14.4 Ozone Generation & Contacting Systems (Subclass 14)
Key Takeaways
- Ozone is generated on site from oxygen in a corona discharge cell and cannot be stored, so generation capacity and demand must be matched continuously.
- Feed gas must be extremely dry, with air-prepared systems requiring a dew point at or below about minus 60 degrees C, because moisture forms nitric acid in the cell and destroys the dielectric.
- Corona discharge generates far more heat than ozone, so cooling water flow and temperature are primary operating variables; ozone production falls sharply as cooling degrades.
- Off-gas from a contactor contains unreacted ozone and must pass through a thermal or catalytic destruct unit before release, with continuous ambient ozone monitoring and alarms in the generator and contactor rooms.
- Ozone leaves no lasting residual and produces biodegradable organic matter, so ozonation is normally followed by biologically active filtration and by a chemical disinfectant for distribution protection.
Ozone Must Be Made Where It Is Used
Ozone is the strongest oxidant in common water treatment use, and it is also unstable, with a half life in water measured in minutes. It cannot be delivered or stored. Every ozone installation is therefore a small chemical manufacturing plant, and the Subclass 14 examination is largely about running that plant.
The process train has five parts: feed gas preparation, generation, contacting, off-gas destruction, and ambient monitoring.
Feed Gas Preparation
| Feed gas source | Description | Typical ozone concentration in gas |
|---|---|---|
| Prepared ambient air | Compressed, cooled, filtered and deeply dried by desiccant dryers | 1 to 3 percent by weight |
| Pressure swing adsorption (PSA) oxygen | Nitrogen removed on site by molecular sieve | 6 to 10 percent by weight |
| Liquid oxygen (LOX) | Delivered cryogenic oxygen, vaporized on site | 8 to 12 percent by weight |
Dryness is not optional. Water vapor in the feed gas reacts with nitrogen in the corona to form nitric acid, which corrodes the cell and attacks the dielectric, and moisture also directly suppresses ozone yield. Air-prepared systems are designed for a dew point at or below roughly minus 60 degrees C, and the dew point monitor is a primary operating instrument. A desiccant dryer that fails to regenerate is one of the most damaging faults in an ozone plant.
Liquid oxygen systems remove the dryer problem but add cryogenic hazards: severe cold burns, oxygen enrichment of the surrounding atmosphere, and a strict prohibition on hydrocarbons near oxygen service, since oil and grease can ignite violently in high-purity oxygen.
The Corona Discharge Generator
Dry feed gas passes through a narrow gap between a high-voltage electrode and a grounded electrode separated by a dielectric, typically glass or ceramic. The electrical discharge splits oxygen molecules, which recombine into ozone.
Operating realities:
- Most of the energy becomes heat, not ozone. Cooling water flow and temperature are therefore direct controls on production, and ozone yield falls markedly as cooling water warms. Summer cooling capacity often sets a plant's maximum ozone output.
- Dielectric failure shows as a sudden drop in production with rising current on one cell and is a scheduled repair item.
- Production control is by adjusting applied power, gas flow or both, paced from plant flow and an ozone residual analyzer.
- Materials in ozone service must be ozone resistant: 316L stainless steel, PTFE and certain fluoroelastomers. Ordinary rubber, carbon steel and many plastics fail rapidly.
Contacting
Ozone must be dissolved and held in contact with the water long enough to do its work.
| Contactor type | Description | Note |
|---|---|---|
| Fine bubble diffuser basin | Deep, baffled, over-under chambers with porous diffusers at the bottom | The traditional design; deep basins improve transfer efficiency |
| Sidestream injection | Ozone injected into a pumped sidestream through a venturi, then blended into the main flow | Compact, high transfer efficiency, common in retrofits |
| Turbine mixer | Mechanical mixer disperses gas | Less common in drinking water |
Transfer efficiency of 85 to 95 percent is typical in a well-designed contactor. Disinfection credit is calculated as CT using the measured dissolved ozone residual and the T10 contact time of the chamber, exactly as for chlorine, and the extremely high potency of ozone against Giardia and Cryptosporidium is what makes short contact times workable.
Off-Gas Destruction and Ambient Safety
Undissolved ozone collects in the headspace of the contactor and must never be vented to the room or to the atmosphere.
- Thermal destruct heats the off-gas to roughly 300 to 350 degrees C to decompose ozone back to oxygen.
- Catalytic destruct uses a metal oxide catalyst at lower temperature and is more energy efficient, but the catalyst is poisoned by moisture, so the gas is heated slightly above the dew point first.
- Ambient ozone monitors are installed in the generator room and around the contactor, alarming and shutting down generation on detection. Ozone is detectable by smell at very low concentrations, well below harmful levels, so any persistent ozone odor is an alarm condition, not background.
- The occupational exposure limit is very low, on the order of 0.1 parts per million as an eight-hour average, and ozone is a severe respiratory irritant.
What Follows Ozonation
Ozone leaves no lasting residual, and it breaks large natural organic molecules into smaller, readily biodegradable organic matter. If that material enters the distribution system it feeds bacterial regrowth and consumes disinfectant residual. Standard practice is therefore to follow ozonation with biologically active filtration, usually a granular activated carbon or anthracite bed operated without a pre-oxidant so that biofilm can consume the assimilable organic carbon, and then to apply a chemical disinfectant for distribution protection. Bromate formation control in bromide-bearing source waters, described with alternative disinfection, constrains how much ozone can be applied in the first place.
An air-fed ozone system experiences a desiccant dryer regeneration failure, and feed gas dew point rises from minus 65 degrees C to minus 20 degrees C. What consequences should the operator expect?
Ozone production at a plant falls noticeably during a July heat wave with no change in feed gas quality or applied power. What is the most likely cause?
Why is ozonation normally followed by biologically active filtration before a chemical disinfectant is applied?