Water | Advanced oxidation processes

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Oxidation techniques: AOP

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 wastewater 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 wastewater 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

Various applications in the AOP techniques also use ultraviolet light (UV). For example, the hydroxyl radicals needed for the advanced oxidation can be produced by a homolytic cleavage of hydrogen peroxide. Hereby the oxygen binding of H2O2 is split into two ·OH radicals by irradiation with UV:

H2O2 + UV → 2·OH

Ozonation (O3) can also be combined with UV radiation with the aim of increasing the efficiency of the intended oxidation. Ozone and hydrogen peroxide are also often combined. In practice, after the dosing of H2O2 or the ozone injection, a UV flow unit with the appropriate specifications is run through.

Another application of UV involves the production of hydroxyl radicals via photocatalytic oxidation with TiO2 as a catalyst. The UV radiation forms an excited or excited electron and an electron cavity on the TiO2 surface. This highly reactive electron cavity reacts with water adsorbed on the surface of the TiO2 bed and produces hydroxyl radicals. This technique is primarily suitable for the 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.