Question

Untreated domestic sewage has a low (a) BOD value (b) \(\%\) dissolved oxygen (c) Amounts of dissolved salts (d) \(\%\) of suspended organic matter

Short answer

b) % dissolved oxygen

Step-by-step solution

Understand the Concept

BOD stands for Biochemical Oxygen Demand. It measures the amount of oxygen required by bacteria to decompose organic matter in water. Higher BOD indicates higher pollution.

Analyze the Dissolved Oxygen Levels

Dissolved oxygen refers to the amount of oxygen available in water. In untreated sewage, bacteria consume a lot of oxygen to break down organic matter, leading to low dissolved oxygen levels.

Consider Dissolved Salts

The amount of dissolved salts typically stays consistent, whether sewage is treated or untreated, and does not significantly reflect the untreated condition.

Evaluate Suspended Organic Matter

Suspended organic matter includes particles of organic material floating in the water. These are usually high in untreated sewage due to the lack of filtration and treatment processes.

Determine the Most Likely Answer

Untreated sewage would have low levels of dissolved oxygen due to high BOD and high amounts of organic matter. Low percentages of dissolved oxygen is the distinctive feature of untreated sewage.

Key concepts

Biochemical Oxygen Demand (BOD)

Biochemical Oxygen Demand (BOD) is a key indicator of water pollution. BOD measures the amount of oxygen required by microorganisms to break down organic matter in water. When organic matter like plant debris or untreated domestic sewage gets into the water, bacteria start using oxygen to decompose it. The higher the BOD, the more polluted the water is because more oxygen is used up, leaving less for aquatic organisms.

Understanding BOD is crucial because:

• It helps determine the level of organic pollution in water bodies.
• High BOD levels can result in oxygen depletion, harming fish and other aquatic life.
• Authorities use BOD measurements to regulate and treat wastewater discharge to protect ecosystems.

So, higher BOD indicates more organic pollution, often from sources like untreated sewage, industrial waste, or agricultural runoff.

Dissolved Oxygen Levels

Dissolved oxygen (DO) levels refer to the amount of oxygen available in water, crucial for the survival of aquatic organisms like fish and invertebrates. DO levels can drop significantly in water bodies receiving untreated domestic sewage. This is because bacteria consume a lot of oxygen to break down the high amount of organic matter present in the sewage.

Key aspects of dissolved oxygen levels include:

• Low DO levels can lead to dead zones where aquatic life cannot survive.
• DO is a critical parameter for assessing water quality.
• Regulating dissolved oxygen is essential for maintaining healthy ecosystems.

In untreated domestic sewage, DO levels are typically low, indicating that the water is heavily polluted and unable to support diverse aquatic life.

Understanding and monitoring DO levels help in assessing the ecological health of water bodies and in implementing measures to reduce pollution.

Untreated Domestic Sewage

Untreated domestic sewage is wastewater that has not gone through any filtration or treatment processes. It usually contains a high load of organic matter, pathogens, nutrients, and chemicals. When released into water bodies, untreated sewage can severely impact water quality and aquatic ecosystems.

The main issues caused by untreated domestic sewage include:

• High BOD, leading to oxygen depletion as microorganisms work to break down the organic matter.
• Low dissolved oxygen levels, making it difficult for aquatic life to survive.
• High levels of pathogens, posing health risks to humans and animals.
• Nutrient overload, which can result in algal blooms, further reducing oxygen levels.

Managing and treating domestic sewage before it enters water bodies is vital for maintaining environmental health and ensuring the safety of water for human use and ecological balance.