September/October – 1997
Story Title: Introduction to Ozone Generation
Author: Steven Carruthers
Ozone is a powerful disinfecting and oxidising agent, successfully used in thousands of water treatment applications. STEVEN CARRUTHERS reports on its use in hydroponics.
What is Ozone? Ozone is a natural component of the earth’s upper atmosphere, where it is primarily formed photochemically. It can be recognised by the clean, fresh odour of air after an electrical thunderstorm, which is the result of low levels of ozone gas produced by lightning.
The odour of ozone was first reported by Van Mauren in 1785, in the vicinity of an electrical discharge. In 1840, Christian Schonbein identified this characteristic odour as a previously undetermined compound. He named it ozone after the Greek word ozein, meaning to smell. Over succeeding decades several researchers experimented with the production and reactions of ozone, however the identity of the structure of the compound was confirmed in 1867 as triatomic oxygen. This simply means the ozone molecule contains three oxygen atoms, having the chemical symbol O3. Ozone is nothing more than another molecular form of oxygen, the chemical symbol for which is O2.
Ozone has been commercially used since 1893 when the first full-scale drinking water treatment application was implemented. Today, it is used in thousands of water treatment applications including the treatment of municipal water and wastewater, cooling towers, ultra pure water, marine aquaria, beverage industries, industrial process water, swimming pools, bottled water plants, aquaculture, food processing, and effluent treatment.
For hydroponic applications, ozone treatment has sometimes proved an expensive and imperfect science. However, according to Brisbane-based ozone specialist, Watertec Engineering Pty Ltd, recent work has demonstrated that ozone treatment is an effective water treatment if used correctly.
According to Technical Director Philip Barlow, ozone is the strongest commercially available oxidising agent we can produce. However, the molecule is unstable and cannot be stored for future use, as can gaseous chlorine.
“For this reason, it must be generated close to the point of application and then used immediately,” he explains.
Recent field work on the Fancyleaf hydroponic farm (see article this issue) has shown that the success of ozone treatment is directly related to the volume of water in a recirculating system, the flow rate, and the number of plants the system supports. Using the Redox Potential (ORP) of the nutrient solution as an indicator, it was found that the appropriate ozone dose for the Fancyleaf operation was between 0.1 and 0.2mg/L, requiring an ozone generator capable of delivering 5-8 grams of ozone per hour.
“If you overdose with ozone you risk damaging the nutrient solution. Also, if you don’t dose sufficient ozone, the desired treatment results will not be achieved,” Philip, a chemical engineer, warns.
Excessive levels of ozone will result in the loss of some micro elements in the nutrient solution, including manganese and iron.
“We’re not generating enough ozone to kill all organisms; rather we’re controlling the organisms that are causing disease problems, by keeping their numbers down,” he added.
Properties of Ozone
According to Philip, the odour threshold of ozone gas is approximately 0.01ppm. Ozone will develop an unpleasant acrid smell at concentrations above 0.1ppm and a faint blue colour at levels exceeding 5ppm.
“Ozone is relatively stable in air, with a half-life of several hours at low concentrations, but this half-life changes significantly once ozone is introduced into water, where many chemical reactions can occur,” Philip explains.
“Because ozone is very reactive in an aqueous environment, it can oxidise material between 10 and 1000 times faster than most oxidants used in water treatment. In some instances of organic oxidation, the material can be completely oxidised to carbon dioxide and water,” Philip added.
On a comparison of oxidation strengths of standard oxidants, ozone is 2.07 volts as compared with chlorine at 1.36 volts and chlorine dioxide at 1.50 volts, versus hydrogen, respectively.
“When ozone is dissolved in water, it can react with contaminants by either direct reaction as the O3 molecule, or by indirect reaction with hydroxyl free radicals. In strong acidic solutions, the direct reaction predominates, but above pH7 the latter reaction predominates.”
As can be seen from Table 1, hydroxyl free radicals have an oxidation potential considerably above that of the ozone molecule itself. The formation of these compounds assists with chemical oxidation, but with a very short half life (microseconds) they do not play a major part in achieving disinfestation.
Ozone is generated by passing oxygen or dry air through a high energy electrical field, where a portion of the oxygen is converted into ozone. There are various methods used to do this, including corona discharge, UV irradiation and electrolysis. For hydroponic applications, corona discharge generation is recommended as practical, safe and economical.
Corona Discharge Generation (CD)
Corona Discharge Generation is capable of producing high concentrations of ozone. Although there are many generation cell designs, the fundamental principal remains the same. Figure 1 details a simplified process flow for a vertical tube ozonator, in which the cooling water is in direct contact with the glass dielectric and used as the grounding electrode.
“Electrons are accelerated across an air gap, so as to give sufficient energy to split the oxygen-oxygen double bond, producing atomic oxygen,” Philip explains.
The oxygen atoms which are produced by the collision react with other diatomic oxygen molecules, to form ozone according to the following equation:
3O2 + Energy = 2O3
The quantity of ozone produced is dependent on several factors, such as the voltage and frequency of the alternating current applied to the CD cells. When enough high energy electrons bombard gas molecules so that they are ionised, a light emitting gaseous plasma is formed, which is commonly referred to as a corona.
Table 1. Oxidising Potential of Various Reagents
Oxidising Reagent – Oxidising Potential (V)
Fluorine – 3.06
Hydroxyl free radicals – 2.80
Atomic Oxygen – 2.42
Ozone – 2.07
Permanganate – 1.67
Hypobromous acid – 1.59
Chlorine dioxide – 1.50
Hypochlorus acid – 1.49
Chlorine – 1.36
Oxygen – 1.23
Bromine – 1.09
Hypochlorite – 0.94
“Ozone generation by corona discharge is an exothermic physio-chemical reaction, where much of the energy used for ozone generation is lost in heat, therefore cooling efficiency is an important factor in generator design,” Philip explains.
“Ozone destruction by decomposition increases as the gas temperature and ozone concentration increase. For this reason, most ozone generator designs are de-rated as the cooling water supply temperature increases. Generator designs in which the cooling water is in direct contact with the dielectric tubes, can tolerate higher cooling water temperatures”, he added.
Typical voltages used in CD ozonators vary between 7 and 20kV. The voltage required for efficient ozone generation is dependent on the generation cell supply frequency and gap between the active electrode and dielectric insulator.
Until recent advances with power electronics, most ozonators operated on mains, or low frequency, being 50 or 60Hz. For these ozonators, voltages of between 12 and 20kV are typical. As the voltages increase, stress on the dielectric material also increases, which has a direct effect on the service life of this component.
“Low frequency ozonators are still quite common and viable, however more modern generator designs, using medium frequency (up to 1000Hz), offer many technical and operational advantages,” says Philip.
Properly designed medium frequency ozonators are now favoured as they provide many benefits over the older low frequency technology. Some of the benefits of medium frequency ozonators include:
– Greater ozone production can be achieved with less electrode surface area. Therefore, for a given ozone output, the equipment is significantly smaller.
– When designed correctly, the power consumption per kg of ozone production is less.
– Using modern power electronics, the generator efficiency may be manipulated by varying frequency, wave form, voltage, etc.
– A greater ozonator turndown is possible and the output is very linear to frequency variations. Using both frequency and voltage control, an ozonator output of between 0-100% may be provided.
– A typical corona discharge ozone system consists of four fundamental components:
– An air preparation or oxygen production unit
– A corona discharge generator
– An ozone diffuser/contactor
– An ozone off-gas destruction system
Fig 1. Typical Vertical Tube Ozonator Process Flow Schematic
1. Air Compressor
2. Air Receiver
4. Refrigerated Cooler
5. 0.01u Coalescing Filter
6. -7°C Desiccant Air Dryer
7. Dust Filter
8. Flow Control Valve
9. Pressure Relief Valve
10. Air Flow Rotameter
11. Cooling Water Jacket a Grounding Electrode
12. Ozone Generator Cell
13. Dielectric Tube
14. High Voltage Electrode
15. To Ozone Diffuser
16. Cooling Water used as
17. H.V Transmitter
18. 4-20mA Control signal
19. Volatage/Frequency Regulator
20. 415V 50Hz Supply
Air preparation is critical for the efficient and reliable operation of corona discharge ozonators. Moisture and particulate matter have a detrimental effect on the generation cell electrodes and dielectric material, and an air dryer is critical to the success of ozone generation.
“Moist air in the ozone generator will cause nitric acid to form, which decreases the ozone production, and corrodes the generation cell components”, Philip warns.
“If wet air is present, white sparks or arcing can be seen within the generator,” he added.
For efficient ozone generation, the final air supply must be filtered to remove particulate matter and any oil or hydrocarbons which may carry over from the compressed air system. Ozone generators are often installed in hot, humid plant rooms. Also, many localities experience ambient conditions with both high temperatures and humidity, placing additional requirements on the air preparation system.
“Pressure fed systems are more suitable than atmospheric pressure dryers”, commented Philip, “as they are able to reliably provide the necessary air quality under all normal climatic conditions.”
Oxygen may be used as the ozonator feed gas, and is generally selected on the basis of higher ozone concentrations being required. The specific advantages of using oxygen is that the generator power consumption is significantly reduced, when compared to an air fed ozonator (typically 6-7kw/kg compared with 18-20kw/kg).
If oxygen is used for economic reasons, oxygen not converted into ozone is normally recovered, dried then recycled back to the ozonator. Due to the cost and operating requirements of these oxygen recovery systems, most applications use air as the feed gas.
Figure 1 details the air preparation typically used for CD generators, particularly where less than ideal ambient conditions are experienced.
Fig 2. Effect of Water Vapour on Ozone Production
Solubility of Ozone
Ozone is only partially soluble in water, therefore the concentration of ozone in the generator gas stream becomes very important. According to Henry’s Law, the efficiency of absorbing a specific gas into an aqueous phase is based on the partial pressure of that particular gas in the total gas flow. Therefore, the higher the concentration of ozone in the generator gas stream, the more complete will be the transfer of ozone into the water. Table 2 shows the theoretical residuals of ozone that can be achieved in water, based on ozone gas concentration and temperature.
As would be expected, ozone solubility decreases with an increase in water temperature. Also, as a consequence of Henry’s Law, the solubility of ozone increases with an increase in ozone concentration in the gas stream. In practical applications, ozone does not have sufficient contact time to achieve equilibrium conditions, resulting in lower solubility levels than shown in Table 2 (Table 2 not shown).
Diffusion of Ozone in Water
Considering that relatively low concentrations of ozone are produced in the generator gas stream, together with the limited solubility of ozone gas, the method of diffusing ozone into the nutrient solution is of great importance. For hydroponic applications, both fine-bubble diffusers and high efficiency injectors are practical, effective and economical. Each system has advantages and disadvantages, therefore selection is based on the requirements and merits of each application.
Fine-bubble diffusers/contactors are widely used due to their ability to operate without additional energy, apart from that of initial gas compression. The technology and design requirements are now well proven and understood, with many years of demonstrated performance, therefore scale up from a theoretical or pilot plant evaluation is not difficult. Figure 3 details a typical ozone contactor using the bubble diffusion technique.
With this system, ozone gas is normally discharged from the ozone generator at pressure of 0.7 – 1.0 Bar, which is sufficient to overcome the hydrostatic head, plus the head loss due to the gas distributor piping and diffusers. This diffusion system may also be used with ozonators operating under negative pressure, using stainless steel liquid ring gas compressors to inject the ozone, under pressure.
Some of the advantages and disadvantages of this type of system are summarised below:
– Maintenance requirements are minimal, as no moving parts are used.
– High efficiencies (>90%) are achievable, however this requires multipoint injection, deep contact chambers and often recycling of off-gas. High diffusion efficiencies are not normally achieved with simple one or two pass contactors.
– Scale up is easily achieved, due to the well established design criteria for this method.
– The systems may be designed to accommodate a low hydraulic head loss through the tanks.
– The diffusion method requires deep contact basins for effective ozone transfer (5-7 metres).
– Clogging of diffusers is possible, particularly when water flow and ozone doses are intermittent, or when iron and manganese oxidation is required.
– Vertical channelling of bubbles is possible, which adversely affects gas/liquid contact.
Fig 3. Typical Fine Bubble Diffusion/Contact System
High Efficiency Injectors
When designed correctly, this injection method will provide high diffusion efficiencies (>95%), however additional energy is required to produce a partial vacuum. Injector selection and design are important in achieving maximum efficiencies with minimum energy.
“This type of injection system has been used throughout the world with both small and large ozonation facilities,” said Philip, “however its use is certainly more common in small and medium size applications.”
With this technique, a side stream of water is normally taken from the main plant flow, where the pressure is boosted to supply a vacuum injector. This involves the water flowing rapidly through a small orifice, creating a venturi effect (partial vacuum), that pulls the gas from the ozonator into the water stream. When designed correctly this provides very effective efficiencies. The ozonated side stream is then injected into the main plant flow, normally via a low head loss static mixing device. Alternatively, this side stream of ozonated water can be injected into a degassing/contact tank which operates at atmospheric pressure.
“With this design, the dissolved ozone is effectively mixed with the main body of water, after which a contact tank would be required to achieve the necessary contact of ozone with the water to be treated,” Philip explains.
In the Fancyleaf system, the ozonated water stream is led along a 20mm pipe located at the bottom of each contact tank. The PVC pipe is riddled with tiny holes to allow the fine bubbles to escape and diffuse into solution.
The following advantages and disadvantages are applicable for this injection technique:
– Both injectors and static mixers (when used), are simple to operate, with low maintenance requirement, due to the lack of moving parts.
– Excellent mixing and mass transfer can be achieved. Depending on the desired detention time, contact/reaction tank depths and volume may often be smaller than those required for bubble diffusion.
– This type of diffusion system may be adapted to any hydraulic plant design.
– Correctly designed counterflow or vortex static mixers impart low hydraulic head loss to the plant.
– Additional electrical energy is required to efficiently operate the vacuum injector, however this is often counteracted by the increase in diffusion efficiency.
– These systems have a turndown capability limited to the capacity of the specific injection device. This problem is minimised where a side stream from the main flow is fed to the injector.
Figure 4 details two typical methods of applying an injector ozone diffusion system. Diagram A would be used for an inline system operating under pressure, whereby Diagram B shows a typical design for an atmospheric tank system.
Fig 4. Injector Ozone Diffusion Methods
1. Water Feed
2. Injector Boost Pump
3. Ozone Fed under Vacum
4. Static Mixer
5. Ozone Contact Tank
6. Off gas Vent Valve
7. Ozone Destructor
8. Treated Water
9. Off Gas
10. Bubble Diffuser
11. Contact Tank
Ozone Off-Gas Destruction
Ozone is an oxidative and reactive gas which is harmful to humans above certain concentrations. Ozone gas is heavier than air, therefore any undissolved ozone must be removed from the system and disposed of correctly. Several methods are available for ozone destruction, but the simplest one used for hydroponic applications is by ozone adsorbtion and reaction with Granulated Activated Carbon (GAC)
Absorption and Reaction with GAC
Activated carbon adsorption is extensively used for small applications where air is the ozonator feed gas. This is an adsorption process whereby the reaction consumes the carbon media. The material used is elemental carbon which has been steam activated to provide a large internal surface area.
“Carbon is a strong reducing agent, therefore upon contact with ozone gas, the carbon is oxidised to carbon monoxide and carbon dioxide, resulting in destruction of the ozone molecule,” Philip explains.
This reaction degrades or powderises the granular activated carbon, therefore it has a finite life. To this end, the ozone consumes the carbon by slow-rate combustion. Due to this consumption of carbon, the media must be replaced regularly. When the carbon is saturated with water and washed with sprays, the reaction becomes partially catalytic, requiring a larger volume of GAC to be used.
“For ozone installations where oxygen is used as the feed gas, carbon adsorption destructors must not be used, due to the dangers of combustion,” Philip warns.
Environmental and Health Aspects
Because ozone has a very short half life in aqueous solutions, and its degradation predominantly results in either oxygen or oxygenated by-products, waters treated with ozone will be less of an environmental hazard than the water left untreated or treated with halogens or related compounds, such as chlorine. Ozone accelerates the natural oxidation process of both atmospheric and biological oxygen take up. Water treated with ozone can generally be recycled in the environment without fear of releasing toxic substances.
Although ozone in the gaseous form is both toxic and reactive, it represents no safety handling problems in properly designed operating systems. Unlike most other oxidants which are stored on site in bulk form, ozone is produced on site in low concentrations and immediately consumed. Consequently, any accidental leakage can be easily controlled, as evidenced by ozone’s long safety history in many applications around the world.
Breathing traces of ozone in air for a few minutes is of little public health concern. Even though throat and lung irritation plus oedema have been observed after extreme exposures to ozone, it is important to recognise that during more than 100 years of commercial use, no deaths related to ozone exposure have ever been reported.
The design and operation of ozone generating and ancillary equipment is both a detailed and complex subject, however this article is intended to give a very brief overview of current thinking and techniques for safely producing and using this unique oxidant for hydroponic applications.
When designed correctly, modern generation equipment will provide a reliable and safe source of ozone, however one must still remember that ozone is a toxic gas, therefore equipment and installations, if incorrectly designed and operated, can present significant dangers to both plant and personnel. No ozone installation should be considered without following the recommendation of organisations who are familiar with and experienced in the generation, use and operation of complete ozonation facilities.
If used correctly, ozone is a valuable tool for disinfecting and improving water quality in hydroponic applications, at the same time addressing the rapidly growing requirements for environmentally friendly or suitable products.
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Ozone in Swimming Pools – Facts and Fallacies,
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