Q1:

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

Part 1: Constituents of Domestic Sewage

Domestic sewage primarily consists of household wastewater generated from daily activities such as bathing, washing, and cooking, alongside human excreta. From a physicochemical standpoint, domestic sewage is approximately $99.9\%$ water and only $0.1\%$ impurities. Despite the seemingly negligible fraction of impurities, this $0.1\%$ is sufficient to render the water severely unfit for human consumption and ecologically disastrous if discharged untreated.

The $0.1\%$ impurities in domestic sewage are categorically divided based on their physical state and solubility:

Part 2: Effects of Sewage Discharge on a River Ecosystem

The indiscriminate discharge of domestic sewage into a river triggers a cascade of detrimental physicochemical and biological changes, fundamentally disrupting the aquatic ecosystem.

1. Alteration of Oxygen Dynamics (BOD and DO)

Domestic sewage is highly rich in biodegradable organic matter. When introduced into a river, it stimulates the explosive proliferation of aerobic decomposers (primarily bacteria and fungi). [Per the principles of aerobic respiration], these microbes consume massive quantities of dissolved oxygen ($O_2$) from the water column to oxidize the organic waste.

• Biochemical Oxygen Demand (BOD): The amount of oxygen required by microorganisms to oxidize the organic matter in one liter of water. Sewage discharge causes a massive, immediate spike in BOD.
• Dissolved Oxygen (DO): The concentration of free, non-compound oxygen present in water. As BOD spikes, DO drops precipitously. The relationship is strictly inverse: $DO \propto \frac{1}{BOD}$.

This localized depletion of oxygen creates hypoxic zones (where DO drops below $4 \text{ mg/L}$), leading to the acute asphyxiation and mass mortality of clean-water organisms, most notably fish and specific aquatic invertebrates.

2. Visualizing the Impact: The Oxygen Sag Curve

The interplay between BOD and DO along the flow of a river following a point-source sewage discharge is graphically represented by the Oxygen Sag Curve.

3. Eutrophication and Algal Blooms

Domestic sewage is highly loaded with phosphates and nitrates resulting from human waste and synthetic detergents. When these limiting nutrients are suddenly introduced into a river in excess, they trigger cultural eutrophication.

• This nutrient saturation promotes the explosive growth of planktonic algae, forming an algal bloom.
• Algal blooms form a thick, often toxic scum on the water's surface, blocking sunlight penetration, which curtails the photosynthetic activity of submerged benthic flora.
• When the algae eventually die, their decomposition demands further massive amounts of oxygen, driving the system towards complete anoxia and resulting in a dead zone.

4. Proliferation of Pathogens and Public Health Hazard

Raw domestic sewage contains fecal matter which serves as a reservoir for pathogenic microorganisms (bacteria, viruses, and protozoa). Discharging untreated sewage turns the river into a vector for severe waterborne diseases. Common pathogens include:

• Vibrio cholerae (Cholera)
• Salmonella typhi (Typhoid fever)
• Entamoeba histolytica (Amoebic dysentery)
• Hepatitis A virus (Jaundice)

5. Changes in Flora and Fauna Biodiversity

The intense environmental stress acts as a stringent selective pressure. Sensitive macro-invertebrates (such as stoneflies and mayflies) and clean-water fish (like trout) are eliminated. Conversely, species tolerant of high organic loads and low DO, such as Tubifex worms (sludge worms) and Chironomus larvae (bloodworms), dominate the affected stretch of the river, drastically altering the natural food web dynamics.

Final Solution: The constituents of domestic sewage comprise $99.9\%$ water and $0.1\%$ impurities (suspended solids, colloidal material, and dissolved nutrients/organics). The discharge of this sewage into a river precipitates a drastic increase in Biochemical Oxygen Demand (BOD), a corresponding lethal drop in Dissolved Oxygen (DO), cultural eutrophication via nutrient loading (causing algal blooms), and the introduction of dangerous fecal pathogens, collectively resulting in severe biodiversity loss and ecological degradation.