Polyaluminium chloride (PAC) is one of the most commonly used coagulants in water treatment. It is applied in drinking water purification plants, industrial wastewater facilities and municipal sewage plants around the world. The reasons are practical: PAC removes turbidity, color and organic matter efficiently, works across a wide range of pH and temperature conditions, and produces stable flocs that are easy to separate. This article explains why PAC has become the preferred choice for so many treatment applications.
Why Is PAC Used in Water Treatment?
PAC is used because it solves the core problem of water treatment: removing finely dispersed particles that do not settle on their own. These particles carry a negative surface charge that keeps them suspended. PAC delivers positively charged aluminum hydrolysis products that neutralize this charge, allowing particles to collide, aggregate and settle. Because the hydrolysis products are pre-formed during manufacturing, PAC acts quickly and requires no separate alkalinity adjustment in many waters, unlike some conventional coagulants.
How PAC Works in Water
The treatment process with PAC follows a well-defined sequence:
Coagulation: PAC is rapidly mixed into the water and hydrolyzes to form cationic species.
Flocculation: gentle mixing brings neutralized particles together into larger flocs.
Separation: flocs are removed by sedimentation, flotation or filtration.
PAC also removes dissolved organic matter such as humic substances through adsorption onto the aluminum hydroxide precipitate, and it captures heavy metals and pathogens along with the flocs.
Key Advantages of PAC
High coagulation efficiency at relatively low doses.
Effective over a wide pH range, typically from 5.5 to 8, with less pH sensitivity than alum.
Good performance in cold water, where many coagulants lose efficiency.
Fast floc formation with larger, denser flocs that settle quickly.
Lower residual aluminum and less sludge in many applications.
Applications in Drinking Water Treatment
In drinking water plants, PAC is used to reduce turbidity and color, remove natural organic matter that forms disinfection by-products, and improve filtration performance. Its ability to work at lower doses reduces chemical costs and minimizes the aluminum residual in finished water, helping plants comply with drinking water standards such as those set by the World Health Organization and national regulations.
Applications in Wastewater Treatment
In municipal and industrial wastewater treatment, PAC is used for phosphorus removal, clarification of secondary effluent, and pre-treatment of high-load streams. It also conditions sludge for better dewatering. Because PAC tolerates fluctuating influent quality, it is well suited to plants receiving variable industrial discharges.
Frequently Asked Questions
Why is PAC better than alum in water treatment?
PAC generally works at lower doses, over a wider pH and temperature range, and produces faster-settling flocs with less sludge and lower residual aluminum.
Is PAC safe for drinking water?
Yes, food-grade and potable-water-grade PAC is approved for drinking water treatment in many countries when used within regulated dose limits and quality standards.
What impurities does PAC remove?
PAC removes suspended solids, turbidity, color, natural organic matter, algae, heavy metals, phosphorus and many pathogens through coagulation and flocculation.
How much PAC is needed?
Typical doses range from about 5 to 50 mg/L depending on raw water quality; jar tests should be run to determine the optimum dose for each water source.
Does PAC affect the pH of treated water?
PAC usually lowers pH slightly due to hydrolysis, and pH may need to be adjusted with alkali in low-alkalinity waters.
Can PAC be used together with polymers?
Yes, PAC is often combined with anionic or cationic polyacrylamide flocculants to strengthen flocs and improve settling in difficult waters.
