An Integrated Approach to Magnesium Hydroxide Use In Water and Wastewater Treatment

Abstract

Magnesium hydroxide has the potential to develop into an essential component of water and wastewater treatment. Its non-hazardous classification and low cost makes it a preferable source of alkalinity for existing processes. Further, the slow release of alkalinity from magnesia slurry allows new processes to be developed that cannot accommodate fast-dissolving additives. Nonetheless, plant operators are often reluctant to use magnesium hydroxide slurries for two reasons: they are concerned about uncertainty surrounding the precipitation of struvite, and, they cannot predict the gradual pH response of dissolving slurry. This research proposes a practical solution to both obstacles. The proposed StrPI model accurately predicted struvite precipitation in both the lab and the field. Calibration can be used to account for local pH spikes, uncertainties in waste composition, and safety factors. This flexibility also allows the model to be applied to both foulant prevention schemes and struvite recovery processes. Similarly, the proposed kinetic model for magnesium hydroxide provides a robust solution for predicting the pH response of dissolving slurries in a buffered environment. These predictions can provide insight into existing processes, which can dramatically improve existing magnesia dosing regimens. The kinetic model can also serve as a framework on which to develop more complex dosing schemes that span multiple processes. This information is valuable to magnesia manufacturers as it can provide guidance into the particle size distributions that will best achieve a desired pH response. Both models can be evaluated and calibrated using a standard jar test apparatus and other common instruments employed by water and wastewater treatment laboratories. As a result, utilities can test the models themselves and need not rely on tools developed using processes and water compositions dissimilar from those in their own plant. Together, the StrPI and kinetic model provide a solution to the primary impediments of widespread magnesia adoption in treatment processes.