In drinking water treatment, harmful bacteria present in the water cannot be effectively destroyed by which of the following methods?

Introduction / Context:
Safe drinking water must be free from harmful bacteria and pathogens. Water treatment plants and households use several methods to disinfect water, such as chlorination, boiling, and ozonisation. Some substances, however, may change the chemical properties of water without reliably killing microorganisms. This question asks you to identify which method among the given options is not effective for destroying harmful bacteria in drinking water under normal treatment conditions.

Given Data / Assumptions:

• The focus is on harmful bacteria present in drinking water.
• Methods mentioned include chlorination, boiling, adding caustic soda, and ozonisation.
• We assume typical practice where methods are applied at reasonable, safe doses for drinking water.
• The goal is to remove or kill bacteria, not just change taste or hardness.

Concept / Approach:
Disinfection methods such as chlorination and ozonisation use strong oxidising agents to kill bacteria and viruses. Boiling water raises the temperature to levels that denature bacterial proteins and enzymes, effectively killing most pathogens. Caustic soda, or sodium hydroxide, is a strong base primarily used for pH adjustment or industrial cleaning, not as a standard drinking water disinfectant. Adding caustic soda in practical, safe amounts may not reliably destroy all harmful bacteria, and high doses would make the water unsafe to drink. Therefore, among the options, adding caustic soda is the method that cannot be relied upon to destroy harmful bacteria in drinking water.

Step-by-Step Solution:
Step 1: Consider chlorination: chlorine is widely used in municipal water supplies as a disinfectant, so it effectively kills bacteria at recommended doses. Step 2: Consider boiling: heating water to boiling temperature for sufficient time kills most bacteria and pathogens, making it a recommended household practice. Step 3: Consider ozonisation: ozone is a powerful oxidising agent used in advanced water treatment plants to disinfect water. Step 4: Consider caustic soda: while it can raise pH and has cleaning uses, it is not standard for safely disinfecting drinking water. Step 5: Conclude that adding caustic soda is the method that cannot be considered a proper way to destroy harmful bacteria for safe drinking water.

Verification / Alternative check:
Public health guidelines from organisations and textbooks recommend chlorination, boiling, filtration, ultraviolet treatment, and ozonisation as accepted methods of disinfecting drinking water. They do not recommend adding sodium hydroxide directly to water for disinfection, because it would alter taste, safety, and chemical balance without providing reliable pathogen kill at acceptable concentrations. Industrial cleaning solutions that contain sodium hydroxide are corrosive and not suitable for consumption. Comparing recommended practices with the given options confirms that caustic soda is the odd one out.

Why Other Options Are Wrong:
Chlorination is a standard, proven method for killing bacteria and has been used successfully for many decades in municipal water supplies. Boiling is a simple and effective household method for disinfection, especially during outbreaks or emergencies. Ozonisation is used in modern water treatment plants where advanced technology is available, and it effectively inactivates many microorganisms. These methods do destroy harmful bacteria when properly applied, so they cannot be the correct answer to the question.

Common Pitfalls:
Students may think that any chemical added to water must kill bacteria, without considering whether it is actually used or safe for drinking water. Another pitfall is to confuse caustic soda with bleaching powder or chlorine compounds, which are genuine disinfectants. To avoid these errors, remember that disinfectants must both kill pathogens and be safe at residual levels for human consumption. Caustic soda fails this test and is not used for routine drinking water disinfection.

Final Answer:
Harmful bacteria in drinking water cannot be reliably destroyed by adding caustic soda (sodium hydroxide).