Chemical Dosing Optimization for Water Treatment Plants
Proper chemical preparation and dosing is essential for optimal treatment performance, chemical cost control, and regulatory compliance. This guide covers best practices for PAC and PAM dosing systems.
PAC Solution Preparation
- Concentration: Prepare 5-10% PAC solution (50-100 g/L). Higher concentrations reduce storage volume but may affect dosing pump accuracy.
- Mixing: Add PAC powder slowly to the vortex of stirring water. Use warm water (20-30C) for faster dissolution.
- Aging: Allow 30 minutes for full hydration and Al species equilibration.
- Storage: PAC solution stable for 2-3 days. Beyond this, polymerization may affect performance.
- Materials: Use HDPE, PP, or FRP tanks and pipes. Avoid carbon steel and aluminium.
PAM Solution Preparation
- Concentration: Prepare 0.05-0.1% PAM solution (0.5-1.0 g/L). Never exceed 0.2% as viscosity becomes unmanageable.
- Dispersion: Use automatic polymer preparation unit or add powder very slowly into vortex. Never pour water onto PAM powder.
- Mixing Speed: 200-400 RPM. Higher speeds break polymer chains, reducing molecular weight and performance.
- Dissolution Time: 60-90 minutes for full hydration. Under-mixed solutions form fish-eyes that clog dosing lines.
- Shelf Life: Use within 24 hours. Aged PAM solution loses 20-50% performance due to chain degradation.
Dosing Point Selection
| Chemical | Dosing Point | Reason |
|---|---|---|
| PAC | Rapid mix chamber or in-line static mixer | Needs intense mixing for 1-2 min |
| PAM (flocculant aid) | After PAC, before flocculation basin | Avoid breaking formed flocs |
| PAM (sludge conditioning) | Before dewatering equipment | Short contact time acceptable |
| pH adjustment | Before coagulant addition | Optimal pH for coagulation: 6.5-7.5 |
Dosing Control Methods
- Flow Proportional: Dose adjusted based on plant flow rate. Simple but does not account for water quality changes.
- Streaming Current Detector (SCD): Measures net charge after coagulant addition, provides real-time feedback.
- Jar Test Guidance: Manual jar testing every shift to validate automatic dosing. Adjust for seasonal raw water changes.
- Zeta Potential: Target zeta potential between -5 and +5 mV for optimal coagulation.
Common Dosing Problems
- Overdosing: Causes charge reversal, restabilization of colloids, high residual Al in treated water
- Underdosing: Incomplete coagulation, carryover floc, high filter effluent turbidity
- Dosing Pump Calibration: Calibrate monthly. Diaphragm pump wear reduces accuracy over time
Contact jingshuicc@gmail.com for dosing optimization support and jar testing protocols.