GENERAL
Generation of ozone is onsite, as it rapidly decomposes to oxygen
(O2 ).
Ozone is usually created by electrically charging dry air or pure
oxygen.
There are five (5) basic components
to an ozonation system:
1) Electrical power supply for the ozone generators. Low frequency is 60 Hz and at medium frequencies
from about 200 to 1,000 Hz. High frequency units generally operate
at about 10,000 Volts and have a frequency range from 1,000 to
3,000 Hz.
2)
Equipment to produce a dry oxygen source.
We may use air (about 95% of all installed systems) or pure oxygen
(5%). Pure oxygen usually does NOT require further moisture removal,
but does usually have filters for particle removal prior to the
gas being sent to the ozone generators (a wise idea.)
Basic Theory: A filtered and very low moisture content oxygen source must be provided. Why? Moisture will result in the formation of nitric acid (HNO3) which will cause severe corrosion of the piping and the ozone generator. Nitric acid can cause a heat sink on the dielectrics which increase the potential for breakage. Nitric acid is usually removed during preventative maintenance shutdowns by scrubbing with soapy water, and then subsequent drying of the equipment. Dust can settle on the dielectrics creating the potential for breakage and can reduce the ozone generator efficiency.
Generally speaking, oxygen gas feed equipment (for air) includes coarse filters for dust, then compressors to provide for the power to move the gas, then followed either by aftercoolers or refigerant dryers to reduce the temperature of the air (gas) which allows for the removal of water vapor. The air is then piped thru pre-desiccant filters, desiccant dryers for final removal of water vapor, and then thru post desiccant filters. At this point it may be fed to the ozone generators.
3)
Ozone generators. The dry oxygen
gas is fed into the generator. The generator may use flat plates
or tubes. I will discuss the tube-type, as that is what I have
used (starting back in 1975) and (also have pictures of from a
local water treatment plant.) The ozone is produced by passing
a high voltage current into a "conducting brush." The
conducting brush is placed in a glass tube that has a metallic
coating on the internal surface (dielectric). The current is rapidly
alternated (high frequency) between the brush and the internal
coating of the tube. This causes negatively charged electrons
to move rapidly between the alternating poles. There must be a
"gap" between the metallic coating and the brush.
When an electron impacts an oxygen molecule (O2) it splits it into two atoms of oxygen ( 2O ).
One of the atoms of oxygen ( O ) may then combine with a molecule of oxygen (O2) to form ozone (O3).
Increasing the voltage will increase the ozone production but this also increases the potential of dielectric failure due to puncture, and also increase the amount of heat generated. Therefore, most ozone generators have cooling water to remove the heat.
If you do NOT remove the heat, then the ozone more quickly degrades to oxygen (O2). (It wants to degrade anyway, the heat just makes the reaction occur much faster!) Ozone basically degrades as shown here:
One way in which to increase ozone production is to increase the frequency rather than the voltage, as it is less damaging than higher voltages. If the frequency is increased, more heat is also generated. Just about 10% of the electrical energy results in ozone production, the rest resulting in heat (the majority), light, and sound!
Optimal Characteristics for the generator: Dielectric constant, thickness of the dielectric, voltage frequency, voltage, peak voltage across the gap, and absolute gas pressure in gap. Higher frequency with lower voltage, lower temperatures, and the combination of gap width and gas pressure which gives best yield for the lower voltage should be used.
The "gas" pressure in the generator, the amount of cooling water/lb of ozone produced, etc. vary among the types and manufacturers. For reference, one large tube-type generally requires a pressure in the vessel between 14 to 20 feet of water pressure (to overcome piping and diffuser frictional losses, and the depth of water in the contactor; and uses about 350 to 500 gallons of cooling water for each pound of ozone produced.
Air source: ozonator discharge contains
0.5% to 3% O3 by weight
Pure Oxygen source: ozonator discharge contains 1.0% to 6% O3
by weight
4) Enclosed ozone contactor and its diffuser. The ozone
contactors are enclosed vessels or tanks. Since ozone is one of
the strongest oxidants, it takes very little to cause harmful
damage to one's respiratory system, etc. Some systems recycle
a percentage of the gas that passes from the water to the chamber
above the water being treated.
Ozone transfer efficiency:
a) as the water quality decreases the transfer efficiency increases
(the demand for ozone is higher.)
b) is affected by pressure, temperature, ozone bubble size ( smaller
is better), contactor flow characteristics.
Contactors are usually constructed to have three or more ozonation zones, where the amount of ozone applied thru the diffusers may be controlled; and to reduce short circuiting, etc. It is not unusual to have 50% of all diffusers in the first zone to meet the initial ozone demand and establish an ozone residual. Later stages further reduce the potential for short circuiting, and to insure maintenance of an ozone residual for the required time, due to its rapid decay, and maybe due to some "latent demand." (Please reference Geo. White page 1235 - 1256 for an excellent discussion.)
5) Contactor ozone exhaust gas destruct apparatus. Remember how we said that we must remove the heat created when we were generating ozone or it would more quickly degrade to oxygen? The first method of ozone destruction utilizes this fact: thermal destruction. Most of the installed ozone destruction equipment work on this principle. They heat the exhaust gases coming out of the contact basins which destroys all of the ozone. Only air source ozone generators may use this type. Pure oxygen types may discharge enough oxygen and ozone to create fires. Pure oxygen may use the other methods which utilize catalytic destruction or thermal/catalytic destruction methods, without adverse consequences.
If the ozone is not destroyed, it will become a source of air pollution and/or a local safety hazard. The spent gas often contains more than 500 ppm of ozone from the contactor.
Most all ozone installations are in potable water treatment facilities. Commonly used for taste and odor reductions, disinfection, and water color reductions. Some facilities also use it to improve coagulation/flocculation performance.
Excellent oxidant for bacterial and also viral disinfection.
Ozone is NOT affected by ammonia or pH.
G. White reports that it may reduce THM precursors (ie: humic acid and malathion).
Ozonation prior to chlorination reduces the formation of THMs in most all cases.
New techniques now make it economically feasible for wastewater disinfection. Additional treatment requirements to meet new effluent discharge regulations in some parts of the country may make ozonation more suitable than other processes.
Added benefits for wastewater:
a) does not require a residual to be removed, as in chlorination
b) breaks down into oxygen in the water, thereby raising the dissolved
oxygen (DO) content.
In water reuse programs, used to remove refractory organics in lieu of carbon-adsorption process.
The amount of ozone consumed, in most wastewater treatment applications, to meet the oxidizable materials is extremely high, non-predictable, and variable throughout the day. (Obviously, the cleaner the water and the less variable the concentrations of the constituents in the water PRIOR to ozonation, the less of an impact there will be.)
Most ozone is consumed or reduced into other forms after 3 to 5 minutes. Ozone may have promise in future uses in water reclamation facilities that produce a "potable water."
Ozonation is equal to chlorine in its ability to kill bacteria and may be superior in inactivating viruses. Of major benefit to advanced wastewater treatment schemes, ozone reduces color and odor, is unaffected by pH and produces an oxygenated effluent.
Safety: Minimum allowable 8 hour working air concentration is 0.1 ppm .
References:
Wastewater Engineering, 3rd Edition; Metcalf & Eddy c 1991
McGraw Hill, Inc
Geo. Clifford White, "Handbook of
Chlorination & Alternative Disinfectants" 4th Edition;
c 1999, John Whiley & Sons, Inc.
Peter J. Lau, "Applying Disinfection
Alternatives to Wastewater Treatment," Pollution Engineering,
1997
Douglas Reed, "Selecting Alternatives
to Chlorine Disinfection," Pollution Engineering, September,
1998
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