Improving water treatment sustainability with magnesium hydroxide

Dangerous chemicals and excessive salinity

Caustic soda is easier to feed and requires less maintenance than other alkalis that typically come in powdered or slurry form. However, caustic soda is extremely dangerous to handle; with a pH of 14, it is the most dangerous of all alkalis. Even diluted concentrations can cause significant burns to unprotected skin, especially with prolonged contact. Therefore, it must be handled with caution, strictly adhering to containment and safety measures. Lime and soda ash are also strong irritants, making them only slightly less dangerous.

Additionally, caustic soda (NaOH) releases sodium salinity into the environment. In order to stabilize the pH, and to favor the growth of beneficial bacteria, wastewater treatment systems must have an adequate level of alkalinity. Unfortunately, NaOH addition releases one ion of sodium (Na+) for every hydroxide ion needed to boost alkalinity. This 1-to-1 ratio of sodium with alkalinity is a serious concern if discharge is to inland streams or land application systems, as excessive salinity can leach into soils and affect the viability of agriculture.

Despite these dangers to both employees and the environment, caustic soda has been the standard treatment for industrial facilities (food and beverage processing, microelectronics, papermaking, mining, etc.), even though a high dose is required to effectively adjust the pH of an acidic wastewater stream. At the same time, a slight overfeed can cause a dramatic pH spike that can severely damage the health of microorganism populations, or damage crops when applied to the land.

For example, in pH adjustment testing of a concentrated organic acid wastewater sample, a dose of 10 mL of 50% NaOH was required to increase the pH of 1000 mL to 6.1. As the goal was pH 7.0, another 0.5 mL was added and the pH spiked up to 12.5 – which is corrosive to skin! For this reason, caustic soda is an excellent cleaner for industrial storage tanks and feed lines, hydrolyzing carbonaceous deposits from surfaces, leaving them completely clean. Caustic can also be used as a shock dose in sewer lines to kill odor-causing microorganisms in the collection system. However, the treatment is not long-lasting nor long-reaching; not all biomass is removed, and odor tends to rapidly return. Again, such harsh treatments are occupationally and environmentally hazardous and offer no long-term benefit.

Replacing caustic soda with a safer, “greener”, and more cost-effective alternative

The ideal replacement for caustic soda (and soda ash and lime) is magnesium hydroxide. Magnesium hydroxide is commonly used in antacids and even skincare products, demonstrating that it’s not only far safer to work with than caustic, but is also more earth-friendly.

Magnesium hydroxide is sodium-free, so by using it the overall sodium levels discharged to the environment can be reduced. It also requires a far lower dose for effective pH adjustment. In the experiment mentioned earlier, a dose of 6 mL of 60% Mg(OH)₂ was able to achieve a pH of 6.2 – as compared to 10 mL for 50% NaOH. This 40% reduction in chemical usage is the primary reason that magnesium hydroxide is so much less expensive on a daily use basis than caustic soda, to go along with the obvious safety and environmental benefits. Rather than being detrimental to the soil, the magnesium cation (Mg2+) is a soil nutrient, being the core element in chlorophyll, which is the key component in photosynthesis that makes all things green.

Additionally, a slight overdose of Mg(OH)₂ from operator error hardly effects the overall wastewater pH, maintaining the wastewater microorganisms in a very healthy environment. The slow-release, buffering nature of magnesium hydroxide allows it to hold hydrogen sulfide in solution for optimal odor and corrosion control performance, delivering both long-term and long-range benefits.

Magnesium hydroxide also assists with solids settling in wastewater treatment by providing coagulation benefits that caustic simply doesn’t. Therefore, it is common when adding Mg(OH)₂ into the aeration process for nitrification pH control, that solids settling in the secondary clarifier is improved, as well as increased % solids in dewatering of digested sludge. Because of the coagulation from Mg2+, many customers will reduce their dewatering polymer usage after transitioning from caustic to magnesium hydroxide.

How Jacobs replaced caustic soda with magnesium hydroxide at Spokane County

The safety, earth-friendly, and cost savings benefits of magnesium hydroxide were recently demonstrated to the Jacobs staff that operate the Spokane County Regional Water Reclamation Facility through the replacement of caustic soda with magnesium hydroxide.

The Spokane County facility processes eight million gallons per day of wastewater from areas around Spokane Valley and has a National Pollutant Discharge System (NPDS) permit to discharge to the Spokane River. Spokane Country has a membrane bioreactor (MBR) system, which allows for a much smaller footprint than conventional wastewater treatment. This is achieved through replacing large clarifier basins with membranes. The process is microbiologically driven, which means controlling the pH and alkalinity reliably and effectively is critical.

Looking for a safer and more cost-effective alternative to caustic soda, Jacobs performed a trial of magnesium hydroxide for 90 days at the Spokane County facility. The result was a 65 percent reduction in chemical usage (by volume) and a significant reduction in digested sludge volume. Roughly half the amount of digested sludge was produced, which lowered sludge hauling fees substantially.

Before switching to magnesium hydroxide, Jacobs was using approximately 1,400 gallons of 25 percent caustic soda every day to treat the wastewater. Replacing this with approximately 450 gallons per day of magnesium hydroxide resulted in chemical reduction cost savings and a reduced impact on the environment when the treated wastewater was discharged into the Spokane River.

Jacobs staff observed that centrifuge cake solids had increased by a few percentage points, which halved the amount of dewatered sludge that were hauled after using magnesium hydroxide. These trial results were replicated and consistently observed. Importantly, no changes to the feed rate of their dewatering polymer had occurred nor any other operational changes in the plant that could otherwise account for the sludge reduction.

This result is due to magnesium hydroxide’s coagulation properties which can’t be replicated by caustic soda. The cost savings delivered through lower volumes of chemical used and less sludge (with lower sludge hauling fees) meant that Jacobs can replace its caustic soda use with magnesium hydroxide with confidence.

Magnesium hydroxide was dosed into the wastewater return activated sludge line using an agitated storage tank and metering pump feed system. Delivered as a highly flowable slurry, magnesium hydroxide is easy to handle and completely safe, with no adverse effects on people, plants, or animals. Pound for pound, it is the least expensive and most powerful alkaline chemical treatment available for pH control and buffering.