Replacement of Caustic Soda with ALKA-Mag⁺

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).

Trials

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.

Trials

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.

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.