Introduction
pH is the single most important variable affecting coagulation performance — more than PAC dose, more than PAM selection, more than mixing intensity. A PAC dose that produces crystal-clear water at pH 7.0 can fail completely at pH 5.5 with the same wastewater. Yet many treatment plants operate without any pH measurement or adjustment before the coagulant dosing point. This guide explains the relationship between pH and coagulation chemistry, and provides practical pH control strategies.
How pH Affects Coagulation — The Chemistry
1. Aluminum Speciation vs pH
When PAC dissolves in water, the aluminum species present depend strongly on pH:
| pH Range | Dominant Al Species | Coagulation Behavior |
|---|---|---|
| <4.0 | Al3+ (free ion, monomeric) | No coagulation. Aluminum remains fully dissolved. No charge neutralization because particles are also positively charged at this pH |
| 4.5-5.5 | Al(OH)2+, Al(OH)2+ (monomeric, partially hydrolyzed) | Weak coagulation. Some charge neutralization but insufficient precipitate for sweep flocculation. High residual soluble aluminum |
| 5.5-7.5 | Alb (medium polymer), Al(OH)3 (amorphous precipitate) | Optimal zone. Charge neutralization from Alb species + sweep flocculation from Al(OH)3 precipitate. Lowest residual Al, fastest floc formation |
| 7.5-8.5 | Al(OH)3 (precipitate), Al(OH)4- (aluminate, soluble) | Coagulation still effective but dominated by sweep floc. Higher dose needed. Residual Al increases as Al(OH)4- forms |
| >8.5 | Al(OH)4- (soluble aluminate) | Coagulation fails. Aluminum redissolves as soluble aluminate. High residual Al, no flocs, or very weak flocs |
2. Particle Surface Charge vs pH
Suspended particles (clay, organic matter, bacteria) carry a surface charge that varies with pH. Most natural particles are negatively charged at neutral pH due to surface -OH and -COOH groups. At very low pH (<3-4), the surface protonates and the zeta potential approaches zero or becomes positive.
Practical implication: At pH 6.5-7.5, particles are negatively charged, PAC Al species are positively charged — charge neutralization works. Outside this range, charge relationships change and coagulation efficiency drops.
3. Alkalinity and pH Buffering
Alkalinity (HCO3-, CO32-) is the water’s natural pH buffer. PAC hydrolysis consumes alkalinity:
Al3+ + 3HCO3- → Al(OH)3 + 3CO2
Each mg/L of Al3+ consumes ~5.6 mg/L of alkalinity as CaCO3. For a 20 mg/L PAC dose (as Al2O3, ~4.2 mg/L Al), this consumes ~24 mg/L alkalinity. In low-alkalinity raw water (<30 mg/L as CaCO3), this can depress pH by 0.5-1.0 units — enough to push pH out of the optimal zone.
Determining Optimal pH — Jar Test Protocol
- Measure raw water pH and alkalinity. If alkalinity <50 mg/L as CaCO3, pH depression from PAC dosing will be significant
- Prepare 6 beakers. Adjust pH of each to a different target: 5.0, 5.5, 6.0, 6.5, 7.0, 7.5, 8.0 (use 7 beakers or skip 5.0). Use 0.1N H2SO4 or 0.1N NaOH for adjustment
- Add the SAME PAC dose to each beaker (use your current plant dose as the baseline)
- Standard jar test procedure: 1 min rapid mix, 10 min slow mix, 15 min settling
- Plot turbidity vs pH: The optimal pH is where turbidity is minimized. The curve is typically U-shaped — turbidity decreases then increases as pH moves away from optimum
- Verify PAC dose at optimal pH: Run a second jar test at the optimal pH, varying PAC dose. This two-step approach (find optimal pH first, then optimize dose) is more efficient than a full matrix
pH Adjustment Strategies
When pH Is Too Low
| Chemical | Dosage Guidance | Pros | Cons |
|---|---|---|---|
| Sodium hydroxide (NaOH, 50% liquid) | ~40g NaOH per 1000 m3 to raise pH by 0.1 (varies with alkalinity) | Fast-acting, easy to automate, no solids addition | Higher cost per kg; hazardous to handle (corrosive) |
| Soda ash (Na2CO3, powder) | ~100g per 1000 m3 to raise pH by 0.1 | Safer to handle; also adds alkalinity | Slower dissolution; more expensive than lime |
| Lime / hydrated lime (Ca(OH)2, powder or slurry) | ~75g per 1000 m3 to raise pH by 0.1 | Lowest cost; also provides calcium for water stability | Requires slurry make-down system; adds TSS (undissolved lime); scaling in pipes; more sludge |
When pH Is Too High
| Chemical | Dosage Guidance | Pros | Cons |
|---|---|---|---|
| Sulfuric acid (H2SO4, 93-98%) | ~50g per 1000 m3 to lower pH by 0.1 | Lowest cost; fast reaction | Extremely hazardous; adds sulfate (corrosion, scaling with Ca2+) |
| Hydrochloric acid (HCl, 30-35%) | ~40g per 1000 m3 to lower pH by 0.1 | No sulfate scaling; fast reaction | Corrosive fumes; adds chloride (corrosion risk for stainless steel) |
| Carbon dioxide (CO2, gas) | Inject into water; forms carbonic acid | Safest option; cannot over-acidify; precise control | Higher capital cost (gas injection system); slower response; requires CO2 supply |
Automated pH Control Systems
For plants treating >500 m3/day, manual pH adjustment is impractical. An automated pH control loop consists of:
- pH sensor: Inline or immersion probe, with automatic temperature compensation (ATC). Clean weekly — fouling is the #1 cause of pH control failure
- pH controller/transmitter: PID controller with 4-20mA output to dosing pump or control valve
- Chemical dosing pump: Variable speed (VFD) or stroke-length adjustment, controlled by the pH controller
- Static mixer or in-line mixer: Ensures rapid, uniform mixing of pH adjustment chemical with the process stream
- Second pH sensor downstream (recommended): Verifies the adjustment and provides feedback trim. Senses probe fouling if readings diverge
PID Tuning for pH Control
pH control is notoriously difficult to tune because the pH response to acid/base addition is highly nonlinear (S-shaped titration curve). Near pH 7, tiny amounts of acid/base cause large pH changes. Standard PID tuning rules often fail. Recommendations:
- Use a “gap” or “deadband” controller: Only dose when pH is >0.2 units from setpoint, to prevent overshoot and oscillation
- Size the dosing pump to correct the maximum expected pH deviation in 5-10 minutes. Over-sized pumps cause oscillation; under-sized pumps can’t keep up during pH spikes
- Consider a two-stage system: bulk neutralization tank (coarse adjustment) followed by trim adjustment (fine) before the coagulant dosing point
pH and PAC — Special Cases
Textile Wastewater
Textile effluent pH can swing from 4 (acid dye bath) to 12 (reactive dye + caustic). Equalization is mandatory. Pre-neutralize to pH 6.5-7.5 before PAC dosing.
Electroplating Wastewater
Metal precipitation requires elevated pH (8.5-10.5 depending on metal). PAC is dosed AFTER pH adjustment for metal precipitation, not before. At pH 9-10, PAC works through sweep flocculation, not charge neutralization — expect to use a higher dose than at neutral pH.
Acid Mine Drainage (AMD)
AMD pH 2-4 is far below PAC’s effective range. Neutralize with lime to pH 8-9 first. PAC coagulates the metal hydroxide precipitates formed by neutralization. Do not dose PAC at pH <4 — aluminum remains fully dissolved and provides zero coagulation.
POME (Palm Oil Mill Effluent)
POME pH 4-5 is suboptimal for PAC. Raise to pH 5.5-6.5 with NaOH or lime before PAC dosing. Some acid-tolerant PAC formulations can work at pH 4.5-5.0 — test before automatically adjusting pH.
Quick Reference: Optimal pH by Coagulant
| Coagulant | Optimal pH Range | Effective Range |
|---|---|---|
| PAC (low basicity, 40%) | 6.0-7.0 | 5.5-7.5 |
| PAC (high basicity, 70-85%) | 6.5-7.5 | 5.5-8.5 |
| Alum (Al2(SO4)3) | 6.5-7.0 | 5.5-7.5 |
| Ferric Chloride (FeCl3) | 4.5-6.0 | 4.0-11.0 |
| Ferric Sulfate (Fe2(SO4)3) | 4.5-6.0 | 4.0-9.0 |
HydroChemix supplies PAC across the full basicity range for varied pH conditions. Contact jingshuicc@gmail.com with your raw water pH, alkalinity, and current treatment challenges for a PAC specification recommendation and free jar test sample.