Presence of carbonaceous matter in the sewage

ANS:D - all (a), (b) and (c).

The correct answer is: all (a), (b) and (c).

Explanation:

The presence of carbonaceous matter in sewage can lead to several issues:

• Reduction in Dissolved Oxygen: The decomposition of organic matter consumes dissolved oxygen, endangering aquatic life.
• Reduction of Sulphate Ions: Carbonaceous matter can reduce sulfate ions to sulfides, causing foul odors.
• Increased Chlorine Demand: Higher levels of organic matter can increase the amount of chlorine needed for effective purification and disinfection.

Thus, all three statements are correct.

Explanation of Reduction in Dissolved Oxygen Content

The presence of carbonaceous matter in sewage significantly impacts the dissolved oxygen (DO) levels in water bodies, leading to serious ecological consequences. Here’s how this process works:

1. Decomposition of Organic Matter

• Microbial Activity: When sewage containing carbonaceous matter enters a water body, microorganisms such as bacteria begin to decompose the organic material.
• Oxygen Consumption: This decomposition process requires oxygen, which is consumed from the water, leading to a reduction in dissolved oxygen levels.

2. Impact on Aquatic Life

• Aquatic Organisms: Fish, invertebrates, and other aquatic organisms rely on dissolved oxygen for respiration. Low DO levels can lead to stress or death in these organisms.
• Ecosystem Imbalance: Reduced oxygen levels can result in shifts in the aquatic ecosystem, favoring anaerobic organisms that may produce harmful substances.

3. Eutrophication

• Algal Blooms: High levels of nutrients from sewage can lead to algal blooms, which further consume oxygen when they decompose, exacerbating the problem.
• Hypoxic Zones: Areas with very low oxygen levels, known as hypoxic zones, can form, creating "dead zones" where aquatic life cannot survive.

4. Long-Term Effects

• Biodiversity Loss: Prolonged low oxygen conditions can lead to a decline in biodiversity as sensitive species are unable to thrive.
• Water Quality Degradation: The overall quality of the water body deteriorates, impacting not only aquatic life but also human activities like fishing and recreation.

Summary

The reduction of dissolved oxygen content due to carbonaceous matter in sewage is a critical environmental issue. It poses a significant threat to aquatic life, disrupts ecosystems, and can lead to long-term degradation of water quality. Effective wastewater treatment is essential to mitigate these impacts and protect aquatic ecosystems.

Explanation of Reduction of Sulfate Ions to Sulfides

The reduction of sulfate ions (SO₄²⁻) to sulfide ions (S²⁻) in sewage can lead to the production of foul odors. Here’s how this process occurs:

1. Anaerobic Conditions

• Oxygen Depletion: In the presence of carbonaceous matter, the decomposition of organic material can deplete dissolved oxygen levels, creating anaerobic (low-oxygen) conditions.
• Microbial Activity: Certain bacteria thrive in these anaerobic environments and can reduce sulfate ions to sulfide ions as part of their metabolic processes.

2. Reduction Process

• Sulfate-Reducing Bacteria (SRB): Specific types of bacteria, known as sulfate-reducing bacteria, facilitate this reduction. They use sulfate as an electron acceptor in the absence of oxygen.
• Chemical Reaction: The reduction can be represented as: SO42−+8 e−+4 H+→S2−+4 H2OSO_4^{2-} + 8 \, e^- + 4 \, H^+ \rightarrow S^{2-} + 4 \, H_2OSO42−​+8e−+4H+→S2−+4H2​O

3. Production of Hydrogen Sulfide (H₂S)

• Formation of Sulfide: The reduced sulfide ions (S²⁻) can further react to form hydrogen sulfide (H₂S), a gas that is highly soluble in water.
• Odorous Nature: Hydrogen sulfide has a characteristic rotten egg smell, which contributes to the obnoxious odors associated with polluted water bodies.

4. Environmental Impact

• Aesthetic Concerns: The presence of hydrogen sulfide can render water bodies unpleasant and unsuitable for recreational use.
• Harm to Aquatic Life: High concentrations of hydrogen sulfide can be toxic to aquatic organisms, further stressing the ecosystem.

5. Management and Mitigation

• Wastewater Treatment: Effective treatment processes are essential to minimize anaerobic conditions and reduce the formation of sulfides. This can include aeration, proper sludge management, and chemical treatments.

Summary

The reduction of sulfate ions to sulfides in sewage occurs under anaerobic conditions, primarily driven by specific bacteria. This process leads to the production of hydrogen sulfide, resulting in obnoxious smells and detrimental effects on water quality and aquatic life. Proper wastewater management is crucial to address this issue.

Explanation of Increased Chlorine Demand for Sewage Purification

The presence of carbonaceous matter in sewage significantly impacts the effectiveness of chlorine used in water treatment processes. Here’s how it affects chlorine demand:

1. Organic Matter Presence

• High Organic Load: Sewage often contains high levels of organic compounds, including carbonaceous matter, which must be reduced before water can be safely discharged or reused.
• Chlorine Demand: Organic matter reacts with chlorine, consuming it during the disinfection process and reducing the amount available for killing pathogens.

2. Chlorination Reaction

• Formation of Chlorinated Byproducts: When chlorine reacts with organic matter, it can form various chlorinated byproducts, some of which may be toxic or undesirable.
• Increased Dosage Requirement: To achieve effective disinfection and ensure sufficient chlorine residual, higher doses of chlorine must be added to account for the chlorine consumed by these reactions.

3. Eutrophication and Algal Blooms

• Nutrient Release: The decomposition of organic matter can release nutrients (like nitrogen and phosphorus) into the water, leading to algal blooms.
• Additional Chlorine Needs: Algal blooms can further increase chlorine demand, as chlorine is also consumed in the process of breaking down algal cells.

4. Operational Challenges

• Water Quality Monitoring: Higher chlorine demands complicate water quality management and require more frequent monitoring and adjustments during treatment.
• Cost Implications: Increased chlorine usage leads to higher operational costs and potential environmental concerns related to chlorine residuals in treated water.

5. Management Strategies

• Pre-treatment: Implementing pre-treatment processes to reduce organic load can help lower chlorine demand and improve the efficiency of disinfection.
• Alternative Disinfectants: In some cases, alternative disinfection methods (such as UV or ozone) may be considered to reduce chlorine demand and minimize the formation of harmful byproducts.

Summary

The presence of carbonaceous matter in sewage increases the quantity of chlorine needed for purification by consuming chlorine during reactions and reducing the amount available for effective disinfection. This necessitates careful management in wastewater treatment to ensure adequate disinfection while controlling costs and environmental impacts.