Polyaluminium Chloride (PAC)
PAC is an inorganic coagulant produced by the partial neutralisation of aluminium chloride with alkali. The result is a family of pre-hydrolysed aluminium species, including the highly effective Al13 tridecameric complex, that are significantly more active as coagulants than the monomeric aluminium ions found in conventional alum (aluminium sulfate).
Compared to alum, PAC coagulates effectively at lower aluminium doses, over a wider pH range (approximately 5.0–8.0), and with less depression of treated water pH. This translates to lower chemical consumption, reduced lime addition for pH correction, smaller sludge volumes, and more stable performance when source water quality fluctuates.
PAC vs. alum: Alum remains in widespread use but requires higher doses to achieve equivalent turbidity removal, produces more sludge, and can drive pH below the optimum coagulation range in poorly buffered source waters. PAC is increasingly the preferred coagulant for modern water treatment plants where operating consistency and sludge handling costs matter.
How Coagulation Works
Raw water contains colloidal particles, clay, silt, organic matter, and biological material, that carry negative surface charges. These charges cause the particles to repel each other and remain stably suspended indefinitely. Gravity settling and filtration alone cannot remove them efficiently. Coagulation destabilises this suspension through two complementary mechanisms:
The combination of these mechanisms makes PAC effective across a wide range of source water conditions, from low-turbidity reservoir water to high-turbidity river water during flood events.
Powder vs. Liquid PAC
Applications
| Application | Treatment Objective | Notes |
|---|---|---|
| Municipal WTP, raw water | Turbidity and colour removal from river and reservoir sources; dosed at rapid mix ahead of settling or dissolved air flotation (DAF) | Jar testing recommended to establish optimum dose for prevailing source water conditions; dose increases significantly during high-turbidity flood events |
| Industrial process water | Pre-treatment of cooling water makeup and boiler feedwater; removal of suspended solids that would foul downstream equipment or reduce heat exchanger efficiency | PAC dose should be verified against source water alkalinity; low-alkalinity waters may require pH adjustment to maintain effective coagulation |
| Effluent treatment (ETP) | Phosphorus removal from industrial or municipal wastewater; removal of suspended solids and colloidal organic matter before biological treatment or final discharge | Effective for phosphorus precipitation at doses producing sufficient Al:P molar ratios; compatible with downstream biological treatment when dosed correctly |
| Paper & textile industry | Process water clarification; colour and turbidity removal from recirculated or discharge streams | High organic loading in textile effluent may require higher PAC doses or combination with organic flocculant for complete treatment |