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In this article we will discuss about the methods and techniques used to sterilise and disinfect water. Learn about:- 1. Treatment Options for Sterilisation of Water 2. Sterilisation Using Ultraviolet Radiation 3. Sterilisation Using Electron Beam 4. Disinfection by Chlorination 5. Disinfection with Interhalogens and Halogen Mixtures 6. Sterilisation Using Ozone.
Contents:
- Treatment Options for Sterilisation of Water
- Sterilisation Using Ultraviolet Radiation
- Sterilisation Using Electron Beam
- Disinfection by Chlorination
- Disinfection with Interhalogens and Halogen Mixtures
- Sterilisation Using Ozone
1. Treatment Options for Sterilisation of Water:
Primary treatment of municipal waste involving settling and retention removes very few viruses. Sedimentation effects some removal. Virus removal of up to 90 per cent (which is a minimal removal efficiency) has been observed after the activated sludge step.
Further physical-chemical treatment can result in large reductions of virus titer, coagulation being one of the most effective treatments achieving as much as 99.9 per cent removal of virus suspended in water. If high pH (above 11) is maintained for long periods of time, 99.9 per cent of the viruses can be removed.
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Of all the halogens, chlorine at high doses (40 mg/l for 10 minutes) is very effective, achieving 99.9 per cent reduction. Lower doses (for example, 8 mg/l) result in no decrease in virus.
As a result of several studies, the following conclusions regarding viruses in sewage warrant consideration:
(i) Primary sewage treatment has little effect on enteric viruses,
(ii) Secondary treatment with trickling filters removes only about 40 per cent of the enteroviruses,
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(iii) Secondary treatment by activated sludge treatment effectively removes 90 per cent to 98 per cent of the viruses, and
(iv) Chlorination of treated sewage effluents may reduce, but may not eliminate, the number of viruses present.
The current concept of disinfection is that the treatment must destroy or inactivate viruses as well as bacillary pathogens. Under this concept, the use of coliform counting as an indicator of the effectiveness of disinfection is open to severe criticism given that coliform organisms are easier to destroy than viruses by several orders of magnitude.
An important concept is that a single disinfectant may not be capable of purifying water to the desired degree. Also, it might not be practicable or cost effective. This has given rise to a variety of treatment combinations in series or in parallel. The analysis further indicates that the search for the perfect disinfectant for all situations is a sterile exercise.
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It has been estimated that in the United States only 60 per cent of municipal waste effluent is disinfected prior to discharge and, in a number of cases, only on a seasonal basis. Coupling this fact with the demonstration that various sewage treatment processes achieve only partial removal of viruses leaves us with a substantial problem to resolve.
Non-Conventional Treatment Methods:
Electromagnetic radiation is the propagation of energy through space by means of electric and magnetic fields that vary in time. Electromagnetic radiation may be specified in terms of frequency, vacuum wavelength or photon energy. For water purification, EM waves up to the low end of the UV band will result in heating the water. (This includes infrared as well as most lasers.)
In the visible range, some photochemical reactions such as dissociation and increased ionisation may take place. At the higher frequencies, it will be necessary to have thin layers of water because the radiation will be absorbed in a relatively short distance.
The conductivity and dielectric constant of materials are, in general, frequency dependent. In case of the dielectric constant, it decreases as 1/wavelength. Hence, the electromagnetic absorption will vary with the frequency of the applied field. There may be some anomalies in the absorption spectra in the vicinity of frequencies that could excite molecules. At those frequencies, the absorption could be unusually large.
Ultraviolet radiation in the region between 0.2 μ to 0.3 μ has germicidal properties. The peak germicidal wavelength is around 0.26 μ. This short UV is attenuated in air and hence, the source must be very near the medium to be treated. The medium must be very thin as the UV will be attenuated in the medium as well.
X-rays and gamma rays are high-energy photons and will tend to ionise anything with which they collide. They could generate UV in air. At higher energies it is possible for the gamma rays to induce nuclear reactions by stripping protons or neutrons from nuclei. This could result in the production of isotopes and/or the production of new atoms.
A sound wave is an alteration in pressure, stress, particle displacement, particle velocity or a combination of these that is propagated in an elastic medium. Sound waves, therefore, require a medium for transmission; that is, they may not be transmitted in a vacuum.
The sound spectrum covers all possible frequencies. The average human ear responds to frequencies between 16 Hz and 16 kHz. Frequencies above 20 kHz are called ultrasonic frequencies. Sound waves in the 50-200 kHz range are used for cleaning and degreasing.
In water purification applications, ultrasonic waves have been used to effect disintegration by cavitation and mixing of organic materials. The waves themselves have no germicidal effect but, when used with other treatment methods, can provide the necessary mixing and agitation for effective purification.
The electron is the lightest stable elementary particle of matter known and carries a unit of negative charge. It is a constituent of all matter and can be found free in space. Under normal conditions, each chemical element has a nucleus consisting of a number of neutrons and protons, the latter equal in number to the atomic number of the element.
Electrons are located in various orbits around the nucleus. The number of electrons is equal to the number of protons and the atom is electrically neutral when viewed from a distance. The number of electrons that can occupy each orbit is governed by quantum mechanical selection rules.
The binding energy between an electron and its nucleus varies with the orbit number and in general the electrons with the shortest orbit are the most tightly bound. An electron can be made to jump from one orbit into another by giving it a quantum of energy.
This energy quantum is fixed for any given transition and whether a transition will occur is again governed by selection rules. In other words, although an electron is given a quantum of energy sufficient to raise it to an adjacent higher state, it will not go up to that state if the transition is not permitted.
In that case, it is theorised that if the electron absorbs the quantum, it will most probably go up to the excited state, remain there for a time allowed by the uncertainty principle, reradiate the quantum and return to its original state. If an electron is given a sufficiently large quantum of energy, it will completely leave the atom.
The electron will carry off as kinetic energy the difference between the input quantum and the energy required to ionise. The remaining atom will now become a positively charged ion and the stripped electron will become a free electron. This electron may have sufficient energy when it leaves the atom (or it may acquire sufficient energy from an external field) to collide with another atom and strip it of an electron.
This is the basis for electric discharge where free electrons are accelerated by an applied field and as they collide with neutral atoms, generate additional free electrons. This process avalanches as the electrons approach the positive electrode.
At the same time, the positively charged ions are accelerated toward the negative electrode. In a vacuum, when a voltage is applied between two electrodes, electrons will move from the cathode to the anode. Of course, in a vacuum there will be no avalanching effects.
Electrons are emitted from the cathode by a number of mechanisms:
1. Thermionic Emission:
Because of the non-zero temperature of the cathode, free electrons are continuously bouncing inside. Some of these have sufficient energy to overcome the work function of the material and can be found in the vicinity of the surface. The cathode may be heated to increase this emission. Also to enhance this effect, cathodes are usually made of or coated with, a low work-function material such as thorium.
2. Shottky Emission:
This is also a thermionic type of emission except that in this case, the applied electric field effectively decreases the work function of the material and more electrons can then escape.
3. High Field Emission:
In this case, the electric field is high enough to narrow the work-function barrier and allow electrons to escape by tunneling through the barrier.
4. Photoemission:
Electromagnetic radiation of energy can cause photoemission of electrons whose maximum energy is equal to or larger than the difference between the photon energy and the work function of the material.
5. Secondary Emission:
Electrons striking the surface of a cathode could cause the release of some electrons and hence, a net amplification in the number of electrons. This principle is used in the construction of photomultipliers where light photons strike a photoemitting cathode releasing photoelectrons. These electrons are subsequently amplified striking a number of electrodes (called dynodes) before they are finally collected by the anode.
In a high-gradient magnetic separator, the force on a magnetised particle depends on the intensity of the magnetising field and on the gradient of the field. When a particle is magnetised by an applied magnetic field, the particle develops an equal number of north and south poles.
Hence, in a uniform field, a dipolar particle experiences a torque, but not a net tractive force. In order to develop a net tractive force, a field gradient is required; that is, the induced poles at the opposite ends of the particle must view different magnetic fields.
In a simplified, one-dimensional case, the magnetomotive force on a particle is given by:
Fm = μ(δH/δx) = MV(δH/δx) = χHV(δH/δx)
Where, μ is the magnetic moment of the particle under field intensity, HδH/δx is the field gradient. The magnetic moment μ is the product of the magnetisation of the particle and its volume (μ = MV). And magnetisation is the product of the particle susceptibility, χ and the field intensity, H. In water purification, this magnetic force may be used to separate magnetisable particles.
Direct and Alternating Currents:
Electrolytic treatment is achieved when two different metal strips are dipped in water and a direct current is applied from a rectifier. The higher the voltage the greater the force pushing electrons across the gap between the electrodes. If the water is pure, very few electrons cross the path between the electrodes.
Impurities increase conductivity, hence decreasing the required voltage. Additionally, chemical reactions occur at both the cathode and the anode. The major reaction taking place at the cathode is the decomposition of water with the evolution of hydrogen gas.
The anode reactions are oxidations by four major means:
(i) Oxidation of chloride to chlorine and hypochlorite,
(ii) Formation of highly oxidative species such as ozone and peroxides,
(iii) Direct oxidation by the anode, and
(iv) Electrolysis of water to produce oxygen gas.
A great deal of interest was generated in the United States prior to 1930 in electrolytic treatment of waste-water, but all plans were abandoned because of high cost and doubtful efficiency. Such systems were based on the production of hypochlorite from existing or added chloride in the waste-water system. A great deal of effort has been made in re-evaluating such techniques.
Reduction in Number of Viable Micro-Organisms by Adsorption onto the Electrodes:
Protein and micro-organism adsorption on electrodes with anodic potential has been documented. Microorganism adsorption on passive electrodes (in the absence of current) has been observed with subsequent electrochemical oxidation. This does not appear to be a major route for inactivation.
Electrochemical Oxidation of the Micro-Organism Components at the Anode:
Oxidation of various viruses due to oxidation at the surface of the working electrode has been indicated, although the peak voltage used in many experiments would not be sufficient for the generation of molecular or gaseous oxygen.
Destruction of the Micro-Organisms by Production of a Biocidal Chemical Species:
It has been shown that NaCl is not needed for effective operation in the destruction of micro-organisms. Biocidal species such as CI, HO–, O, CIO and HOCI occur but have very low diffusion coefficients. Hence, if this phenomenon occurs, the probability is that organisms are destroyed at the electrode surface rather than in the bulk solution.
Destruction by Electric Field Effects:
It has been observed that some organisms are killed in midstream without contact with the electrodes. The organisms were observed to oscillate in phase with the electric field. Hence, micro-organism kill can also be ascribed to changes caused by changing electromotive forces resulting from the impressed AC.
In the typical operation, a magnetised fine-particle seed (typically iron oxide) and a flocculent (typically aluminum sulphate) are added to the waste-water, prompting the formation of magnetic microflocs. The stream then flows through a canister packed with stainless steel wire and a magnetic field is applied. The stainless steel wool captures the floes by magnetic forces.
2. Sterilisation
Using Ultraviolet Radiation:
It has been shown that:
1. Ultraviolet radiation around 254 mm renders bacteria incapable of reproduction by photochemically altering the DNA of the cells.
2. A fairly low dose of ultraviolet light can kill 99 per cent of the fecal coliform and fecal streptococcus.
3. Bacterial kill is independent of the intensity of the light but depends on the total dose.
4. Simultaneous treatment of water with UV and ozone results in higher micro-organism kill than independent treatment with both UV and ozone.
5. When ultrasonic treatment was applied before treating with the UV light, a higher bacteria kill was obtained.
6. The UV dose required to reduce the survival fraction of total coliform and fecal streptococcus to 102 (99 per cent removal) is approximately 4×10 ff Einsteins/ml.
Some limitations are associated with UV radiation for disinfection.
These include:
(i) The process performance is highly dependent on the efficacy of upstream devices that remove suspended solids,
(ii) Another key factor is that the UV lamps must be kept clean in order to maintain their peak radiation output and,
(iii) A further drawback is associated with the fact that a thin layer of water (< 0.5 cm) must pass within 5 cm of the lamps.
One way of implementing the UV disinfection process at existing activated sludge plants involves suspending the UV lights (in the form of low-pressure mercury arc UV lamps with associated reflectors) above the secondary clarifiers. The effluent is exposed to the UV radiation as it rises over the wire in a thin film.
3. Sterilisation
Using Electron Beam:
The idea of using ionising radiation to disinfect water is not new. Ionising radiations can be produced by various radio-active sources (radioisotopes), by X-ray and particle emissions from accelerators and by high-energy electrons. The advances in reliable, relatively low-cost devices for producing high-energy electrons are more significant.
Unlike X-rays and gamma rays, electrons are rapidly attenuated. The maximum range of a 1 million- volt electron is about 4 m in air and about 5 cm in water. In transit in matter, an electron loses energy through collisions that ionise atoms and molecules along its path.
Bacteria and viruses are destroyed by the secondary ionisation products produced by the primary traversing electron. The energetic electrons dissociate water into free radicals H+ and OH–. These may combine to form active molecules-hydrogen, peroxides and ozone.
These highly active fragments and molecules attack living structures to promote their oxidation, reduction, dissociation and degradation. Studies have indicated that 4,00,000 rads would be adequate for sewage disinfection.
At 100 ergs per gram rad, 4,00,000 rads would raise the temperature of the water or sludge by 1°C. At this dose, each cm2 of moving sludge would receive about 12 x 1012 electrons, each electron producing some 30000 secondary ionisations.
4. Disinfection by Chlorination
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Disinfection has received increased attention over the past several years from regulatory agencies through the establishment and enforcement of rigid bacteriological effluent standards. In upgrading existing waste-water treatment facilities, the need for improved disinfection as well as the elimination of odour problems are frequently encountered.
Adequate and reliable disinfection is essential in ensuring that waste-water treatment plants are both environmentally safe and aesthetically acceptable to the public. Chlorine is the most widely used disinfectant in water and waste-water treatment. It is used to destroy pathogens, control nuisance micro-organisms and for oxidation.
As an oxidant, chlorine is used in iron and manganese removal, for destruction of taste and odour compounds and in the elimination of ammonia nitrogen. It is, however, a highly toxic substance and recently concerns have been raised over handling practices and possible residual effects of chlorination.
Germicidal Destruction by Chlorination:
Chlorine’s ability to destroy bacteria and various micro-organisms results from chemical interference in the functioning of the organism. Specifically, it is the chemical reaction between HOCl and the bacterial or viral cell structure which inactivates the required life processes.
The high germicidal efficiency of HOCl is attributed to the ease by which it is able to penetrate cell walls. This penetration is comparable to that of water and is due both to its low molecular weight (that is, it’s a small molecule) and its electrical neutrality. Organism fatalities result from a chemical reaction of HOCl with an enzyme system in the cell which is essential to the metabolic functioning of the organism.
The enzyme attacked is triosephosphate dehydrogenase, found in most cells and essential for digesting glucose. Other enzymes also undergo attack. However, triosephosphate dehydrogenase is particularly sensitive to oxidising agents. The OCl– ion resulting from the dissociation is a relatively poor disinfectant because of its inability to diffuse through a micro-organism’s cell walls.
This is because of its negative charge. The sensitivity of bacteria to chlorination is well-known. However, the effect on protozoans and viruses has not been entirely delineated. Protozoal cysts and enteric viruses are more resistant to chlorine than are coliforms and other enteric bacteria.
Water chlorination is carried out by using both free and combined residuals. The latter involves chlorine application to produce chloramine with natural or added ammonia. Anhydrous ammonia is used if insufficient natural ammonia is present in the waste-water.
Although the combined residual is less effective than free chlorine as a disinfectant, its most common application is as a post-treatment following free residual chlorination to prove initial disinfection.
Free residual chlorination establishes a free residual through the destruction of naturally present ammonia. High dosages of chlorine applied during treatment may result in residuals that are esthetically objectionable or undesirable for industrial water use.
Dechlorination is sometimes performed to reduce the chlorine residual by adding a reducing agent (called a dechlor). Sulphur dioxide is often used as the dechlor in municipal plants. Aeration by submerged or spray aerators also diminishes the residual chlorine concentration.
The chlorine used for disinfestation is available in three forms- liquified compressed gas, calcium hypochlorite or sodium hypochlorite and chlorine bleach solutions. Liquid chlorine is shipped in pressurised steel cylinders with sizes typically 100 and 500 lb; one-tonne containers are used in large installations.
There are two types of chlorine dispensing system- direct feed and solution feed. The first involves metering dry chlorine gas and conducting it under pressure to the water. Solution-feed systems meter chlorine gas under vacuum and dissolve it in a small amount of water, forming a concentrated solution which is then applied to the water being treated.
At 20°C, 1 volume of water dissolves 2.3 volumes of chlorine gas (about 7000 mg/l). At concentrations of total chlorine below 1000 mg/l, none of the gas exists in solutions as Cl2; all of it is present as HOCl or dissociated ions calcium hypochlorite is a dry bleach which is available in granular and tablet forms.
Calcium hypochlorite is relatively stable under normal conditions; however, it can undergo reactions with organic materials. It should be stored in an isolated area. Sodium hypochlorite is available in liquid form. It is marketed in carboys and rubber- lined drums for small quantities.
Sodium hypochlorite solutions are highly corrosive, unstable and require storage at temperatures below 85°F. Sodium hypochlorite can either be delivered to the site in liquid form in 500-5000 gallon tank cars or trucks or manufactured on site. It is normally sold at a concentration of 12 per cent to 15 per cent by weight of available chlorine. It can be manufactured on site from salt or from sea water.
The main component in a chlorine gas feed is the variable orifice inserted in the feed line to control the rate of flow out of the cylinder. The orifice basically consists of a grooved plug sliding in a fitted ring. Feed rate is adjusted by varying the V-shaped opening.
Since a chlorine cylinder pressure varies with temperature, the discharge through such a throttling valve does not remain constant without frequent adjustments of the valve setting. Also, conditions on the outlet side vary with pressure changes at the point of application.
Therefore, a pressure-regulating valve is used between the cylinder and the orifice, with a vacuum-compensating valve on the discharge side. A safety pressure-relief valve is held closed by vacuum.
Chlorine feeders can be controlled either manually or automatically based on flow or chlorine residual or both. In manual mode, a continuous feed rate is established. This is satisfactory when chlorine demand and flow are relatively constant and where operators are available to make adjustments.
Automatic proportional control equipment is used to adjust the feed rate to provide a constant pre-established dosage for all rates of flow. This is accomplished by metering the main flow and using a transmitter to signal a chlorine feeder. An analyser located downstream from the point of application is used to monitor the chlorinator.
Combined automatic flow and residual control maintain a present chlorine residual in the water that is independent of the demand and flow variations. The feeder is designed to respond to signals from both the flow meter transmitter and the chlorine residual analyser.
For hypochlorite solutions, positive-displacement diaphragm pumps (either mechanically or hydraulically actuated) are used. The hypochlorinator consists of a water-powered pump paced by a positive-displacement water meter. The meter register shaft rotates proportionately to the main line flow and controls a cam-operated pilot valve.
This in turn regulates water now discharged of hypochlorite that is proportional to the main flow. Admitting main pressure behind the pumping diaphragm balances the water pressure in the pumping head. The advantage of this system is that the pump does not need electrical power. The hypochlorite dosage can be manually adjusted by changing the stroke length setting of the pump.
5. Disinfection with Interhalogens and Halogen Mixtures
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The interhalogen compounds are the bromine- and iodine-base materials. It is the larger, more positive halogen that is the reactive portion of the interhalogen molecule during the disinfection process. Although only used on a limited basis at present, there are members of this class that show great promise as environmentally safe disinfectants.
Disinfection with Bromine Chlorine:
In chlorination, chlorine’s reaction with ammonia forms chloramines, greatly reducing its bactericidal and virucidal effectiveness. The biocidal activity of monochloramine is only 0.02-0.01 times as great as that of free chlorine.
Typical ammonia concentrations found in secondary sewage range from 5-20 ppm, which is about an order of magnitude greater than the amount needed to form monochloramine from normal chlorination dosages (which requires about 5-10 ppm).
Therefore, monochloramine is the major active chlorine constituent in chlorinated sewage plant effluents. In contrast, BrCl ammonia reactions produce the major product bromamines. Bromamines have disinfectant characteristics which are significantly different than chloramines.
Disinfection with Iodine Compounds:
Two interhalogens having strong disinfecting properties are iodine monochloride (ICl) and iodine bromide (IBr). Iodine monochloride has found use as a topical antiseptic. It may be complexed with nonionic or anionic detergents to yield bactericides and fungicides that can be used in cleansing or sanitising formulations.
These generally have a polymer structure which establishes its great stability, increased solubility and lower volatility. By reducing the free halogen concentration in solution, polymers reduce both the chemical and bactericidal activity. Complexes of ICl are useful disinfectants which compromise lower bactericidal activity with increased stability.
Iodine monochloride is itself a highly reactive compound, reacting with many metals to produce metal chlorides. Under normal conditions it will not react with tantalum, chromium, molybdenum, zirconium, tungsten or platinum. With organic compounds, reactions cause iodination, chlorination, decomposition or the generation of halogen addition compounds.
In water, ICI hydrolyses to hypoiodous and hydrochloric acids. In the absence of excess chloride ions, hypoiodous acid will disproportionate into iodic acid and iodine. Iodine bromide has a chemistry similar to ICl. Iodine bromide reacts with aromatic compounds to produce iodination in polar solvents and bromination in nonpolar solvents.
It has complex chemical properties, as its solubility is increased more effectively by bromide than by chlorided ions. Primary hydrolysis takes place in the presence of hydrobromic acid. As a disinfectant, IBr is used in its complexed or stabilised forms.
Unfortunately, it undergoes hydrolysis and dissociation reactions in aqueous solutions, both reactions being major limitations. Its disinfecting properties are similar to ICl and as in the case of ICl, germicidal activity should not be reduced by haloamine formation since bromamines are highly reactive and iodoamines are not generated.
Upon application of prepared solutions to control micro-organisms, the complex releases IBr gradually. This process forms free iodine during the decomposition of IBr (the decomposition takes place as fast as the IBr is released).
Disinfection with Halogen Mixtures:
Two approaches that have been investigated recently for disinfection are mixtures of bromine and chlorine and mixtures containing bromide or iodide salts. Some evidence exists that mixtures of bromine and chlorine have superior germicidal properties than either halogen alone.
It is believed that the increased bacterial activity of these mixtures can be attributed to the attacks by bromine on sites other than those affected by chlorine.
The oxidation of bromide or iodide salts can be used to prepare interhalogen compounds or the hypollalous acid in accordance with the following reaction:
HOCl + NaBr ® HOBr + HCl
It has been reported that the rate of bacterial sterilisation by chlorine in the presence of ammonia is accelerated with small amounts of bromides. As little as 0.25 ppm of bromamines can be significant under some conditions.
However, if chloramines are produced prior to contact with bromide ions, the reaction and subsequent effect are reduced. Improved germicidal activity has also been shown for mixtures containing bromides and iodides with various chlorine releasing compounds.
Bromide improves the disinfecting properties of dichloroisocyanuric acid and hypochlorite against several bacteria. Bromine-containing compounds are useful for their combined bleaching and disinfectant properties. There has been the concern that the use of interhalogen compounds in waste-water disinfection could produce unknown organic and inorganic halogen-containing substances.
In the case of iodine, concern has been expressed over the physiological aspects in water supplies. Extensive studies have been reported on the role played by iodine and iodides in the thyroid glands of animals and man. Information on acute inhibition of hormone formation by excessive amounts of iodine is well known.
Despite the fact that no strong evidence exists that iodine is harmful as a water disinfectant, only limited use has been attempted. Chronic bromide intoxication from continuous exposure to dosages above 3-5 gram is called bromism.
Typical symptoms are skin rash, glandular excretions, gastrointestinal disturbances and neurological disturbances. Bromide can be absorbed from the intestinal tract and contaminate the body in a manner very similar to that for chloride.
Brominated drinking water does not, however, significantly increase the amount of bromine admitted internally. The amount of additional bromine in chlorobrominated waters will not significantly increase human bromine concentrations nor result in bromism.
6. Sterilisation Using Ozone
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Ozone (O3) is a powerful oxidant and application to effluent treatment has developed slowly because of relatively high capital and energy costs compared to chlorine. Energy requirements for ozone are in the range of 10 to 13 kWh/lb generated from air, 4 kWh/lb from oxygen and 5.5 kWh/lb from oxygen- recycling systems. Operating costs for air systems are essentially the electric power costs; for oxygen systems the cost of oxygen must be added to the electrical cost.
Actual uses of ozone include odour control, industrial chemicals synthesis, industrial water and waste-water treatment and drinking water. Lesser applications appear in fields of combustion and propulsion, foods and pharmaceuticals, flue gas-sulphur removal and mineral and metal refining. Potential markets include pulp and paper bleaching, power plant cooling water and municipal waste-water treatment.
The odour control market is the largest and much of this market is in sewage treatment plants. Use of ozone for odour control is comparatively simple and efficient. The application is for preservation of environmental quality; in addition, alternative treatment schemes requiring either liquid chemical oxidants (like permanganate or hydrogen perioxide) or incineration can significantly increase capital and costs.
Ozone applications in the United States for drinking water are far fewer than in Europe. However, the potential market is large, if environmental or health needs ever conclude that an alternate disinfectant to chlorine should be required.
Although energy costs of ozonation are higher than those for chlorination, they may be comparable to combined costs of chlorination dechlorination-reaeration, which is a more equivalent technique. One of ozone’s greatest potential uses is for municipal waste-water disinfection.
Technical, economic and environmental advantages exist for ozone bleaching of pulp in the paper industry as an alternate to hypochlorite or chlorine bleaching which yields deleterious compounds to the environment.
Ozone has been used continuously for nearly 90 years in municipal water treatment and the disinfection of water supplies. This practice began in France, then extended to Germany, Holland, Switzerland and other European countries and in recent years to Canada and other developing countries.
Ozone is a strong oxidising substance with bactericidal properties similar to those of chlorine. In test conditions it was shown that the destruction of bacteria was between 600 and 3000 times more rapid by ozone than by chlorine. Further, the bactericidal action of ozone is relatively unaffected by changes in pH while chlorine efficacy is strongly dependent on the pH of the water.
Ozone’s high reactivity and instability as well as serious obstacles in producing concentrations in excess of 6 per cent preclude central production and distribution with its associated economies of scale.
In the electric discharge (or corona) method of generating ozone, an alternating current is imposed across a discharge gap with voltages between 5 and 25 kV and a portion of the oxygen is converted to ozone. A pair of large-area electrodes are separated by a dielectric (1-3 mm in thickness) and an air gap (approximately 3 mm). Although standard frequencies of 50 or 60 cycles are adequate, frequencies as high as 1000 cycles are also employed.
The mechanism for ozone generation is the excitation and acceleration of stray electrons within the high-voltage field. The alternating current causes the electron to be attracted first to one electrode and then to the other. As the electrons attain sufficient velocity, they become capable of splitting some oxygen molecules into free radical oxygen atoms. These atoms may then combine with O2 molecules to form O3.
Besides the disinfection of sewage effluent, ozone is used for sterilising industrial containers such as plastic bottles, where heat treatment is inappropriate. Breweries use ozone as an antiseptic in destroying pathogenic ferments without affecting the yeast.
It is also used in swimming pools and aquariums. It is sometimes used in the purification and washing of shellfish and in controlling slimes in cooling towers. Ozone has also been shown to be quite effective in destroying a variety of refractory organic compounds.
Principles of Ozone Effluent Treatment:
Ozone was first discovered by the Dutch philosopher Van Marun in 1785. In 1840, Schonbein reported and named ozone from the Greek word ozein, meaning to smell. The earliest use of ozone as a germicide occurred in France in 1886, when de Meritens demonstrated that diluted ozonised air could sterilise polluted water.
In 1893, the first drinking water treatment plant to use ozone was constructed in Oudshorm, Holland. Other plants quickly followed at Wiesbaden and Paderborn in Germany. In 1906, a plant in Nice, France, was constructed using ozone for disinfection.
Today, there are over 1000 drinking water treatment plants in Europe utilising ozone for one or more purposes. In the United States, the first ozonation plant was constructed in Whiting, Indiana, in 1941 for taste and odour control.
Over 100 years ago it had been demonstrated that ozone (O3), the unstable triatomic allotrope of oxygen, could destroy moulds and bacteria and by 1892 several experimental ozone plants were in operation in Europe.
In the 1920s, however, as a result of wartime research, during World War I, chlorine became readily available and inexpensive and began to displace ozone as a purifier in municipalities throughout the United States.
Most ozone studies and developments were dropped at this time, leaving ozonation techniques, equipment and research at a primitive stage. Ozone technology stagnated and the development and acceptance of ozone for water and waste-water treatment was discontinued.
In addition to the popular use of chlorination as a waste-water disinfectant and the consequent technology lag in ozonation research, there was a third impediment to ozone commercialisation: the comparatively high cost of ozonation in relation to chlorination.
Ozone’s instability requires on-site generation for each application, rather than centralised generation and distribution. This results in higher capital requirements, aggravated by a comparatively large electrical energy requirement. Ozone’s low solubility, in water and the generation of low concentrations, even under ideal conditions, also necessitates more elaborate and expensive contacting and recycling systems than chlorination.
In spite of such obstacles there is interest from time to time in the use of ozone, particularly for waste-water treatment. The technology for the destruction of organics and inorganics in water has not kept pace with the increasingly more sophisticated water pollution problems arising from greater loads, new products and new sources of pollutant entry into the environment and increased regulation.
The growing trend toward water reuse and the fact that some highly toxic pollutants may be refractory to conventional treatment methods has spurred investigation into new treatments, including ozonation.
A significant impetus from time to time for developing new methods is dissatisfaction with chlorination. Chlorine affects taste and odour and produces chloramines and a wide variety of other potentially hazardous chlorinated compounds in waste-waters.
It seriously threatens the environment with an estimated 1000 tonnes per year of chlorinated organic compounds discharged into U.S., waters (chloramines are not easily degradable and pose a hazard to the environment) and is questionable as a drinking water viricidal disinfectant. Ozone’s development, on the other hand, could parallel a greater environmental awareness and a resulting demand for higher-quality effluents, as its potential for overcoming these problems is possible.
Ozonation Equipment and Processes:
Ozonation systems are comprised of four main parts, including a gas-preparation unit, an electrical power unit, an ozone generator and a contactor which includes an off-gas treatment stage. Ancillary equipment includes instruments and controls, safety equipment and equipment housing and structural supports.
A high level of gas preparation (usually air) is needed before ozone generation. The air must be dried to retard the formation of nitric acid and to increase the efficiency of the generation. Moisture accelerates the decomposition of ozone. Nitric acid is formed when nitrogen combines with moisture in the corona discharge.
Since nitric acid will chemically attack the equipment, introduction of moist air into the unit must be avoided. Selection of the air-preparation system depends on the type of contact system chosen. The gas-preparation system will, however, normally include refrigerant gas cooling and desiccant drying to a minimum dew-point of -40°C. A dew-point monitor or hygrometer is an essential part of any air preparation unit.
Conversion efficiencies can be greatly increased with the use of oxygen. However, the use of high- purity oxygen far ozone generation for disinfection is cost effective.
Electrical power supply units vary considerably among manufacturers. Power consumption and ozone-generation capacity are proportional to bath voltage and frequency. There are two methods to control the output of an ozone generator: vary voltage or vary frequency.
Three common electrical power supply configurations are used in commercial equipment:
1. Low frequency (60 Hz), variable voltage.
2. Medium frequency (600 Hz), variable voltage.
3. Fixed voltage, variable frequency.
The most frequently used is the constant low-frequency, variable-voltage configuration. For larger systems, the 600 Hz fixed frequency is often employed as it provides double ozone production with no increase in ozone generator size.
The electrical (corona) discharge method is considered to be the only practical technique for generating ozone in plant-scale quantities. In principle, an ozone generator consists of a pair of electrodes separated by a gas space and a layer of glass insulator.
An oxygen-rich gas is passed through the empty space and a high voltage alternating current is applied. A corona discharge takes place across the gas space and ozone is generated when a portion of the oxygen is ionised and then becomes associated with non-ionised oxygen molecules.
Typical horizontal tube-type ozone generator unit is preferred for larger systems. Water-cooled plate units are often used in smaller operations. However, these require considerably more floor space per unit of output than the tube-type units.
The air-cooled Lowther plate type is a relatively new design. It has the potential for simplifying the use of ozone-generating equipment. However, it has had only limited operating experience in water treatment facilities.
After the ozone has been generated, it is mixed with the water stream being treated in a device called a contactor. The objective of this operation is to maximise the dissolution of ozone into the water at the lowest power expenditure. There is a variety of ozone contactor designs.
Principal ones employed in waste-water treatment facilities include:
1. Multistage porous diffuser contactors, which involve a single application of an ozone-rich gas stream and application of fresh ozone gas to second and subsequent stages with off-gases recycled to the first stage.
2. Eductor-induced, ozone vacuum injector contactors, which include total or partial plant flow through the eductor; and subsequent stages with off-gases recycled to the first stage.
3. Turbine contactors, which involve positive or negative pressure to the turbine.
4. Packed-bed contactors, which include concurrent or countercurrent water/ozone-rich gas flow.
Two-level diffuser contactors, which involve application of ozone-rich gas to the lower chamber.
Lower chamber off-gases are applied to the upper chamber. Off-gas treatment from contactors is an important consideration. Methods employed for off-gas treatment include dilution, destruction via granular activated carbon, thermal or catalytic destruction and recycling.