Magnesium Hydroxide Feed Reliability – How Low Can You Go?

DOUG KELLEY \ November 7, 2022

Anyone who has ever worked with a thick, slurry product like magnesium hydroxide knows that the biggest challenge is to figure out a way to achieve reliable feed.  Magnesium hydroxide has some fantastic benefits, such as operator safety, improved microorganism activity, improved solids dewatering, and cost savings as compared to caustic soda and lime…but you can’t take advantage of these benefits if you can’t figure out how to reliably feed the product!  This story shares our exciting findings from a trial to replace caustic soda with IER’s new ALKA-Mag+ (60% Mg(OH)2) product under some of the worst possible conditions for chemical feed – extremely low usage rate and cold winter temperatures.

The Liberty Lake Water and Sewer District (LLWSD) recently completed an upgrade to their Water Reclamation Facility (WRF) which included the addition of membrane filtration and improved dewatering chemicals and equipment, required to allow their treatment process to achieve new reduced limits on nitrogen (N) and phosphorous (P) discharge into the Spokane River, which is a very clean, pristine mountain stream that is still recovering from decades of contamination from previous unregulated mining operations.

In order to maintain the necessary pH and alkalinity for nitrogen removal (nitrification), and to overcome the acidity of Alum (aluminum sulfate) being used for P removal, they were feeding caustic soda (50% NaOH) at a rate of about 36-72 gallons per day.

Based on the obvious safety benefit of Magnesium Hydroxide, along with possible coagulation benefits of the magnesium ion (Mg2+) in dewatering, LLWSD decided to perform an on-site evaluation of ALKA-Mag+.  They were also anticipating chemical cost savings due to the ability to provide the same pH and alkalinity using 0.6 of a 60% Mg(OH)2 product as compared to every 1.0 gallon of 50% NaOH.  In fact, it was decided to begin at a 1.0-to-0.5 ratio – meaning that we would begin the trial at 0.75 gph (18 gpd) of ALKA-Mag+.

For the trial, IER supplied a 500-gallon agitated cone-bottom poly storage tank and peristaltic metering pump, injecting ALKA-Mag+ directly into the aeration basin with 3/8 inch tubing (see Figure 1).

The biggest technical concern was to determine if we could maintain a reliable feed of this heavy, slurry product at such a low feed rate. Since slurries tend to settle out under laminar flow conditions, like in a slow-moving chemical feed line, we were concerned that we would struggle with feed line plugging.

The trial: Installing the ALKA-Mag + storage tank

The trial was initiated in October 2021 with an initial feed rate of 0.75 gph. The feed system ran smoothly and steadily. As the weather became colder, the LLWSD staff wrapped the chemical feed line with heat tape and insulation and placed the pump within a “dog house” with a space heater (see Figure 2). However, the liquid bulk product was not protected from the cold, simply stirring within the continuously agitated storage tank, and the staff proactively flushed the suction and discharge feed lines on a weekly basis.

Amazingly, despite numerously experiencing morning low temperatures below 10oF that winter, the product fed very reliably night and day with no plugging issues.  There was one instance that caused serious concern for the LLWSD staff. They came in one cold morning and realized that the power to the plant had been lost. They went to the ALKA-Mag+ storage tank and saw that the mixer was off and the slurry product had settled, having about 10 inches of a clear water layer above the settled magnesium hydroxide slurry. When power was restored, they turned the mixer back on and the product was remixed for about 15 minutes. They flushed the feed lines with water and restarted the pump. Again, the product fed perfectly with no issues, and they have not had any feed inconsistencies since. They now proactively flush the feed lines every other week and experience no plugging.

The results: Significant improvement by magnesium hydroxide vs caustic soda

In order to optimize P removal during the seasons, there are times when LLWSD needed to increase their dosage of Alum from 3 gph to 5 gph. Due to this addition of this acidic product, they needed to adjust their ALKA-Mag+ feed from 0.75 gph to 1.25 gph. With this combination of Alum and ALKA-Mag+, along with a significant improvement in their polymer sludge dewatering application, they have found that their overall microbiological activity and sludge handling processes have become incredibly steady. For instance, when using caustic soda they commonly saw wide variations in their mixed liquor concentrations, ranging from 1400 to 2800 mg/L, while with ALKA-Mag+ they are maintaining 1800-2000 mg/L year round – with no need to adjust the sludge wasting rate for summer and winter seasonality. Correspondingly, they have also seen an improvement in the % solids for dewatered sludge.  With NaOH, and poor polymer dewatering equipment, the % solids would vary widely around an average of ~13%, but now is “locked in” at a very tight ranging of ~16%.

Finally, by reducing their alkali treatment from an average of 50 gpd of 50% NaOH to about 25 gpd of ALKA-Mag+, they have been rewarded with a significant chemical cost reduction. Most importantly, the LLWSD staff greatly appreciated the benefits of replacing such a hazardous chemical with one that is inherently safe.

Trial tank on liberty lake site

Figure 1. Trial tank outdoor location on LLWSD property which was decided after consultation.

Figure 2. Insulation protects the chemical feed lines to avoid flow difficulties during colder seasons.
Conclusion: Installing a permanent ALKA-Mag+ storage and feed system

This on-site study provided the opportunity to demonstrate a highly reliable feed of a 60% Magnesium Hydroxide product at a very low dosage rate – even under very cold wintery conditions! IER’s ALKA-Mag+ is unique in its reactivity and stability properties compared to any other magnesium hydroxide product in the market!

Due to the chemical cost savings, operator safety, reliability of feed, and the overall improvements in microorganism health and process stability, LLWSD decided to install a permanent ALKA-Mag+ storage and feed system which is coming online in November of 2022.

If you’re interested in learning more about caustic replacement in your wastewater system with something that is occupationally and environmentally safer and much more cost-effective, don’t hesitate to get in touch with us today!

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