Drinking water disinfection – modern physical method: Which physical method is widely used as a germicidal treatment step in modern potable water systems?

Introduction / Context:
Safe drinking water requires pathogen inactivation. While chemical disinfectants are common, a modern physical method that leaves no chemical residual and is effective against many microbes is ultraviolet (UV) irradiation, typically at germicidal wavelengths around 254 nm.

Given Data / Assumptions:

• Continuous flow UV reactors sized for target UV dose (mJ/cm^2).
• Pre-treatment to reduce turbidity for adequate UV transmittance.
• Compliance with local regulatory standards for disinfection.

Concept / Approach:
UV radiation inactivates microorganisms by damaging nucleic acids, preventing replication. It is a physical method, contrasting with chlorination or bleaching powder (chemical) and permanganate (oxidant). UV is often paired with a secondary disinfectant residual for distribution safety.

Step-by-Step Solution:
Categorize options: chemical vs physical.Identify UV radiation as the sole physical germicidal option listed.Select UV as the correct answer.Note: utilities may use UV plus chloramination for residual.

Verification / Alternative check:
Utility practice and standards acknowledge UV systems for primary disinfection, especially effective against protozoa such as Cryptosporidium, which are chlorine-resistant.

Why Other Options Are Wrong:
Chlorination, bleaching powder, and permanganate are chemical oxidants/disinfectants, not physical methods.

Common Pitfalls:

• Assuming UV provides a residual; a downstream chemical residual is usually still required.
• Ignoring UV transmittance limitations in colored/turbid waters.

Final Answer:
Ultraviolet (UV) radiation