Question

When domestic sewage mixes with river water
A. The river water is still suitable for drinking as impurities are only about 0.1%.
B. Small animals like rats will die after drinking river water.
C. The increased microbial activity releases micronutrients such as iron.
D. The increased microbial activity uses up dissolved oxygen.

Answer 1

Domestic sewage is the waste material or the sewage that is discharged from homes and commercial as well as residential buildings. Domestic sewage is mostly biodegradable and has lots of microbes in it. When this mixes with the river water then the BOD of the water increases.

Complete answer: Domestic sewage is the most common source of water pollution. In our homes, in cities and towns, everything is washed off into the drains. Only 0.1% of impurities make domestic sewage unfit for human use. The sewage usually consists of biodegradable pollutants like fecal matter, animal wastes, dissolved organic compounds, etc. Other substances like carbohydrates, proteins, fats, urea, inorganic salts such as phosphates of detergents. During the sewage treatment, the solids are readily removed whereas the dissolved salts persist. The biodegradable organic matter is easily decomposed by bacteria and microorganisms. Now higher is the amount of organic wastes, the higher is the rate of decomposition in the water leading to higher consumption of oxygen. This causes the dissolved oxygen level of the water body to decrease. Hence, the BOD or the biochemical oxygen demand of the water increases. Thus, based on the above information we can conclude that when domestic sewage mixes with river water the increased microbial activity uses up dissolved oxygen.
Hence, the correct answer is option (D).

Note: The composition of wastewater –
Wastewater is 99.9% water. The rest 0.1% is impurities. These impurities consist of suspended solids like sand, silt, and clay. Other than that, they contain colloidal material like fecal matter, bacteria, cloth and paper fibers, etc. It also contains some dissolved materials like nutrients, for example, nitrate, ammonia, phosphate, sodium, calcium, etc.