Properties of Magnesium Hydroxide for Wastewater Treatment - Wastewater Plants

DOUG KELLEY \ December 16, 2022

Many wastewater operators consider themselves “zoologists”.  However, unlike at zoos, where the species being looked after are lions, tigers, and bears (oh, my!), wastewater operators are responsible for maintaining the stability and growth of “critters” they can’t even see – microorganisms. But, oh, what amazing life and activity that can be seen when examining a wastewater sample under a microscope! It’s a whole different world!

The processes involved in recovering clean water and valuable carbon, nitrogen, and phosphorous by-products from wastewater are actually very elegant. In the traditional, conventional approach, as wastewater enters a facility, the biggest contaminants are removed using grit filters and screens, followed by primary clarifiers where the heaviest of the suspended solids are allowed to settle to the bottom as “primary sludge”, allowing the remaining soluble and suspended contaminants to travel onward with the water. In the next stage, hungry microorganisms consume the remaining suspended organic contaminants in the wastewater through aerobic and anoxic processes, and in so doing multiply to tremendous numbers – called “activated sludge”. In a properly designed secondary treatment process, the activated sludge so effectively removes the wastewater contaminants that the bacteria can actually become cannibalistic toward each other. Upon recycling this activated sludge to the front end of the process, the extremely hungry bacteria are ready to consume the fresh wastewater contaminants entering the system, and the whole cycle repeats itself – with extremely clean water being the end result.

Over time, the volume of activated sludge grows so large that some need to be removed from the system. Therefore, a percentage of the activated sludge is separated from the secondary system and joined with the primary sludge from the beginning of the process. These two sludges are typically fed into another microbiological system called an anaerobic digester, to be reduced in number and converted into methane–biogas, “green” energy.

How wastewater microorganisms can lower the pH

All microorganisms display certain types of metabolic activity, meaning that they have the ability to chemically react with certain contaminants in the wastewater to reduce the contaminant into a smaller substrate. These chemical reactions driven by wastewater microorganisms can have an impact on the pH of the overall water system. For instance, the microbiological activities involved in removing nitrogen from wastewater are called nitrification and denitrification. In nitrification, bacteria convert ammonia and similar nitrogen compounds into nitrite (NO2) and nitrate (NO3).  These reactions consume alkalinity, which can result in a drop in pH. If not countered by feeding an alkaline additive into the wastewater, the pH can fall to a level that is below that for the bacteria to continue to perform their functions. If the pH falls too far, it can actually lead to microorganism death. 

As was mentioned previously, the industry standard for treating low pH is to feed caustic soda (NaOH), which is hazardous for the employees needing to store and feed it. It is also hazardous for the microorganisms, as NaOH kills any bacteria it encounters upon entering the wastewater stream until it becomes diluted to the desired pH. At that point, its job has been completed and it has no lingering buffering ability. Lastly, the increased sodium ion (Na+) concentration can become a detriment to microorganism activity and can interfere with the formation of beneficial flocs for solids settling and dewatering. 

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 4oC as compared to 25oC.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.

Conclusion: Multiple benefits of IER’s ALKA-Mag+ for treatment of your wastewater

In summary, there are numerous benefits in the transition from the use of caustic soda to magnesium hydroxide when managing a wastewater treatment process. The most obvious is operator safety. However, improved process stability from the increased buffering strength of magnesium hydroxide is also a tremendous operational asset. In some cases, improved sludge dewatering can be observed which can translate into significant savings in sludge hauling fees.  Finally, in membrane bioreactor systems, the replacement of NaOH with Mg(OH)2 can result in the ability to maintain a maximum flow rate even through the coldest months of the year.  

In order to enjoy these benefits, care must be taken to design and install a proper storage and feed system. IER takes pride in providing agitated trial storage tanks of various sizes (150 to 1500 gallons), allowing our customers to experience the performance and reliable feed of our products prior to committing to installing a permanent bulk storage system. The 40% reduction in chemical usage when switching from 50% NaOH to 60% Mg(OH)2 is typically enough to cover the costs of installing the storage and feed system with an ROI of less than one year. Then, in years two and onward the cost savings can be dramatic.

Sources:

1) Glasdam S.M., Glasdam S., Peters G.H.  The Importance of Magnesium in the Human Body: A Systematic Literature Review. Adv. Clin. Chem. 2016, 73, 169-193. 

2) De Baaij J.H., Hoenderop J.G., Bindels R.J.  Magnesium in man: Implications for health and disease. Physiol. Rev. 2015, 95, 1-46.

3) Weihua Li, Muhammad Saboor Siddique, Nigel Graham, and Wenzheng Yu*.  Influence of Temperature on Biofilm Formation Mechanisms Using a Gravity-Driven Membrane (GDM) System: Insights from Microbial Community Structures and Metabolomics, Environ. Sci. Technol.2022, 56, 12, 8908-8919.

4) Keren DemishteinRam Reifen, and Moshe Shemesh*Antimicrobial Properties of Magnesium Open Opportunities to Develop Healthier Food, Nutrients. 2019, 11, 10, 2363.

5) Pasvolsky R., Zakin V., Ostrova I., Shemesh M.  Butyric acid released during milk lipolysis triggers biofilm formation of Bacillusspecies. Int. J. Food Microbiol. 2014, 181, 19–27.

6) Ben-Ishay N., Oknin H., Steinberg D., Berkovich Z., Reifen R., Shemesh M.  Enrichment of milk with magnesium provides healthier and safer dairy products. NPJ Biofilms Microb.  2017, 3, 24.

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This article serves as the final piece in our series of articles on the properties of magnesium hydroxide in relation to wastewater treatments. 

Want to know more? Check out the rest of this three-part series below:

Part One: Industrial Wastewater Pretreatment

Part Two:  Wastewater Collection System

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