Magnesium hydroxide as a substitute for caustic soda and lime

Improving the Sustainability of Water Treatment

Ralph Lloyd-Smith \ June 27, 2020
Home \ Water & Wastewater \ Magnesium hydroxide as a substitute for caustic soda and lime

Magnesium hydroxide is the least expensive and most powerful alkaline chemical treatment available on the market for pH control applications. At the same time, it is by far the safest and most gentle to use. By replacing the sodium in caustic soda or soda ash with the magnesium ion, the treated wastewater is transformed from having to deal with a detrimental form of salinity (Na+) to the benefit of a macronutrient (Mg2+). Mg2+ is the core element in chlorophyll that drives photosynthesis.

Controlling the pH of industrial wastewater is important to prevent harm to the wastewater collection/transmittance infrastructure by minimising the corrosive effect of gaseous acidic contaminants (such as H2S) or, if applied to farm land, to prevent harm to the irrigated crops. Once the wastewater enters a treatment plant, the control pf pH is essential to optimise aerobic and anaerobic microorganism activity. In this article, we explore three ways magnesium hydroxide has cost-effectively replaced caustic soda and lime in different wastewater treatment applications.

1) Safety is the driver

Caustic soda and lime are historically the two most industrially available choices for wastewater pH control, however, both have significant limitations. Caustic freezes at 15ºC to plug feed lines and is extremely hazardous to handle, causing severe chemical burns.

Lime is even better known for the plugging of feed lines, as well as wastewater transport lines due to the formation of calcium mineral scale, and over-exposure can also result in severe burns to operators’ hands.

2) Replacing caustic soda in a membrane bioreactor system

The membrane bioreactor (MBR) process allows for a much smaller footprint as compared to conventional wastewater treatment, due to the replacement of large clarifier basins with membranes. Recent innovations in membrane technology have reduced the cost of MBR systems, making them a more attractive choice in the market.

 

 

As the MBR process is microbiologically driven, the control of pH is critical, and there is no alkaline additive that is safer both for operators and microorganisms than magnesium hydroxide.

3) Improved solids settling

The wastewater stream for many industrial processes can be quite unpredictable and inconsistent. Dramatic changes in pH can have adverse effects on the settleability of suspended solids. To stabilise the pH, the wastewater is commonly neutralised with alkalis. The nature of the alkali selected can have a dramatic impact on the ability to settle suspended solids. Caustic soda typically provides no settling benefit and often results in gelatinous sludge that does not dewater easily. With lime (Ca(OH)2) there is typically a significant increase in sludge volume due to the formation of calcium minerals, like calcium carbonate (CaCO3).

In contrast, magnesium hydroxide assists with solids settling, providing a more compact sludge with a boost in alkaline pH to improve subsequent secondary treatment and anaerobic digestion performance.

ACTI-Mag for Caustic replacement

A safe and cost effective alternative to the use of sodium hydroxide (caustic soda) for wastewater alkalinity and pH adjustment.

Find out more about ACTI-Mag

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