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Magnesium Hydroxide vs. Caustic Soda for Wastewater Treatment

Choosing between magnesium hydroxide and caustic soda for municipal and industrial wastewater treatment is a critical decision that requires thorough evaluation. It is imperative to understand the differences between these two options, despite their similar functions.

This post will provide:

  • an overview of wastewater treatment

  • a comparison of the two more common treatment chemicals

  • a little about the cost

By going through this information, you can make a knowledgeable decision.



Local municipalities and various industries utilize technical processes that generate wastewater that somebody must treat before being released into the local sewage collection system. The treatment involves neutralizing the acidic wastewater byproduct and removing all hazardous materials.

Biological Purification

Plant operators at wastewater treatment facilities know that maintaining a specific pH level and adequate alkalinity is vital to the health of microorganisms and optimal plant performance. Each of these factors plays a crucial role in promoting stable biological activity. Thus, it is essential to treat and maintain both while ensuring that the treatment of one does not negatively impact the other. It is a delicate scientific balance. This balance can also determine the appropriate treatment chemical to use.


This post will compare the two most commonly used treatment chemicals - magnesium hydroxide and sodium hydroxide.


Magnesium hydroxide, or milk of magnesia, is a highly effective alkaline powder often dosed as a liquid in the treatment process. Its low solubility in water makes it a slow-acting but long-lasting neutralizer of acidic wastewater. Magnesium hydroxide boosts the water's pH and removes heavy metals and suspended solids through precipitation, improving its alkalinity. Additionally, it has the advantage of being a natural mineral compound with minimal environmental impact.

Magnesium hydroxide does have some drawbacks to consider. While its slurry may be prone to separation and require constant mixing or circulation, this issue can be easily resolved by selecting appropriate equipment. Storing magnesium hydroxide may be more challenging than the other options, but its self-buffering properties ensure more stable environments for biological organisms involved in wastewater treatment. This allows for consistent reaction times and optimal efficiency without harming these organisms.


Sodium hydroxide, commonly called caustic soda, possesses remarkable alkaline properties. Its swift reaction with acidic wastewater makes it an effective neutralizer; however, its water solubility can lead to unforeseen reactions, such as a surge in pH levels exceeding the limits of healthy biological parameters.

Despite its higher efficiency in neutralizing highly acidic wastewater than magnesium hydroxide, sodium hydroxide's corrosive nature poses a higher risk during handling and transportation. It is also essential to consider the solution's temperature and percentage of the caustic used. If the concentration surpasses 50%, the freezing point becomes 60°F, which either confines its usage to warmer environments or demands heating tanks.

For prolonged low to mid-level acidity, magnesium hydroxide is a preferable option. However, if high-level acidity needs to be neutralized quickly, caustic soda may be more effective. When deciding between magnesium hydroxide and caustic soda, it's essential to consider the type of wastewater, the required pH level, and any potential environmental, climate or safety concerns along with withe discharge requirements of the facility.



When deciding based on price, it's important to note that they can fluctuate frequently, making it difficult to determine the best option. Currently, sodium hydroxide's cost is increasing more rapidly than magnesium hydroxide, making Mg(OH)2 the more economical choice. Considering equipment and storage requirements, chemical usage, and sludge disposal costs is also essential.

As for chemical usage, magnesium hydroxide requires roughly 27% less material than caustic but depends on variables such as the volume of wastewater treatment required and the desired pH level.

When used properly in a treatment system, magnesium hydroxide creates a denser sludge that can be more easily dewatered with a lower disposal cost. This sludge is more particulate in nature because of the longer reaction time. On the other hand, sludge produced with sodium hydroxide has a more gelatinous consistency due to its quick precipitation.


To achieve the desired outcomes, it's crucial to thoroughly evaluate the entire treatment process and consider various factors before opting for the usage of mag or caustic soda.

While magnesium hydroxide can enhance the alkalinity of a microbial wastewater system, it can pose a storage challenge if not handled properly. Nevertheless, it has no adverse impact on pH levels and facilitates a more favorable environment for the bioremediation of BOD, nitrogen, and phosphorus.

For more details on phosphorus removal in wastewater, please click here, and stay tuned for our upcoming post on nitrogen removal.


Garrison Minerals manufactures AlkapHix® High Purity Magnesium Hydroxide as an alkalinity source and for pH control in municipal and industrial wastewater systems.

To learn more about our products or discuss your treatment needs with one of our talented professionals, visit our website or message us at


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