The correct option is (c).
Explanation
Sewage is a complex mixture containing suspended solids, organic and inorganic impurities, nutrients, saprotrophic and disease-causing bacteria, and other microbes. The classification of these impurities depends on their chemical composition, specifically whether they are carbon-based (organic) or mineral-based (inorganic).
Statement-wise Analysis:
- Statement 1 is Correct: Urea (a major component of urine) and pesticides (agrochemicals) are complex carbon-based compounds. Consequently, they are classified as organic impurities.
- Statement 2 is Incorrect: Nitrates and phosphates are mineral salts and nutrients often derived from detergents and fertilizers. They are classified as inorganic impurities, distinct from carbon-based organic matter.
- Statement 3 is Correct: Vegetable waste (kitchen refuse, fruit peels) and animal waste are biological in origin and biodegradable. These are fundamentally composed of carbon compounds and are classified as organic impurities.
Key Takeaway:
Organic impurities in sewage include human faeces, animal waste, oil, urea, pesticides, herbicides, and fruit/vegetable waste. Inorganic impurities include nitrates, phosphates, and metals.