How to Use Aluminum Sulfate (Alum) in Water Treatment

Overview

Aluminum Sulfate — commonly known as alum — is one of the oldest and most widely used inorganic coagulants in water treatment. With an Al₂O₃ content of 15.8–17% and a 1% solution pH of 3.0–3.5, it works by hydrolyzing in water to form aluminum hydroxide flocs that adsorb and sweep colloidal particles out of suspension. While it has been largely supplemented by Polyaluminum Chloride (PAC) in modern plants, alum remains a cost-effective choice for large-volume municipal drinking water treatment, lake and pond restoration, and paper sizing where its ultra-low iron content (≤ 0.005%) prevents color contamination.

The non-ferric grade described here is specifically formulated for color-sensitive applications. Its exceptionally low iron content means no yellowing of treated water — a critical requirement for drinking water and food-grade process water.

Preparation & Dissolution

Aluminum sulfate is supplied as white flakes or granules and must be dissolved before dosing into the treatment system. Follow these steps:

1. Prepare a stock solution: Dissolve alum flakes in clean water at a ratio of 1:10 to 1:20 (10–5% w/v solution). Always add alum to water — never water to alum — to avoid localized heating and splashing.
2. Use warm water (30–40°C) to speed dissolution. Cold water below 10°C significantly slows the dissolution rate and can leave undissolved particles.
3. Stir continuously during dissolution. For large batches, use a paddle mixer or recirculation pump for 20–30 minutes until the solution is fully clear.
4. Check solution pH: A properly dissolved 1% alum solution should show pH 3.0–3.5. If significantly higher, check for product degradation.
5. Do not prepare stock solutions stronger than 15% as precipitation may occur, especially at lower temperatures.

Dissolution tank sizing: For a plant dosing 10 mg/L alum into a 10,000 m³/day flow, you need 100 kg/day of alum. A 1 m³ dissolution tank with 10% solution provides 100 kg capacity, allowing one batch per day.

Dosing Guide

Application	Typical Dose (mg/L as product)	Optimal pH Range	Notes
Drinking water — low turbidity (< 10 NTU)	5–20 mg/L	6.5–7.5	Jar test required
Drinking water — medium turbidity (10–100 NTU)	20–50 mg/L	6.0–7.2	Flash mix critical
Drinking water — high turbidity (> 100 NTU)	50–150 mg/L	5.8–7.0	May need lime for pH
Municipal wastewater	30–80 mg/L	5.5–7.0	Higher SS reduces dose
Lake / pond restoration	10–50 g/m² (surface)	6.0–8.0	Single shock dose
Swimming pool	50–100 mg/L	6.8–7.4	Shock clarification
Paper sizing (acidic sizing)	200–500 mg/L as process chemical	4.5–5.5	Different process

Application Procedure

1. Conduct a jar test before full-scale application. Prepare 6 beakers with the raw water, dose alum at 10, 20, 30, 50, 80, and 100 mg/L, apply rapid mixing (150–200 rpm) for 1 minute, slow mixing (30–40 rpm) for 15–20 minutes, and settle for 30 minutes. Measure residual turbidity and select the optimal dose.
2. Adjust pH if necessary. Alum lowers water pH (consumes alkalinity). If raw water alkalinity is below 50 mg/L as CaCO₃, add lime or soda ash to maintain pH 6.5–7.5 for optimal coagulation. A ratio of 1 mg/L alum consumes approximately 0.5 mg/L alkalinity as CaCO₃.
3. Flash mixing: Dose alum at the point of maximum turbulence — inlet to a flash mixer, pump suction, or hydraulic jump. G-value of 300–1000 s⁻¹ for 30–60 seconds. Poor flash mixing is the single biggest cause of coagulation failure.
4. Flocculation: After flash mixing, reduce mixing intensity (G = 20–70 s⁻¹) for 20–30 minutes to allow floc growth without shearing.
5. Sedimentation or flotation: Allow 1–4 hours settling time in a conventional clarifier, or 20–30 minutes in a DAF (Dissolved Air Flotation) unit.
6. Filtration: Pass through sand/anthracite filters to capture residual fine floc. Target filtered water turbidity < 0.3 NTU for drinking water.

Monitoring & Control

Parameter	Frequency	Target / Action
Raw water turbidity	Continuous	Adjust dose proportionally
Treated water turbidity	Continuous	< 1 NTU pre-filter; < 0.3 NTU post-filter
Coagulation pH	Every 2 hours	6.5–7.5 for optimal Al floc
Residual aluminum (treated water)	Daily	< 0.2 mg/L (WHO guideline)
Sludge blanket depth	Daily	Keep below 60% of clarifier depth
Jar test	Weekly or after raw water change	Re-optimize dose
Alkalinity (raw water)	Daily	> 50 mg/L as CaCO₃; add lime if lower

Common Mistakes

• Dosing dry alum directly: Never add alum flakes or granules directly to the water channel without pre-dissolving. Undissolved alum sinks to the bottom and dissolves unevenly, creating localized high-acid zones that impair coagulation and can corrode downstream equipment.

• Ignoring alkalinity: Alum consumes alkalinity. In soft water (alkalinity < 30 mg/L), uncontrolled alum dosing can drop the coagulation pH below 5.5, preventing proper floc formation. Always check raw water alkalinity and supplement with lime if necessary.

• Incorrect flash mixing: Dosing alum before a slow mixer, or at a point with insufficient turbulence, wastes chemical. The initial hydrolysis reaction happens within milliseconds — alum must contact all the water at once with intense rapid mixing.

• Over-dosing: Excessive alum doses (beyond the sweep floc zone) redissolve the floc and re-stabilize colloids through charge reversal. This results in a paradoxically worse effluent despite using more chemical. Always find the optimal dose via jar test.

• Neglecting seasonal adjustment: Raw water temperature, turbidity, and organic load change with seasons. A dose optimized for summer conditions will likely be wrong in winter when colder water slows floc formation and increases the optimal dose by 20–50%. Re-run jar tests quarterly.

Storage & Handling

• Store in a dry, covered warehouse away from moisture. Alum is hygroscopic — exposure to humidity causes caking.
• Bags should be stacked on pallets, not directly on concrete floors.
• FIFO (first-in, first-out) rotation to avoid long-term storage degradation.
• Wear chemical-resistant gloves and eye protection when handling. Alum is acidic (pH 3.0–3.5 in solution) and causes skin/eye irritation.
• Keep spill containment equipment nearby. Neutralize spills with soda ash or lime before washing away.
• Do not mix with alkaline chemicals (lime, caustic soda) in storage — risk of exothermic reaction and product degradation.
• Shelf life: 2 years in sealed bags in dry conditions.