Magnesium oxide (MgO) is commonly used to control the pH and promote struvite precipitation in wastewater treatment systems. The amount of MgO needed to be added to a struvite precipitation reactor can be calculated using the following formula:

MgO (g) = (mg/L of Mg) x (volume of wastewater (L)) / 40

Where mg/L of Mg is the concentration of magnesium ions in the wastewater and 40 is the molecular weight of MgO.

For example, if the magnesium concentration in the wastewater is 10 mg/L and the reactor volume is 1000 L, the amount of MgO needed to be added to the reactor would be:

MgO (g) = (10 mg/L) x (1000 L) / 40 = 250 g

It's important to note that the amount of MgO needed to be added will also depend on the pH of the wastewater and the desired pH for struvite precipitation. In general, a pH range of 8.5 - 9.5 is optimal for struvite precipitation, so the pH of the wastewater should be adjusted to this range before adding the MgO.