Ozone water treatment has made great progress in the world, not only because of its effective ability to remove impurities and sterilize, but also because it will not produce secondary pollution (residual poison) in the treated water, and excess ozone will be decomposed into oxygen faster than chlorine agent, forming carcinogens such as chloramine and chloroform in the water, so it is recognized as the safest disinfectant in the world. There is no large-scale promotion in developing countries, because the fixed assets investment and operating power consumption of ozone treatment are too high. In countries lacking funds, since the mid-1980s, due to the high water quality standard and high economic benefits of bottled water, many domestic bottled water plants have adopted ozone treatment, and small ozone generators have been widely promoted. Most bottled water plants that use ozone to treat water correctly can meet the international double zero standard (the total number of Escherichia coli and bacteria is zero).
Second, several basic factors affecting the sterilization effect of ozone water treatment
Because ozone water treatment is a new thing, people are not familiar with it. Some manufacturers, construction units and ozone users mistakenly believe that disinfection is completed by pressing the button to blow ozone gas into water. This misunderstanding makes the application of ozone ineffective, and even leads some people to doubt the sterilization ability of ozone itself.
Some manufacturers use a simple ozone generator to treat bottled water, but they know nothing about the ozone concentration produced by it and the water-soluble ozone concentration after treatment. The actual effect of sterilization is unbelievable. It is difficult to apply. The author also interviewed a mineral water plant, which used 5 tons of water per hour. The design unit chose 100g03/h ozone generator, and the water stayed in the contact absorption device for only a few seconds. Results The treated water was unqualified, and a large amount of ozone tail gas overflowed in the filling room, making workers unable to work.
There are also some domestic water processors produced by some manufacturers, both oxygen concentration and treatment time are not enough. It is doubtful whether this water processor can produce qualified drinking water.
Therefore, it is extremely important to correctly understand the physical and chemical process of ozone in water and the biochemical process of ozone sterilization. Because the mechanism of ozone dissolving in water and the influence of ozone on the material exchange of biological cells are extremely complicated, this paper can't discuss it in detail, but only discuss ozone sterilization in general.
1, water-soluble ozone concentration and retention time are necessary conditions for sterilization.
Professor Ma Yilun, Institute of Military Health, Academy of Military Medical Sciences, etc. Experiments on ozone treatment of Bacillus anthracis and Bacillus subtilis were carried out. Niger, and summarized the empirical formula of sterilization kinetics:
dN/dt=-KNtMCN
Where: n: number of bacteria t: time c: ozone concentration m in water, n is the index of t, and c k: efficiency constant, which can also indicate bacterial resistance.
As can be seen from the above formula, the sterilization amount per unit time is the ratio of ten treatments to ozone concentration in water and treatment time. It can be seen that under the condition of constant K and N, it is necessary to ensure the ozone concentration in water and a certain contact time.
2. The necessity of ensuring ozone concentration in water
There are many conditions to ensure ozone concentration in water, such as water temperature, air pressure, relative velocity of gas and liquid, partial pressure of ozone gas acting on liquid surface, surface area of ozone gas, viscosity, density and surface tension of water. Some factors, such as water temperature, air pressure and the partial pressure of ozone gas acting on the liquid surface, are very important. Some, such as the density, viscosity and surface tension of water. , which is invariant under certain conditions, can be ignored. This relationship is briefly introduced as follows:
The mass transfer intensity between gas and liquid depends on the diffusion speed and turbulence of molecules, which can be expressed by the general mass transfer formula:
u=dG/dt=KF △C
Where: u: mass transfer speed, which can be determined by the amount of ozone g transferred from gas phase to liquid phase in t time, i.e. DG/dt. K: mass transfer coefficient, f: contact surface area of gas phase and liquid phase. The △ c power in the mass transfer process can be determined by the difference between the actual situation and the ozone concentration at equilibrium (that is, the greater the difference between the ozone concentration in water and the ozone concentration in the ozone source, the greater the mass transfer speed).
Through the analysis of the general mass transfer equation, we can know that, firstly, to make ozone dissolve into water as much as possible, it is necessary to increase the contact surface area f between ozone and water as much as possible, which is determined by the contact device.
Secondly, △C shows that the higher the concentration of ozone generator, the more beneficial it is for water to absorb ozone.
Thirdly, the mass transfer coefficient k is related to many factors. K (total mass transfer coefficient) is the sum of gas-phase mass transfer coefficient K and liquid-phase mass transfer coefficient K, while ozone is a gas with low solubility, and K gas can be ignored. According to Henry Dalton's law, K liquid is a compound function of many physical parameters.
K solution =f(T, p, u, w, p, ó)
Among them, dissolved ozone is directly proportional to air pressure P and inversely proportional to water temperature T. ..
With the increase of the relative linear velocity of two phases, the contact surface area f and its renewal speed of gas-liquid two phases also increase, but the contact time between each bubble and liquid will decrease. Therefore, from the comprehensive effect, it is better to maintain the relative linear velocity of gas and liquid in a range.
Viscosity u, density p of liquid and interfacial surface tension between gas and liquid. The increase of K can reduce the updating speed of interface and K solution, so Km is inversely proportional to U, P and O, which can be ignored for all kinds of drinking water.
In application, we should pay attention to two parameters, temperature and air pressure, and pay attention to the relative speed of water flow and air flow, especially the temperature, when designing the contact device, because high temperature not only reduces the absorption effect of ozone by water, but also decomposes ozone itself because of high temperature. China once tried to treat the water temperature of 70℃ with ozone, but it didn't have any effect.
In 1894, Mailfert calculated the ozone concentration in water according to previous experimental reports as follows:
Temperature (Celsius) o11.81519 27 40 55 60
Solubility (L gas /L water) 0.64 0.5 o.456 0.381o.27 0.112o.031o.
This set of data is generally linear, which shows that the solubility of ozone in water is about 0/5 times that of oxygen.
Venosa and Opatken pointed out that the basic relationship that determines the solubility of ozone (or any gas) in liquid is Henry's law, that is, at a certain temperature, the weight of any gas dissolved in a known volume of liquid will be proportional to the partial pressure of the gas acting on the liquid.
Moreover, this law can conclude that the solubility of ozone is 13 times that of oxygen at standard temperature and pressure.
According to Henry's law, in order to improve the solubility of ozone in water, it is necessary to increase the partial pressure of ozone gas in the whole gas source, that is, to increase the concentration of ozone source. If the concentration of ozone source is not enough and the treatment time is long, the concentration of ozone in water will not increase (because the concentration balance has been reached).
From the above discussion, we can draw the conclusion that:
1, in order to ensure the sterilization effect, it is necessary to ensure a certain ozone concentration and treatment time in the water.
2, in order to ensure a certain concentration of ozone in water, must ensure that:
A. ozone source concentration.
A certain temperature.
C. the water temperature should not be too high.
D. The specific surface area of ozone gas put into water should be as large as possible, so that there are more opportunities for ozone to contact with water.
According to the application experience at home and abroad, the recommended parameters for disinfection treatment of drinking water with general water quality are: water-soluble ozone concentration of 0.4 mg/L, contact time of 4 minutes, that is, ct value 1.6. The ozone dosage is 1-2 mg/L, and the water temperature is preferably below 25 degrees Celsius. The former Soviet Union standard stipulated that the ozone concentration in drinking water should not be lower than 0.3 mg/L, and the bottled water industry in China recommended that the ozone concentration in bottled water should be 0.3 mg/L..
Three commonly used contact devices at present and their effects
As mentioned above, the basic purpose of the contact device is to ensure the maximum solubility of ozone in water. Therefore, it is necessary to make the contact area between ozone gas and water as large as possible and have enough contact time. Therefore, the basic requirements for contact devices are:
1, which can ensure the best ozone absorption effect.
2. When the contact device works, the process parameters are easy to control, the work is stable and the safety is good.
3. The energy consumption (power required for stirring or conveying water and gas) is the lowest.
4. The smallest volume has the largest production capacity.
5. Simple structure, cheap materials and low manufacturing and maintenance costs.
There are three commonly used contact devices: bubble column or pool, water jet (Venturi tube) and fixed spiral mixer (used alone or in combination), agitator or screw pump, and two or more of them are used in series. A brief introduction is as follows:
L, bubbling method: large-scale water treatment bubbling pool, commonly used in small-scale water treatment bubbling tower, requires that the bubbler has a small aperture (aperture of several microns to several tens of microns) to increase the specific surface area of ozone, and the gas in the aperture is required to be evenly distributed, so that the water vapor can fully contact. Especially when multiple gas distributors are used in the bubbling pool, the water depth from the water surface to the surface of the gas distributors is generally required to be not less than 4-5m, so as to facilitate the full contact of gas and water.
Its advantages are convenient operation, easy change of operation parameters, no influence on dosing effect and working stability, less power consumption, simple structure of bubble column and convenient maintenance.
However, its volume is too large, the pool type occupies a large area, and the tower type requires higher factory cost.
2. The water ejector (Venturi tube) sucks ozone gas in the negative pressure area generated by high-speed water flow in the variable-diameter tube to form turbulence and achieve the mixing effect.
The fixed spiral mixer behind the venturi tube can further stir water and gas, and maintain a turbulent state in a long distance to enhance absorption.
Because the mixing time of this device is very short, it is often necessary to add a water storage tank behind its output pipeline to increase the contact time between water and gas and reduce the flow rate of water to precipitate tail gas.
Compared with bubble column, its structure is greatly simplified and its production cost is low. However, it is necessary to add a water pump to ensure the injection speed of water, and the process parameters are not easy to master, so the treated water quantity cannot be adjusted at will, otherwise gas-liquid separation will occur and the absorption effect will be affected.
3. Stirring mode: The agitator produced in the early days is similar to a single-cylinder washing machine, except that the motor is placed on it, and the outer cylinder is made into a polygon, and the bubbles are broken and dissolved into the liquid by the vortex generated by stirring. This stirring method has poor effect, high energy consumption and smaller volume than bubbling method, but the cost is not low. Because of mechanical movement and ozone corrosion, the life of the machine is short and the maintenance cost is high.
In recent years, a turbopump has been put into the market, which has good mixing effect, small volume and easy operation of technical parameters, but it has complex structure, high cost, high power consumption and complex maintenance, so it is necessary to set a water storage tank behind its pipeline.
Fourth, ozone concentration test.
Because ozone is a gas with extremely unstable chemical properties, it is difficult to collect and measure its content in air and water in a short time. As mentioned above, in order to ensure the purification and disinfection of water by ozone, it is necessary to control various parameters, among which the measurement of ozone concentration is difficult. Some ozone generator manufacturers can't detect it themselves and don't know the ozone concentration produced by their products. Some manufacturers use the difficulty of testing to exaggerate the performance of products, which has a very bad influence and even affects people's trust in ozone sterilization ability.
It should be said that it is not difficult to detect ozone concentration now. In practical application, ozone concentration is the basis of ensuring disinfection effect, and it is also a necessary means to identify the real performance of ozone generator. Therefore, while promoting the application of ozone, we should also promote the ozone measurement method.
This article does not intend to discuss ozone testing in detail. Interested comrades can refer to the relevant articles published by Li Hanzhong at the Fifth National Disinfection Academic Exchange Conference, and only make a brief introduction here.
L, iodometry: the most classic measurement method in the past, using ozonized gas to make iodine in potassium iodide solution free and develop color, and then titrating with sodium thiosulfate to reduce it to colorless, and calculating ozone concentration according to the amount of sodium thiosulfate consumed. This method is intuitive in color development and cheap in equipment, but it is inconvenient to use chemical inspection equipment such as drugs, bottles, measuring cylinders, balances and burettes, and it is easily interfered by other oxidants (such as NO, CL, etc.). ) At present, I ratio method is still the standard measurement method in China.
2. Ultraviolet absorption method: using the maximum absorption value of ozone to ultraviolet light with wavelength of =254nm, the ultraviolet light is attenuated in ozone atmosphere, and then the data is output through photoelectric elements and electronic circuits (comparison circuit, data processing and digital-to-analog conversion). This method is accurate and can be used for on-line continuous measurement. It has been selected as the standard method by advanced industrial countries such as the United States, but the instrument is expensive and is generally used as a testing unit and a production and scientific research unit.
3. Electrochemical method: Using the electrochemical reduction of ozone in water on the electroactive surface, the current change curve in the electrochemical circuit is directly proportional to the ozone concentration in the solution. The instrument has the function of data output, which can measure online and realize the closed-loop feedback control of ozone generator. It is cheaper and smaller than ultraviolet method. At present, it is used in large-scale water treatment projects.
4. Colorimetry: Like iodometry, it is a chemical method, which uses the color development or decoloration phenomenon when ozone reacts with chemical reagents to determine the ozone concentration. Various chemicals such as potassium iodide, o-toluidine or indigo dye can be used, which can be directly observed by naked eyes and compared with standard color tubes or color plates, and can also be detected by spectrophotometer. This method is simple and easy, and the cost is not high. At present, it is suitable for popularization in China, but the experimental drug is disposable consumables.
5.DPD ozone water concentration test reagent: DPD reagent in the box is packed with double-layer aluminum foil paper, which contains disintegrant and can be dissolved quickly. This product is highly sensitive to ozone, accurate to 0.05ppm. The colorimetric card is made of precise color separation and equipped with special colorimetric tube, which has the advantages of convenient use, long shelf life, stable and reliable quality and so on. The prepared DPD method corresponds to colorimetric solution, which is compared with KIO3 standard solution and determined. This method is especially suitable for on-site analysis, which can be compared with imported similar products, and has broad application prospects in water industry, food industry, beverage and pharmaceutical industry. At present, DPD ozone determination kit has been widely used by hundreds of well-known mineral water and purified water enterprises in China, such as Robust, Wahaha, Ipoh, Nongfu Spring, Jing Tian and Yili.