Degradation of organic matter takes place in many processes within a wastewater treatment plant (WWTP). The need for optimisations and extended degradation processes for both organics and nitrogen will increase as outlet demands are strengthened and increasing loads are desired. The purpose of this work was to study the degradation of organic matter at WWTPs within two areas in particular; 1) degradation potential of mixed carbon sources for extended denitrification efficiency and 2) degradation and reduction of refractory organic matter by use of chemical oxidation methods. For biodegradability analysis, oxygen uptake rate (OUR) measurements were used in both parts of the study. Enhanced denitrification in activated sludge was established when mixtures of acetate and methanol were used. It resulted in nitrate uptake rates (NUR) higher than for the two individual carbon sources alone. Application of microautoradiography (MAR) and fluorescence in situ hybridisation (FISH) showed that bacteria belonging to the genus Azoarcus were the main responsible group for uptake of both methanol and acetate, which indirectly indicates that single cells belonging to this genus were simultaneously degrading both methanol and acetate. These observations together with increased NUR for mixed carbon sources demonstrate a potential for enhanced denitrification by addition of a mixed carbon source. A post-ozonation step reduced the amount of refractory organic matter in the effluent from a municipal WWTP well below discharge limits. According to OUR measurements, the biodegradability of the organic matter increased after ozonation up to doses around 250 mg O3/l (supplied ozone). Higher ozone doses result in less biodegradable organic matter probably due to total mineralization of the organics. The following two aerobic biofilters (Kaldnes Moving Bed™ Bioreactor and a submerged filter Biocarbone) reduced all the biodegradable organic matter produced in the ozone stage and both systems seem to be a good complement to the ozonation plant in order to reduce refractory organic matter in the wastewater. A screening for potential methods suitable for degradation and/or reduction of refractory organics was performed on landfill leachate. It was demonstrated that biological pre-treatment of landfill leachate resulted in larger reduction of organic matter compared to untreated leachate when the same ozone doses were applied. Ozonation in combination with hydrogen peroxide was established to be the altogether most efficient oxidation method for the tested leachate when results from both chemical and biological processes in combination were taken into account.