PAC stands for Polyaluminum Chloride (also commonly known as PACl or Aluminum Chlorohydrate). It is an inorganic polymer widely used as a coagulant in water and wastewater treatment.
Here's a detailed breakdown of what PAC is:
Not a Single Compound: Unlike simple aluminum salts (e.g., alum), PAC is not a single, well-defined chemical. It's a complex mixture of various aluminum hydroxy species in different stages of polymerization.
General Formula: Often represented as [Al₂(OH)ₙCl₆₋ₙ]ₘ where n is typically between 1 and 5, and m represents the degree of polymerization.
Key Feature: It contains pre-formed, highly charged cationic polymers like Al₁₃O₄(OH)₂₄⁷⁺ (often called the "Al₁₃ Keggin ion"), which are responsible for its superior coagulation performance compared to traditional coagulants.
PAC is produced by partially neutralizing aluminum chloride (AlCl₃) with a base (like sodium hydroxide - NaOH or calcium hydroxide - Ca(OH)₂) under controlled conditions. The degree of neutralization (often expressed as the Basicity or OH/Al ratio) is a critical parameter:
Basicity: Typically ranges from 40% to 85%. Higher basicity means more hydroxide ions have been added, leading to greater pre-polymerization and larger Al₁₃ clusters.
| Property | Advantage |
|---|---|
| Higher Molecular Weight & Charge | Pre-formed polymeric species provide superior charge neutralization and bridging capabilities, often requiring lower dosages than alum. |
| Wider Effective pH Range | Effective over a broader pH range (typically ~5.5 to 8.5), offering more operational flexibility. |
| Less Sensitive to Low Temperatures | Performs better than alum in cold water conditions. |
| Lower Final pH Impact | Generates less free H⁺ (acid) upon hydrolysis, so it causes less reduction in pH and often requires less alkalinity addition (e.g., lime) for pH correction. |
| Reduced Sludge Production | Produces lighter, denser, and more compact sludge that settles faster and has lower volume compared to the bulky sludge from alum. This reduces sludge handling costs. |
| Faster Floc Formation & Settling | Forms large, dense flocs quickly, improving clarification efficiency. |
Drinking Water Treatment: Primary coagulant for removing turbidity, color, algae, and pathogens.
Municipal Wastewater Treatment: Removing suspended solids, phosphorus (as aluminum phosphate), and organic matter.
Industrial Wastewater Treatment: Treating effluents from various industries (textile, printing, food processing).
Swimming Pools & Recreational Water: As a clarifier.
Paper Manufacturing: As a retention and drainage aid.
Liquid: Most common form, supplied as a concentrated solution (typically 8-12% as Al₂O₃). It's usually clear to yellowish or brownish.
Solid: Available as a white or yellowish powder or granules, which must be dissolved before use.
vs. Alum (Al₂(SO₄)₃): PAC is generally superior in terms of dosage, pH range, sludge production, and performance in cold water. It doesn't add sulfate ions.
vs. Ferric Salts (FeCl₃, Fe₂(SO₄)₃): PAC doesn't cause the reddish-brown staining associated with iron and produces less colored sludge. Iron can be better for very low pH water or specific contaminants.
vs. Organic Polymers (PolyDADMAC, CPAM): PAC is inorganic, adds no organic carbon (avoiding DBP concerns), but produces more sludge than pure organic coagulants. They are sometimes used together.
In summary, PAC (Polyaluminum Chloride) is an advanced, pre-polymerized aluminum-based coagulant that offers significant performance and operational advantages over traditional aluminum sulfate (alum), making it one of the most popular coagulants in modern water treatment facilities. Its effectiveness stems from its high charge density and large molecular size due to the presence of polymeric aluminum species.
