FeSO4 for Wastewater Treatment — Dosage, Coagulation Mechanism & PAC, FeCl3 Comparison

TL;DR: FeSO4 (ferrous sulfate, CAS 7782-63-0) is the lowest-cost iron-based coagulant with superior phosphate precipitation — reducing 85–95% PO₄³⁻. Optimal pH 8.0–9.0, dosage 200–500 mg/L depending on wastewater type. Compared to PAC, FeSO4 forms heavier flocs with faster settling but generates more sludge. Loc Thien supplies FeSO4 granular 98%, 50kg bags, nationwide delivery to industrial zones. Call 0979 891 929 for a quote within 15 minutes.

What is FeSO4? Coagulation Mechanism in Wastewater Treatment

FeSO4 (ferrous sulfate, iron(II) sulfate) is a divalent iron salt, existing as pale green crystals. When dissolved in water, Fe²⁺ is oxidized to Fe³⁺ — the primary coagulation agent.

Coagulation proceeds through 2 stages:

1. Fe²⁺ → Fe³⁺ oxidation: Fe²⁺ + O₂ + H⁺ → Fe³⁺ + H₂O. This process requires dissolved oxygen and pH ≥ 7.5.
2. Hydrolysis to form Fe(OH)₃ floc: Fe³⁺ + 3OH⁻ → Fe(OH)₃↓. This precipitate entraps suspended solids, phosphate, and heavy metals.

Loc Thien Specialist — Tech. Vo Thi Nhu Hoa: “FeSO4 is the most economical choice for medium to large-scale WWTPs needing phosphate removal. Cost is only 40–60% of FeCl3 at equivalent iron content. However, correct operation at pH 8.0–9.0 and sufficient dissolved oxygen are essential for complete Fe²⁺→Fe³⁺ conversion.”

Technical Specifications

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

Dosage by Wastewater Type

Dosing flow formula: W = Q × C × 24 / (1000 × P) — where Q: flow rate (m³/h), C: dosage (mg/L), P: prepared solution concentration (%).

FeSO4 vs FeCl3 vs PAC Comparison

→ Choose FeSO4 when: phosphate removal needed, tight budget, aeration system available. Choose FeCl3 when: rapid coagulation needed without oxygen. Choose PAC when: treating drinking water, minimal sludge desired.

Preparation and Injection Procedure

Step-by-step preparation

1. Prepare 5–10% solution: Dissolve 50–100 kg FeSO4 granules into 1,000 L clean water, stir mechanically for 15–20 minutes until fully dissolved. The solution will have a characteristic pale green color and low pH (3–4).

2. Storage: FeSO4 solution is susceptible to oxidation — use within 24 hours of preparation, store away from direct sunlight. Upon prolonged exposure to air, Fe²⁺ oxidizes to Fe³⁺, causing a yellow-brown discoloration and reduced coagulation efficiency. For bulk use, prepare only what can be consumed within one shift.

3. Injection point: Position after the equalization tank, before the coagulation tank. Install a static mixer or in-line flash mixer at the injection point to ensure rapid dispersion. Aeration or rapid mechanical mixing is essential at the injection point to supply dissolved oxygen for Fe²⁺→Fe³⁺ conversion.

4. pH adjustment: Maintain coagulation pH at 8.0–9.0 using NaOH (caustic soda) or Ca(OH)₂ (hydrated lime). If pH drops below 7.5, the Fe²⁺→Fe³⁺ conversion is severely inhibited, resulting in poor floc formation and elevated dissolved iron in the effluent. Lime is often preferred over NaOH for FeSO4 coagulation because it simultaneously supplies OH⁻ ions and provides additional alkalinity buffering.

5. Polymer aid (optional): For difficult wastewater, inject 0.5–2 mg/L anionic polymer 30–60 seconds after FeSO4 injection. The polymer bridges the Fe(OH)₃ flocs into larger, denser aggregates that settle faster.

Dosage adjustment guidelines

FeSO4 vs FeCl3 vs PAC: Detailed Analysis

→ Choose FeSO4 when: phosphate removal is the primary goal, budget is constrained, and aeration system is already installed. FeSO4 is the coagulant of choice for municipal wastewater plants with phosphorus discharge limits below 1 mg/L.

→ Choose FeCl3 when: rapid coagulation is needed without oxygen, the wastewater has low alkalinity, or high color removal is required. FeCl3 is more acidic (pH 1–2 undiluted) and requires careful corrosion management.

→ Choose PAC when: treating drinking water supply, minimal sludge generation is desired, or operating at low temperatures where FeSO4 performance degrades.

FeSO4 Usage in Specific Industrial Effluents

Textile dyeing wastewater

Textile wastewater contains high color, surfactants, and recalcitrant dyes. FeSO4 at 300–500 mg/L achieves 60–80% color removal through precipitation of dye-Fe complexes. Reactive and direct dyes show the highest removal rates, while disperse dyes respond better when combined with PAC in a dual-coagulant system. Operating at pH 8.5–9.0 maximizes dye precipitation while maintaining acceptable effluent quality.

Electroplating wastewater

FeSO4 at 200–400 mg/L at pH 8.5–9.5 achieves >90% removal of heavy metals including Cr, Ni, Cu, Zn through co-precipitation with Fe(OH)₃ flocs. The Fe(OH)₃ matrix adsorbs and enmeshes metal hydroxides, producing a dense, stable sludge that passes the TCLP (Toxicity Characteristic Leaching Procedure) test for landfill disposal. For Cr⁶⁺ reduction to Cr³⁺, pre-treatment with sodium bisulfite or ferrous sulfate (taking advantage of Fe²⁺‘s reducing properties) is required before the main coagulation step.

Livestock farming wastewater

FeSO4 at 400–600 mg/L is highly effective for livestock wastewater with high phosphate content (50–150 mg/L PO₄³⁻). The Fe:P molar ratio of approximately 2:1 ensures near-complete phosphate removal. Combined with a polymer flocculant, TSS removal exceeds 90% and phosphate removal reaches 85–95%, allowing the treated effluent to meet discharge standards for agricultural reuse.

Case Study: Industrial Zone Wastewater Treatment — Detailed Results

A centralized WWTP in My Phuoc Industrial Park, Binh Duong Province — design capacity 5,000 m³/day — was experiencing phosphate exceeding QCVN 40 discharge limits. Inlet PO₄³⁻ averaged 8.5 mg/L, with outlet levels of 5–6 mg/L using PAC at 400 mg/L. The plant switched to FeSO4 at 350 mg/L combined with aeration enhancement:

Pre-implementation (PAC):

• PAC dosage: 400 mg/L
• Inlet PO₄³⁻: 8.5 mg/L
• Outlet PO₄³⁻: 5.2 mg/L (removal ~39%)
• Chemical cost index: 100 (baseline)

Post-implementation (FeSO4 + aeration):

• FeSO4 dosage: 350 mg/L
• Inlet PO₄³⁻: 8.5 mg/L
• Outlet PO₄³⁻: 1.8 mg/L (removal ~79%)
• Chemical cost index: 55 (−45% vs. PAC baseline)
• Sludge volume increase: 15%
• Sludge disposal cost increase: equivalent to ~7% of the baseline chemical cost

Net savings: −45% chemical cost, partly offset by higher sludge disposal (+~7% of baseline) = ~38% net monthly reduction

The plant maintained TSS below QCVN 40 limits and COD within acceptable range. The switch also reduced effluent iron concentration from 7–10 mg/L (with PAC carryover concerns) to <2 mg/L Fe residual after proper pH adjustment.

Frequently Asked Questions

Why does FeSO4 require high pH and dissolved oxygen?

FeSO4 contains divalent iron (Fe²⁺), which is not yet a coagulant. For coagulation, Fe²⁺ must first be oxidized to Fe³⁺ then form Fe(OH)₃. This process requires oxygen and pH ≥ 7.5. Without sufficient oxygen, Fe²⁺ remains in solution causing yellow discoloration without floc formation.

Can FeSO4 be used with polymer?

Yes. FeSO4 + anionic polymer is an effective combination: FeSO4 coagulates phosphate and suspended solids, while anionic polymer aids flocculation to form large, fast-settling flocs. Polymer dosage: 0.5–2 mg/L, injected 30–60 seconds after FeSO4.

How to verify complete Fe²⁺→Fe³⁺ conversion?

Take a sample from the post-coagulation basin, filter out flocs, add a few drops of H₂O₂ — if a red-brown precipitate appears, residual Fe²⁺ remains. Solution: increase retention time or boost aeration.

Does FeSO4 corrode equipment?

FeSO4 is less corrosive than FeCl3 because it contains no Cl⁻ ions. However, its solution has low pH (3–4) before dilution — use PP/HDPE tanks or 316 stainless steel for preparation and storage.

Where to buy FeSO4 98%?

Loc Thien Chemical supplies FeSO4 98% granular, 50kg bags, COA per batch. Nationwide delivery from 6 warehouses. Call 0979 891 929 — quote within 15 minutes.

Loc Thien INVESTMENT DEVELOPMENT CO., LTD — Tax ID: 0313650856 · Hotline: 0979 891 929 · hoachatlocthien.com

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]