Orthophosphate and other similar chemicals are used for corrosion control in drinking water systems. Orthophosphate is a corrosion control and sequestering agent that helps prevent lead and copper from leaching into the water.
Many of these chemicals are proprietary blends, so it's important to get the SDS directly from the supplier.
Here is a Product Safety Summary of an orthophosphate blend from Carus, a supplier of water treatment chemicals.
Contact with skin or eyes can cause irritation, and inhalation of dusts, vapors or mists can cause respiratory system irritation.
Be sure to wear all appropriate PPE as recommended in the SDS.