Which of the following is a part of inorganic impurities of the sewage? 

Phosphate

The correct answer is option 3. Phosphate.

Sewage is wastewater that contains a variety of impurities and pollutants, including both organic and inorganic substances. Inorganic impurities in sewage can come from various sources, including industrial discharges, runoff from roads, and household activities.

Phosphate is one of the inorganic impurities commonly found in sewage. Phosphates are compounds containing the phosphate ion \((PO_4^{3-})\). They often enter sewage through various sources, such as detergents, fertilizers, and certain industrial discharges. Phosphates are commonly used in fertilizers and are present in agricultural runoff. In urban areas, detergents used in households and industrial processes can contribute to phosphate levels in sewage.

In summary, phosphate is one of the inorganic impurities commonly found in sewage, and its presence can have environmental consequences if not properly managed through effective sewage treatment processes.