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.
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.
Example: Life extension of metalworking fluids
In several projects we investigated whether the efficient reduction of microbial contamination in cooling lubricant emulsions can be achieved by means of ultrasound or ultrasound in combination with specific AOP processes. Depending on the type and concentration of the microbial contamination we were able to achieve a sustained reduction, without impairing the quality of the cooling lubricant.