What is Magnesium Hydroxide?

July 13, 2022
Magnesium hydroxide is an effective agent for treating potable water and wastewater.

Municipal wastewater requires alkalinity to encourage organic waste to biologically convert, and insufficient alkalinity prevents optimal bacterial growth conditions from being maintained. Magnesium hydroxide is a base used to raise the pH of extremely acidic potable water or wastewater.

This guide will explore the definition and uses of magnesium hydroxide, and the pros and cons of this material. 

Magnesium Hydroxide: A Definition

Magnesium hydroxide, known as Mg(OH)2, is a white powder that is produced by the addition of calcium hydroxide in seawater. This raw material forms “milk of magnesia” in water, which is used as a laxative and an antacid. 

Magnesium hydroxide is a chemical inorganic compound, and non-hazardous product that has several uses in water treatment. 

Uses of Magnesium Hydroxide

There are two common uses of magnesium hydroxide in water treatment: to precipitate dissolved heavy metals and to filter out suspended soilds, ultimately boosting water’s pH. 

Magnesium oxide can be used in place of sand in existing water treatment equipment to remove heavy metals from water and improve alkalinity. 

Magnesium oxide is most commonly used in large-scale wastewater treatment applications, but it may also be used as a point of use application for individual homes or businesses. 

How Magnesium Hydroxide Corrects Corrosive Water Conditions

Magnesium hydroxide neutralizes the free carbon dioxide in water, which corrects acidic water conditions. 

Acidic water is much more corrosive than neutral or alkaline water, and is more prone to metal leaching. A reactive magnesium hydroxide solution can substantially increase pH and restore water to a non-corrosive condition.  

Magnesium Hydroxide for Wastewater Treatment

Magnesium hydroxide is becoming a popular alternative to caustic soda and lime in wastewater treatment, particularly in the nitrification and anaerobic digestion processes.

Bacteria and other organisms that are involved in wastewater treatment are most capable when water has a neutral or slightly alkaline pH of between 7 and 8. To allow optimal pH conditions to be maintained, alkali will usually need to be added to the water (the exact amount depending on the type of water treatment used and the level of pollution in the water).

Wastewater treatment facilities can use magnesium hydroxide slurry for alkalinity and biological treatment. Magnesium is a nutrient supply for the bacteria, and the slurry’s buffering ability boosts water’s pH levels.

Previously, caustic soda and hydrated lime were considered the best options for boosting pH in wastewater treatment. However, magnesium oxide is becoming a more popular choice today because it is superior to both caustic soda and lime in controlling pH and providing alkalinity. 

What makes magnesium hydroxide slurry unique from the previously used products? It has an extremely low water solubility, so it produces limited hydroxyl ions, allowing for better pH control and longer-lasting water alkalinity. 

Pros and Cons of Magnesium Hydroxide Water Treatment


Some of the advantages of magnesium hydroxide in water treatment are: 

  • Magnesium oxide has been found to be superior to garnet sand and silica sand, producing lower turbidities and allowing for longer filter runs in the filtration of a number of particulates. 
  • If heavy metals are present in water, magnesium oxide can precipitate the metals effectively, enabling them to be removed from the water. Magnesium oxide can also enable the removal of phosphorus. 
  • Not only does magnesium oxide provide alkalinity; it is also a source of magnesium. This micro-nutrient is essential for controlled bacterial growth in wastewater treatment. 
  • Magnesium oxide sludge is more easily dewatered or filtered, and it precipitates metals more effectively than lime. 
  • Because magnesium oxide supplies more alkalinity per gallon of water, a smaller amount of slurry can be used compared to hydrated lime and caustic soda, allowing businesses to operate more cost-effectively.
  • Magnesium oxide makes it quicker and easier to control water’s pH with no risk of exceeding a pH of 9 (the maximum allowed in many wastewater treatment applications), all while maintaining the optimal conditions for bacterial growth.
  • Unlike hydrated lime, magnesium oxide does not cause scaling, so water treatment equipment using this agent will require less frequent maintenance and cleaning.
  • Finally, magnesium hydroxide is safe to handle. It i not toxic or corrosive, unlike hydrated lime and caustic soda.  


Some of the setbacks of using magnesium oxide in water treatment are: 

  • Magnesium oxide is typically more expensive upfront than most alternatives.
  • Magnesium oxide is not as widely available as caustic soda and hydrated lime. 


Magnesium hydroxide is an effective agent for treating potable water and wastewater. This product is becoming a popular alternative to caustic soda and hydrated lime for wastewater treatment, and silica sand and garnet sand for water filtration. Magnesium oxide is particularly beneficial when used in wastewater nitrification and anaerobic digestion processes. 

While magnesium hydroxide is slightly more expensive than the cost of similar materials, the benefits of this agent — including the savings in sludge handling — far outweigh this single setback.

About the Author

Brian Campbell

Brian Campbell is the founder of WaterFilterGuru.com, where he blogs about all things water quality. His passion for helping people get access to clean, safe water flows through the expert industry coverage he provides. Follow him on twitter @WF_Guru or contact him by email [email protected].