6 minute read · Published by Swhift Stainless Fabrication, building stainless tanks in Sandpoint, Idaho since 1980
Ozone contact tanks are one of the things we build most, and they are also the product buyers most often arrive knowing least about — usually because the ozone system was specified by someone else and the tank is the last piece to be sourced.
What the tank actually does
Ozone (O₃) is a powerful oxidiser used for disinfection and for oxidising dissolved contaminants out of water. Unlike chlorine it cannot be stored or shipped — it is unstable and decays back to ordinary oxygen within minutes. So it is generated on site, on demand, and used immediately.
The sequence is: generate ozone, inject it into the water stream, then give it time to work. That last step is what the contact tank is for. It is a controlled holding volume that keeps water in the presence of dissolved ozone long enough for the reaction to complete before the water moves downstream.
How contact time determines size
The governing relationship is straightforward: required volume equals design flow rate multiplied by required contact time. A system running 20 gallons per minute that needs five minutes of contact requires roughly 100 gallons of contact volume.
| Flow rate | 2 minutes | 5 minutes | 10 minutes |
|---|---|---|---|
| 5 gpm | 10 gal | 25 gal | 50 gal |
| 10 gpm | 20 gal | 50 gal | 100 gal |
| 25 gpm | 50 gal | 125 gal | 250 gal |
| 50 gpm | 100 gal | 250 gal | 500 gal |
| 100 gpm | 200 gal | 500 gal | 1,000 gal |
Real vessels also need allowance for the fact that flow through a tank is not perfectly uniform. Baffling and port placement influence how closely actual residence time matches the theoretical figure, which is why inlet and outlet locations matter as much as volume.
Why stainless steel
Ozone is a strong oxidiser, and that is not selective — it attacks the vessel as readily as the contaminants. Many plastics and elastomers degrade under continuous ozone exposure, becoming brittle and cracking over time. It is a well-known material compatibility issue and it is the main reason contact vessels are normally stainless even in treatment systems that use plastic pipework elsewhere.
- Stainless holds up to continuous ozone service
- No lining or coating to degrade and expose a substrate
- Smooth, non-porous surfaces that clean down properly
- FDA-approved materials — 100% tasteless and odorless — for potable service
- Can be built as a pressure vessel where the system runs pressurised
304 or 316 for ozone service?
Both handle ozone. The deciding factor is usually the water chemistry rather than the ozone — specifically chlorides, and temperature. Our warranty covers T-304 in water at 1.5 ppm chlorine or less and T-316 up to 8 ppm, and those limits are the practical guide.
What to specify when you order
- Design flow rate, and the contact time your treatment scheme requires
- Whether the vessel runs pressurised or vented — this changes the whole construction
- Operating pressure and temperature if pressurised
- Water chemistry, particularly chloride or chlorine content
- Injection, vent, drain and sample port sizes and locations
- Any envelope the vessel has to fit inside — an existing skid, a plant room, a doorway
- Whether a sanitary interior finish is required
Where these end up
Municipal and private drinking water treatment, commercial ozone laundry systems, bottling and beverage water, well water treatment for iron, manganese and sulfur, and CIP and sanitation water. The construction is broadly the same across all of them; what changes is the volume, the port arrangement and whether it holds pressure.