Steps in Process
Function
Redox
Application
Optimisation
Advanced oxidation
Flow diagram
Sizing of systems |
Ozone is an unstable gas comprising of three oxygen atoms, the gas will readily degrade back to oxygen, and during this transition a free
oxygen atom, or radical is form.
The free oxygen radical is highly reactive and short lived, under normal conditions it will only survive for milli seconds, and during this
time frame it will oxidise virtually any chemical species. For example it will oxidise iron, manganese and may other heavy metals, it will
crack the carbon double bond of organic molecules such as dissolved proteins. It will also oxidise the the proteins in the the cell wall of
bacteria, the cilia of protozoa, the shell of viri. In fact ozone has 7 times the oxidising capacity of free chlorine, but does not
produce any toxic residuals.
Function of ozone in aquaculture and water treatment
When ozone degrades back to oxygen and free radicals, the potential of the water to oxidise chemical species will increase. This ability to
oxidise is measure as REDOX potential or ORP (oxidation reduction potential). The higher the redox potential, the higher the
concentration of free radicals. As the redox potential is increased a greater variety of chemical species that will be oxidised.
The redox potential of water
Different bacteria, viri, parasites, fungi etc, have unique environmental requirements. For examples some organism can only survive in
zero oxygen (anoxic) conditions with a low redox potential, e.g. anaerobic bacteria in septic tanks. As the oxygen level or redox potential is
increased, this environment become toxic or less favourable to these organisms. The redox potential is in effect selectively disinfecting
the water. Viri, fungi and many pathogenic bacteria favour a low redox potential, therefore simply by increasing the level, the water quality can be
improved, and many of the the pathogenic organisms are removed. This level of redox is not technically disinfecting the water, it is just making
it better for fish, aquatic life, beneficial bacteria.
In freshwater and marine systems, the optimum ORP is between 300 and approximately 330mv. If the redox potential is increased above 400mv,
then physical oxidation starts to occur and a higher percentage of the bacteria will be removed from the water. There may also be some damage to
the gills of fish and other aquatic animals. At levels above 500mv, a high percentage of the water will be disinfected, and there will be
potentially serious damage to fish. At levels of 750mv, you have complete disinfection of the the the water of all organisms,
including fish and aquatic life. This is the same level used by swimming pool operators to keep pool water sterile.
Application
There are basically two applications for ozone,
- Used to improve water quality and minimise pathogenic reducing bacteria, fungi and viri. The low level will not disinfect the water,
it just makes it more difficult for the pathogens to develop.
- High ozone concentrations with ORP levels above 700mv, will give complete disinfection, but you may have problems getting the level
back down to 300mv, or a safe level for the aquatic life.
Optimisation of ozone
Technically it is not the ozone increasing the ORP and cracking the organics, it is the free radicals from the degradation of the gas that is
actually doing the work. 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.
If the ozone can be made to degrade quickly it will produce a high concentration pulse of free radicals. This can be achieved using a number of
different techniques such as a catalytic AFM filter, hydrogen peroxide, or by using UVc light. When UVc light is used, the process is
referred to Advanced Photo Oxidation.
Advanced Photo Oxidation
When ozonated water enters a UVc chamber, the UVc light provides the energy required to initiate the degradation of ozone. This
reaction happens very quickly, and virtually all of the ozone will be converted to oxygen, and a very high concentration of free radicals.
If there is any bacteria, viri or chemical species present in the the water that can be oxidised, then the high concentration pulse of free
radicals will crack the molecules and disinfect the water. The ozone:UVc combination can be several times more effective hat either UVc light or
ozone on its own. If there are no chemical species present in the water then the free radicals will simply combine to for, oxygen.
When ozone is used, we always recommend the use of advanced photo oxidation techniques.
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