Environmental Water Treatment Knowledge: What Ozone Is and What It Does
Ozone is an allotrope of oxygen. At room temperature it is a pale blue gas with a distinctive pungent odour. The English word "ozone" comes from the Greek ozon, meaning "to smell".
The following excerpts are taken from Collected Papers on Ozone Technology Applications, edited by Tsinghua University. They classify ozone concentrations by safe exposure level, airborne application level, in-water application level, environmental level and odour-perception level, so that the figures can be looked up quickly in practice. Safe level: the concentration people are permitted to be exposed to is no higher than 0.2 mg/m3.
(1) For air deodorisation and sterilisation a relatively low ozone concentration is sufficient, for example 0.25 ppm (0.5 mg/m3), whereas surface disinfection of objects (killing micro-organisms and removing chemical contamination) requires a concentration dozens of times higher.
(2) Killing efficiency improves at low temperature and high humidity. Humidity is the decisive factor: below 45% relative humidity ozone has almost no killing effect on airborne micro-organisms; the effect increases gradually at 60% and reaches its maximum at 95%.
(3) Cold-storage disinfection calls for 6 to 10 ppm. After the generator is shut down and the store sealed for more than 24 hours, the bacterial kill rate is about 90% and the mould kill rate about 80%.
(4) In water applications ozone solubility ranges from 0.1 mg/L to 10 mg/L. The lower value is the minimum concentration required for water disinfection and purification; the upper value is the concentration that "ozone water disinfectant" can actually reach.
(5) Disinfection with ozonated water is extremely fast and takes place almost instantaneously. Once the required concentration is reached in clean water, bacteria are killed within 0.5 to 1 minute; at 4 mg/L the hepatitis B virus is 100% inactivated within 1 minute.
(6) Herbold reported that at 20 degrees C an ozone concentration of 0.43 mg/L in water destroys 100% of E. coli, and that at 10 degrees C only 0.36 mg/L is needed for complete destruction.
(7) Bottled water treatment should reach an ozone solubility of 0.3 to 0.5 mg/L, which requires an ozone output of 2 g O3 per cubic metre of water. In practice, an ozone generation concentration above 8 mg/L makes this level easy to achieve.
Ozone is produced when oxygen in the atmosphere undergoes photochemical reaction.
(1) Ozone is very unstable: it decomposes slowly at room temperature and rapidly at 200 degrees C. It is a stronger oxidiser than oxygen. It oxidises metallic silver to silver peroxide and lead sulphide to lead sulphate, oxidises organic compounds and bleaches many organic colourants such as indigo. It attacks rubber and readily oxidises unsaturated organic compounds. The oxidation potentials (reduction potentials) of ozone, chlorine and hydrogen peroxide are 2.07 V, 1.36 V and 1.28 V respectively, which shows that ozone is the strongest oxidising agent available for water treatment. Ozone oxidation ruptures unsaturated organic molecules: the ozone molecule adds across the double bond to form an ozonide, which then splits spontaneously into a carbonyl compound and a zwitterion bearing acidic and basic groups. The zwitterion is unstable and decomposes into an acid and an aldehyde. The reaction is as follows:
Ozone is a harmful gas. At 6.25 x 10-6 mol/L (0.3 mg/L) it irritates the eyes, nose and throat; at (6.25-62.5) x 10-5 mol/L (3 to 30 mg/L) it causes headache and local paralysis of the respiratory organs; at 3.125 x 10-4 to 1.25 x 10-3 mol/L (15 to 60 mg/L) it is harmful to human health. Toxicity also depends on exposure time. For example, long-term exposure to less than 1.748 x 10-7 mol/L (4 ppm) can cause permanent cardiac disturbance, whereas exposure to less than 20 ppm for no more than 2 hours produces no permanent harm. The permissible ozone concentration is therefore set at 4.46 x 10-9 mol/L (0.1 ppm) for 8 hours. Because ozone has such a strong odour, 4.46 x 10-9 mol/L (0.1 ppm) is already perceptible. Ozone has been used worldwide for more than a hundred years and no death from ozone poisoning has ever been reported.
Ozone is strongly oxidising and attacks almost every metal except gold and platinum. Aluminium, zinc and lead are strongly oxidised on contact with ozone, whereas chromium-containing iron alloys are essentially unaffected. For this reason, chromium-iron alloy containing 25% Cr (stainless steel) is commonly used for the parts of ozone generators and dosing equipment that come into direct contact with ozone.
High concentrations of ozone near ground level irritate and damage the eyes, the respiratory system and other mucous membranes and have an adverse effect on human health. Experts point out that, from long-term observation, although ozone exceedances are concentrated in the sunny months from April to September, experience abroad suggests that as nationwide efforts to control PM2.5 intensity and air visibility improves, the probability of ozone exceedance will keep rising, and controlling ozone pollution will be harder than controlling PM2.5.
Ozone (O3) is an allotrope of oxygen (O2). It is a pale blue gas with a distinctive odour. Its molecular structure is triangular with a bond angle of 116 degrees; its density is 1.5 times that of oxygen and its solubility in water is 10 times that of oxygen. Ozone is a strong oxidiser with a redox potential in water of 2.07 eV, second only to fluorine (2.5 eV) and higher than chlorine (1.36 eV) and chlorine dioxide (1.5 eV). It destroys and ruptures bacterial cell walls, penetrates the cell quickly and oxidises and decomposes glucose oxidase and other enzymes that bacteria need to metabolise glucose. It can also act directly on bacteria and viruses, damaging the cell and ribonucleic acid (RNA) and decomposing macromolecular polymers such as deoxyribonucleic acid (DNA), RNA, proteins, lipids and polysaccharides, so that bacterial metabolism and reproduction are disrupted. Bacteria are killed by rupture of the cell membrane in a process known as cell dissipation, caused by the cytoplasm being shattered in water; under dissipation conditions the cell cannot regenerate. It should be noted that, unlike hypochlorite disinfectants, the bactericidal power of ozone is unaffected by pH changes and by ammonia. Its bactericidal power is 600 to 3,000 times that of chlorine, and its sterilisation and disinfection effect is almost instantaneous: at an ozone concentration of 0.3 to 2 mg/L in water, bacteria are killed within 0.5 to 1 minute. To achieve the same sterilisation result (for example a 99% E. coli kill), the ozone dose required is only 0.0048% of that of chlorine.
Ozone also oxidises and decomposes pollutants in water and is highly effective in water treatment for odour removal, decolourisation, sterilisation and removal of phenol, cyanide, iron and manganese, as well as for lowering COD and BOD.
(3) For water treatment, ozone is normally applied through a dissolving and dosing unit. The main types are aeration (direct aeration or oxidation tower), venturi injector, vortex negative-pressure suction and HICHINE pump mixing. Dissolution efficiency increases in that order, and the HICHINE pump type can exceed 95%.
GMP-validated air disinfection in the pharmaceutical industry [1]: ozone is used for the central air-conditioning and purification system of clean production areas, for ozone sterilisation cabinets, and for sterilising production spaces that have no central air-conditioning. A considerable number of Chinese pharmaceutical companies use ozone as their main means of disinfection and air sterilisation, including Shenyang Jishi Pharmaceutical, Dalian Zhen'ao Pharmaceutical, Shanghai Qing'an Pharmaceutical, Jilin Aodong Pharmaceutical Group Yanji and Fujian Xing'an Pharmaceutical. Ozone as the primary air-disinfection method has made an important contribution to the GMP validation of these companies.
The half-life of ozone is only 30 to 60 minutes. Because it is unstable and decomposes easily it cannot be stored as an ordinary product and must be generated on site. Ozone generated from air typically reaches 10 to 20 mg/L, and from oxygen 20 to 40 mg/L. Air containing 1% to 4% ozone by mass can be used for water disinfection.
Low-voltage electrolysis produces ozone by passing low-voltage direct current through the positive and negative poles of a solid membrane electrode to electrolyse deionised water. At the special anode interface the water is separated into hydrogen and oxygen in the form of proton exchange; hydrogen is discharged directly at the cathode interface, while oxygen molecules at the anode interface gain energy from electrons excited by the high-density current and combine into ozone. The electro-oxidation method is a low-voltage electrolysis method. A low-voltage electrolytic ozone generator uses pure water as raw material, a solid precious-metal polymer as electrolyte and a cation-exchange mode, and obtains ozone by low-voltage electrolysis without any auxiliary material or additive. The ozone concentration produced exceeds 20%, the accompanying gas is oxygen, and there is no secondary pollution.
For this reason the pharmaceutical industry abroad has added total organic carbon (TOC) monitoring to pharmaceutical water systems. Unfortunately, when raw water heavily contaminated with organic matter is treated with ozone, large organic molecules are broken into fragments that become nutrients for microbial metabolism; if the ozone concentration in the distribution network is not maintained, this actually increases slime and degrades water quality [1].
(3) Killing efficiency is better at high humidity. UV sterilisation falls off sharply above 60% relative humidity and above 80% may even revive bacteria. Ozone behaves in the opposite way: the higher the humidity, the better the killing effect, because at high humidity the cell membrane swells and thins and its structure is more easily destroyed by ozone. This property suits the high-humidity environments common in the food industry [1].
Atoms of oxygen then combine with surrounding oxygen molecules to form ozone, which contains three oxygen atoms. More than 90% of atmospheric ozone is in the upper atmosphere or stratosphere, 10 to 50 km above the ground, and this is the ozone layer that needs protecting. A small fraction of ozone molecules drift near ground level and still help block ultraviolet radiation. Some experts have found, however, that ozone concentrations in the air near the ground are rising rapidly, which is a cause for concern. Ozone plays a vital role in protecting people and the environment in the stratosphere, but rising concentrations in the troposphere are harmful to human health. Ozone irritates the eyes and respiratory tract, affects lung function, and at higher concentrations is also harmful to plants.
French researchers believe that the main cause of rising ozone levels in soil is nitrogen oxides produced when mineral fuels such as petroleum products are burned. These nitrogen oxides drift in the air, and some of their oxygen atoms slowly combine with atmospheric oxygen to form ozone made of three oxygen atoms. They stress that sunlight accelerates this chemical reaction, so the effect of soil ozone on plant growth differs between regions with different climates. In water treatment systems, tanks, exchange columns, filters, membranes and piping continuously support the growth and multiplication of bacteria. Disinfection methods all provide some ability to remove bacteria and micro-organisms, but none of them can remove all bacteria and water-soluble organic contamination in a multi-stage water treatment system. To date the best way to remove bacteria and viruses continuously in high-purity water systems is ozone.
Ozone has been used in water treatment since 1905. It is superior to chlorine treatment because it removes halides from the water. In China this method is only at an early stage in water systems, whereas abroad the disinfection method is very common because ozone leaves no harmful residue.
Ozone was first used to disinfect hospital rooms in Paris in 1856; in 1902 Hurion defended at the Paris medical school a thesis on inhaling ozone to treat whooping cough, becoming the first person to obtain a doctorate through the application of ozone. In 1936 the French physician P. Aubourg first advocated rectal insufflation of ozone to treat chronic colitis. Over the past 60 years more than ten kinds of medical and health applications of ozone have been developed. Ozone development in China began in 2000, after which ozone medicine took root and flourished, with notable achievements driven by well-known specialists such as Ni Jiaxiang and He Xiaofeng. Ozone can be used in surgery, internal medicine and other fields of health care.
2. Methods of use. (1) Seed treatment. Bubble ozone gas into clean water while stirring continuously; after 10 minutes an ozone solution is obtained. Soak the seeds in it for 15 to 20 minutes to kill viruses, pathogens and insect eggs on the seed surface. (2) Control of pests and diseases in greenhouses. (a) Chamber fumigation before planting: 10 days before transplanting, in combination with high-temperature chamber sealing, release ozone from a generator into the chamber for at least 2 hours. (b) Control in seedling beds: seal the bed tightly, release ozone for 1 minute per 10 square metres, keep the chamber closed for 10 minutes, then ventilate for 30 minutes. (c) Control after transplanting vegetables: after the seedlings have recovered, release ozone for 7 to 10 minutes per mu of greenhouse, keep it closed for 15 to 20 minutes, then ventilate for 30 minutes. For chambers free of pests and diseases, release once every 5 to 7 days for 5 consecutive applications, adding 5 minutes to the release time every 2 to 3 applications until it reaches 25 minutes per application; the fumigation time is likewise increased by 5 to 10 minutes every 2 to 3 applications. Trials show that ozone gives good control of tomato grey mould, leaf mould, early blight and late blight, cucumber downy mildew and blight, and of greenhouse whitefly, leaf miners and aphids. It has no effect on soil-borne pests and diseases in the greenhouse, however, because too little ozone gas penetrates the soil and the concentration there is too low.
Experiments along the coast of Africa suggest that under enhanced ultraviolet B radiation the photosynthesis of plankton is weakened by about 5%. Enhanced UV-B can also alter freshwater ecosystems by killing micro-organisms in the water and thereby reduce the self-purification capacity of water bodies. Enhanced UV-B can also kill juvenile fish, small shrimps and crabs. If the plankton in Antarctic waters declined sharply, marine life as a whole would change greatly. Some plankton are very sensitive to ultraviolet light and others are not; the degree of DNA damage caused by ultraviolet light differs by a factor of 100 between organisms.
Growth of crops and trees is seriously impeded. Some plants, such as peanuts and wheat, tolerate UV-B relatively well, while others, such as lettuce, tomato, soybean and cotton, are very sensitive. Tremolada of the Agricultural Biotechnology Center at the University of Maryland observed six soybean varieties under solar lamps and found three of them extremely sensitive to UV radiation: photosynthesis in the leaves declined, reducing yield, and the protein and oil content of the seed also fell. A 1% loss of atmospheric ozone reduces the soybean harvest by 1%.
Worldwide, purified water and natural water (spring water, mineral water and groundwater prepared by filtration and other processes) are now commonly disinfected with ozone. In the purification of tap water with ozone, the conventional international standard is a solubility of 0.4 mg/L maintained for 4 minutes, that is, a CT value of 1.6.
9. HACCP China: Practical Guide to HACCP, Guide to Implementing an HACCP System in Food Manufacturing Enterprises and Consulting and Auditing of HACCP Systems in Food Manufacturing Enterprises.
