Problem 49 - A Complete Resource Book in Chemistry for JEE Main

Question

Untreated domestic sewage has a low (a) persentage of dissolved oxygen (b) persentage of suspended organic matter (c) \(\mathrm{BOD}\) value (d) amount of dissolved salts

Step-by-Step Solution

Answer

Option (a) 'percentage of dissolved oxygen' is low in untreated domestic sewage.

Step 1: Understanding the Problem

We are trying to identify which of the given properties of untreated domestic sewage is low. The options are related to the characteristics of water quality: dissolved oxygen, suspended organic matter, BOD, and dissolved salts.

Step 2: Analyzing the Options

Let's analyze each option: - (a) Dissolved oxygen: Untreated sewage usually has low dissolved oxygen due to biological degradation. - (b) Suspended organic matter: Usually high because of waste content. - (c) BOD value: High in untreated sewage as it represents the amount of oxygen required to break down organic compounds. - (d) Dissolved salts: These can vary but are not characteristically low.

Step 3: Choosing the Correct Option

Based on the analysis, low dissolved oxygen, option (a), is a characteristic associated with untreated sewage, since the biological breakdown process consumes oxygen. Suspended organic matter and BOD are typically high, and dissolved salts can be variable.

Step 4: Final Conclusion

From the evaluation of all the given options, option (a) 'persentage of dissolved oxygen' is indeed low in untreated domestic sewage, making it the correct answer.

Key Concepts

Untreated Domestic Sewage

Untreated domestic sewage refers to wastewater that has not undergone any form of processing to remove contaminants. It is typically an accumulation of household waste products like food scraps, soaps, and paper. The characteristic features of untreated sewage make it a substantial environmental concern. Since it has not been treated, it holds many pollutants that can have harmful effects on natural water bodies if discharged directly. When untreated sewage is released into bodies of water, it can significantly alter the aquatic ecosystem, causing damage to water quality and affecting the organisms living there. This is largely due to its high content of pollutants, including pathogens, organic waste, and various chemicals, which can lead to water pollution. Key points about untreated domestic sewage include:

• High concentrations of organic matter.
• Presents potential health risks due to bacterial and viral pathogens.
• Can cause oxygen depletion in water bodies due to organic load.
• Negatively impacts water quality characteristics.

Biological Degradation

Biological degradation is a natural process where microorganisms such as bacteria and fungi break down organic materials. In the context of untreated domestic sewage, biological degradation plays a crucial role. During this process, bacteria consume organic matter in the waste, which results in the consumption of dissolved oxygen. This is why untreated sewage typically has a low percentage of dissolved oxygen. Understanding biological degradation is important because it:

• Reduces organic pollutants by transforming them into simpler substances.
• Involves the use of microbes which need oxygen to break down the waste materials.
• Can lead to deoxygenation of water if excessive organic matter is present.
• Is essential for the natural self-purification of water bodies, although only effective to a point.

This process is beneficial in controlled environments like sewage treatment plants, but can cause environmental issues if sewage is untreated and released directly into ecosystems.

Water Quality Characteristics

Water quality characteristics refer to the various parameters used to assess the condition or quality of water. In understanding the impact of untreated domestic sewage, several key characteristics are considered: The primary water quality characteristics affected by untreated sewage include:

• Dissolved Oxygen (DO): This is often low in areas affected by sewage due to the biological degradation process consuming oxygen.
• Biochemical Oxygen Demand (BOD): High in untreated sewage, indicating a high level of organic pollution.
• Suspended organic matter: Unusually high in untreated sewage due to unfiltered waste content.
• Nutrients: Could be high leading to eutrophication, which causes excessive plant growth and decay.

These characteristics are crucial in assessing water pollution levels and help in determining the necessary treatment processes needed to restore and maintain healthy environments.

BOD (Biochemical Oxygen Demand)

Biochemical Oxygen Demand (BOD) is a critical measure used to assess the effect of organic pollutants in water. It represents the quantity of dissolved oxygen needed by aerobic biological organisms to break down organic material present in a given water sample. In untreated domestic sewage, BOD levels are often high because:

• There is a significant amount of organic waste present that requires decomposition.
• Microbial activity is increased due to an abundant organic matter, leading to a greater consumption of oxygen.
• It indicates high levels of pollution; a high BOD value suggests that the water is heavily polluted with waste.

Managing BOD levels is central to wastewater treatment processes in order to prevent the depletion of oxygen in receiving water bodies, which could harm aquatic life.

Suspended Organic Matter

Suspended organic matter in water refers to tiny particles of organic compounds that are not dissolved but suspended throughout the water through which light cannot pass. In untreated domestic sewage, the levels of suspended organic matter are notably high. This matter consists of:

• Small particles of food waste, paper, and other organic substances.
• Can lead to increased turbidity or murkiness in water bodies.
• Forms a food source for bacteria and other microorganisms, further influencing BOD levels.
• If not removed, can settle at the bottom of water bodies, impacting aquatic life.

Treating sewage reduces suspended organic matter, thus improving water clarity and quality, essential for maintaining the health of aquatic ecosystems.