Water treatment with ozone is nothing new. The method has been used for more than a century and its efficiency is well documented and proven. Ozone technology, however, has improved in the last couple of years as high-concentration ozone generators have hit the market.
"The high ozone concentration and high gas pressure of modern ozone generators have opened up a whole new market for ozone treatment," said Arash Golshenas, Head of R&D at Primozone. "Heavily polluted waters that before were very hard to treat are now treatable and actually at a lower cost than before."
High Gas Pressure
It all comes down to the physical law formulated by William Henry in 1803, also known as Henry's Law:
| "At a constant temperature, the amount of a given gas that dissolves in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid." |
This principal also applies to dissolving ozone in water. The only difference is that ozone reacts with the pollutants once it has been dissolved. An ozone generator with high gas pressure will therefore be very efficient at dissolving the ozone gas into water.
"The amount of ozone that can be dissolved is directly proportional to the pressure," explained Golshenas. "When going from 1 bar (g) pressure at the dissolution point to 2 bar (g), the application is able to dissolve approximately 60% more ozone."
High Ozone Concentration
High ozone concentration is another important factor for improving dissolution. The ozone concentration depends upon the ratio of ozone and oxygen in the outgoing gas mixture from the ozone generator. Depending on the construction of the ozone generator, the ozone concentration that can be achieved varies. This also means that the amount of oxygen needed to feed the system varies. If the ozone concentration is doubled, the amount of oxygen needed to feed the system is reduced to half (see Fig. 1).
"Tests show that doubling the ozone concentration also doubles the dissolution ability of the ozone into water," explained Golshenas. "This means that high- concentration ozone generators have a better efficiency, which reduces the cost of treatment by not having surplus ozone produced."
The cost efficiency of high concentration ozone generators is due to a couple of factors:
- Less oxygen is needed to feed the system (50%).
- The increased dissolvability enables smaller contact systems (half the size).
- Decreased energy consumption of the oxygen generator (50% compared to other systems).
Complete Ozone System Design
To take full advantage of the benefits of high ozone concentration and high gas pressure, there are some factors to consider when designing a complete ozone system. One is the way the ozone is dissolved into the water. The aim is to create the smallest bubbles possible to maximize the reaction area. Maximizing the reaction area is limited as the water eventually will be saturated.
Another consideration is the pressure of the dissolution system. This pressure needs to be lower than the discharge pressure of the ozone generator. But increased pressure at the dissolution point improves dissolution as it can push the saturation limit; hence, the discharge pressure of the ozone generator is crucial.
Efficiency Cuts Costs
The bottom line is that higher discharge pressure and higher ozone concentration mean better dissolution of ozone into water. This enables significant energy savings.
"At Primozone, we have seen a total energy savings capability of up to 50% and even more when an ozone solution is correctly designed," said Golshenas. "Heavily polluted water can now be treated at a lower cost, which is important when, for example, municipalities are imposed to remove pharmaceutical residue and other hard-to-treat pollutants."
Primozone is exhibiting at AWWA's ACE14 expo in Booth 601.
