Advanced Oxidation Processes (AOP)

Oxidative water treatment (AOP, advanced oxidation processes) is understood as processes for chemical water treatment in which hydroxyl radicals are formed. These highly reactive radicals are available for chemical decomposition reactions and react with organic or inorganic substances that are not easy to break down biologically. They can be formed by adding oxidative substances such as ozone and hydrogen peroxide, or by introducing energy by means of UV radiation, ultrasound or electric current via inert electrodes as well as by a combination of these processes.

At present, catalytic, photochemical, and electrochemical processes as well as plasma processes for oxidative water treatment are investigated at Fraunhofer IGB. Various experimental set-ups for continuous, semi-continuous and batch trials are available for this purpose.

Application areas

AOP processes are always used when a biological decomposition is not feasible or cannot be carried out efficiently, for example because the contaminations contain persistent substances. Also, AOP processes are the method of choice when the process wastewater has a toxic effect on the microorganisms of a biological purification stage or occurs extremely discontinuously. The energy required for operating the system can be provided by electricity from renewable sources such as solar and wind power.

Example: Quantification of methylene blue degradation

A problem in the field of oxidative wastewater treatment is the formation of degradation by-products, some of which are hazardous or are not sufficiently evaluated toxicologically. However, the formation of toxic by-products can be avoided in almost all cases by choosing suitable process parameters. In order to quantify the reaction mechanisms and degradation products of various AOP methods in the AOP research facility, tests with the model substance methylene blue (C16H18Cl N3S) were carried out. In addition to the decoloration (measurement at 664 nm), the formation of by-products was observed using HPLC, coupled with UV and mass spectrometry. In a comparison of anodic oxidation, ozone treatment und UV treatment, the ozone treatment turned out to be the best method for this wastewater model.

Example: Decoloration of organic dyes by UV / H2O2 and anodic oxidation

As models for real wastewater from the textile industry, a dissolved organic dye and a particulate organic dye were discolored by more than 90 percent – until the liquids were transparent to the human eye. The study also served to determine the most energy-efficient process parameters and compared the decomposition products produced by each method.

Batch reactor for UVC/H2O2 treatment of water.
Batch reactor for UVC/H₂O₂ treatment of water.
Determination of transferred ozone, foaming risk and treatment cost.
© Fraunhofer IGB
Determination of transferred ozone, foaming risk and treatment cost.
Dipl.-Ing. Christiane Chaumette in the laboratory.
© Fraunhofer IGB
Dipl.-Ing. Christiane Chaumette in the laboratory.


  • Complete mineralization of pollutants possible
  • Staff savings and increased reliability
  • No increase in salinity, which enables recirculation
  • No disinfection by-products – in particular no halogenated compounds
  • Less handling of hazardous chemical agents
  • Little or no sludge formed
  • Hygienic outflow water
  • Robust process – discharge criteria can be met reliably
  • Available quickly – standby operation possible
  • Suitable for varying quantities and qualities of wastewater



AOP research facility

We offer our AOP pilot plant for the development of optimized treatment processes. Modular units (ozone generator, ozone reactor, UV reactor, ultrasonic units, electrolytic cells) can be freely combined.


Reactor for the elimination of micropollutants in wastewater by oxidation

The degradation of man-made pollutants in low concentrations, so-called organic trace substances or micropollutants, is increasingly gaining priority in water treatment. In Switzerland, the corresponding expansion of larger wastewater treatment plants is already mandatory, in California and other industrialized regions it is partly implemented as a precautionary measure.


Combination and integration of oxidative and electrolytic processes

Oxidative and adsorptive processes such as electrophysical precipitation can be combined, depending on the problems to be solved. By doing this, results can be achieved that exceed the sum of the results of the individual processes. A further advantage of these processes is that they are suited to standby operation and can be switched on or off at any time. Integration into existing plants and automation including autonomous operation or remote control are feasible without any problems. Continuous online logging of organic carbon (TOC, total organic carbon) can be effected, enabling requirement- based and thus energy-optimized treatment.

Removal of trace organics and hard COD

Pollutant of the month

„The new pollutant of the month“ chemists in water analytics often joked when they had matched another one of the many recurring peaks in the analytical spectrum of wastewater samples to a specific substance. If it is an artificial, man-made substance that is or could be toxic such an identification can provoque a wave of media coverage and in case of real concern a wave of further measurements and political initiatives.  

Regulated monitoring of micropollutants in water

On a European level the process is more systematic. REACH regulation, EU-monitoring lists, regular reporting as part of the water framework directive and regular updates of the EU directives for drinking water and urban wastewater include risk evaluation and assess systems as a whole. The limit values of EU directives are then successively implemented into national legislation. The German Federal Center for Trace Substances at⁠ UBA, located in Leipzig, was founded in 2021. It connects interest groups with the goal to reduce critical substances emissions to water even faster.

Many German wastewater treatment plants are currently already planning or building an additional treatment step called the 4th stage to further reduce emissions – usually ozonation and filtration or active carbon adsorption. This is already obligatory in Switzerland and in wastewater treatment plants which indirectly feed into drinking water reservoirs.

However, the lists of substances to monitor are growing longer, as are the lists of individual substances of potential concern. Intensive research is currently being conducted into the avoidance, replacement and removal of per- and polyfluorinated substances (PFAS), which are man-made and have been proven to be detrimental to human health in some cases.

Fraunhofer IGB is part of this and offers independent scientific consulting

The IGB offers independent scientific consulting on pollutants and trace pollutants removal to public and industrial customers. Our technicians regularly test degradation of biologically inert organic pollutants called hard COD by ozone and UVC-H2O2. Because accumulation of hard COD prevents closed water cycles in many production lines. Our engineers and scientists compare these treatment options to our customer’s respective alternatives, e.g. filtration and adsorption units.

Development and application of emerging technologies along with their critical assessment in view of environmental and economic benefits is our daily task in several research projects. These national and international cooperations of several partners are often partially supported by public funds.   

Plasmaoxidation, electrooxidation, 172-nm UV-irradiation and catalytic degradation induced by UVA or sunlight can remove water pollutants at concentrations of several hundred mg/l but also at trace levels of a few ng/l. This is however only useful in those cases in which pollution prevention and biological treatment are inefficient.