Compositions and methods and systems for treating wastewater within a wastewater treatment system of described. In an embodiment, a composition for treating wastewater within a wastewater treatment system and for preventing or reducing the occurrence of struvite scale formation within the wastewater treatment system may include an admixture including a magnesium compound. The admixture may also include a dispersing agent. The admixture may further include at least one of a free magnesium control agent and a seed material. Other compositions and methods are described.