The study evaluated the energy recovery potential of denim processing wastewater when treated by a membrane activated sludge, (MAS), implementing a specific model derived for this configuration. Emphasis was also placed upon particle size distribution (PSD) analysis. The results were benchmarked against sewage. They revealed an energy conservation rate of around 50 % for denim wastewater for super-fast operation at sludge age (θX) 1.0–2.0 d, declining to 40 % for high-rate operation at sludge age 3.0–4.0 d. The corresponding rates were higher for sewage, indicating that energy conservation was not a function of wastewater strength, but depended more on the PSD of COD. The results revealed that MAS was the best option for energy conservation, which was optimized when it was operated at super-fast mode (θX < 2.0 d). They also showed that the conventional activated sludge (AS) system with gravity settling had inherent drawbacks, because stability of operation necessary to ensure energy conservation and effluent quality at this operation range cannot be maintained. PSD analysis should be recognized and implemented as a new and indispensable tool for wastewater characterization.