Phosphate
Explanation
Sewage is a complex liquid mixture containing suspended solids, organic and inorganic impurities, nutrients, saprophytes, and disease-causing bacteria. These impurities are collectively referred to as contaminants. The classification of these contaminants is based on their chemical nature and origin.
- Urea is incorrect: Urea is a nitrogenous compound found in urine. It is chemically an organic compound containing carbon, nitrogen, oxygen, and hydrogen. Therefore, it is classified as an organic impurity.
- Vegetable waste is incorrect: Vegetable waste consists of biodegradable matter derived from plants. It is carbon-based and is classified as an organic impurity.
- Phosphate is correct: Phosphates are salts or esters of phosphoric acid. In the context of sewage, they often originate from detergents, soaps, and fertilizers. They do not possess the carbon-hydrogen structure typical of organic compounds and are classified as inorganic impurities, alongside nitrates and metals.
- Pesticide is incorrect: Most pesticides used in agriculture and households are complex organic chemical compounds. In standard wastewater classification, pesticides and herbicides are grouped under organic impurities.
Key Takeaway: In sewage analysis, inorganic impurities primarily include nitrates, phosphates, and metals, whereas organic impurities include human excreta, animal waste, oil, urea, pesticides, and vegetable waste.