Properties of Magnesium Hydroxide for Wastewater Treatment - Pretreatment

DOUG KELLEY \ December 14, 2022

Significant industrial users (SIUs) are regulated industries within a municipality or sewer district that discharge either: 

1) a volume of wastewater that is above a minimum threshold as being representative of a certain percentage of flow entering the treatment facility, or 

2) a wastewater stream containing a contaminant(s) that could adversely impact the operations of the treatment facility.  

SIUs are commonly regulated on wastewater contaminants such as TSS (total suspended solids), BOD (biological oxygen demand), pH, and flow.  For instance, a common pH range for acceptable discharge is from 6.0 to 9.0.  Therefore, if an industry has a wastewater stream that has a pH less than 6.0, it will be required to feed an alkaline additive to bring the final effluent pH to above 6.0 prior to discharge to the municipality.

More recently, sewer districts are requiring SIUs to increase the pH of their effluent wastewater, perhaps from a minimum of 6.0 up to 6.5 or 7.0.  This is being done to minimize the impacts of further pH reduction and corrosion that can occur while the sewage passes through the collection system (which will be discussed in the next section).

The industry standard alkaline additive used to increase the pH of acidic wastewater is sodium hydroxide (NaOH), also called caustic soda, and it is typically sold as a 50% concentrate.  In colder climates, the typical product concentration is 25% NaOH, which freezes at the same temperature as water, while 50% NaOH freezes at 60oF.  

All about caustic soda: the benefits and the drawbacks 

One of the benefits of caustic soda is that it is relatively inexpensive, being a byproduct of the chlor-alkali process that produces chlorine gas.  Another benefit is that it is a pure liquid and a strong base that reacts instantly within the wastewater stream to give the desired pH within seconds.  As a liquid, it is relatively easy to feed using a standard chemical metering pump. 

The negatives of caustic soda are also related to its high rate of reactivity.  First, if the dose in the wastewater is too high, the pH can readily spike up to a value above 12, resulting in a need to feed an acid-based product to bring the wastewater pH back down into regulatory compliance.  That means the wastewater operator has to handle two hazardous products – a strong base and a strong acid.  This brings us to the next concern with sodium hydroxide:  it is a highly hazardous chemical to handle.  The primary industrial use for NaOH is as a cleaner, because of its rapid hydrolysis reaction to break down proteins and carbohydrates that can line the walls of storage tanks and transfer lines.  That is why “caustic cleaners” are extremely common products in the sanitation market.  Unfortunately, our skin and eyes consist of proteins and carbohydrates that are equally susceptible to the extreme reactivity of caustic soda, and serious caustic burns are far too common in the industry.  For this reason, most wastewater operators do not enjoy working with caustic soda.  

In addition, an industrial wastewater operator typically does not have the same amount of resources, support, or training available to them as compared to a municipal operator.  At a municipal wastewater treatment facility, the entire purpose is to achieve a high-quality effluent stream that meets or exceeds their state permit.  The effluent being discharged to the environment is their “product” – something to be extremely proud of.  However, for industries like food processing or metal plating, a clean wastewater stream is not a sellable product.  Their products are pork chops and chrome fenders, and the need to pretreat the wastewater for discharge is purely an overhead expense.  Therefore, the Environmental Manager responsible for the industry’s pretreatment process often runs a leaner operation with less personnel and training.  For these reasons, it’s in their best interest to help their wastewater operator to work with less hazardous chemicals.  From the very beginning of IER, this has been our primary mission: to help industrial wastewater operations convert from the use of caustic soda to magnesium hydroxide, to make the life of the wastewater operator a little safer each day.

All about IER’s ALKA-Mag+ 

The biggest benefit of IER’s ALKA-Mag+ (60% Magnesium Hydroxide) is that it is completely nonhazardous, which is a huge feather in the cap for an Environmental Manager interested in making the switch from caustic soda.

In addition, the dose required for ALKA-Mag+ to provide a certain pH boost to a wastewater stream is far lower than that for 50% NaOH.  There are two reasons for this:

  • ALKA-Mag+ contains 60% total active molecules versus 50% in caustic soda
  • Magnesium hydroxide releases two moles of OH from a single molecule of Mg(OH)2, while NaOH only releases one 

Calculating the stoichiometric number of moles of hydroxide that can be released from each product results in a projected usage rate reduction of 0.6 lbs of 60% Mg(OH)2 for every 1.0 lb of 50% NaOH.  The following equations show how the moles of OH released from 100 lbs of 50% NaOH and from 60 lbs of 60% Mg(OH)2 are nearly identical: 

Moles of OH from 100 lbs of 50% NaOH:

(100 lbs) x (50 lbs NaOH/100 lbs) x (453.5 g/lb) x (1 mole NaOH/40 g) x (1 mole OH/mole NaOH) = 567 mole OH

Moles of OH from 60 lbs of 60% Mg(OH)2:

(60 lbs) x (60 lbs Mg(OH)2/100 lbs) x (453.5 g/lb) x (1 mole Mg(OH)2/58 g) x (2 mole OH/mole Mg(OH)2) = 563 mole OH

This 40% reduction in chemical usage can result in dramatic cost savings when switching from caustic soda to magnesium hydroxide. 

Likewise, when comparing 25% NaOH with 60% Mg(OH)2, the calculation shows that every 1.0 lb of 25% NaOH can be replaced with 0.3 lbs of ALKA-Mag+ to provide the same number of moles of OH (as a side note, both 25% NaOH and ALKA-Mag+ freeze at the same temperature as water).

Trial tank on liberty lake site

500 gallon ALKA-Mag+ feed System

Vinegar Test Data

 

Magnesium Hydroxide vs Caustic Soda in a customer trial

We were once challenged by a potential customer about these usage estimations. The customer runs a plant that made condiments in the Midwest, having a large amount of vinegar on-site as a raw material in their products. 

They performed an experiment that involved diluting vinegar 20-to-1 with city water and filling two 2000 mL flasks.  They then proceeded to add incremental doses of 50% NaOH into one of the flasks and 60% Mg(OH)2 into the other, while stirring.  The data provided in the table demonstrates that the expected 1.0-to-0.6 ratio is accurate. For instance, after adding 5 mL of 50% NaOH the pH had climbed to 4.92, while it only took 3 mL of 60% Mg(OH)2 to achieve roughly the same pH. Likewise, it took a dose of 6.5 mL of 50% NaOH to achieve roughly the same pH as 4 mL of 60% Mg(OH)2.

This demonstration of the actual 40% reduction in chemical use resulted in chemical cost savings that when coupled with the dramatic safety benefit resulted in this customer rapidly making the switch. That was about 15 years ago, and they remain a happy customer to date.

This same customer also realized another pretreatment benefit of magnesium hydroxide that they were not expecting.  When using caustic soda, whenever the plant would release a large volume of concentrated acids into the wastewater stream, the pH in their flow equalization tank (EQ Tank) would drop precipitously.  This is because caustic soda provides little to no buffering benefit. 

However, after switching to magnesium hydroxide, the wastewater pH in the EQ Tank was only marginally decreased after a similar release of acids from the plant, allowing them to remain within their compliance range.  This strong buffering effect was also seen when an operator accidentally overfed magnesium hydroxide into the EQ Tank.  Instead of the pH shooting up over 12, as would have happened with NaOH, the pH climbed only into the mid-8s, which was still within their compliance range. 

So, if magnesium hydroxide is so perfect then why isn’t everyone using it?  This brings us to the primary negative aspect of magnesium hydroxide.  It is a slurry that is unstable and prone to settling over time – like old paint in a can.  It needs to be well mixed in storage in order to feed reliably through a small metering pump.  Because past chemical suppliers did not work with their customers to develop reliable magnesium hydroxide storage and feed systems, this product has an unfortunate history of being “impossible to feed” or “constantly plugging”.  It’s hard for an operator to enjoy the benefits of this inexpensive and nonhazardous product if they can’t feed it!

Conclusion: How IER Water can help you 

Therefore, in order for an industry to have a smooth and successful transition from caustic soda to magnesium hydroxide, they will need to figure everything out on their own…or, select a magnesium hydroxide supplier that provides technical support in the design and installation of a robust and reliable storage and feed system, as well as on-site service to make sure it performs optimally. Since reliable feed is the primary challenge with magnesium hydroxide, IER has made it our responsibility to provide our customers with robust and reliable agitated storage and feed systems.

For instance, as a first step toward the transition away from caustic soda, we provide customers with a trial storage tank ranging from 150 to 1500 gallons that, as shown in the photo, can be simply set into place to demonstrate performance without being intrusive to the existing wastewater pretreatment process. In this way, the customer can focus their attention on the actual performance comparison of magnesium hydroxide versus sodium hydroxide, rather than having to fight with the feed of a new, unknown slurry product.

Another difference between NaOH and Mg(OH)2 is how rapidly they react to neutralize acids.  As mentioned previously, caustic soda requires only a few seconds to boost a wastewater pH from 4 to 7, while it may require 10 minutes for magnesium hydroxide to do the same.  Therefore, consideration might need to go into finding a dosing location upstream of the caustic feed point in order to allow enough time for magnesium hydroxide to do its job.  Alternatively, we’ve worked with customers that have redirected their wastewater flows into a large holding tank prior to discharge, allowing the necessary time for Mg(OH)2 to do its job.  In such cases, the costs to add a “detention time” holding tank, along with the agitated magnesium hydroxide storage and feed system, were still lower than that of continuing to feed caustic soda because of the reduction in chemical usage as discussed previously.

In summary, there are numerous benefits to transitioning from the use of caustic soda to magnesium hydroxide when managing a wastewater pretreatment process.  The most obvious is safety.  However, process stability from the increased buffering strength of magnesium hydroxide is also a tremendous operational asset.  In order to enjoy these benefits, care must be taken to design and install proper storage and feed system. The 40% reduction in chemical usage is typically enough to cover the costs of installing the storage and feed system with an ROI of less than one year. Then, in year two and onward the cost savings can be dramatic.

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Interested in finding out more about the properties of magnesium hydroxide in wastewater treatment? Click the links below to read the next articles in this series:

Part Two:  Wastewater Collection System

Part Three:  Wastewater Treatment Plants

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