Ozone disinfection in water treatment: Which is the main drawback that limits its use as the sole final disinfectant in distribution systems?

Introduction / Context:
Ozone is a powerful oxidant and disinfectant used for primary disinfection and oxidation of taste-and-odour compounds, iron/manganese, and organic precursors. However, its application must consider distribution-system protection against re-contamination.

Given Data / Assumptions:

• Potable water treatment with post-ozonation.
• Conventional distribution network where intrusion/regrowth risk exists.

Concept / Approach:

Unlike free chlorine or chloramine, ozone decomposes rapidly and provides no persistent residual in the distribution system. Therefore, although it achieves excellent primary disinfection, it cannot by itself protect against downstream contamination or biofilm regrowth; a secondary disinfectant residual (e.g., chlorine/chloramine) is typically required.

Step-by-Step Solution:

Verification / Alternative check:

Operational experience and guidelines emphasize maintaining a measurable residual in the network; ozone cannot provide this due to its short half-life in finished water.

Why Other Options Are Wrong:

• Removal of colour/odour is beneficial, not a drawback.
• “Adds taste” is not typical; ozone generally improves taste/odour.
• Efficiency: ozone is generally more potent than chlorine for primary inactivation; the limitation is residual, not efficacy.

Common Pitfalls:

• Relying solely on ozonation without secondary residual protection.
• Ignoring bromate formation control when ozonating bromide-bearing waters.

Final Answer:

Ozone leaves no lasting residual; it vanishes before water reaches consumers.