Which one of the following consequences will happen if large quantities of domestic sewage are continuously discharged in a stream ?

The continuous discharge of large quantities of domestic sewage into a stream primarily leads to a significant reduction in dissolved oxygen (DO) levels [2]. Domestic sewage contains high amounts of organic matter, which serves as a substrate for aerobic bacteria. As these microorganisms decompose the organic waste, they consume the oxygen dissolved in the water, a process measured as Biochemical Oxygen Demand (BOD) [t2]. When the BOD exceeds the rate of natural aeration and photosynthesis, the DO level drops, often creating an 'oxygen sag curve' [t5]. While sewage can eventually lead to algal blooms due to nutrient enrichment (eutrophication) [c2][t3], the most immediate and direct consequence of the organic load itself is the depletion of oxygen, which can reach lethal levels for aquatic life [t2][t7]. Furthermore, decomposition of organic matter typically increases carbon dioxide levels rather than reducing them [c3][t1].

Sources

1. [2] Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 18: Ocean Acidification > ii. Eutrophication > p. 264