Mulifunctional disinfectant, de-odouriser and coagulant which leaves no residues
By now, everybody in the HVAC industry will be aware that the draft regulations concerning the phasing out and management of ozone depleting substances in South Africa were published in the government gazette on 14th January this year. The fact that South Africa is accepting its share of responsibility for ensuring the continuing self-regeneration of the damaged ozone layer is commendable. Ground-based and satellite monitoring continue to measure the steady progress of the ozone layer with precision as it recovers from the damages caused by ozone depleting substances, particularly chlorinated fluorocarbon chemicals [CFCs] which were first manufactured commercially way back in the 1930s. By the early 1980s, recognition of the potentially catastrophic global consequences of the ozone layer becoming ineffective focussed attention on ozone itself which had, for nearly 100 years been applied to various processes with only limited and sporadic successes. If ozone was succeeding so well in protecting our existence on earth why was it not showing more successes in other applications? To answer this question many researchers began to look again at ozone itself.
What is ozone?
Chemists say that oxygen forms “allotropes”, which is just a way of saying that oxygen can exist in different forms. Normal oxygen gas [or liquid under high pressure] is the familiar stable O2; two atoms of oxygen joined together; the 20% of the atmosphere which reacts with [oxidises] iron, copper and other metals. Single atoms of oxygen, O1, called “nascent” or “atomic” oxygen, exist but only very briefly. A good example illustrating this is when water, which always contains small amounts of dissolved oxygen, heats up and releases atoms of O1 from solution which immediately either combine with each other making O2 or oxidise any metals they come into contact with causing an extremely rapid form of corrosion known as “oxygen corrosion”. Under the influence of UV radiation or electrical arcs, O2 will react to form the ozone allotrope, O3, which is also not stable but changes back to O2 more slowly than O1 does. Incoming UV radiation from the sun causes two different effects in the ozone layer around the earth. Shorter wavelength UV creates two O3 atoms from three O2 atoms whilst longer wavelength UV is the energy absorbed by O3 atoms when cracking them back to become O2 atoms again. The balance between these two UV effects maintains the total amount of ozone in the layer which serves as a barrier absorbing much of the UV from the sun which would otherwise be severely damaging to life on the surface of the earth.
In the atmosphere, ozone is slightly heavier than oxygen so it tends to sink slowly towards the ground. However, because ozone is inherently unstable, by the time it reaches ground level its concentration in the atmosphere is generally less than 1 part per million which is below the normal odour detection threshold of people. Similar small amounts of ozone are formed at ground level by residual solar UV causing reactions between nitrates and sulphates in the air and hydrocarbons of animal and plant origin. During the second half of the twentieth century, due to the build-up of nitrate and sulphate airborne pollutants in urban industrial areas prior to legislation limiting polluting emissions, ozone concentration increased to a level where its odour became noticed and was erroneously labelled as the odour of “pollution” resulting in a perception that ozone was the main culprit in smog formation. In reality, ozone has a characteristically “clean” aroma when it is formed in nature by lightening, or is artificially manufactured in an unpolluted environment. It is theoretically possible but not practical to store ozone in very high pressure cylinders. Consequently, ozone is manufactured on site either by specific UV lamps, water electrolysis or the now widely used method of corona discharge. Electrolysis is not an energy-efficient process so the two most common methods for ozone production are UV for small amounts, mainly used for air purification, and corona discharge when larger quantities are required. Figure 1 is a schematic of a corona discharge unit which is fed either by air in which the 20% oxygen content is sufficient for the amount of ozone needed, or, if higher amounts of ozone are required then pure oxygen is supplied to the corona unit.
Figure 1. Schematic of a corona discharge unit for manufacturing Ozone. Picture courtesy of Ozone Service Industries Pty Ltd
Alternating electrical potentials measured in kV are applied between the high voltage and earth electrodes at kHz frequencies. Total electrical power demand varies from small fractional kilowatt units, increasing for larger units according to rates of ozone production required. Corona discharge systems using air produce ozone in gas phase concentrations of 1 - 5% by weight and up to 14% by weight if using high purity oxygen instead of air.
The Electrochemical Oxidation Potential [EOP] of a substance, measured in volts, is a good indicator of how strongly and how quickly the substance will act as a disinfectant. Figure 2 shows 9 of the highest EOP values of disinfectant substances. Fluorine has the highest EOP but is very costly and difficult to use within current technology. The next highest EOP substance, the Hydroxyl-radical is not a substance by itself but is produced when air contacts the surfaces of a titanium grid under specific wavelength UV radiation. Disinfecting activity of the Hydroxyl-radical is rapid but confined to micro-organisms coming into contact with the titanium grid surfaces. Next comes Oxygen [atomic], the O1 previously noted which, in practice, reverts back to the O2 form too rapidly to provide practical disinfection. Only slightly below Oxygen [atomic] is Ozone with an EOP value of 2.08V which is almost twice the EOP of normal O2 at 1.23V. [As a disinfectant, O2 oxygen is poisonous only to anaerobic bacteria which limits its use to a few specialised applications.]
Figure 2. Provided by Ozone Service Industries Pty Ltd.
A more accurate way to assess the disinfecting power of ozone is to compare its EOP with that of chlorine which is still the most commonly used disinfectant worldwide. The EOP of ozone at 2.42V is nearly twice the 1.36 EOP of chlorine indicating that ozone has almost twice the oxidising power of chlorine. Furthermore, many direct practical tests have shown that ozone disinfects by oxidation at a faster rate than chlorine. Also, ozone oxidises the organic debris resulting from dead bacteria and other micro-organisms as well as any other organic and inorganic substances with which it comes into contact. When all this happens in an aqueous environment then ozone delivers another benefit. It acts as a micro-flocculent, coagulating non-soluble particles which sink down leaving the bulk of the water with a clean appearance. The widely shown pictures of ozone treated Olympic swimming pool water in television coverage of the 1984 Olympic Games in Los Angeles showed pool water that was not only brilliantly clean but also had to comply with the stringent disinfection specifications of the International Olympic Committee.
Objectionable odours are associated with air and vapour from volatile liquids but they also emanate from non-volatile liquids such as water. The oxidation and coagulation abilities of ozone in water occur in a similar manner in air as well, either removing offending odour-producing particles completely or altering them to larger flocculated forms which are much easier to remove with electrostatic scrubbers or filters. In other words, ozone removes odours, not by masking them but by reacting with and coagulating the substances producing the odours. In this respect, ozone is again superior to chlorine which does remove some odours but may, in fact, add to odour problems by reacting with dissolved metals such as iron commonly occurring in aqueous solutions.
Chlorine does have practical handling and application advantages over ozone. Chlorine can be stored as solid calcium hypochlorite [HTH] granules, in liquid form as sodium hypochlorite or as a gas in cylinders under pressure. A functional advantage which chlorine also has is its stability in water and air which prolongs its disinfection activity into days, weeks or even months. By comparison, the half life time of ozone in air is generally a maximum of about 30 minutes and in water solutions open to atmosphere even shorter unless water circuits are specifically designed to ensure longer ozone contact times. One of the important research areas into disinfecting and purifying water supplied by municipalities and other water authorities has been to determine optimum methods of applying ozone to do the initial purifying and then adding chlorine in amounts small enough not to adversely affect the taste of the water but enough to protect the water against re-infection and growth of micro-organisms during storage and reticulation.
The progress of ozone application since the beginning of the twentieth century has, in similar fashion to many other practical technologies, been reliant on advances in other technical fields. UV lamps designed to produce ozone did not become commercially viable until the 1990s. Now special UV lamps are even available purely for breaking down any excess ozone emitted from treatment vessels. Like other strong oxidising agents, ozone corrodes non-stainless steel as well as other metals commonly used in water circuits. Therefore, the advent of plastic and reinforced fibreglass pipes and vessels allowed ozone solutions, as well as many other astringent liquids, to be far more widely and safely used.
Recent developments in metallurgy and dielectric materials have also greatly advanced the efficiency and reliability of corona discharge ozone manufacturing units. In the past a serious drawback of corona units was their short life due to dielectric breakdown causing arcing and corrosion across the electrodes. This electrode corrosion was exacerbated by low capacity desiccants which did not remove water vapour efficiently from air fed into the unit with the result that high voltage between the electrodes created nitric acid from airborne nitrogen oxides and water vapour. As these problems were overcome with improved designs and materials, successful ozone disinfection and purifying applications increased to the extent that the US Food and Drug Administration [FDA] the Department of Agriculture [USDA] and Environmental Protection Agency [EPA] issued successive Ozone Use Regulations during the period shown in Figure 3 covering a relatively recent 22 year period from 1982 to 2004.
Figure 3. Provided by Ozone Service Industries Pty Ltd.
Since 2004 many more advances have been made in both the technology and technique of manufacturing ozone and getting the best results from its powerful oxidising, coagulating and cleaning properties. Ozone is rapidly becoming the product of choice for the primary treatment of potable water, disinfecting and sanitising air, removing all sorts of odours, for example after fire damage, and cost-effectively achieving these results without leaving any sort of residue. A highly versatile tool indeed with 100% green credentials [except for needing small amounts of electrical power]. It has taken time to learn how to utilise ozone correctly and safely. The next article details the latest ozone production units available and recommendations by Ian Wright, the CEO of Ozone Service Industries Pty Ltd, as to how they can be optimally applied in HVAC systems, either alone or in combination with chlorine, other halogens such as bromine, or various types of UV radiation.