Replacement of Caustic Soda with ALKA-Mag⁺

In a Membrane Bioreactor Wastewater Treatment Process

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The Background

The Brightwater Treatment Plant Operated by King County is located near Woodinville WA and has the capacity to treat 36MG of wastewater via a membrane bioreactor (MBR) process. The plant location is in an extremely sensitive area for both environmental and aesthetic factors, requiring the Brightwater staff to optimally design, implement, and maintain their processes, to which they have succeeded – being consistently recognized for design and compliance with state and federal permit limits.

Due to the naturally low alkalinity present in the Pacific Northwest wastewater, since the beginning of operations in 2011 the plant has needed to add an alkalinity supplement in order to maintain a stable nitrification process. To this end, 25% Caustic Soda (NaOH) has been used due to cost and the temperature stability of the product (50% NaOH freezes at <55°F).

The Problem

A consistent challenge that the plant experienced was the difficulty to maintain stable and effective pH and alkalinity for full nitrification using caustic soda. This was especially observed when dewatering, where the ammonia-rich centrate was being returned to the aeration process, resulting in a dramatic decrease in pH that required a rapid increase in the caustic feed rate. This translated into pH swings that were not optimal for microorganism activity.

In addition, during periods of high-demand the plant was requiring 1-2 truckloads/day of 25% NaOH to meet the pH control needs, causing logistical problems, concerns about handling the highly hazardous chemical, and contributing to a larger carbon footprint.

While the MBR process typically performed well during the summer months, with the onset of winter’s cold weather the filterability of the MBR system would significantly decline, resulting in a reduction of hydraulic capacity that often required rerouting sewage to other King County wastewater treatment plants at an increased cost of operations.

IER Water plant

Water truckload

The Solution

In exploring different options to achieve a more stable alkalinity and to improve membrane filterability, it was hypothesized that replacing the monovalent sodium ion (Na+) in caustic soda with the divalent magnesium ion (Mg2+) of magnesium hydroxide might improve the microbiology to achieve the desired operational goals.

Brightwater was also interested in chemical cost savings due to the ability of 0.30 lbs of 60% Mg(OH)2 to provide the same number of moles of hydroxide (OH-) as 1.0 lb of 25% NaOH. This proposed usage rate reduction could translate into a reduction in truckload deliveries from 6-12/week to 3-4/week. This would be a tremendous reduction in Brightwater’s carbon footprint, greatly reducing their logistical pressures, and replacing a hazardous chemical with one that is completely nonhazardous.

The Brightwater staff reached out to IER to begin the process of trialing a 60% Mg(OH)2 product


Pilot MBR

The performance of IER’s ALKA-Mag⁺ was evaluated in three separate trials, with each performed under larger volume and longer timed operations. The first trial was performed in 2015-2017 using a pilot-sized MBR process (see Fig.1), using an IER-supplied 175 gallon agitated storage tank and metering pump. The results from this testing showed an improvement from the replacement of NaOH with Mg(OH)2, but the degree of improvement was not very quantifiable. The staff reported enjoying the safe handling of magnesium hydroxide versus the hazards of caustic soda.

Alkalinity Trial


Carnation MBR Plant

King County began operations of a MBR wastewater plant in the small town of Carnation, WA in 2008, as a precursor to building the Brightwater plant a few years later. The Carnation Plant (Fig. 2) was designed to treat only ~0.5 MGD, as opposed to Brightwater at 36 MGD. Therefore, this MBR facility was a perfect test-bed to perform the next level of evaluation on the replacement of NaOH with Mg(OH)2. The initial trial was performed in 2018 from September through December to capture the transition from dry to rainy season. IER provided a 1000 gallon agitated storage tank and metering pump to allow for reliable product feed. It was during this evaluation that King County was able to begin to quantify the improvements in chemical cost savings and filterability (Fig. 3). Additionally, they observed stronger buffering ability of Mg(OH)2 that allowed for simpler controls and less concerns of dramatic pH drops and spikes with NaOH. These results provided the confidence to move forward with full-scale trials at Brightwater.

Alka-Mag Trial

Temperature permeability


Full-Scale Brightwater Trial

In order to maintain a steady supply of ALKA-Mag⁺ for the trial, IER provided to King County a 10,000 gallon agitated storage tank and metering pump, and worked with Brightwater staff on the operational set-up to ensure reliable feed and product inventory (Fig. 4). The trial was initiated in July of 2020, using an average of 3-4 truckloads/week of ALKA-Mag⁺ consistent with the expected usage rate reduction. Not long after start-up it was observed that the daily sCOD ( superfine” chemical oxygen demand) values were some of the lowest ever observed (Fig. 5). The sCOD process control test gave operators information on how the number of particulates in the wastewater might be blinding the membranes. A lower sCOD value indicates that there are fewer superfine particles in the stream, indicating less impact on the membranes and a reduction in membrane cleaning frequency.

Carnation MBR Plant

Flow permeability

This reduction in sCOD prompted the Brightwater staff to compare floc characteristics of wastewater treated with NaOH and ALKA-Mag⁺. As can be seen in Figures 6a and 6b, there was a dramatic improvement in floc structure and a corresponding decrease in filamentous and dispersed bacteria with the transition to magnesium hydroxide.

Perhaps the most interesting finding was that this improvement in floc formation and reduction in sCOD was observed all throughout the 2020-2021 winter months, allowing for the membrane filterability to remain unchanged year round, unlike with caustic soda where every winter season there was a significant loss in membrane filterability and treatment capacity. This ability to maintain treatment capacity through the winter and early spring months is a tremendous benefit, as those are also the months of the year with the highest rate of influent flow (rain and snow melt), when the need for optimum capacity is most important.

Finally, the ability to run at full capacity meant less cleaning cycles for the membranes, resulting in significant energy savings (Fig. 7) and helping to further reduce the Brightwater carbon footprint.

Improvement treated wastewater

Membrane area energy use

The Conclusion

Due to the extremely positive results observed in 2020/2021, the trial was continued for another year, allowing the Brightwater staff to observe the same performance trends as the year before. With two consecutive years of data showing improved pH and alkalinity control, improved membrane filterability and treatment capacity, and reduced cost (both chemical and energy savings), Brightwater made the decision to make a permanent transition from caustic soda to ALKA-Mag⁺

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