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|Ozone Generators and Systems|
|Ozone Generator Sizing Guide|
Ozone was first used in water treatment in the late 1800s and, with recent improvements in technology, is now becoming an attractive water treatment alternative. It is a powerful oxidizing agent that can reduce levels of many impurities in water, including color, taste and odor. Ozone can also be used as a disinfectant and is a good alternative to chlorine for some applications.
Ozone is an unstable gas comprising of three oxygen atoms. As a result of this instability, the gas will readily degrade
back to oxygen, and during this transition a free oxygen atom, or free radical, is formed. The free oxygen radical is highly reactive and short lived. Under normal conditions it will only
survive for milliseconds. Ozone is a colorless gas that has an odor similar to the smell of the air after a
Ozone has a greater disinfection effectiveness against bacteria and viruses
compared to chlorination. In addition, the oxidizing properties can also
reduce the concentration of iron, manganese, sulfur and reduce or eliminate
taste and odor problems. Ozone oxides the iron, manganese, and sulfur in
the water to form insoluble metal oxides or elemental sulfur. These
insoluble particles are then removed by post-filtration. Organic particles
and chemicals will be eliminated through either coagulation or chemical
oxidation. Ozone is unstable, and it will degrade over a time frame ranging from a few seconds to 30 minutes. The rate of
degradation is a function of water chemistry, pH and water temperature.
The Ozonation Process
The transformation of oxygen into ozone occurs with the use of energy. This process is carried out by an electric discharge field as in the CD-type ozone generators (corona discharge simulation of lightning), or by ultraviolet radiation as in UV-type ozone generators (simulation of the ultraviolet rays from the sun). Oxygen molecules (O2) are split into individual oxygen atoms. In this state, oxygen is very unstable so it joins with other oxygen
molecules to form O3, which is ozone.
Because ozone gas is so unstable (a property that gives ozone it's extraordinary oxidizing
capabilities), it cannot be packaged or stored and must be generated on site. In most common water treatment applications, this means a simple wall-hung ozone generator combined with a compact air preparation unit and a venturi injector to safely get the ozone into the water. This means no drums to store, record, report or dispose of.
UV Ozone Generation
In this case, ozone is produced when air (usually ambient) is passed over an ultraviolet lamp, which splits
oxygen molecules in the air stream. Light is measured on a scale called an electromagnetic spectrum and its increments are referred to as nanometers. Ultraviolet ozone generators use a mostly 185 nanometer lamp because that
wavelength happens to be the most efficient for producing ozone. However, the ozone produced by this "photo dissociation" of oxygen molecules is comparatively weak because the concentration of ozone available in the output
gas is very low - commonly in the range of .01 - .1% by weight. At the same time, ultraviolet ozone generators do not require the feed gas air preparation that the other ozone equipment technology needs.
Corona Discharge (CD) Ozone Generation
In this case, ozone is produced with a high voltage electrical discharge. Called "corona
discharge" or "CD", it is the method most commonly used to generate usable amounts of ozone for most water treatment applications. The idea is to actually create a small, controlled lightning storm, which involves producing a constant,
controlled spark (corona) across an air gap through which a prepared feed gas is passed. This feed gas may be air that has simply had most of its moisture removed or air with enhanced oxygen levels.
The key to an efficient, reliable CD ozone generator is making sure that the feed gas is dried to a dew point of at
least -60° F. This is important because as the electrical discharge splits the oxygen molecules, nitrogen molecules are also being split, forming several species of nitrogen oxides. They are normally benign, but if combined with moisture
(ordinary humidity), these oxides form a very corrosive substance called nitric acid. Consequently, proper air preparation is critical.
The relative strength of corona discharge ozone expressed as a percentage of
concentration by weight is commonly 0.5 - 1.7% for systems using dried air and
1.0 - 6.0% when an oxygen-enhanced feed gas is used.
The raw water is then passed through a venturi injector which creates a vacuum and pulls the ozone gas into the water or the air is then bubbled up through the water being treated. Since the ozone will react with metals to create insoluble metal oxides, post filtration is required.
After injection, the ozone must have time to do its work. This time is known as contact time. Contact time is the time ozone has to oxidize and disinfect,
or the time the water is allowed to hold the disinfectant. For ozone, contact time is measured from the point of injection to the filter. Suppose you inject ozone into a 25-gallon contact vessel followed by a filter, and the flow rate is 5 gpm. The contact
time in this situation would be 25 gallons divided by 5 gpm, or five minutes contact time.
The contact time needed varies with
the matter to be oxidized. Time required for oxidation ranges from almost instantly to 10 minutes or more. A general rule of thumb is no less than four minutes contact time with a measurable residual of 0.1 mg/L ozone.
All gasses inducted through the venturi must be properly disposed of or
“off-gassed.” At a bare minimum, this is accomplished by simply venting the contact tank to the outside.
Filtration of Oxidized Particles
In most cases, a properly
sized multi-media depth filter is very well suited to filtering objectionable oxidized contaminants and will give years
of operation with little maintenance required.
Benefits and Applications of Ozone
- Ozone is effective over a wide pH range and rapidly reacts with bacteria, viruses, and protozoans and has stronger germicidal properties then chlorination. Has a very strong oxidizing power with a short reaction time. Ozone creates none of the trihalomethanes commonly associated with chlorine compounds and properly matched to the application, ozone will reduce most organic compounds to carbon dioxide, water and a little heat.
- The treatment process does not add chemicals to the water.
- Ozone can eliminate a wide variety of inorganic, organic and microbiological problems and taste and odor problems. The microbiological agents include bacteria, viruses, and protozons (such as: Giardia and Cryptosporidium).
- Bottled water, perishable goods (seafood, fruit, vegetables, etc.) and well water disinfection are examples of ideal ozone applications. The fact that ozone efficiently oxidizes the organics that cause taste, odor and color problems
without leaving a high residual helps to simplify many water treatment trains.
- Ozone's ability to kill algae (planktons) with low doses makes it a popular treatment method for ponds and water features.
- Ozone oxidizes and precipitates many metals and destroys some pesticides without leaving a trace.
- Ozone functions as a preoxidizer of iron, manganese and sulfide compounds, allowing for their
removal by simple direct filtration. Ozone acts quickly and easily, and the water quality resulting from its use is unmatched.