36. The maximum BOD and minimum DO for pure drinkingwater should be
    (1) 25, 5 (2) 2, 5
    (3) 3, 9 (4) 0, 6

Ensuring the safety and quality of drinking water is a fundamental requirement for public health. Two critical parameters used to assess water quality are Biochemical Oxygen Demand (BOD) and Dissolved Oxygen (DO). These indicators help determine whether water is safe for consumption and free from harmful pollutants. This article explores the maximum BOD and minimum DO values specified for pure drinking water, the significance of these standards, and the broader context of water quality management.

Understanding BOD and DO

Biochemical Oxygen Demand (BOD)

BOD measures the amount of oxygen required by microorganisms to decompose organic matter present in water. High BOD indicates significant organic pollution, which can lead to oxygen depletion and harm aquatic life. For drinking water, low BOD is essential to ensure that the water is free from excessive biodegradable pollutants.

Dissolved Oxygen (DO)

DO refers to the amount of oxygen present in water, which is crucial for the survival of aquatic organisms and for maintaining aerobic conditions. High DO levels are desirable in drinking water sources to support healthy ecosystems and ensure that the water is well-aerated and safe for consumption.

Standards for Pure Drinking Water

Water quality standards are established by regulatory agencies to ensure that drinking water is safe for human consumption. These standards specify the maximum permissible levels of various pollutants and the minimum acceptable levels of essential parameters like dissolved oxygen.

Maximum BOD for Pure Drinking Water

According to national and international guidelines, the maximum BOD for pure drinking water should be as low as possible. Ideally, safe drinking water should have a BOD of zero, indicating the absence of biodegradable organic matter. However, in practical terms and according to many water quality criteria, the BOD for drinking water should not exceed 2 mg/L for water sources requiring minimal treatment and 3 mg/L for sources requiring conventional treatment.

Minimum DO for Pure Drinking Water

The minimum dissolved oxygen (DO) level for pure drinking water is typically set to ensure that the water is well-aerated and supports healthy aquatic life. For drinking water sources without conventional treatment, the minimum DO is 6 mg/L or more. For sources requiring treatment, the minimum DO is 4 mg/L or more. However, for pure drinking water (considered at the highest standard), the minimum DO is often referenced as 5 mg/L or more in some guidelines.

Interpreting the Options

Let’s review the options provided in the context of these standards:

• (1) 25, 5:

  • BOD = 25 mg/L is far too high for drinking water and indicates severe pollution.

  • DO = 5 mg/L is acceptable for some guidelines but not for the highest standards.

• (2) 2, 5:

  • BOD = 2 mg/L is the maximum for drinking water sources without conventional treatment.

  • DO = 5 mg/L is acceptable for some guidelines but not the strictest standards.

• (3) 3, 9:

  • BOD = 3 mg/L is the maximum for drinking water sources with conventional treatment.

  • DO = 9 mg/L is much higher than required and not a standard minimum.

• (4) 0, 6:

  • BOD = 0 mg/L is ideal for safe drinking water, indicating no organic pollution.

  • DO = 6 mg/L is the minimum for drinking water sources without conventional treatment.

Which Option Is Correct?

The most accurate answer depends on the strictness of the standard and the context of the question:

• If the question asks for the ideal maximum BOD and minimum DO for the purest drinking water:

  • BOD should be as low as possible, ideally 0 mg/L.

  • DO should be at least 6 mg/L for the highest standards.

  • Therefore, option (4) 0, 6 is the best answer for the strictest standard.

• If the question allows for practical standards (where BOD up to 2 mg/L is acceptable):

  • Option (2) 2, 5 is acceptable in some contexts, but not for the purest standard.

Given that the question specifies pure drinking water, the strictest standard applies, making option (4) 0, 6 the most accurate.

Why Are These Standards Important?

Health Implications

• Low BOD ensures that the water is free from harmful organic pollutants and pathogens.

• High DO ensures that the water is well-aerated, tastes better, and supports healthy aquatic ecosystems.

Environmental Protection

• Maintaining low BOD and high DO protects aquatic life and prevents the growth of harmful bacteria and algae.

• These standards help prevent waterborne diseases and ensure sustainable water resources.

Regulatory Compliance

• Water utilities and authorities use these standards to monitor and manage water quality.

• Compliance ensures that drinking water is safe for all consumers.

Additional Water Quality Parameters

While BOD and DO are critical, other parameters are also monitored to ensure drinking water safety:

• Microbial Contaminants: Fecal coliforms and E. coli should be absent or below detectable limits.

• Chemical Contaminants: Limits are set for heavy metals, nitrates, fluoride, and other chemicals.

• Physical Parameters: Turbidity, color, and odor should be within acceptable ranges.

Summary Table

Key Takeaways

• The maximum BOD for pure drinking water should ideally be 0 mg/L, but up to 2 mg/L is acceptable for sources without conventional treatment.

• The minimum DO for pure drinking water should be at least 6 mg/L for the highest standards.

• Option (4) 0, 6 is the best answer for the strictest standard of pure drinking water.

• Maintaining low BOD and high DO is essential for public health and environmental protection.

Conclusion

For pure drinking water, the maximum BOD should be as low as possible, ideally 0 mg/L, and the minimum DO should be 6 mg/L or more. These standards ensure that the water is free from harmful organic pollutants, well-aerated, and safe for human consumption. While practical standards may allow slightly higher BOD and lower DO in some contexts, the strictest and most accurate answer is: