Q.(a) What would happen if a large volume of untreated sewage is discharged into a river?

Sewage is not just dirty water — it carries organic waste, pathogens, and nutrients that can wreak havoc on aquatic ecosystems if released untreated.

(a) When a large volume of untreated sewage enters a river, microorganisms in the water rapidly multiply to decompose the organic matter. This microbial activity consumes dissolved oxygen at an alarming rate, leading to a sharp drop in oxygen levels. Fish and other aerobic organisms suffocate and die. The river becomes polluted, foul-smelling, and unfit for drinking or bathing. Essentially, the natural balance collapses under the organic load. …

(a) The Catastrophe of Untreated Sewage in a River

When a large volume of untreated sewage enters a river, it sets off a chain of ecological disasters rooted in one critical problem: oxygen depletion. Sewage is rich in organic matter—human waste, food scraps, detergents, and other biodegradable materials. Aerobic bacteria in the river water immediately begin decomposing this organic load, but the process consumes dissolved oxygen at an alarming rate. The river's biochemical oxygen demand (BOD) skyrockets.

As oxygen levels plummet, the consequences cascade through the ecosystem. Fish and other aquatic organisms that depend on dissolved oxygen for respiration begin to suffocate. Sensitive species die first, followed by more tolerant ones if the pollution persists. The river transforms from a living, breathing habitat into what is effectively a dead zone—murky, foul-smelling, and devoid of most life forms.

The water quality deteriorates in other ways too:

• Eutrophication accelerates: Nutrients like nitrogen and phosphorus in sewage fuel explosive algal blooms. When these algae die and decompose, they further drain oxygen reserves.
• Pathogens proliferate: Untreated sewage carries disease-causing bacteria, viruses, and parasites (cholera, typhoid, dysentery agents), making the water unsafe for drinking, bathing, or irrigation.
• Aesthetic and economic damage: The river becomes visually repulsive, emits noxious odours, and loses its value for recreation, fisheries, and as a water source for downstream communities.

In essence, the river's self-purification capacity is overwhelmed. What should be a gradual, natural breakdown of small amounts of organic waste becomes an environmental emergency when the volume is too large and the sewage untreated.

(b) The Vital Role of Anaerobic Sludge Digestion

Sewage treatment generates a thick, semi-solid byproduct called sludge—a mixture of bacteria, organic matter, and inorganic particles that settles out during primary and secondary treatment. Left untreated, this sludge is bulky, odorous, and still rich in pathogens and organic content. Anaerobic sludge digestion solves these problems elegantly.

In large, closed tanks called digesters, anaerobic bacteria (those that thrive without oxygen) break down the organic matter in sludge over several weeks. This process is important for three interconnected reasons:

Volume reduction: The bacteria decompose a significant portion of the organic material, shrinking the sludge volume substantially. What remains is more stable and easier to handle, transport, and dispose of—often dried and used as manure. …

(a) as a cause-effect chain; (b) as a resource-recovery framing -- rather than two descriptive paragraphs.

(a) Chain: large organic load enters river -> aerobic bacteria in the river multiply to decompose it -> dissolved oxygen is consumed faster than it's replenished -> oxygen-dependent aquatic life (fish, etc.) suffocates -> the river also carries untreated pathogens (cholera, typhoid agents) -> water becomes unfit for drinking/bathing/irrigation. …