight) or sequential aids to form stable flocs.

3.2 Dissolved Organics (Humic Substances, Organic Colloids, etc.)

Organic colloids may strongly compete with PolyDADMAC for adsorption, reducing effective polymer concentration post-adsorption. This necessitates increased dosage or a strategy of first neutralising organic colloids with low molecular weight products before employing high molecular weight products to enhance bridging. For water bodies with high colour/high COD, co-formulation with adsorbents or PAC-type inorganic coagulants is often recommended.

3.3 Metal Ions (Fe³⁺, Al³⁺, Ca²⁺, Mg²⁺, etc.)

Trivalent metals (Fe³⁺, Al³⁺): Typically exhibit good synergistic effects with PolyDADMAC. Their hydrolysed polyvalent hydroxyl cations (e.g., Al₁₃⁺) form flocculation nuclei, enhancing floc density and sedimentation rate.

Divalent metals (Ca²⁺, Mg²⁺): Generally beneficial for flocculation. They reduce electrostatic repulsion between particles by compressing the double electric layer, thereby promoting destabilisation and aggregation. This can partially reduce PolyDADMAC dosage.

However, note that under high alkalinity/high carbonate conditions, Ca²⁺/Mg²⁺ may form carbonate precipitates (CaCO₃/MgCO₃), which can affect subsequent filtration, appearance, or cause pipeline scaling.

Engineering Considerations: Identify the types and concentrations of metal ions in the raw water. If containing Al/Fe, a combination strategy typically reduces total chemical consumption and improves sedimentation. If containing high Ca/Mg, consider whether softening is required or if carbonate precipitation issues need to be avoided.

3.4 Anionic Components (Sulphate, Carbonate, Humic Acid, etc.)

High concentrations of anions/chelating agents (e.g., humic acid, carbonate) may coordinate with or neutralise PolyDADMAC, increasing dosage requirements or altering floc properties. For surface water containing humic acid, initial treatment with low molecular weight rapid neutralisation followed by medium/high molecular weight bridging is often more effective.

4. Field/Laboratory Optimisation Process (Practical Recommendations)

4.1 Recommended Jar Test Protocol (for determining optimal pH/temperature/dosage)

Collect representative water samples (≥1 L × multiple groups).

Design variables: pH (e.g., 4, 6, 7.5, 9), temperature (ambient/low-temperature simulation/high-temperature e.g., 40°C), PolyDADMAC molecular weight grades and concentration gradients (e.g., 0.5, 1, 2, 5, 10 mg/L), and test sequential combinations with/without PAC or CPAM.

Agitation: Rapid dispersion 1–2 min → Medium speed 2–5 min → Slow speed 5–10 min → Settling 10–30 min.

Record: Effluent turbidity, settling velocity, floc size and stability (visual observation + hand-rubbing test); conduct small-scale filter press tests if dewatering is a concern.

Conduct zeta potential measurements (where feasible) to identify the ‘near-zero’ potential range, typically the optimal charge neutralisation zone.

4.2 Field Dosage and Control Considerations

Dosage sequence: When used with inorganic coagulants, common sequence: inorganic coagulant (e.g., PAC) → PolyDADMAC (enhancement) or vice versa (depending on system); For waters containing substantial organic colloids, a strategy of first neutralising with PolyDADMAC followed by PAC is also common. Pilot testing is recommended.

Dissolution Method: High molecular weight liquids or powders require low-shear dissolution per supplier recommendations to avoid polymer chain breakage (reducing bridging capacity) caused by high shear.

pH adjustment: Make incremental corrections to avoid excessive single-step adjustments. For potable water applications, strictly adhere to regulatory limits for residual dosing agents and effluent parameters.

Temperature and energy control: Adjust mixing energy and contact time appropriately under high or low temperature conditions to prevent floc destruction by excessive shear.

5. Conclusion

The field performance of PolyDADMAC is jointly determined by pH, temperature, and water quality conditions. Understanding these factors enables better control of dosage, optimisation of process flows, and significant enhancement of flocculation and purification efficiency.

Recommendation: During project implementation, conduct systematic pilot testing (Jar Test + zeta measurement) and validate under actual operating conditions (temperature, raw water ion composition). This forms the basis for determining suitable molecular weight, dosage, and pH/temperature control strategies, thereby achieving a balance between cost and effectiveness.