Polyaluminum chloride (PAC) is one of the most widely used coagulants in modern water treatment, valued for its broad effective pH range and lower sludge production compared to traditional alum. However, achieving optimal performance requires careful dose adjustment as raw water turbidity fluctuates — a challenge particularly acute for surface water treatment plants that experience seasonal variations from monsoon events, snowmelt, or agricultural runoff.
The relationship between PAC dose and turbidity removal is not linear. At low turbidity (< 10 NTU), charge neutralization is the dominant mechanism, and even small overdoses can restabilize particles, causing turbidity to increase. At moderate turbidity (10–100 NTU), sweep flocculation becomes dominant, and slightly higher doses are tolerable. At high turbidity (> 100 NTU), the high particle concentration itself aids floc formation, and the required dose per unit of turbidity actually decreases — a phenomenon known as the 'self-flocculation effect.'
A practical approach to dose optimization involves establishing a dose-turbidity curve for your specific water source through systematic jar testing across the expected turbidity range. Plot the optimal PAC dose against raw water turbidity on a log-log scale. For most surface waters treated with PAC at 10–12% Al₂O₃ concentration, the relationship follows a power-law curve: Dose (mg/L) = k × Turbidity^n, where k is typically 2–5 and n is 0.4–0.6. This curve can then be programmed into SCADA-controlled dosing systems for automatic adjustment.
Beyond turbidity, operators must account for seasonal changes in alkalinity, pH, temperature, and organic loading. Cold water (< 10°C) significantly slows floc formation kinetics, often requiring 20–30% higher doses or the addition of a cationic polymer to assist. High organic loading (elevated UV254 or DOC) competes for coagulant demand and may necessitate dose increases of 15–40% compared to low-organic conditions. Regular jar testing — at minimum monthly, or weekly during transitional seasons — remains essential for validating and refining the dose-turbidity curve.