FeCl₃ vs Aluminum Sulfate — 10-criteria comparison table | Hóa Chất Lộc Thiên

FeCl₃ vs Aluminum Sulfate — 10-criteria comparison table

TL;DR: FeCl₃ and Al₂(SO₄)₃ (Aluminum Sulfate) are both common coagulants in water and wastewater treatment. FeCl₃ performs better across a wider pH range, at low temperatures, for color removal and phosphate removal. Aluminum Sulfate has lower cost and is familiar to many plants. The 10-criteria comparison table below helps engineers choose the right chemical for each application.

10-criteria comparison table — FeCl₃ vs Al₂(SO₄)₃

1. Optimal pH range

FeCl₃ has a wider optimal pH range than Al₂(SO₄)₃. FeCl₃ works effectively from pH 4.5 to 8.0, while Al₂(SO₄)₃ narrows to pH 5.5–7.5. Outside this range, Al₂(SO₄)₃ quickly loses effectiveness because Al(OH)₃ becomes soluble again at pH < 5.0 and > 8.0.

For wastewater with fluctuating pH (e.g. textile and food wastewater), FeCl₃ maintains more stable performance without requiring frequent pH adjustment.

2. Floc weight

FeCl₃ floc has a higher specific gravity (Fe(OH)₃ density ~3.4–3.9 g/cm³ vs Al(OH)₃ ~2.4 g/cm³). As a result, FeCl₃ floc settles faster, reducing the load on sedimentation tanks.

In jar-test trials, the settling rate of FeCl₃ floc during the slow-mix phase is 1.1 to 2.3 times higher than Al₂(SO₄)₃ depending on dosage.

3. Low-temperature performance

At temperatures < 10°C, the hydrolysis of Al₂(SO₄)₃ slows down significantly, reducing coagulation efficiency. FeCl₃ is less temperature-sensitive because Fe³⁺ hydrolyzes rapidly and the reaction is less dependent on ambient temperature.

At 4°C, FeCl₃ retains 85–90% of its coagulation efficiency, while Al₂(SO₄)₃ retains only 60–70% compared to performance at 25°C.

4. Sludge generation

Al₂(SO₄)₃ generates about 10–15% less sludge than FeCl₃. However, FeCl₃ sludge has higher solids content and better dewatering characteristics. Al₂(SO₄)₃ sludge is more difficult to dewater due to the colloidal nature of Al(OH)₃.

FeCl₃ sludge has higher density, reducing storage and transport volume compared to Aluminum Sulfate sludge of the same dry mass.

5. Residual Al/Fe after treatment

Soluble Fe³⁺ residual is very low at pH > 6.7 (below 0.3 mg/L). Al³⁺ has a narrower existence range — Al(OH)₃ re-dissolves at pH < 5.5 and > 8.5, causing aluminum residual in treated water.

WHO recommends aluminum residual in drinking water below 0.2 mg/L due to neurological risk concerns. For industrial wastewater, aluminum thresholds are typically controlled at 0.5–1.0 mg/L.

6. Equipment corrosion

FeCl₃ is more corrosive than Al₂(SO₄)₃. FeCl₃ creates a Cl⁻ and Fe³⁺ environment with oxidizing properties, requiring plastic (PP, PVC, HDPE) or 316L stainless steel tanks and piping.

Al₂(SO₄)₃ is less corrosive and can be used with 304 stainless steel or composite materials.

7. Wastewater color removal

FeCl₃ removes color better than Al₂(SO₄)₃, especially with textile wastewater containing reactive and disperse dyes. Fe(OH)₃ floc adsorbs dye molecules effectively due to its large surface area and positive charge at pH 5.0–6.5.

For high-COD textile wastewater, FeCl₃ removes 38–46% of COD while Al₂(SO₄)₃ removes 27–36%.

8. Phosphate removal

FeCl₃ achieves 85–95% phosphate removal at pH 5.0–6.0, higher than Al₂(SO₄)₃ (80–90%) under the same conditions. FePO₄ has a lower solubility product than AlPO₄, meaning a more stable precipitate that is less prone to re-dissolution.

If the treatment target is deep phosphorus removal (< 0.5 mg/L), FeCl₃ is the more suitable choice.

9. Chemical cost

Al₂(SO₄)₃ is the cheapest coagulant, about 10–30% lower than FeCl₃ per unit mass. However, considering treatment efficiency (cost per kg of pollutant removed), FeCl₃ can be more competitive due to lower dosage requirements and higher performance in many applications.

10. COA certification

Both FeCl₃ and Al₂(SO₄)₃ from Hóa Chất Lộc Thiên come with batch COA, meeting ZDHC MRSL Level 1. QC is controlled by Phan Cẩm Thủy (incoming) and Võ Thị Như Hòa (outgoing), ensuring every batch has complete test documentation.

When to use FeCl₃ vs when to use Aluminum Sulfate?

Choose FeCl₃ if:

• Wastewater has low or strongly fluctuating pH (4.5–6.5)
• Wastewater temperature is low (< 10°C)
• Deep color removal is required (textile, paper)
• Deep phosphate removal is required (< 0.5 mg/L P)
• Fast-settling floc is needed to increase sedimentation tank capacity

Choose Al₂(SO₄)₃ if:

• Chemical budget is the priority
• Wastewater has stable neutral pH (6.5–7.5)
• Deep phosphorus removal is not required
• Existing equipment cannot withstand FeCl₃ corrosion

Summary table: FeCl₃ is suitable for wastewater with low/fluctuating pH, low temperature, deep P removal and color removal requirements. Al₂(SO₄)₃ is suitable for stable neutral pH wastewater, limited budgets, and equipment that cannot withstand Cl⁻ corrosion.

In actual operation, many plants combine both — using FeCl₃ for primary physico-chemical treatment (P removal, color removal) and Al₂(SO₄)₃ for secondary coagulation after the biological treatment stage. Running a jar-test with actual wastewater is a mandatory step before deciding to switch coagulants.

Quick jar-test comparison guide

When comparing FeCl₃ and Al₂(SO₄)₃ on the same wastewater source:

1. Prepare 1% FeCl₃ solution and 1% Al₂(SO₄)₃ solution (based on active ingredient)
2. Set up 6 jar-test cups, 3 cups per chemical at dosages of 50, 100, 200 ppm
3. Rapid mix at 100 rpm for 1 minute, slow mix at 30 rpm for 15 minutes
4. Settle for 30 minutes
5. Measure pH, turbidity, PO₄³⁻, color, COD of settled water
6. Select the chemical and dosage yielding the highest performance on target parameters

Recent comparative studies

Kertil et al. (2026) compared FeCl₃ and Al₂(SO₄)₃ on high-COD industrial wastewater (2,136 mg/L COD, 1,085 mg/L BOD, 798 mg/L TSS):

• FeCl₃ achieved higher COD, BOD, TSS, and TP removal than Al₂(SO₄)₃ at the same dosage
• Both have a narrow optimal dosage range; overdosing causes floc instability
• FeCl₃ has 15–25% lower Global Warming Potential (GWP) and Cumulative Energy Demand (CED) compared to Al₂(SO₄)₃
• No scenario where Al₂(SO₄)₃ had lower GWP than FeCl₃

A study on the Nakdong River (South Korea) showed coagulation efficiency order: FeCl₃ > PAC (r=2.2) > PAC (r=1.2) > Al₂(SO₄)₃ for both turbidity and organic matter.

Frequently asked questions

Which is more expensive, FeCl₃ or Aluminum Sulfate?

FeCl₃ is 10–30% more expensive than Aluminum Sulfate per unit mass. However, its higher treatment efficiency means the actual cost can be equivalent or lower.

Can Aluminum Sulfate be replaced with FeCl₃?

Yes, but pH must be controlled and equipment materials must be checked (FeCl₃ is more corrosive). Always run a jar-test before switching.

Is FeCl₃ safe for drinking water?

Yes, FeCl₃ is used in drinking water treatment in many countries. Iron residual is low at neutral pH. COA is required to control impurities.

Does Al₂(SO₄)₃ have toxic aluminum residual?

Al can be neurotoxic at high concentrations. WHO recommends < 0.2 mg/L in drinking water. Controlling pH at 6.5–7.5 helps reduce residual Al.

Does using FeCl₃ increase water color?

If overdosed, residual Fe³⁺ creates a yellow-brown color. At optimal dosage and pH > 6.7, Fe precipitates completely and water remains colorless.

🛒 Need FeCl₃ or Aluminum Sulfate? Request quote for FeCl₃ 38% — batch COA/MSDS, nationwide delivery. | Request quote for Aluminum Sulfate Al₂(SO₄)₃ — batch COA. Hotline 0979 891 929.