Relationship between structural conversion of refractory pollutants and their biodegradability improvement was clarified under various hydrothermal conditions to know the usability of treated refractory pollutants in biological treatment methods. Hydrothermal reaction was also verified by using real wastewater. High-molecular-weight structured poly vinyl alcohol and hydrocarbon structured chloroacetic acids were partially fractured by hydrolysis and dehydration at the beginning of hydrothermal reaction. Then partially fractured refractory pollutants were rapidly converted to easily degradable substances by thermal decomposition depending on hydrothermal conditions. This significant change occurred within 3 min from the moment reaching the desired conditions at 350 and 400oC. These results were very similar to the results of soybean milk, except dyeing wastewater involving aromatic compounds. Excess sludge treated by hydrothermal reaction was also converted to easily degradable substrate as carbon sources in a bioreactor. Hydrothermal reaction should be adjusted to increase the amount of biodegradable products in reactants, without much reduction of total organic carbon. The content change of refractory pollutants and excess sludge by hydrothermal reaction was the most important factor on the viewpoint of treated reactant reuse, for the following biological treatment methods.

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