Synopsis: Ozone oxidation is a powerful natural sanitation process driven by the strong tendency for the third oxygen atom in ozone, to combine with other substances. Ozonation is a type of advanced oxidation process, involving the production of very reactive oxygen species, able to attack a wide range of organic compounds and all microorganisms.
Ozone Oxidation Capabilities
Discovered in the 19th Century, ozone, a natural form of activated oxygen (allotropy), is produced during lightning storms and continuously occurs in the stratosphere due to action of ultraviolet (UV) light. It can be artificially produced by the action of high voltage discharge in air or oxygen.
O2 + O ——-energy —————> O3
Ozone is highly unstable and must be generated on site. Its’ oxidation potential (-2.07V) is greater than that of hypochlorite acid (-1.49V) or chlorine (-1.36V). The latter agents being widely used in water treatment practice. Ozone is thought to decompose accordingly (Miller 1978, 167-168):
O3 + H2O -> HO3 + OH-
HO3+ + OH -> 2HO2
O3 + HO2 -> HO + 202
HO + H02 -> H2O + O2
This unstable form of oxygen breaks down to oxygen molecules and oxygen atoms, which have high oxidation potential. If we examine the oxidation power of ozone by measuring the REDOX potential will find out that ozone is about 5 times more oxidizing than oxygen and about twice as much as chlorine. These high potentials increase its reactivity with other elements and compounds. The reactivity of ozone is about 20 to 50 times more reactive than chlorine and permanganates and is well documented in the case of the high kill rate of micro-organisms (Fungi, Bacteria & Viruses). This high kill rate means smaller retention times, and smaller storage tanks are required to do the same disinfecting as other oxidants. In other words, the capital cost of building these tanks and treatment plants are reduced considerably. Ozone will reduce chemical handling, storage, transportation, infrastructure and production facilities. Ozone requires only electricity, which is readily available from hydro, solar, wind or fuel electric generators. In many instances, ozone will allow decentralization of services which will provide better flexibility and better cost management.
Ozone chemical free treatments and applications:
• Wastewater effluents
• Industrial /Agriculture Food Industry
• Domestic/Municipal Cooling towers treatment
• Drinking & water bottling
• Smoke & odor treatment
• Pulp & paper
• Boiler water treatment
• Grain silo disinfecting
• Semiconductor wafer cleaning
• Mining (Cyanide, Arsenic)
• Chilled water treatment
• Fruit & vegetable storage
• Laundry water recycling
• (Phenol) Cutting fluids recycling
• Meat storage
• Medical instrument sterilization
• Barn disinfection (air/water)
• Slaughterhouse disinfection
• Hospital air sterilization
• Hydroponics Fruits & vegetable wash Aquaculture
• Animal waste treatment
• Food container sterilization
• Paper pulp bleach
• Wine/Beer SO2 replacement
• Sour gas desulfurization
• Heavy metal precipitation
• Animal drinking water
• Chicken egg wash
• Zebra mussel’s treatment
• Landfill leachates
• Irrigation water
• Ozonated meat grinders
• Rubber recycling, etc.
Ozonation Design considerations
Organic and Inorganic Load:
The reaction of ozone with most organic compounds may be modeled using a first order kinetic equation:
ln (C/Co) = -kt (1)
Where k is the reaction rate constant. The value of k is found by carrying out laboratory experiments. Typical values range from 4×10-3 sec-1 to 4×10-4sec-1. The value of k measured under ambient conditions and neutral pH may be adjusted to account for the effects of pressure, temperature and pH as follows:
k’’=k’(b P/Pa)(c T/Ta)(d pH/7)
Where the subscript (a) represents ambient conditions. Although equation 2 implies that, the reaction rate increases with increasing temperature, as is the case generally, it must be kept in mind that ozone solubility is adversely affected by increasing temperature. Typically, for organic contaminants 0.1 to 1.6 g O3/g COD is needed. The oxidation of heavy metals such as iron and manganese if present alone generally occurs in stoichiometric proportions.
Ozone Dissolution (Mass Transfer):
For the diffusion of ozone from a gas bubble to an aqueous fluid the boundary conditions are such that the Fick’s law simplifies to NA = dL*C (6) where C is the ozone concentration within the bubble and dL is the mass-transfer coefficient as defined in Eq. 7, dL= 2DAB/Dp + N (7) where Dp is the bubble diameter and N is a function of the Schmidt Number.
Equations 6 and 7 reveal two very important characteristics of ozone dissolution.
1) O3 dissolution increases with the gaseous ozone concentration
2) O3 dissolution increases with decreasing bubble diameter.
To take advantage of these two points the ozonator must be designed to efficiently dissipate heat which would otherwise cause the premature conversion of the ozone to oxygen thus lowering its concentration. Also, an oxygen feed may be used to yield higher ozone concentrations than those obtained from air. (See Figure 2) Secondly, the ozone/water contact should be made under pressure in order to produce small bubbles. (See Figure 3) The maximum number of moles of O3 transferred to the solution may be calculated from, MO3 = NASt (8) where S=total bubble surface area t = contact time
This reveals a third important ozone dissolution characteristic:
3) O3 dissolution increases with increasing retention time
Equation 8 also confirms point (2) above.
In summary when ozone is used to treat water or wastewater, it must be transferred from the gas phase, in which it is generated, to the liquid phase. Ozone is 12.5 times more soluble in water than oxygen. The single most important variable that affects ozone mass transfer is the concentration of dissolved ozone- reactive materials in the water. Our ozonators will produce O3
concentrations from about 4 to 10 % with Oxygen feed however, the optimum concentrations for generating ozone are 5-6% wt with oxygen used as the feed gases.
The principle methods currently used to introduce ozone into water and wastewater are:
1) Contact column
2) Venture injection
3) Centrifugal injection
The ozonation treatment step is usually followed by :
1) Clarification: to precipitate oxidized organic and inorganic matter
2) Filtration: (nano, sand, charcoal): to remove precipitants. The use of activated carbon filters has the advantage of absorbing the excess, un-reacted, un-recycled ozone and allows it to convert back to oxygen.
3) O3 destruct in air vents: may be accomplished using thermal, catalytic or ultraviolet destruction
The free radicals (H02 and HO) react with a variety of impurities such as metal salts, organic matter including micro organisms, hydrogen and hydroxide ions. They are more potent germicides than hypochlorite acid by factors of 10 to 100 fold and disinfect 3125 times faster than chlorine (Nobel 1980). Oxidation potential does not indicate the relative speed of oxidation nor how complete the oxidation reactions will be. Complete oxidation converts a specific organic compound to carbon dioxide and water. Oxidation reactions that take place during water treatment are rarely complete, due to the large quantity of contaminants and relatively short durations of time in which to oxidize the water pollutants. Therefore, partially oxidized organic compounds, such as aldehydes, Carboxylic organic acids are produced during the relatively short reaction periods.
These aldehydes and carboxylic acids can be removed by other means prior to complete mineralization to reduce the amount of Ozone needed for complete oxidation of these chemicals.There are three fundamental mechanisms which apply to the oxidation of organic compounds reacting with an oxidizer. Each mechanism is unique as to how organic compounds react with an
oxidizer. But, in some cases, oxidants will react with organic compounds by all three mechanisms, although in sequential steps.
First. The addition mechanism which occurs with organic compounds containing aliphatic unsaturated, such as olefin. Ozone can add across a double bond to form an ozonide. This reaction occurs readily in nonaqueous solvents, but as soon as water is added, the ozone hydrolyzes to other products, with cleavage of the former double bond. Second. The substitution mechanism involves replacement of one atom or functional group with another. This specific reaction also can be viewed as an insertion reaction, whereby oxygen is inserted between the ring carbon and hydrogen to form the hydrogen group on the ring.
Oxidation also can involve cleavage of carbon-carbon bonds to produce fragmented organic compounds.
Effect of Temperature and PH:
It is clear that other parameters affect the reaction mechanism and rates as found by various studies. The effectiveness of ozone to oxidize organic and inorganic compounds is function of to the temperature of the water and pH levels. In wastewater, applications there are many variables such as: water temperature, pH, COD, BOD, TSS, heavy metals, which need to be considered. Ozone, at low pH levels (less than 7), reacts primarily as the O3 molecule by selective and sometimes relatively slow reactions. Ozone at elevated pH (above 8) rapidly decomposes into hydroxyl free radicals, which react very quickly. Many compounds that are slow to oxidize will oxidize rapidly when the pH is adjusted to the Alkaline side. It was found that PH=8-10 is most suitable for organic molecules oxidation.
The initial step of the decomposition of ozone is the reaction between ozone and hydroxide ion to form ozone ion and hydroxyl radical (OH-): The hydroxyl radical then reacts further. This process would explain the increased dissociation of ozone with increasing alkalinity. Recently, studies have shown experimentally that as pH increases, the kinetics of ozonation of organic compounds changes. Hydroxyl radicals may be the important active species in ozonation as has
been concluded (Glaze 1980).
Therefore, the alkalinity of the water is a key parameter in advanced oxidation processes. Ozone then decomposes rapidly in water with a half life of a few minutes about 20 minutes in room temperature but could be much faster less than 10 minutes in the presence of bicarbonate and carbonate ions which are excellent scavengers for free radicals. In addition, carbonate ions are 20 to 30 times more effective in scavenging for hydroxyl free radicals than bicarbonate ions. For that reason we stated that the pH of 8.0 to 10.0 is most appropriate for ozonation as it was found that at that PH > 10 the bicarbonate ions convert to carbonate ions (EPA 1989).
Effect of catalysts:
There are several catalysts used in conjunction with Ozone such as semi-precious and precious metals, ultrasonic agitation, H2O2,electro-coagulation but the most commonly utilize ed and well documented are the ultraviolet rays at wave length of 254 nm.. Ultra-Violet has been found in the past 10 to 15 years that when this treatment is combined with Ozone there is a rather «explosive» reaction as the two in a sense «destroy» each other creating a highly reactive Hydroxyl ion. The end results of this is that many compounds that neither UV or Ozone independently can remove, however with the combination of these compounds can be removed and the rate of removal is extremely rapid, sometimes in as little as two seconds. The produced free radicals of this reaction contain at least four radicals: (O) Excited Atomic Oxygen species, (HO) Hydroxy radicals, (H0Û) Hydroperoxy radicals and Excited Carbon containing species. Compounds normally refractory or slow reacting to Ozone or UV alone but which react to the combination of such organo metallic complexes, Cyanides, Phosphorous and nitrogen compounds.
Ultrasounds: has not been fully exploited as applied to waste water treatment nor fully understood nor documented at present. It is known that in many cases it works with the Ozonation treatment in ways quite similar to UV. Principally it excites the ionic structure of both the water and the contaminants in the waste stream and as a result the Ozonation process is expedited (either in speed of reaction or it actually causes a reaction where the contaminant had
been refractory to the Ozone treatment).
Electro Coagulation: is the process of applying a direct or alternating electrical current and voltage of varying strength to electrodes contacting water or waste water which results in the formation of Floc which can be filtered to remove suspended and flocculated substances. The mechanism by which this flocculating occurs are complicated and involve the inter conversion of electrical and chemical energies reacting with and on substances in the water or wastewater. Wastewaters, which are slightly reactive to Ozone show excellent reaction with the Electrochemical method such as for domestic and textile wastewaters.
This quick synopsis about ozone usage and applications shows that it is simple and easy to use once it is understood. We can now predict the ozone quantities required to treat a certain flow rate of a given chemical analysis. It is now obvious that Ozonation should be considered in treating difficult wastewaters (heavy metals, textile effluent, plating, with no, or minimal, use of other chemicals).