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DISINFECTION WITH OZONE
With the increase of the world-wide population a greater demand of water and food take place, but also the fact that we are more people in this planet produce an increasing of the contamination of the resources available. The water used by man for domestic, industrial and agricultural activities returns to channels from the rivers and the lakes with a load of physical, chemical and biological polluting agents, among them the microorganisms, in a considerable amount.
It is said that the greater enemy of the nature is the man; the different industrial activities developed for obtaining new industrial advances sometimes are the cause of the contamination in superficial and underground waters. The polluting agents can have different origin although they always cause the same effect; they can come from an intensive agriculture due to the use of fertilizers and pesticides: the excessive and incorrect application of fertilizers and the irrigation activities produce the nitrate washing, its incorporation to the superficial water and its infiltration to the water-bearing subterranean, on the other hand some of chemical agents used in the control of the diseases of the plants have a remarkable resistance to the degradation, staying a long period of time in the ground.
Other polluting agents come from industry and mines. The industrial sector produces a variety of organic and inorganic substances that when they are spilled in an uncontrolled way, can give rise to important contaminations like heavy metals. Another type of contamination also to consider is the one that comes from the domestic activity derived from the use of detergent and clinquants chemical agents.
The residual waters without treating and the solid remainders, coming from the urban nuclei, are the main sources of microbiological contamination, originating diseases transmitted by the water. The water is a natural habitat for a great number and diversity of microorganisms, which do not suppose a risk for the health, mainly if they are in low number. However, the bacteria that arrive at the water come from the man and the animals are the real cause of numerous diseases.
DISEASES TRANSMITTED BY WATER
From before Christ it was already suspected that water could transmit diseases for that reason Hippocrates (460 - 354 a .C) recommended to boil it.
Nowadays we know several types of pathogenic microorganisms that can be transmitted by water and produce a great variety of diseases. As an example we can mention the gastroenteritis that can be caused by diverse microorganisms or viral agents (Escherichia coli, Aeromonas hydrophila, rotavirus, astrovirus).
But we wonder how they are able to survive and to be a cause of disease if the water is not the best way for cultivate them and the bacteria rarely can multiply, well, the answer is very easy, many of them have the capacity to survive in fresh water what it is defined as persistence, this one depends on several factors like the time, the temperature, organic matter, pH, salinity, oxygen, and the presence of other microbial competitors. The bacteria usually have a brief persistence (one week) or average (a month) with some exception that can survive up to two months. The virus has a prolonged persistence, of several months, mainly for example the Hepatitis A and increases in water with organic matter, due to the protective effect of the particles to which they are adhering. As far as the parasites that also are another form of transmission of diseases some can get to survive a year.
Some pathogenic microorganisms are resistant to chemical agents, like the chlorine, which is the most disinfectant used in treatments of purification of water. In order to give an idea: the bacteria are little resistant to chlorine and they are destroy with the habitual doses (1 mg/l), with the exception of Legionella which needs greater doses (5-10 mg/l). The virus also needs greater doses (6mg/l) and the parasites like the Cryptosporidium has a so high resistance to chlorine that survives to disinfection treatments.
DISINFECTION
In practical terms, to disinfect the water means to eliminate the microorganisms which are able to produce diseases. In the disinfection a physical or chemical agent is used to destroy the pathogenic microorganisms, which can transmit diseases using the water like passive vehicle.
The disinfection is a selective process: it does not destroy all the present organisms in the water and it does not always eliminate all the pathogenic organisms. For that reason it requires previous process that can eliminate them by means of the coagulation, sedimentation and filtration.
But which is the utility of the disinfection: The use of disinfection as a part of a process of water treatment can obey to the following objectives:
a) To reduce the initial content of microbiological polluting agents in the crude water (predisinfection). This process is used single in special cases.
b) To disinfect the water after filtration. It constitutes the most important use.
c) Simple disinfection of a water free of physic and chemical polluting agents that does not require another treatment.
There are a lot of physical and chemical disinfectants agents. The most known is the chlorine, since who has not used the phrase: 'this water tastes to chlorine'. What it is wanted to present through these lines, is that at the present time it is a method of disinfection much more powerful and effective than chlorine but that still he is a great stranger, it is ozone.
OZONE
Ozone is enriched oxygen which consists on three oxygen atoms, it is unstable and it is disturb with certain facility in normal oxygen and rising oxygen, that it is a strong oxidant. Due to this characteristic, it acts with great efficiency as disinfectant and it is constituted like the most serious competence of chlorine.
Ozone is a slightly blue gas with a characteristic scent that can be perceived after electrical storms. It is little soluble in the water and very volatile. It can be able to stay in the single water some minutes; in their application, approximately 10% are lost by volatilization. The doses necessary to disinfect the water vary according to the quality of the same one.
It is considered that ozone is the disinfectant of greater microbicida efficiency and it requires quite short times of contact. It has been shown that when ozone is transferred to the water by means of a mixer in line without movement, the bacteria are destroyed in two seconds. For that reason, the time of contact in the ozonization does not have greater importance.
The speed that ozone has to kill the bacteria is quite greater than the one of chlorine, three thousand times greater, because, although both are oxidating, the mechanism of action i s different:
Ozone kills bacteria through the rupture of the cellular membrane. This process, known like destruction of cells by lisina, produces the dispersion of the cellular cytoplasm in the water: the unsaturated lipids are the majority components of the cytoplasm membrane that has the bacteria; ozone attacks the olefin connections and forms the ozonide. This action begins the destruction of the capacity of the cell to work and it can be enough for causing the death of weaker cells. This ozonide has a high potential of oxidation, he is unstable, and it exerts his own action of disinfection attacking enzymes, sulphydric groups or aldehydes, releasing peroxiles compounds that are also disinfectants. All of these produce the dispersion of the cytoplasm and therefore to the death of the microorganism.
However, the chlorine must be introduced through the cellular wall of the bacterium and spread within the cytoplasm, action that depends on the time of contact.
Due to its great oxidating power, the use of ozone can be recommendable in the water pre-cure for the reduction of dissolved metals and the removal of organic matter, which allows saving coagulants and times of retention. Experimentally, it has been researched that less amount of ozone than of chlorine in similar process of pre-cure are required. Ozone, besides to attack the precursors of the trihalometanes and to reduce its concentration in the water, destroys these compounds already formed. Another advantage in front of chlorine is that it does not distribute to the water colour, scent or flavour and we can avoid other toxicologic aspects coming from the chloration (trihalometanes, chlorofenoles and chloramines).
Next we can see a clear example of the effect of ozone in a culture of spores:
These are the same spores 30 seconds after to be dealt with ozonized water.
