This study presents the development and validation of a comprehensive activated sludge model for degradation of the organic solvent N,N-dimethylacetamide (DMAc) under aerobic conditions in wastewater treatment plants. Through systematic batch experiments measuring oxygen uptake rates (OUR), we identified the need to incorporate of incorporating two distinct biomass fractions to accurately capture the degradation kinetics observed in practice. Model development and calibration is based on substrate pulse experiments with intermediate substances. The degradation pathway proceeds via dimethylamine (DMA) with acetic acid and methanol as intermediate products. Two distinct biomass fractions were required for an accurate simulation: one for the direct growth kinetics on methanol degradation and another for the simultaneous storage and growth processes of acetate degradation. Model calibration revealed unusually high growth and decay rates compared to those of conventional municipal activated sludge systems, indicating the presence of specialized microbial populations adapted to using DMAc as the sole carbon source. The model successfully reproduced the complex respiratory patterns observed during the substrate pulse experiments involving both single compounds and combinations. This work provides valuable insights for designing and operating biological treatment systems for DMAc-containing industrial wastewaters. The respiration-based method of model development can be adapted to other single-substrate degradation processes involving comparable substrates.