Properties of Magnesium Hydroxide for Wastewater Treatment - Wastewater Plants

Multiple benefits of magnesium hydroxide through out the wastewater treatment plant

Alternatively, magnesium hydroxide is completely nonhazardous, is slow to dissolve in the wastewater system, and provides an extremely strong buffering effect. In addition, the magnesium ion (Mg²⁺) is a beneficial macronutrient. It is the core element in chlorophyll, the reaction center in green plants that drives photosynthesis. Within bacterial cells, the Mg²⁺ ion is a cofactor in enzymatic reactions supporting phosphorylation, carbohydrate and lipid metabolism, and nucleic acid and protein synthesis, and plays an active role in the transport of calcium and potassium ions across cell membranes._1-2

The buffering benefit of magnesium hydroxide within the various microbiological wastewater treatment processes can be a dramatic benefit. In numerous instances, treatment plants struggling to maintain pH and alkalinity using NaOH have made the switch to Mg(OH)₂ to find that the resulting pH and alkalinity remain incredibly stable. The tremendous buffering strength of magnesium hydroxide stems from its relative insolubility at pH levels at or above neutral. By maintaining a pH in the low 7s, a small percentage of the magnesium hydroxide slurry will not readily dissolve but will instead become entrained within the activated sludge (or anaerobic sludge within a digester). Then, as bacterial processes occur that produce acid in the system, the residual Mg(OH)₂ particles present within the sludge release hydroxide ions (OH^–) to neutralize the acid – holding the pH steady.  

The magnesium ion also has coagulation properties that Na⁺ does not. With a quick glance at the periodic table, one can see that the Mg²⁺ ion resides between sodium and aluminum (Al³⁺).  While increasing concentrations of Na+ are detrimental to coagulation and floc formation, the use of aluminum salts, such as alum (aluminum sulfate), PAX (polyaluminum chloride), and ACH (aluminum chlorohydrate) are common industrial coagulants. The enhanced coagulation from Na⁺ to Mg²⁺ to Al³⁺ is related to a phenomenon called charge density.  With each ion in progression possessing one less valence electron, the increased positive (+) charge in the nucleus pulls the remaining valence electrons in toward the nucleus of the atom. Therefore, the size, or ionic radius, of the ions is reduced as the positive charge is increased.  In other words, the size of the Al³⁺ ion is much smaller than that of Mg²⁺, which is smaller than Na⁺. Since the Al³⁺ ion has a higher (+)-charge in a smaller ionic space, it is said to have a higher charge density.  Simply put, ions having high charge density, such as Al³⁺ and Fe³⁺, are excellent coagulants, attracting negatively charged suspended particles to them to form pin flocs and even larger flocs.

Residing between the two extremes of Na⁺ and Al³⁺, the Mg²⁺ ion possesses some coagulation benefits, though not nearly as strong as the aluminum salts.  However, it is not uncommon when transitioning from the use of caustic soda to magnesium hydroxide for pH and alkalinity control that the sludge dewatering properties at the back end of the wastewater treatment process are improved. In one membrane bioreactor wastewater facility, the dewatered centrifuge solids were increased from the low 20% range into the upper 20% range simply from the transition from 25% NaOH to 60% Mg(OH)₂. This improved sludge dewatering resulted in reducing the number of truckloads/month of dry cake solids from ~34 to ~18. With the cost for each truckload being about $1000, this translated into significant monthly cost savings, along with the cost savings from using only about 30% of the daily dosage of alkali products. While we have never experienced a situation where the sludge dewatering process was negatively impacted by the switch from caustic soda to magnesium hydroxide, at the same time the dramatic benefit mentioned above is not always observed. However, this is also true for standard coagulants like alum and PAX. As every wastewater operator knows, every plant seems to have different wastewater properties.

Magnesium hydroxide’s effect on flow rate through membrane bioreactor

The last observation to discuss was probably the least expected, but in hindsight makes sense based on the chemistry of magnesium hydroxide and the Mg²⁺ ion.  We have recently observed in a membrane bioreactor (MBR) wastewater treatment facility that in switching from NaOH to Mg(OH)₂ the flow rate through the membrane was maintained steady year-round, while when using NaOH the flow rate always declined significantly during the winter months. When discussing this phenomenon with membrane manufacturers, the feedback is that it is normal to see a decrease in flow through the membrane during the colder months of the year. This is consistent with recent biofilm research by Weihua et al showing denser biofilm formation and reduced flux at 4^oC as compared to 25^oC.₃ However, when feeding IER’s ALKA-Mag⁺ (60% Mg(OH)₂) the flow rate through the membrane remained constant even during the coldest months of the year. This is likely related to the effect of the Mg²⁺ ion on inhibiting the formation of biofilm,₄ that can become dense during cold weather months and inhibit flux through the membrane. While the exact mechanism of how magnesium prevents biofilm growth remains unclear, evidence for this phenomenon goes as far back as antimicrobial studies in 1915. Regardless of the mechanism, the presence of a certain concentration of Mg²⁺ results in a reduction in biofilm.

Recent studies were performed on milk, which contains nutrients such as lactose, proteins, and lipids, and has a nearly neutral pH, making it an ideal medium for growing bacteria. Since milk microorganisms can pose health risks the industry is subject to stringent regulations, including pasteurization at high temperatures to kill most bacteria and storage at low temperatures to limit bacteria growth.  Research on several Bacillus strains has shown that milk can trigger the formation of biofilm which might make the bacteria more resistant to pasteurization.₅ A recent study has shown that supplementation of milk with 5 mM MgCl₂ significantly impairs biofilm formation.₆ Therefore, if the Mg²⁺ ion can inhibit the formation of biofilm in milk, the chemistry of magnesium hydroxide and the Mg²⁺ ion is likely to perform a similar function to inhibit the formation of biofilm in wastewater membrane bioreactor systems. This mechanism of biofilm inhibition is likely the same activity that is being observed with the use of ALKA-Mag⁺ in the collection system for odor control – with the observed removal of existing biofilm and FOG to keep the treated sewer line clean.