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Procesos avanzados de oxidación (AOP)

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Purpose

  • Degradation of recalcitrant components in wastewater at various industries
  • Physical disintegration return sludge for better settleability and reduced sludge disposal
  • Use of ozone for disinfection of water in swimming pools, replacing irritating chlorine products

Principle

AOP stands for "advanced oxidation processes" and brings together a number of environmental technologies for specific applications which can aid traditional water treatment technologies or even completely replace them. It mainly concerns situations where contaminated water is laden with difficult degradable components such as pesticides, dyes and specific aromatic compounds, organochlorine compounds, ...

Common feature of AOP techniques is that in most cases it comes down to the production of hydroxyl radicals (OH ·). These hydroxyl radicals have a very powerful oxidising potential.

There are ten or so different AOP techniques which are distinguishable. In many cases a distinction is made between photochemical and non-photochemical AOP techniques and this depending on whether there is light energy used (usually UV) for the production of the hydroxyl radicals. Some of these are mentioned:

  • Ozonation (O3)
  • Ozone / hydrogen peroxide (O3 / H2O2)
  • Fenton reagents (Fe2+ / H2O2)

Applications

The main scope of AOP is the destruction of specific and difficult biodegradable (persistent) pollutants in groundwater, surface water and industrial effluents. In an increasing number of cases, the combination of AOP is applied with biological techniques. When the waste water in addition to the persistent pollutants primarily contains good biodegradable compounds then the AOP technique will be used after the biological treament in order to conserve energy and chemicals. If the waste water is primarily contaminated with recalcitrant compounds and if those pollutants can be converted into biodegradable compounds then however the AOP technology will be installed prior to the biological treatment. AOP techniques can also be considered (post-treatment) for the removal of flavor and fragrance from potable water or for odor control.

Ozonisation

There are several techniques to generate ozone artificially. Each of these techniques provides for a supply of energy to the oxygen (O2) in order to achieve the formation of ozone (O3):

3O2 => 2O3

Ozone is an unstable gas which is used because of its oxidative properties for water treatment. In water ozone can react either directly by oxidation with target components or decompose into hydroxyl radicals which oxidize with, among others, target components with formation of by-products.

Ozone dosage for water treatment is typically applied to raw waste water (pre-ozonation) or after sedimentation. Pre-treatment is used in order to increase the biological degradability of waste water while ozonation is applied as a post-treatment when the focus is on the removal of recalcitrant COD succeeding biological treatment. The ozone demand depends strongly on the properties of the water to be treated.

An additional use of ozone is to dose it in the return sludge, or into a fraction of it, with the aim to prevent floating sludge. Ozone molecules react faster with thread formers than the sludge flocs that have relatively less (contact) surface area per volume.

Scheme

Fenton reagens

In this technique, the · OH radical is obtained from H2O2 by means of a reaction with a chemical catalyst, Fe2+ in the waste water. The net reaction is the formation of 2-OH radicals and water from 2 molecules of hydrogen peroxide, with Fe2+ as a catalyst. The OH radical can oxidize different molecules, wherein each time new radicals are formed.

OH· + RH => H2O + R·

The concentration of catalyst (Fe) will help determine the expiration of the reactions. A useful range for Fe: H2O2 is 1: 5-25. The use of Fenton's reagent is strongly pH-sensitive which means that the formation of radicals can only take place within a pH range of 3.5 to 5.0. The necessary reaction times vary from 30 minutes to several hours, depending on the composition and concentration of the substrate.

UV

Ultraviolet light (UV) is applied in different AOP techniques. The required hydroxyl radicals for the advanced oxidation can be produced by means of a  homolytic bond cleavage of hydrogen peroxide. Here the oxygen bond of H2O2 is broken by UV radiation and two ·OH-radicals are produced:

H2O2 + UV => 2·OH

UV radiation can also be applied in combination with ozonation (O3) in order to achieve an increase of the efficiency of the aimed oxidation. Often also a combination of ozone and hydrogen peroxide is applied. In practice the treated water is flowing through a UV-unit with appropriate specifications after dosage of H2O2 or ozone injection.

Another application of UV concerns the production of hydroxyl radicals by means of photocatalytic oxidation with TiO2 as a catalyst. The UV radiation on the TiO2 surface leads to an excited electron and an electron gap. The highly reactive electron gap reacts with water adsorbed onto the surface of the TiO2-bed and ·OH-radicals are produced. This technique is mainly appropriate for removal of micro-contaminants or the production of ultrapure water. 

Operational costs

Ozone generation is an energy-intensive process in which one also has to calculate the cost of the dioxygen. The installation itself must be made of materials which are resistant to ozone in solution or gas phase. To assess the costs and the effectiveness of the required ozone dosage a representative laboratory test on the designated wastewater is preformed.

Approach Trevi

Prior to considerating AOP techniques Trevi will first examine whether through separation of different waste streams or organic (pre) treatment savings can be realized.

Based on a representative wastewater sample different types of laboratory setups for ozonation, Fenton, and / or peroxidation are investigated and possibly compared with activated carbon tests. Not only the removal efficiency of specific recalcitrant components are of interest in the evaluation of AOP techniques but also the final BOD / COD ratio of the treated effluent.