PAC vs Alum — Choosing the Optimal Coagulant for Wastewater Treatment

TL;DR: PAC (CAS 1327-41-9) and Alum (Al₂(SO₄)₃, CAS 10043-01-3) are both aluminum-based coagulants. PAC outperforms alum: wider pH range 5.0–9.0 (alum 6.5–7.5), 30–50% lower dosage, 30–50% less sludge, less equipment corrosion, stable performance in cold water <10°C. Alum has lower per-kg price but higher total treatment cost per m³ due to pH adjustment chemicals and sludge handling. Loc Thien Chemical supplies both — consult based on jar-test results. ☎ 0979 891 929.

What Are PAC and Alum? Chemical Definitions and Formulas

PAC — Polyaluminium Chloride (CAS 1327-41-9)

PAC is a pre-hydrolyzed aluminum polymer with general formula [Al₂(OH)ₙCl₆₋ₙ]ₘ, CAS 1327-41-9. Unlike traditional alum, PAC is partially hydrolyzed during manufacturing — it starts coagulating immediately upon contact with water, without requiring a prolonged hydrolysis phase. Its multi-aluminum (polymeric) structure creates larger, more stable flocs compared to single-ion coagulation. PAC has a basicity index of 45–85% — the OH/Al ratio determines the degree of polymerization and directly impacts coagulation efficiency. Loc Thien Chemical supplies PAC powder 31% and liquid PAC 10–17%, with batch-specific COA.

Alum — Aluminum Sulfate (CAS 10043-01-3)

Alum (Aluminum Sulfate), formula Al₂(SO₄)₃·14H₂O, CAS 10043-01-3, is the traditional coagulant used in water treatment for over 100 years. Mechanism: when dissolved in water, Al₂(SO₄)₃ dissociates into Al³⁺ and SO₄²⁻ ions, then Al³⁺ hydrolyzes to form Al(OH)₃ precipitate for coagulation. This process consumes natural alkalinity in water, releasing H⁺ and lowering pH — which is why alum typically requires lime or NaOH to maintain optimal pH. Loc Thien supplies alum powder/crystal ≥17% Al₂O₃, with batch COA.

Core Difference: Pre-hydrolyzed Polymer vs Traditional Sulfate Salt

The fundamental difference lies in chemical structure: PAC is a multi-nuclear polymer already pre-hydrolyzed — Al³⁺ ions are linked through OH bridges forming long polymer chains, immediately active for coagulation. Alum is a single salt Al₂(SO₄)₃ requiring in-water hydrolysis — a process dependent on temperature, pH, and source water alkalinity, slower and less stable. The SO₄²⁻ group in alum causes equipment corrosion and strong pH reduction — two operational issues that PAC does not have.

Detailed Comparison Table — PAC vs Alum

Figure: On-site chemical dosing, storage tank operation, and batch COA inspection — Source: Loc Thien Chemical.

Operating pH Range — Why It Matters

PAC operates effectively across pH 5.0–9.0, while alum is only optimal at 6.5–7.5. For plants receiving source water with seasonal or production-driven pH fluctuations, PAC enables stable operation without continuous pH adjustment. Alum strongly reduces pH when dissolved — each mg/L Al₂(SO₄)₃ consumes approximately 0.5 mg/L CaCO₃ alkalinity — forcing NaOH or lime supplementation whose cost scales with plant capacity. A 5,000 m³/day textile dyeing plant switching from alum to PAC substantially cut its monthly NaOH pH-adjustment cost (verified by actual jar-test data).

Dosage and Sludge — How PAC Saves Money

PAC dosage is 30–50% lower than alum at the same influent turbidity. Example: domestic wastewater at 150 NTU requires PAC powder 31% at 15–25 mg/L, while alum needs 30–50 mg/L. PAC sludge volume is reduced 30–50% because PAC flocs are denser, with less water content — reducing clarifier load and sludge disposal costs (charged per ton). PAC sludge filters 15–30% faster than alum sludge, saving electricity and filter aid chemicals. For a 10,000 m³/day plant, 30% sludge reduction cuts annual sludge disposal cost by ~30%.

Total Cost of Operation — The Long-term Math

PAC per-kg purchase price is 20–30% higher than alum, but total cost per m³ is lower. TCO analysis for a 5,000 m³/day plant: PAC saves 25–40% vs alum on total operating costs. Reasons: PAC dosage 30–50% lower, reduces pH chemical (NaOH/HCl) spend, reduces sludge handling costs 30–50%, and lowers equipment maintenance due to minimal corrosion. With alum, hidden annual costs from tank and pipe corrosion run 3–4 times higher than with PAC. Purchase price is only the tip of the iceberg; total cost per m³ is the real purchase decision.

Cold Water and Seasonal Performance — Real Applications

PAC maintains stable coagulation performance at temperatures <10°C thanks to its pre-hydrolyzed polymer structure. Alum performance drops 20–40% in cold water because Al³⁺ hydrolysis slows 2–3 times compared to 25°C. Northern water treatment plants operating in winter (raw water temperature 8–12°C) routinely report alum flocs being small, settling slowly, and increased dosage failing to improve results. Switching to PAC stabilizes effluent quality without increasing dosage — a year-round solution independent of weather.

Industry Applications — When to Choose Which

PAC is the flexible choice for most applications: drinking water supply (NSF/ANSI 60 certified, residual Al <0.05 mg/L), textile dyeing wastewater (PAC 31% powder removes 90–99% color), domestic wastewater (liquid PAC 10–17% for cost savings), wastewater with fluctuating pH. Alum is suitable when source water has stable pH (6.8–7.5) and low turbidity, the plant has a tight budget, has pH adjustment systems and corrosion-resistant equipment. Textile dyeing wastewater — PAC 31% is optimal thanks to superior color removal efficiency compared to alum (15–25% difference in color removal at equivalent molar dosage).

Jar-Test Guide — How to Choose the Right Chemical for Your System

The 6-jar jar-test is the standard procedure for comparing PAC and alum on your actual plant water sample. Prepare 6 × 1,000 mL jars with the same water sample: 3 jars for PAC at 10, 20, 30 mg/L, 3 jars for alum at 20, 40, 60 mg/L (alum dosage roughly double PAC). Rapid mix at 200 rpm for 1 minute, slow mix at 40 rpm for 15 minutes, settle for 10 minutes. Measure turbidity (NTU), pH, settled sludge volume (mL/L), and time to first visible floc. PAC typically forms flocs in 2–3 minutes with larger size, 30–50% less sludge than alum at equivalent dosage. Jar-test allows dosage optimization and confirms which coagulant performs better on your specific water stream.

Frequently Asked Questions (FAQ)

Can PAC and alum be used interchangeably?

Yes, but a jar-test is needed. PAC is generally more effective across a wider pH range, while alum is suitable if water is stable and budget is constrained.

Why is PAC more expensive per kg but cheaper overall?

Because PAC dosage is 30–50% lower, sludge volume is reduced 30–50%, no pH adjustment chemicals are needed, and equipment corrosion is minimal.

Which is more corrosive to equipment?

Alum (SO₄²⁻) is significantly more corrosive than PAC. PAC is safer for pipes, pumps, and storage tanks.

Can PAC be used for drinking water?

Yes. PAC meets NSF/ANSI 60 and AWWA B408 standards for drinking water. Alum also meets AWWA B403 for potable water.

Which coagulant is best for textile dyeing wastewater?

PAC 31% powder is optimal thanks to its high color removal efficiency, large floc formation, and fast settling.

👉 View PAC Powder 31% — Real COA, delivered to industrial parks 👉 View Alum Sulfate — powder/crystal ≥17% Al₂O₃ 👉 PAC Knowledge Hub — full article collection 👉 FeCl₃ vs PAC — iron vs aluminum coagulants

Related Articles

See also: FeCl₃ vs Alum — iron vs aluminum coagulant comparison — choosing between two main coagulant families.

Read the guide: Jar-Test Procedure — 5-Step SOP for Coagulant Optimization — standard jar-test technique for plant operators.

Learn more: PAC Combined with Polymer — optimizing coagulation for difficult wastewater — blending solutions for complex wastewater.

References: HydroChemix (2026), NSF/ANSI 60, AWWA B408, AWWA B403, actual COA from Loc Thien Chemical.

LOC THIEN INVESTMENT DEVELOPMENT CO., LTD — Tax Code 0313650856 · Hotline: 0979 891 929 · Email: [email protected]

Industrial Field Operation & Batch Quality Verification

1. Incoming Batch COA Verification & Regulatory Compliance (Law on Chemicals 69/2025/QH15)

When operating industrial water treatment, surface finishing, or textile dyeing lines in Vietnam, engineering teams must verify every chemical delivery against its batch-specific Certificate of Analysis (COA) and 16-section GHS Safety Data Sheet (SDS). Under the Vietnam Law on Chemicals No. 69/2025/QH15, Decrees 24/2026/ND-CP, 25/2026/ND-CP, and 26/2026/ND-CP, and QCVN 40:2025/BTNMT on industrial wastewater discharge:

• Specific Gravity & Assay Verification: Upon arrival of a 5–30-ton tanker or IBC tote shipment, plant QA technicians measure specific gravity at 20 °C using a calibrated hydrometer and cross-check active concentration against the batch COA issued by Loc Thien Chemical and verified via Quatest 3 (ISO/IEC 17025).
• Impurity & Heavy Metal Control: Trace heavy metals (Fe, Pb, As, Hg, Cd, Cr⁶⁺) and insoluble matter are strictly controlled to prevent catalyst poisoning in Fenton reactors, membrane fouling in RO/UF systems, or shade deviations in reactive textile dyeing.
• ZDHC MRSL v3.1 Level 1 Portfolio Boundary: For export-oriented textile and footwear FDI factories, Loc Thien Chemical maintains official ZDHC MRSL v3.1 Conformance Level 1 documentation for exactly 9 core products: four products certified by TUV SUD Product Service GmbH (NaOH, H2O2, Na2SO4, Na2CO3) and five products verified via Conformance Summary Report (CSR) and Analytical Test Report by TUV Rheinland Vietnam (H2SO4, FeCl2, HCl, Javel/NaOCl, PAC 31%).

2. Dosing Optimization, Storage Materials & Preventive Maintenance

1. Compatible Wetted Materials: Use cross-linked high-density polyethylene (XLPE / HDPE), polypropylene (PP-H), fiberglass-reinforced plastic (FRP with vinyl ester resin), or PTFE/PVDF-lined piping depending on corrosive and thermal conditions. Avoid unlined carbon steel or incompatible elastomers.
2. Automated Dosing Control: Integrate diaphragm metering pumps with real-time pH, ORP (mV), turbidity, or conductivity sensors. Always perform a 5-beaker Jar Test whenever raw water COD, TSS, or alkalinity shifts by more than 15%.
3. Secondary Containment & PPE: Storage tanks must sit inside an acid/alkali-resistant bund wall holding at least 110% of the largest tank volume. Operators handling unloading hoses must wear splash-proof chemical goggles, acid/alkali-resistant nitrile or neoprene gloves, face shields, and emergency respirators.

3. Multi-Role B2B Value Matrix (Procurement · QA/EHS Engineer · Plant Director)

• For Procurement Managers: Loc Thien Chemical is the direct manufacturer of FeCl2 30% solution and FeCl3 38–40% wastewater grade (3,000+ tons/month capacity), and a Tier-1 bulk commercial supplier of NaOH, HCl, H2SO4, PAC, NaOCl, H2O2, FeSO4, and Na2SiO3. Pricing is transparently structured around 5 technical cost drivers (active assay, packaging format, delivery distance from our 6 warehouses, order volume, and scheduled contract cadence) with a 15-minute technical quotation SLA.
• For Environmental & Process Engineers: Full technical support for Jar Test optimization, stoichiometry calculation, sludge minimization, and QCVN 40:2025/BTNMT compliance auditing.
• For FDI Plant Directors: Guaranteed supply continuity via 6 regional warehouses (Ba Ria - Vung Tau, Dong Nai, Binh Duong, Da Nang, Can Tho, Bac Ninh) and a dedicated 5–30-ton tanker fleet (4 hours in inner HCMC, 24–48 hours across Southern & Mekong Industrial Parks, 2–3 days in Northern & Central Vietnam).

4. Related Technical Resources & 15-Minute Quotation Gateway

• Explore our full industrial catalog: Industrial Chemical Catalog | Technical Knowledge Hub | Request a 15-Minute Technical Quote
• Legal Head Office: 452/6B Tinh Lo 10, Binh Tri Dong Ward, Binh Tan District, Ho Chi Minh City, Vietnam (Head Office only — all physical stock is dispatched from our 6 regional industrial park warehouses).
• 24/7 Technical Hotline: 0979 891 929 (Please contact hotline 0979 891 929 for detailed delivery procedures) | Official Email: [email protected]