Carbone attivo o ozonizzazione? Un quadro di riferimento per la scelta della tecnologia di quarto stadio

If you have established that your plant falls within the scope of Directive (EU) 2024/3019, the next question is technology. The directive deliberately does not prescribe a specific solution – it sets a performance target and leaves the choice to operators. 
This article gives you a structured way to compare the two dominant fourth-stage technologies – activated carbon adsorption and ozonation – and a framework for evaluating not just the technology itself, but the operational reality of running it for 20 years. 

The two technologies side by side 

Activated carbon (PAC or GAC) 
How it works: micropollutants adsorb onto the high surface area of the carbon. 
Strengths: broad spectrum of micropollutants captured, low energy demand, no harmful by-products, well-established design guidelines. 
Weaknesses: ongoing consumable cost; dosing reliability depends on robust powder handling; spent carbon must be disposed of or regenerated. 
Ozonation 
How it works: ozone oxidizes micropollutants directly in the water. 
Strengths: no continuous consumable to dose; compact footprint. 
Weaknesses: high electricity demand; can form by-products (such as bromate) requiring a downstream biological filter; more complex process control. 
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 hidden decision: powder handling 

Most published comparisons stop at the technology level. But for plants choosing activated carbon, the real operational risk lies one step down: how reliably can the carbon actually be dosed?

Powdered activated carbon (PAC) is fine (10–50 µm), cohesive, hygroscopic, and electrostatically active. On the dry side, these properties cause exactly the bulk‑handling problems you see in practice: bridging and ratholing in silos and hoppers, feeders that struggle at very low throughputs, dust and residue on internal surfaces. In the aeration or contact tank, the same powder can also show a “cocoa effect” – PAC floating and clumping on the surface before it fully wets and sinks – but that in‑water behaviour is primarily about mixing and tank hydrodynamics, not about the bulk‑handling equipment itself. 
 
Under‑dosing or because the dry system bridges or starves, or over-dosing because the system surges, can both compromise the fourth‑stage performance. Even if the chemistry and tank are correctly designed, unstable dry‑side handling will show up as inconsistent removal. 
 
“What really matters is dosing accuracy in the chosen regime,” says Florian Stiller, technical expert at IB (Sales Support). “With long retention times in the aeration tank, robust discontinuous dosing of precisely defined batches is often more reliable than trying to run extremely small continuous flows through very delicate devices.” 

A framework for evaluating dosing systems 

1. Dosing accuracy over time, not just at commissioning.  

Dosing accuracy over time, not just at commissioning. Whether your process uses continuous or discontinuous PAC addition, the key is dose accuracy and repeatability at the dosing regime you choose – not the smallest possible continuous rate. Ask for documented dose accuracy over 6–12 months of operation. Commissioning data alone does not reveal how the system behaves once PAC has built up on internal surfaces. 

2. Containment and dust control. PAC dust is a housekeeping and health risk. A robust dosing system should keep the powder contained from delivery to dosing point without operator intervention. 
3. Cleaning and maintenance access. Buildup is inevitable. The question is how easily it can be removed without taking the dosing line offline for extended periods.

What decision-grade evidence looks like 

• When you collect references and trial data at this stage, look for: 
• Documented dosing accuracy (±%) over an extended operating period. 
• Intervention frequency – how often operators have to step in during normal operation. 
• Maintenance hours per year on the dosing system. 
• Evidence that PAC reaches the injection point as intended (no chronic bridging, no uncontrolled surges), so that the plant’s mixing and tank design can deliver the desired dispersion. 

If a supplier cannot provide this, you do not yet have the information you need to decide. 

What to do next 

Use this framework internally with your engineering and operations teams. The output should be a shortlist of two to three solutions where the technology choice is settled and the remaining decision is about supplier robustness and lifecycle cost. 

For more background on the core technologies and operational risks discussed here, see the explainers "Activated carbon vs. ozonation – a quick comparison for fourth-stage planning" and "Why powdered activated carbon is difficult to handle and dose reliably".

Next in the series: how to translate this into a defensible business case for management and finance.