Magnesium Hydroxide Feed Reliability – How Low Can You Go?

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 10^oF 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.

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.