Aluminium sulphate (alum) is one of the most commonly used coagulants in water treatment. When dosed correctly, it helps remove suspended solids and improves water clarity. But calculating the right dosage isn’t always straightforward—especially in regional and remote systems where conditions can vary widely.

Here’s a quick breakdown of how alum dosing works:

Why Dose Alum?

Alum reacts with natural alkalinity in water to form aluminium hydroxide flocs, which trap particles and settle them out. The effectiveness of this process depends on:

• Raw water quality (turbidity, pH, alkalinity)
• Desired treatment outcomes
• System design and flow rates

How to Calculate Alum Dosage

To calculate the correct alum dose, you’ll need:

• Flow rate of the system (in ML/day or L/sec)
• Target alum concentration (typically in mg/L)
• Alum solution strength (e.g. 8.3% or 10%)

The basic formula is:

Dose (L/day) = [Flow (ML/day) × Dose (mg/L) × 1000] ÷ [Alum strength (%) × 10000]

This gives you the volume of liquid alum to dose per day.

Try It Yourself – Test Our Alum Calculator

We’ve created a free online calculator to help you run the numbers quickly and accurately. It’s designed for operators, technicians, and engineers working in real-world conditions.

Common Mistakes to Avoid

Even experienced operators can run into issues. Here are some of the most frequent dosing errors we see:

• Overdosing: More isn’t always better. Excess alum can lower pH too much, leading to corrosion and poor floc formation.
• Underdosing: Insufficient alum fails to remove turbidity, leaving water cloudy and non-compliant.
• Ignoring pH and alkalinity: Alum requires adequate alkalinity to react properly. Always check these parameters before dosing.
• Using outdated flow data: Flow rates can change seasonally or with demand—make sure your inputs are current.
• Not adjusting for solution strength: Commercial alum solutions vary. Using the wrong strength in your calculation can throw off your dose significantly.

This guide is part of our ongoing effort to support water professionals across Australia with practical, field-ready resources. If you have questions about alum dosing or want to dive deeper into treatment optimisation, feel free to reach out.