Problem 12 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Biochemical oxygen demand(BOD) value can be a measure of water pollution caused by the organic matter. Which of the following statements is correct? (a) Polluted water has BOD value higher than 10 ppm (b) Aerobic bacteria decreases the BOD value (c) Anaerobic bacteria increases the BOD value (d) Clean water has BOD value higher than \(10 \mathrm{ppm}\)

Step-by-Step Solution

Answer

Polluted water has BOD higher than 10 ppm, so (a) is correct.

Step 1: Understanding BOD

Biochemical oxygen demand (BOD) measures the amount of oxygen that microorganisms will consume while decomposing organic matter present in water. This makes BOD an important indicator of water quality.

Step 2: Analyzing Polluted vs. Clean Water

High BOD values indicate high levels of organic pollution that consume oxygen, which means polluted water typically has a higher BOD value than clean water. Clean water generally has a lower BOD.

Step 3: Examining Each Option

Option (a) states that polluted water has BOD higher than 10 ppm. This is correct, as polluted water usually exhibits high BOD values. Option (b) states aerobic bacteria decrease BOD, which is incorrect; aerobic bacteria contribute to reducing oxygen by consuming it. Option (c) suggests anaerobic bacteria increase BOD, which is also incorrect; they don't increase BOD. Option (d) indicates clean water has BOD higher than 10 ppm, which is incorrect: clean water typically has lower BOD values.

Key Concepts

Water Pollution

Water pollution is an issue that affects the entire planet, impacting not just natural ecosystems but human health. At its core, it involves the introduction of harmful substances into water bodies. This includes chemicals, waste, and other pollutants that degrade water quality. Water pollution can lead to the destruction of aquatic habitats and can make water unsafe for drinking, swimming, and other recreational activities.
Common sources of water pollution include:

• Industrial discharges
• Agricultural runoff containing pesticides and fertilizers
• Sewage and waste from human activities
• Plastic waste and marine litter

One way to measure the extent of water pollution is through Biochemical Oxygen Demand (BOD). This measures how much oxygen is needed by aerobic organisms to break down organic material in the water. High BOD indicates a high level of pollution because higher oxygen consumption means more organic material needing to be decomposed. Understanding BOD levels can help in assessing the quality of water and in implementing measures to restore polluted water bodies.

Aerobic Bacteria

Aerobic bacteria are microorganisms that require oxygen to survive and break down organic matter. They play a critical role in the process of biodegradation, which is the breakdown of organic matter in water. This process is important for reducing the effects of pollution, as these bacteria help in decomposing pollutants into less harmful substances.
In a water body, aerobic bacteria consume oxygen as they digest organic substances. This consumption increases the Biochemical Oxygen Demand (BOD) because more oxygen is needed as these bacteria proliferate to deal with increased levels of organic pollutants.
Despite their role in increasing BOD, the actions of aerobic bacteria can be beneficial. By breaking down organic waste, they help to clean up polluted water. However, if the BOD is too high, it can deplete oxygen levels, leading to an environment where fish and other aquatic organisms cannot survive.

Anaerobic Bacteria

Anaerobic bacteria differ from aerobic bacteria as they do not require oxygen to decompose organic matter. This allows them to thrive in environments where oxygen is scarce or absent, such as deep sediments and in some polluted water bodies.
These bacteria utilize other molecules, like nitrates or sulfates, instead of oxygen, to carry out digestion. As a result, anaerobic bacteria can produce gases like methane and hydrogen sulfide as byproducts, some of which have offensive smells associated with pollution.
Anaerobic bacteria can help decompose organic matter under specific conditions, but they do not increase the BOD directly. Instead, they operate in environments where oxygen has already been depleted, typically following aerobic processes. Understanding the role of anaerobic bacteria can help environmental scientists develop strategies for managing and treating water pollution, particularly in heavily polluted environments.