What Is PAC? Complete Guide to Coagulant for Water Treatment

--- ## 1. What is PAC? Definition, CAS number, chemical formula PAC stands for Poly Aluminium Chloride — a polymerized aluminum salt, with CAS number 1327-41-9 and general formula [Al₂(OH)ₙCl₆₋ₙ]ₘ. Here, n represents the number of hydroxyl groups (OH⁻) replacing chloride ions (Cl⁻) on the aluminum molecule, while m is the degree of polymerization — the number of Al₂ units linked together in long chains. Unlike alum (Al₂(SO₄)₃), which is a simple salt, PAC has been processed to form polymer chains with stronger coagulation capability even at low dosages. The most important technical parameter is Al₂O₃ content: liquid form reaches 10% by weight, bright yellow powder form reaches 28.5% - 31%. The basicity of PAC ranges from 45–85%, and the pH of a 1% solution falls within 3.5–5.0. Thanks to its pre-polymerized structure, PAC does not require extended hydrolysis time like alum — coagulation effectiveness appears immediately after dosing into water, significantly shortening water retention time in reaction tanks. PAC's characteristic of causing less pH reduction helps plants save on alkalinity adjustment chemical costs while reducing sludge generation compared to traditional alum. An average sludge reduction of 25–35% means lower costs for sludge treatment and transport to landfills — a significant saving for high-capacity plants. On a large scale, switching from alum to PAC reduces 20–30% of reaction and sedimentation tank volume due to shorter retention time, or increases treatment capacity by 1.3 times without expanding tanks. --- ## 2. Coagulation mechanism of PAC — Charge neutralization and polymer bridging PAC's coagulation mechanism operates through two simultaneous stages. Stage one — charge neutralization: highly positively charged polynuclear aluminum ions (Al₁₃O₄(OH)₂₄⁷⁺, Al₈(OH)₂₀⁴⁺) attract negatively charged colloidal particles in water (silt, clay, organic matter), destabilizing surface charges and breaking electrostatic repulsion between particles. Stage two — polymer bridging: long polymer chains [Al₂(OH)ₙCl₆₋ₙ]ₘ bridge between neutralized particles, binding them into flocs measuring 0.5–3 mm. PAC flocs are denser than alum flocs due to the compact polymer structure, settling 2–3 times faster in sedimentation basins. Water retention time in sedimentation basins drops from 90–120 minutes to 45–60 minutes, allowing plants to increase treatment capacity or reduce basin size in new designs. In retrofit systems, switching from alum to PAC increases treatment capacity by 30–40% without building additional sedimentation tanks. One important technical advantage: PAC causes less pH reduction in treated water. Alum consumes 6–7 equivalents of alkalinity per equivalent of aluminum, while PAC consumes only 4–5 equivalents. As a result, plants save 20–40% on pH adjustment chemicals (lime, NaOH). This is why PAC is preferred in low-alkalinity water treatment. Furthermore, PAC's molecular structure allows it to work effectively across a wide temperature range from 5°C to 40°C, whereas alum performance notably drops when water temperature falls below 15°C — a particular advantage for drinking water treatment plants in northern regions during winter. PAC dosage is only 60–70% of alum dosage to achieve the same coagulation efficiency, reducing chemical inventory and transportation volume at scale. Additionally, PAC flocs have higher compressibility, accelerating sludge thickening in sludge thickeners, reducing sludge moisture after thickening from 98% to 95%, and decreasing sludge volume for landfill transport. When using PAC in water with high dissolved organic content (humic surface water, eutrophic lake water), PAC's DOC (Dissolved Organic Carbon) removal efficiency is 15–30% higher than alum, reducing trihalomethanes (THM) — carcinogenic precursors in drinking water. --- ## 3. Common PAC forms — Powder, liquid 10% and liquid 17% PAC is available on the market in three main forms, differing in active content, transportation cost, and operational requirements. The choice depends on distance from plant to supply source, monthly consumption volume, existing infrastructure, and initial investment budget. **PAC powder 31% (Al₂O₃ content 28.5% - 31%):** Bright yellow powder, 25 kg bags, suitable for long-distance transport and export. Saves 60–70% of transport costs compared to liquid PAC with equivalent active content. Shelf life 12 months in sealed bags, dry warehouse. Requires a 5–10% solution preparation system before use. **PAC liquid 10% (Al₂O₃ 10%):** Clear to light yellow solution, pH 3.5–4.5, specific gravity 1.19–1.20 g/cm³. Directly pumped from storage tanks or drums, no preparation needed. Shelf life 6 months. Suitable for plants near supply sources with stable consumption. **PAC liquid 17% (Al₂O₃ 17%):** More concentrated solution, specific gravity ~1.375 g/cm³, direct pumping. An intermediate choice between powder and liquid 10% for plants with storage tanks that want to reduce transport volume. Compared to PAC liquid 10%, the 17% form reduces transport volume by 30% for the same Al₂O₃ mass. See detailed comparison: [Powder vs Liquid PAC comparison](https://hoachatlocthien.com/en/pac-bot-vs-pac-long) — comprehensive cost analysis, pros and cons, and selection guide. --- ## 4. PAC applications in water treatment fields PAC is present in most industrial and municipal water treatment processes. Here are five key applications: **Drinking water treatment:** PAC is the preferred coagulant in water treatment plants for its ability to remove turbidity, organic matter, and microorganisms at low dosages. Using PAC instead of alum reduces sludge volume by 30% and pH adjustment chemical costs by 20–40%. A 10,000 m³/day water plant can save 15–25 million VND per month when switching from alum to PAC. **Industrial wastewater treatment:** From textile dyeing, paper, food processing to seafood processing wastewater — PAC effectively treats COD, TSS, color, and heavy metals. Dosage ranges 200–800 mg/L depending on pollution load. In textile wastewater, PAC combined with flocculant polymer achieves over 90% color removal efficiency. **Swimming pool water treatment:** PAC powder is used to clarify swimming pool water — forming flocs that settle to the bottom for vacuuming by cleaning robots. Dosage 5–15 g/m³ water, advantages include no foaming and no significant pH change. PAC liquid 10% is also used for commercial pools with automatic dosing systems. **Textile dyeing wastewater treatment:** The textile dyeing industry uses PAC to coagulate reactive dye color and dyeing auxiliaries in wastewater, particularly effective with stubbornly colored wastewater. High-basicity PAC (70–85%) achieves high decolorization efficiency. In denim fabric dyeing wastewater, PAC combined with cationic polymer reduces color from 1,200 Pt-Co to below 100 Pt-Co after just one coagulation stage. **Paper production:** PAC is used as a retention aid and pitch control agent in paper production, improving the whiteness and strength of finished paper. PAC also helps control white water system pH in paper machines. See detailed analysis: [PAC applications in water treatment](https://hoachatlocthien.com/en/ung-dung-pac-trong-xu-ly-nuoc). --- ## 5. How to choose the right PAC grade and read COA Choosing the right PAC grade is crucial for treatment efficiency and operational costs. Engineers need to evaluate three factors: source water quality, treatment objectives, and existing operational system. **Step 1 — Identify water source and treatment requirements:** High-turbidity surface water (100–500 NTU) requires PAC with 70–85% basicity. Textile dyeing wastewater needs PAC with low iron (Fe) content to avoid darkening fabric color. Drinking water treatment requires PAC meeting potable water standards (heavy metal content such as As, Pb, Hg, Cd, Cr⁶⁺ within TCVN limits). Seafood processing wastewater typically needs high-dose PAC (400–800 mg/L) combined with anionic polymer. Paper wastewater with high color and COD requires high-basicity PAC combined with FeCl₃ to boost treatment efficiency. **Step 2 — Run jar test:** This is the only method to precisely determine the optimal PAC dosage. The 5-step jar test procedure: (1) prepare 1% PAC solution, (2) dose increasing amounts into 6 sample jars, (3) rapid mix at 200 rpm for 2 minutes, (4) slow mix at 40 rpm for 15 minutes, (5) settle for 30 minutes — compare clarity. Reference: [Standard jar test procedure](https://hoachatlocthien.com/en/quy-trinh-jar-test-sop-5-buoc-toi-uu-coagulant). **Step 3 — Read COA (Certificate of Analysis):** When purchasing PAC, COA is a mandatory document. Five parameters to check: (1) Al₂O₃ (%) — must match product label, tolerance ≤0.5%; (2) Basicity (%) — 45–85%, higher means faster coagulation; (3) Water-insoluble matter (%) — <0.5% for powder, <0.2% for liquid — higher values indicate impurities in the Al(OH)₃ raw material; (4) Fe content (mg/kg) — <500 for drinking water — high Fe causes yellow water post-treatment, affecting product quality in textile and paper industries; (5) Heavy metals (As, Pb, Hg, Cd, Cr⁶⁺) — meeting TCVN standards. Potable-grade PAC costs 10–20% more than technical-grade PAC but is mandatory for drinking water treatment plants. When purchasing PAC in bulk, request the supplier to provide COA with each shipment for verification — each production batch has different parameters. --- ## 6. Case study — 2,500 m³/day textile dyeing plant at Song Than Industrial Park A textile dyeing plant at Song Than Industrial Park (Binh Duong) was using alum for its 2,500 m³/day wastewater treatment system. Problems: small flocs, slow settling, effluent pH fluctuating 5.0–5.5 requiring lime dosing to raise to 6.5–8.5, average lime cost 12 million VND/month. Loc Thien engineer (Technician Phan Cam Thuy) advised switching to PAC powder 31%, basicity 72%. After jar testing determined optimal dosage of 350 mg/L, the plant saw: settling time reduced from 45 minutes to 25 minutes, effluent pH naturally reached 6.3–7.0 without lime. Sludge volume reduced by 30%, monthly chemical costs dropped by 18 million VND (PAC savings + no lime needed). The plant switched to bulk PAC purchases — Loc Thien delivers 5–30 ton tankers weekly, with batch COA and regular technical support from Technician Vo Thi Nhu Hoa. After 3 months, the plant reported additional benefits: 25% reduction in sludge requiring disposal, reduced dosing pump maintenance frequency as PAC is less corrosive than alum. Lesson learned: seasonal jar testing is key — wastewater quality changes with production season, requiring corresponding PAC dosage adjustments. This plant now saves 216 million VND per year from switching to PAC. --- ## 7. Frequently asked questions --- ## 8. Loc Thien — Your bulk PAC supply partner --- See also: [PAC Technology — basicity, production, powder vs liquid comparison](https://hoachatlocthien.com/en/ky-thuat-va-san-xuat-pac) · [Buy industrial PAC — pricing and ordering](https://hoachatlocthien.com/en/mua-pac-cong-nghiep) · [PAC applications in water treatment](https://hoachatlocthien.com/en/ung-dung-pac-trong-xu-ly-nuoc) · [PAC powder vs PAC liquid](https://hoachatlocthien.com/en/pac-bot-vs-pac-long) · [Jar test SOP procedure](https://hoachatlocthien.com/en/quy-trinh-jar-test-sop-5-buoc-toi-uu-coagulant) --- **LOC THIEN INVESTMENT DEVELOPMENT COMPANY LIMITED** MST: 0313650856 — Address: 452/6B Tinh Lo 10, P. Binh Tri Dong, Q. Binh Tan, TP.HCM Hotline: [0979 891 929](tel:0979891929) — Email: [email protected] Website: [hoachatlocthien.com](https://hoachatlocthien.com)