+ Reduce bacteria below minimum standards
+ Do not lead to toxic wastewater
+ Do not lead to increased risk to human health or the environment caused by the transport, storage or treatment of chemical disinfectants.
In principle, the process of disinfection can be physical or chemical.
1. Physical method
1.1. Heat method
Higher temperature treatment of microorganisms denigrates molecules (structure, function) of microbial cells. When boiling water at 1000C, most microorganisms are killed. . In rare cases when the temperature rises, the sporangia is transformed into a strong protective layer. To kill the spore, it is necessary to boil water up to 1200C or to boil in the following order: under normal conditions for 15 to 20 minutes, allow the water to cool to below 350C and keep within two hours for the spores to grow back, then boil again water.
The simple but energy-consuming method of heating is usually applied only on a small scale.
1.2. UV sterilization
Ultraviolet (UV) radiation is an electromagnetic radiation with a wavelength of about 4 to 400 nanometers (nanometer). The wavelength of ultraviolet radiation outside the detection area, the perception of the naked eye. Ultraviolet rays for sterilization do not change the chemical and physical properties of water.
Ultraviolet strikes change the DNA of bacterial cells, the nucleic acids absorb 240 – 280 nm of wavelength energy and inhibit the reproduction and growth of bacterial cells. Therefore, ultraviolet radiation of 254nm wavelength has the highest disinfection ability.
The disadvantage of this method is that high operating costs, turbidity of the water and mucus sticking to the lamp can prevent ultraviolet rays from affecting the bacteria, reducing the effectiveness of disinfection.
1.3. Ultrasonic method
An ultrasound with an intensity of no less than 2W / cm2 over a period of 5 minutes is capable of destroying all microorganisms in water. However, the installation and use of this method is often complicated and difficult to control.
1.4. Filtration
Most microorganisms in the water (except the virus) are 1 to 2 μm in size. If filtered through a filter with a slot size smaller than 1 μm can eliminate most bacteria. Common filter used are porcelain plates, porcelain plates with very small hollow slot. With this method, filtered water must have a concentration of less than 2mg / l. The disadvantage is that often the filter need to be replaced and the nature of the method does not destroy the microorganism.
Disinfection by physical methods has the fundamental advantage of not changing the physical and chemical properties of water, without causing side effects. However, due to its low efficiency, it is usually only applied on a small scale with the economic and technical conditions.
2. Chemical methods
The basis of the chemical method is to use strong oxidants to oxidize the yeast of bacterial cells and destroy them. The most commonly used chemicals are chlorine, bromine, iodine, chlorine dioxide, hypochlorite and its salts, ozone, potassium permanganate, hydrogen peroxide. Due to high efficiency, chemical disinfection is now widely applied in all sizes.
2.1. Disinfect water with chlorine and its compounds
– This is a simple, inexpensive and highly effective method. People often use the following types of chlorine for disinfection:
+ Cl2 liquid chlorine
+ NaClO sodium hypochlorite
Calcium hypochlorite CaCl2 (ClO) 2.2H2O
– Chlorine is a strong oxidant, in any form, pure or compound, when used with water to form hypoclorite molecules. HOCl has a strong antiseptic effect.
HCl is very weak, unstable and easily decomposed into HCl and oxygen or H + + OCl- :
Cl2 + H2O -> HCl + HOCl
HOCl -> HCl + O
HOCl -> H+ + OCl–
Hoặc CaCl2O + 2H2O -> Ca(OH)2 + 2HClO + CaCl2
The process of killing microorganisms occurs in two stages: first, the disinfectant diffuses through the microbial cell shell, then reacts with the enzyme inside the cell and destroys the metabolism leading to cell death. destruction of the cell.
– The speed of the disinfection process is determined by the kinetics of the disinfectant dispersion through the cellular shell and the kinetics of the enzyme breakdown.
– The disadvantage of this method is the effect of pH and temperature, can reduce the pH of water.
2.2. Disinfect water with iodine
Iodine is a strong oxidizer and is often used to disinfect water in swimming pools. As the substance is difficult to dissolve so iodine is used in the form of saturated solution.
When the pH of water is less than 7, the dosage of iodine used is from 0.3 to 1 mg / l. If used higher than 1.2mg / l will make water taste iodine.
2.3. Sterilize with ozone
Ozone is a gas with strong oxidizing properties. In water, ozone is broken down into reactions:
O3 + H2O -> HO3 + OH-
HO3 + OH-> 2H2
O3 + HO2 -> HO + 2O2
HO + HO2 -> H2O + O2
Ozone is created by letting oxygen or air pass through the sparking device. To provide enough ozone for the water treatment plant, use an electric spark generator consisting of two metal electrodes spaced about a distance from the air. Supply alternating current to the electrodes to create an arc, while blowing clean air through the gap between the electrodes to convert part of oxygen into ozone.
The advantage of this method is that it reduces the oxygen demand of water, decreases the concentration of organic matter, the concentration of the active substances, the decolourization, phenol and cyanide; does not cause odor; increase dissolved oxygen concentration; not harmful products; increase suspended sediment velocity; It is less influenced by temperature and pH and does not require a suturing such as chlorine.
However, this method has high initial capital and consumes a lot of electrical energy.