Activated carbon vs. ozonation – a quick comparison for fourth-stage planning

The EU directive does not prescribe a specific technology for the fourth treatment stage. The two dominant options are adsorption on activated carbon (powdered or granular) and oxidation by ozone. Each has trade-offs. 

Activated carbon (PAC or GAC) 

• How it works: micropollutants adsorb onto the high surface area of the carbon.
• Strengths: well understood, broad spectrum of micropollutants captured, low energy demand, no harmful by-products. 
• Weaknesses: ongoing consumable cost; dosing reliability depends on robust powder handling (see the cocoa effect); spent carbon must be disposed of or regenerated.  

Ozonation 

• How it works: ozone oxidizes micropollutants directly in the water.
• Strengths: no consumable to dose continuously; compact footprint. 
• Weaknesses: high electricity demand; can form by-products (such as bromate) requiring a downstream biological filter; more complex process control.  

The hybrid trend 

A growing number of plants combine both: ozonation followed by activated carbon polishing, or activated carbon as the main stage with ozone reserved for specific contaminants. The optimal choice depends on influent profile, energy prices, and operational preferences.

Where IB fits 

IB's strength is in the powder handling side – specifically, robust dosing of activated carbon at full plant scale, drawing on experience from activated carbon dosing in waste incineration plants. The operational reliability of the dosing system is often what separates a fourth-stage plant that performs from one that struggles. 
This explainer is referenced from the articles "EU Directive 2024/3019: what the fourth treatment stage means for your plant", "Activated carbon or ozonation? A framework for choosing your fourth-stage technology", and "Building the business case for a fourth-stage upgrade: 20-year thinking in a 20-month decision".