The correct option is Urea : Organic impurity
Explanation
Sewage is a complex liquid waste containing suspended solids, organic and inorganic impurities, nutrients, saprophytes, and disease-causing bacteria. The classification of these impurities is based on their chemical composition, specifically whether they are carbon-based (organic) or mineral-based (inorganic).
- Option (a) is Incorrect: Nitrates ($NO_3^-$) and Phosphates are mineral salts. They are classified as inorganic impurities. These substances often act as nutrients that accelerate algal growth in water bodies.
- Option (b) is Incorrect: Animal waste, similar to human faeces, fruit and vegetable waste, and oil, consists of complex carbon compounds. Therefore, it is classified as an organic impurity.
- Option (c) is Correct: Urea ($NH_2CONH_2$) is a major nitrogenous waste product found in urine. Since it is a carbon-based compound, it is correctly classified as an organic impurity.
- Option (d) is Incorrect: Metals (such as lead, mercury, and arsenic) are elemental substances derived from minerals. They are classified as inorganic impurities.
Key Takeaway: Organic impurities in sewage include human faeces, animal waste, urea, oil, pesticides, and herbicides. Inorganic impurities include nitrates, phosphates, and metals.