Magnesium Hydroxide in Water Purification: Benefits, Dosage & Best Practices

Quick Takeaways

• Magnesium hydroxide (Mg(OH)₂) raises pH while adding low alkalinity to water.
• It works as a gentle neutralizer, flocculant aid and hardness reducer.
• Typical dosing ranges from 0.5 to 3 kg per m³, depending on raw water quality.
• Compared with lime, it produces less sludge and requires lower storage space.
• Monitoring pH, alkalinity and residual magnesium is key to avoiding over‑correction.

When a treatment plant needs to swing the pH of acidic water without flooding the system with sodium or calcium, magnesium hydroxide steps in. Below is a practical walk‑through of what the chemical does, how to size it, and when it shines brighter than alternatives.

Magnesium hydroxide is a white, slightly soluble inorganic compound (chemical formula Mg(OH)₂) that behaves like a weak base in water. Because its solubility is about 0.009 g L⁻¹ at 25 °C, it dissolves slowly, delivering a gradual increase in pH and a modest rise in alkalinity. The slow release makes it ideal for processes that require steady neutralization without a sudden shock.

In the world of Water purification, magnesium hydroxide is prized for three reasons: it lifts pH, it promotes precipitation of metals, and it assists flocculation when paired with polymers. Each of those functions ties directly to the chemistry of the water being treated.

How Magnesium Hydroxide Adjusts pH

Acidic influent can result from industrial discharge, acid rain, or natural runoff. Raising the pH to the neutral range (≈7.0 ± 0.2) is often the first step before downstream processes like filtration or disinfection. When magnesium hydroxide dissolves, it releases hydroxide ions (OH⁻) that combine with hydrogen ions (H⁺) to form water, thus reducing acidity:

Mg(OH)₂ + 2 H⁺ → Mg²⁺ + 2 H₂O

The reaction also generates magnesium ions (Mg²⁺), which are harmless at typical treatment concentrations. Because the solid dissolves sparingly, the pH rises slowly, letting operators fine‑tune the dose in real time.

pH Neutralization vs. Alkalinity Control

Alkalinity is the water’s capacity to resist pH changes. Magnesium hydroxide adds a small amount of alkalinity (about 0.13 meq L⁻¹ per g Mg(OH)₂). In contrast, traditional lime (Ca(OH)₂) delivers 2.5 meq L⁻¹ per g, a much stronger buffering effect. The modest alkalinity of magnesium hydroxide means you can correct pH without overshooting the buffer, which is useful for streams that already have high natural alkalinity.

Role in Flocculation and Coagulation

Flocculation aggregates fine particles into larger flocs that settle out in clarifiers. While polymers are the primary flocculants, magnesium hydroxide can act as a co‑agulant aid. The Mg²⁺ ions neutralize surface charges on colloids, reducing repulsion and allowing polymers to bridge particles more effectively.

When paired with a polymer such as polyacrylamide, a typical dosage might be 0.5 kg Mg(OH)₂ per m³ of water plus 5 mg L⁻¹ of polymer. The result is clearer effluent and lower polymer consumption.

Hardness and Metal Removal

Magnesium hydroxide precipitates with certain metals (e.g., iron and manganese) as hydroxide solids. In a mildly acidic stream, adding Mg(OH)₂ drives the following reactions:

Fe³⁺ + 3 OH⁻ → Fe(OH)₃ (solid)

Mn²⁺ + 2 OH⁻ → Mn(OH)₂ (solid)

These precipitates settle in the same clarifier used for flocs, providing a dual‑benefit step. For facilities battling iron staining or manganese taste, magnesium hydroxide can replace separate heavy‑metal precipitants.

Calculating the Correct Dose

The dosage hinges on three measured parameters: initial pH, target pH, and alkalinity demand (measured as mg CaCO₃/L). A simple step‑by‑step method works for most municipal plants:

1. Measure the raw water’s pH and alkalinity.
2. Determine the desired pH (usually 7.2 ± 0.2 for downstream disinfection).
3. Use the following formula to estimate Mg(OH)₂ needed:
   Dose (kg/m³) = [(Target pH - Initial pH) × Alkalinity (mg CaCO₃/L) × 0.018] / 0.13
   0.018 converts alkalinity from mg CaCO₃/L to meq/L; 0.13 is the meq contribution per gram of Mg(OH)₂.
4. Round up to the nearest 0.1 kg and add the chemical gradually while stirring.
5. Check the pH after each addition; stop when the target is reached.

For a water source with pH 5.8, alkalinity 20 mg CaCO₃/L, and a target pH 7.2, the calculation yields roughly 1.3 kg Mg(OH)₂ per m³. Operators typically start with 0.8 kg/m³ and adjust based on online pH probes.

Comparison with Other Alkalinity Agents

Below is a side‑by‑side look at three common bases used in treatment plants. Magnesium hydroxide stands out for its low sludge volume and modest storage footprint.

For plants constrained by space or dealing with high calcium hardness, magnesium hydroxide offers a cleaner footprint.

Real‑World Implementation: A Municipal Case Study

A mid‑size city in the Pacific Northwest faced occasional pH dips to 6.4 after heavy rain events, which interfered with chlorination. The existing lime feed over‑corrected the water, causing high calcium scaling. The plant switched to magnesium hydroxide in 2023.

Key steps taken:

• Installed a screw feeder calibrated for 0.8 kg/m³.
• Added an online pH sensor linked to a PLC that modulates feed every 5 minutes.
• Monitored residual magnesium - stayed under 0.2 mg/L, below taste thresholds.
• Reduced sludge handling costs by 40 % because the Mg(OH)₂ precipitate is lighter and settles faster.

After six months, the plant reported stable pH (7.1 ± 0.1) and a 15 % drop in chlorine demand, translating into $120 k annual savings.

Common Pitfalls and How to Avoid Them

Even a simple chemical can cause headaches if you miss a step. Here are the usual suspects:

• Underdosing: Because Mg(OH)₂ dissolves slowly, you may think the pH is rising fast enough and stop early. Keep a 10‑minute observation window after each addition.
• Over‑dosing: Excess magnesium can cause a slight increase in water hardness, which may affect downstream softening units. Check Mg²⁺ levels regularly.
• Clogged feed lines: The powder can form bridges in narrow pipes. Use a vibratory feeder or install a small agitator.
• Temperature sensitivity: Solubility rises with temperature; in warm summer months the same dose may raise pH faster. Adjust the set‑point based on seasonal water temperature.

Addressing these issues early keeps the process smooth and avoids costly re‑tuning.

Frequently Asked Questions

By understanding the chemistry, dosing methods, and operational nuances, you can harness magnesium hydroxide to keep water clear, safe, and within regulatory limits. The chemical’s gentle yet effective nature makes it a go‑to solution for many modern treatment facilities.