Water Processing Disinfection by Ozonation and UV

Transcription

1 memberresources Water Processing Disinfection by Ozonation and UV Disinfection, when applied, is always the last step before bottling. No further steps can be taken once the water has been disinfected. The principal disinfectant used in bottled water production is ozone, through ultraviolet radiation is sometimes used. Ozone is an unstable, colourless gas. Ozone is a powerful oxidizer and a potent germicide with a much higher disinfection potential than other disinfectants. Chemically, ozone consists of 3 atoms of oxygen, while normal oxygen gas has only 2 atoms of oxygen. Ozone, with its 3 atoms of oxygen, is very unstable. It quickly decomposes to normal oxygen plus 1 free oxygen atom. This extra oxygen is responsible for much of the oxidizing effect (O 3 O 2 + O). Factors Affecting Ozone Residual Ozone is used in bottled water industry because of its ability to rapidly kill bacteria in water. Ozone can remove turbidity, colour, tastes/odours, and organics without leaving a residual taste, as chlorine does. Ozone can be used to remove iron and manganese from water. Variables in determining the effectiveness of ozone killing micro-organisms are contact time, ozone output, and residual ozone concentration in the water. Ozone residual is dependent on how much ozone is added to the water, minus how much ozone demand is in the water. Ozone demand refers to amount of ozone consumed by oxidisable material in the water. Ozone residual is also dependent on the total dissolved solids in the water. The higher the TDS, the less ozone can be dissolved in the water. Ozone solubility in water is also affected by ph and temperature. The lower the ph, or the lower the temperature, the higher ozone s solubility. In cold water, ozone residual can be maintained for several days in bottled water. Proper design of an ozonation system depends on the quality of water to be treated. Sufficient ozone must be added to maintain an ozone residual of ppm in the bottle immediately after filling. Methods of dispensing ozone gas into the contact tank can greatly affect the efficiency of ozone dissolution in the water. Smaller bubbles increase the surface area of ozone. Bubbles of 1 millimetre (mm) diameter have nearly 32 times more surface area in contact with water than do bubbles of 10 mm. In a typical operation after a theoretical dose of 2 mg/l, ozone concentration in-line will actually be mg/l and 0.3 mg/l in a just-filled 15 litre bottle. The contact time is 3-4 minutes. Ozone degrades fairly rapidly in the bottled product, as illustrated in Table 2B-12.

2 Table 1 - Disappearance of Ozone in Bottled Water (20 C) Time for Disappearance Water Type Ozone Half - life 0.64 ppm O ppm O ppm O ppm TDS 5. min min min. 1.1 min ppm TDS 29 min hrs. 2.9 hrs. 2.2 hrs. 1 ppm TDS 9 min hrs..9 hrs. 6.0 hrs. A two-stage application, split dose, of ozone can be used when water contains high levels of oxidisable matter. This is another means of increasing contact time. At the other extreme, taste problems can arise if too much ozone is present in the bottled product. Ozone has a low taste and odour threshold, ppm. One solution to this problem is to hold inventory to allow ozone to disappear. During cold weather, water can be heated to increase ozone disappearance. In no instance should the ozone residual in a just-filled bottle fall below 0.1 ppm, the minimum acceptable level for product disinfection. Ozone Disinfection Capability Ozone is the strongest germicide available, over 3,000 times faster than chlorine at killing bacteria. Ozone rapidly kills pathogenic viruses and cysts. An ozone residual of 0.28 ppm for 1 minute inactivated % of Coxsackie virus B3. Data on ozone dose/time to kill 99% of polio virus and coliform are presented in Table 2B-13. Table 2-99% Kill of Coliform and Polio Virus Test Ozone Contact Time X Temp. Microorganisms Mg/L) Time (min.) (mg/l) ph ( C) E. coli Polio 1 < < < < Unofficial studies performed at the USA EPA Laboratories gives further indication of ozone, disinfection efficiency when compared to free chlorine, chloramine, and chlorine dioxide. As with bacteria and viruses, ozone CT values for inactivation of Giardia cysts, were lower than any of the chlorine-based disinfectants considered. Page 2 of 6

3 Table 3 illustrates the comparative effect of ozone on G. Lamblia and G. Muris. Table 3 - Cut values for 99% Inactivation of G.muris and G.lamblia Cysts by Ozone* Species G. muris G. lamblia Temp (⁰C) 5 5 ph Disinf. Conc (mg/l) Time (min) CT Mean CT No. of G. muris G. muris G. lamblia Reference: Wickramanayake et. al. (1984, 1985) Exp n (slope) Along with chlorine, ozone is generally recognized as one of the most potent germicidal agents used in the treatment of water. Chemical disinfectants kill or inactivate micro-organisms by physical or chemical disruption of a critical cellular structure or function. In the case of ozone, a chemical reaction takes place that is oxidative in nature. Thus, the oxidative potential of ozone is an important factor with regard to its capabilities as a disinfectant. The chemical state of ozone, that is microbiocidally active, is not known but it may be the OH (hydroxyl radical) or the HO 2 - (hydroperoxyl radical). The presence of other species, such as O 3 - (ozonide radical), O 2 - (superoxide radical), and O - (oxide radical), have been demonstrated, but their role in germicidal activity is not known. Most experimental data indicate that the efficiency of ozone as a disinfectant varies with ph, and that it is somewhat more efficient at a lower ph (6.0) than at a higher ph (10.0). This result is probably caused by the greater stability of ozone in water at lower ph s. Data shows that ozone dissipates rapidly when used in water, and that the rate of decay of residual ozone in water varies inversely with the amount of oxidisable materials present. Care of Ozonator An ozone generator (ozonator) must be used to create ozone at the time it is to be injected, because ozone is unstable and cannot be stored. Air is typically used a s the feed gas to supply oxygen to the ozonator. Pure oxygen can be used if a higher concentration of ozone is desired. The use of pure oxygen will, approximately, double the ozone concentration in the gas stream. In an ozonator, the ozone generator shell is the heart of the system. However, with out a good air preparation system, operating efficiency and life of an ozonator will be decreased. The ozonator consists of two metal electrodes separated by an air gap (called a discharge gap), and an insulator (dielectric) made of specially formulated glass. Feed gas, consisting of air oxygen, passes through the discharge gap while a high voltage alternating current is applied to the active electrode, creating a corona discharge. The incoming gas is instantly ionised as electrons are drawn back and forth across the discharge gap at a rate equal to the electrical supply frequency (normally 60 Hertz). Some of the oxygen molecules in the gas stream are bombarded by the electrons and separate into free oxygen atoms by the following reaction: O 2 > e - = 20 + e - Some of these free oxygen atoms recombine with intact oxygen molecules and form ozone (O 3 ) by the reaction: O + O 2 > O 3 The glass dielectric serves as an electric current insulator to prevent concentrated discharges of energy sparking or arcing between the two electrodes. The result is a silent, evenly diffused, discharge of energy called the corona discharge. It is this even dispersion of energy which creates ozone. The corona is purple in colour when the gas stream is air and white when the gas is oxygen. If the feed gas is wet, the corona will be interlaced with many sparks, giving the corona the appearance of a small lightning storm. Air coming into an ozonator contains roughly 9% nitrogen, 21% oxygen and trace amounts of dust, water vapour and other gases. The air preparation system (consisting of an air dryer, filters, etc.) is designed to remove dust Page 3 of 6

4 and water vapour from the feed air. Air passes through the air preparation system and enters the ozone generator shell. In the shell some of the generated ozone reacts with nitrogen gas in the air to form nitrogen pentoxide (N 2 O 5 ). Nitrogen pentoxide by itself is not a problem, but if the air is moist, it reacts with the water vapour to form nitric acid (HNO 3 ). The nitric acid then deposits on the generator shell and piping, causing corrosion of the metal surface and possible contamination of product water (raising conductivity). Corrosion is the most serious problem resulting from wet air. Yet another problem with wet air is the reduction in ozone yield from the ozonator. For example, if the dewpoint of the incoming air increases from -60 F to - 40 F, the production rate of the ozonator will drop about 10%. With care and attention, an ozone generator can last 20 years or more. It is especially important to keep operating records and pay particular attention to the feed gas dewpoint. A proper dewpoint will improve operating efficiency of the ozonator, reduce the need for dielectric cleaning, and increase the life of the ozonator. Therefore, it is essential to regularly measure moisture in the feed air. There are three methods to do this: Blind Method, Lightning Storm Method, and Dewpoint Cup Method. Blind Method The Blind Method consists of comparing data on operating log sheet with values recommended in the operating manual. Important things to note on the log sheet regarding wet air are: Dryer pressure should be between 80 and 100 psig. Gas flow should be at the design rate and should be steady. Air dryer should be operating and regenerating at design pressure, switching from cell to cell every 30 seconds. The air preparation system, when properly designed, will dry the incoming air to a dewpoint of -60 C or lower. The drier the air, the more ozone is produced and the less maintenance is required. If one of the air preparation components is not working properly, it is unlikely that the overall air preparation system is maintaining the desired dewpoint. A daily log of the key operating points is well worth the time invested. Lightning Storm Method As mentioned earlier, if air is wet the corona around the dielectrics will be interlaced with many small sparks. Examine the corona every week and record it s appearance. If a small lightning storm is observed, make sure it surrounds all dielectrics more or less EVENLY BEFORE CONCLUDING THAT AIR IS WET. If the sparks surround only some dielectrics and not others, it may mean that the sparking dielectrics are dirty and should be cleaned. Dewpoint Cup Method The Dewpoint Cup Method is the most accurate and foolproof way of determining air moisture. Of the three methods, this is the only one that measures exactly the dewpoint of feed gas. The Dewpoint Cup Method requires a dewpoint cup (usually made of polished aluminium), a thermometer, dry ice and acetone. A mixture of dry ice and acetone is used to reduce the temperature of the cup, as an air stream (from the sample ozone outlet) is blown on the outer polished surface of the cup. At the first sign of a dew or moisture on the polished surface of the cup the temperature is read from the thermometer. This reading is the dewpoint. The dry ice/acetone mixture allows readings as low as -38 C. The Dewpoint Cup Method is simple to use and the equipment is relatively inexpensive (about $300). Check feed gas dewpoint once a month to make sure the air is properly dry. An operating dewpoint of -60 C or lower is recommended. Ultraviolet Disinfection Ultraviolet light (UV), light of wavelengths from 240 to 280 nanometres (nm), is an effective agent for killing bacteria. Low-pressure mercury-arc lamps provide a UV source to disinfect water by emitting high intensity radiation at 253. nm. Most UV disinfection employs a system where the lamps are sealed from the water. Lamps are positioned parallel to the flow of water. A cold cathode UV lamp must be maintained at 38 C, because its efficiency drops 50% with temperature fluctuation of + 4 C. Disinfection Dosage for UV The dose of UV absorbed by a micro-organism depends on: - the energy output of UV source - distance from source (energy dispersion) Page 4 of 6

5 - depth of water - UV absorption of water - UV losses in contact unit Table 4 provides data on UV dose needed to kill certain micro-organisms. Note that UV will not kill Giardia cysts or Cryptosporidium oocysts. Table 4 - Ultraviolet Energy Necessary to Inactivate Various Microorganisms Test Microorganism Lethal Dose (µ W s/cm 2 ) Escherichia coli Staphylococcus aureus Serratia marcescens Sarcina lutea Bacillus globiggii spores T3 coliphage Polio virus Vaccinia virus Semliki Forrest virus EMC virus ,250 1, A minimum dosage of 16,000 microwatt-second per square centimetre (µ W s/cm 2 ) with a maximum water depth of.5 cm (distance from lamp) is recommended by U.S. Public health Service (PHS). An excerpt of the U.S. PHS bulletin, Criteria for the Acceptability of a Ultra-Violet Disinfecting Unit, is provided: Ultraviolet radiation at a level of 253. nm must be applied at a minimum dosage of 16,000 microwatt-seconds per square centimetre at all points throughout the water disinfection chamber. Maximum water depth in the chamber, measured from the tube surface to the chamber wall, shall not exceed three inches. The ultraviolet tubes shall be: Jacketed so that a proper operating tube temperature of about 40 C is maintained, and The jacket shall be of quartz or high silica glass with similar optical characteristics. A time delay mechanism shall be provided to permit a two-minute tube warm-up period before water flows from the unit. One manufacturer recommends a 5-minute warm-up period. The unit shall be designed to permit frequent mechanical cleaning of the water contact surface of the jacket without disassembly of the unit. An automatic flow control valve, accurate within the expected pressure range, shall be installed to restrict flow to the maximum design flow of the treatment unit. An accurately calibrated ultraviolet intensity meter, properly filtered to restrict its sensitivity to the disinfection spectrum, shall be installed in the wall of the disinfection chamber at the point of greatest water depth from the tube or tubes. A flow diversion valve or automatic shut-off valve shall be installed which will permit flow into the potable water system only when at least the minimum ultra-violet dosage is applied. When power is not being supplied to the unit, the valve should be in a closed (fail-safe) position so as to prevent the flow of water into the potable system. An automatic audible alarm shall be installed to warn of malfunction or impending shutdown if considered necessary by the Control or Regulatory agency. The materials of construction shall not impart toxic materials into the water, either as a result of the pressure of toxic constituents in the materials of construction or as a result of physical or chemical changes resulting from exposure to ultra-violet energy. The unit shall be designed to protect the operator against electrical shock or excessive radiation. Page 5 of 6

6 Maintenance of UV System The following routine maintenance is recommended for UV disinfection systems. Wipe the quartz jacket of the lamp at least once a month. Let the ultraviolet lamps warm up for at least five minutes before allowing their use in treating water. Lamps should be replaced when the intensity meter indicates less than 50% of the rated lamp intensity. Check UV intensity of intensity of 253. nm daily. Replace quartz jacket when a new lamp intensity is diminished. (2-3 years) References: Ghuyre, G., and D.A. Clifford, Laboratory Study on the Oxidation of Arsenic (III), submitted to the USEPA, June 2000 Dennis Clifford, Removing Arsenic from Water The Importance of ph, Background Contaminants and Oxidation, Water Conditioning and Purification Magazine, August 2000, p.30 Arsenic in Drinking Water Treatment Technologies: Removal, USEPA Internet Site, July 2001 For Further Information: Australian Beverages Council Ltd info@australianbeverages.org Correct as at 1th October, Page 6 of 6