Struvite in Municpal Wastewater plants and the role of magnesium hydroxide

Magnesium hydroxide provides required alkalinity (acid neutralization) for the biological treatment, but solubility is limited by pH. Magnesium hydroxide is supplied in a slurry form; by definition slurry contains undissolved particles suspended in a liquid (water) medium. The suspended magnesium hydroxide particles dissolve best in acid conditions, dissolve slowly in near-neutral pH, and the particles have very low solubility in the high pH levels needed for struvite creation. That means magnesium hydroxide is an excellent source of alkalinity to support the biological treatment process, but may not provide sufficient soluble magnesium ions to support struvite formation.

For the nutrient reuse-recycle process to be beneficial, the fertilizer formed when magnesium, ammonium, phosphate and water combine to form struvite (magnesium ammonium phosphate hexa-hydrate) must stay where you want it, keep away from where you don’t want it, and be processed in a saleable form in a reasonable period of time. An old saw in chemistry is that any chemical reaction is driven forward by time, temperature, and concentration. Take away any of these and the reaction will not proceed. When all three dissolved reactants – magnesium, ammonium, and phosphate – are at the right conditions and concentrations, struvite can be formed and potentially recovered.

https://magnesiaspecialties.com/reduce-reuse-recycle-struvite/