Advanced Oxidation Processes (AOP) are a class of processes that create the hydroxyl radical. In general these processes require combining at least two different chemicals or a chemical and UV to complete the reaction. Some of the common AOP are ozone/UV, peroxide/UV, Fe/peroxide and the subject of this article ozone/peroxide.
The hydroxyl radical, the neutral state of the OH- ion, is a short-lived compound with one of the highest electrochemical oxidation potentials. this makes it highly reactive to virtually all organic compounds except those that are highly chlorinated.
In water treatment the hydroxyl radical can be used to oxidize many organic compounds all the way to CO2. This can be useful for materials that can not be treated any other way. An example would be acetone, which is the oxidation product of isopropyl alcohol (IPA). This compound is not easily stripped or absorbed onto carbon, but it can be oxidized to CO2 using the hydroxyl radical. Unlike membrane processes which transfer contaminants from one stream to another, i.e. concentrate the contaminant, oxidation processes that completely remove the contaminant from water.
All advanced oxidation processes make hydroxyl radicals. The selection of the appropriate process is an engineering analysis to minimize the cost of formation with the most efficient use of the hydroxyl radicals produced. Ozone/UV AOP require that the water have a relatively high UV transmittance, i.e. that the UV light can readily pass through the liquid. In applications where this is not possible another oxidation process should be employed.
The ozone peroxide process combines ozone from an ozone generator that has been dissolved into water with hydrogen peroxide to form one hydroxyl radical. the combination of ozone and peroxide is sometimes call peroxone. The advantage of this process is that it is not dependent on the UV transmittance of the water and does not require the added capital cost associated with the UV reactor. After reaction nothing is added to the water except oxygen. So, the process is sustainable.
The ozone, a gas produced in an ozone generator, is often transferred to the water via fine bubble diffuser or venturi injector. Peroxide is added via metering pump. The venturi injector is becoming a more common option since i can provide excellent mixing of the ozone, water and peroxide due to the high shear forces inside the venturi. The injection can be done at a single point or multiple points. The latter may be necessary if the amount of ozone to be injected is large versus the volume of water.
The amount of peroxide required relative to the ozone is 0.5:1 to 1:1 on a weight basis. This ratio varies from application to application. The proper amount is determined from experimentation. It is important to note that both ozone and peroxide can react with organic matter in water directly, although not at the rate of the hydroxyl radical. Given the complexity of most applications careful laboratory and pilot studies are recommended.
It is also important to note that since hydroxyl radicals are not selective oxidants, they will react with any oxidizable species. Since the radicals are expensive to form, less expensive oxidants or methods should be used to deal with the easier to deal with materials that could waste the hydroxyl radicals. Examples would include iron, manganese, hydrogen sulfide, etc. The advanced oxidation process should be targeted at the higher value refractory materials in solution.
Ozone peroxide has been used in a number of applications commercially including the treatment of 1,4 Dioxane. A difficult to treat organic solvent found in contaminated groundwater. it has also been approved to treat municipal wastewater for reuse.
Given the simplicity and flexibility of the ozone peroxide process and the lack of any byproducts, it is a good option for wastewater treatment applications where refractory organics are present.
