Q.What are the various constituents of domestic sewage? Discuss the effects of sewage discharge on a river.

Constituents of domestic sewage

Domestic sewage — the wastewater generated by homes — is about 99% water by weight, with the remaining 1% consisting of:

• Organic matter: faecal matter and other human excreta, food waste, oils and fats, soaps and detergents.
• Suspended and dissolved solids: silt, clay particles, and dissolved salts, nitrates and phosphates (largely from detergents and excreta).
• Microorganisms: a large number of bacteria, many of them disease-causing (pathogenic) — e.g., coliform bacteria are used as an indicator of faecal contamination — along with some viruses and protozoan parasites.

Effects of untreated sewage discharge on a river

• Rise in Biochemical Oxygen Demand (BOD): the organic matter in sewage is degraded by aerobic microorganisms, which consume large amounts of dissolved oxygen from the river water in the process. A river receiving heavy sewage load therefore shows a sharp rise in BOD.
• Eutrophication: the nutrients (nitrates, phosphates) in sewage over-fertilise the water, triggering excessive growth of algae ("algal bloom") on the surface.
• Oxygen depletion and death of aquatic life: as the algal bloom ages and dies, its decomposition by microbes further depletes dissolved oxygen. The combined effect of high BOD and eutrophication can drive dissolved oxygen so low that fish and other oxygen-dependent aquatic organisms die in large numbers.
• Spread of disease: the pathogenic bacteria, viruses and parasites in sewage contaminate the river water, making it unsafe for drinking, bathing or irrigation and spreading water-borne diseases to people who depend on the river.
• Loss of biodiversity and water quality: the combined stress of low oxygen, turbidity and toxicity reduces the diversity of aquatic life and makes the river unfit for many of its natural uses.

Split the answer into two clearly separate parts as the question asks: (1) list the constituents of sewage under organic matter, solids/nutrients, and microorganisms; (2) trace the chain of effects — organic load → BOD rise → oxygen consumption, and nutrient load → eutrophication → algal bloom → oxygen depletion — ending in death of aquatic life and disease spread.

• Forgetting that domestic sewage is ~99% plain water and treating it as if it were mostly solid waste.
• Confusing BOD (oxygen consumed by microbes decomposing organic waste) with eutrophication (nutrient over-enrichment causing algal blooms) — both matter, but they are distinct mechanisms.
• Omitting the disease/public-health angle (pathogenic microorganisms) and discussing only the oxygen-depletion effect.