Enhanced Biological Phosphorous Removal Using ALKA-Mag⁺

The Problem

In order to achieve this elevated discharge pH, the BWRRF needed to add a supplemental alkaline chemical. After researching numerous options, they realized that the most cost-effective choice would be magnesium hydroxide (Mg(OH)₂).

The BWRRF Supervisor had a previous positive experience with IER’s magnesium hydroxide products and services at a different facility, and he contacted us to perform a trial.

The Benefit of Data

Because of the sensitive nature of the White River, Buckley had been
proactively monitoring their influent and effluent water for ammonia (NH₃), total Phosphorous (total-P), and orthophosphate (ortho-P), along with the other traditional wastewater parameters (flow, TSS, BOD, etc.) since 2021. Therefore, they had strong knowledge of operational performance at the lower pH range without supplemental feed.

The Solution

IER’s ALKA-Mag⁺, containing 60% Mg(OH)₂, was selected as the optimum treatment to provide pH and alkalinity for his process. BWRRF very much liked the nonhazardous nature of ALKA-Mag⁺, as compared to the serious skin or eye hazards associated with caustic soda, or the high sodium content of soda ash.

For the trial IER, provided BWRRF with the 1000 gallon agitated storage tank and metering pump shown in the photo, with ALKA-Mag⁺ being fed into the influent stream of their activated sludge process (“oxidation ditch”). Note the lack of containment on the chemical storage tank, due to the nonhazardous nature of ALKA-Mag⁺.

Results

The ALKA-Mag⁺ trial was performed from November of 2022 through June of 2023. Prior to the trial, the final effluent pH was typically between 6.5 and 7.0. During the trial the pH was maintained between 7.0 and 7.3. Figure 1 shows the magnesium hydroxide buffering effect on the final effluent alkalinity values, providing roughly a 100% increase in alkalinity. Pre-trial values ranged from about 70 ppm in the summer to about 40 ppm in the winter (affected by rain). Alkalinity values during the trial ranged from 100-180 ppm, with the highest levels being applied in the months of May and June of 2023 (months 23 & 24).

Figure 2 shows the dramatic increase in phosphorous removal that corresponded to the onset of ALKA-Mag⁺ feed. Prior to the trial, the total-P and ortho-P % removal values ranged widely from month-to-month from a low of 64% up to a high of 93%.

Interestingly, the natural alkalinity of the wastewater during the two months immediately prior to the trial was 84 and 98 ppm, respectively; the highest on measure. Correspondingly, the total-P and ortho-P % removal values during those months were the highest pre-trial values recorded.

During the trial, the total-P and ortho-P % removal values were consistently <93%, with typical readings of 93-97% for total-P and 97-99% for ortho-P. During May and June of 2023, when the ALKA-Mag⁺ dose was highest, the % removal values were the highest recorded during the trial.

Further evidence of the direct correlation between ALKA-Mag⁺ feed and improved P removal was shown when the chemical was stopped in July and August of 2023 and restarted in September. In July and August, the total-P and ortho-P % removal values dropped into the mid 80% range despite the natural alkalinity in the wastewater still being in the 80-105 ppm range. Once the ALKA-Mag⁺ feed was resumed, the total-P and ortho-P removal values returned into >90% range.