This study characterized sonicated waste activated sludge (WAS) liquids as a possible carbon source for nitrogen removal. In this context, the effect of sonication density on chemical oxygen demand (COD) and nitrogen release was determined by particle size distribution (PSD) analysis and anoxic batch experiments. The increase in ultrasonic density from 0.8 W/mL to 1.6 W/mL had a slight impact on the soluble COD/total COD ratio. The high ultrasonic energy input increased the solubilization of nitrogenous organic substances and resulted in a low COD/TKN (total Kjeldahl nitrogen) ratio, which is not appropriate for nutrient removal systems. The change in ultrasonic power had a significant effect on COD fractionation of sonicated WAS liquid. The COD fraction at the size ranges higher than 1600 nm decreased from 44% to 3% as the energy input increased. The increase in specific energy raised the COD fraction, at the size ranges of <2 nm, from 11% to 23%. The PSD-based COD fractionation showed that increasing the sonication density markedly changed the size distribution. The anoxic batch tests indicated that the specific denitrification rate of sonicated WAS liquid was in the range of that reported for the slowly biodegradable fraction of the domestic wastewater and higher than those reported for agro-food wastewater.